Advances in Agronomy, Volume 101

ADVANCES IN AGRONOMY Advisory Board

PAUL M. BERTSCH

RONALD L. PHILLIPS

University of Kentucky

University of Minnesota

KATE M. SCOW

LARRY P. WILDING

University of California, Davis

Texas A&M University

Emeritus Advisory Board Members

JOHN S. BOYER

KENNETH J. FREY

University of Delaware

Iowa State University

EUGENE J. KAMPRATH

MARTIN ALEXANDER

North Carolina State University

Cornell University

Prepared in cooperation with the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America Book and Multimedia Publishing Committee DAVID D. BALTENSPERGER, CHAIR LISA K. AL-AMOODI

CRAIG A. ROBERTS

WARREN A. DICK

MARY C. SAVIN

HARI B. KRISHNAN

APRIL L. ULERY

SALLY D. LOGSDON

Academic Press is an imprint of Elsevier 525 B Street, Suite 1900, San Diego, CA 92101-4495, USA 30 Corporate Drive, Suite 400, Burlington, MA 01803, USA 32 Jamestown Road, London, NW1 7BY, UK Radarweg 29, PO Box 211, 1000 AE Amsterdam, The Netherlands First edition 2009 Copyright # 2009 Elsevier Inc. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise without the prior written permission of the publisher Permissions may be sought directly from Elsevier’s Science & Technology Rights Department in Oxford, UK: phone (+44) (0) 1865 843830; fax (+44) (0) 1865 853333; email: [email protected]. Alternatively you can submit your request online by visiting the Elsevier web site at http://elsevier.com/locate/permissions, and selecting Obtaining permission to use Elsevier material Notice No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Because of rapid advances in the medical sciences, in particular, independent verification of diagnoses and drug dosages should be made ISBN-13: 978-0-12-374817-1 ISSN: 0065-2113 (series) For information on all Academic Press publications visit our website at elsevierdirect.com Printed and bound in USA 09 10 11 12 10 9 8 7 6 5 4 3 2 1

CONTRIBUTORS

Numbers in Parenthesis indicates the pages on which authors’ contributors begin

Asher Bar-Tal ( 315) Department of Soil Chemistry and Plant Nutrition, Institute of Soils, Water and Environmental Sciences, Agricultural Research Organization, The Volcani Center, Bet-Dagan 50250, Israel Shahzad M. A. Basra ( 351) Department of Crop Physiology, University of Agriculture, Faisalabad 38040, Pakistan Kevin Coleman (1) Department of Soil Science, Rothamsted Research, Harpenden, Herts AL5 2JQ, United Kingdom Benjamin O. Danga ( 315) Department of Soil Chemistry and Plant Nutrition, Institute of Soils, Water and Environmental Sciences, Agricultural Research Organization, The Volcani Center, Bet-Dagan 50250, Israel, and Department of Crops, Horticulture and Soils, Egerton University, Njoro, Kenya M. Farooq ( 351) International Rice Research Institute (IRRI), Metro Manila, Philippines, and Department of Agronomy, University of Agriculture, Faisalabad 38040, Pakistan Y. J. Gao (123) Northwestern Science and Technology University of Agriculture and Forestry, Yangling, Shaanxi 712100, P.R. China S. Heuer (59) International Rice Research Institute, Metro Manila, Philippines G. Howell (59) International Rice Research Institute, Metro Manila, Philippines T. T. Hu (123) Northwestern Science and Technology University of Agriculture and Forestry, Yangling, Shaanxi 712100, P.R. China A. Ismail (59) International Rice Research Institute, Metro Manila, Philippines

ix

x

Contributors

O. Ito ( 351) Japan International Research Center for Agricultural Sciences, Tsukuba, Japan S. V. K. Jagadish (59) International Rice Research Institute, Metro Manila, Philippines A. Edward Johnston (1) Lawes Trust Senior Fellow, Rothamsted Research, Harpenden, Herts AL5 2JQ, United Kingdom N. Kobayashi ( 351) International Rice Research Institute (IRRI), Metro Manila, Philippines S. X. Li (123) Northwestern Science and Technology University of Agriculture and Forestry, Yangling, Shaanxi 712100, P.R. China K. P. Prabhakaran Nair (183) Distinguished Visiting Scientist, Indian Council of Agricultural Research, New Delhi, India Josephine P. Ouma ( 315) Department of Crops, Horticulture and Soils, Egerton University, Njoro, Kenya H. Pathak (59) International Rice Research Institute, New Delhi, India Paul R. Poulton (1) Department of Soil Science, Rothamsted Research, Harpenden, Herts AL5 2JQ, United Kingdom E. Redona (59) International Rice Research Institute, Metro Manila, Philippines R. Serraj (59) International Rice Research Institute, Metro Manila, Philippines R. K. Singh (59) International Rice Research Institute, Metro Manila, Philippines B. A. Stewart (123) Dryland Agriculture Institute, West Texas A&M University, Canyon, TX 79016, USA K. Sumfleth (59) International Rice Research Institute, Metro Manila, Philippines A. Wahid ( 351) Department of Botany, University of Agriculture, Faisalabad 38040, Pakistan

Contributors

xi

Isaiah I. C. Wakindiki ( 315) Department of Crops, Horticulture and Soils, Egerton University, Njoro, Kenya Z. H. Wang (123) Northwestern Science and Technology University of Agriculture and Forestry, Yangling, Shaanxi 712100, P.R. China R. Wassmann (59) Research Center Karlsruhe (IMK-IFU), Garmisch-Partenkirchen, Germany, and International Rice Research Institute, Metro Manila, Philippines

PREFACE

Volume 101 continues the rich tradition of the previous 100 volumes of Advances in Agronomy, containing six comprehensive and contemporary agronomic reviews. Chapter 1 deals with soil organic matter and its significance in sustainable agriculture and carbon dioxide fluxes. Chapter 2 discusses impacts of climate change on rice production and the physiological and agronomic basis for adaptation strategies. Chapter 3 covers the management of nitrogen in dryland soils of China. Chapter 4 provides a thorough review on agronomic and economic aspects of important industrial crops with emphasis on areca, cashew, and coconut. Chapter 5 reviews legume– wheat rotation effects on residual soil moisture, nitrogen, and wheat yield in tropical regions. Chapter 6 provides strategies for increasing rice production with less water including genetic improvements and different management systems. I thank the authors for their excellent contributions. DONALD L. SPARKS Newark, Delaware, USA

xiii

C H A P T E R

O N E

Soil Organic Matter: Its Importance in Sustainable Agriculture and Carbon Dioxide Fluxes A. Edward Johnston,* Paul R. Poulton,† and Kevin Coleman† Contents 1. Introduction 2. Some Aspects of the Nature and Behavior of Soil Organic Matter 2.1. The nature and determination of soil organic matter 2.2. Relationship between amount and C:N ratio of added plant material and organic matter in soil 2.3. Equilibrium levels of soil organic matter 3. Changes in the Organic Content of Soils and Their Causes 3.1. Effects of fertilizer and manure inputs on soils of different texture where cereals are grown each year 3.2. Effects of short-term leys interspersed with arable crops 3.3. Effect of different types of organic inputs to soils growing arable crops 3.4. Effects of straw incorporation 3.5. Effect of different arable crop rotations on the loss of soil organic matter 3.6. Increases in soil organic matter when soils are sown to permanent grass 4. Soil Organic Matter and Crop Yields 4.1. Arable crops grown continuously and in rotation 5. Explaining the Benefits of Soil Organic Matter 5.1. Organic matter, soil structure, and sandy loam soils 5.2. Separating nitrogen and other possible effects of soil organic matter 5.3. Soil organic matter and soil structure 5.4. Soil organic matter and soil phosphorus and potassium availability 5.5. Soil organic matter and water availability

* {

2 5 5 6 8 11 11 15 22 25 26 27 28 28 37 37 38 40 43 45

Lawes Trust Senior Fellow, Rothamsted Research, Harpenden, Herts AL5 2JQ, United Kingdom Department of Soil Science, Rothamsted Research, Harpenden, Herts AL5 2JQ, United Kingdom

Advances in Agronomy, Volume 101 ISSN 0065-2113, DOI: 10.1016/S0065-2113(08)00801-8

#

2009 Elsevier Inc. All rights reserved.

1

2

A. Edward Johnston et al.

6. Modeling Changes in Soil Organic Matter 7. Disadvantages from Increasing Soil Organic Matter Acknowledgments References

46 52 54 54

Abstract Soil organic matter is important in relation to soil fertility, sustainable agricultural systems, and crop productivity, and there is concern about the level of organic matter in many soils, particularly with respect to global warming. Longterm experiments since 1843 at Rothamsted provide the longest data sets on the effect of soil, crop, manuring, and management on changes in soil organic matter under temperate climatic conditions. The amount of organic matter in soil depends on the input of organic material, its rate of decomposition, the rate at which existing soil organic matter is mineralized, soil texture, and climate. All four factors interact so that the amount of soil organic matter changes, often slowly, toward an equilibrium value specific to the soil type and farming system. For any one cropping system, the equilibrium level of soil organic matter in a clay soil will be larger than that in a sandy soil, and for any one soil type the value will be larger with permanent grass than with continuous arable cropping. Trends in long-term crop yields show that as yield potential has increased, yields are often larger on soils with more organic matter compared to those on soils with less. The effects of nitrogen, improvements in soil phosphorus availability, and other factors are discussed. Benefits from building up soil organic matter are bought at a cost with large losses of both carbon and nitrogen from added organic material. Models for the buildup and decline of soil organic matter, the source and sink of carbon dioxide in soil, are presented.

1. Introduction The following quotation taken from Sanskrit literature was written perhaps 3500 or 4000 years ago and yet it is as relevant today as it was then. Besides emphasizing the importance of the soil upon which food is grown, the phrase ‘‘surround us with beauty’’ brings to the fore issues about the environment: Upon this handful of soil our survival depends. Husband it and it will grow our food, our fuel and our shelter and surround us with beauty. Abuse it and the soil will collapse and die taking man with it

The decline and collapse of many ancient civilizations is clear evidence of the truth of these statements. In Mesopotamia, the Sumerian society, which started about 3000 BC, became the first literate society in the world, but then gradually perished as its agricultural base declined as the irrigated soils on which its food was produced became so saline that crops could no longer be

Soil Organic Matter: Its Importance in Sustainable Agriculture and Carbon Dioxide Fluxes

3

grown. In Mesoamerica, the earliest settlements of the Mayan society date from about 2500 BC. Intellectually this society was remarkable, particularly in its study of astronomy, yet its decline started once internal and external factors led it to give too little attention to managing its intensive agriculture in terraced fields on the hillsides and raised fields in swampy areas. Although soil cultivation and growing crops produce food for people and animals, the appreciation and understanding of the processes involved took many centuries. It was in 1840 that Liebig (1840) presented his report entitled ‘‘Organic Chemistry in its Application to Agriculture and Physiology’’ to the British Association for the Advancement of Science. In it he noted that: ‘‘The fertility of every soil is generally supposed by vegetable physiologists to depend on . . . humus. This substance (is) believed to be the principle nutriment of plants and to be extracted by them from the soil.’’ The hypothesis was that plant roots have tiny mouths and ingest small fragments of humus directly. Liebig demolished this hypothesis and he expressed the view that humus provides a slow and lasting source of carbonic acid. This could be absorbed directly by the roots as a nutrient or it could release elements like potassium (K) and magnesium (Mg) from soil minerals. The importance of soil organic matter (SOM) in soil fertility was questioned by the early results from the field experiments started by Lawes and Gilbert at Rothamsted between 1843 and 1856. The results showed that plant nutrients like nitrogen (N), phosphorus (P), and K, when added to soil in fertilizers and organic manures, like farmyard manure (FYM), were taken up by plant roots from the soil. As the annual applications of fertilizers and FYM continued, the level of SOM in FYM-treated soils increased relative to that in fertilizer-treated soils, but even into the 1970s, yields of cereals and root crops were very similar on both soils (see later). This gave rise to the belief that, provided plant nutrients were supplied as fertilizers, extra SOM was of little importance in producing the maximum yields of the crop cultivars then available. It should be noted, however, that Lawes and Gilbert never said that fertilizers were better than FYM. They realized that no farmer would ever have the amount of FYM they were using (35 t ha
1 annually on each FYM-treated plot) to apply to every field every year. However, what they appreciated was that by using fertilizers, there was the possibility that farmers could produce the increasing amounts of food that would be necessary to feed the rapidly increasing population of the UK at that time. Very much more recently, Holmberg et al. (1991), like many others, have talked about the importance of agricultural sustainability: Sustainable agriculture is not a luxury . . . When an agricultural resource base erodes past a certain point, the civilisation it has supported collapses . . . There is no such thing as a post-agricultural society. (Holmberg et al., 1991)

Any definition of sustainability related to the managed use of land must include physical, environmental, and socioeconomic aspects. No agricultural

4

A. Edward Johnston et al.

system will be sustainable if it is not economically viable both for the farmer and for the society of which he is a part. But, economic sustainability should not be bought at the cost of environmental damage, which is ecologically, socially, or legally unacceptable or physical damage that leads to irreversible soil degradation or uncontrollable outbreaks of pests, diseases, and weeds. Within these boundaries, food production requires fertile soils, the level of fertility needed depending on the farming system practiced in each agroecological zone. Irrespective of the level required, soil fertility depends on complex and often incompletely understood interactions between the biological, chemical, and physical properties of soil. Of these various properties, the role of SOM has been frequently discussed. Russell (1977) noted that: It has long been suspected, ever since farmers started to think seriously about raising the fertility of their soils from the very low levels that characterised mediaeval agriculture, that there was a close relationship between the level of organic matter, or humus, in the soil and its fertility. In consequence good farmers have always had, as one of their goals of good management, the raising of the humus content of their soils.

Russell went on to point out that present-day economic factors have caused farmers to adopt practices which may cause the level of SOM to decline. Consequently, he stressed that the research community must seek to explain and quantify the effects of SOM in soil fertility and crop production to help farmers develop cropping systems that will prevent or minimize any adverse effect that a lowering of SOM levels may bring about. Thus, there are three important topics to which answers have to be sought, namely:  

Is SOM important in soil fertility? Over what time scales and with what farming practices do SOM contents change?  Can the various soil factors that might/can contribute to the ‘‘organic matter effect’’ be identified, separated, and quantified? Here, we attempt to provide answers to these questions by presenting data on the effects of fertilization and cropping systems on the level and rate of change of organic matter in soils of the long-term experiments at Rothamsted and Woburn. We show how SOM affects crop productivity in these experiments and discuss ways in which SOM has caused and/or affected these changes. Examples of the use of these long-term data sets to provide models for the turnover of SOM are given because of their use in discussions of carbon dioxide fluxes. The soil at Rothamsted is a well- to moderately well-drained silty clay loam classified as Batcombe Series (Soil Survey of England and Wales, SSEW), as an Aquic Paleudalf (USDA) and as a Chromic Luvisol (FAO). The soil at Woburn is a well-drained, sandy

Soil Organic Matter: Its Importance in Sustainable Agriculture and Carbon Dioxide Fluxes

5

loam classified as Cottenham Series (SSEW), as a Quartzipsammetric Haplumbrept (USDA) and as a Cambric Arenosol (FAO).

2. Some Aspects of the Nature and Behavior of Soil Organic Matter 2.1. The nature and determination of soil organic matter Soil organic matter consists of organic compounds containing carbon (C), hydrogen (H), oxygen (O), nitrogen (N), sulfur (S), and phosphorus (P). Most agronomic studies of SOM are interested in it as a possible source of N, S, and P or in its contribution to the biological and physical properties of soil and these are discussed in this chapter. The constituents of SOM can range from undecomposed plant and animal tissues through ephemeral decay products to fairly stable brown and black material often called humus. The latter is usually the largest proportion and it contains no trace of the anatomical structure of the material from which it was derived. Percent SOM is measured by multiplying percent organic C (%C) by the factor 1.724, derived from the %C in peat. The determination of %C includes C in the soil microbial biomass, but this usually accounts for less than 5% of the total soil organic carbon so this does not greatly affect the estimate of SOM. Throughout this chapter %C is % total organic C. The surface layer of many soils growing arable crops contains 1–3%C as SOM while grassland and forest soils usually contain somewhat more. The ratio (by weight) of organic C to organic N in SOM is relatively constant and ranges between about 9:1 and 14:1 for different soils under different management conditions, but excluding strongly acid and poorly drained soils. Why the ratio falls within such narrow limits is unclear. It may relate to the fact that SOM is largely a fairly uniform end product from the microbial decomposition of plant and animal residues together with material that is very resistant to such attack. The C:N ratio of material added to soil determines whether N will be released or fixed in SOM as the material decomposes. For example, the Market Garden experiment started in 1942 on the sandy loam at Woburn compared four organic manures. They and their C:N ratios were FYM, 13.0:1; vegetable compost, 13.8:1; sewage sludge (biosolids), 9.5:1; and a compost of biosolids and straw, 11.6:1. After 25 years, the C:N ratio of the differently treated soils ranged from only 10.0:1 to 11.1:1 ( Johnston, 1975). All but the biosolids would have released some N as the result of their decomposition by microbial activity, but the biosolids would have fixed some mineral N. Similarly, straw with a C:N ratio of 100:1 requires mineral N from the soil for its decomposition but N-rich crop residues like those of lucerne (alfalfa) or clover with a C:N ratio less than 40:1 release N as they are decomposed.

6

A. Edward Johnston et al.

2.2. Relationship between amount and C:N ratio of added plant material and organic matter in soil In the Woburn Market Garden experiment mentioned earlier, the four organic manures were each applied at the same two amounts of the fresh material but because of differences in composition and percent dry matter, different amounts of organic matter were added between 1942 and 1967. These amounts (in t ha
1) for the single and double application were, respectively, FYM, 138 and 276; biosolids, 165 and 330; vegetable compost, 118 and 236; and biosolids/straw compost, 118 and 236. There was a linear relationship between the amount of organic matter added and %C in soil (Fig. 1) that accounted for 82% of the variance ( Johnston, 1975). However, much C and N was lost from the soil following the addition of these different manures. At the end of 25 years, 75% of the C added in FYM had been lost; similar losses from added FYM occurred in the Woburn Green Manuring experiment ( Johnston, 1975 using data from Chater and Gasser, 1970). After 18 years, of the C added in biosolids, 64% had been lost and about 60% from the composts. Much the same proportions of added N were lost as for C, that is, the losses were appreciable. Thus, there is a major cost in terms of the losses of C and N from the soil, with associated environmental impacts, when building up SOM from additions of organic manures. It has been noted that SOM is the end product of microbial decomposition of organic material added to soil which could explain its fairly constant

Organic C in soil, 0–23 cm, %

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0

50

100

150 200 250 Organic matter added, t ha−1

300

350

Figure 1 Relationship between organic matter added (t ha
1) during 1942–1950 and 1942–1960 and percent organic carbon (%C) in the top 23 cm of a sandy loam soil in 1951 and 1960. Market Garden experiment, Woburn. FYM, single □, double ▪; biosolids, single △, double ▲; FYM compost, single ○, double ; biosolids compost, single e, double ^. Manure applied as fresh material, single and double rate 37.5 and 75.0 t ha
1 each year. (Adapted from Johnston et al., 1989.)

Soil Organic Matter: Its Importance in Sustainable Agriculture and Carbon Dioxide Fluxes

7

C:N ratio. The uniformity of composition is illustrated again by data from the Woburn Market Garden experiment discussed earlier. The treatment with biosolids and biosolids/straw compost ceased in 1961 because of concerns about heavy metal additions in these two materials, and no further organic manures were applied to these plots. The use of vegetable compost ended also in 1961 and was replaced by FYM, but both FYM treatments ceased in 1967. The different types and amounts of organic manures applied had increased SOM to different levels (Fig. 1) by the time the additions ceased; SOM then began to decline from these different levels starting in 1962 for the two biosolids treatments and in 1968 for the FYM treatments. The soil on each plot was sampled and %C determined for a number of years and an individual carbon decay curve was produced for each plot. Visual observation suggested that these individual decay curves were sections of a single decay curve and an exponential decay model was then fitted to each individual curve; by using horizontal shifts (in years) all eight decay curves were brought into coincidence (Fig. 2). The shifts required to bring the curves into coincidence were related only to the different starting levels of SOM and not to the different organic manure added. Thus, the microbial decomposition of these different manures had produced SOM that decayed at the same rate suggesting a very uniform composition. The half-life of the SOM, relative to the asymptotic %C, was calculated to be 20.1 years from the fitted C decay curve (Fig. 2). The half-life for organic N (not shown) was calculated to be 12.4 years. The half-life for C and N was calculated relative to the equilibrium level of soil C

Carbon, t ha−1, in soil 0–23 cm

90 80 70 60 50 40 30 20 10

Fitted curve Asymptote

0 −20 −15 −10 −5 0 5 10 Years, shifted to fit model

15

20

Figure 2 Decline in soil organic carbon (t ha
1) in the top 23 cm of a sandy loam soil. Market Garden experiment, Woburn. Individual decline curves for each treatment shifted horizontally to fit model (see text). FYM, single □, double ▪; biosolids, single △, double ▲; FYM compost, single ○, double ; biosolids compost, single e, double ^. (Adapted from Johnston et al., 1989.)

8

A. Edward Johnston et al.

and N that would be reached eventually. Thus, it would take 20.1 years for organic C to decline by half between any starting level and the equilibrium level for soil C on this soil type and with this cropping system. The shorter half-life for organic N suggests that N-rich constituents of SOM decompose more quickly than those with less N. In another experiment on the sandy loam soil at Woburn, three amounts of peat were added for a number of years to build up different levels of SOM where horticultural crops were grown ( Johnston and Brookes, 1979). Once peat applications ceased, the decline in %C was monitored during a number of years and again the three individual C decay curves could be brought into coincidence by horizontal shifts ( Johnston et al., 1989); the half-life of the peat-derived soil C was 12.4 years. The difference in the C half-lives in the two experiments is interesting. Possibly, it relates to the different C:N ratios of the organic materials (45:1 for peat and a range from 9.5 to 13.8:1 for the other organic manures) and this could lead to different equilibrium levels of SOM in the two experiments on the same soil type.

2.3. Equilibrium levels of soil organic matter The concept of equilibrium levels of SOM, introduced in the paragraph above, is crucially important. It is not always appreciated that SOM changes toward an equilibrium level in any farming system and the level will vary with a number of factors. Supporting evidence for this statement is presented in this chapter. However, there is a paucity of appropriate data because in temperate climates SOM changes slowly and long-term experiments with unchanged cropping and management are required to monitor such changes and determine the appropriate equilibrium level. Existing evidence shows that the amount of organic matter in soils depends on:    

The input of organic material and its rate of oxidation The rate at which existing SOM decomposes Soil texture Climate

The first two factors depend on the farming system practiced. In addition to the aboveground crop residues that are ploughed-in, there will also be different amounts of root remaining in the soil. Root weights are difficult to determine but some indication of the differences can be seen in the different root length densities in the top 20 cm soil, which can vary from 0.8 to 12.2 cm cm
3 for broad beans and winter wheat, respectively ( Johnston et al., 1998; Table 8). Decomposition of added and existing organic matter in soil is by microbial activity and the extent and speed of decomposition depends on a carbon source for the microbes, temperature, and the availability of oxygen and water. Thus, activity in the northern hemisphere will be greater in autumn when C from crop residues is incorporated into warm

Soil Organic Matter: Its Importance in Sustainable Agriculture and Carbon Dioxide Fluxes

9

soil and rainfall provides adequate moisture. In addition, the extent of soil cultivation affects oxygen availability and hence microbial activity. Consequently, SOM will decline more quickly when soil is cultivated too frequently and unnecessarily. Soil cultivation and a lack of organic inputs, for example, when soils are fallowed (i.e., grow no crop) to control weeds can lead to an appreciable loss of SOM. In the Broadbalk Winter Wheat experiment at Rothamsted, the plots were divided into five sections in 1925 so that weeds could be controlled by fallowing the individual sections in sequence. In 1968, the five sections were each divided into two to give ten sections, so that wheat continued to be grown each year on some sections while on others there were two rotations, one included a fallow year, the other potatoes. Between 1925 and 2000, the number of years that the different sections had been fallowed or grown potatoes ranged from 8 to 24 and by 2000, %C in the top 23 cm on fertilizer-treated plots was strongly linearly related (R2 = 0.9266) to the number of fallow and potato years. From the linear relationship, soil with least fallowing contained 1.16%C and this declined to 0.91%C with most fallowing. Soil texture, besides affecting some of these properties, is also important because clay helps to stabilize SOM and limit its decomposition. Besides rainfall, the other important climatic factor is temperature because it greatly affects the rate of organic matter decomposition. When Jenkinson and Ayanaba (1977) prepared a bulk sample of 14C-labeled plant material and added part to similar textured soils, one in the UK and the other in Nigeria, the decomposition curve for the labeled material was the same in both soils. But the rate of decomposition was four times faster in Nigeria than in the UK due to the difference in temperature at the two sites. Excessive rainfall can create anaerobic conditions in soil and then, especially at low ambient temperature, plant material decomposes very slowly leading to the formation of peat. The four factors listed above interact so that the equilibrium level of SOM is specific to the farming system, soil type, and climate. In general under similar climatic conditions, for any one cropping system, the equilibrium level of SOM in a clay soil will be larger than in a sandy soil, and for any one soil type the equilibrium level will be larger under permanent grassland than under continuous arable cropping. Examples are given later. The fact that SOM changes toward an equilibrium value dependent on the interaction of the four factors listed above does not seem to have been appreciated and mentioned in two recent papers, one by Khan et al. (2007) and the other by Bellamy et al. (2005). Khan et al. (2007) discussing the effect of N fertilization on C sequestration in soil, support their contention that the application of N fertilizers causes a decrease in soil C by presenting, very briefly (Khan et al., 2007; Table 4) results from two long-term Rothamsted experiments ( Jenkinson, 1991; Jenkinson and Johnston, 1977) and one at Woburn (Christensen and Johnston, 1997). There was an initial decline in soil C in the first few years of the Rothamsted

10

A. Edward Johnston et al.

experiments where NPK fertilizers were applied but the decline was less than on plots with PK but no N. Khan et al. (2007) suggest that comparing %C on soils with NPK and PK only is unacceptable, but why? For any one comparison of a with and without N treatment, the result is ‘‘a snapshot in time’’ and a perfectly valid comparison can be made between soils with and without fertilizer N and the effect on %C in soil. For example, in the Broadbalk Winter Wheat experiment at Rothamsted, there are plots which, since 1852, have had PKMg either without or with 144 kg N ha
1 each year. Percent organic C in these soils without and with N has been at equilibrium, about 0.93 and 1.12%C, respectively, during the last 100 years. Additional N treatments testing 240 and 288 kg N ha
1 were started in 1985 on plots that had received smaller amounts of fertilizer N previously. Since 1985, %C has increased by about 16%, to 1.22 and 1.29% C on plots with 240 and 288 kg N ha
1, respectively, concentrations larger than that in the soil getting 144 kg N ha
1; adding more fertilizer N has increased %C. Similar data showing that SOM is increased where fertilizer N is applied comes from many long-term experiments (Glendining and Powlson, 1995). Applying N increases both crop yield and the return of plant residues to the soil and more carbon is retained in the soil. The initial decline in soil C in the Rothamsted and Woburn experiments noted by Khan et al. (2007) was not due to the use of N fertilizer; it was because there was a change in farming system. For many decades prior to the establishment of the experiments, the fields had grown arable crops in rotation: turnips (Brassica napus), spring barley, a forage or grain legume, and winter wheat. Besides crop residues, there were two additional inputs of organic matter, from occasional applications of FYM to the turnips and from weeds, which grew in all four crops, were difficult to control at that time, and often made considerable growth after harvest of the crop and before ploughing. It is most probable that the very small amount of SOM in the soils getting only fertilizers in the experiments on arable crops started by Lawes and Gilbert in the 1840s–1850s compared to the amount in other soils growing arable crops on the Rothamsted farm is largely due to the fact that weeds were controlled very efficiently in the experiments. Changes in the soil C status of the Morrow plots at Illinois presented by Khan et al. (2007; Fig. 2) could equally well be explained due to the changes in husbandry and cropping leading to different C inputs and SOM changing toward a new equilibrium level associated with the new system. This would be especially so for plots where organic manure inputs had ceased some years previously. We agree with Khan et al. (2007) when they assert that when long-term sustainability of an agricultural system is discussed then changes in SOM over time are important. But the importance is related to the equilibrium level of SOM, the speed with which it is reached, and the productivity of the soil at the equilibrium level. For example, in the two Rothamsted experiments referred to above, there was a decline in SOM initially, more

Soil Organic Matter: Its Importance in Sustainable Agriculture and Carbon Dioxide Fluxes

11

without than with applied N, but the new equilibrium level of SOM in these soils has been maintained for the last 100 years (see later), and, where NPK fertilizers are applied yields have increased over time as discussed later. It seems to us that much of the current discussion about soil carbon sequestration is related to interest in carbon trading. Such discussion should be based on acknowledging that, for any farming system and its management, including fertilizer and manure inputs, there is an equilibrium level of SOM dependent on the interactions of the four factors listed above. In any soil, the level of SOM does not increase indefinitely. The experimental data presented here from experiments in a temperate climate show that in different farming systems with acceptable fertilizer inputs, increases and decreases in SOM are often small and in most cases the new SOM equilibrium level has been reached only after many years. Achieving significant increases in the equilibrium level of SOM in most farming systems requires very large inputs of organic matter and these have to be maintained if SOM is not to decline. Similarly, in a recent paper discussing C losses from all soils across England and Wales during the period 1978–2003, Bellamy et al. (2005) make no mention of the fact that where C has been lost this is most probably because of changes in farming systems. Such changes have included the ploughing of grassland and growing arable crops with a decrease in annual C inputs and decline in SOM as it changes toward a new equilibrium value. The authors used data from the National Soil Inventory of England and Wales, which holds soil data for 5662 soils sampled 0–15 cm at the intersections of an orthogonal 5-km grid in 1978–1983. Sufficient subsets of the sites were resampled at intervals from 12 to 25 years after the original sampling to be able to detect changes in C content with 95% confidence (Bellamy et al., 2005). While the authors highlight losses of soil carbon, they make little mention of the fact that for soils originally under arable cropping and maintained in mainly arable cropping, the C content of these soils remained largely unchanged or had increased slightly. These soils had reached the appropriate SOM equilibrium value when the initial sample was taken and have remained at this level subsequently. The loss of C from soils will only be halted if farming systems change and any change must be financially viable for the farmer and continue to provide food and feed in both amount and quality.

3. Changes in the Organic Content of Soils and Their Causes 3.1. Effects of fertilizer and manure inputs on soils of different texture where cereals are grown each year The effect of organic matter inputs and soil texture on the level of SOM and the rate of change as it moves toward the appropriate equilibrium level is well illustrated by changes in %C in the top 23 cm of soil during more than

12

A. Edward Johnston et al.

3.5

Organic C in soil, 0–23 cm, %

3.0 2.5 2.0 1.5 1.0 0.5 0.0 1840

1860

1880

1900

1920

1940

1960

1980

2000

Figure 3 Changes in percent organic carbon (%C) in the top 23 cm of a silty clay loam soil, Broadbalk Winter Wheat experiment, Rothamsted. Annual treatments: unmanured since 1844, x; PKMg plus 144 kg N ha
1 since 1852, ▪; 35 t ha
1 FYM since 1844, ▲; 35 t ha
1 FYM since 1885 plus 96 kg N ha
1 since 1968, ^. A

100 90

Organic C in soil, t ha−1

80 70

B 100 90 80 70

60

60

50

50

40

40

30

30

20

20

10

10

0 1840 1860 1880 1900 1920 1940 1960 1980 2000 Year

0 1840 1860 1880 1900 1920 1940 1960 1980 2000 Year

Figure 4 Changes in organic carbon (t ha
1) in the top 23 cm of a silty clay loam soil. (A) Hoosfield Continuous Barley experiment, Rothamsted. Annual treatments since 1852: unmanured ▲; NPK fertilizers ; 35 t ha
1 FYM ▪; 35 t ha
1 FYM 1852–1871 none since ^. (Adapted from Jenkinson and Johnston, 1977 with additional data). (B) Woburn; continuous cereals given inorganic fertilizers only ○; manured four-course rotation ▲. (Adapted from Mattingly et al., 1975.)

100 years of cropping, mainly with cereals, at Rothamsted and Woburn (Figs. 3 and 4). The Broadbalk Winter Wheat experiment was started in autumn 1843 on a field that had probably been in arable cropping for several

Soil Organic Matter: Its Importance in Sustainable Agriculture and Carbon Dioxide Fluxes

13

centuries; the soil is a silty clay loam. Winter wheat has been grown on all or most of the experiment each year since then. Changes in %C with four contrasted treatments are shown in Fig. 3. On the unfertilized plot, SOM probably declined a little initially and has then remained essentially constant at about 0.85%C, its equilibrium level, for about 150 years. Applying 144 kg N ha
1 together with P and K each year gave larger crops and organic matter returns in stubble and roots have been greater than on the unfertilized plot. In this soil, SOM has remained largely unchanged at its equilibrium level, about 1.12%C, for many years and it now contains about 25% more SOM than the unfertilized control. Where 35 t ha
1 FYM has been applied annually since autumn 1843, %C increased rapidly at first and then more slowly as SOM approached the equilibrium level for this treatment. This soil now contains about 2.82%C, some 2.5 times more than the unfertilized soil. A second FYM treatment (also 35 t ha
1) was started in 1885 and the change in SOM on this plot closely mirrors that on the original FYM plot. Currently this soil contains about 2.65%C, some 2.4 times more than that in the control soil. On the two FYM plots, %C declined between 1914 and 1936 (the data points for these 2 years are joined by dotted lines) because there were major changes in this period. FYM continued to be applied each year until 1925 so SOM was still increasing. Then in 1925, it was decided to take steps to control weds by occasional fallow years with frequent soil cultivation to kill germinating seedlings. The experiment was divided into five sections and from 1926 to 1929 each section was fallowed in 2 of the 4 years, the soil was cultivated intensively and no FYM was applied in the fallow year. From 1931, each section was fallowed and no FYM was applied 1 year in five. Thus, as a consequence of fallowing, intensive soil cultivation and not applying FYM, SOM had declined by 1936. Fallowing 1 year in five and not applying FYM continued until 1967. The less frequent fallowing with less soil cultivation allowed SOM to increase again after 1936. Not having soil samples in 1925 was unfortunate but it highlights the need to take samples before major changes in husbandry practices when monitoring changes in soil fertility. The apparent convergence in %C on the two FYM treatments in recent years may be due to the extra N fertilizer added, since 1968, to the treatment which had received FYM since 1885. This extra N has increased yields and hence the return of organic residues to the soil. One aspect of change that can be followed occurred in 1968. The five sections were each halved so that a comparison could be made between wheat grown continuously on some half-sections and wheat grown in rotation on the others. The rotation included some fallow years and growing potatoes and field beans. The extra soil cultivations for these crops and fallowing caused SOM to decline by about 16% in the rotation soils between 1966 and 2000 compared with the SOM in soils continuously

14

A. Edward Johnston et al.

cropped with wheat. However, yields of the first and second wheat crops grown after a 2-year break always exceeded those of wheat grown continuously. Thus, any possible adverse effect of a small decrease in SOM due to rotational cropping was more than balanced by the beneficial effect of controlling soil pathogens, especially take-all. Figure 4A shows data from the Hoosfield experiment where spring barley has been grown each year since 1852 (Warren and Johnston, 1967). Jenkinson and Johnston (1977) showed that on the unmanured and fertilizer-treated plots of this experiment, %C declined a little initially and has then remained constant for more than 100 years at the equilibrium value for this farming system on this soil type. In the fertilizer-treated soil, %C is about 10% larger than in the unfertilized soil and has been for more than 100 years because annually more organic matter is ploughed-in as stubble and root residues from the larger crops grown with N fertilizer. Annual applications of FYM (35 t ha
1) increased %C rapidly at first and then more slowly as the equilibrium value for this input and cropping system was approached (Fig. 4A). The very slow decline in %C on the plot that received the same amount of FYM for the first 20 years and none since is very interesting. Even after 130 years, the level of SOM has not declined to that on the plot that receives fertilizers only (Fig. 4A). Presumably some SOM very resistant to microbial decomposition was accumulated from the applied FYM. The buildup of SOM with the FYM treatment in the long-term Rothamsted experiments accounts for only a fraction of the applied C and N, much of both has been lost, and the annual losses have increased as the SOM level approached the equilibrium level. Evidence for this comes from the Broadbalk experiment at Rothamsted where winter wheat has been grown each year since 1843 ( Johnston and Garner, 1969). The amount of FYM applied annually was 35 t ha
1 and the buildup of SOM is shown in Fig. 3. An estimated N balance and the average annual accumulation of soil N can be calculated for four periods using the N added in FYM and by aerial deposition and that removed in grain plus straw (Table 1). Nitrogen inputs increased in periods 3 and 4, and the N offtake increased as yield increased on the FYM plot until the 1980s. However, gradually less N has been retained as SOM approached the equilibrium level. Over the whole period of the experiment, although more N has been removed in the increasing yields of grain plus straw, this has not compensated for the declining retention of N in SOM. Consequently, the amount of N not accounted for has increased gradually from about 110 to 170 kg N ha
1 (Table 1; Johnston et al., 1989 with additional data). Rosenani et al. (1995) considered leaching of nitrate to be the dominant process causing these losses. On this experimental site leaching usually ceases in spring, however, even small anaerobic sites would lead to denitrification provided there was a C source for the denitrifying bacteria and Rosenani et al. (1995) did observe more denitrification on the FYM-treated soil rather than fertilizer-treated soil.

Soil Organic Matter: Its Importance in Sustainable Agriculture and Carbon Dioxide Fluxes

15

Table 1 Nitrogen balance and increase in soil nitrogen at various periods in the FYMtreated plot on the Broadbalk Winter Wheat experiment, Rothamsteda N input inb

a b

Period

FYM

Atmosphere

N in crop kg ha
1 each year

1852–1861 1892–1901 1970–1978 1996–2006

225 225 250 230

20 20 45 30

65 90 125 86

Increase in soil N

N not accounted for

70 30 5 5

110 125 165 169

Adapted from Johnston et al. (1989) with later additions. Atmospheric N inputs specific to Rothamsted: pre-1901 are estimates; 1970–1978 from Powlson et al. (1986) and 1996–2006 from Jenkinson et al. (2004).

Adding organic manures to soil can lead to large losses of C and N when the SOM level is near the equilibrium level. The effect of soil texture on SOM is illustrated by comparing changes in SOM in long-term experiments growing arable crops at Rothamsted with those at Woburn (Fig. 4). The sandy loam soil at Woburn contained more SOM at the start of the experiments there in 1876 than did the silty clay loam at Rothamsted in 1852 (cf. Fig. 4A and B) but with all-arable cropping at Woburn, SOM declined more quickly than it did at Rothamsted to approach an equilibrium level lower than that in the heavier textured soil at Rothamsted. At Woburn, even with a well-manured four-course rotation with good yields for the period (Fig. 4B, triangles), the decline in SOM was very similar to that where cereals were grown continuously (Fig. 4B, open circles). The difference in %C at the start of the long-term experiments at Rothamsted and Woburn relates to the previous cropping and manuring histories of the fields on which the experiments were established. The fields at Rothamsted had a long history of arable cropping with occasional applications of small amounts of FYM and ploughed-in weeds. The field at Woburn had been in grass before it was ploughed some years before the experiments started but it is probable that large amounts of FYM were added for the arable crops grown after ploughing the grass. The effects of growing grass for long and short periods on SOM are discussed in the following sections.

3.2. Effects of short-term leys interspersed with arable crops Traditional farming practice in the UK was to have some fields on the farm growing arable crops continuously whilst others were in permanent grass. This, in part, was probably because of the difficulty of quickly establishing

16

A. Edward Johnston et al.

productive grass swards on arable fields. From the 1930s, high-yielding cultivars of grasses and clovers that established well given good soil conditions were being introduced. This allowed the development of Ley–arable farming systems in which 3- or 4-year leys (grass or clover or mixtures of both) were interspersed with a few years of arable crops, that is, a cycle of ley, arable, ley, arable cropping. The perceived benefit was that the ‘‘restorative ley’’ would increase SOM and increase yields of arable crops that followed. Experiments testing this concept were started at Woburn in 1938 (Boyd, 1968; Mann and Boyd, 1958), then at Rothamsted in 1949 (Boyd, 1968). Similar experiments were started in the early 1950s on six of the Experimental Husbandry Farms belonging to the UK’s National Agricultural Advisory Service (Harvey, 1959); regrettably with the current interest in SOM these were not continued. At Woburn, four different ‘‘treatment’’ cropping systems, each lasting 3 years, were compared and their effects were measured on the yields of two ‘‘test’’ crops that followed ( Johnston, 1973). Each phase of the treatment and test cropping was present each year; there was no permanent grass treatment. Initially the treatment cropping had two arable rotations and two ley treatments, and all were followed by two arable test crops, which changed during the course of the experiment. The arable rotations differed only in the crop grown in the third year; in one it was a 1-year grass ley (Ah), the grass seed being undersown in the preceding cereal; in the other it was a root crop (Ar) usually carrots. The two leys were lucerne (alfalfa) harvested for hay (Lu) and grass–clover grazed by sheep (L). There was a half-plot test of FYM (38 t ha
1) applied only to the first test crop, that is, every fifth year. Each treatment sequence and the half-plot test of FYM continued on the same plots (‘‘Continuous Rotations’’). The soil, 0–25 cm, was sampled at the end of the third treatment year to determine %C (Table 2). Initially the soil had 0.98%C. After 33 years there was 1.04%C in the soil of the Ah rotation, that is, SOM had increased slightly. Replacing the 1-year grass ley with a root crop resulted in a small loss of SOM, %C declined to 0.90%, presumably due to a smaller input of C from the root crop compared to the 1-year grass ley, and autumn ploughing and spring soil cultivation before sowing the carrots and cultivations to control weeds. After 33 years with the grazed ley in 3 years of the 5-year cycle, %C increased to 1.26%C but there was very little increase in %C where lucerne was grown as the ley. The very small effect of lucerne in increasing SOM was also found in the Rothamsted Ley–arable experiment. We can offer no reason except to note that the lucerne was grown in rows 25 cm apart and the plant has little fibrous root compared to grass. For all these treatment sequences, the increase in %C from applying FYM (38 t ha
1) ranged from 6% to 14%, the larger values being on the plots with leys (Table 2). In the early 1970s, it was decided to simplify the experiment while providing additional information and changes were phased in over a period

Table 2 Effect of cropping sequences on percent organic carbon (%C) in the 0–25 cm plough layer of a sandy loam soil, Ley–arable experiment, Woburn Perioda 1955–1959

a b

Crop rotation

No FYM

FYM

Arable with roots Arable with hay Grass ley grazed Lucerne for hay

0.91 0.98 1.10 1.00

0.99 1.07 1.21 1.14

1960–1964 b

No FYM

FYM

0.90 0.94 1.09 0.96

0.97 1.07 1.28 1.11

1965–1969 b

Soil sampled at the end of the third treatment year, mean of five plots, one sampled each year. FYM, 38 t ha
1 applied once in 5 years to the first test crop.

No FYM

FYM

0.88 0.95 1.13 0.95

0.98 1.04 1.32 1.13

1970–1974 b

No FYM

FYMb

0.90 1.04 1.26 1.03

0.99 1.10 1.44 1.20

18

A. Edward Johnston et al.

of 5 years. The arable rotations became barley, barley, beans (AB, after Ah) and fallow, fallow, beans (AF, after Ar); the ley rotations became grass with N fertilizer (Ln3, after L) and grass–clover (Lc3, after Lu). The test of FYM was stopped. A test of 8-year leys (Ln8 and Lc8) was introduced to compare the benefit, if any, of having longer leys. Changes in %C for four main treatments during the 60 years since the start of the experiment are in Fig. 5. Three treatments have remained relatively unchanged, AB, AF, and Ln3 while one, Lc3 followed the lucerne ley. On this plot there was no increase in SOM during the period when lucerne was grown and it is only since the early 1970s under the 3-year grass/clover (Lc) ley that SOM has increased (Fig. 5). On this sandy loam soil, changes in SOM due to differences in cropping have been relatively small over many years as the level of SOM in each system has changed toward its equilibrium value. An overall summary of the changes in %C during almost 60 years is in Table 3 . From a starting level of 0.98%C, most SOM was lost (25%) in an all-arable cropping rotation which initially had cereals and root crops and then after 35 years had 2 year fallow in each 5-year cycle. Arable cropping with mainly cereals and initially a grass crop for 1 year in five has resulted in a smaller decline in SOM. Growing grass or clover for 3 years followed by two arable crops in a 5-year cycle, increased % C but only by 10–15% after 60 years. The more recent introduction of an

1.6 1.4

%C, 0–25 cm

1.2 1.0 0.8 0.6 0.4 0.2 0.0 1930

1940

1950

1960

1970

1980

1990

2000

Year

Figure 5 Changes in percent organic carbon (%C) in the top 25 cm of a sandy loam soil under continuous arable and Ley–arable cropping, Ley–arable experiment, Woburn. Continuous arable, AB ^; Continuous arable, AF ▪; 3-year all-grass ley, Ln ▲; 3-year grass/clover ley, Lc . For treatment details see text. (Adapted from Johnston, 1973 with recent data added.)

Soil Organic Matter: Its Importance in Sustainable Agriculture and Carbon Dioxide Fluxes

19

Table 3 Percent organic carbon (%C) in 0–25 cm soil after 58 years of different cropping sequences, Ley–arable experiment, Woburn

Cropping sequencea

Ar that became AF after 35 years

FYM treatmentb

No Yes Ah that became AB after 35 years No Yes L that became Ln3 after 35 years No Yes Lu that became Lc3 after 35 years No Yes L that became Ln8 after 35 years No Yes Lu that became Lc8 after 35 years No Yes

a b c

%C in 1995– 1999

Change from initial valuec

0.74 0.76 0.87 0.92 1.09 1.17 1.14 1.18 1.22 1.30 1.16 1.27


0.24
0.22
0.11
0.06 þ0.11 þ0.19 þ0.16 þ0.20 þ0.24 þ0.32 þ0.18 þ0.29

For treatment symbols, see text. FYM at 38 t ha
1 to the first test crop, only five applications in the first 25 years. Initial value 0.98%C.

8-year ley followed by two arable crops further increased SOM, but by only a small amount (Table 3). Today, when much is said about the importance of SOM in soil fertility it is not always appreciated that changes in SOM over time are small unless there are major modifications in cropping practice to achieve a large change (see later). These comparatively small changes in acceptable farming systems over many years are very similar to those in long-term experiments on a similar sandy loam soil at Askov in Denmark (Christensen and Johnston, 1997). At Rothamsted, there are two Ley–arable experiments in which the treatment cropping lasts for 3 years followed by three test crops ( Johnston, 1973). One experiment (Highfield) was sited on what had been an old arable field that was sown to grass in 1838, so that by 1949 SOM would be reaching the equilibrium value for less-intensively managed grassland; the soil (0–23 cm) contained about 2.75%C. The other experiment (Fosters) was sited on a field that had been in permanent arable cropping for many decades and the soil contained about 1.65%C. On Highfield some plots were left in the original permanent grass sward (Permanent Grass). In both experiments some plots were sown to grass that was to remain unploughed (Reseeded Grass), on Highfield this treatment was established on plots where the original sward was ploughed-in autumn 1948 and the same grass

20

A. Edward Johnston et al.

mixture as used on Fosters was sown in spring 1949. Common to both experiments were three types of ley and one arable treatment. Initially the 3-year leys were lucerne, grass–clover grazed by sheep and grass given N fertilizer and cut for conservation. The arable treatment rotation was sugar beet, oats and 1-year grass undersown in the oats and cut for hay. The test crops grown in rotation were winter wheat, potatoes, and spring barley. In these experiments each phase of the 6-year cycle was present in duplicate each year and the soil, 0–23 cm, was sampled at the end of each third treatment year. Figure 6 shows the changes in t organic C ha
1 for two treatments on each field, permanent grass and permanent arable on Highfield and permanent arable and reseeded grass on Fosters for a period of some 50 years. Changes in total organic C are used in Fig. 6 rather than changes in %C because this allows for the differences and changes in bulk density in the differently treated soils (see later for an explanation). Under arable cropping, the amount of organic C remained essentially constant on the old arable field (Fosters) but declined steadily where the old grassland soil was ploughed (Highfield) and the amounts of organic C in these two soils are now similar but the soil weight on Highfield is slightly

100 90

Organic C in soil, t ha−1

80 70 60 50 40 30 20 10 0 1940

1960

1980

2000

Year

Figure 6 Changes in organic carbon (t ha
1) in the top 23 cm of a silty clay loam soil, Ley–arable experiment, Rothamsted, 1949–2002. Highfield old grassland soil: kept in grass □; ploughed and kept in arable cropping ○. Fosters old arable soil: kept in arable ; sown to grass and kept in grass ▪. (Adapted from Johnston, 1973 with recent data added.)

Soil Organic Matter: Its Importance in Sustainable Agriculture and Carbon Dioxide Fluxes

21

less than on Fosters. Where the permanent grass was left undisturbed on Highfield, organic C slowly increased toward a new equilibrium level as a result of more intensive management and increased N applications that increased aboveground yields and consequently greater root growth and decay that increased organic matter inputs. Where the old arable soil was sown to grass on Fosters, the amount of C increased slowly but after about 50 years it was still much less than in the permanent grass plots on Highfield. The effect of the different 3-year leys that were compared at the start of the experiment on %C after 36 years was remarkably small (Table 4). After this long period and compared to the all-arable soil in each experiment, %C was increased by about 18% under the two grass leys but by only 6% under the lucerne. The cumulative buildup of SOM was small because most of the organic matter accumulated during the 3 years of ley was decomposed during the following 3 years of arable cropping. The important effect of soil texture on SOM is seen again in these data sets from the Ley–arable experiments at Rothamsted and Woburn when the cropping and management of the experiments were very similar. The lowest level of SOM in the continuous arable plots on the silty clay loam (25% clay) at Rothamsted (Fig. 6) is still larger than the highest level of SOM achieved on the sandy loam (12% clay) at Woburn with the largest input of organic matter from an 8-year ley followed by two arable crops (Table 3).

Table 4 Effect of 3-year leys compared to all-arable cropping on percent organic carbon (%C) in the 0–23 cm depth of a silty clay loam after 36 years, Ley–arable experiments, Rothamsted Cropping sequence Continuous arable

3 years arable preceded by 3 years Lucerne

Old grassland soil % organic carbon in soila Increase in %C due to ley Old arable soil % organic carbon in soila Increase in %C due to ley a

Grass/ Clover

Grass þN

1.70

1.80 þ0.10

2.06 þ0.36

1.99 þ0.29

1.43

1.52 þ0.09

1.66 þ0.23

1.66 þ0.23

Soil sampled in the third year of the ley before ploughing, for initial values see text. %C measured at the end of the sixth 3-year period in ley in the ley and arable cropping sequence.

22

A. Edward Johnston et al.

3.3. Effect of different types of organic inputs to soils growing arable crops In 1964 the Organic Manuring experiment was started on the sandy loam at Woburn to test the effects of different types of organic matter inputs on SOM and crop yields (Mattingly, 1974). Six organic treatments were compared with two fertilizer-only treatments. For the first 6 years, the two fertilizer treatments and four of the organic treatments had arable crops grown in rotation: spring barley, potatoes, winter wheat, sugar beet, field beans (Vicia faba), and winter rye. Three of the organic treatments applied annually during the first 6 years were FYM (about 50 t ha
1) and straw and peat (both at 7.5 t ha
1 dry matter). The fourth organic treatment was ‘‘green manures’’; these were undersown in the three cereal crops and allowed to grow until the soil was ploughed for the next spring-sown crop. Four rates of N were also tested on the arable crops. In addition there were two ley treatments, one grass–clover and the other grass with fertilizer N and these were not ploughed in the first 6 years. The amounts of organic matter added during the first 6-year treatment phase and their effect on %C in soil are in Table 5 . In 1971, the two fertilizer-treated soils contained, on average, 0.69%C. The largest increase in %C was with peat; the next largest was with the FYM treatment. The leys and straw increased %C by the same amount but there was only a very small increase where green manures were incorporated. Although SOM accumulated with these treatments, there were varying and often large losses of C and N. About 50% of the C added in FYM was lost and the loss was even larger with straw and green manures (Table 5). Much of the C added in peat was retained, presumably because most of the readily decomposable organic matter had already gone, so that the C:N ratio of the peat was about 10:1. Estimating the amount of the organic matter accumulated under the leys was difficult but Mattingly et al. (1974) considered that in 1971 much of the C accumulated under the leys had been retained in the soil. Arable crops were grown in rotation with an eight-level N test (see page 31) during the next 8 years (1973–1980) to assess the effects of the increased levels of SOM achieved by the organic amendments. During this period the only organic inputs were ploughed-in roots and cereal stubble and the level of SOM declined on all plots, more where there had been organic amendments than on fertilizer-treated plots. This period was followed by another treatment phase from 1981 to 1986, but with some modifications. The fertilizer, FYM, straw, and grass/clover ley treatments were continued but the green manure, peat, and grass ley with N treatments were all replaced with a grass/clover ley, that is, half the plots were in grass/clover ley, half in arable crops and of the latter, two had organic matter additions, FYM and straw. Again, SOM increased with the organic treatments and leys but continued to decline slowly where only fertilizers were applied.

Table 5 Changes in percent organic carbon (%C) in the top 23 cm of a sandy loam after applying different organic matter amendments for 6 years and percent retention of applied carbon, Organic Manuring experiment, Woburn (adapted from Mattingly et al., 1974) Organic treatment 1965–1971

%C in top 23 cm in 1971 Increase in %C compared to fertilizer Amount of organic matter added (t ha
1) % organic matter retained in topsoil

Fertilizer

Straw

FYM

0.69

0.92 0.23 43.4

1.04 0.35 41.8

13

50

Grass/clover

0.92 0.23 9.0 100

Grass þ N

0.92 0.23 10.5 120

Green manure

0.79 0.10 6.1 35

Peat

1.33 0.64 36.2 90

24

A. Edward Johnston et al.

This treatment phase from 1981 to 1986 was followed by another 8-year test phase in 1987–1994 when six rates of N were tested on the arable crops. Then from 1995 to 2002, arable test cropping continued but with only two rates of N being tested. In 2002 all the plots were sampled before another treatment phase started. The effects of the different treatments on SOM during the period 1965–2002 are shown in Fig. 7. At the last sampling in 2002, %C had apparently increased on all plots by much the same amount; we cannot offer an explanation for this apparent increase, it may be due to sampling or analysis. Soil sampling should always be as consistent as possible following agreed protocols for an experiment. Changing analytical techniques poses a problem; much of the earlier C data presented here were determined using a wet digestion technique that was later replaced by an automated combustion technique. Archived soil samples have been used for cross checking but to reanalyze all samples would be a major undertaking. During the 38-year period, SOM declined slowly for the first 20 years to reach an equilibrium value about 0.65%C where arable crops were grown only with fertilizers. All the organic treatments increased SOM initially by varying amounts (Table 5), but SOM then declined once the input of organic matter, over and above that is ploughed-in as crop residues, ceased. During the second 6-year organic treatment phase, SOM increased again, more with the FYM treatment than any other, and then declined again when the extra organic inputs ceased. Interestingly, although the initial

1.60

% C in top-soil, 0–23 cm

1.40

Organic treatment

Organic treatment

1.20 1.00 0.80 0.60 0.40 0.20 0.00 1960

1965

1970

1975

1980 1985 Year

1990

1995

2000

2005

Figure 7 Changes in percent organic carbon (%C) in the top 23 cm of a sandy loam soil, Organic Manuring experiment, Woburn, 1965–2002. Fertilizers only □, ▪; Straw dry matter 7.5 t ha
1, ▲; Grass/clover ley, ^; FYM 50 t ha
1, x; Peat dry matter 7.5 t ha
1, .

Soil Organic Matter: Its Importance in Sustainable Agriculture and Carbon Dioxide Fluxes

25

large increase in SOM from applying peat was not maintained once the peat applications ceased, there was nevertheless a residue of very resistant organic matter that has maintained a higher level of SOM on this treatment than on any other even though peat was not applied after autumn 1970. Although there was an appreciable increase in SOM from applying FYM, the amount applied annually was far larger than that which would be available in many farming systems unless very large numbers of animals are kept. As in the Ley–arable experiments described above, interspersing leys with arable crops in this experiment increased SOM by about 30%, a worthwhile increase, but the adoption of such a farming system requires that it is financially viable.

3.4. Effects of straw incorporation Incorporating plant residues from grain crops, like cereal straw and maize stover, is one means by which farmers can add organic matter to soil. Experiments to test the effects of straw incorporation compared to its removal by burning were started at Rothamsted and Woburn in 1985. Chopped straw was incorporated either by ploughing to 20 cm (inversion tillage) or by tine cultivator (noninversion tillage) to 10 or 20 cm. About 4 t ha
1 of straw was incorporated each year for 17 years before the 0–10 and 10–20 cm soil depths were sampled in 2001 (Table 6). There was no measurable increase in %C where straw was incorporated by ploughing at Rothamsted but at Woburn there was a small increase in both soil horizons. Where straw was incorporated by tine cultivator to 10 cm, there was a small increase in %C at both depths at Rothamsted but no effect at Woburn. Such differences in the change in %C between sites and methods of incorporation are difficult to explain. The effects of straw incorporation on %N were more consistent (Table 6). The difference between C and N is because during the microbial decomposition of straw, with its wide C:N ratio, there is a greater loss of C than of N to reach the C:N ratio of about 10:1 for SOM. Thus, while only about 10% of the added C was retained in the soil, 70–100% of the added N could be accounted for at both Rothamsted and Woburn. These straw incorporation experiments were stopped in 2001. However, to assess any long-term effect of straw incorporation on SOM, it was decided in 1986 to plough-in the straw produced each year on the plots of Section O of the Broadbalk Winter Wheat experiment. After 14 years, changes in %C and %N have been small but mainly positive where straw has been incorporated on plots getting fertilizer N each year. In both these experiments, it is difficult to explain why so little C has been retained in the soil after 14–17 years of straw addition on plots that have received sufficient N fertilizer to grow acceptable yields of grain crops. However, anecdotal evidence from farmers who have been incorporating straw for some years

26

A. Edward Johnston et al.

Table 6 Effect of straw incorporation for 17 years (1985–2001) on percent soil organic carbon (%C) and total N (%N) on two contrasted soil types Rothamsted silty clay loam, 20% clay

Treatment

Ploughed Tined

Ploughed Tined a b

a

Depth sampledb (cm)

0–10 10–20 0–10 10–20 0–10 10–20 0–10 10–20

Woburn sandy loam, 13% clay

Straw Burnt

Incorporated

Organic C (%) 1.84 1.87 1.86 1.85 2.28 2.40 1.86 2.02 Total N (%) 0.150 0.160 0.152 0.161 0.179 0.201 0.160 0.173

Straw Burnt

Incorporated

Organic C (%) 1.08 1.28 1.14 1.26 1.54 1.58 1.23 1.18 Total N (%) 0.093 0.108 0.096 0.104 0.117 0.134 0.098 0.106

Straw was either burnt or chopped and incorporated by ploughing to a depth of 20 cm or by tine cultivation to a depth of 10 cm. Soils sampled in autumn 2001.

invariably suggests that there has been a benefit in terms of ease of ploughing. Possibly incorporation of crop residues by inversion or noninversion tillage prevents the soil becoming seriously compacted.

3.5. Effect of different arable crop rotations on the loss of soil organic matter Different arable crop rotations can have different effects on SOM. At Rothamsted two different arable rotations followed the ploughing of old grassland soil that contained 3.0%C. One rotation had four root crops and two cereals in 6 years; the other had three cereals, two root crops, and a 1-year grass ley in the 6 years. In both rotations crop residues like straw and sugar beet tops were removed after each harvest, and no organic manures were applied. Changes in SOM with these two rotations were compared with those where no crop was grown after ploughing the grass and weeds were controlled by soil cultivation, the fallow treatment. All soils were sampled periodically to 23 cm and %C determined. Where the soil was continuously fallowed, the decline in %C was exponential, about 50% of the original SOM was lost in the first 20 years and about 60% had been lost after 40 years. While such losses were expected there were also large losses on the soils growing the arable crop rotations. During the first 20 years after ploughing the grass, SOM declined by 40% in the rotation with most root

Soil Organic Matter: Its Importance in Sustainable Agriculture and Carbon Dioxide Fluxes

27

crops and by 30% in the rotation with more cereal crops ( Johnston, 1986). Presumably the extra soil cultivations to prepare for sowing root crops and to control weeds caused the larger decline in SOM.

3.6. Increases in soil organic matter when soils are sown to permanent grass Comment has been made about the difficulty of increasing SOM but appreciable increases are possible when permanent grass is established and maintained on soils with little SOM as a consequence of growing arable crops for very many years. At various times in the 1870s–1880s, a number of fields on the Rothamsted farm were sown to grass and periodically the soils were sampled 0–23 cm and the total N determined by Lawes and Gilbert. Their data in the Rothamsted archive were published by Richardson (1938). In the 1960s a few of these fields were still in grass and they were sampled again and the soil analyzed for total N. Lawes and Gilbert’s and our data for the 1960s were combined to show the buildup of soil N over time ( Johnston and Poulton, 2005; Fig. 5). Subsequently more data related to the buildup of N in soil with time have been collected and are shown in Fig. 8. The approximately 220- and 350-year values in Fig. 8 are from soils from the Park Grass experiment at Rothamsted (Warren and Johnston, 1964). This experiment was started in 1856 on a site that had been in grass for at least 200 years so the ‘‘220 year’’ %N was for soil sampled in 1876 and the ‘‘350 year’’ %N was that in 2002, 150 years after the start. Adding in more 0.350 0.300

Total N, %

0.250 0.200 0.150 0.100 0.050 0.000 0

50

100

150

200 250 Years in grass

300

350

400

Figure 8 Buildup of organic nitrogen (%N) in the top 23 cm of a number of silty clay loam soils that had been in arable cropping and were then sown to grass at various times and for various periods at Rothamsted.

28

A. Edward Johnston et al.

data has inevitably increased the scatter shown in Fig. 8. The scatter in percent N appears to be related to management; grassland that is intensively managed, harvested more frequently and given more N seems to accumulate more N than extensively managed grassland. However, the underlying principle is unaltered, namely for the silty clay loam at Rothamsted it takes about 100 years for the equilibrium %N content, typical of an old arable soil to increase to the equilibrium %N of a soil under permanent grass. However, Fig. 8 also shows that on this soil type under the prevailing climatic conditions, it takes about 25 years to increase SOM to a level half-way between that of an old arable soil and a permanent grassland soil. Even under this ideal condition for SOM accumulation, SOM increases only slowly.

4. Soil Organic Matter and Crop Yields 4.1. Arable crops grown continuously and in rotation 4.1.1. Experiments before the 1970s Comment has already been made that in the early years of the Rothamsted experiments Lawes and Gilbert showed that it was possible to get the same yields of winter wheat, spring barley, and mangels (Beta vulgaris var. esculenta) with fertilizers, providing the right amounts of N, P, and K were applied, as with FYM applied at 35 t ha
1 annually. As these experiments continued the annual applications of FYM gradually increased SOM so that these soils contained 2.5–3.0 times more SOM in the 1970s than soils getting fertilizers only. Yet throughout the period from the 1850s to the mid-1970s, yields were the same with the two contrasted treatments (Table 7) leading to an oft repeated comment that SOM was unimportant provided sufficient nutrients were applied as fertilizers. The wheat and barley experiments did not, at that time, include a treatment with FYM plus N, but this was a treatment on Barnfield where root crops were grown each year. Applying 96 kg ha
1 fertilizer N with FYM appreciably increased yields of both mangels and sugar beet (Table 8). Presumably N mineralized from the large annual application of FYM and any N mineralized each year from SOM were not sufficient to meet the N requirements of these root crops. This result led subsequently to a test of FYM plus additional amounts of fertilizer N in many experiments at Rothamsted. Until the 1970s, other results from long-term experiments confirmed the lack of benefit from the extra SOM shown in Table 7, for example, those in the Rothamsted Ley–arable experiments ( Johnston and Poulton, 2005; Fig. 6). Where N fertilizer was not applied, yields of potatoes, winter wheat, and spring barley were larger following ploughing a 3-year grass/ clover than those following arable crops. However, where fertilizer N at 100 and 90 kg ha
1 was given to the wheat and barley, respectively,

Soil Organic Matter: Its Importance in Sustainable Agriculture and Carbon Dioxide Fluxes

29

Table 7 Yields of winter wheat and spring barley grain and roots of mangels and sugar beet at Rothamsted (adapted from Johnston and Mattingly, 1976) Yield (t ha
1) with Experiment

Crop

Period

FYMa

NPK fertilizersa

Broadbalk

Winter wheat

Hoosfield

Spring barley

Barnfield

Mangels

1852–1861 1902–1911 1970–1975 1852–1861 1902–1911 1964–1967 1876–1894 1941–1959 1946–1959

2.41 2.62 5.80 2.85 2.96 5.00 42.2 22.3 15.6

2.52 2.76 5.47 2.91 2.52 5.00 46.0 36.2 20.1

Sugar beet a

FYM, 35 t ha
1; N to wheat, 144 kg ha
1; to barley, 48 kg ha
1 but 96 kg ha
1 in 1964–1967; to mangels and sugar beet, 96 kg ha
1.

Table 8 Yields (t ha
1), roots of mangels, 1941–1959, and sugar beet, 1946–1959, Barnfield, Rothamsted (adapted from Johnston, 1986) Mangels

a b

Sugar beet

Treatment

No N

þNa

No N

þNa

PK FYMb

6.8 22.3

36.2 50.2

4.5 15.6

20.1 27.9

96 kg N ha
1 as sodium nitrate. 35 t ha
1.

the yields of both cereals were the same following the ley and arable cropping. Also, when comparing yields in both experiments, although the soil on Highfield contained 2.1%C compared to 1.6%C on Fosters, the larger amount of SOM in Highfield soils did not affect the yields of the cereals provided sufficient fertilizer N was applied. However, the yields of potatoes were always larger on Highfield with more SOM than Fosters. There was a ‘‘crop effect’’ in the response to SOM. 4.1.2. Experiments after the 1970s Having shown that one amount of fertilizer N applied with FYM increased the yields of mangels and sugar beet in the Barnfield experiment (Table 8), this experiment was modified in 1968, to test four amounts of N on

30

A. Edward Johnston et al.

potatoes, sugar beet, spring barley, and spring wheat grown three times in rotation on all plots between 1968 and 1973. Irrespective of the amount of N applied, the largest yields of the root crops were always on FYM-treated soils that contained more SOM and the benefit of the extra SOM was smaller for spring barley and spring wheat. However, for all four crops less fertilizer N was needed to achieve the optimum or near optimum yield when the crops were grown on the plots with more SOM (Table 9). Similar benefits on crop yields from extra SOM were evident on the sandy loam at Woburn from the early 1970s. Yields of red beet in the Market Garden experiment were larger on soils with more SOM even though as much as 450 kg N ha
1 was applied to fertilizer-only plots ( Johnston and Wedderburn, 1975). In the Ley–arable experiment sugar yields were about 0.6 t ha
1 larger when the beet followed a 3-year lucerne ley than in an all-arable rotation even though 220 kg N ha
1 was applied ( Johnston, 1986). Cereals and potatoes were both grown between 1973 and 1980 in an experiment where two levels of SOM were established by adding peat ( Johnston and Brookes, 1979). Peat was chosen as the source of organic matter because it would add little or no mineral nutrients. Four amounts of N appropriate to the crop were tested and yields of the spring crops, potatoes, and barley were always larger on the soil with more organic matter irrespective of the amount of N applied, but yields of winter-sown cereals were independent of SOM (Table 10). Spring-sown crops have to Table 9 Yields of potatoes and sugar beet, spring barley, and spring wheat in 1968– 1973 on soils treated with PK fertilizers or FYM since 1843a, Barnfield, Rothamsted (adapted from Johnston and Mattingly, 1976) Fertilizer N appliedb N0 Crop

Treatment

Potatoes, tubers

FYMc PK FYM PK FYM PK FYM PK

Sugar beet, roots Spring barley, grain Spring wheat, grain a b c

N1

N2

N3


1

Yields (t ha )

24.2 11.6 27.4 15.8 4.18 1.85 2.44 1.46

38.4 21.5 43.5 27.0 5.40 3.74 3.73 2.97

44.0 29.9 48.6 39.0 5.16 4.83 3.92 3.53

44.0 36.2 49.6 45.6 5.08 4.92 3.79 4.12

PK- and FYM-treated soils contained 0.10 and 0.25%N, respectively. N applied: N0, N1, N2, N3: 0, 48, 96, 144 kg ha
1 to cereals; 0, 72, 144, 216 kg ha
1 to root crops. FYM, 35 t ha
1 annually.

Soil Organic Matter: Its Importance in Sustainable Agriculture and Carbon Dioxide Fluxes

31

Table 10 Yields of potatoes, spring barley, winter wheat, and winter barley, 1973– 1980, Peat experiment, Woburn (adapted from Johnston and Brookes, 1979 and Johnston and Poulton, 1980) Fertilizer N applieda

Crop

%C in soil

Potatoes, tubers, 1973 and 0.76 1975 2.03 Spring barley grain, 1978 0.76 1.95 Winter wheat grain, 1979 0.76 1.95 Winter barley grain, 1980 0.76 1.95 a

N0

N1

N2

N3


1

Yields (t ha )

25.7 27.1 2.19 2.58 3.54 4.81 3.05 3.57

35.6 40.6 5.00 5.12 7.32 7.21 6.01 5.92

41.7 50.7 6.73 6.85 8.05 8.09 7.32 7.00

43.2 59.0 7.05 7.81 7.82 8.08 7.83 7.98

N applied: N0, N1, N2, N3: 0, 100, 200, 300 kg N ha
1 for potatoes; 0, 50, 100, 150 kg N ha
1 for cereals.

develop a sufficiently large root system quickly to acquire nutrients and water and for this a good soil structure, which is related to SOM, is required. Autumn-sown crops have a long period to develop an adequate root system. In this experiment all operations were done by hand so there was no effect of SOM on soil compaction. The effect of management and a range of organic inputs on SOM in the Woburn Organic Manuring are described on page 22. In the first test cropping phase potatoes, winter wheat, sugar beet, and spring barley were grown in rotation and on each crop eight amounts of N were tested. The two fertilizer treatments had received different amounts of P, K, and Mg to allow for the very different amounts applied in FYM and the other organic amendments, and this resulted in differences in readily plant-available P, K, and Mg in the two soils. However, crop yields were almost identical on these treatments and as the upper and lower values spanned the range in plots testing the organic inputs this suggests that yields on the latter were not limited by these nutrients. Yields of all four crops, averaged over the four lowest and four largest amounts of N fertilizer, were always larger on soils with more organic matter ( Johnston, 1986). After the first test phase there was another treatment phase (see page 24) followed by another test phase in which only potatoes and wheat were grown in rotation and six amounts of N were tested. The response of wheat and potatoes to N on the four treatment sequences common to both treatment phases is shown in Fig. 9. Yields were always smallest on soil with least SOM and generally largest on soils ploughed out from a grass/clover ley. Some of the benefit from N-rich

32

A. Edward Johnston et al.

B

A

80

8

60 Tubers, tha−1

Grain, tha−1 at 85% dry matter

10

6 4

40

20

2 0

0 0

50 100 150 200 250 N applied, kgha−1

0

50 100 150 200 250 300 350 N applied, kgha−1

Figure 9 Yields (t ha
1) of test crops in the Organic Manuring experiment, Woburn. Annual organic treatment from 1965 to 1971 and again in 1981–1986: fertilizers only, ^; 7.5 t ha
1 straw, ▲; 50 t ha
1 FYM, ▪; grass/clover ley, . (A) Winter wheat in 1987 and 1988; (B) potatoes in 1988 and 1989.

clover ley residues ploughed-in the previous autumn could derive from the availability of N, by mineralization of the residues, late in the growing season and at positions in the soil profile difficult to mimic with applications of fertilizer N. Good yields were given by 50 t ha
1 FYM but very few farms have such quantities available for application every year to build up SOM to the levels in this experiment. For both wheat and potatoes in the second test phase, yields following grass/clover leys exceeded those given by fertilizers with the largest amount of N, in most other cases less N was required to achieve maximum yield on the soils with organic amendments compared to those on fertilizer-only plots. Of considerable interest is the benefit from ploughing in straw each year at a rate that a good crop of cereals should produce. That yield benefits continue to be measured with this treatment suggests that on soils with little SOM, straw incorporation will increase SOM sufficiently to have beneficial effects. Straw incorporation is one method readily available to farmers for increasing or maintaining SOM, or perhaps preventing it declining to very low levels where there could be adverse effects on crop yields. 4.1.3. Recent data from long-term experiments In 1968 a number of major changes were made to the experiment on winter wheat on Broadbalk and that on spring barley on Hoosfield. Besides growing wheat or barley continuously, a three-course rotation of potatoes, field beans, and wheat or barley was started to estimate the effects of soil borne pathogens on the yields of the cereal crop. Modern, short-strawed cultivars of either wheat or barley were also introduced.

Soil Organic Matter: Its Importance in Sustainable Agriculture and Carbon Dioxide Fluxes

33

On Hoosfield, where spring barley has been grown in all but 4 years since 1852, all plots were divided into four subplots to test four rates of fertilizer N on all treatments including the FYM- and fertilizer-treated plots. By the 1960s the FYM-treated soil contained 2.5 times more SOM than did the fertilizer-treated plot but in 1964–1967 this extra SOM did not increase yield provided the optimum amount of fertilizer N was applied, see Table 7. The first of the modern cultivars, Julia, was introduced in 1968 together with the increased rates of N. Grain yield was larger when 48 kg N ha
1 was applied in spring to the FYM-treated soil than with the largest amount of N on the fertilizer-treated soil (Fig. 10A). Yields were the

A

B 9 Grain, tha−1 at 85% dry matter

Grain, tha−1 at 85% dry matter

9 8 7 6 5 4 3 2 1 0

7 6 5 4 3 2 1 0

0

48 96 N applied, kgha−1

144

C

0

48 96 N applied, kgha−1

144

0

48 96 N applied, kgha−1

144

D 9

9 Grain, tha−1 at 85% dry matter

Grain, tha−1 at 85% dry matter

8

8 7 6 5 4 3 2 1 0

8 7 6 5 4 3 2 1 0

0

48 96 N applied, kgha−1

144

Figure 10 Yields of spring barley grain (t ha
1) Hoosfield Continuous Barley, Rothamsted. Annual treatment 1852–2006: PK fertilizers, ^; 35 t ha
1 FYM, ▪; annual treatment only from 2001 to 2006: 35 t ha
1 FYM, □. (A) cv. Julia, 1976–1979, (B) cv. Triumph, 1988–1991, (C) cv. Cooper, 1996–1999, and (D) cv. Optic 2004–2007.

34

A. Edward Johnston et al.

same on both treatments when 144 kg N ha
1 was given. In the following years cultivars with a larger yield potential were grown and the difference in yield between the FYM- and fertilizer-treated soils increased. In 1996– 1999, cv. Cooper yielded as much as 2.5 t ha
1 more grain on the soil with more organic matter (Fig. 10C). Interestingly, the maximum yield of each cultivar grown on the fertilizer-treated soil has not declined since the mid1970s, it has remained largely unchanged. Rather it is the yields on the soil with more SOM that have been larger as the yield potential of the cultivar grown has increased. We believe that much of the difference in yield between these soils with different levels of SOM is because the extra SOM improves soil structure, although additional N, mineralized late in the growing season and deeper in the soil profile, may have contributed to the larger yield. A better soil structure allows a spring-sown crop to quickly develop an adequate root system for maximum water and nutrient uptake. The shape of the N response curve on the soil with less SOM does not indicate that applying more N would increase yield to that on the FYMtreated soil. In 2001 a new FYM-treated plot was started within the Hoosfield experiment with annual applications of 35 t ha
1. Yields on this plot, which also tests four rates of N, have increased very rapidly to be intermediate between those on the long-continued fertilizer- and FYM-treated soils (Fig. 10D). This shows that even a small increase in SOM together with N from the current application of FYM and fertilizer N has improved yield on a soil that had been in cereal cropping for 150 years and contained little SOM. On Broadbalk, where a range of fertilizer N rates was already being tested, changes in 1968 included testing extra fertilizer N, 96 kg ha
1, on one of the FYM plots, and a comparison of wheat grown each year (continuous wheat) with wheat grown in a rotation designed to minimize any adverse effect of the soil borne pathogen Gaeumannomyces graminis, which causes take-all in wheat. As on Hoosfield, modern, short-strawed cultivars, with an improved grain: straw ratio, were also introduced. Now the cultivar grown is reviewed periodically and a new one introduced when appropriate. The yields of the different cultivars of wheat grown continuously and in rotation with fertilizers, FYM and FYM þ 96N since 1968 are in Fig. 11. The yields of continuous wheat with either PK þ 144 kg N ha
1 or 35 t ha
1 FYM have remained closely similar as they have from the beginning of the experiment in 1843 (Fig. 11A). However, as the yield potential of the cultivar grown has increased, and, since 1979, where that yield potential has been protected by the use of fungicides, grain yield has increased where more N has been applied. Consequently, the maximum yield with both ‘‘PK þ best N’’ and FYM þ 96N is now about 2 t ha
1 larger than with PK þ 144 kg N and FYM alone, respectively (Fig. 11A). (The yield with the ‘‘PK þ best N’’ treatment is the largest yield given by

Soil Organic Matter: Its Importance in Sustainable Agriculture and Carbon Dioxide Fluxes

35

A

Grain, tha−1 at 85% dry matter

12 10 8 6 4 2 0 1970

1975

1980

1985

1990

1995

2000

2005

2010

B

Grain, tha−1 at 85% dry matter

12 10 8 6 4 2 0 1970

Cappelle

1975

Flanders Brimstone Apollo

1980

1985

1990

Hereward

1995

2000

2005

2010


1

Figure 11 Average yields of winter wheat grain (t ha ) with different cultivars on the Broadbalk Winter Wheat experiment, 1970–2006. Annual treatment: PK þ 144 kg N ha
1, ^; FYM 35 t ha
1, ▪; ‘‘Best’’ NPK, e; FYM 35 t ha
1 plus 96 kg N ha
1, □. (A) Wheat grown year after year; (B) wheat grown after a 2-year break.

either 192, 240, or 288 kg N ha
1 each year; this yield has been averaged for each group of years.) For each of the cultivars shown in Fig. 11, the effect of growing ‘‘a first’’ wheat after a 2-year break has been to increase comparable treatment yields by about 2 t ha
1. Thus, there is a large benefit from minimizing the adverse effects of take-all. In many cases, the yield of cv. Hereward grown between 1996 and 2007 has declined both when grown continuously and in rotation. In part, this can be explained by some poor growing seasons in this period

36

A. Edward Johnston et al.

and also it appears that this cultivar has a high demand for N. For example, in 1996–2000, both when grown continuously and in rotation, the yield with the ‘‘PK þ best N’’ and FYM þ 96N treatments were similar to those of cultivars Brimstone and Apollo. Then, with all treatments, there was a serious decline in yield in 2001–2004 with very poor growing seasons. Yields improved somewhat in 2005–2007, more so with wheat grown in rotation than continuously, where the best yields with ‘‘PK þ best N’’ and FYM þ 144 kg N ha
1 were closely similar; adding 144 kg fertilizer N with FYM was first introduced in 2005. It is difficult to see why the N available from the mineralization of the extra SOM in the FYM-treated plot plus that from a fresh application of 35 t ha
1 FYM requires an extra 144 kg ha
1 fertilizer N to meet the N requirement of Hereward. The yields of the three cultivars grown between 1979 and 1995 with FYM þ 96N were always larger than those with the ‘‘PK þ best N’’ treatment, especially when the wheat was grown in rotation. This suggested that there was a benefit from the extra SOM, probably through an improvement in soil structure. However, with the increased amounts of fertilizer N applied to cv. Hereward these two treatments have given very similar yields suggesting no benefit from the extra SOM accumulated from FYM. This change is difficult to explain. A number of changes have been made in the test and treatment crops in the Woburn Ley–arable experiment over the period of the experiment. In 1981–1991, winter wheat and spring barley, each testing four amounts of N, were grown as first and second test crops following the 3-year treatment cropping. Wheat yields following ploughed-in leys were always larger than those following arable crops at all levels of N except the largest (Fig. 12).

B

A

10

8

Barley grain, tha−1

Wheat grain, tha−1

10

6 4 2 0

8 6 4 2 0

0

50

100

150

200

N applied, kgha−1

250

0

50

100

150

200

N applied, kgha−1

Figure 12 Yields (t ha
1 grain) of test crops in the Ley–arable experiment, Woburn. Three-year treatment cropping: arable crops, ^; grass ley þ N, □; grass/clover ley, ▲. (A) Winter wheat, 1981–1990; (B) spring barley, 1982–1991. (Adapted from Poulton and Johnston, 1996.)

Soil Organic Matter: Its Importance in Sustainable Agriculture and Carbon Dioxide Fluxes

37

However, less fertilizer was needed to get maximum yield following the leys and more N was mineralized from the grass/clover ley than the all-grass ley to give a larger increase in yield of wheat but not of barley. In terms of N fertilizer use there is a benefit for arable crops that follow ploughed-in leys but within a farming enterprise the use of the leys has to be financially viable.

5. Explaining the Benefits of Soil Organic Matter As mentioned in Section 1, Russell (1977) noted that ‘‘the major problem facing the agricultural research community is to quantify the effects of soil organic matter on the complex of properties subsumed under the phrase soil fertility. . .’’ Soil organic matter can/may contribute to soil fertility in a number of ways, namely: 

During its microbial decomposition it may release N, P, and S and some trace elements at times during the growing season and positions within the soil profile when it is difficult to mimic the effect with a fertilizer application.  Stabilize soil structure especially in poorly structured soils.  Increase cation and anion exchange capacity especially in light textured soils.  Increase water-holding capacity, especially that of available water. It is difficult to identify and quantify the interrelationships of these factors with the biological, chemical, and physical properties of soil especially when there are few appropriate techniques to use in the laboratory and setting-up field experiments with plots with different levels of SOM on the same soil type takes many years and can be very expensive. Here, results from some long- and short-term experiments at Rothamsted are used to try to tease out some of these effects and interactions.

5.1. Organic matter, soil structure, and sandy loam soils Soil organic matter could improve soil structure through a range of mechanisms like bonding mineral particles into crumbs or peds and then stabilizing them, so that the formation of large pores would increase the rate of water infiltration and speed the exchange of gases. However, these mechanisms do not seem to work on sandy loam soils and the following results from field experiments suggest that generalizing about short-term effects of SOM is not easy.

38

A. Edward Johnston et al.

Observations on the behavior of the sandy loam soil at Woburn suggest that the buildup of SOM from long-continued applications of FYM does not seem to create more stable crumbs than those on fertilizer-treated soils. With both treatments soil aggregates can be created during seedbed preparation, but the impact of heavy rain disintegrates them and small amounts of silt and clay particles fill the voids between the sand-sized particles. As the surface soil dries a ‘‘crust’’ is formed through which young seedlings have to emerge. On fertilizer-treated soils the crust is ‘‘hard’’ and seedlings emerge with difficulty resulting in less than optimum plant populations. On FYMtreated soil the extra SOM appears to form a thin film around sand grains decreasing friction between them, so that emerging seedlings can more easily push them apart to establish a plant population giving acceptable yields. When peat was incorporated into the soil surface to minimize the formation of a crust and compared with peat dug into the top 25 cm soil, the yields of globe beet but not carrots were increased by the surface application while the dug-in peat increased yields of carrots but not globe beet ( Johnston et al., 1997). Attempts to simulate the effects of the extra root mass when grass leys are ploughed-in was tried by incorporating coir fiber that looks like fine roots. The intimate distribution of fine roots within the soil mass was difficult to mimic with the coir fiber and the seedbed remained very ‘‘open,’’ dried very quickly and lack of moisture decreased seedling emergence. Consequently the yields of sugar from beet and of globe (red) beet were smaller than those on the control plot ( Johnston et al., 1997). After producing a range of crumb sizes by cultivation during seedbed preparation, these were stabilized by coating them with a (hydrolyzed poly (acrylonitrile)) that was available as ‘‘Krilium,’’ produced by Monsanto Chemicals. It stabilized the soil crumbs against rain but not against mechanical impact. Effects on yields were variable, compared to the untreated soil, those of globe beet were increased, sugar yields were the same but lettuce yields were decreased, the latter probably because the surface soil remained ‘‘too open’’ and rapid drying adversely affected germination of the smallseeded lettuce ( Johnston et al., 1997).

5.2. Separating nitrogen and other possible effects of soil organic matter 5.2.1. Nitrogen, crop rotation, and soil organic matter effects In Tables 9 and 10 and Figs. 9 and 10, the yields in the absence of applied fertilizer N are all larger on soils with more SOM. This could be due solely to N released by the mineralization of the organic matter but a component of this benefit could also have been due to an improvement in soil structure or some other factor affecting yield. Following the changes to the Broadbalk experiment in 1968 wheat was grown, either continuously, or as fallow,

39

Soil Organic Matter: Its Importance in Sustainable Agriculture and Carbon Dioxide Fluxes

W1F 6 6

W2F 6

5

WC 6 W1Be 6

5

5

Grain, t ha−1

5

4

5

3

4

2

3

1

2

0

1

4

4 4

3

3 3

2

2 2

1

1 1

0

0

48

96

144

0

192 0

0

48

96

WC 192

144

W1Be

0

0

48

96

144

192 W1F

0 0

48

48 96

96 144

144 W2F 192

192

Nitrogen, kg ha−1

Figure 13 Broadbalk Winter Wheat experiment 1970–1978. Relationship between nitrogen applied and mean yield of grain (t ha
1) when wheat was grown either: continuously, ▲; after a 2-year break, ○; after a 1-year fallow, △; or as a second wheat after a 1-year fallow, . (A) Individual fitted N response curves; (B) fitted N response curves brought into coincidence by vertical and horizontal shifts.

wheat, wheat or potatoes, beans, wheat; N was tested on each wheat crop at 0, 48, 96, 144, and 192 kg ha
1 (Dyke et al., 1983). Thus, there were four grain yield/N response curves (Fig. 13A) on soils with similar levels of SOM in the 8 years, 1970–1978. However, visual inspection of these curves suggested that each curve could be a segment of a single N response curve, which would be expected in terms of the biochemistry and physiology of the N nutrition of the plant. Fitting an exponential plus linear model as the response function produced a maximum yield for each response curve and these maximum yields could be brought into coincidence by appropriate horizontal and vertical shifts to produce a single N response curve (Fig. 13B). Horizontal shifts were interpreted as differences in available N, vertical shifts as differences in potential yield. Relative to continuous wheat, the first wheat after field beans (the second crop in the 2-year break) benefited by 23 kg ha
1 available N and produced 0.51 t ha
1 more wheat, probably because the adverse effect of take-all was decreased after a 2-year break. The first wheat after a 1-year fallow benefited by 53 kg ha
1 available N but produced 0.36 t ha
1 less grain because the adverse effects of take-all is more severe immediately following a 1-year break than in continuous wheat (Dyke et al., 1983).

40

A. Edward Johnston et al.

A similar exercise was done on the fertilizer- and FYM-treated plots, with their different levels of SOM, and the N response curves were brought into coincidence. The average horizontal shift, 69.2 kg N ha
1 represents the fertilizer N equivalent of the extra SOM while the average vertical shift, 1.39 t ha
1 grain, represents a unique benefit of extra SOM that did not equate to an application of N fertilizer in spring ( Johnston, 1987). 5.2.2. Nitrogen and organic matter effects from short-term leys A similar approach to that above was taken to try to separate the N effects from other factors affecting yield in an experiment on the sandy loam soil at Woburn. Following the ploughing of 1–6-year old grass/clover leys, the yields and N response of the four following arable crops were measured ( Johnston et al., 1994). The four crops, grown in rotation, were winter wheat, potatoes, a second winter wheat, and finally field beans and on each crop, except the beans, there was a test of nil and five amounts of fertilizer N applied in spring. For each of the four test crops a linear plus exponential N response model was fitted to the yields given by each ley treatment. The six N response curves were then brought into coincidence by vertical and horizontal shifts with that for the 1-year ley (Fig. 14). For the first test crop winter wheat, most of the shift was horizontal, suggesting that the differences between the preceding ley treatments was largely due to the N released from the ploughed-in crop residues. The available N after the 4- and 5-year leys was equivalent to about 85 kg N ha
1 applied as one application in spring, while for the 6-year old ley it was about 126 kg N ha
1. The vertical shift represents some unique, but undefined effect of ploughing in the 2–5-year old leys was just less than 1.0 t ha
1 grain. For the second test crop potatoes, little horizontal shift was required, the range was 2–6 kg N ha
1, suggesting that there was much less mineral N available from the mineralization of the ploughed-in ley residues. The vertical shift, range 6–10 t ha
1 tubers, suggested an appreciable organic matter effect, which has not been defined. Yields of winter wheat grown in the third year after ploughing the leys (not shown) showed little residual N effect.

5.3. Soil organic matter and soil structure In an experiment on a silty clay loam soil at Rothamsted, plots were established over a 12-year period with two levels of SOM and at each level of SOM, 24 levels of Olsen P. After 12 years both the SOM and the Olsen P were well incorporated into the 23 cm plough layer. Potatoes, spring barley, and sugar beet were then each grown twice in rotation. The yields of tubers, grain, and sugar were plotted against Olsen P at each level of SOM and from the fitted response curve the yield at 95% of the asymptote and the Olsen P associated with this yield was estimated (Table 11). The soil

Soil Organic Matter: Its Importance in Sustainable Agriculture and Carbon Dioxide Fluxes

Grain, t ha–1

A

B

10

10

10

9

9

8

8

7

7

6

6

5

5

4

4

10 10 10 10

9

9

9

9

9

8

8

8

8

8

7

7

7

7

7

6

6

6

6

6

5

5

5

5

5

4

4

4

4

4 O Tw ne 0 o

50

100 150

0

50

100

3 50

0

100

41

Th

150 200 250

re

e

u Fo

50

0

200 250

150 200 250 100 150 200 250

r ve Fi x

Si

C

D

80

80

Potato tubers, t ha–1

70 70

80 80

80

80 70

70 70 70

60 60

60

60 60 60

50 50 40 40 30 30 0

70

50

50 50 50 40 40

40

40 30

30 30 30

140 210 280 350

0

50

0

50

100 150 200 250

0

50

100 150 200 250

100 150 200 250

80

70

60

50

40

30

O Tw ne 0 o 0 re e 0

Th

70

140 210 280 350

u Fo

70

140 210 280 350

70

140 210 280 350

r Fi ve

0

x

Si

Nitrogen applied, kg ha–1

0

70 70

0

70

140 210 280 350 140 140

210 280 350 210 280

350

Figure 14 Effect of age of a grass/clover ley ploughed-in before growing winter wheat and potatoes in succession on their response to nitrogen fertilizer. Ley age in years: one, ○; two, ; three, □; four, ▪; five, △; six, ▲. (A) and (C) Individual N response curves for wheat (A) and potatoes (C); (B) and (D) individual N response curves brought into coincidence by appropriate horizontal and vertical shifts for wheat (B) and potatoes (D).

on which this experiment was made is one of the most difficult to cultivate on the Rothamsted farm, particularly for early spring drilling of cereals. The yield of spring barley at 95% of the asymptote was appreciably smaller

42

A. Edward Johnston et al.

Table 11 The effect of soil organic matter on yield responses to Olsen P, Agdell, Rothamsted

a

Crop

Organic C (%)

Yield at 95% of the asymptote (t ha
1)

Spring barley grain (t ha
1) Potatoes tubers (t ha
1) Sugar beet sugar (t ha
1)

1.40 0.87 1.40 0.87 1.40 0.87

5.00 4.45 44.7 44.1 6.58 6.56

Ryegrass dry matter (g pot
1)

1.40 0.87

Olsen P associated with 95% yield (mg kg
1)

Field experiments 16 45 17 61 18 32 Pot experiment 6.46a 23 6.51 25

Variance accounted for (%)

83 46 89 72 87 61 96 82

The response curves at the two levels of SOM were not visually different.

on the soil with less SOM compared to that where there was more SOM. For the potatoes and sugar, the 95% yields were very similar because there was time in spring to produce good seedbeds for both crops. Of great importance, however, the level of Olsen P associated with the 95% yield was very much lower on the soil with more SOM compared to the Olsen P on the soil with less SOM and the percentage variance accounted for in the yield/Olsen P relationship was very much larger where there was more SOM. These differences were most probably due to the effects of SOM on soil structure, which was improved where there was more SOM so that roots grew more freely and more thoroughly explored the soil to find nutrients, especially P. Hence less Olsen P was required to achieve the optimum yield. To test this, soil samples from all 48 plots (2 levels SOM  24 levels Olsen P) were brought to the laboratory, air-dried, and ground to pass a 2 mm sieve before being put in pots and cropped with ryegrass given adequate N, K, and Mg. The grass was harvested four times and the total yield of dry matter plotted against Olsen P. The response curves at the two levels of SOM were not visually different and the Olsen P associated with the 95% yield was essentially the same at both levels of SOM (Table 11). Because any soil structure effects were minimized under the conditions of the pot experiment, we consider that the differences in the critical Olsen P values seen in the field experiment were due to differences in soil structure under the field conditions.

Soil Organic Matter: Its Importance in Sustainable Agriculture and Carbon Dioxide Fluxes

43

The experiments discussed here have also been used to measure the effects of SOM on other aspects of soil structure. For example, the draught required for inversion ploughing to 23 cm was assessed on Broadbalk. Although the largest differences were related to clay content, the small (10%) increase in SOM on plots that had received more than 96 kg N ha
1 decreased draught appreciably (Watts et al., 2006). Other examples include effects on soil friability (Watts and Dexter, 1998), soil aggregation (Watts and Dexter, 1997; Watts et al., 2001), aggregate stability (Williams, 1978), and water infiltration (Blair et al., 2006).

5.4. Soil organic matter and soil phosphorus and potassium availability 5.4.1. Availability of soil phosphorus Soil organic matter contains both anion and cation exchange sites able to hold readily plant-available P and K. Comparing the retention of P in soils with different levels of SOM in the Rothamsted long-term experiments shows some interesting differences. Soil samples taken in the 1950s and 1970s from the 0–23 cm soil horizon of the unmanured, fertilizer- and FYM-treated soils were analyzed for total P, Olsen P, and P soluble in 0.01 M CaCl2. The latter solution has about the same ionic strength as the soil solution in neutral and slightly calcareous soils like those at Rothamsted, so that the P in the extract would be similar to that in the soil solution. Much more P was extracted by all three reagents from the fertilizer- and FYMtreated soils than from the control (Table 12) and the amounts of total P and Olsen P in the two P-treated soils were similar. However, there was appreciably more CaCl2 P extracted from the FYM-treated soils than from those given superphosphate. Only on the Barnfield experiment was superphosphate and FYM applied on the same plot. With this treatment the increase in both total and Olsen P was equal to the sum of the increases on plots getting only superphosphate or FYM, but the increase in CaCl2 P was larger than the sum of the increase on plots getting either superphosphate or FYM (Table 12). This suggests that the extra SOM on the FYM-treated soils was providing a larger number of low energy bonding sites holding P and where superphosphate was added with FYM some of the P from the superphosphate was also held on these low energy bonding sites. The importance of SOM in retaining readily plant-available P is seen in the data from the Exhaustion Land experiment (Table 12). Some plots had superphosphate from 1856 to 1901, others FYM from 1876 to 1901, and there was a control (no P) treatment. All plots were sampled in 1903 and the increase in total P, Olsen P, and CaCl2 P followed the same pattern as in the other experiments (Table 12). No more superphosphate or FYM was applied after 1901 and SOM gradually declined in the previously FYMtreated plots ( Johnston and Poulton, 1977). When the plots were sampled

Table 12 Total, Olsen, and CaCl2-soluble P in 0–23 cm topsoil from three long-term experiments at Rothamsted (adapted from Johnston and Poulton, 1993)

a b c

Experiment and year started

Soil sampled

Treatmenta

Total P (mg kg
1)

Olsen P (mg kg
1)

CaCl2 P (mg l
1)

Barnfield, 1843

1958

Hoosfield, 1852

1966

Exhaustion Land, 1856

1903

Exhaustion Land, 1856

1974c

Control P FYM FYM þ P Control P FYM Control P FYM Control P residues FYM residues

670 1215 (545)b 1265 (595) 1875 (1205) 630 1175 (545) 1340 (710) 530 885 (355) 860 (330) 480 595 (115) 630 (150)

18 69 (51) 86 (68) 145 (127) 6 103 (97) 102 (96) 8 65 (57) 66 (58) 2 10 (8) 12 (10)

15 93 (78) 396 (381) 691 (676) 9 446 (437) 787 (778) 6 173 (167) 297 (291) 3 6 (3) 9 (6)

P single superphosphate at 33 kg P ha
1; FYM, 35 t ha
1. Figure in parenthesis is the difference from the control. No superphosphate or FYM applied after 1901.

Soil Organic Matter: Its Importance in Sustainable Agriculture and Carbon Dioxide Fluxes

45

again in 1974, the total P and Olsen P had both declined but were still very much the same in both soils, but interestingly, there was now little difference in CaCl2 P. The decline in SOM had depleted the number of low energy bonding sites on which CaCl2 P was held. Other examples of SOM holding more CaCl2 P were given by Johnston and Poulton (1993). 5.4.2. Availability of soil potassium Soil organic matter has cation exchange sites that hold exchangeable K, thus extra SOM can increase the plant-available K in soil. As in many field experiments in temperate climates, Addiscott and Johnston (1971) showed a very strong linear relationship between exchangeable K and K balance (K applied minus K removed in the harvested product) in many long-term Rothamsted experiments. Interestingly they showed that K retention in soil as exchangeable K by SOM appeared to be related to differences in the selectivity of clay and organic matter for K relative to calcium (Ca). Where K was applied in FYM, the K was already held on exchange sites. Where K was applied in fertilizer to a permanent grass sward on a slightly calcareous soil, there was competition between K and Ca for exchange sites on SOM as it was produced in the soil. In consequence, the ratio of K:Ca was larger in SOM derived from FYM than in SOM derived from grass roots in a slightly calcareous soil.

5.5. Soil organic matter and water availability The effect of SOM on increasing the available water capacity (AWC) in the top 30 cm of soil has been assessed in a number of experiments at Rothamsted and the increases, ranging from 4 to 10 mm, are very small (Salter and Williams, 1969). These authors compared the AWC in soils of fertilizer- and FYM-treated plots from two of the long-term experiments at Rothamsted where there was a well-established difference in SOM. The AWC in the silty clay loam of fertilizer- and FYM-treated plots was, respectively, 49 and 58 mm on Broadbalk and 44 and 48 mm on Barnfield. On the sandy loam soil at Woburn the comparison was between a soil growing cereals continuously and one just ploughed from a 3-year grass/ clover ley, the AWC was 45 and 55 mm, respectively. Later, D. Hall (personal communication) measured the AWC in the 10–15 cm layer of the fertilizer- and FYM-treated plots, which are ploughed annually to 23 cm, in the Broadbalk and Barnfield experiments where there is 2.5 times more SOM where FYM is applied compared to where fertilizers are used. Hall found that for the fertilizer- and FYMtreated soils, the AWC was 32 and 44 mm, respectively, that is, an increase of 12 mm, in soils with extra SOM from long-continued applications of FYM. He also found that the easily available water was only increased by 8 mm, from 17 to 25 mm. Such small differences might be sufficient to

46

A. Edward Johnston et al.

mitigate against the adverse effect of short-term drought on young plants until they develop a root system capable of finding water in the deeper soil horizons.

6. Modeling Changes in Soil Organic Matter The soil is a major sink for carbon dioxide (CO2) in the form of SOM. Thus, there is considerable interest in modeling changes in SOM because, as the amount increases or decreases, CO2 will be either retained in or lost from the soil. The large amount of data on changes in SOM in Rothamsted experiments has made it possible for Jenkinson and his coresearchers to develop models to describe such changes, and some examples are given here. The current Rothamsted model (ROTHC-26.3; Jenkinson, 1990; Jenkinson et al., 1994) is a five compartment model. Added plant material is initially divided between two input compartments: decomposable plant material (DPM) and resistant plant material (RPM). Both DPM and RPM are retained in the soil and gradually decompose by first-order processes, which have characteristic (and different) rates, to CO2 (lost from the system), and to microbial biomass (BIO) and humified organic matter (HUM), which are also retained in the soil. Both microbial biomass and humified organic matter decompose at their characteristic rates by firstorder processes to give more CO2, biomass, and humified matter. The soil is also assumed to contain a small organic compartment (IOM) that is inert to biological attack. Decomposition processes in the model work in monthly intervals and allow for the effects of temperature (mean monthly air temperature), soil moisture content (calculated from rainfall and evaporation), plant cover (decomposition being faster in bare soil than under vegetation; Jenkinson, 1977), and soil clay content (from which is calculated the moisture held in a soil layer between field capacity and wilting point and the proportion of CO2 that is evolved). Data on both sequestering C and its release from soil are calculated in t organic C ha
1, and this can be done for the top 23 cm of soil in the longterm experiments at Rothamsted. Lawes and Gilbert originally sampled the top 23 cm of soil although initially perhaps only the top 12.5 cm was ploughed; perhaps they thought roots took up nutrients from this deeper layer of soil. Sampling to this depth has continued so it is now possible to make direct comparisons for total element content of the soil from any plot throughout the period of the experiment. However, there is a complicating factor. Where SOM is increasing, soil bulk density is decreasing and, conversely, where SOM is deceasing bulk density is increasing. The first situation arises where large amounts of FYM have been added each year, and the second where permanent grass has been ploughed. Where SOM has

Soil Organic Matter: Its Importance in Sustainable Agriculture and Carbon Dioxide Fluxes

47

increased, then at each sampling occasion the top 23 cm of soil did not include some of the mineral soil that was part of the top 23 cm soil at the start of the experiment. So, to estimate the total C content in the same weight of mineral soil on each sampling occasion, it is necessary to add an amount of C in the appropriate weight of ‘‘unsampled original 23 cm soil.’’ It is possible to do this for these experiments because soil weights and %C and %N in the 0–23 and 23–46 cm depths were determined on a number of occasions. This ‘‘correction’’ only applies to the FYM-treated plots and those sown to and kept in grass for many years or old arable sites which have been abandoned and have since reverted to woodland and it has been made to the data given here. Soil weights on fertilizer-treated plots have changed very little, but often %C has also changed very little also. A similar allowance for change in bulk density has been made where permanent grass was ploughed for continuous arable cropping. Here, the increasing bulk density as a result of loss of SOM has been used because SOM in the soil layer below 23 cm at the start of the experiment has been incorporated into the 23 cm plough layer over time and the organic matter it contained has been subject to microbial decomposition. Thus, it is necessary to add, for the earlier samplings, an amount of C in the appropriate weight of ‘‘unsampled original 23 cm soil.’’ The fit of the model to the observed changes in SOM for three treatments in the Hoosfield Barley experiment is good (Fig. 15), with the exception of the first few years where SOM was declining after the addition of FYM for 20 years. Jenkinson et al. (1987, 1994) give other examples. It should be emphasized that Fig. 15 is a true test of the model because no data from the Hoosfield experiment were used to set the model parameters and no adjustments were made to improve the fit. Figure 16 shows the fit of the model to changes in SOM in two contrasted situations, namely increasing and decreasing SOM, on closely similar soils less than 500 m apart. In 1847, Lawes and Gilbert started a field experiment in Geescroft in which field beans (V. faba) were grown year after year with three fertilizer treatments. Over time, yields declined and in many years the crop failed; the experiment was stopped in 1878. After 4 years bare fallow followed by 3 years when clover was grown, part of the experimental site was fenced off and allowed to revert to natural vegetation. The sequence of vegetation has been herbaceous plants followed by shrubs and now semimature oaks with an understorey of holly (Harmer et al., 2001; Poulton et al., 2003). The first soil sample, 0–23 cm, taken in 1883 had 1.07%C and pHwater 7.1; with natural acidifying inputs the pH had fallen to 4.4 in 1999. Estimated changes in C inputs throughout the period from 1883 have been used to model the accumulation of soil C and the fit of the model to the measured data is good (Fig. 16). In 1949, an area of permanent grass near to Geescroft and adjacent to the Highfield Ley–arable experiment was ploughed, has not grown a crop since,

48

A. Edward Johnston et al.

100 Organic C in soil, t C ha–1

90 80

FYM annually

70 60

FYM 1852–1871 nothing thereafter

50 40 30 20

Estimated

Unmanured

10 0 1840

1860

1880

1900

1920

1940

1960

1980

2000

2020

Year

Figure 15 Organic carbon (t ha
1) in the top 23 cm from three plots growing spring barley on the Hoosfield Barley experiment, Rothamsted. Treatments are unmanured, ▲; FYM, 35 t ha
1 annually, ▪; FYM 35 t ha
1 annually 1852–1871, none since, ^. The data points are adjusted for changes in soil bulk density (see text) and the solid lines are the model output. The FYM, ploughed-in in February 1852–1930 and in late autumn after 1932, was assumed to contain no biomass but DPM, RPM, and HUM in the proportions 0.49, 0.49, and 0.02, respectively. The incoming plant residues were assumed to have DPM and RPM in the proportion 0.59 and 0.41, respectively. The IOM for these treatments contained 2.7 t C ha
1. See text for explanation of DPM, etc. To obtain a (modeled) value for the amount of carbon in the soil at the start of the experiment, a plant debris input of 1.69 t C ha
1 was used. Thereafter, the annual C inputs (t ha
1) were unmanured plot, 1.28 (from plant debris); FYM plot, 2.8 (from plant debris) plus 3.0 (from FYM); FYM residues plot as FYM plot 1852–1871 then 2.0 (from plant debris) after 1872.

and has been kept weed free by soil cultivation—Highfield Bare Fallow. In this case without having to estimate any C inputs the model describes the decline in soil carbon very well (Fig. 16). The results from these two experiments contrast sharply. Under a bare fallow system, more SOM has been lost in 50 years from the top 23 cm soil than has been built up in the same depth of soil under regenerating natural woodland in 120 years. Under regenerating woodland there has been some accumulation of soil C below 23 cm and in 1999, the total C in the top 69 cm soil was 105 t ha
1. This amount of C in the top 69 cm soil is only about half of that (200 t ha
1 C) which has accumulated in the trees during the 120 years (Poulton et al., 2003). This is a significant amount if one is looking to sequester C and mitigate against the effects of global warming, but the aboveground biomass will not accumulate C indefinitely and at some time a new equilibrium value for C in the soil will be reached and further accumulation of C will cease.

Soil Organic Matter: Its Importance in Sustainable Agriculture and Carbon Dioxide Fluxes

49

100 90

Organic C in soil, t Cha–1

80 70 60 50 40 30 Geescroft 20

Bare fallow

10 0 1870 1890 1910 1930 1950 1970 1990 2010 Year

Figure 16 Organic carbon (t ha
1) in the top 23 cm on Geescroft Wilderness and Highfield Bare Fallow, Rothamsted. The data points are adjusted for changes in soil bulk density (see text) and the solid lines are the model output. The incoming plant residues were assumed to have DPM and RPM in the proportion 0.59 and 0.41, respectively. The IOM for these sites contained 2.5 and 3.0 t C ha
1 on Geescroft and Highfield, respectively. See text for explanation of DPM, etc. To obtain a (modeled) value for the amount of carbon in the soil at the start of the experiment, a plant debris input of 1.48 and 3.0 t C ha
1 for Geescroft and Highfield, respectively, was used. Thereafter, the annual C inputs (t ha
1) were Geescroft, ▪, 2.5 (from plant debris); Bare Fallow, △, zero.

The fit of the model to the observed changes in SOM for two contrasted treatments in the Rothamsted Ley–arable experiments on Highfield and Fosters (see also Fig. 6) is shown in Fig. 17. Figure 17A shows, for the experiment on Highfield, the changes in SOM on the permanent grass plots and the continuous arable plots after ploughing out the grass. For the first 12 years, the grass was grazed by sheep before the treatment changed to a grass/clover sward harvested three or four times per year for conservation. Different annual C inputs were estimated for the two periods and the fit of the model to the observed amounts of soil C is good. The fit is not so good where the grassland soil was ploughed to grow a rotation of arable crops. An average annual C input of 1.4 t C ha
1 has been assumed and the model predicts a slower rate of decline in SOM than that observed. The fit of the model to the data is good if the input was 0.6 t C ha
1, but this is probably too small. The fit of the model to the observed data for the Fosters experiment (Fig. 17B) is good, the change from grazing to harvesting herbage for

50

A. Edward Johnston et al.

B

100

100

90

90

80

80 Organic C in soil, t C ha–1

Organic C in soil, t C ha–1

A

70 60 50 40 30

70 60 50 40 30

20

20

10

10

0 1940

1960

1980 Year

2000

2020

0 1940

1960

1980

2000

2020

Year

Figure 17 Organic carbon (t ha
1) in the top 23 cm on Highfield and Fosters Ley– arable, Rothamsted. The data points are adjusted (for each site) for changes in soil bulk density (see text) and the solid lines are the model output. The incoming plant residues were assumed to have DPM and RPM in the proportion 0.59 and 0.41, respectively. The IOM for these experiments was 3.0 t C ha
1. See text for explanation of DPM, etc. To obtain a (modeled) value for the amount of carbon in the soil at the start of the experiments, a plant debris input of 2.7 and 2.1 t C ha
1 for Highfield and Fosters, respectively, was used. Thereafter, the annual C inputs (t ha
1) from plant debris were (A) Highfield grass, ▲, 5.0 for 12 years then 4.0; Highfield arable, ▪, 1.4; and (B) Fosters grass, ▲, 5.0 for 12 years then 4.0; Fosters arable, ▪, 1.4.

conservation being well modeled. On the plots that remained in continuous arable the fit of the model to the observed data was good. The same annual input of C was used for plots growing arable crops on both Highfield and Fosters because the yields were very similar on both experiments. Further work is needed to see whether altering the parameters for the rate of decline of SOM will give a better fit to the observed decline in SOM on Highfield. It should be noted that all the model parameters in Figs. 16 and 17 were exactly the same as those used in the initial model developed by Jenkinson, which gave the fit to the data shown in Fig. 15; the only driving variable was the annual input of organic carbon. Many of the aberrant observed points in all four relationships are probably, in part, due to soil sampling issues. This approach to modeling, which can be perceived as ‘‘bottom-up,’’ that is, a single site studied in great detail, has the benefit that the parameters in the model can be determined on the basis of well-estimated data and

Soil Organic Matter: Its Importance in Sustainable Agriculture and Carbon Dioxide Fluxes

51

then, as other data sets become available, the parameters can be adjusted so that the model adequately describes changes in a wider range of soils, farming systems and climates. Climate change will influence the stock of SOM in two ways: by altering plant production, thus altering the annual return of plant debris to the soil, and by changing the rate at which this input decays in or on the soil. Global warming will increase decomposition rates and if inputs remain unchanged the world stock of SOM will decline releasing CO2 to the atmosphere. A similar positive feedback will be caused by an increase in rainfall (except for wetlands) in those situations where decomposition is currently restricted by drought. In reality, however, inputs of organic matter may increase, sequestering CO2 in SOM. Models describing change based on past well-known events can be good as shown here, predicting change when there is doubt about the magnitude of change in any one compartment of a model and its possible interaction with other compartments is much more difficult. The model described earlier, as with most other models for SOM turnover, was designed for use in topsoils. However, if half of the world’s organic C held in the top meter of soil (estimated at 1600 Gt by Prentice, 2001) is in the 25–100 cm layer ( Jobba´gy and Jackson, 2000) then any effect of global warming on this subsoil C could be important. Thus, realistic models dealing with the turnover of subsoil C need to be developed. This has been done for sites from four contrasted systems of land management at Rothamsted, namely continuous arable, permanent grassland, and regenerating woodland on both calcareous and acidic soils. Crucially these soils had been sampled in the 1870s by 9 in. (23 cm) depths to 36 in. and were sampled again by these depths recently; in presenting the C data here the metric equivalent for sampling depth is used. All these samples were analyzed for organic C and 14C to develop a C turnover model for the top 91 cm soil. This model, Roth PC-1, is based on the earlier model, ROTHC26.3, originally developed to model C turnover in topsoil and used to provide the data presented above. Two extra parameters have been added to the original model; one allows for movement of C down the profile by advection, the other slows decomposition of that C with depth. Jenkinson and Coleman (2008) describe in detail Roth PC-1 while the data used to develop and test it are given by Jenkinson et al. (2008). Jenkinson and Coleman (2008) also compared the new multilayer model and the single layer version to see how they respond to a possible increase in global warming of 0.25  C per decade over the next 100 years. The model runs strongly suggest that treating the top meter of soil as a single homogeneous layer overestimates the decomposition of the SOM it contains due to global warming. More realistic estimates of SOM decomposition, and hence the release of CO2, will be obtained from multilayer models such as Roth PC-1.

52

A. Edward Johnston et al.

7. Disadvantages from Increasing Soil Organic Matter The benefits of increasing the amount of SOM are bought at a cost and this should be realized. Data given here show how much C and N is lost during the microbial decomposition of added organic matter and, that, at the equilibrium level of SOM for any soil, climate, and farming system, all the C and N in further additions of organic matter will be lost. There are some further problems too. The loss of nitrate from soil in autumn is an issue that has attracted much attention because of possible environmental risks, but there is also a financial cost to the farmer if N fertilizers are not used efficiently. Fertilizer- and FYM-treated soils on the Hoosfield Barley experiment were sampled to 110 cm on eight occasions between September 1986 and early May 1987 and the total mineral N content determined. Throughout the period, FYMtreated soil contained much more mineral N than did fertilizer-treated soil (Powlson et al., 1989) due to the mineralization of existing SOM and there was little contribution to the mineral N pool from the FYM ploughed-in in autumn until March (Fig. 18). The large amounts of mineral N in the FYMtreated soil were at risk to loss by leaching whenever excess rainfall caused through drainage. Goulding et al. (2000) also showed that, on the Broadbalk

250

Inorganic N, 0–110cm, kgha–1

Cultivated and drilled 200

FYM applied and ploughed

150

100

50

0 26/08/1986 15/10/1986 04/12/1986 23/01/1987 14/03/1987 03/05/1987 22/06/1987 Date sampled
1

Figure 18 Inorganic N (kg ha ) in the soil to 110 cm in autumn/winter 1986/1987, Hoosfield Continuous Barley experiment, Rothamsted. Annual treatment since 1852: NPK fertilizers, ^; FYM 35 t ha
1, ▪. Both soils had received 96 kg N ha
1 in spring 1986. (Adapted from Powlson et al., 1989.)

Soil Organic Matter: Its Importance in Sustainable Agriculture and Carbon Dioxide Fluxes

53

Inorganic N, 0–50 cm, kgha–1

Wheat experiment, more inorganic N was at risk of loss by leaching under soils with a history of FYM addition. Many field experiments with cereals have shown that when fertilizer N is applied to achieve the economic optimum grain yield the amount of mineral N remaining in the soil at harvest is often only a little larger than that in soil to which no fertilizer N was applied (Glendining and Powlson, 1995). To assess the relative efficiency of added fertilizer N and fertilizer N added to soil with extra SOM, some 15N experiments have been done on long-term experiments at Rothamsted. In the Hoosfield experiment, labeled fertilizer N was applied to spring barley grown on both fertilizerand FYM-treated soil. The labeled N was taken up with similar efficiency on both soils and at harvest less than 4% of the added fertilizer N was present in inorganic form in either soil (Glendining et al., 1997). Thus, it appeared that fertilizer N was taken up preferentially to soil N even though there was more soil N in FYM-treated soil. A similar result was found where winter wheat was grown after ploughing in leys in the Woburn Ley–arable experiment (Fig. 19). Wheat given 140 kg ha
1 labeled fertilizer N gave good yields and at harvest only about 3 kg ha
1 of the labeled N was present as inorganic N in the top 50 cm of soil. However, while the total mineral N in this depth of soil was about 16 kg ha
1 following all-arable cropping, the unlabeled mineral N following ploughed-in leys was much larger, up to about 60 kg ha
1. Thus, the overwhelming majority of the mineral N in soil at harvest was not the residue of the fertilizer N applied in spring but came from the mineralization of SOM (Macdonald et al., 1989).

70 60 50 40 30 20 10 0

Arable

Grass + N

Grass/clover

Previous cropping

Figure 19 Inorganic N (kg ha
1) in the soil to 50 cm after wheat following different rotations, Ley–arable experiment, Woburn. Total inorganic N, unfilled þ filled bars; inorganic N derived from spring-applied 15N-labeled fertilizer, filled bars. (Adapted from Macdonald et al., 1989.)

54

A. Edward Johnston et al.

ACKNOWLEDGMENTS We thank Rodger White for fitting the trend lines to the observed changes in soil carbon in Figs. 4A and 6. Rothamsted Research receives grant-aided support from the Biotechnology and Biological Sciences Research Council and the Lawes Agricultural Trust.

REFERENCES Addiscott, T. M., and Johnston, A. E. (1971). Potassium in soils under different cropping systems. II. The effects of cropping systems on the retention by soils of added K not used by crops. J. Agric. Sci. (Cambridge) 76, 553–561. Bellamy, P. H., Loveland, P. J., Bradley, R. I., Lark, R. M., and Kirk, G. J. D. (2005). Carbon losses from all soils across England and Wales 1978–2003. Nature 437, 245–248. Blair, N., Faulkner, R. D., Till, A. R., and Poulton, P. R. (2006). Long-term management impacts on soil C, N and physical fertility. Part 1. Broadbalk experiment. Soil Till. Res. 91, 30–38. Boyd, D. A. (1968). Experiments with ley and arable farming systems. Rothamsted Exp. Station Rep. 1967, 316–331. Chater, M., and Gasser, J. K. R. (1970). Effects of green manure, farmyard manure and straw on organic matter in soil and of green manure on available nitrogen. J. Soil Sci. 21, 127–137. Christensen, B. T., and Johnston, A. E. (1997). Soil organic matter and soil quality: Lessons learned from long-term experiments at Askov and Rothamsted. In ‘‘Soil Quality for Crop Production and Ecosystem Health’’ (E. G. Gregorich and M. R. Carter, Eds.), pp. 399–430. Elsevier, Amsterdam. Dyke, G. V., George, B. J., Johnston, A. E., Poulton, P. R., and Todd, A. D. (1983). The Broadbalk Wheat experiment 1968–78. Rothamsted Exp. Station Rep. 1982. Pt. 2, 5–44. Glendining, M. J., and Powlson, D. S. (1995). The effects of long continued application of inorganic nitrogen fertilizer on soil organic nitrogen—A review. In ‘‘Soil Management: Experimental Basis for Sustainability and Environmental Quality’’ (R. Lal and B. A. Stewart, Eds.), pp. 385–446. CRC Press, Boca Raton. Glendining, M., Poulton, P. R., Powlson, D. S., and Jenkinson, D. S. (1997). Fate of 15Nlabelled fertilizer applied to spring barley grown on soils of contrasting nutrient status. Plant Soil 195, 83–98. Goulding, K. W. T., Poulton, P. R., Webster, C. P., and Howe, M. T. (2000). Nitrate leaching from the Broadbalk Wheat experiment, Rothamsted, UK, as influenced by fertilizer and manure inputs and the weather. Soil Use Manage. 16, 244–250. Harmer, R., Peterken, G., Kerr, G., and Poulton, P. (2001). Vegetation changes during 100 years of development of two secondary woodlands on abandoned arable land. Biol. Conserv. 101, 291–304. Harvey, P. N. (1959). The disposal of cereal straw. J. R. Agric. Soc. England 120, 55–63. Holmberg, J., Bass, S., and Timberlake, L. (1991). ‘‘Defending the Future’’, p. 13. Earthscan/IIED, London. Jenkinson, D. S. (1977). Studies on the decomposition of plant material in soil V. J. Soil Sci. 28, 424–434. Jenkinson, D. S. (1990). The turnover of organic carbon and nitrogen in soil. Philos. Trans. R. Soc. B 329, 361–368. Jenkinson, D. S. (1991). The Rothamsted long-term experiments: Are they still of use? Agron. J. 83, 2–10.

Soil Organic Matter: Its Importance in Sustainable Agriculture and Carbon Dioxide Fluxes

55

Jenkinson, D. S., and Ayanaba, A. (1977). Decomposition of carbon-14 labeled plant material under tropical conditions. Soil Sci. Soc. Am. J. 41, 912–915. Jenkinson, D. S., and Coleman, K. (2008). The turnover of organic carbon in subsoils. Part 2. Modelling carbon turnover. Eur. J. Soil Sci. 59, 400–413. Jenkinson, D. S., Hart, P. B. S., Rayner, J. H., and Parry, L. C. (1987). Modelling the turnover of organic matter in long-term experiments at Rothamsted. INTECOL Bull. 15, 1–8. Jenkinson, D. S., and Johnston, A. E. (1977). Soil organic matter in the Hoosfield Continuous Barley experiment. Rothamsted Exp. Station Rep. 1976, Pt. 2, 87–101. Jenkinson, D. S., Bradbury, N. J., and Coleman, K. (1994). How the Rothamsted classical experiments have been used to develop and test models for the turnover of carbon and nitrogen in soil. In ‘‘Long-Term Experiments in Agricultural and Ecological Sciences’’ (R. A. Leigh and A. E. Johnston, Eds.), pp. 117–138. CAB International, Wallingford, UK. Jenkinson, D. S., Poulton, P. R., Johnston, A. E., and Powlson, D. S. (2004). Turnover of nitrogen-15-labeled fertilizer in old grassland. Soil Sci. Soc. Am. J. 68, 856–875. Jenkinson, D. S., Poulton, P. R., and Bryant, C. (2008). The turnover of organic carbon in subsoils. Part 1. Natural and bomb radiocarbon in soil profiles from the Rothamsted long-term field experiments. Eur. J. Soil Sci. 59, 391–399. Jobba´gy, E. G., and Jackson, R. B. (2000). The vertical distribution of soil organic carbon and its relation to climate and vegetation. Ecol. Appl. 10, 423–436. Johnston, A. E. (1973). The effects of ley and arable cropping systems on the amounts of soil organic matter in the Rothamsted and Woburn Ley–Arable experiments. Rothamsted Exp. Station Rep. 1972, Pt. 2, 131–159. Johnston, A. E. (1975). The Woburn Market Garden experiment, 1942–69. II. Effects of the treatments on soil pH, soil carbon, nitrogen, phosphorus and potassium. Rothamsted Exp. Station Rep. 1974, Pt. 2, 102–131. Johnston, A. E. (1986). Soil organic matter, effects on soils and crops. Soil Use Manage. 2, 97–105. Johnston, A. E. (1987). Effects of soil organic matter on yields of crops in long-term experiments at Rothamsted and Woburn. INTECOL Bull. 15, 9–16. Johnston, A. E., and Brookes, P. C. (1979). Yields of, and P, K, Ca, Mg uptakes by, crops grown in an experiment testing the effects of adding peat to a sandy loam soil at Woburn, 1963–77. Rothamsted Exp. Station Rep. 1978, Pt. 2, 83–98. Johnston, A. E., and Garner, H. V. (1969). Broadbalk: Historical introduction. Rothamsted Exp. Station Rep. 1968, Pt. 2, 12–25. Johnston, A. E., Hewitt, M. V., Poulton, P. R., and Lane, P. W. (1997). Peat – a valuable resource. In ‘‘Humic Substances in Soils, Peats and Waters’’ (M. H. B. Hayes and W. S. Wilson, Eds), pp. 368–383. Royal Society of Chemistry, Cambridge, UK. Johnston, A. E., and Mattingly, G. E. G. (1976). Experiments on the continuous growth of arable crops at Rothamsted and Woburn Experimental Stations. Effects of treatments on crop yields and soil analysis and recent modifications in purpose and design. Ann. Agron. 27, 927–956. Johnston, A. E., and Poulton, P. R. (1977). Yields on the Exhaustion Land and changes in the NPK contents of the soils due to cropping and manuring, 1852–1975. Rothamsted Exp. Station Rep. 1976, Pt. 2, 53–85. Johnston, A. E., and Poulton, P. R. (1980). Effects of soil organic matter on cereal yields. Rothamsted Exp. Station Rep. 1979, Pt. 1, 234–235. Johnston, A. E., and Poulton, P. R. (1993). The role of phosphorus in crop production and soil fertility: 150 years of field experiments at Rothamsted, United Kingdom. In ‘‘Phosphate Fertilizers and the Environment’’ ( J. J. Schultz, Ed.), pp. 45–63. International Fertilizer Development Center, Muscle Shoals, AL.

56

A. Edward Johnston et al.

Johnston, A. E., and Poulton, P. R. (2005). Soil organic matter: Its importance in sustainable agricultural systems. Proc. 565, Int. Fert. Soc., York, UK, 48pp. Johnston, A. E., and Wedderburn, R. W. M. (1975). The Woburn Market Garden experiment, 1942–69. I. A history of the experiment, details of treatments and the yields of crops. Rothamsted Exp. Station Rep. 1974, Pt. 2, 79–101. Johnston, A. E., McGrath, S. P., Poulton, P. R., and Lane, P. W. (1989). Accumulation and loss of nitrogen from manure, sludge and compost: Long-term experiments at Rothamsted and Woburn. In ‘‘Nitrogen in Organic Wastes Applied to Soils’’ ( J. A. A. Hansen and K. Henriksen, Eds.), pp. 126–139. Academic Press, London. Johnston, A. E., McEwen, J., Lane, P. W., Hewitt, M. V., Poulton, P. R., and Yeoman, D. P. (1994). Effects of one to six year old ryegrass–clover leys on soil nitrogen and on subsequent yields and fertilizer nitrogen requirements of the arable sequence winter wheat, potatoes, winter wheat, winter beans (Vicia faba) grown on a sandy loam soil. J. Agric. Sci. (Cambridge) 122, 73–89. Johnston, A. E., Barraclough, P. B., Poulton, P. R., and Dawson, C. J. (1998). Assessment of some spatially variable soil factors limiting crop yields. Proc. 419, Int. Fert. Soc., York, UK, 46pp. Khan, S. A., Mulvaney, R. L., Ellsworthy, T. R., and Boast, C. W. (2007). The myth of nitrogen fertilization for soil carbon sequestration. J. Environ. Qual. 36, 1821–1832. Macdonald, A. J., Powlson, D. S., Poulton, P. R., and Jenkinson, D. S. (1989). Unused nitrogen fertiliser in arable soils—Its contribution to nitrate leaching. J. Sci. Food Agric. 46, 407–419. Mann, H. H., and Boyd, D. A. (1958). Some results of an experiment to compare ley and arable rotations at Woburn. J. Agric. Sci. (Cambridge) 50, 297–306. Mattingly, G. E. G. (1974). The Woburn Organic Manuring experiment: I. Design, crop yields and nutrient balance, 1964–72. Rothamsted Exp. Station Rep. 1973, Pt. 2, 98–133. Mattingly, G. E. G., Chater, M., and Poulton, P. R. (1974). The Woburn Organic Manuring experiment: II. Soil analyses, 1964–72, with special reference to changes in carbon and nitrogen. Rothamsted Exp. Station Rep. 1973,Pt. 2, 134–151. Mattingly, G. E. G., Chater, M., and Johnston, A. E. (1975). Experiments made on Stackyard Field, Woburn, 1876–1974. III. Effects of NPK fertilisers and farmyard manure on soil carbon, nitrogen and organic phosphorus. Rothamsted Exp. Station Rep. 1974, Pt. 2 61–77. Poulton, P. R., and Johnston, A. E. (1996). The long-term effect of ley–arable cropping on soil organic matter and crop yield. In “Transactions of the 9th Nitrogen Workshop,” Braunschweig, September, 1996, pp. 437–440. Poulton, P. R., Pye, E., Hargreaves, P. R., and Jenkinson, D. S. (2003). Accumulation of carbon and nitrogen by old arable land reverting to woodland. Glob. Change Biol. 9, 942–955. Powlson, D. S., Pruden, G., Johnston, A. E., and Jenkinson, D. S. (1986). The nitrogen cycle in the Broadbalk Wheat experiment: Recovery and losses of 15N-labelled fertilizer applied in spring and inputs of nitrogen from the atmosphere. J. Agric. Sci. (Cambridge) 107, 591–609. Powlson, D. S., Poulton, P. R., Addiscott, T. M., and McCann, D. S. (1989). Leaching of nitrate from soils receiving organic and inorganic fertilizers continuously for 135 years. In “Nitrogen in Organic Wastes Applied to Soils” ( J. A. A. Hansen and K. Henriksen, Eds.), pp. 334–345. Academic Press, London. Prentice, I. C. (2001). The carbon cycle and atmospheric carbon dioxide. In ‘‘Climate Change 2001: The Scientific Basis’’ ( J. T. Houghton, Ed.), pp. 183–237. Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge. Richardson, H. L. (1938). The nitrogen cycle in grassland soils with special reference to the Rothamsted Park Grass experiment. J. Agric. Sci. (Cambridge) 28, 73–121.

Soil Organic Matter: Its Importance in Sustainable Agriculture and Carbon Dioxide Fluxes

57

Rosenani, A. B., Powlson, D. S., Webster, C. P., Goulding, K. W. T., and Poulton, P. R. (1995). The dynamics of nitrate fertilizer in soil in the autumn and the effect of farmyard manure. In ‘‘Nuclear Methods in Soil-Plant Aspect of Sustainable Agriculture’’, pp. 51–59. International Atomic Energy Authority, Vienna. Russell, E. W. (1977). The role of organic matter in soil fertility. Philos. Trans. R. Soc. Lond. B 281, 209–219. Salter, P. J., and Williams, J. B. (1969). The moisture characteristics of some Rothamsted, Woburn and Saxmundham soils. J. Agric. Sci. (Cambridge) 73, 155–158. von Liebig, J. (1840). Organic Chemistry in Its Application to Agriculture and Physiology. Taylor and Walton, London. Warren, R. G., and Johnston, A. E. (1964). The Park Grass experiment. Rothamsted Exp. Station Rep. 1963, 240–262. Warren, R. G., and Johnston, A. E. (1967). Hoosfield Continuous Barley. Rothamsted Exp. Station Rep. 1966, 320–328. Watts, C. W., and Dexter, A. R. (1997). The influence of organic matter in reducing the destabilization of soil by simulated tillage. Soil Till. Res. 42, 253–275. Watts, C. W., and Dexter, A. R. (1998). Soil friability: Theory, management and the effects of management and organic carbon content. Eur. J. Soil Sci. 49, 73–84. Watts, C. W., Whalley, W. R., Longstaff, D. J., White, P. R., Brookes, P. C., and Whitmore, A. P. (2001). Aggregation of a soil with different cropping histories following the addition of organic materials. Soil Use Manage. 17, 263–268. Watts, C. W., Clark, L. J., Poulton, P. R., Powlson, D. S., and Whitmore, A. P. (2006). The role of clay, organic carbon and long-term management on mouldboard plough draught measured on the Broadbalk wheat experiment at Rothamsted. Soil Use Manage. 22, 334–341. Williams, R. J. B. (1978). Effects of management and manuring on the physical properties of some Rothamsted and Woburn soils. Rothamsted Exp. Station Rep. 1977, Pt. 2, 37–54.

C H A P T E R

T W O

Climate Change Affecting Rice Production: The Physiological and Agronomic Basis for Possible Adaptation Strategies R. Wassmann,*,† S. V. K. Jagadish,* S. Heuer,* A. Ismail,* E. Redona,* R. Serraj,* R. K. Singh,* G. Howell,* H. Pathak,‡ and K. Sumfleth* Contents 1. Introduction 2. Stress Physiology and Possible Adaptation Mechanisms to Climate Induced Stresses 2.1. High temperature and humidity 2.2. Drought 2.3. Salinity 2.4. Submergence 3. Comparative Assessment of Rice Versus Other Crops (In Terms of Vulnerability and Adaptation Options) 3.1. Advantages/disadvantages in warmer climates 3.2. Advantages/disadvantages under worsening water stress 3.3. Advantages/disadvantages in deteriorating soils 3.4. Flexibility for adjusting and coping with climate changes 4. Outlook: Current Advances and Future Prospects References

60 63 63 80 93 97 102 102 106 107 108 109 110

Abstract This review addresses possible adaptation strategies in rice production to abiotic stresses that will aggravate under climate change: heat (high temperature and humidity), drought, salinity, and submergence. Each stress is discussed regarding the current state of knowledge on damage mechanism for rice plants as well as possible developments in germplasm and crop

* { {

International Rice Research Institute, Metro Manila, Philippines Research Center Karlsruhe (IMK-IFU), Garmisch-Partenkirchen, Germany International Rice Research Institute, New Delhi, India

Advances in Agronomy, Volume 101 ISSN 0065-2113, DOI: 10.1016/S0065-2113(08)00802-X

#

2009 Elsevier Inc. All rights reserved.

59

60

R. Wassmann et al.

management technologies to overcome production losses. Higher temperatures can adversely affect rice yields through two principal pathways, namely (i) high maximum temperatures that cause—in combination with high humidity—spikelet sterility and adversely affect grain quality and (ii) increased nighttime temperatures that may reduce assimilate accumulation. On the other hand, some rice cultivars are grown in extremely hot environments, so that the development of rice germplasm with improved heat resistance can capture an enormous genetic pool for this trait. Likewise, drought is a common phenomenon in many rice growing environments, and agriculture research has achieved considerable progress in terms of germplasm improvement and crop management (i.e., water saving techniques) to cope with the complexity of the drought syndrome. Rice is highly sensitive to salinity. Salinity often coincides with other stresses in rice production, namely drought in inland areas or submergence in coastal areas. Submergence tolerance of rice plants has substantially been improved by introgressing the Sub1 gene into popular rice cultivars in many Asian rice growing areas. Finally, the review comprises a comparative assessment of the rice versus other crops related to climate change. The rice crop has many unique features in terms of susceptibility and adaptation to climate change impacts due to its semiaquatic phylogenetic origin. The bulk of global rice supply originates from irrigated systems which are to some extent shielded from immediate drought effects. The buffer effect of irrigation against climate change impacts, however, will depend on nature and state of the respective irrigation system. The envisaged propagation of irrigation water saving techniques will entail benefits for the resilience of rice production systems to future droughts. We conclude that there are considerable risks for rice production stemming from climate change, but that the development of necessary adaptation options can capitalize on an enormous variety of rice production systems in very different climates and on encouraging progress in recent research.

1. Introduction Rice is consumed by about 3 billion people and is the most common staple food of a large number of people on earth, in fact it feeds more people than any other crop (Maclean et al., 2002). Ninety percent of the world’s rice is produced and consumed in Asia, where irrigated and rainfed rice ecosystems form the mainstay of food security in many countries (Fig. 1). Rice production under flooded conditions is highly sustainable and has apart from emissions of the greenhouse gas methane fewer adverse environmental impacts than other production systems, for example, less soil erosion, high soil organic matter content, and so on (Bouman et al., 2007). Climate change, however, could seriously threaten production levels required to feed future generations in Asia and other continents. Climate change has many facets, including changes in long term trends in temperature and

Climate Change Affecting Rice Production

61

1 Dot = 10,000 ha

Irrigated rice

1 Dot = 10,000 ha

Rainfed rice

Figure 1 Irrigated and rainfed rice in East, South and Southeast Asia (data source: Huke and Huke, 1997).

rainfall regimes as well as increasing variability in extreme events. The impacts of these changing conditions on agriculture are already being seen, yet there are still considerable gaps in our knowledge of how agricultural systems will be affected by both short- and long-term changes in climate, and what implications these changes will have for rural livelihoods, particularly among the most vulnerable. Despite some projected increase in photosynthesis caused by higher concentrations in CO2 ([CO2]), increased temperature may result in reduced productivity. For some regions and

62

R. Wassmann et al.

crops, there will be opportunities for increased production, but all in all, there is no doubt that net agricultural production will be adversely affected by climate change (IPCC, 2007a). Future farming and food systems will have to be better adapted to a range of abiotic and biotic stresses to cope with the direct and indirect consequences of a progressively changing climate. To this end, intensively managed cropping systems such as rice production offer a variety of entry points to adjust to projected climate change (Aggarwal and Mall, 2002; Butt et al., 2005; Easterling et al., 2003; Challinor et al., 2007; Howden et al., 2007; Travasso et al., 2006). Climate change will aggravate a variety of stresses for rice plants, namely heat, drought, salinity, and submergence. Improved tolerance to these abiotic stresses has always been at the heartland of research institutions, such as the International Rice Research Institute (IRRI), dealing with agricultural production in unfavorable environments. For rice production, research on adaptation to climate change can broadly capitalize on the enormous progress made in disentangling the traits associated with tolerance and in developing DNA-based technologies for precise and speedy breeding of more adapted varieties. The new challenge of climate change, however, will require stepping up these activities to unprecedented levels. The resilience of rice production systems has to be increased in a two-pronged approach, (i) increasing tolerance to individual stresses and at the same time (ii) achieving multiple stress tolerance. While we do not see crop technology as the ultimate solution to all threats posed by climate change, we remain convinced that germplasm development and improved agronomic practices should be a center piece of climate change adaptation in agriculture. These approaches have proven track records in achieving more resilience to climate variability and extremes. Superimposed on Climate Change effects, agriculture is confronted with other rapid socioeconomic changes resulting in labor shortages, rising costs of energy, and so on. Competition for water, for instance, will increase the pressure on rice land and favor the adoption of cropping systems or practices that consume less irrigation water. The Green Revolution has improved rice productivity across monsoon Asia through a combination of new high-yielding varieties with increased input use, such as stable water supply from new irrigation systems, fertilizer, and biocide use (Hossain and Fischer, 1995). Because of this increased productivity, and an increase in cropped area, total rice production over the last 40 years has more than kept pace with the tremendous growth in population in Asia and now stands at about 550–600 million tones annually (Maclean et al., 2002). After a 3-decade long period of low rice prices, however, rice prices have soared in 2007/2008. The world price of Thai export grade rice has almost tripled from December 2007 to April 2008 this year. A major reason for this price increase is the slowing growth in production, which declined from 2.7% per year in 1970–1990 to 1.2%

Climate Change Affecting Rice Production

63

per year in 1990–2007. In the foreseeable future, rice will continue to be the main staple food of Asia (Rosegrant et al., 2001; Sombilla et al., 2002). To fight poverty and provide food security, rice production must increase dramatically in spite of climatic change impacts.

2. Stress Physiology and Possible Adaptation Mechanisms to Climate Induced Stresses 2.1. High temperature and humidity Rice, like other cultivated crops, has relative variable temperature preferences over the growing season. Deviation from the stage-dependent optimum temperature will alter the physiological activities or lead to a different developmental pathway (Downton and Slatyer, 1972). The response of rice to high temperatures differs according to the developmental stage with high temperature tolerance at one developmental stage may or may not necessarily lead to tolerance during other stages. Similarly, cold tolerance at the booting stage was shown to have no relationship to the flowering stage tolerance in high-yielding rice varieties (Goto et al., 2008). However, an independent extreme heat episode during vegetative stages was shown to have no influence on reproductive stage (Porter and Semenov, 2005). Hence, the effect of high temperature during different developmental stages has to be partitioned and evaluated separately for assessing, identifying, characterizing for genetic manipulation of tolerance mechanisms (Wahid et al., 2007). The crop growth cycle of rice can be broadly divided into three stages namely vegetative, reproductive, and grain filling or ripening phase (Fig. 2), and their response to high temperatures with extra emphasis on the most sensitive reproductive stage is explained in this section. 2.1.1. Heat stress at different ontogenetic stages 2.1.1.1. Vegetative phase During vegetative stage, rice can tolerate relatively high temperatures (35/25
C; expressing day/night temperature regime). Temperatures beyond this critical level could reduce plant height, tiller number and total dry weight (Yoshida et al., 1981). In a temperature gradient chamber study, rice exposed to 3.6 and 7.0
C higher temperature than ambient, from heading to middle ripening stage, reduced photosynthesis by 11.2–35.6%, respectively (Oh-e et al., 2007). This decline in the photosynthesis can be attributed to structural changes in the organization of thylakoids (Karim et al., 1997) and more particularly due to loss of stacking of grana in the chloroplast or its ability to swell (Wahid et al., 2007). Moreover, membranes that house these cell organelles are extremely important as high temperatures increase the kinetic energy, in turn the molecular movements to loosen the bonds between biological membranes

64

R. Wassmann et al.

Vegetative 60 DAG

Reproductive

Grain filling

30 DAG

30 DAG

Pollen mother cell formation ~80 DAG

Meiosis I (tetrad formation) Microsporogenesis Microspore stage Pollen formation stage (mitosis) Microgametogenesis Mature starchy pollen stage

90 DAG

90% heading Anthesis/spikelet opening Anther dehiscence

50–80 min Pollination Pollen germination Spikelet closes Fertilization

Figure 2 Partitioning crop growth cycle of rice variety (120 days) into three major phases with extra emphasis on heat sensitive stages during the reproductive stage. DAG, Days after germination.

(Wahid et al., 2007). Such rapid movements will lead to increase in fluidity of lipid layer (Savchenko et al., 2002) resulting in increased solute leakage and membrane instability. Quantitative electrolyte leakage or cellular membrane thermostability (CMT) has been used as a measure of heat tolerance during the vegetative stage in many crops (see Prasad et al., 2006; Tripathy et al., 2000). A positive association between CMT and heat tolerance at flowering has been found in cowpea (Vigna unguiculata L.) (Ismail and Hall, 1999). However, a poor correlation (r = 0.02) between reproductive stage tolerance measured by spikelet fertility and heat tolerance during vegetative stage measured by CMT in 14 rice genotypes was observed (Prasad et al., 2006). Accordingly, in peanuts (Arachis hypogea) a similar relation has been found (Craufurd et al., 2003; Kakani et al., 2002), indicating different responses to heat at vegetative and reproductive stages in rice and peanuts.

65

Climate Change Affecting Rice Production

2.1.1.2. Reproductive phase Reproductive stage in rice is more sensitive to heat than the vegetative stage (Yoshida et al., 1981). Anthesis/flowering, identified with the appearance of the anthers, is the most sensitive process during reproductive stage to high temperature (Nakagawa et al., 2002; Satake and Yoshida, 1978) followed by microgametogenesis (Fig. 3). Reciprocal studies with manual shedding of pollen from control plants on to the stigma exposed to high temperature and vice versa showed that the ability of the pistil to be fertilized remained unaffected even over a period of 5 days at 41
C (Yoshida et al., 1981). Similarly, wheat spikelet fertility was increased from 30 to 80% by pollinating heat stressed pistil with unstressed pollen (Saini and Aspinall, 1982). Hence, the male reproductive organ is mainly responsible for spikelet sterility under high temperature and has been targeted for increasing tolerance to warmer climates. Mature male reproductive unit or pollen formation is a result of:

(i) Pollen mother cell formation from diploid sporophytic cells in the anther (ii) Formation of haploid unicellular microspores from pollen mother cells (microsporogenesis) (iii) Microspores to microgametophytes with gametes and (microgametogenesis) (iv) Male gametophytes developing into mature starchy pollen (Fig. 2) Processes close to the meiotic stage during tetrad formation and young microspore stage are most sensitive to high temperature during microsporogenesis (Yoshida et al., 1981), similar to drought (Sheoran and Saini, 1996) and cold stress (Imin et al., 2004). A significant reduction in pollen production at 5
C above ambient air temperature (Prasad et al., 2006) was

Spikelet fertility (%)

100 80 60 Microsporogenesis 40 20 0

Anthesis –25 –20 –15 –10 –5 0 5 Day relative to anthesis

10

Figure 3 Spikelet fertility of BKN6624–46–2 exposed to high temperature of 35
C during different stages of panicle development for 5 days (Yoshida et al., 1981; redrawn by P. Craufurd).

66

R. Wassmann et al.

attributed to impaired cell division of microspore mother cells (Takeoka et al., 1992). At lower temperature (20
C), wheat spikelets had 93% of viable pollen in dehisced anther while at a higher temperature of 30
C for 3 days dehisced anthers had significantly lower percent (59%) of viable pollen (Saini and Aspinall, 1982). Accordingly, high temperatures (35
C) during microsporogenesis resulted in 34% decline in spikelet fertility (Fig. 3). Heat stress during anthesis leads to an irreversible effect with stagnation in panicle dry weight even with subsequent improvement in the environment (Oh-e et al., 2007). However, rice genotypes can either escape or avoid high temperatures during anthesis, by heading during the cooler periods of the season (macroescape), by anthesing during cooler hours of early morning (microescape, O. glaberrima spp. Yoshida et al., 1981), altered flowering pattern or by increased transpiration cooling of the canopy. Advancing peak anthesis toward early hours of the morning (Prasad et al., 2006), is an efficient strategy to escape high temperatures during later hours of the day. Significant genotypic variation for early morning peak anthesis exists in rice germplasm with O. glaberrima (CG14) having the ability to flower immediately after dawn, potentially escaping high temperatures during the later hours of the day (Fig. 4). The early morning flowering advantage of O. glaberrima has been exploited in interspecific crosses between O. glaberrima and O. sativa to advance peak flowering time of the day by 1h toward early morning (Yoshida et al., 1981). Moreover, rice has the ability to monitor and control the rate of flowering as an escape mechanism under high temperature. The concept of spenders and savers with reference to rate of flowering in rice has been mentioned ( Jagadish, 2007, Jagadish et al., 2007), wherein a 20% increase and 36% decline in the rate of flowering was seen in cultivars IR64 and Azucena, respectively, at 38
C and 60% relative humidity over three consecutive days. Rice plants when exposed to high temperatures during critical stages can avoid heat by maintaining their microclimate temperature below critical levels by efficient transpiration cooling. Moreover, the effect of high temperature is closely related to the ambient relative humidity and hence the level of transpiration cooling is determined by vapor pressure deficit than temperature per se. Using ultra thin copper constantan thermocouples, Jagadish et al. (2007) recorded spikelet tissue temperatures of 29.6, 33.7, and 36.2
C, that is, 0.4, 1.3, and 1.8
C below ambient air temperatures of 30, 35, and 38
C, respectively. Similar differences were observed elsewhere in rice (Satake, 1995) and in peanut flowers in the same growth cabinets used by Jagadish et al. (2007) (Vara Prasad et al., 2001). Lower relative humidity of 60% at 38
C leads to a higher vapor pressure deficit of 2.65 facilitating the plant to exploit its transpiration cooling ability ( Jagadish, 2007; Jagadish et al., 2007). Similarly, Abeysiriwardena et al. (2002) recorded a 1.5
C increase in spikelet temperature by increasing RH from 55–60% to 85–90% at a constant temperature regime of 35/30
C.

67

Climate Change Affecting Rice Production

250 29.6⬚C 36.2⬚C

Number of spikelets opened

IR64 200

150

100

50

0 0

1

2

3 4 5 Hours after dawn

6

7

8

350 Number of spikelets opened

CG14

29.6⬚C 36.2⬚C

300 250 200 150 100 50 0 0

1

2

3 4 5 Hours after dawn

6

7

8

Figure 4 Flowering patterns of O. sativa cv. IR64 and O. glaberrima cv.CG14 under both control and high temperature (bars indicates SE; adapted from Jagadish et al., 2008).

Moreover, Weerakoon et al. (2008) using a combination of high temperatures (32–36
C) with low (60%) and high (85%) RH recorded high spikelet sterility with simultaneous increase in temperature and RH. Hence it can be concluded that the reduction in spikelet temperature in relation to RH is avoidance while the performance of a variety at a given spikelet temperature to be true tolerance. On the basis of the interaction between high temperature and relative humidity, rice cultivation regions in the tropics and sub-tropics can be classified into hot/dry or hot/humid regions. It can be assumed that rice cultivation in hot/dry regions where temperatures may exceed 40
C

68

R. Wassmann et al.

(e.g., Pakistan, Iran, India) has been facilitated through unintentional selection for efficient transpiration cooling under sufficient supply of water. Furthermore, the effect of transpiration cooling was assessed using a simple heat budget model for a typical microclimate in paddy fields resulting in a 0.6
C lower canopy temperature at an ambient temperature of 30
C and RH of 60%. The results showed higher cooling under hot and dry condition with 2.5
C lower canopy temperature at an ambient temperature of 34
C and 60% RH, 4.6
C at 30
C and 20% RH, and 6.9
C at 34
C and 20% RH (Matsui et al., 2007). An exceptionally high temperature difference of 6.8
C between crop canopy and ambient air temperature (34.5
C) was recorded in Riverina region of New South Wales, Australia which was primarily due to extremely low humidity of 20% (VPD=4.32), resulting in strong transpiration cooling mainly driven by high wind velocity of 3.2– 4.2ms1 (Matsui et al., 2007). Introduction, acceptance and wide spread cultivation of semidwarf improved varieties, with better canopy architecture could be one major reason for adjusting rice to existing temperature changes and could play an important role in adapting to future extreme temperatures. Moreover, improved varieties that have panicles surrounded by plant leaf canopy unlike traditional varieties, are immensely benefited by combined transpiration cooling during the sensitive anthesis period (Fig. 5). Heat avoiding genotypes thrive well in hot and dry rice cultivation regions of the world while for hot and humid regions either heat escape

Figure 5

Plant architecture of the rice plant.

69

Climate Change Affecting Rice Production

or true tolerance is essential to maintain productivity. However, with predicted increased mean surface air temperature rather than just increased maximum temperature the rice plant could be exposed to increased day and night temperatures further indicating the importance of true heat tolerance. Increased heat tolerance is most needed in O. sativa spp. (IR64; Fig. 4), compared to O. glaberrima spp., (CG14; Fig. 4) which exhibit peak anthesis during late morning till midafternoon (Yoshida et al., 1981), exposing the heat sensitive reproductive organs to high temperatures invariably leading to increased spikelet sterility ( Jagadish et al., 2008; Prasad et al., 2006). Moreover, O. sativa spp, occupy major rice growing regions of Asia and is exponentially increasing in the African continent. High temperatures induce sterility, if the sensitive physiological processes (anther dehiscence, pollination, pollen germination on the stigma, pollen tube growth or the early events of fertilization) are affected. Anthesis in rice is extremely sensitive to high temperature and spikelets opening on any flowering day during the flowering period (5–7 days) could be affected differently depending on the duration of exposure (Fig. 6);

A C

B

D E

100

Spikelet fertility (%)

80

60

40

20

0 –4 –2 0 2 4 Flowering time in relation to anthesis (h)

Figure 6 The extreme sensitivity of high temperature during anthesis leading to spikelet sterility: (A) high temperature for 4 h, (B) high temperature for 1 h, (C) 1 h before the onset of high temperature, (D) 1 h immediately after high temperature exposure, and (E) beyond 1 h of high temperature exposure (modified from Satake and Yoshida, 1978).

70

R. Wassmann et al.

(i) High temperature exposure for 4 h coinciding with anthesis reduced spikelet fertility from 90 to <20% (Yoshida et al., 1981; see period ‘‘A’’ in Fig. 6). (ii) Spikelet tissue temperatures >33.7
C (ambient air temperature of 35
C) for  half an hour induced sterility indicating extreme sensitivity of rice to high temperature at anthesis ( Jagadish, 2007; Jagadish et al., 2007; see period ‘‘B’’ in Fig. 6). (iii) Spikelets opening an hour before the onset of high temperature were unaffected with the subsequent high temperature exposure ( Jagadish, 2007; Jagadish et al., 2007; see period ‘‘C’’ in Fig. 6). (iv) Spikelets opening within an hour after the high temperature exposure were partially affected as the function of the pollen sac (anther) itself would be affected by the preceding high temperatures ( Jagadish, 2007; Jagadish et al., 2007; Matsui et al., 2000a; see period ‘‘D’’ in Fig. 6). (v) Spikelets opening beyond 1 h of the high temperature exposure are unaffected (see period ‘‘E’’ in Fig. 6). Anther dehiscence is the most susceptible process during anthesis under high temperature (Matsui et al., 1999b). High temperature results in increased vapor pressure deficit, enhancing evaporation from the anthers, thereby depriving the crucial moisture needed for pollen grain swelling which is inevitable for anther dehiscence. Genotypic differences in anther characteristics between susceptible and tolerant rice genotypes exist (Table 1). Artificial spikelet opening triggered rapid pollen swelling, resulting in anther dehiscence and subsequent pollen shedding from apical and basal pores (Matsui et al., 1999a,b). The anther basal pore length is considered to have a significant contribution toward pollination under high temperature because of its close proximity to the stigmatic surface (Matsui and Kagata, 2003). The importance of the apical pore under high temperature is not well understood. Alternatively, in some water stressed anthers of IR64, the basal pore failed to open while in the other anthers with open pores the pollen failed to shed from the opened apical pore, which was attributed to increased pollen stickiness (Liu et al., 2005). A similar mechanism could operate in anthers of heat sensitive genotypes, which warrants a detailed study. Dehiscence of the anther leading to pollen deposition on the stigma is called as pollination. After pollination if takes about 30min for the pollen tube to reach the embryo sac and fertilization will be completed in 1.5–4 h (Cho, 1956). Rice pollen is extremely sensitive to temperature and relative humidity (Matsui et al., 1997b) and looses its viability within 10 min of shedding (Song et al., 2001). Spikelets having >20 germinating pollen on the stigma showed good agreement with fertility at high temperature of 38
C (Matsui et al., 1997a). The tolerant cultivar Shanyou63 showed significantly slower reduction in pollen activity, pollen germination and rate of floret fertility compared to the susceptible cv. Teyou559 at 39
C (Tang et al., 2008). Developmental

71

Climate Change Affecting Rice Production

Table 1 Anther characteristics affecting dehiscence in tolerant and susceptible rice genotypes (modified from Jagadish et al., 2007) Anther traits of tolerant genotypes

Anther traits of susceptible genotypes

Longer anthers

Comparatively shorter anthers Two cell layers (degrading Three cell layers (degrading tapetum, or degraded tapetum and endothecium cells, endothecium cells) and parenchyma cells) separate the locule from separate the locule the lacuna, allowing for from the lacuna, easy anther dehiscence hindering anther dehiscence Easy and homogeneous Abnormal or no anther anther dehiscence dehiscence

Anthers dehisce within the spikelet on short filaments, shedding more pollen on the stigma

Longer basal pore length

Anther do not dehisce or they may dehisce outside the spikelet on loose sagging filaments, with less pollen shed on the stigma Shorter or no basal pore opening

Reference

Matsui et al. (2001) Matsui and Omasa (2002)

Matsui et al. (1997b) Yoshida et al. (1981) Satake and Yoshida, (1978) Satake and Yoshida (1978)

Matsui and Kagata (2003)

processes beyond pollen germination are sensitive to heat and have been shown in rice (Enomoto et al., 1956; Satake and Yoshida, 1978; Yamada, 1964) and other crops (A. hypogea: Kakani et al., 2002, 2005; Glycine max: Salem et al., 2007). The mechanisms of anther dehiscence are well understood (Matsui et al., 2000a,b) but the physiological and biochemical reasons for reduced pollen activity and germination are not yet clear. However, endogenous hormones are known to play an important role in determining male fertility. Tang et al. (2008) quantified the growth hormones in the anthers and found a decrease in indole acetic acid (IAA), gibberellic acid (GA3), free proline and soluble proteins but a significant increase in absisic acid (ABA) content.

72

R. Wassmann et al.

They concluded that low levels of IAA and GA and higher levels of ABA lead to pollen abortion, a major reason for male sterility. Simultaneous decline in free proline and soluble proteins in the susceptible cultivar enhanced stress resulting in floret sterility. Furthermore, accumulation of compatible osmolytes like sugars, sugaralcohols (Sairam and Tyagi, 2004), glycine betaine (Sakamoto and Murata, 2002) plays an adaptive role under extreme temperatures by buffering cellular redox potential (Wahid and Close, 2007). Screening for heat tolerant donors Prasad et al. (2006) identified heat tolerance in both sub spp. of O. sativa and it cannot be generalized that either indica or japonica sub spp. are more tolerant than the other based on the place of origin. An aus variety N22 has consistently shown tolerance to high temperature during anthesis ( Jagadish et al., 2008; Prasad et al., 2006; Yoshida et al., 1981). However N22 is also known to be highly drought tolerant with enhanced levels of reactive oxygen scavenging enzymes resulted in lower H2O2 levels in water stressed panicles of N22 compared to the susceptible N118. Since, N22 is consistently proved to be truly heat tolerant under diverse experimental conditions, a similar reactive oxygen scavenging mechanism could operate leading to heat tolerance, which needs further experimental evidence for confirmation. Variable heat tolerance thresholds among rice genotypes during flowering are known (Yoshida et al., 1981). A 3
C difference in critical temperature causing 50% spikelet sterility between the tolerant genotype (40
C; Akitakomachi) compared to the susceptible genotype (37
C; Hinohikari) is recorded (Matsui et al., 2001). Although genotypic difference to critical heat thresholds in rice is known (Yoshida et al., 1981), experimental evidence for interaction between high temperature and duration of exposure was recently documented ( Jagadish, 2007; Jagadish et al., 2007). An interaction between high temperature and duration of exposure in a heat sensitive genotype (Azucena) but not in a moderately tolerant IR64 was identified ( Jagadish, 2007; Jagadish et al., 2007), indicating the importance of temperature and duration interactions in actual field experiments and inclusion in future crop models. Present crop models have the genotypic difference in critical temperature thresholds causing sterility incorporated in them, the possibility of an interaction between temperature and duration of exposure is assumed to be nonsignificant. Generally, response of rice to high temperature has been modeled using daily mean temperature (Horie et al., 1995; Kropff et al., 1995), number of days with maximum temperature >34
C (Challinor et al., 2007) and more recently using daily minimum and maximum temperature (Krishnan et al., 2007) but anthesis is extremely sensitive to hourly time course of temperature. Hence, flowering models with hourly temperature changes are needed, which can be incorporated into crop models for better prediction. The interactive effect could be included into crop models by adopting the cumulative temperature

73

Climate Change Affecting Rice Production

response above a threshold temperature of 33
C for rice ( Jagadish, 2007; Jagadish et al., 2007; Nakagawa et al., 2002). A similar response was seen in ground nut and has been quantified by cumulative temperature approach (Vara Prasad et al., 1999). Accumulated temperature or thermal time above a threshold can be calculated by TT = (T33
C)  t, where T is the day temperature and t is the duration of the treatment (Fig. 7). Furthermore, quantification of high temperature impact on future crop yields based on predictions is in its infancy (Challinor et al., 2007) due to less experimental data available. 2.1.1.3. Ripening phase High temperature affects cellular and developmental processes leading to reduced fertility and grain quality (Barnabas et al., 2008). Decreased grain weight, reduced grain filling, higher percentage of white chalky rice and milky white rice are common effects of high temperature exposure during ripening stage in rice (Osada et al., 1973; Yoshida et al., 1981). In addition, increased temperature causes serious reduction in grain size and amylase content (Yamakawa et al., 2007; Zhu et al., 2005) further reducing the potential economic benefits farmers can derive from rice cultivation due to depression in farm-gate and/or milled grain prices. High temperature during grain-filling period accelerates the demand for more assimilates to avoid milky white kernels (Kobata and

0

Spikelet fertility (logit %)

–1

–2

–3

–4

–5

0

5 10 15 20 Accumulated hours with temperature >33⬚C

25

Figure 7 Relation between spikelet fertility and accumulated hourly temperature >33
C in Azucena. Key: open symbols ¼ 33.7
C; closed symbols ¼ 36.2
C; 2003 (^, e); 2004duration: ○, 1 h; □, 2 h; △, 4 h; ▽, 6h. Fitted line: Y ¼ 6.50 1.67X, r2 ¼ 0.88. ( Jagadish et al., 2007).

74

R. Wassmann et al.

Uemuki, 2004). The reduced grain weight under high temperature is attributed to excessive energy consumption to meet the respiratory demand of the seeds (Tanaka et al., 1995). Alternatively, the reduction in grain weight is attributed to higher grain dry matter accumulation rate together with a shortened grain-filling period (Kobata and Uemuki, 2004). High temperature during grain-filling period is a critical factor to reduce grain filling/ripening but this effect could be magnified by lower assimilate supply (Kobata and Uemuki, 2004). They concluded that the reduction in grain weight could be overcome if sufficient assimilates were supplied to meet higher grain dry matter accumulation rate under high temperatures. Following the work of Kobata and Moriwaki (1990) and Kobata et al. (2000) the plant density was reduced to half by thinning and found that in thinned plants significantly more assimilates were produced than required to meet higher accumulation rates even when the plants were exposed to high temperatures, mainly due to reduced competition (Kobata and Uemuki, 2004). They recorded >100% increase in grain dry weight even under high temperature exposure (1–4
C higher than outside temperature) over 2 years. Similarly, by shading or panicle clipping Tsukaguchi and Yusuke (2008) showed a significant reduction in milky white and white belly kernels with increased assimilate supply under high temperature during the initial grain-filling period. Cultivar Koshihikari has been identified to meet the increased grain dry matter accumulation rates during grain-filling period and shows reduced percentage of milky white and white belly kernels with sufficient assimilate supply even under high temperatures (Kobata and Uemuki, 2004). Hence simple agronomic measures like optimum plant densities, that is, single seedling per hill could be useful in sufficient assimilate supply during grain filling under high temperature to overcome a large proportion of chalky grain and reduction in grain weight. 2.1.2. Temperature and CO2 interaction Although elevated CO2 could enhance photosynthesis, especially in C3 crop like rice, it is a potential component to trap the short wave radiations from the earth surface only to be redirected back to increase the global surface mean temperature. Increased biomass production due to elevated CO2 could potentially increase yield, provided microsporogenesis, flowering, and grain-filling are not disrupted by environmental stresses such as drought or high temperature. Biochemically, increase in CO2 concentration stimulates increase in RuBisCO and photorespiration is reduced. Hence, increasing temperature could result in higher net photosynthesis and CO2 uptake (Potvin, 1994). Moreover, rice grains are a significant sink for assimilates and removal or restriction of this carbon sink will fail to exploit the elevated CO2 due to photosynthetic insensitivity (Stitt, 1991; Webber et al., 1994). Accordingly, Ziska et al. (1996) recorded a significant increase in root/shoot ratio

Climate Change Affecting Rice Production

75

with elevated CO2 with increasing temperature and hinted at alternative sinks becoming active recipients with reduced carbon sink capacity of the grains due to spikelet sterility from high temperature exposure. Furthermore, Matsui et al. (1997a) studying the interaction of CO2 and temperature at reproductive stage recorded an increase in canopy temperature due to stomata closure at high CO2 concentrations. They concluded that the critical air temperature for spikelet sterility (as determined from the number of germinated pollen grains on the stigma) was reduced by 1
C at elevated concentrations of carbon dioxide (ambient +300 ml1CO2) which could have been due to low transpiration cooling majorly driven my stomata closure. Increasing temperatures from 28/21 to 37/30
C decreased grain yield from 10.4 to 1.0 Mgha1 even under 660 mmol of CO2 mol1 of air (Baker et al., 1992). Ziska et al. (1996) recorded 70 and 22% increase in biomass at elevated CO2 treatment under 29/ 21
C and 37/29
C, respectively, while grain yield of 17 contrasting cultivars recorded <1% filled spikelets. Hence this indicates that increasing CO2 concentration could limit rice yield if average air temperature increased simultaneously. Hence interaction of CO2 and temperature at both vegetative and reproductive stages has to be further explored to exploit the increasing CO2 for increasing yields. 2.1.3. High night temperature Peng et al. (2004) analyzed weather data at the International Rice Research Institute farm from 1979 to 2003 to examine the temperature trends and the relationships between rice yields and temperature. Annual mean maximum and minimum temperatures increased by 0.35 and 1.13
C, respectively, for the above period and a close correlation between rice grain yield and mean minimum temperature was observed. Grain yield declined by 10% for each 1
C increase in minimum temperature in the dry season whereas the effect of maximum temperature was insignificant. Similarly, Pathak et al. (2003) estimated that the rate of change in the potential yield trend of rice from 1985 to 2000 ranged from 0.12 to 0.05 Mgha1yr1. Negative yield trends were observed at six of the nine sites, four of which were statistically significant (P < 0.05). The decrease in radiation and increase in minimum temperature were identified as the reasons for the yield decline. Although, high temperature at both day and night reduced the duration of grain growth, the rate of growth was lower in the early or middle stages of grain filling, and also reduced cell size midway between the central point and the surface of endosperm at high night temperature (22/34
C) than at high day temperature of 34/22
C (Morita et al., 2005). However, research into the effect of high night temperature is not been understood well and should be prioritized with much higher mean night temperatures’ predicted.

76

R. Wassmann et al.

2.1.4. Breeding rice for warmer world The development and adoption of high-yielding and semidwarf rice varieties during the ‘‘Green Revolution’’ period starting in the 1960s more than doubled the rice production from 256 million (m) tons in 1965 to around 600 mtons in 2007. The new varieties developed were more responsive to fertilizer inputs, were lodging resistant, had 2–3 times higher yield potential than traditional varieties, and possessed multiple resistances to biotic and abiotic stresses leading to stable yields (Khush, 1999). Higher production achieved through improved varieties led to the lowering of rice prices by about 40%, thus benefiting the poor population of developing nations who spend 50–60% of their income on food (Khush, 2001). Moreover, shifting from traditional to modern varieties increased farmers’ yield by 2.1 tons ha1, on average, and resulted in an annual economic benefit estimated at US $10.8 billion (Hossain et al., 2002). Evenson and Gollin (1997) estimated that, between 1975 and 1995, widespread adoption of modern varieties reduced rice importation by 8%, malnutrition by 1.5–2%, and saved millions of hectares of forests and fragile ecosystems from being converted into rice areas. Plant breeding, rice varietal improvement can potentially avert—at least in part—the negative effects of climate change on rice production. Although farmers can adapt to climate change by shifting planting dates, selecting varieties with different growth durations, or changing crop rotations, these coping mechanisms may result in lower yields and with delayed or changed plantings may slow down rice yield growth. Developing germplasm with higher tolerance to climate-induced stresses through breeding is a sound climate change adaptation strategy. 2.1.4.1. Genetic improvement for heat tolerance Breeding rice varieties tolerant to high temperature has so far received little attention as compared to other abiotic stresses like drought and salinity. After one comprehensive study in the early 1980s (Mackill, 1981; Mackill et al., 1982; Mackill and Coffman, 1983), high temperature tolerance of rice has only been treated within regionspecific breeding programs with limited success. In Sindh (Pakistan), IR6 has been introduced in the year of 1969 and is still the prevalent cultivar, although a local research institute has released several varieties since 1982 to cope with high temperatures (Naich and Mari, 2007). Under temperatures that regularly exceed 36
C during the flowering period, however, IR 6 has outperformed these new varieties in terms of spikelet sterility (5% for IR6 vs >15% for newer varieties) and yield (>7 tha1 for IR6 vs<7 tha1 for newer varieties). Evidence in crops such as tomato, peanut, cotton, and cowpea clearly indicate that plant breeding can yield varieties adapted to high temperature stress (Hall, 1992, 2004). Two distinct concepts of breeding can be explored, that is, (i) breeding rice varieties that can tolerate higher

Climate Change Affecting Rice Production

77

temperatures per se or (ii) that escape high temperatures either by having shorter growing seasons or flowering/maturity or by flowering earlier during the day (Redon˜a et al., 2007). For the latter, the most promising approach is to shift the time of flowering from the usual 10 AM to noon (Yoshida et al., 1981) to an earlier daytime when the temperatures would be expected to be lower. Wahid et al. (2007) noted that to be successful in improving agricultural productivity under a stress environment, emphasis should be put on developing cultivars that can both tolerate environmental stresses as well as maintain economic yield. Therefore, genes or quantitative trait loci (QTL) underlying heat tolerance or avoidance need to be identified and then combined with traits such as high yield, resistance to multiple stresses, and acceptable grain quality, among others. The breeding process could be complex and may involve several steps, such as 1. Identification of genetic donors 2. Hybridization and recombination 3. Phenotypic and/or molecular marker-aided selection of desired genotypes from segregating populations 4. Preliminary evaluation of elite breeding lines in unreplicated trials 5. Extensive multi-environment (both temporal and spatial) testing 6. On-farm trials and participatory varietal selection 7. Varietal release and production of breeder, foundation, registered, and certified seeds 8. Frontline demonstration and promotion of the newly approved cultivars The varietal development process requires the active involvement of multidisciplinary teams comprised of breeders, geneticists, pathologists, entomologists, physiologists, biotechnologists, agronomists and cereal chemists, among others. Recent experience in breeding for biotic and other abiotic stresses ( Jena and Mackill, 2008) suggests–while the process may be complex–it should be possible to transfer major QTLs for high temperature tolerance, once identified, into locally adapted genotypes or new genotype combinations using either conventional breeding approaches or molecular maker assisted selection techniques. 2.1.4.2. Genetic donors for heat tolerance and avoidance While heat tolerance in rice has been determined to be a highly heritable trait as early as the 1970s (IRRI, 1976), most of the breeding work done so far have focused on germplasm screening and evaluation (IRRI, 1980; Yoshida et al., 1981). Genetic variability for high temperature tolerance has been observed in rice (Mackill et al., 1982; Yoshida et al., 1981). High temperature tolerant lines were found to have higher pollen shedding under optimal temperatures than the intolerant lines. Jennings et al. (1979), for example, found the variety Hoveyzeh from southern Iran to be tolerant at temperature higher than 45
C when other varieties were already

78

R. Wassmann et al.

completely sterile. Moradi and Gilani, (2007) reported the local Iranian landraces Anbori and Hoveaze (probably the same as or a variant of Hoveyzeh), to be also tolerant to high temperatures. Yoshida et al. (1981) reported that varieties such as Agbede, Carreon, Dular, N22, OS4, P1215936, and Sintiane Diofor have fertility percentages ranging from 84 to 90% even at high temperatures. Some other promising heat tolerant and avoidance entries identified are documented ( Jagadish et al., 2008; Mackill, 1981; Matsui and Omasa, 2002; Matsui et al., 2001, 2007; Redon˜a et al., 2007; Yoshida et al., 1981). Genetic variation for time of day flowering (TDF) has also been reported in rice. Yoshida et al. (1981) noted that O. glaberrima flowers early in the morning and proposed that this trait be incorporated into O. sativa through breeding. Furthermore, they reported that derivatives from O. glaberrima and O. sativa crosses flowered earlier than O. sativa. Sheehy et al. (2005) also found O. glaberrima accessions to have early TDF while noting that Chhalangpa had the earliest TDF of 0915 h among selected O. sativa cultivars. Prasad et al. (2006) confirmed the observations of Yoshida et al. (1981) that cultivars of O. glaberrima (CG-14 and CG-17) and interspecific hybridization derivative lines (WAB-12 and WAB-16) flowered early (0700 and 0830 h). However, despite early flowering, it was noted that the spikelet fertilities of CG-14, CG-17, WAB-12, and WAB-16 were decreased by high temperature suggesting different genetic control for the heat tolerance and early TDF traits. Among O. sativa cultivars that flowered earlier than 1000h were IR-8, IR-72, and N-22. 2.1.4.3. Selection indexes for heat tolerance and avoidance Proper screening techniques and procedures under the right environments are crucial for determining the true value of a given genotype to be used for both genetics and plant breeding applications. The choice of a field screening environment, for example, can influence the reliable detection of morphological and agronomic characters conferring high temperature tolerance (Hall, 1992). Several parameters have been proposed as selection indexes in breeding for heat tolerance and avoidance in rice. Some important selection indexes used for heat tolerance and avoidance are

(i) Early morning flowering to escape heat damage and screening for high temperature tolerant lines done at 38
C while 35
C can be used to eliminate heat susceptible materials (Satake and Yoshida, 1978) (ii) High pollen shedding (i.e., expressed as pollen number on the stigma) (Mackill and Coffman, 1983; Prasad et al., 2006) (iii) Pollen production in the anthers and high spikelet fertility for heat tolerance during the reproductive phase (Prasad et al., 2006). (iv) Grain weight heat susceptibility index [GWHSI=(grain weight at optimum temperature-grain weight at high temperature)/grain weight

Climate Change Affecting Rice Production

79

at optimum temperature 100] to evaluate tolerance of rice to heat stress (Zhu et al., 2005) (v) Six hour high temperature exposure encompassing the peak anthesis period for the flowering day to reduce the possibility of escape ( Jagadish et al., 2008) (vi) Length of basal dehiscence could be used as morphological marker for selecting high temperature tolerant genotypes (Matsui et al., 2005, 2007). They found out that the length of basal dehiscence was highly correlated with the pollination viability under hot conditions. Furthermore, they observed that long basal dehiscence helps the pollen grains to fall from theca into stigma, thereby increasing reliability of pollination under hot and normal environmental conditions. 2.1.4.4. Genetics of heat tolerance Heat tolerance is controlled by not only one major gene but several genes (Mackill, 1981; Maestri et al., 2002). Mackill and Coffman (1983) reported that the genetic control of high pollen shedding in rice is recessive and influenced by different genes. In contrast, Yoshida et al. (1981) observed that most of the genetic variation for pollen shedding is additive. Their results showed significant broad sense and narrow sense heritabilities of 76 and 71%, respectively, while finding a high correlation between spikelet fertility and pollen shedding. QTL analysis, correlation and co-segregation approaches, and the use of genetic stocks were most applicable in studying the genetic basis of heat tolerance in cereals (Maestri et al., 2002). In rice, QTL mapping for heat tolerance at grain-filling stage revealed three QTLs controlling the trait (Zhu et al., 2005). These QTLs were detected on chromosomes 1, 4, and 7 with LOD scores of 8.16, 11.08, and 12.86, respectively (LOD stands for logarithm of the odds, for example, LOD score of three means the odds are 103/1 in favor of genetic linkage), and correspondingly explaining 8.9, 17.3, and 13.5%, of the phenotypic variance. The QTL in chromosome 4 showed no interaction with environment and epistatic effect, suggesting stable expression over different environments and genetic backgrounds. The QTLs on chromosomes 1 and 7, on the other hand, had significant GE interactions. Moreover, eight pairs of QTLs with epistatic effects were detected. Other QTL mapping studies designed to identify major QTLs from known donors such as N22 into popular indica varieties are currently underway at IRRI. Traditional breeding methods comprise pedigree and bulk selection based on morphological markers such as percent fertility. These approaches have been successfully used in breeding for heat tolerance in other crops (Hall, 1992) whereas IRRI breeding programs have focused on other traits than heat tolerance. However, as the molecular genetic basis of heat tolerance in rice is elucidated and QTLs are identified and suitable markers developed, molecular breeding approaches are expected to be

80

R. Wassmann et al.

utilized for developing superior heat tolerant varieties more precisely and expeditiously. A shuttle-breeding strategy was successfully used in developing rainfed lowland rice varieties in Eastern India while also facilitating the exchange of breeding materials of diverse origin (Mallik et al., 2002). This type of program is envisioned for validating the field performance of promising breeding lines in hotspot areas while shortening the breeding cycle. In field screening, the confounding effects of relative humidity on heat-induced spikelet sterility (Weerakoon et al., 2008) also need to be accounted for. Thus, it may be necessary to stratify the target breeding environments into hot and humid versus hot and dry zones and tailor the selection protocols accordingly. Also, incorporation of a heat tolerance breeding objective into ideotype as well as intersubspecific heterosis breeding programs for raising yield potential (Peng et al., 2008) could be adopted as a strategy for increasing rice productivity and breaking the yield ceiling, even under various climate change scenarios.

2.2. Drought The recent IPCC Technical Paper on Climate Change and Water stated with high confidence that ‘‘the negative impacts of future climate change on freshwater systems are expected to outweigh the benefits’’ (Bates et al., 2008). As compared to the current situation, we will see much more land where the water stress will aggravate and only a small portion of the land where the water stress situation will be alleviated. In spite of an increased total water supply, the effects of increased precipitation variability and seasonal runoff shifts, water quality, and flood risks are likely to prevail in their impact on food production. It has been shown that the production of rice, maize, and wheat has declined in many parts of Asia in the past few decades, due to increasing water stress, arising partly from increasing temperatures, increasing frequency of El Nin˜o events and reductions in the number of rainy days (Aggarwal et al., 2000; Fischer et al., 2002; Tao et al., 2004). In turn, this will decrease food security and increase vulnerability of poor rural farmers, especially in the arid and semiarid tropics (Bates et al., 2008). Because of its semiaquatic phylogenetic origin and the diversity of rice ecosystems and growing conditions, current rice production systems rely on ample water supply and thus, are more vulnerable to drought stress than other cropping systems (O’Toole, 2004). Drought stress is the largest constraint to rice production in the rainfed systems, affecting 10 million ha of upland rice and over 13 million ha of rainfed lowland rice in Asia alone (Pandey et al., 2007). At the whole plant level, soil water deficit is an important environmental constraint influencing all the physiological processes involved in plant growth and development. Drought is conceptually

Climate Change Affecting Rice Production

81

defined in terms of rainfall shortage compared to a normal average value in the target region. However, drought occurrence and effects on rice productivity depend more on rainfall distribution than on the total seasonal rainfall. A typical example was given in a recent screening experiment at IRRI during the wet season 2006, when seasonal rainfall exceeded 1000 mm, including a major typhoon (International name: Xangsane) with around 320 mm rainfall in a single day. Yet, a short dry spell that coincided with the flowering stage resulted in a dramatic decrease of grain yield and harvest index, compared to the irrigated control (Serraj et al., 2008). Beyond the search for global solutions to a generic ‘‘drought,’’ the precise characterization of droughts in the target population of environments (TPEs) is a prerequisite for better understanding their consequences on crop production (Heinemann et al., 2008). 2.2.1. The present situation of catastrophic, chronic, and inherent droughts Drought definitions depend on the disciplinary outlook, including meteorological, hydrological, and agricultural perspectives. Agricultural drought occurs when soil moisture is insufficient to meet crop water requirements, resulting in yield losses. Depending on timing, duration, and severity, this can result in catastrophic, chronic, or inherent drought stress, which would require different coping mechanisms, adaptation strategies and breeding objectives. The 2002 drought in India could be described as a catastrophic event, as it affected 55% of the country’s area and 300 million people. Rice production declined by 20% from the inter-annual baseline trend (Pandey et al., 2007). Similarly, the 2004 drought in Thailand affected over 8 million people in almost all provinces. Severe droughts generally result in starvation and impoverishment of the affected population, resulting in production losses during years of complete crop failure, with dramatic socioeconomic consequences on human populations (Pandey et al., 2007). Production losses to drought of milder intensity, although not so alarming, can be substantial. The average rice yield in rainfed eastern India during ‘‘normal’’ years still varies between 2.0 and 2.5 tha1, far below achievable yield potentials. Chronic dry spells of relatively short duration, can often result in substantial yield losses, especially if they occur around flowering stage. In addition, drought risk reduces productivity even during favorable years in drought-prone areas, because farmers avoid investing in inputs when they fear crop loss. Inherent drought is associated with the increasing problem of water scarcity, even in traditionally irrigated areas, due to rising demand and competition for water uses. This is, for instance the case in China, where the increasing shortage of

82

R. Wassmann et al.

water for rice production is a major concern, although rice production is mostly irrigated (Pandey et al., 2007). Increasing rice productivity in the drought-prone rainfed areas requires adapted solutions and strategies in response to the different types of droughts, based on precise characterization of the TPEs. With milder chronic droughts being generally more frequent than the catastrophic ones, overall crop productivity in rainfed areas would probably benefit more from breeding for enhanced water productivity and resistance to the chronic type of water deficits. 2.2.2. Interaction of drought and CO2 on crop yield and physiological responses The potential impacts of increasing [CO2] on photosynthesis have been well documented for many crops (e.g., Allen et al., 1994). Because soil water availability is the most limiting environmental factor for crop growth (Boyer, 1982), it is crucial to analyze the possible interactions of water deficits and [CO2] upon major crops such as rice. If there is a fundamental change in plant responses to soil water content, then plant growth under climate changes associated with less precipitation might be either aggravated or lessened as compared to what is expected using response functions developed for current CO2 levels. Most of the carbon stored in the mature rice grains originates from CO2 assimilation during the grain-filling period, with the flag leaf as the most photosynthetically active, factors that lower the photosynthesis rate of the flag leaf during this period could potentially limit grain yield. Baker et al. (1997a) analyzed the growth and grain yield responses of rice to drought stress under carbon dioxide concentration [CO2] enrichment. Rice (cv. IR-72) was grown to maturity in plant growth chambers under naturally sunlit, in atmospheric [CO2] of 350 and elevated (700 mmol CO2 mol1 air). The [CO2] enrichment increased plant growth, number of panicles per plant and grain yield. Drought accelerated leaf senescence, reduced leaf area and above-ground biomass and delayed crop ontogeny. The [CO2] enrichment allowed 1–2 days more growth during drought-stress cycles. It was concluded that in the absence of air temperature increases, future global increases in [CO2] should promote rice growth and yield while providing a modest reduction of near 10% in water use and so increase drought avoidance (Baker et al., 1997a). Similarly, a recent study in wheat (Manderscheid and Weigel, 2007) showed that CO2 enrichment enhanced final biomass and grain yield by less than 10% under well-watered conditions and by more than 44% under drought-stress conditions, respectively. This indicated that the increase in atmospheric CO2 concentration will be likely to attenuate the effects of drought stress on wheat grain yield. The analysis of potential acclimation of rice photosynthesis to long-term [CO2] growth treatments, by comparison of canopy photosynthesis rates

Climate Change Affecting Rice Production

83

across a wide range of short-term [CO2] showed essentially no acclimation response (Baker et al., 1997b). Photosynthetic rate was found to be a function of current short-term [CO2] rather than long-term [CO2] growth treatment. Carbon dioxide enrichment significantly increased both canopy net photosynthetic rate and water-use efficiency while reducing evapotranspiration by about 10%. This water saving under [CO2] enrichment allowed photosynthesis to continue for about 1–2 days longer during drought in the enriched compared with the ambient [CO2] control treatments (Baker et al., 1997b). Widodo et al. (2003) have confirmed that elevated CO2 delays the effects of drought stress and accelerates recovery of rice leaf photosynthesis. At elevated [CO2], midday leaf photosynthetic and concentrations of chlorophyll (Chl) were increased, whereas total soluble protein (TSP) decreased, compared with plants at ambient [CO2]. Furthermore, elevated [CO2] increased midday leaf sucrose-phosphate synthase (SPS) activity and enhanced midday leaf sucrose and starch accumulation during early reproductive phases, but not during later reproductive phases. Water deficit caused substantial decreases in midday photosynthesis and concentrations of Chl and TSP, with concomitant reductions in photosynthetic primary products and SPS activity. However, these drought-induced effects were more severe for plants grown at ambient than at elevated [CO2], as the latter ones were able to maintain leaf photosynthesis longer into the drought period than plants grown at ambient [CO2]. In addition, leaf photosynthesis recovered from water deficit more rapidly in the elevated [CO2] treatment. It was concluded that in the absence of other potential climate stresses, rice grown under future increases in atmospheric [CO2] may be better able to tolerate drought situations (Widodo et al., 2003). Similarly in wheat, Manderscheid and Weigel (2007) reported that CO2 enrichment stimulated the green area index under drought stress and the seasonal radiation absorption was only decreased by 16%. Radiation use efficiency was reduced by drought and increased by CO2 elevation and the CO2 effect was higher under drought (+60%) than under well-watered conditions (+32%). Robredo et al. (2007) analyzed the impact of elevated [CO2] on water relations, water use efficiency (WUE) and photosynthetic gas exchange in barley (Hordeum vulgare L.) under wet and drying soil conditions. They concluded that the improved water status of droughtstressed plants grown at elevated CO2 was the result of stomatal control rather than of osmotic adjustment. Photosynthesis under drought was maintained at higher rates for longer with elevated [CO2]. The reduction of stomatal conductance and transpiration, and the enhancement of carbon assimilation by elevated [CO2], increased instantaneous and whole plant WUE in both irrigated and drought-stressed plants. Thus, the metabolism of barley plants grown under elevated [CO2] and moderate or mild water deficit conditions was benefited by increased photosynthesis and lower transpiration (Robredo et al., 2007). Previous studies in soybean showed

84

R. Wassmann et al.

that elevated [CO2] decreased water loss rate and increased leaf area development and photosynthetic rate under both well-watered and droughtstressed conditions (Serraj et al., 1999). There was, however, no significant effect of CO2 concentration in the response relative to soil water content of normalized leaf gas exchange and leaf area. This study also demonstrated that drought response based on soil water content for transpiration, leaf area, and photosynthesis provides an effective method for describing the responses of the physiological processes to the available soil water, independent of CO2 concentration (Serraj et al., 1999). 2.2.3. Genetic basis of grain formation failure under drought Although drought affects all stages of rice growth and development, water stress during the flowering stage depresses grain formation much more than drought at other reproductive stages (Boonjung and Fukai, 1996). Therefore, screening for tolerance near flowering stage has been considered to be more useful in breeding for improved drought resistance. The strong effects of drought on grain yield are largely due to the reduction of spikelet fertility and panicle exsertion. Several studies have found that reproductive development from meiosis in the spore mother cells to fertilization and early seed establishment was extremely sensitive to various stresses, including drought. These stresses cause various structural and functional disruptions in reproductive organs, leading to failure of fertilization or premature abortion of the seed (Saini, 1997; Saini and Westgate, 2000). Drought can inhibit the development of reproductive organs, such as the ovary (Saini et al., 1983) and the pollen at meiosis stage (Saini, 1997); but it can also inhibit processes such as anther dehiscence, pollen shedding, pollen germination, and fertilization (Ekanayake et al., 1990; Satake and Yoshida, 1978). The drought-induced inhibition of panicle exsertion has been identified as a consequence of a decrease in peduncle elongation, which can usually account for 70–75% spikelet sterility under water deficit (O’Toole and Namuco, 1983). Drought stress slows down the peduncle elongation, and re-watering can only partially restore elongation. Recent studies at IRRI found that drought significantly delayed the peduncle elongation, trapped a significant fraction of panicle within the flag leaf sheath due to the repression of the expression of cell-wall invertase genes ( Ji et al., 2005). The spikelets left inside the leaf sheath are usually sterile, resulting in a poor yield, which indicates that peduncle elongation may play a major role in panicle exsertion and spikelet fertility under stress. Mutant studies showed that the cause of spikelet sterility can be of two types: inhibition of starch accumulation in pollen grains and failure of anther dehiscence and/or synchronization with anthesis due to suspension of septum degradation and stomium breakage (Zhu et al., 2004). If drought stress occurs during these processes, the reproductive organs will be abnormal and damaged and then grain set will be sterile. Liu et al. (2005) reported a significant difference in number of

Climate Change Affecting Rice Production

85

pollen grains between IR64 and Moroberekan in the four top rachis under drought conditions. The variation in spikelet fertility between the genotypes was mainly due to the difference in locule wall structure, and then to variation of the number of pollen grains on stigma (Liu et al., 2005). 2.2.4. Genetic enhancement of drought-stress resistance The success in breeding for improved drought resistance depends essentially on the choice of parents, selection criteria and robustness of the managed screening protocols. Successful breeding programs must have clear objectives, namely to produce a cultivar that is superior to farmers’ varieties in a particular TPE. The objectives of a screening system are to focus on the TPEs and adaptation to major stress occurrence scenarios, and to minimize field variability for detecting heritable differences in drought resistance. Comparing several drought screening protocols in the upland or in drained lowland paddies, Lafitte and Courtois (2002) found that intermittent stress, imposed by withholding irrigation during the period bracketing the entire flowering and grain-filling stages, is generally reliable for ranking cultivars’ performance under drought, similarly to stress targeted precisely at the flowering period of individual cultivars. Recent research findings at IRRI have demonstrated the feasibility of direct selection for yield under drought (Kumar et al., 2008). Since yield under stress is a function of yield potential, escape, and drought response, the use of the Drought Resistance Index (DRI) can help to distinguish drought resistance from escape and yield potential (Ouk et al., 2006) and therefore further enhance the precision and reproducibility of drought screening. While breeding for upland and aerobic rice has recently made significant progress in developing new rice cultivars for water-short environments (Bernier et al., 2008), the progress in rainfed lowlands has been relatively slow. Most improved cultivars grown in drought-prone rainfed lowlands were originally bred for irrigated conditions and were never selected for drought tolerance (Kumar et al., 2008). Drought escape has been exploited in the drought-prone areas of eastern India and Bangladesh, through shortduration varieties, mainly of the aus germplasm group. But most of these varieties are not necessarily drought resistant. The slow progress in the genetic improvement of grain yield in the rainfed lowland was explained by two major factors: the complexity of the target genotype  environment system and the insufficiency of genetic resources available to the breeding programs (Cooper et al., 1999). Large genetic variation exists within rice and its wild Oryza relatives for performance under drought stress, but progress in developing improved cultivars has been relatively slow. Many parental lines and donors have been identified for drought resistance in upland (Atlin et al., 2006), but only a few have been reported for the more extensive rainfed lowland system. The identification of parental materials and development of new populations was

86

R. Wassmann et al.

a major target for the IRRI rainfed lowland breeding program in the 1990s, focusing on the major target environments in eastern India and northeast Thailand (Sarkarung and Pantuwan, 1999). Breeding populations were developed in the backgrounds of Mahsuri, Safri17, and Sabita for eastern India and KDML105 for northeast Thailand. An extensive G  E study in rainfed lowland by Wade et al. (1999) analyzed the interactions of 37 genotypes across 36 environments in India, Bangladesh, Thailand, Indonesia, and the Philippines from 1994 to 1997. Only a small group of genotypes were stable across environments. The cultivar NSG19 was found to be adapted to environments with rapid-onset late drought, whereas Sabita and KDML105 showed adaptation to environments with late maturity or recovery after drought. Stress-sensitive mega varieties are still widespread across South and Southeast Asian rainfed rice production systems, including Swarna, Sambha Mahsuri, IR36, IR64, BR11, and MTU 1010. These varieties are generally preferred by farmers for their yield potential and quality traits are not tolerant to drought. As they were bred for the irrigated ecosystem, these varieties provide high yield in non-drought years, but they show a highyield reduction in mild to moderate drought years and collapse completely in severe drought-stress years (Kumar et al., 2008). In field experiments conducted at IRRI during the dry seasons of 2006– 2008, large scale field-managed drought screening has been focusing on the confirmation of drought-resistant breeding lines and identification of new potential donors of drought resistance within gene bank germplasm collections, molecular breeding lines, Oryza glaberrima introgression lines, hybrids, and their parental lines (Serraj et al., 2008), in addition to mutants and transgenic lines (Herve and Serraj, 2009). 2.2.5. Agronomic approaches to cope with less water The cultivation of rice in flooded fields requires about 2500–3000 m3 water to produce 1 ton of rice grain versus around 1000 m3 to produce 1 ton of wheat grain. In Asia, more than 80% of the developed freshwater resources are used for irrigation purposes, mostly for rice production. Thus, even small savings of water due to a change in current practices will translate into a significant bearing on reducing the total consumption of fresh water for rice farming. By 2025, 15–20 million hectares of irrigated rice will experience some degree of water scarcity (Bouman et al., 2007). Many rainfed areas are already drought-prone under present climatic conditions and are likely to experience more intense and more frequent drought events in the future. Thus, water saving techniques are absolutely essential for sustaining— and possibly increasing—future rice production under climate change. The potentials and constraints of different water-saving approaches have recently been discussed in detail within a review published in this journal (Bouman

Climate Change Affecting Rice Production

87

et al., 2007), so that this presentation here provides only a brief overview of the available approaches and highlights a few aspects with specific relevance to climate change adaptation. The period of land preparation encompasses various options to save water, namely lining of field channels, land leveling, improved tillage, and bund preparation. Likewise, crop establishment can be optimized under water scarcity by direct seeding which reduces turnaround time between crops and may tap rainfall. Finally, the crop growth period offers essentially three alternative management practices to save irrigation water: saturated soil culture (SSC), alternate wetting and drying (AWD), and aerobic rice. In SSC, the soil is kept as close to saturation as possible, thereby decreasing seepage and percolation losses. A meta-analysis of field experiments showed that water input decreased on average by 23% (range: 5–50%) as compared to continuously flooded check, with a nonsignificant yield reduction of 6% on average (Bouman and Tuong, 2001). Figure 8 provides three examples from the Philippines indicating higher WUE through SSC. In AWD, irrigation water is applied in certain intervals leading to episodes of non-flooded soil conditions in the fields. The intervals of nonflooded periods can vary from 1 day to more than 10 days depending on the specific management regime and soil/climate conditions. In almost all field experiments, AWD resulted in slightly lower yields (see Fig. 8). However, AWD is consistently increasing WUE, that is, the amount of grain produced per unit of water input. The efficiency of AWD also depends on the soil type. AWD is a mature technology for lowland rice areas with heavy soils and shallow groundwater tables has been widely adopted in those areas in China (Li and Baker, 2004). In loamy and sandy soils with deeper groundwater(e.g., in Northern India) water inputs can be even reduced up to 50%, but yield losses are generally high (more than 20%) as compared to flooding (Sharma et al., 2002; Singh et al., 2002; Tabbal et al., 2002). AWD is also an integral part of the System of Rice Intensification (SRI), an approach developed in Madagascar and now intensively advocated in many rice-growing countries (Uphoff, 2007). Aerobic rice is a very distinct way of growing rice as compared to paddy fields; in fact, it is grown like other cereals, such as wheat, in non-flooded, non-saturated (aerobic) soil with supplementary irrigation. Growing aerobic rice eliminates the water losses that are typical for flooded rice (seepage, percolation, and evaporation from the standing water layer). On the other hand, it requires various adjustments to obtain high yields under nonflooded conditions, namely special input-responsive rice cultivars adapted to aerobic soils and new management practices. Field experiments at IRRI indicated a wide range of yield losses (Fig. 8). Peng et al. (2006) attributed these differences to cultivar effects and their specific performance in the dry and wet seasons, but also indicated that the range of yield losses was higher

88

R. Wassmann et al.

A 4000

10

Yield (t/ha)

8

3000

6 2000 4 1000

2

Total water input (mm)

Yield Total water input

0 Fl o ae ode ro d b Fl ic oo ae de ro d b Fl ic oo ae de ro d b Fl ic oo ae de ro d b Fl ic oo ae de ro d bi c

Fl

oo de SS d Fl C oo de d SS Fl C oo de d SS C

0

<=================Philippines================><====China===> B 10

4000 (b)

Yield (t/ha)

Yield Total water input

3000

6 2000 4 1000

2

0

Fl

oo

d AWed Fl D oo d AWed Fl D oo d AWed Fl D oo d AWed Fl D oo d AWed Fl D oo d AWed Fl D oo d AWed Fl D oo d AWed Fl D oo d AWed D

0

Total water input (mm)

8

<============Philippines=========><===China===><===India===>

Figure 8 Yields and total water inputs under flooded conditions and water saving techniques; (A) for saturated soil culture (data from Tabbal et al., 2002) and aerobic rice (data from Bouman et al., 2005 and Yang et al., 2005); (B) for alternate wetting-drying (data from Belder et al., 2004; Mishra et al., 1990; Tabbal et al., 2002).

when aerobic rice was grown over several consecutive seasons. In spite of some encouraging evidence, e.g., aerobic varieties in Brazil with a yield potential of up to 6 tons ha1 (Pinheiro et al., 2006), the implementation of aerobic rice in farmers’ fields still has to overcome several obstacles. Irrespective of the existing problems, research until now has synthesized opportunities for further development of aerobic rice.

Climate Change Affecting Rice Production

89

Irrigation can be perceived as a buffer against drought effects and thus, as rendering some degree of resilience to irrigated rice production under climate change. Between the 1960s and the turn of the century the irrigated area in Asia doubled, but this development encompassed very different irrigation systems (Table 2). Historically irrigation has developed in four distinct but overlapping phases: (A) community irrigation, (B) river diversion schemes, (C) storage dams, and (D) pumps for access and control of surface and ground water (Barker and Molle, 2004). Community and pumping systems can be found throughout Asia whereas the other types are confined to specific hydrological conditions, that is, high water discharge from large rivers and man made reservoirs for irrigation. One of the decisive features to determine the buffer effect by a given irrigation type is the respective size. Community systems typically have small catchments and water storage capacities, so that rainfall deficits can hardly be attenuated through these systems. In contrast, river diversion schemes have an inherently larger catchment area, especially in the mega deltas, that will compensate for local rainfall anomalies within a broader area. The buffer effect of storage dams and pumping systems can be assessed as being intermediate between community systems and river diversion schemes (see details in Table 2). Apart from size, the effectiveness of irrigation schemes to ensure water supply under drought also depends on the current state of infrastructure. Most community systems and storage dams are rather old and are often dilapidated entailing high water losses in the canal/tube systems. The use of pumps is becoming more and more popular in many rice-growing regions. Pumps are nowadays affordable for many farmers and their use is extremely flexible, but the scope of irrigation is only patchy due to limited capacity of individual pumping units. Insofar, they may provide only limited cushion against severe dry spells. As compared to other irrigation types, irrigation schemes that divert water from large rivers, namely in deltaic regions, represent the best cushion against droughts. Apart from having large and heterogeneous catchments, many of these irrigation systems have extensive canal systems with good accessibility throughout the delta as well as a reliable hydro-technological infrastructure (e.g., sluices) to optimize water supply. Rice production in the deltas of Mekong, Red River, Irrawaddy, and Ganges–Brahmaputra is of outstanding importance for food security in Vietnam, Myanmar and Bangladesh, respectively; the Mekong Delta is also a major source of rice traded internationally. On the other hand, the deltaic areas are exposed to high risks associated with climate change. Rice production in the deltaic regions will directly be affected by sea level rise that increase the risk of inundation (Wassmann et al., 2004). Insofar, the future challenge for irrigation systems in deltaic regions may be primarily about the prevention of salinity intrusion (see Section 2.3) and excessive flooding (see Section 2.4) as opposed to compensating for dry spells.

90

Table 2

Typology of irrigation systems (after Barker and Molle, 2004) and their potential role under climate change

Historical development

Geographic distribution

(A) Community systems

(B) River diversion schemes

In most cases old irrigation systems (some several centuries old); in many cases deteriorating Pervasive throughout Asia

Typically small catchment and irrigation schemes (<1000ha) LOW: Buffer effect to – Small catchments local/shortterm droughts – Low water storage capacities (plus and – Poor infrastructure minus) discriminates ‘‘end of the pipe’’ farmers in drought situations

Scale of catchment/ irrigation

(C) Large storage dams

(D) Pumps for access and control of water

Old origin, but have continuously been upgraded over recent decades

Most dams were constructed from 1950s to 1980s (with a peak in the 1970s); recent constructions in China

In semiarid regions since the 1960s; now increasingly popular throughout monsoon Asia

In major river deltas (e.g., Mekong, Chao Phraya, and so on)

Scattered in effectively every Asian country; in many cases with deteriorated water transport systems Varying in size; catchment in upland areas and irrigation in lowland areas MODERATE: + Potential water storage from wet to dry season/year + Uplands often with high heterogeneity in rainfall – In most cases only medium sized catchment – Poor infrastructure discriminates ‘‘end of the

Pervasive throughout Asia

Very large catchment and irrigation area

HIGH: + Large and heterogeneous catchment + Extensive canal systems with good accessibility + Hydro-technological infrastructure (e.g., sluices) to optimize water supply

Very small irrigation (<1 ha) by individual unit, but frequently used MODERATE: + Very flexible use in time and space + Pumping of groundwater if needed + Independent from (delayed) policy reaction to CC

Possible Improvement to cope Climate Change impact

* Reduction of water losses/canal lining * Water diversion/ conjunctive use * Temporary use of groundwater * Water pricing * Greater equity among water recipients * Micro-irrigation * Innovative water harvesting

– Risks for delta regions due to sea level rise and vulnerability to climate extremes

pipe’’ farmers in drought situations

* New sluices to reduce salinity affects * Micro-irrigation/ improved varieties/ crop rotations * Basin-wide frameworks for water allocation

* Reduction of water losses/canal lining * New reservoirs * Trans basin diversion * Improved dam management * Sectoral re-allocation prioritizing agriculture at drought events * Micro-irrigation/ improved varieties/crop rotations * Innovative water harvesting

– Only patchy amelioration of drought in landscape – Constrains for poor farmers due to purchasing and operating costs * Collective use of groundwater by several farmers * Micro-irrigation * Subsidies for pumping in drought situations * Innovative water harvesting

91

92

R. Wassmann et al.

2.2.6. An integrated research strategy for drought resistance improvement under climate change scenarios The need for precise characterization of the drought-prone TPE has long been emphasized, but this effort is yet to be done systematically, across the drought-prone rice environments in South and Southeast Asia and SubSaharan Africa taking into account the dynamics and risks of climate change. Under future scenarios of climate change, simulation models can play a role both in the characterization and in enhancing the precision and integration of phenotyping either by linking model coefficients directly to or more heuristically to guide integrated phenotyping approaches. Increased crop yield and water productivity require the optimization of the physiological processes involved in the initial critical stages of plant response to soil drying, WUE and dehydration avoidance mechanisms (Serraj et al., 2008). Overall, it is now well accepted that the complexity of the drought syndrome can only be tackled with a holistic approach integrating plant breeding with physiological dissection of the resistance traits and molecular genetic tools together with agronomical practices that lead to better conservation and utilization of soil moisture and matching crop genotypes with the environment. Some of the steps involved in this multidisciplinary approach are described below: (i) Define the target drought-prone environment(s), and identify the predominant type(s) of drought stress and the rice varieties preferred by farmers. Define the phenological, and morphological traits that contribute substantially toward adaptation to drought stress(es) in the target environment(s). A critical research aspect is dissecting the interactions between drought, CO2 and temperature. (ii) Use simulation modeling and systems analysis to evaluate crop response to the major drought patterns under variable CO2 and temperature scenarios, and assess the value of candidate physiological traits in the target environment. (iii) Develop and refine appropriate screening methodologies for characterizing genetic stocks that could serve as donor parents for the traits of interest. (iv) Identify the genetic stocks for various putative, constitutive and inducible traits in the germplasm and establish genetic correlations between the traits of interest and the degree of adaptation to the targeted drought stress. (v) Harness functional genomics, transgenics and reverse genetics tools to understand the genetic control of the relevant traits. (vi) Use mapping populations and/or linkage disequilibrium mapping to identify genetic markers and QTLs for traits that are critical for stress resistance.

Climate Change Affecting Rice Production

93

(vii) Incorporate some of the components of relevant physiological traits into various agronomic genetic backgrounds to provide a range of materials with specific traits of interest for improving adaptation to drought and abiotic stresses in locally adapted varieties.

2.3. Salinity Increasing threat of salinity has become an essential issue linked to the consequences of climate change. Increased CO2 concentration per se may not have the detrimental effects on crop growth but the indirect effect of increased temperature on sea level rise, much larger areas of coastal wetlands may be affected by flooding and salinity in the next 50–100 years (Allen et al., 1996). A rise of 1000 mm sea level due to thermal expansion is estimated for 3.58
C increase in temperature. This excludes the additional expected increase in sea level due to melted ice leading to increased coastal salinity and further yield reduction, even in previously favorable areas (Manabe and Stouffer, 1994; Wassmann et al., 2004). Furthermore, greater than half (55%) of total ground water is naturally saline (Ghassemi et al., 1995). Secondary salinization, specifically due to the injudicious use of water and fertilizers in irrigated agriculture could increase the percentage of brackish ground water. The ground water table, if it rises and is brackish in nature, becomes ruinous to most of the vegetation. Higher temperature aggravates the situation by excessive deposition of salt on surface due to capillary action which is extremely difficult to leach below the rooting zone. The increased temperature will also disrupt weather patterns, leading to more frequent occurrence of problems associated with floods, drought, and salinity. Rice can be categorized as a moderately salt sensitive crop with a threshold electrical conductivity of 3 dS m1 (Maas and Hoffman, 1977). Recently, many new rice varieties have been developed worldwide with enhanced level of tolerance both for saline and sodic soils. Rice is usually monocropped in tropic and subtropic coastal areas during wet seasons due to its adaptation to waterlogged environments while tolerating salinity up to a certain extent. Soil sodicity is a different kind of problem soil, as a result of high salt concentration and low infiltration rate and poor hydraulic conductivity. This forces water stagnation on the soil surface which in turn do not allow any crop except rice to survive. Hence, rice is recommended as the first crop to be planted during reclamation of sodic soils. The high adaptability of rice under salt-affected areas makes it the most preferred crop for growing in these unfavorable environments. At elevated CO2 concentration, there is greater WUE, improved plant water status and more rapid leaf production in the vegetative growth phase

94

R. Wassmann et al.

provided there is non-limiting supply of the nutrients (Ackerly et al., 1992; Grashoff et al., 1995). In saline conditions, increased WUE potentially could reduce the salt uptake by plant. Partial stomatal closure under such future environmental conditions could be an ideal mechanism for salt tolerance (Bowman and Strain, 1987; Rozema et al., 1991). Alternatively, high temperature at the plant canopy level will increase transpiration by changing the vapour pressure deficit at the leaf surface, accelerate ageing of the foliage, and also shorten the growing season or grain-filling period which is very critical for the grain yield (Kenny et al., 1993). Rising temperature will accelerate the crop development for most of the cultivars that may lead to a reduced water use over the shortened growth period, but also to a loss of potential yield. Further studies related to shortening of growth duration and amount of water transpired during high temperature with reference to salt uptake is needed. 2.3.1. Mechanisms of salinity stress There are several mechanisms operating for salt tolerance in crop plants of which major ones are 

Ionic balance: Ionic balance is a major contributor for the tolerance mechanism. Rice on exposure to salt stress in the soil, ions of the soluble salts, specially Na+, K+, and Cl, are generally taken up along with the water uptake through transpirational stream. For normal functioning of the cells, high K+/Na+ ratio is essential which is usually the case under non-salt stress conditions. However under salt stress environment, rice intakes excessive amount of Na+ as cheap cation at the cost of energy consuming uptake of K+ and Ca2+. Higher passive uptake and increased load of Na+ in the xylem ultimately enters in the tissues/cells to disrupt the physiological and biochemical activities. This accumulation leads to disturbed Na+/K+ ratio with toxic levels of Na+ in the plant cells which also impairs the enzymatic activity inside the cell leading to the ultimate death of cell, tissue or the organ (Flowers and Yeo, 1981; Yeo and Flowers, 1983; Yeo et al., 1990). Rice can tolerate 50–100 mM Na+ in cytosol, beyond which either the cell has to further sequester the Na+ to tonoplast or cell membrane through antiporters otherwise the cell succumbs to high salt concentration. Therefore, different mechanisms of ionic control through partitioning are seen. Mainly two mechanisms are known for Na+ entry into the root cell cytosol. Either, it may be through cation channels or transporters. It has been shown in different systems that high affinity K+ transporters (HKT) act as low affinity Na+ transporters facilitating the entry of Na+ into the root cells under high salinity stress. Further, Na+ may also enter root xylem stream through apoplastic pathways as shown in rice by Yadav et al. (1996).

Climate Change Affecting Rice Production

95

Compartmentation: Many studies have documented the beneficial effects of sodium compartmentation in the cells, tissues, or organs in the plant system. In order to adapt to the changing environment and to minimize the deteriorating effects, precise regulation of ion transport system in the plant at cell, tissue, or whole plant level is critical for salt tolerance. Some of the mechanisms occurring in the plants to maintain desirable K+/Na+ ratios in the cytosol include (i) regulation of K+ uptake and/or prevention of Na+ entry (ii) efflux of Na+ from the cell, and (iii) utilization of Na+ for osmotic adjustment. Osmotic regulation is maintained either by Na+ compartmentation into the vacuole or by the biosynthesis and accumulation of compatible solutes (Sharma and Singh, 2008). This is called cell level compartmentation. However, specific plant tissue or organ is aimed to store the toxic ions that can be sacrificed by active metabolic activity. Plants usually deposit the excess toxic ions like Na+ in their old leaves and leaf sheaths because they are easy to sacrifice after rendering them inactive (Yeo and Flowers, 1983; Yeo et al., 1990).  Organic compatible osmolytes: Besides accumulation of ions for osmotic adjustment plants also synthesize organic osmolytes to help in maintaining water uptake and cell turgor under osmotic stress situations. These osmolytes are localized in cytoplasm, and the inorganic ions such as Na+ and Cl are preferentially sequestered into vacuole, thus leading to the turgor maintenance for the cell under osmotic stress (Bohnert et al., 1995; Flowers et al., 1977). A range of osmotic solutes namely proline, betaine, polyols, sugar alcohols, and soluble sugars has been reported in different plants upon their exposure to salt and water stress conditions. Glycine betaine and trehalose act as osmoprotectants by stabilizing quaternary structures of proteins and highly ordered states of membranes. Mannitol serves as free radicle scavenger. Proline serves as a storage sink for carbon and nitrogen and a free radicle scavenger. Due to their properties, these organic osmolytes are known a osmoprotectants (Bohnert and Jensen, 1996; Chen and Murata, 2000).  Oxidative stress management: Under salinity stress, the accumulation of reactive oxygen species (ROS) including superoxide radicals, H2O2, and hydroxyl radicals has been termed as an important cause of damage to the plant cell (Apse et al., 2003). Alleviation of these oxidative stresses reduces the cell level damage and enhances the level of salt tolerance. The tolerant plants produce more antioxidants like ascorbic acid and reduce glutathione and various reactive oxygen scavenging enzymes than the sensitive plants. Production of stress proteins as well as the accumulation of compatible osmolytes has been reported which probably detoxify the plants by scavenging reactive oxygen specien or preventing them from damaging cellular structures (Apse et al., 2003; Ismail et al., 2007; Moradi and Ismail, 2007). Plants with high activity of such detoxifying enzymes will be naturally selected for future climates. 

96

R. Wassmann et al.

2.3.2. Adaptive mechanisms for salinity tolerance With the projected increase in high temperature, the rice genotypes in saltaffected soils will have to adapt to avoid yield losses. Salinity tolerance is important both at seedling and reproductive stage under high temperature regimes. High evapotranspirational demands due to increased temperature will favor the plants which can withstand the higher accumulation of salt under salt-stress environment. This would be relevant to the tropical and subtropical areas but more so to salt-stressed arid and subarid regions where RH is lower than usual. Pyramiding genes/QTLs for salinity tolerance and heat tolerance using marker-assisted breeding would be the ideal choice. There are existing landraces which can withstand very high level of salt tolerance and could be a good candidate for high temperature and salt affected regions but inherently they are poor yielder. Although salinity tolerant genotypes are available in the improved background considering the future climate projections, salinity tolerance has to be further enhanced. This could be achieved by pyramiding of the component mechanisms for salinity tolerance like development of good excluder with better tissue tolerance. For example, Na+ entry in cytosol is restricted or minimized through root cells, it will reduce transportation of toxic ions to shoots from the roots, thereby lowering the salt load on to the plant system. Tissue tolerance reflects the capacity of the genotype to withhold salt load and maintaining its high photosynthetic activity (Flowers et al., 1985; Yeo and Flowers, 1983, 1986; Yeo et al., 1990). Developing genotypes with different sodium transporters that could provide the needed ion homeostasis during salt stress opens the possibility of engineering crop plants with improved salt tolerance. This is possible by enhanced vacuolar H+-pumping to provide additional driving force for vacuolar sodium accumulation via the vacuolar Na+/H+ antiporter. This has been demonstrated in transgenic tomato plants by overexpressing AtNHX1, the A. thaliana vacuolar Na+/H+ antiporter (Zhang and Blumwald, 2001; Zhang et al., 2001) and also in rice using the rice homolog OsNHX1 (Fukuda et al., 2004). Coastal rice ecosystems usually receive heavy rainfall during the wet season. This may coincide with strong sea disturbances, inundating the coasts because of high tides. Due to combined high rainfall and high tide, the rice crop in the coastal areas experiences submergence with moderately saline water, specifically during early crop growth. With the projected seawater rise, such saline water inundation in the coastal areas would be more frequent in the future. Therefore, initial 5–6 weeks are more crucial for the survival of the plants. To cope with this companion stress problem— salinity and submergence, rice plants need to have tolerance to both stresses. The major QTL for the submergence tolerance in rice has already been identified (Xu and Mackill, 1996; Xu et al., 2000) and the technique for transferring the gene for submergence tolerance (Sub1) in different rice

Climate Change Affecting Rice Production

97

varieties through MAS has already been tested successfully (Neeraja et al., 2007). Therefore it seems possible to develop salinity and submergence— dual tolerant rice varieties. Ideally, a rice variety for salt stressed environments should have heat, salinity, and submergence tolerance to perform sustainability in future changing climates.

2.4. Submergence Submergence is an important abiotic stress affecting about 10–15 million ha of rice fields in South and South East Asia causing yield losses estimated to US $1B every year (Dey and Upadhyaya, 1996). This number is anticipated to increase considerably in the future given the increase in seawater level, as well as an increase in frequencies and intensities of flooding caused by extreme weather events (Bates et al., 2008). Although a semiaquatic plant, rice is generally intolerant of complete submergence and plants die within few days when completely submerged. This is also the case for deep water rice that escapes complete submergence by rapid internode elongation that pushes the plants above the water surface where it has access to oxygen and light to resume the mitochondrial oxidative pathway and photosynthesis. There are, however, few varieties that are tolerant to complete submergence capable of surviving under water for about 14 days and to recover after the water recedes (Fig. 9). Tolerant rice varieties have been identified already in the 1970s (Vergara and Mazaredo, 1975) and have been used as donors of tolerance by breeders, and studies on the tolerance mechanisms ever since. The most widely used variety is FR13A, a tall, photoperiod-sensitive variety of the aus-type rice from India. Other tolerant varieties are Kurkarrupan and Goda Heenati from Sri Lanka. The chromosomal region conferring most of the tolerance in FR13A, designated submergence 1 (Sub1), has been mapped to Chr. 9 by independent groups (Nandi et al., 1997; Toojinda et al., 2003; Xu and Mackill, 1996), and the Sub1 locus has recently been fine mapped and sequenced in an FR13A-derived tolerant line (Xu et al., 2006). The information on the genes located in the Sub1 locus now facilitates in-depth analyses of the molecular and physiological tolerance mechanisms, and, more importantly, triggered a breakthrough in marker-assisted breeding of submergence tolerant rice varieties (for a time lapse series video visit http:// www.irri.org/timelapse.asp). 2.4.1. Physiology and molecular biology of submergence tolerance The hormonal control and physiological basis of submergence tolerance and submergence escape (deep water rice) have been studied in detail and the data have recently been summarized in several excellent reviews

98

R. Wassmann et al.

(Bailey-Serres and Voesenek, 2008; Fukao and Bailey-Serres, 2008; Sarkar et al., 2006). The major difference between the two responses to submergence is that internode elongation is induced in deep water rice, whereas cell elongation is suppressed in submergence tolerant rice. Of central importance to both, submergence escape and tolerance, is therefore the regulation of energy providing processes. Under submergence, plants are exposed to low oxygen conditions and the final electron acceptor O2 in the electron transport chain in mitochondria is limited. Plants therefore need to recycle NADH via an alternative pathway to maintain glycolysis. This is mainly achieved by ethanolic fermentation converting pyruvate to ethanol which regenerates one molecule of NAD+. This is transiently preceded by conversion of pyruvate to lactate leading to a drop in cellular pH and regeneration of one NAD+ molecule. Whereas ethanol is benign since it can diffuse out of the cell, the formation of acetaldehyde as a toxic intermediate is problematic. Detoxification of acetaldehyde, probably by the mitochondrial aldehyde dehydrogenase OsAdh2 (Nakazono et al., 2000), is therefore important to avoid cell death under prolonged submergence. Several other cellular processes are altered under low oxygen conditions and the interested reader might be referred to the reviews cited above for further details. Given the poor energy (ATP) production of the anaerobic pathways (2–4 mol ATP versus 30–36 mol ATP under aerobic conditions), starch reserves are rapidly depleted. Physiological studies of a submergence tolerant near isogenic Sub1 line (M202-Sub1) showed that tolerance is associated with a significantly higher transcript level and in vitro activity of key enzymes of the ethanolic fermentation pathway (pyruvate decarboxylase, PDC, and alcohol dehydrogenase, ADH), in conjunction with delayed starch degradation and maintenance of a higher level of soluble sugars until 14 days of submergence (Fukao et al., 2006). At the same time, submergence tolerant plants show less elongation under submergence and therefore require less energy. Suppression of cell elongation has been associated with a lower expression of cell wall loosening expansion genes in the Sub1 near isogenic lines (Fukao et al., 2006). In summary, these data suggest that Sub1 confers tolerance via an optimized maintenance metabolism and suppression of the energy consuming escape response. This enables plants to survive under water for about 14 days and to retain sufficient carbohydrate CH reserves for regeneration of growth once the water recedes (Fig. 9). The above outlined processes are mainly controlled by the phytohormone ethylene and a fine balance of the antagonistic hormones, that is gibberellic acid (GA) and abscisic acid (ABA). Both, ethylene and GA stimulate cell division and cell elongation, whereas ABA acts in an antagonistic way and is rapidly degraded under submergence (Das et al., 2005; Ella et al., 2003; Saika et al., 2007; for review see Fukao and Bailey-Serres, 2008). Sequencing of the Sub1 locus on Chr. 9 recently revealed the presence of

99

Climate Change Affecting Rice Production

IR64-Sub1 Samba-Sub1 Samba

IR49830 (Sub1 ) Samba

IR64 IR42

IR42 IR64

IR49830 (Sub1) IR49830 (Sub1)

IR64-Sub1

IR64

Samba

Samba-Sub1

IR64-Sub1 IR42

IR49830 (Sub1)

IR42 IR64-Sub1

Samba

IR49830 (Sub1)

Samba-Sub1 IR64

Figure 9

New Sub1 lines after 17 days submergence in field at IRRI.

three ethylene-responsive transcription factor (ERF) genes (Sub1A, Sub1B, and Sub1C) and the Sub1A-1 allele has been identified as the major determinant of tolerance (Xu et al., 2006). Interestingly, the Sub1A gene is absent from all analyzed japonica varieties, including the Nipponbare reference genome, and located on ‘‘chromosome unknown’’ in the indica reference genome of 93–11. This finding shows the limitations of the current reference genomes and the importance of sequencing major QTLs in the respective donor parent even when obvious candidate genes are present in the syntenic region in Nipponbare. Detailed sequence and expression analyses of the three ERF genes revealed tolerant-specific alleles and expression pattern for Sub1A and Sub1C, but not for Sub1B. In general, high Sub1A and low Sub1C expression is observed in tolerant varieties, whereas low Sub1A and high Sub1C expression is detected in intolerant varieties (Xu et al., 2006). In addition, both genes carry characteristic single nucleotide polymorphism (SNPs) that created a putative kinase phosphorylation site in Sub1A-1 and mutated a putative phosphorylation site in Sub1C-1. These SNPs are now being targeted by allele specific markers used for molecular breeding (see below). Although the function of the tolerant specific Sub1C-1 allele remains to be finally clarified by transgenic approaches, gene expression analyses in a range of different tolerant and intolerant rice varieties suggest that this gene is not a major determinant of tolerance (Septiningsih et al., 2008; Dang et al., manuscript in preparation). In contrast, it was shown that over expression of Sub1A-1 in an intolerant variety (Liaogeng) that naturally lacks the Sub1A genes, confers tolerance by suppressing elongation growth under submergence (Xu et al., 2006). It was further shown that Sub1A is induced by ethylene, but not by GA treatment.

100

R. Wassmann et al.

In summary, these data suggest that Sub1A is an upstream regulator that acts as a suppressor of the ethylene induced escape response. 2.4.2. Molecular breeding of submergence tolerant rice varieties Sub1 is an exceptionally strong QTL that shows a large effect in diverse genetic backgrounds and environments. This indicates that Sub1 acts far upstream in the stress response pathway and that the factors interacting with Sub1 are highly conserved in all target varieties. However, slight differences in tolerance levels are observed between Sub1-introgression lines (Septiningsih et al., 2008) suggesting the existence of modifying factors or additional tolerance genes (QTLs) with small effects. Although submergence tolerant breeding lines were developed already in the 1980s they were never adopted by farmers since they were not locally adapted and did not meet farmers’ and consumers’ expectations on grain quality (Mackill, 2006). A novel marker-assisted backcrossing (MAB) approach was therefore developed that facilitate introgression of Sub1 into the background of widely grown rice varieties, so-called mega varieties. On the basis of the Sub1 sequence information, PCR-based allele specific Sub1A and Sub1C markers (foreground markers) were developed that facilitate the distinction of the tolerant and intolerant Sub1 haplotype (Neeraja et al., 2007; Septiningsih et al., 2008; Dang et al., manuscript in preparation). These markers are now routinely being used in conjunction with Sub1 flanking and background markers. Background markers are being used to restore, as much as possible, the genetic background of the recipient parent and to remove undesirable additional introgressions from the Sub1 donor. Flanking markers are used to select for double cross over plants upstream and downstream of Sub1 thereby minimizing the size of the Sub1 introgression (Fig. 10). Flanking and background markers, both, need to be developed for each individual cross whereas Sub1 foreground markers can be used for most crosses. With this new technique, submergence tolerant plants can be developed by two to three backcrosses (BC2F3 or BC3F2) to the recipient mega variety (Septiningsih et al., 2008). The resulting Sub1 varieties are indistinguishable from the original intolerant variety with respect to yield, grain quality and other desirable agronomic traits. The main advantage of using mega varieties as recipient parent is that farmers’ and consumers’ preferred traits present in these varieties are preserved, and the risk of introducing undesirable traits is considerably reduced. Indeed it is being discussed if Sub1 versions of mega varieties can enter into an accelerated national release pipeline to speed up out-scaling for the benefit of farmers in submergence-prone areas. Sub1 versions of six important rice varieties (IR64, Swarna, BR11, TDK1, Samba Mahsuri, and CR1009; Septiningsih et al., 2008) have been developed, and Swarna-Sub1 has already been tested by national institutes in more than seven Asian countries. In parallel, management options and fertilizer recommendations

101

Climate Change Affecting Rice Production

Tolerant donor

Intolerant local variety 1 2 3 4 5 6 7 8 9 10 11 12

Chr. 1 2 3 4 5 6 7 8 9 10 11 12

X 1 2

F1 generation Backcross (BC) to local variety and marker selection starting in BC1F1

2nd (and 3rd) BC to local variety, marker selection, 1–2 x selfing

1 2 3 4 5 6 7 8 9 10 11 12

1 2 3 4 5 6 7 8 9 10 11 12

1

1 2

2

Final product Progeny plant with target QTLs and background introgressions from donor

Local variety with pyramided QTLs and restored genetic background

Figure 10 Pyramiding of stress tolerance QTL(s) by marker-assisted backcrossing. A tolerant donor variety carrying one or two hypothetical target QTLs (black squares with white numbers) located on a given rice chromosome (grey bars) is crossed to an intolerant recipient parent, e.g., a locally adapted rice cultivar. Three types of molecular markers are applied earliest in the first back cross (BC) generation to select for plants carrying the target QTLs in the genetic background of the recipient parent: QTL-specific foreground markers (black arrows), flanking markers to select for plants with double cross over (grey arrows), and background markers on all chromosomes with ~5 Mb spacing (triangles) to select against the genetic background of the donor parent. BC2F3 or BC3F2 progenies are almost identical to the original recipient variety but are tolerant.

are being developed and tested in collaboration with national agricultural institutes (Ella and Ismail, 2006). So far, Sub1 varieties performed well in almost all trials and data from field experiments in 2007 in India with natural flooding events between 10 and 30 days showed a 36% average yield advantage of Swarna-Sub1 over the intolerant original Swarna variety (IRRI, unpublished data). 2.4.3. Moving beyond Sub1 A germplasm screening has been conducted at IRRI to identify novel sources of submergence tolerance distinct from Sub1. The Sub1 haplotype and submergence tolerance level of over 200 rice accessions has been determined revealing few tolerant accessions with the intolerant Sub1 haplotype (Sub1A-2, Sub1C-2). However, subsequent gene expression analyses revealed unexpectedly high expression of Sub1A-2 in tolerant

102

R. Wassmann et al.

accessions, in contrast to low Sub1A-2 expression in intolerant accessions (Dang et al., manuscript in preparation). These data suggest that high expression of Sub1A, regardless of the specific allele, is sufficient to confer at least some level of tolerance. In-depth analyses of the Sub1A-1 and Sub1A-2 promoter regions are now ongoing and will reveal the regulatory factors acting upstream of Sub1A (N. Singh, IRRI, unpublished data).

3. Comparative Assessment of Rice Versus Other Crops (In Terms of Vulnerability and Adaptation Options) Rice is a C3 grass that evolved in semiaquatic, low-radiation habitats and is currently grown in wider range of environments from humid tropics to arid and semiarid conditions and even to temperate zones. As such, it carries a peculiar range of adaptations to existing and changing environments compared with other crop species. This broader adaptation will make rice more amenable for manipulation to adjust to climate changes as a consequence of global warming. However, to cope with these changes, adjustments will be necessary both in breeding strategies to develop suitable and more robust varieties, as well as in management strategies.

3.1. Advantages/disadvantages in warmer climates As discussed in Section 2.1, especially rice is sensitive to heat stress during reproductive development. Consequently, the currently available varieties are not fitting for future climates, particularly in areas where striking shifts in either or both night and day temperatures are expected. These conditions could endure substantial reduction and alternation in rice growth and development. Accelerated development during reproductive stage for example, will shorten the duration of grain filling, reducing grain yield in some cases, as observed for other crop species (Hall et al., 1997). Besides, rice is grown under climatic and socioeconomic conditions that differ from other major crop species such as maize and wheat. The unique feature of growing rice in flooded soils (ca. 90%), and mostly being confined to lower latitudes (30
N and 30
S), may suggest that rice will probably be subjected to different challenges as a consequent of global warming. However, the overall effects of heat stress on rice could be of lesser impacts under certain circumstances, (1) in areas where sufficient good quality water is available, (2) in drier environments where aerial humidity is low to promote evaporative cooling, and (3) given that new high-yielding varieties were developed that can maintain high stomatal and hydraulic conductance to maintain transpiration in hotter climates to cool sensitive tissues and organs.

Climate Change Affecting Rice Production

103

The degree of high temperature damage depends to a large extent on the crop species and stage of development. Generally, high soil temperatures can reduce the required time for emergence and crop establishment, but with large variation in the maximum threshed between warm-season and cool-season annuals. For example, the threshold maximum seed zone temperature for cowpea is about 37
C, but only 25–33
C for lettuce. High temperature damage during crop establishment in rice could be avoided by transplanting in standing water in hotter climates to mitigate the effects of hotter dry soil in the root zone experienced with dry land crops. Under such conditions, transplanting will be a better option than direct seeding. However, for direct seeding systems, varieties that germinate under water (Ismail et al., 2008) may be useful in areas where surface soils become hot during crop establishment, as a shallow water film or even water logging will mitigate this heat effect during germination. During the vegetative stage, high day temperatures can damage the components of leaf photosynthesis, particularly those of photosystem II, as well as membrane properties (Ismail and Hall, 1999). Studies comparing responses to heat in contrasting species indicated that photosystem II of wheat, a cool season species, is more sensitive to heat stress than photosystem II of rice and pearl millet, which are adapted to higher temperatures (Al-Khatib and Paulsen, 1999). In extreme cases, heat stress can cause mortality, but with large variation depending on species. For instance, temperatures above 35
C for sufficient duration are lethal to pea, whereas cowpea can produce substantial biomass when grown in one of the hottest crop production environments (maximum shelter temperature of 50
C), although its vegetative development may exhibit some abnormalities. The strong ability of some rice genotypes to undergo evaporative cooling will be advantageous under such conditions, provided that sufficient high quality irrigation water is available with sufficient vapor pressure deficit between the canopy and air to drive the transpiration process and enhance cooling of the canopy. Reproductive development of many crops is more sensitive to heat stress than vegetative growth (Hall, 1992, 1993). Here we will briefly review the current understanding of the responses to heat stress in cowpea, one of the most hardy crop species adapted to dry and hot environments, since ample information is available that might have some implications for other systems such as in rice. High temperature seems to cause relatively less damage to cowpea during vegetative growth, but more so during reproductive development, and the damage is greater when stress occurs during the night than during the day, which is similar to the observations made on rice (Peng et al., 2004). Being mostly indeterminate, the effects also vary with the stage of development of specific reproductive structures in cowpea. Heat stress can negatively impact floral bud development, flower development, pod set, grain filling and even grain quality, and these responses will be reviewed in brief.

104

R. Wassmann et al.

Floral bud development is greatly suppressed or completely hindered by heat stress, with consequent inhibition of flower production. Two weeks or more of consecutive or interrupted hot nights during the first month after germination can cause this damage (Ahmed and Hall, 1993). This suppression occurs under long days but not under short days in the field, and is dependent on light quality. Floral buds were not suppressed in long-day high night temperature conditions when light with high red/far-red ratio of about 1.9 was used in growth chambers, but pod set was low. However, when growth chambers were used with lighting systems with a red/far-red ratio of 1.3–1.6, floral bud suppression was similar to what seen under longday high night temperatures in the field, where sunlight has a red/far-red ratio of about 1.2. These findings caution against the use of controlled conditions with artificial lighting systems, as it can result in either artifacts or methodological advantages when studying responses to heat stress. Extreme prudence should be taken when setting these conditions. In view of these effects, further studies are needed to elucidate the interactive effects of light quality and duration with high temperature during the early stages of panicle development in rice. However, these effects might be of lesser significance if the developing rice panicles were kept closer to floodwater in paddy fields during early development. Flower development in cowpea is also sensitive to high temperature, with the sensitive stage occurring at about 9–7 days before anthesis (Ahmed et al., 1992; Warrag and Hall, 1984), which is after meiosis and coincides with the release of pollen microspores from the tetrads. High night temperature at this stage causes premature degeneration of the tapetal layer that provides nutrients to developing pollen, resulting in infertile pollen and even hinders anther dehiscence in some cowpea genotypes. These damages also inhibit transport of proline from the tapetal layer to developing pollen grains (Mutters et al., 1989a). The association between genetic differences in sensitivity to heat stress and rapid leakage of electrolytes from tissues subjected to high temperature observed in cowpea (Ismail and Hall, 1999) may suggest that, this heat-induced malfunction of cellular membranes could impact other processes such as pollen development. Subjecting cowpea shoot to moderately high night temperature can damage pod set (Warrag and Hall, 1984), however, much hotter day temperatures did not, and reciprocal artificial pollinations between plants grown under high and optimal night temperatures indicated the low pod set was caused by male sterility while the pistils did not appear to be affected, which is similar to observations made on cereals, as wheat (Saini and Aspinall, 1982) and rice (Yoshida et al., 1981). This effect was also demonstrated in the field using enclosure systems (Nielsen and Hall, 1985). The injury was later demonstrated to occur during the last 6h of the night, where plants subjected to high temperature during this period exhibited substantial increase in pollen sterility; but not when plants were subjected to high

Climate Change Affecting Rice Production

105

temperature during the first 6h of the night (Mutters and Hall, 1992), suggesting that specific heat sensitive processes during pollen development occur in the late night period or at predawn when temperatures are the coolest and are probably under circadian control. These effects were also found to be more severe under longer days and are perhaps phytochromemediated (Mutters et al., 1989b). These findings might have some relevance to rice, where cultivars that flower earlier in the morning (Sheehy et al., 2005; Yoshida et al., 1981) are expected to be more tolerant to heat stress, possibly by completing these sensitive processes earlier, during the cooler predawn period. Heat stress also causes embryo abortion resulting in fewer seeds per pod. High day and high night temperatures as well as other stresses, such as drought, reduce the number of ovules that produce seeds resulting in fewer seeds per pod. Grains produced under high temperature also can have asymmetrically twisted cotyledons (Warrag and Hall, 1984) with brown seed coat discoloration in some cultivars, which reduces the grain market value. Whether similar effects on grain quality could be experienced in rice remain to be seen. The accelerated reproductive development under night-time heat stress may negatively influence productivity. Under the cool nights of subtropical California (min. temperatures of 16
C), individual pods of cowpea took 21 days from anthesis to mature dry pod, but only 14 days when the same cultivar was grown under higher night temperatures of the tropics (min. temperatures of 26
C). This rapid pod development may increase the extent of embryo abortion and result in smaller grains. The more rapid development of individual grains also shortens the overall reproductive period, causing lower grain yields of cowpea cultivars grown with optimal management in tropics than in the subtropics (Hall et al., 1997). Apart from the effect of solar radiation, low night temperatures might partially explain why the maximum grain yield of warm season crops, such as rice, is usually higher in the subtropics and middle latitudes. In cowpea, evidence for heatinduced yield reduction was demonstrated under natural farmers’ fields using a set of cultivars with similar genetic backgrounds, evaluated under optimal management over eight environments contrasting in temperature regimes, but with similar high levels of solar radiation (Ismail and Hall, 1998), to provide a more realistic evaluation of heat stress effects. Grain yield was negatively correlated with mean minimum night temperatures during the 3-week period beginning 1 week before first flowering. For minimum night temperatures greater than 16.5
C grain yield decreased by 14% per
C, associated with a similar decrease (12% per
C) in number of pods per peduncle, but only a small decrease (6% per
C) in shoot biomass production. This confirms the earlier observations that effects of heat stress are more detrimental on reproductive development in cowpea, as was also observed in rice (Yoshida et al., 1981). In fact, sensitive varieties that showed complete suppression of reproductive development were very vigorous

106

R. Wassmann et al.

under field conditions, and a system was proposed involving the use of sensitive cowpea varieties as cover crops in hot environments. In rice, enhanced male sterility will be useful for hybrid seed production. Whether incorporating heat tolerance into crop plants will have any negative consequences on agronomic or quality traits should also be considered carefully. For example, incorporating heat tolerance in cowpea resulted in more compact genotypes, and this becomes more apparent with increasing temperature during the growing season. This may require new management strategies to avoid reduction in yield under field conditions, such as adjusting the planting density (Ismail and Hall, 2000), as was also reported in rice (Baloch et al., 2006). Apparently, heat stress could have variable consequences during different developmental stages of crop plants, with strong interaction in some cases, with other factors such as photoperiod and light quality. Whether similar responses are experienced by rice awaits further studies.

3.2. Advantages/disadvantages under worsening water stress Global warming is expected to impact the extent and severity of other environmental stresses such as drought, salinity, and frequency of storms and floods. Varieties that withstand the consequent high temperatures may also need to encompass tolerances of other stresses relevant to particular areas or ecosystems. Besides, traits associated with tolerance to some of these stresses may also help mitigate the effects of high temperatures. For instance, mechanisms of tolerance of drought and salinity that result in higher water uptake and maintenance of stomatal function under stress will bear a consequent cooling effect. Certain protective mechanisms such as ability to upregulate the antioxidant pathways to scavenge reactive oxygen species generated during stress are useful under most abiotic stresses and substantial genetic variability in these traits were reported in rice as well as in other crops (Moradi and Ismail, 2007). Roles of compatible solutes in reducing tissue desiccation and protecting macromolecules and cellular membranes under stress are becoming more apparent, and our understanding of their physiological and molecular basis is progressing fast (Svensson et al., 2002). More severe water shortages as well as increased wetness are anticipated in different regions with global warming, and both conditions will have substantial consequences on productivity of food crops, particularly rice. Implications of water shortage as well as rice responses to such conditions were thoroughly reviewed in Section 2.2. Given the high sensitivity to water stress in rice, significant changes in either the farming systems and/or cropping sequences will become necessary, including measures like shifting from paddy production to aerobic rice systems (Bouman et al., 2002), and the adjustment of cropping calendar to escape the hotter periods during the year.

Climate Change Affecting Rice Production

107

In some cases with severe incidences of water shortage, shifting from rice to other crops that are less demanding for water might be necessary; particularly during the dry season or in areas that are predominantly irrigated. Substantial efforts are needed to develop such future varieties to suit these evolving systems.

3.3. Advantages/disadvantages in deteriorating soils The upshot of climate change in worsening soil problems, particularly salt stress, is becoming obvious with the rising sea level; increasing storm incidences and reliance on water resources of poor quality as fresh water become more precious. Salt-affected soils in both coastal and inland areas already have low productivity and provide fewer options for food security and livelihood for local farmers. Salinity in coastal areas is more difficult to handle through reclamation or long-term infrastructure investments because of its dynamic nature and the complex relations between users at this fresh-saline water interface, particularly for agriculture versus aquaculture uses. In inlands, salinity, and alkalinity, either inherent or induced via improper irrigation practices have been mounting in recent years and are expected to worsen further with lack of good quality water and excessive irrigation to cope with the rising temperatures. The unique feature of rice to thrive in flooded soils made it one of the few crops that can be used to rehabilitate most salt affected coastal and inland soils, despite being sensitive to salt stress (Maas and Hoffman, 1977). This will help in leaching of harmful salts, in addition to its high potential for genetic improvement. Besides, rice is the only possible crop in some coastal areas because of excessive wetness due to tidal movements and/or monsoon rain. Nonetheless, rice productivity in salt-affected areas is currently very low, 1–1.5 tons ha–1, but can reasonably be raised by at least 2 tons ha–1 (Ponnamperuma, 1994), providing food for millions of the poorest people. Salt stress negatively affects growth and productivity of most crop plants, and recent research has begun to unravel the complexities of traits involved in its tolerance. The most common mechanisms across crop plants are control of sodium transport, cellular ion homeostasis, and salt response signaling (Hasegawa et al., 2000; Horie and Schroeder, 2004; Zhu, 2003; Tester and Davenport, 2003). The fundamental knowledge of salt response mechanisms in plants forms the basis for strategies to improve salt tolerance in crop species such as rice (Flowers, 2004; Ismail et al., 2007; Yamaguchi and Blumwald, 2005). As with other crops, tolerance of salt stress in rice is complex and varies with the stage of development, being relatively more sensitive during the early vegetative and reproductive stage (Akbar et al., 1972), and tolerance at these two sensitive stages is weakly associated (Moradi et al., 2003). Hence, discovering and combining suitable tolerance

108

R. Wassmann et al.

traits at both stages as well as for various other mechanisms are essential for developing resilient varieties with broader adaptation. The vast genetic variability in tolerance to salinity in rice (Akbar et al., 1972; Flowers and Yeo, 1981) makes it amenable for further genetic improvements. In recent years, good progress was made in unraveling the traits associated with tolerance to salt stress, and in breeding (Ismail et al., 2007; Moradi and Ismail, 2007). The current efforts at IRRI target major QTLs and candidate genes for the development and use in marker-assisted backcrossing system to combine major QTLs into suitable genetic backgrounds, and this implies that developing highly tolerant varieties for future challenges is feasible. Longer-term strategies will involve combining multiple tolerances of salinity at different stages for conditions where salt stress is expected any time during the season.

3.4. Flexibility for adjusting and coping with climate changes The multifaceted abiotic stresses experienced in unfavorable rice ecosystems (high salinity and other soil problems, submergence, stagnant flooding, and drought), forced farmers to grow mostly a single crop to rice during the monsoon season. Local rice varieties have some level of tolerance to these conditions but their productivity is low. During the rest of the year, large areas remain fallow due to high soil and water salinity and lack of good quality irrigation water. Combining tolerance of multiple stresses such as flash-flooding through incorporation of Sub1A gene, tolerance to longerduration partial flooding, tolerance of salt stress conferred by different traits, and so on will help in developing more robust varieties with wider adaptation. This is particularly feasible in rice because of the enormous progress made in disentangling the traits associated with tolerance and in developing DNA-based technologies for precise and speedy breeding of more adapted varieties. Once these varieties were developed, they are normally more responsive to management practices that can further boost and stabilize their performance. Incorporating some of these tolerance mechanisms will also help cope with heat stress, such as varieties with extreme discrimination against toxic salts, but with ability to maintain high stomatal conductance and transpiration cooling. In some areas, adjustments of the cropping sequence will be necessary to match the changing ecosystem boundaries. Rice ecosystems might shift north and southward, and the cropping calendar might need to be adjusted within the season in some areas. Earlier planting will help avoid high temperature during the most sensitive reproductive stages but this will have implications on subsequent crops and will entail development of suitable varieties of both rice as well as non-rice crops to suit the new climate patterns. In some areas rice might not be feasible and other crops might be of better choice in terms of adaptation and economic returns,

Climate Change Affecting Rice Production

109

while in others, rice might emerge as a new option. In all cases, rice will remain the crop of choice in areas with increasing wetness, but less so in areas where water become progressively inadequate.

4. Outlook: Current Advances and Future Prospects Traditional breeding methods that comprise pedigree and bulk selection based on morphological markers have been successfully used in rice breeding for stress tolerance and will remain important as a standard technique in many rice breeding institutions scattered throughout Asia. However, as the molecular genetic basis of heat tolerance in rice is elucidated and QTLs are identified and suitable markers developed, molecular breeding approaches are expected to be utilized for developing superior heat tolerant varieties more precisely and expeditiously. The availability of tolerant rice germplasm with major QTLs for abiotic stresses anticipated to increase in a future adverse climate is an encouraging accomplishment. The development of rice varieties that can tolerate these stresses will help—at least to some extend—limiting yield losses and major food shortages in the future. With the identification and fine mapping of major QTLs for important abiotic stresses (e.g., submergence, stagnant flooding, salinity, drought, heat) in conjunction with improved marker technologies, it will become possible in the near future to pyramid these major tolerance QTLs into any genetic background. The MAB approach developed for Sub1 now facilitates the restoration of the genetic background of the recipient parent. Using widely grown, high-yielding varieties with good grain quality as recipient parent furthermore reduces the risk of adverse traits, speeds up outscaling, and guarantees rapid adoption by farmers. To assure success in this strategy, the combined efforts of plant breeders, molecular biologists, plant physiologists, and agronomists, among other scientists, would be essential. Moreover, the role of the informal seed sector and alternative seed systems as a key seed supplier in the rice research to production chain, particularly in the unfavorable areas affected by heat stress, would become more important (Bishaw and Turner, 2008) even as rice breeding programs may continue to strongly rely on the formal seed sector that played a key role in spurring and sustaining the first ‘‘Green Revolution.’’ One of the key features for successful implementation, however, will be the combination of these crop technology options with advanced climatology tools. Although climate change is a global phenomenon, it will manifest itself as locally variable impacts. This variation carries uncertainty about the nature of change at local scale that cannot be addressed by the inherently coarse spatial resolution of global climate models. More targeted approaches

110

R. Wassmann et al.

for climate change adaptation will have to rely on higher-resolved data, that is, for identifying regional hot-spots of aggravating stresses in rice production. IRRI has initiated the integration of advanced GIS, and climatology approaches into ongoing and envisaged research and extension programs for improving rice production systems. The dissemination of seeds with the flood resistant Sub1 gene rice seed is streamlined through GIS analysis of flood prone areas under present conditions and future climate scenarios (Hijmans, personal communication). Apart from individual projects, IRRI has launched the ‘‘Rice and Climate Change Consortium’’ in 2007 as a strategic platform in the endeavor to fuse different disciplines related to impact assessment, adaptation to climate change impacts as well as mitigation of Greenhouse Gas emissions. Germplasm improvement and natural resource management have a proved track record of decreasing susceptibility of agricultural systems to individual stresses and will offer increasingly important solutions for adapting to progressive climate change. These measures, however, need to be adapted, both individually and at landscape level, to new combinations of stresses that a changing climate will impose. Risks not only include immediate impacts on production and livelihood, but also long-term degradation/conservation issues for soil, water, and biodiversity. The uptake of innovative management strategies must be greatly accelerated particularly in those regions where persistent poverty contributes to high vulnerability of food security to climate change impacts. Adaptive management to continually refine these strategies will be required and can be supported by the predictive capacity of downscaled global climate models. The challenge is made more difficult by the lead time required to develop, test, and disseminate scientific/technical agricultural innovations, and the substantial uncertainty about the magnitude and, in the case of rainfall patterns, often the direction of climatic shifts over the coming decades. While we do not ignore the enormous task of adaptation to climate change, we remain convinced that the basic understanding of stress physiology and agronomy of the rice production systems—as synthesized in review—can serve as a great asset for the work ahead.

REFERENCES Abeysiriwardena, D. S. D. Z., Ohba, K., and Maruyama, M. (2002). Influence of temperature and relative humidity on grian sterility. J. Natl. Sci. Found. Sri Lanka 30, 33–41. Ackerly, D. D., Coleman, J. S., Morse, S. R., and Bazzaz, F. A. (1992). CO2 and temperature effects on leaf-area production in 2 annual plant-species. Ecology 73, 1260–1269. Aggarwal, P. K., and Mall, R. K. (2002). Climate change and rice yields in diverse agroenvironments of India. II. Effect of uncertainties in scenarios and crop models on impact assessment. Clim. Change 52, 331–343.

Climate Change Affecting Rice Production

111

Aggarwal, P. K., Bandyopadhyay, S. K., Pathak, H., Kalra, N., Chander, S., and Kumar, S. (2000). Analysis of yield trends of the rice–wheat system in north-western India. Outlook on Agriculture 29, 259–268. Ahmed, F. E., and Hall, A. E. (1993). Heat injury during early floral bud development in cowpea. Crop Sci. 33, 764–767. Ahmed, F. E., Hall, A. E., and DeMason, D. A. (1992). Heat injury during floral development in cowpea (Vigna Unguiculata, Fabaceae). Amer. J. Bot. 79, 784–791. Akbar, M., Yabuno, T., and Nakao, S. (1972). Breeding for saline resistant varieties of rice. I. Variability for salt tolerance among some rice varieties. Jpn. J. Breed. 22, 277–284. Al-Khatib, K., and Paulsen, G. M. (1999). High-temperature effects on photosynthetic processes in temperate and tropical cereals. Crop Sci. 39, 119–125. Allen, J. A., Pezeshki, S. R., and Chambers, J. L. (1996). Interaction of flooding and salinity stress on baldcypress (Taxodium distichum). Tree Physiol. 16, 307–313. Allen, L. H., Valle, R. R., Mishoe, J. W., and Jones, J. W. (1994). Soybean leaf gas-exchange responses to carbon-dioxide and water-stress. Agron. J. 86, 625–636. Apse, M. P., Aharon, G. S., Snedden, W. A., and Blumwald, E. (1999). Salt tolerance conferred by overexpression of a vacuolar Na+/H+ antiport in Arabidopsis. Science 285, 1256–1258. Apse, M. P., Sottosanto, J. B., and Blumwald, E. (2003). Vacuolar cation/H+ exchange, ion homeostasis, and leaf development are altered in a T-DNA insertional mutant of AtNHX1, the Arabidopsis vacuolar Na+/H+ antiporter. Plant J 36, 229–239. Atlin, G. N., Lafitte, H. R., Tao, D., Laza, A., Amante, A., and Courtois, B. (2006). Developing rice cultivars for high-fertility upland systems in the Asian tropics. Field Crops Res. 97, 43–52. Bailey-Serres, J., and Voesenek, L. (2008). Flooding stress: Acclimations and genetic diversity. Annu. Rev. Plant Biol. 59, 313–339. Barker, R., and Molle, F. (2004). Evolution of Irrigation in South and Southeast Asia. Comprehensive Assessment of WatED Management in Agriculture, Research Report No. 5. Colombo: IWMI. Baker, J. T., Allen, L. H., and Boote, K. J. (1992). Response of rice to carbon-dioxide and temperature. Agric. For. Meteorol. 60, 153–166. Baker, J. T., Allen, L. H., Boote, K. J., and Pickering, N. B. (1997a). Rice responses to drought under carbon dioxide enrichment.1. Growth and yield. Glob. Chang. Biol. 3, 119–128. Baker, J. T., Allen, L. H., Boote, K. J., and Pickering, N. B. (1997b). Rice responses to drought under carbon dioxide enrichment. 2. Photosynthesis and evapotranspiration. Glob. Chang. Biol. 3, 129–138. Baloch, M. S., Awan, I. U., and Hassan, G. (2006). Growth and yield of rice as affected by transplanting dates and seedlings per hill under high temperature of Dera Ismail Khan, Pakistan. J. Zhejiang Univ. Sci. B. 7, 572–579. Barnabas, B., Jager, K., and Feher, A. (2008). The effect of drought and heat stress on reproductive processes in cereals. Plant Cell Environ. 31, 11–38. Bates, B. C., Kundzewicz, Z. W., Wu, S., and Palutikof, J. P., Eds. (2008). “Climate Change and Water,” 210 pp. Technical Paper of the Intergovernmental Panel on Climate Change, IPCC Secretariat, Geneva. http://www.ipcc.ch/ipccreports/tp-climate-change-water. htm. Belder, P., Bouman, B. A. M., Cabangon, R., Lu, G., Quilang, E. J. P., Li, Y. H., Spiertz, J. H. J., and Tuong, T. P. (2004). Effect of water-saving irrigation on rice yield and water use in typical lowland conditions in Asia. Agric. Water Manage. 65, 193–210. Bernier, J., Atlin, G. N., Serraj, R., Kumar, A., and Spaner, D. (2008). Breeding upland rice for drought resistance (A review). J. Sci. Food Agric. 88, 927–939.

112

R. Wassmann et al.

Bishaw, Z., and Turner, M. (2008). Linking participatory plant breeding to the seed supply system. Euphytica 163, 31–44. Bohnert, H. J., and Jensen, R. G. (1996). Strategies for engineering water-stress tolerance in plants. Trends Biotechnol. 14, 89–97. Bohnert, H. J., Nelson, D. E., and Jensen, R. G. (1995). Adaptations to environmental stresses. Plant Cell 7, 1099–1111. Boonjung, H., and Fukai, S. (1996). Effects of soil water deficit at different growth stages on rice growth and yield under upland conditions.2. Phenology, biomass production and yield. Field Crops Res. 48, 47–55. Bouman, B. A. M., Humphreys, E., Tuong, T. P., and Barker, R. (2007). Rice and water. In “Advances in Agronomy,” Vol. 92, pp. 187–237. IRRI, Los Ban˜os, Philippines. Bouman, B. A. M., Peng, S., Castaneda, A. R., and Visperas, R. M. (2005). Yield and water use of irrigated tropical aerobic rice systems. Agricultural Water Management 74, 87–105. Bouman, B. A. M., and Tuong, T. P. (2001). Field water management to save water and increase its productivity in irrigated lowland rice. Agric. Water Manage. 49, 11–30. Bouman, B. A. M., Xiaoguang, Y., Huaqi, W., Zhiming, W., Junfang, Z., Changgui, W., and Bin, C. (2002). Aerobic rice (Han Dao): A new way of growing rice in water-short areas. Proceedings of the 12th International Soil Conservation Organization Conference, 26–31 May, 2002, Beijing, China. Tsinghua University Press, pp. 175–181. Bowman, W. D., and Strain, B. R. (1987). Interaction between co2 enrichment and salinity stress in the c-4 nonhalophyte andropogon-glomeratus (walter) bsp. Plant Cell Environ. 10, 267–270. Boyer, J. S. (1982). Plant productivity and environment. Science 218, 443–448. Butt, T. A., McCarl, B. A., Angerer, J., Dyke, P. T., and Stuth, J. W. (2005). The economic and food security implications of climate change in Mali. Clim. Change 68, 355–378. Challinor, A. J., Wheeler, T. R., Craufurd, P. Q., Ferro, C. A. T., and Stephenson, D. B. (2007). Adaptation of crops to climate change through genotypic responses to mean and extreme temperatures. Agric. Ecosyst. Environ. 119, 190–204. Chen, T. H. H., and Murata, N. (2000). Enhancement of tolerance of abiotic stress by metabolic engineering of betaines and other compatible solutes. Curr. Opin. Plant Biol. 5, 250–257. Cho, N. (1956). Double fertilization in Oryza sativa L. and development of the endosperm with special reference to the aleurone layer. Bull. Natl. Inst. Agric. Sci. 6, 61–101. Cooper, M., Fukai, S., and Wade, L. J. (1999). How can breeding contribute to more productive and sustainable rainfed lowland rice systems? Field Crops Res. 64, 199–209. Craufurd, P. Q., Prasad, P. V. V., Kakani, V. G., Wheeler, T. R., and Nigam, S. N. (2003). Heat tolerance in groundnut. Field Crops Res. 80, 63–77. Das, K. K., Sarkar, R. K., and Ismail, A. M. (2005). Elongation ability and non-structural carbohydrate levels in relation to submergence tolerance in rice. Plant Sci. 168, 131–136. Dey, M., and Upadhyaya, H. (1996). In ‘‘Rice research in Asia: Progress and Priorities’’ (R. Evenson, R. Herdt, and M. Hossain, Eds.), pp. 291–303. CAB International, Oxon, UK. Downton, J., and Slatyer, R. O. (1972). Temperature dependence of photosynthesis in cotton. Plant Physiol. 50, 518–522. Easterling, W. E., Chhetri, N., and Niu, X. Z. (2003). Improving the realism of modelling agronomic adaptation to climate change: Simulating technological submission. Clim. Change 60, 149–173. Ekanayake, I. J., Steponkus, P. L., and Dedatta, S. K. (1990). Sensitivity of pollination to water deficits at anthesis in upland rice. Crop Sci. 30, 310–315. Ella, E., Kawano, N., Yamauchi, Y., Tanaka, K., and Ismail, A. (2003). Blocking ethylene perception enhances flooding tolerance in rice. Funct. Plant Biol. 30, 813–819.

Climate Change Affecting Rice Production

113

Ella, E. S., and Ismail, A. M. (2006). Seedling nutrient status before submergence affects survival after submergence in rice. Crop Sci. 46, 1673–1681. Enomoto, N., Yamada, I. and Hozumi, K. (1956). On artificial germination of pollen in rice plants. IV. Temperature limits of pollen germination in rice varieties. (In Japanese with English summary). Proc. of the Crop Science Society of Japan, pp. 25–69. Evenson, R. E., and Gollin, D. (1997). Genetic resources, international organizations, and improvement in rice varieties. Econ. Dev. Cult. Change 45, 471–500. Fischer, G., Shah, M., and Velthuizen, H. V. (2002). “Climate Change and Agricultural Vulnerability,” 152 pp. International Institute for Applied Systems Analysis, Laxenburg. Flowers, T. J. (2004). Improving crop salt tolerance. J. Exp. Bot. 55, 307–319. Flowers, T. J., Duque, E., Hajibagheri, M. A., McGonigle, T. P., and Yeo, A. R. (1985). The effect of salinity on leaf ultrastructure and net photosynthesis of 2 varieties of rice— Further evidence for a cellular-component of salt-resistance. New Phytol. 100, 37–43. Flowers, T. J., Troke, P. F., and Yeo, A. R. (1977). Mechanism of salt tolerance in halophytes. Annu. Rev. Plant Physiol. Plant Mol. Biol. 28, 89–121. Flowers, T. J., and Yeo, A. R. (1981). Variability in the resistance of sodium-chloride salinity within rice (oryza-sativa-l) varieties. New Phytol. 88, 363–373. Fukao, T., and Bailey-Serres, J. (2008). Ethylene—A key regulator of submergence responses in rice. Plant Sci. 175, 43–51. Fukao, T., Xu, K. N., Ronald, P. C., and Bailey-Serres, J. (2006). A variable cluster of ethylene response factor-like genes regulates metabolic and developmental acclimation responses to submergence in rice (W). Plant Cell 18, 2021–2034. Fukuda, A., Nakamura, A., Tagiri, A., Tanaka, H., Miyao, A., Hirochika, H., and Tanaka, Y. (2004). Function, intracellular localization and the importance in salt tolerance of a vacuolar Na+/H+ antiporter from rice. Plant Cell Physiol. 45, 146–159. Ghassemi, F., Jakeman, A. J., and Nix, H. A. (1995). ‘‘Salinisation of Land and Water Resources’’. CAB International, Wallingford. Goto, A., Sasahara, H., Shigemune, A., and Miura, K. (2008). Estimation of cool tolerance at the booting and flowering stages in indica rice varieties. Jpn. J. Crop Sci. 77, 167–173. Grashoff, C., Dijkstra, P., Nonhebel, S., Schapendonk, A., and VandeGeijn, S. C. (1995). Effects of climate change on productivity of cereals and legumes; model evaluation of observed year-to-year variability of the CO2 response. Glob. Chang. Biol. 1, 417–428. Hall, A. E. (1992). Breeding for heat tolerance. Plant Breed Rev. 10, 129–169. Hall, A. E. (1993). Physiology and breeding for heat tolerance in cowpea, and comparison with other crops. In “Adaptation of Food Crops to Temperature and Water Stress” (C. G. Kuo, Ed.), pp. 271–284. Publ. No. 93–410. Asian Vegetable Research and Development Center, Shanhua, Taiwan. Hall, A. E. (2004). Breeding for adaptation to drought and heat in cowpea. Eur. J. Agron. 21, 447–454. Hall, A. E., Singh, B. B., and Ehlers, J. D. (1997). Cowpea breeding. Plant Breed. Rev. 15, 215–274. Hasegawa, P. M., Bressan, R. A., Zhu, J. K., and Bohnert, H. J. (2000). Plant cellular and molecular responses to high salinity. Annu. Rev. Plant Physiol. Plant Mol. Biol. 51, 463–499. Heinemann, A. B., Dingkuhn, M., Luquet, D., Combres, J. C., and Chapman, S. (2008). Characterization of drought stress environments for upland rice and maize in central Brazil. Euphytica 162, 395–410. Herve´, P., and Serraj, R. (2009). Gene technology and drought: A simple solution for a complex trait. Afr. J. Biotech. (in press). Horie, T., Nakagawa, H., Centeno, H. G. S., and kropff, H. J. (1995). “The rice Crop Simulation Model SIMRIW and Tts Testing.” CABI in association with IRRI, Los Ban˜os, Philippines. Horie, T., and Schroeder, J. I. (2004). Sodium transporters in plants. Diverse genes and physiological functions. Plant Physiol. 136, 2457–2462.

114

R. Wassmann et al.

Hossain, M., and Fischer, K. S. (1995). Rice research for food security and sustainable agricultural development in Asia: Achievements and challenges. GeoJournal 35, 286–298. Hossain, M., Gollin, D., Cabanilla, V., Cabrera, E., Jonson, N., Khush, G. S., and McLaren, G. (2002). International research and genetic improvement in rice—Evidence from Asia and Latin America. In ‘‘Crop Variety Improvement and Its Effect on Productivity—The Impact of International Agricultural Research’’ (R. E. Evenson and D. Gollin, Eds.), pp. 71–107. CABI Publishing, UK. Howden, S. M., Soussana, J. -F., Tubiello, F. N., Chhetri, N., Dunlop, M., and Meinke, H. (2007). Adapting agriculture to climate change. Pnas 104(5), 19691–19696 http:// www. pnas.org/cgi/doi/10.1073/pnas.0701890104. Imin, N., Kerim, T., Rolfe, B. G., and Weinman, J. J. (2004). Effect of early cold stress on the maturation of rice anthers. Proteomics 4, 1873–1882. International Rice Research Institute. (1976). ‘‘Symposium on Climate and Rice.’’ Los Ban˜os, Philippines. International Rice Research Institute. (1980). Annual Report, 1979, Los Ban˜os, Philippines. IPCC. (2007a). IPCC Fourth Assessment Report (AR4). Comprises the AR4 Synthesis Report (online at http://www.ipcc.ch/ipccreports/ar4-syr.htm); Working Group I Report ‘The Physical Science Basis (online at http://www.ipcc.ch/ipccreports/ ar4-wg1.htm); Working Group II Report ‘Impacts, Adaptation and Vulnerability’ (online at http://www.ipcc.ch/ipccreports/ar4-wg2.htm); and Working Group III Report ‘Mitigation of Climate Change’ (online at http://www.ipcc.ch/ipccreports/ar4wg3.htm). IPCC-WG1. (2007b). “Climate Change 2007: The Scientific Basis.” WMO/UNEP, Geneva. IPCC-WG2. (2007c). “Climate Change 2007: Impacts, Adaptation and Vulnerability.” WMO/UNEP, Geneva. Ismail, A. M., Ella, E. S., Vergara, G. V., and Mackill, D. J. (2008). Mechanisms associated with tolerance of flooding during germination and early seeding growth in rice (Orayza sativa). Ann. Bot. 103, 107–209. Ismail, A. M., and Hall, A. E. (1998). Positive and potential negative effects of heat-tolerance genes in cowpea. Crop Sci. 38, 381–390. Ismail, A. M., and Hall, A. E. (1999). Reproductive-stage heat tolerance, leaf membrane thermostability and plant morphology in cowpea. Crop Sci. 39, 1762–1768. Ismail, A. M., and Hall, A. E. (2000). Semidwarf and standard-height cowpea responses to row spacing in different environments. Crop Sci. 40, 1618–1623. Ismail, A. M., Heuer, S., Thomson, M. J., and Wissuwa, M. (2007). Genetic and genomic approaches to develop rice germplasm for problem soils. Plant Mol. Biol. 65, 547–570. Jagadish, S. V. K. (2007). Molecular and physiological dissection of heat tolerance during anthesis in rice (Oryza sativa L.). Thesis, The University of Reading, UK. Jagadish, S. V. K., Craufurd, P. Q., and Wheeler, T. R. (2007). High temperature stress and spikelet fertility in rice (Oryza sativa L.). J. Exp. Bot. 58, 1627–1635. Jagadish, S. V. K., Craufurd, P. Q., and Wheeler, T. R. (2008). Phenotyping parents of mapping populations of rice (Oryza sativa L.) for heat tolerance during anthesis. Crop Sci. 48, 1140–1146. Jena, K. K., and Mackill, D. J. (2008). Molecular markers and their usein marker-assisted selection in rice. Crop Sci. 48, 1266–1276. Jennings, P. R., Coffman, W. R., and Kauffman, H. E. (1979). ‘‘Rice Improvement.’’ IRRI, Los Ban˜os, Philippines. Ji, X. M., Raveendran, M., Oane, R., Ismail, A., Lafitte, R., Bruskiewich, R., Cheng, S. H., and Bennett, J. (2005). Tissue-specific expression and drought responsiveness of cell-wall invertase genes of rice at flowering. Plant Mol. Biol. 59, 945–964.

Climate Change Affecting Rice Production

115

Kakani, V. G., Prasad, P. V. V., Craufurd, P. Q., and Wheeler, T. R. (2002). Response of in vitro pollen germination and pollen tube growth of groundnut (Arachis hypogaea L.) genotypes to temperature. Plant Cell Environ. 25, 1651–1661. Kakani, V. G., Reddy, K. R., Koti, S., Wallace, T. P., Prasad, P. V. V., Reddy, V. R., and Zhao, D. (2005). Differences in in vitro pollen germination and pollen tube growth of cotton cultivars in response to high temperature. Ann. Bot. 96, 59–67. Karim, M. A., Fracheboud, Y., and Stamp, P. (1997). Heat tolerance of maize with reference of some physiological characteristics. Ann. Bangladesh Agri. 7, 27–33. Kenny, G. J., Harrison, P. A., Olesen, J. E., and Parry, M. J. (1993). The effects of climate change on land suitability of grain maize, winter wheat and cauliflower in Europe. Eur. J. Agron. 2, 325–338. Khush, G. S. (1999). Green revolution: Preparing for the 21st century. Genome 42, 646–655. Khush, G. S. (2001). Challenges for meeting the global food and nutrient needs in the new millennium. Proc. Nutr. Soc. 60, 15–26. Kobata, T., and Moriwaki, N. (1990). Grain growth rate as a function of dry-matter production rate: An experiment with two rice cultivars under different radiation environments. Jpn. J. Crop Sci. 59, 1–7. Kobata, T., Sugawara, M., and Takatu, S. (2000). Shading during the early grain filling period does not affect potential grain dry matter increase in rice. Agron. J. 92, 411–417. Kobata, T., and Uemuki, N. (2004). High temperatures during the grain-filling period do not reduce the potential grain dry matter increase of rice. Agron. J. 96, 406–414. Krishnan, P., Swain, D. K., Bhaskar, B. C., Nayak, S. K., and Dash, R. N. (2007). Impact of elevated CO2 and temperature on rice yield and methods of adaptation as evaluated by crop simulation studies. Agric. Ecosyst. Environ. 122, 233–242. Kropff, M. J., Matthews, R. B., Van Laar, H. H., and Ten Berge, H. F. M. (1995). ‘‘The rice model Oryza 1 and its testing.’’ CABI in association with IRRI Los Ban˜os, Philippines. Kumar, A., Bernier, J., Verulkar, S., Lafitte, H. R., and Atlin, G. N. (2008). Breeding for drought tolerance: Direct selection for yield, response to selection and use of droughttolerant donors in upland and lowland-adapted populations. Field Crop Res. 107, 221–231. Lafitte, H. R., and Courtois, B. (2002). Interpreting cultivar  environment interactions for yield in upland rice: Assigning value to drought-adaptive traits. Crop Sci. 42, 1409–1420. Lampayan, R. M., Bouman, B. A. M., De Dios, J. L., Lactaoen, A. T., Espiritu, A. J., Norte, T. M., Quilang, E. J. P., Tabbal, D. F., Llorca, L. P., Soriano, J. B., Corpuz, A. A., Malasa, R. B., et. al. (2005). Transfer of water saving technologies in rice production in the Philippines. In ‘‘Transitions in Agriculture for Enhancing Water Productivity.’’ (T. M. Thiyagarajan, H. Hengsdijk, and P. Bindraban, Eds.), pp 111–132. Proceedings of the International Symposium on Transitions in Agriculture for Enhancing Water Productivity, 23–25 September 2003, Kilikulam, Tamil Nadu Agricultural University, Tamil Nadu, India. Li, Y. H., and Baker, R. (2004). Increasing water productivity for rice field irrigation in China. Rice Field Water Environ. 2, 187–193. Liu, J. X., Liao, D. Q., Oane, R., Estenor, L., Yang, X. E., Li, Z. C., and Bennett, J. (2005). Genetic variation in the sensitivity of anther dehiscence to drought stress in rice. Field Crops Res. 97, 87–100. Maas, E. V., and Hoffman, G. J. (1977). Crop salt tolerance—Current assessment. ASCEJ Irrig Drain Div. 103, 115–34. Mackill, D. J. (1981). Studies on the mechanism and genetics of high temperature tolerance in rice. Ph.D. Dissertation, University of California Davis, California, USA. Mackill, D. J. (2006). Breeding for resistance to abiotic stresses in rice: The value of quantitative trait loci. In ‘‘Plant Breeding: The Arnel R Hallauer International Symposium’’ (K. R. Lamkey and M. Lee, Eds.), pp. 201–212. Blackwell Pub, Ames, IA.

116

R. Wassmann et al.

Mackill, D. J., and Coffman, W. R. (1983). Inheritance of high temperature tolerance and pollen shedding in a rice cross. Z Pflanzesuchtg 91, 61–69. Mackill, D. J., Coffman, W. R., and Rutger, J. N. (1982). Pollen shedding and combining ability for high temperature tolerance in rice. Crop Sci. 22, 730–733. Maclean, J.L., Dawe, D., Hardy, B., and Hettel, G.P. (Eds.), ‘‘Rice Almanac,’’ p. 253. International Rice Research Institute, Los Ban˜os, Philippines. Maestri, E., Klueva, N., Perrotta, C., Gulli, M., Nguyen, H. T., and Marmiroli, N. (2002). Molecular genetics of heat tolerance and heat shock proteins in cereals. Plant Mol. Biol. 48, 667–681. Mallik, S., Mandal, B. K., Sen, S. N., and Sarkarung, S. (2002). Shuttle-breeding: An effective tool for rice varietal improvement in rainfed lowland ecosystem in eastern India. Curr. Sci. 83, 1097–1102. Manabe, S., and Stouffer, R. J. (1994). Multiple-century response of a coupled oceanatmosphere model to an increase of atmospheric carbon-dioxide. J. Climate 7, 5–23. Manderscheid, R., and Weigel, H. J. (2007). Drought stress effects on wheat are mitigated by atmospheric CO2 enrichment. Agron. Sustainable Dev. 27, 79–87. Matsui, T., and Kagata, H. (2003). Characteristics of floral organs related to reliable self pollination in rice (Oryza sativa L.). Ann. Bot. 91, 473–477. Matsui, T., and Omasa, K. (2002). Rice (Oryza sativa L) cultivars tolerant to high temperature at flowering: Anther characteristics. Annals of Botany 89, 683–687. Matsui, T., Kobayashi, K., Kagata, H., and Horie, T. (2005). Correlation between variability of pollination and length of basal dehiscence of the theca in rice and a hot-and-humid condition. Plant. Prod. Sci. 8, 109–114. Matsui, T., Kobayasi, K., Yoshimoto, M., and Hasegawa, T. (2007). Stability of rice pollination in the field under hot and dry conditions in the Riverina region of New Soth wales, Australia. Plant Prod. Sci. 10, 57–63. Matsui, T., Namuco, O. S., Ziska, L. H., and Horie, T. (1997a). Effect of high temperature and CO2 concentration on spikelet sterility in indica rice. Field Crops Res. 51, 213–219. Matsui, T., Omasa, K., and Horie, T. (1997b). High temperature induced florets sterility of japonica rice at flowering in relation to air temperature, humidity and wind velocity condition. Jpn. J. Crop Sci. 66, 449–455. Matsui, T., Omasa, K., and Horie, T. (1999a). Rapid swelling of pollen grains in response to floret opening unfolds locule in rice (Oryza sativa L.). Plant Prod. Sci. 2, 196–199. Matsui, T., Omasa, K., and Horie, T. (1999b). Mechanism of anther dehiscence in rice (Oryza sativa L.). Ann. Bot. 84, 501–506. Matsui, T., Omasa, K., and Horie, T. (2000a). High temperature at flowering inhibit swelling of pollen grains, a driving force for thecae dehiscence in rice (Oryza sativa L.). Plant Prod. Sci. 3, 430–434. Matsui, T., Omasa, K., and Horie, T. (2000b). Mechanism of septum opening in anthers of two rowed barley. Ann. Bot. 86, 47–51. Matsui, T., Omasa, K., and Horie, T. (2001). The difference in sterility due to high temperatures during the flowering period among japonica rice varieties. Plant Prod. Sci. 4, 90–93. Mishra, H. S., Rathore, T. R., and Pant, R. C. (1990). Effect of intermittent irrigation on groundwater table contribution, irrigation requirement and yield of rice in Mollisols of the Tarai Region. Agric. Water Management 18, 231–241. Moradi, F., and Gilani, A. (2007). Impact of global warming and high temperature stress on Iranian rice cultivars: An overview. Proc. Intl. Workshop on Cool Rice for a Warmer World, Huazhong Agricultural University, Wuhan, Hubei, China. 26-30 March 2007. Moradi, F., and Ismail, A. M. (2007). Responses of photosynthesis, chlorophyll fluorescence and ROS-Scavenging systems to salt stress during seedling and reproductive stages in rice. Ann. Bot. 99, 1161–1173.

Climate Change Affecting Rice Production

117

Moradi, F., Ismail, A. M., Gregorio, G. B., and Egdane, J. A. (2003). Salinity tolerance of rice during reproductive development and association with tolerance at the seeding stage. Ind. J. Plant Physiol. 8, 105–116. Morita, S., Yonemaru, J., and Takanashi, J. (2005). Grain growth and endosperm cell size under high night temperatures in rice (Oryza sativa L.) Ann. Bot. 95, 695–701. Morris, M. L., and Bellon, M. R. (2004). Participatory plant breeding research: Opportunities and challenges for the international crop improvement system. Euphytica 136, 21–35. Mutters, R. G., Ferreira, L. G. R., and Hall, A. E. (1989a). Photoperiod and light quality effects on cowpea floral development at high temperatures. Crop Sci. 29, 1501–1505. Mutters, R. G., and Hall, A. E. (1992). Reproductive responses of cowpea to high temperature during different night periods. Crop Sci. 32, 202–206. Mutters, R. G., Hall, A. E., and Patel, P. N. (1989b). Photoperiod and light quality effects on cowpea floral development at high temperatures. Crop Sci. 29, 1501–1505. Nakagawa, H., Horie, T., and Matsui, T. (2002). Effects of climate change on rice production and adaptive technologies. In ‘‘Rice Science: Innovations and Impact for Livelihood’’ (T. W. Mew, D. S. Brar, S. Peng, D. Dawe, and B. Hardy, Eds.), pp. 635–657. International Rice Research Institute, China. Nakazono, M., Tsuji, H., Li, Y. H., Saisho, D., Arimura, S., Tsutsumi, N., and Hirai, A. (2000). Expression of a gene encoding mitochondrial aldehyde dehydrogenase in rice increases under submerged conditions. Plant Physiol. 124, 587–598. Naich, A. N., and Mari, N. (2007). Research work on the development of rice varieties suitable for high temperature at Rice Research Institute Dokri, Sindh, Pakistan. Proc. Intl. Workshop on Cool Rice for a Warmer World, Huazhong Agricultural University, Wuhan, Hubei, China. 26–30 March 2007. Nandi, S., Subudhi, P. K., Senadhira, D., Manigbas, N. L., SenMandi, S., and Huang, N. (1997). Mapping QTLs for submergence tolerance in rice by AFLP analysis and selective genotyping. Mol. General Genet. 255, 1–8. Neeraja, C. N., Maghirang-Rodriguez, R., Pamplona, A., Heuer, S., Collard, B. C. Y., Septiningsih, E. M., Vergara, G., Sanchez, D., Xu, K., Ismail, A. M., and Mackill, D. J. (2007). A marker-assisted backcross approach for developing submergence-tolerant rice cultivars. Theor. Appl. Genet. 115, 767–776. Nielsen, C. L., and Hall, A. E. (1985). Responses of cowpea (Vigna unguiulata [L.] walp.) in the field to high night temperatures during flowering. II. Plant responses. Field Crop Res. 10, 181–196. Oh-e, I., Saitoh, K., and Kuroda, T. (2007). Effects of high temperature on growth, yield and dry-matter production of rice grown in the paddy field. Plant Prod. Sci. 10, 412–422. Osada, A., Sasiprada, V., Rahong, M., Dhammanuvong, S., and Chakrabandhu, M. (1973). Abnormal occurrence of empty grains of indica rice plants in the dry, hot season in Thailand. Proc. Crop Sci. Soc. Jpn. 42, 103–109. O’Toole, T. C. (2004). Rice water: The final frintier. In “First International Conference on Rice for the Future”, Aug. 31–Sept. 2, 2004 Bangko, Tailand. O’Toole, J. C., and Namuco, O. S. (1983). Role of panicle exsertion in water stress induced sterility. Crop Sci. 23, 1093–1097. Ouk, M., Basnayake, J., Tsubo, M., Fukai, S., Fischer, K. S., Cooper, M., and Nesbitt, H. (2006). Use of drought response index for identification of drought tolerant genotypes in rainfed lowland rice. Field Crops Res. 99, 48–58. Pandey, S., Bhandari, H., and Hardy, B. (2007). “Economic Costs of Drought and Rice Farmers’ Coping Mechanisms: A Cross-country Comparative Analysis,” 203 pp. International Rice Research Institute, Manila. Pathak, H., Ladha, J. K., Aggarwal, P. K., Peng, S., Das, S., Singh, Y., Singh, B., Kamra, S. K., Mishra, B., Sastri, A., Aggarwal, H. P., Das, D. K., et al. (2003). Trends

118

R. Wassmann et al.

of climatic potential and on-farm yields of rice and wheat in the Indo-Gangetic Plains. Field Crops Res. 80, 223–234. Peng, S., Bouman, B. A. M., Visperas, R. M., Castaneda, A., Nie, L., and Park, H. K. (2006). Comparison between aerobic and flooded rice: Agronomic performance in a long-term (8-season) experiment. Field Crops Res. 96, 252–259. Peng, S. B., Huang, J. L., Sheehy, J. E., Laza, R. C., Visperas, R. M., Zhong, X. H., Centeno, G. S., Khush, G. S., and Cassman, K. G. (2004). Rice yields decline with higher night temperature from global warming. Proc. Natl. Acad. Sci. USA 101, 9971–9975. Peng, S. B., Khush, G. S., Virk, P., Tang, Q. Y., and Zou, Y. B. (2008). Progress in ideotype breeding to increase rice yield potential. Field Crops Res. 108, 32–38. Pinheiro, B., da, S., de Castro, E., da, M., and Guimaraes, C. M. (2006). Sustainability and profitability of aerobic rice production in Brazil. Field Crops Res. 97, 34–42. Ponnamperuma, F. N. (1994). Evaluation and improvement of lands for wetland rice production. In “Rice and Problem Soils in South and Southeast Asia” IRRI Discussion Paper Series No. 4 (D. Senadhira, Ed.), pp 3–19. International Rice Research Institute, Manila, Philippines. Porter, J. R., and Semenov, M. A. (2005). Crop responses to climatic variation. Philos. Trans. Roy. Soc.B Biol. Sci. 360, 2021–2035. Potvin, C. (1994). Interactive effects of temperature and atmospheric CO2 on physiology and growth. In ‘‘Plant Responses to the Gaseous Environment, Molecular, Metabolic and Physiological Aspects’’ (R. B. Alscher and A. R. Wellburn, Eds.), pp. 39–54. Chapman & Hall, London. Prasad, P. V. V., Boote, K. J., Allen, L. H., Sheehy, J. E., and Thomas, J. M. G. (2006). Species, ecotype and cultivar differences in spikelet fertility and harvest index of rice in response to high temperature stress. Field Crops Res. 95, 398–411. Redon˜a, E. D., Laza, M. A., and Manigbas, N. L. (2007). Breeding rice for adaptation and tolerance to high temperatures. Proc. Intl. Workshop on Cool Rice for a Warmer World, Huazhong Agricultural University, Wuhan, Hubei, China, 26–30 March 2007. Ren, Z. H., Gao, J. P., Li, L. G., Cai, X. L., Huang, W., Chao, D. Y., Zhu, M. Z., Wang, Z. Y., Luan, S., and Lin, H. X. (2005). A rice quantitative trait locus for salt tolerance encodes a sodium transporter. Nat. Genet. 37, 1141–1146. Robredo, A., Perez-Lopez, U., de la Maza, H. S., Gonzalez-Moro, B., Lacuesta, M., MenaPetite, A., and Munoz-Rueda, A. (2007). Elevated CO2 alleviates the impact of drought on barley improving water status by lowering stomatal conductance and delaying its effects on photosynthesis. Environ. Exp. Bot. 59, 252–263. Rosegrant, M. W., Paisner, M. S., Meijer, S., and Witcover, J. (2001). “Global Food Projections To 2020: Emerging Trends and Alternative Futures,” p. 206. International Food Policy Research Institute, Washington, D.C. Rozema, J., Lenssen, G. M., Arp, W. J., and van de Staay, J. W. M. (1991). Global change, the impact of the greenhouse effect (atmospheric CO2 enrichment) and the increased UV-B radiation on terrestrial plants. In ‘‘Ecological Responses to Environmental Stresses’’ ( J. Rozema and J. A. C. Verkleij, Eds.), pp. 220–231. Kluwer Academic, Dordrecht. Saika, H., Okamoto, M., Miyoshi, K., Kushiro, T., Shinoda, S., Jikumaru, Y., Fujimoto, M., Arikawa, T., Takahashi, H., Ando, M., Arimura, S., Miyao, A., et al. (2007). Ethylene promotes submergence-induced expression of OsABA8ox1, a gene that encodes ABA 8-Hydroxylase in rice. Plant Cell Physiol. 48(2), 287–298. Saini, H. S. (1997). Effects of water stress on male gametophyte development in plants. Sexual Plant Reprod. 10, 67–73. Saini, H. S., and Aspinall, D. (1982). Abnormal sporogenesis in wheat (triticum-aestivum l) induced by short periods of high-temperature. Ann. Bot. 49, 835–846.

Climate Change Affecting Rice Production

119

Saini, H. S., Sedgley, M., and Aspinall, D. (1983). Effect of heat-stress during floral development on pollen-tube growth and ovary anatomy in wheat (triticumaestivum-l). Aust. J. Plant Physiol. 10, 137–144. Saini, H. S., and Westgate, M. E. (2000). Reproductive development in grain crops during drought. ‘‘Advances in Agronomy,’’ Vol. 68, pp. 59–96. Sairam, R. K., and Tyagi, A. (2004). Physiology and molecular biology of salinity stress tolerance in plants. Curr. Sci. 86, 407–421. Sakamoto, A., and Murata, N. (2002). The role of glycine betaine in the protection of plants from stress: Clues from transgenic plants. Plant Cell Environ. 25, 163–171. Salem, M. A., Kakani, V. G., Koti, S., and Reddy, K. R. (2007). Pollen-based screening of soybean genotypes for high temperatures. Crop Sci. 47, 219–231. Sarkar, R. K., Reddy, J. N., Sharma, S. G., and Ismail, A. M. (2006). Physiological basis of submergence tolerance in rice and implications for crop improvement. Current Science 91, 899–906. Sarkarung, S., and Pantuwan, G. (1999). Improving rice for drought-prone rainfed lowland environments. In ‘‘Genetic Improvement of Rice for Water-Limited Environments’’ (O. Ito, J. C. O’Toole, and B. Hardy, Eds.), pp. 57–70. International Rice Research Institute, Los Banos, The Philippines. Satake, T. (1995). High temperature injury. In ‘‘Science of the Rice Plant, Vol. 2. Physiology’’ (T. Matsuo and K. Hoshikawa, Eds.), pp. 805–812. Food and Agricultural Policy Research Centre, Tokyo. Satake, T., and Yoshida, S. (1978). High temperature-induced sterility in indica rice at flowering. Jpn. J. Crop Sci. 47, 6–17. Savchenko, G. E., Klyuchareva, E. A., Abrabchik, L. M., and Serdyuchenko, E. V. (2002). Effect of periodic heat shock on the membrane system of etioplasts. Russ. J. Plant Physiol. 49, 349–359. Septiningsih, E. M., Pamplona, A. M., Sanchez, D. L., Neeraja, C. N., Vergara, G. V., Heuer, S., Ismail, A. M., and Mackill, D. J. (2008). Development of submergence tolerant rice cultivars: The Sub1 locus and beyond. Ann. Bot. 103, doi:10.1093/aob/mcn206. Serraj, R., Dimayuga, G., Gowda, V., Guan, Y., He, Hong, Impa, S., Liu, D. C., Mabesa, R. C., Sellamuthu, R., and Torres, R. (2008). Drought-resistant rice: Physiological framework for an integrated research strategy. In ‘‘Drought frontiers in rice – Crop improvement for increased rainfed production’’ (R. Serraj, J. Bennett, and B. Hardy, Eds.), World Scientific Publishing (in press). Serraj, R., Allen, L. H., and Sinclair, T. R. (1999). Soybean leaf growth and gas exchange response to drought under carbon dioxide enrichment. Glob. Chang. Biol. 5, 283–291. Sharma, P. K., Bhushan, L., Ladha, J. K., Naresh, R. K., Gupta, R. K., Balasubramanian, B. V., and Bouman, B. A. M. (2002). Crop water relations in rice– wheat cropping under different tillage systems and water management practices in a marginally sodic, medium-textured soil. In ‘‘Water-wise Rice Production. Proceedings of the International Workshop on Water-Wise Rice Production,’’ (B. A. M. Bouman, H. Hengsdijk, B. Hardy, T. P. Toung, and J. K. Ladha, Eds.), Los Bano˜s, Philippines. 11 April 2002, pp. 223–235. Sharma, G., Patil, S. K., Buresh, R. J., Mishra, V. N., Das, R. O., Haefele, S. M., and Shrivastava, L. K. (2005). Rice establishment method aVects nitrogen use and crop production of rice–legume systems in drought-prone eastern India. Field Crops Res. 92, 17–33. Sharma, P. C., and Singh, R. K. (2008). Approaches to enhance salt tolerance in crop plants: Progress and prospects. In ‘‘Crop Improvement: Strategies and Applications’’ (R. C. Setia, H. Nayyar, and N. Setia, Eds.), pp. 179–205. I.K. International Publishing House Pvt. Ltd., New Delhi. Sheehy, J., Elmido, A., Centeno, G., and Pablico, P. (2005). Searching for new plants for Climate Change. J. Agric. Meteorol. 60, 463–468.

120

R. Wassmann et al.

Sheoran, I. S., and Saini, H. S. (1996). Drought-induced male sterility in rice: Changes in carbohydrate levels and enzyme activities associated with the inhibition of starch accumulation in pollen. Sexual Plant Reprod. 9, 161–169. Singh, A. K., Choudhury, B. U., and Bouman, B. A. M. (2002). EVects of rice establishment methods on crop performance, water use, and mineral nitrogen. In ‘‘Water-Wise Rice Production’’ (B. A. M. Bouman, H. Hengsdijk, B. Hardy, P. S. Bindraban, T. P. Tuong, and J. K. Ladha, Eds.), pp. 237–246. International Rice Research Institute, Los Banos, Philippines. Sombilla, M. A., Rosegrant, M. W., and Meijer, S. A. (2002). Along-term outlook of rice supply and demand balances in South, Southeast and East Asia. In ‘‘Developments in the Asian Rice Economy’’ (B. Sombilla, Hossain, and M. Hardy, Eds.), pp. 291–316. IRRI, Los Ban˜os, Philippines. Song, Z. P., Lu, B. R., and Chen, J. K. (2001). A study of pollen viability and longevity in Oryza rufipogon, O. sativa and their hybrids. IRRN 26, 31–32. Stitt, M. (1991). Rising CO2 levels and their potential significance for carbon flow in photosynthetic cells. Plant Cell and Environment 14, 741–762. Svensson, J., Ismail, A. M., Palva, T. E., and Close, T. J. (2002). ‘‘Cell and Molecular Responses to Stress,’’ Elsevier Science, Amsterdam. Tabbal, D. F., Bouman, B. A. M., Bhuiyan, S. I., Sibayan, E. B., and Sattar, M. A. (2002). On-farm strategies for reducing water input in irrigated rice; case studies in the Philippines. Agric. Water Manage. 56, 93–112. Takeoka, Y., Al Mamun, A., Wada, T., and Kaufman, P. B. (1992). Primary features of the effect of environmental stress on rice spikelet morphogenesis. Reproductive adaptation of rice to environmental stress. “Developments on Crop Science,” Vol. 22, Chapter 5, pp. 113–141. Japan Scientific Societies Press, Elsevier, Tokyo. Tanaka, K., Kasai, Z., and Ogawa, M. (1995). Physiology of ripening. In ‘‘Science of the Rice Plant. Vol. 2. Physiology’’ (T. Matsuo, K. Kumazawa, K. I. Ishii, and H. Hirata, Eds.), pp. 07–118. Food and Agriculture Policy Research Center, Tokyo. Tang, R. S., Zheng, J. C., Jin, Z. Q., Zhang, D., Huang, H., and Chen, L. G. (2008). Possible correlation between high temperature-induced floret sterility and endogenous levels of IAA, GAs and ABA in rice (Oryza sativa L.). Plant Growth Regul. 54, 37–43. Tao, F., Yokozawa, M., Zhang, Z., Hayashi, Y., Grassl, H., and Fu, C. (2004). variability in climatology ang agricultural production with the East Asia summer monsoon and El Nin˜o South Oscillation, clim. Res. 28, 23–30. Tester, M., and Davenport, R. (2003). Na+ tolerance and Na+ transport in higher plants. Ann. Bot. 91, 503–527. Thomson, M., Septiningsih, E., Suwardjo, F., Santoso, T., Silitonga, T., and McCouch, S. (2007). Genetic diversity analysis of traditional and improved Indonesian rice (Oryza sativa L.) germplasm using microsatellite markers. Theor. Appl. Genet. 114, 559–568. Toojinda, T., Siangliw, M., Tragoonrung, S., and Vanavichit, A. (2003). Molecular genetics of submergence tolerance in rice: QTL analysis of key traits. Ann. Bot. 91, 243–253. Travasso, M. I., Magrin, G. O., Baethgen, W. E., Castao, J. P., Rodriguez, G. R., Rodriguez, R., Pires, J. L., Gimenez, A., Cunha, G., and Fernandes, M. (2006). Adaptation measures for maize and soybean in Southeastern South America. Working Paper No. 28, Assessments of Impacts andAdaptations to Climate Change (AIACC), 38 pp. Tripathy, J. N., Zhang, J., Robin, S., Nguyen, T. T., and Nguyen, H. T. (2000). QTLs for cell-membrane stability mapped in rice (Oryza sativa l.) under drought stress. Theor. Appl. Genet. 100, 1197–1202. Tsukaguchi, T., and Yusuke, L. I. D. A. (2008). Effects of assimilate supply and high temperature during grain filling period on the occurrence of various types of chalky kernels in rice plants (Oryza sativa L.). Plant Prod. Sci. 11, 203–210.

Climate Change Affecting Rice Production

121

Uphoff, N. (2007). Agroecological alternatives: Capitalizing on existing genetic potentials. J. Dev. Stud. 43, 218–236. Vara Prasad, P. V., Craufurd, P. Q., Kakani, V. G., Wheeler, T. R., and Boote, K. J. (2001). Influence of temperature during pre- and post-anthesis stages of floral development on fruit-set and pollen germination in groundnut (Arachis hypogaea L.). Aust. J. Plant Physiol. 28, 233–240. Vara Prasad, P. V., Craufurd, P. Q., and Summerfield, R. J. (1999). Fruit number in relation to pollen production and viability in groundnut exposed to short episodes of heat stress. Ann. Bot. 84, 381–386. Venuprasad, R., Sta Cruz, M. T., Amante, M., Magbanua, R., Kumar, A., and Atlin, G. N. (2008). Response to two cycles of divergent selection for grain yield under drought stress in four rice breeding populations. Field Crops Res. 107, 232–244. Vergara, B. S., and Mazaredo, A. (1975). Screening for resistance to submergence under greenhouseconditions. In ‘‘Proceeding of the International Seminar on Deepwater Rice’’ pp. 67–70. Bangladesh Rice Res. Inst., Dhaka. Wade, L. J., McLaren, C. G., Quintana, L., Harnpichitvitaya, D., Rajatasereekul, S., Sarawgi, A. K., Kumar, A., Ahmed, H. U., Sarwoto, A. K., Singh, A. K., Rodriguez, R., Siopongco, J., et al. (1999). Genotype by environment interactions across diverse rainfed lowland rice environments. Field Crops Res. 64, 35–50. Wahid, A., and Close, T. J. (2007). Expression of dehydrins under heat stress and their relationship with water relations of sugarcane leaves. Biol. Plant. 51, 104–109. Wahid, A., Gelani, S., Ashraf, M., and Foolad, M. R. (2007). Heat tolerance in plants: An overview. Environ. Exp. Bot. 61, 199–223. Warrag, M. O. A., and Hall, A. E. (1984). Reproductive responses of cowpea (vignaunguiculata (l) walp) to heat-stress.1. responses to soil and day air temperatures. Field Crops Res. 8, 3–16. Wassmann, R., Hien, N. X., Hoanh, C. T., and Tuong, T. P. (2004). Sea level rise affecting the Vietnamese Mekong Delta: Water elevation in the flood season and implications for rice production. Clim. Change 66, 89–107. Webber, A. N., Nie, G. Y., and Long, S. P. (1994). Acclimation of photosynthetic proteins to rising atmospheric co2. Photosynthesis Res. 39, 413–425. Weerakoon, W. M. W., Maruyama, A., and Ohba, K. (2008). Impact of humidity on temperature-induced grain sterility in rice (Oryza sativa L). J. Agron. Crop Sci. 194, 135–140. Widodo, W., Vu, J. C. V., Boote, K. J., Baker, J. T., and Allen, L. H. (2003). Elevated growth CO2 delays drought stress and accelerates recovery of rice leaf photosynthesis. Environ. Exp. Bot. 49, 259–272. Xu, K., and Mackill, D. J. (1996). A major locus for submergence tolerance mapped on rice chromosome 9. Mol. Breed. 2, 219–224. Xu, K., Xu, X., Fukao, P., Canlas, R., Maghirang-Rodriguez, R., Heuer, S., Ismail, A. M., Bailey-Serres, J., Ronald, P. C., and Mackill, D. J. (2006). Sub1 A is an ethyleneresponse-factor-like gene that confers submergence tolerance. Nature 442, 705–708. Xu, K., Xu, X., Ronald, P. C., and Mackill, D. J. (2000). A high-resolution linkage map of the vicinity of the rice submergence tolerance locus. Sub1. Mol. Gen. Genet. 263, 681–689. Yadav, R., Flowers, T. J., and Yeo, A. R. (1996). The involvement of the transpirational bypass flow in sodium uptake by high- and low-sodium-transporting lines of rice developed through intravarietal selection. Plant Cell Environ. 19, 329–336. Yamada, I. (1964). Studies on artificial germination and germination physiology of rice pollen. Ph.D. Thesis Kyoto University, Kyoto. pp. 1–122 Yamaguchi, T., and Blumwald, E. (2005). Developing salt-tolerant crop plants: Challenges and opportunities. Trends Plant Sci. 10, 615–620.

122

R. Wassmann et al.

Yamakawa, H., Hirose, T., Kuroda, M., and Yamaguchi, T. (2007). Comprehensive expression profiling of rice grain filling-related genes under high temperature using DNA microarray. Plant Physiol. 144, 258–277. Yang, X., Bouman, B. A. M., Wang, H., Wang, Z., Zhao, J., and Chen, B. (2005). Performance of temperate aerobic rice under different water regimes in North China. Agricultural Water Management 74, 107–122. Yeo, A. R., and Flowers, T. J. (1983). Varietal differences in the toxicity of sodium-ions in rice leaves. Physiol. Plant. 59, 189–195. Yeo, A. R., and Flowers, T. J. (1986). Salinity resistance in rice (oryza-sativa-l) and a pyramiding approach to breeding varieties for saline soils. Aust. J. Plant Physiol. 13, 161–173. Yeo, A. R., Yeo, M. E., Flowers, S. A., and Flowers, T. J. (1990). Screening of rice (oryzasativa-l) genotypes for physiological characters contributing to salinity resistance, and their relationship to overall performance. Theor. Appl. Genet. 79, 377–384. Yoshida, S., Satake, T., and Mackill, D. (1981). High temperature Stress. IRRI Res. Pap. 67, 1–15. Zhang, H. X., and Blumwald, E. (2001). Transgenic salt-tolerant tomato plants accumulate salt in foliage but not in fruit. Nat. Biotechnol. 19, 765–768. Zhang, H. X., Hodson, J. N., Williams, J. P., and Blumwald, E. (2001). Engineering salttolerant Brassica plants: Characterization of yield and seed oil quality in transgenic plants with increased vacuolar sodium accumulation. Proc. Natl. Acad. Sci. USA 98, 12832–12836. Zhu, J. K. (2003). Regulation of ion homeostasis under salt stress. Curr. Opin. Plant Biol. 6, 441–445. Zhu, Q. H., Ramm, K., Shivakkumar, R., Dennis, E. S., and Upadhyaya, N. M. (2004). The ANTHER INDEHISCENCE1 gene encoding a single MYB domain protein is involved in anther development in rice. Plant Physiol. 135, 1514–1525. Zhu, C., Xiao, Y., Wang, C., Jiang, L., Zhai, H., and Wan, J. (2005). Mapping QTL for heat-tolerance at grain filling stage in rice. Rice Sci. 12, 33–38. Ziska, L. H., and Manalo, P. A. (1996). Increasing night temperature can reduce seed set and potential yield of tropical rice. Aust. J. Plant Physiol. 23, 791–794. Ziska, L. H., Weerakoon, W., Namuco, O. S., and Pamplona, R. (1996). Influence of nitrogen on the elevated CO2 response in field-grown rice. Aust. J. Plant Physiol. 23, 45–52.

C H A P T E R

T H R E E

Nitrogen in Dryland Soils of China and Its Management S. X. Li,* Z. H. Wang,* T. T. Hu,* Y. J. Gao,* and B. A. Stewart† Contents 1. Introduction 2. Contents and Distribution of N in Dryland Soils 3. Ways for N Loss and Gain in Dryland Areas 3.1. Mineral N loss by volatilization 3.2. Mineral N loss by denitrification 3.3. Mineral N loss by leaching 3.4. Mineral N gains from wet deposition 4. Rational Application of N Fertilizer to Dryland Soils 4.1. Application of N fertilizer with OF 4.2. Application of N fertilizer with P fertilizer 4.3. Deep application of N fertilizer 4.4. Timing of N fertilization 4.5. Determination of N rate based on soil N-supplying capacity 4.6. Choice of suitable form of N fertilizer for different crops 5. Strategies for Managements of Soil N on Drylands 5.1. Adequate supply of N fertilizer 5.2. Crop rotation with legumes 5.3. Full use of biological materials as nutrient sources 5.4. Improving crop health for better use of nutrients 6. Conclusions Acknowledgments References

126 131 134 135 136 136 137 137 138 139 142 146 149 158 164 165 165 166 167 169 170 170

Abstract As an essential nutritional element, nitrogen (N) plays extremely important roles in agriculture. It is needed by plants in large amounts while almost all soils worldwide are deficient in this element, resulting in a great gap between its demand and its supply from soil. Application of N fertilizers has strongly

* {

Northwestern Science and Technology University of Agriculture and Forestry, Yangling, Shaanxi 712100, P.R. China Dryland Agriculture Institute, West Texas A&M University, Canyon, TX 79016, USA

Advances in Agronomy, Volume 101 ISSN 0065-2113, DOI: 10.1016/S0065-2113(08)00803-1

#

2009 Elsevier Inc. All rights reserved.

123

124

S. X. Li et al.

helped promote agricultural development for supplying food to man and fodder to animals, and enabled the enormous and unprecedented expansion of the global human population. More than 55% of the increase of crop production in developing countries is from the use of chemical fertilizers with N fertilizers being dominant. China has consumed about 30% of the N fertilizer in the world and this is one of the reasons for the Chinese success to feed 21.8% of the world’s population with only 6.8% of the world arable land. However, nitrogen is also an important pollutant. Inputs of large amounts of N fertilizer have brought about many malpractices: low N use efficiency and low N fertilizer recovery have led to low economic returns, and the processes of N behavior such as ammonia volatilization, wet and dry deposition of the volatilized ammonium N to lands and waters, eutrophication in rivers, lakes and sea mouths, nitrate N formation by nitrification and nitrate N leaching, nitrate-N accumulation in water and plants (especially in vegetables), and transformation of nitrate N into N2O, NO, and N2 by denitrification. These processes have exerted a great impact on environmental pollution, ecosystem deterioration, and biodiversity as well as on human health. This situation not only affects agricultural outcome at the present, but will have a major impact on agricultural development in the future. Drylands are a large part of agricultural production areas in China, and the arable lands constitute a large proportion of the total cultivated lands. With the growth of population and the decrease of land and water resources, the drylands become more and more important for the Chinese agricultural development and sustainability. Due to poor plant growth induced by shortage of water supplies, serious wind and water erosion, and low input of fertilizes by farmers, the arable dryland soils in China are low in organic matter (OM) and thus in total N that varies from a minimum of 0.001% to a maximum of 0.178% depending on soil type and environmental conditions. Thus, N deficiency is a major nutritional constraint for crop production. In addition to wind and water erosion that causes organic N loss, nitrogen loss from dryland soils and fertilizers added to soil is mainly in mineral forms. Ammonia volatilization, and nitrate N accumulation in soil profile in and beyond root zones are the major pathway for N loss while denitrification loss is negligible. Despite N deficiency in most of drylands, excessive addition of N fertilizer has occurred in some places and resulted in great attention to the rational management of soil N. For improvement of nitrogen fertilizer recovery (NFR) and nitrogen use efficiency (NUE) while providing adequate N for crop production, some achievements have been made and various measures have been adopted. Application of N fertilizer with organic fertilizer (OF) can significantly increase the NUE, and at the same time significantly increase crop yield and water use efficiency (WUE). Since N in OF is slowly mineralized, P is too high for plant requirements. Thus, OF should be applied together with N fertilizers, but separately from P fertilizer. In most dryland areas, P deficiency has limited crop production and N fertilizer efficiency, and in some lands even prevented crops from responding to N fertilizer. Combining the use of N fertilizer with P

Nitrogen in Dryland Soils

125

fertilizer can increase NFR and NUE as well as WUE. If N fertilizer is mixed with acid P fertilizer, N loss by volatilization can be reduced. Deep application can place N fertilizer in the layer where more water is available, nutrients are deficient, and more roots are present for efficiently using N from the fertilizer applied and the moisture from the soil in addition to reduction of N loss by volatilization, and thus can increase crop yield, and fertilizer use efficiency. Deep application can be conducted with deep plowing so that N fertilizer can be placed in a suitable layer for plant use. In areas without supplemental irrigation, early application of N fertilizer to wheat and other autumn-sown crops should be encouraged while for maize with full irrigation, N fertilizer should be divided into four portions with one portion applied at sowing, one portion at elongation, and two portions before heading. A crop often obtains up 45%–70% of its total N from the soil. Therefore, N fertilizer applications should be made according to the soil nitrogen supplying capacity (SNSC). Several biological and chemical procedures have been used for evaluating the SNSC, yet none of the methods has proven suitable for agricultural practice. A large number of field results demonstrated that the cumulative amount of nitrate N from the 0 to 1 m soil layer was significantly correlated to crop uptake N with a correlation coefficient of 0.908, giving a very satisfactory index of soil availability. Due to high nitrate N accumulated in soil profile, the potentially mineralizable N, estimated by either incubation or chemical reagent extraction, did not show a good correlation. In contrast, in soils with low amounts of nitrate N accumulated in soil profile, some methods for determining mineralizable N did exert a positive role in reflecting the SNSC, having certain potentials for use. Crop responses to N forms depend on soil pH and plant species. In dryland soils, nitrate N is the major form existing in soil and due to high pH buffering capacity, it is also the major form taken up by plants, and the major crops, wheat and maize, have responded better to nitrate N than to ammonium N. Since ammonium-based N fertilizer, particularly urea, is the major form produced by industrial processes, enhancing nitrate nutrition may be difficult for some countries or some regions. For solving this problem, rapid nitrification of ammonium N may be a solution. This can be achieved by pretreating the soil with a small quantity of NH4þ-salt before a large amount of urea is applied. For sustainable agriculture and eliminating N fertilizer pollution of the environment, different strategies have been proposed. Roughly, two ways are noted: agricultural and industrial. The former is improvement of crop growing conditions for efficient use of the N fertilizer whereas the latter improvement of N fertilizer characteristics. Of these, agricultural strategies are fundamental and basic. Applying adequate rates of N fertilizer, rotating legumes in cropping sequences, using organic materials in combination with chemical fertilizers, and improving crop health for better use of nutrients are some important aspects for consideration in the future.

126

S. X. Li et al.

1. Introduction Of plant nutritional elements, N may be the most important. As an essential element, it is used by living organisms to produce a number of complex organic molecules such as amino acids, proteins, and nucleic acids that play extremely important roles in their lives (Mengel and Kirkby, 2001), and it is needed by plants in large amounts. In contrast, almost all of soils worldwide are deficient in N supply, and there is a great gap between its demand and supply. For much of human history, N supply limited crop production, and still does in many subsistence and low-input farming systems. Since the beginning of the twentieth century, the use of N fertilizer produced industrially by chemical reduction of atmospheric (gaseous) N into forms that can be directly used by plants has increased massively in importance. Between 1900 and 1960 N fertilizer use increased from 1 to 10 Mt, then to 15 Mt in 1966 and to 50 Mt in 1977, reaching 80 Mt in 1990 (Bock and Hergert, 1991; Bumb, 1995). From 1966 to 1977, developed (industrial) agricultural countries increased fertilizer use by 70%, but developing (less industrial) countries by 200%. In 2006, N fertilizer use reached 101 Mt (FAO, 2008). The application of chemically fixed N has strongly helped promote agricultural development for supplying food to man and fodder to animals (Evans, 1998), enabling the enormous and unprecedented expansion of the global human population (currently 6.6 billion). Although adequately fed in the main (Bacon, 1995; Evans, 1998), many people in the developing world are still undernourished, particularly with respect to proteins. World population is expected to reach 10 billion or even 12 billion (Bumb, 1995) in the new epoch of world history. It is estimated by FAO that more than 55% of the increase of crop production in developing countries is from the use of chemical fertilizers, and N fertilizers play a dominant role in this aspect. Such a situation is particularly true for China. China is one of the largest and most important agricultural countries in the world; its total production of chemical fertilizers has already reached 22% of the world and its total consumption reached 25% (FAO, 2005). Of the chemical fertilizers, N fertilizer makes up 60–70%. In 1961, N fertilizer used in China was only 5% of the world total. However, in 1980, it had increased to 20%. China consumed 1.74  107 Mg N of chemical fertilizer in 1990 (ECCAY, 1991) and 2.22  107 Mg N in 1995 (ECCAY, 1996), accounting for more than 25% of the world total. Since 1996, N fertilizer use in China has exceeded 25  106 Mg (FAO, 2001), accounting for more than 70% of the total chemical fertilizers in China and about 30% N in the world. China was the leading country in the world in both production and consumption of N fertilizer. The crop yield increase was closely related with chemical

Nitrogen in Dryland Soils

127

fertilizer input, especially N fertilizer (Li and Lin, 1995). In 1949, the grain production in China was 11,318  104 Mg while in 1995, it was 46,500  104 Mg. In 1949, cotton yield was 44.4  104 Mg while in 1995, it was 450  104 Mg. Experts estimate that for sustainable crop yields, nutrients supplied by chemical fertilizers should be kept greater than 50% of the total supply, and N fertilizer should occupy 65–70% of the chemical fertilizer (Li et al., 1997). However, inputs of large amounts of N fertilizer have brought about many malpractices: low use efficiency (defined as yield produced per unit N, i.e., = yield per N uptake) and low N fertilizer recovery (NFR) (defined as N uptake from N fertilizer per N input  100%) resulting in low economic returns constitute a major issue, in addition to environmental pollution and ecosystem deterioration (Hong and Huang, 1994; Keeney, 1982). This situation not only affects agricultural outcome at the present, but will have a major impact on agricultural development in the future. The investment of chemical fertilizer occupies a large component of cost of agricultural production in the world, and wheat, maize, and rice are the major crops for consumption of chemical fertilizers. For cereal crops, the average recovery of N fertilizer in the world was estimated to be 33%, far less than the 50% generally reported (Hardy and Havelka, 1975). Based on such an estimation, a 1% increase in NFR for cereal production worldwide would lead to a $234,658,462 savings in N fertilizer costs, and an increase in NFR of 20% would result in a savings in excess of $ 4.7 billion per year (Raun and Johnson, 1999). In China, fertilizer cost is the most expensive input and accounts for 50% of the total crop production costs including seed, fertilizer, pesticides, machinery, and irrigation (Lin et al., 1999). The low N recovery has caused great economic loss. Since 1961, chemical fertilizer, particularly N, increased 15-fold, while average cereal grain yields in China increased only fivefold. From 1984 to 1994, chemical fertilizer usage increased 93.6% whereas total grain yields increased 27%. Such a difference between the fertilizer increase and the grain yield increase has become even greater since the 1980s (Ye and Rozelle, 1994). According to long-term experimental results from the National Fertilizer Experiment Network and those from the Institute of Soil Science, Chinese Academy of Sciences, the current NFR by plants is only 30–35%, much lower than it was 20 years ago, and the trend is downward. When the late 1950s are compared to the 1980s, the rice yield increase from 1 kg added N was reduced from 15–20 to 9.1 kg, wheat from 10–15 to 10 kg, maize from 20–40 to 13.4 kg (ISF, 1994). At present, China uses about 25,000,000 Mg of N, and if the average N loss is estimated as 45%, the total N lost is 11,250,000 Mg having a value of about 34.2 billion RMB (about $ 4.87 billion). The environmental pollution induced by the low recovery of N fertilizer has attracted great attention. Nitrogen is not only a major nutritional

128

S. X. Li et al.

element, but also an important pollutant as well. When ammonium N fertilizer or urea is applied to soil, especially to calcareous soil, ammonia is easily released and volatilized to the atmosphere. This is not only a significant loss of valuable N fertilizer, but also a contributor to environmental degradation. Though a little amount of ammonia volatilized to the atmosphere may be removed by photodissociation to form NH2 that rapidly reacts with O2 and finally forms H2O, N2, and N2O ( Jayanty et al., 1976), most of it will return by wet and dry deposition to lands and waters close to or around the place where it volatilized because ammonia stays in the atmosphere only a short time and cannot become fully mixed with air and leaves far away from the source areas (Fenn, 1998; Su et al., 2005a). The deposited NHþ 4 –N increases soil available N and benefits plant growth and agricultural production. In a low deposit area, almost all of N deposited (>95%) remain in the soil (Grennfelt and Hultberg, 1986) and play a role as N fertilization. However, this will become a secondary source of N2O and NO and affects the capacity of soil methane sink (Mosier et al., 1998). Further, a large amount of deposited N input can lead to various effects on the eco-environmental systems of forestry, agriculture, and waters (CuestaSantos et al., 2001; Matson et al., 2002; Pryor et al., 2001; Salahi et al., 2001; Skeffington, 1990) such as intensification of N saturation in some terrestrial ecosystem, acidification of soil (Bartnicki and Alcamo, 1989), and decline of biodiversity and ecosystem functioning. Overloading of wet deposited NHþ 4 in terrestrial ecosystems and a high rate of ammonium uptake can also induce imbalanced uptake of cations by plants. The rhizospheric zone could be quickly acidified, and cations such as Kþ, Ca2+ and Mg2+ could become inhibited. In a soil rich in N, the increase of wet deposited N may exceed plant uptake, and some soil NHþ 4 may be nitrified, resulting in high concentration of nitrate N. Nitrate N leaching is a strong process of acidification and can lower soil pH, change the release and migration of soil base cations and Hþ, Al3+, increase Al and Mn activity, decrease soil fertility, reduce tree growth, and damage the forest ecological stability (Bergkvist and Felkeson, 1992). In water systems, the increase of wet deposited ammonium N has further intensified eutrophication in rivers, lakes and sea mouths (Galloway and Cowling, 2002; Vitousek et al., 1993), and caused damages to fishes, other aquatic animal and plants. All this is harmful to plant growth, particularly to forest plants (Matson et al., 2002; Pryor et al., 2001). In the past several years, due to burning a large amount of mineral oil and coal, N fertilization and development of livestock, N compounds emitted to the atmosphere have been rapidly increased, and the wet deposit is proportional to such an increase (Galloway and Cowling, 2002; Vitousek et al., 1993; Wright and Rasmussen, 1998). Therefore, at the same time, wet deposited N is considered as a source for utilization and more concern should be paid to its possible negative effect on the environment.

Nitrogen in Dryland Soils

129

In aerobic dryland soils, when either ammonium fertilizer or urea is applied, most of the N will be transformed into nitrate N through nitrification (Li et al., 1993b). In dryland soils, nitrate N is the major form for plant uptake, and a large portion of nitrate N entering plants is reduced by nitrate and nitrite reductases and participates in a series of metabolic processes, being assimilated to amino acids and finally to proteins. If too much N fertilizer is applied, or the soil has a high capacity to supply N, a large amount of nitrate N may be accumulated in plants. The accumulated nitrate has no detrimental effect on plants, but it may be harmful to human beings as well as to other living organisms, particularly when it accumulates in vegetables (Wang et al., 1998, 2001). Nitrate N can also be leached from topsoil to deep soil layers, and ultimately to groundwater, or it can be carried by runoff to surface waters such as rivers and lakes, thereby polluting the groundwater and surface water supplies (Keeney, 1982; Ma, 1992) that people use for drinking as well as for producing food. It is estimated that 50–60% N originating from chemical fertilizers is not recovered in plants and enters rivers, lakes, and the atmosphere (Li, 1999), being one of the most important sources of environmental pollution. Of 532 rivers in China, 82% was polluted by N at different levels (Zhang et al., 2003); 92 and 88% of N entering to Yangtze and Yellow rivers annually originate from agriculture, and chemical fertilizer N contributes 50% of it (Zhu et al., 2005). With large amounts of water and N inputs, soil nitrate N leaching to the groundwater becomes a more and more serious issue. Zhang et al. (1995) reported that the groundwater nitrate concentration in some suburban areas of large cities was well correlated with N fertilizer rates. The pollution of surface waters accelerates the process of eutrophication resulting in abundant growth of algae that consumes the oxygen in water and makes it anaerobic that results in aquatic animals and hydrophytes dying and even becoming extinct. The pollution of groundwater by nitrate N is more serious, and in many places it has exceeded the standard-drinking water level. During the 1970s, fertilizer N was increased by 6.49%, while nitrate N in groundwater was increased by some 23% in some areas of China. (ISF, 1994). After entering the human body either from water or from vegetables,  nitrate (NO 3 ) can be reduced to nitrite (NO2 ) by bacteria and some enzymes existing in the human digestion system. Nitrite then enters the blood system where it combines with the hemoglobin and oxidizes the Fe2+ present to Fe3+. This prevents the blood system from transferring oxygen. This condition is known as methemoglobinemia induced by deficiency of oxygen. This is particularly hazardous for babies and there is a concern that with very young infants, drinking water containing high nitrate concentrations can give rise to what is known as the blue baby syndrome, being assumed to be a form of nitrate poisoning. Also, nitrate can cause abnormal development and thyroid gland disease (Rockman and Granli, 1991; Zhu, 1995). More seriously, nitrite can form nitrosamine and nitrosamide,

130

S. X. Li et al.

well-known cancer-causing compounds by reacting with amines and amides (Choi, 1985; Gangolli et al., 1994; Minotli, 1978; Walker, 1990). At present, of 120 nitrosamines and nitrosamides, 90 of them have been identified as cancer-causing compounds (Chen et al., 1989). It gives rise to increasing incidence of stomach cancers in the human beings. Investigation in 11 countries of the world has shown that the occurrence rate of gastric cancer of human is well correlated with the daily intake of nitrate amount (Fine, 1982). For these reasons, the World Health Organization has proposed a series of standards for maximum nitrate levels allowed in drinking water. The European standards introduced in 1970 recommended a maximum level of 50 mg L1 nitrate but considered the range 50–100 mg NO3 L1 as acceptable. Levels of over 100 mg NO3 L1 were not recommended for drinking water. The European Community directive on quality of water for human consumption introduced in 1980 was more stringent. This has a maximum acceptable level of 50 mg NO3 L1 in drinking water and a guideline of 25 mg NO3 L1 that should not be exceeded if possible. For the same reasons, many countries have also set up nitrate concentration limits for vegetables. In Germany, a maximum of 791 mg nitrate N kg1 is set as being acceptable for fresh spinach and 791–1017 mg nitrate N kg1 for fresh lettuce (Schwemmer, 1990). In 1997, the European Union officially established maximum limits of nitrate for some vegetables such as spinach and lettuce (Santamaria et al., 1998). In China, Shen et al. (1982) proposed 700 mg nitrate N kg1 as a maximum limit for leafy and root vegetables. Although recently there have been greatly different viewpoints about the detrimental effect of nitrate on human health (Addiscott and Benjamin, 2004; L’hirondel and L’hirondel, 2001), the presence of too much nitrate in water and vegetables will be a problem (Li, 1999). The nitrate N can be transformed into N2O, NO, and N2, a process known as denitrification, in an anaerobic condition with adequate organic matter (OM) as energy source. Nitrogen fertilizer and organic fertilizer contribute 55–80% to the total N2O emission (OECD, 1998). All gases produced by denitrification can be emitted to the atmosphere. Since N2O is a greenhouse gas, its emission can increase global warming and intensify damage to the ozone layer. This will induce more problems for the environment as well as for human beings existence. The dryland is a large part of agricultural production areas in China, and the arable lands constitute a large proportion of the total cultivated lands. It has made a great contribution to China’s agriculture and will make an increasingly important contribution to the Chinese agricultural development and sustainability. However, most dryland soils are deficient in N, and application of N fertilizer has been regarded as a priority for supplying an adequate nutrient. As a result, agriculture development in dryland areas is closely associated with the use of N fertilizer as in other regions, and N fertilizer has an indissoluble bond with agriculture. Due to water deficit

Nitrogen in Dryland Soils

131

being a major issue in the dryland areas, N fertilizer use efficiency and its recovery are much lower than those in other areas. Thus, improvement of N fertilizer efficiency and recovery rate is particularly urgent in dryland areas, and it is of critical significance for the development of dryland agriculture and for the protection of the fragile ecosystems and environment (Li and Wang, 2006; Li and Xiao, 1992). The vicious circle of N has shaken the very foundations of agricultural production, affected environmental quality, and threatened human’s health and existence. This situation will be continued if the circle is not improved. Due to such considerations, emphasis has focused on N processes by governments in all countries of the world, and agronomists, pedologists, environmental scientists, and plant nutrition scientists have given a special concern to this problem. For avoiding negative consequences of N, the key is rational N fertilization based on scientific findings, thus improving N use efficiency (NUE) and N fertilizer recovery (NFR). Only if N, either from N fertilizer, organic fertilizer, soil OM or other sources, can be more fully utilized by crops can the residual N left in soil be reduced, and its ill effects on the environment and ecosystems be eliminated. In this review paper, achievements made in management of dryland soil N are discussed. Although it is not feasible to assume that all the measures discussed can be applied to other areas, the principles might be useful for other regions.

2. Contents and Distribution of N in Dryland Soils In dryland areas, pollution problems caused by N are not serious, and often not even an issue. In contrast, its deficiency is a serious issue for agricultural production. OM is the major source of soil N, and more than about 95% N in soil exists in OM if the mineral-fixed ammonium is not considered. Since in dryland areas, the soil OM is low, even extremely low in some soils, N is thus deficient. Contents of OM and N are different between virgin lands and cultivated lands. As shown in Table 1, virgin lands with natural vegetation that have not suffered greatly from serious erosion and detrimental impact of human activities are high in OM contents and thus high in N. Under such natural vegetation, soil N content mainly depends on the relative degree of biomass accumulation to its decomposition, and hence on the vegetation type and factors affecting plant growth and microorganism activities. Of the factors, water regimes and temperature have the greatest influences on soil OM and soil N content. Seen from a large scale, this is very clear. From east to west in the northern territory of China, with the increase of aridity (defined as evaporation/precipitation),

132

S. X. Li et al.

Table 1 Range of organic matter, nitrogen, and C/N ratios in major soils in arid, semiarid and subhumid areas of northwest China under natural vegetation

Soil type

Dark Chernosem Chestnut Brown rendzina and sierosem Desert White muddy Dark brown earth Drab and Brown earth Yellow brown earth

Range of organic matter (%)

Range of total N (%)

Range of C/N ratio

5.22–13.9 2.62–6.79 1.24–3.65 0.60–1.98

0.256–0.695 0.129–0.431 0.078–0.197 0.040–0.105

11.2–11.9 9.5–13.7 7.2–12.0 6.5–10.6

0.24–0.74 3.03–5.71 5.66–11.9 1.46–2.88

0.028–0.073 0.144–0.346 0.168–0.364 0.060–0.145

6.2–9.1 11.8–13.4 12.1–14.4 9.0–13.5

1.24–2.91

0.060–0.148

9.3–13.1

Modified from Nanjing Institute of Soil Science, Chinese Academy of Sciences (1978).

vegetation becomes sparser and net biomass accumulation becomes less and less. Consequently, the OM and N contents decrease. The decrease is associated with soil types, declining from chernozems in northeast China to chestnut soils, to brown soils, and to sierozem soils in northwest China. Also, OM and N contents gradually decrease southward from dark brown soils to brown soils, drab soils and yellow brown earth due to the gradual increase of decomposition of OM caused by the increase of water and temperature (Table 1). In contrast, as a whole, soil OM and N contents for arable soils are lower than those with natural vegetation (Table 2). With acceleration of mineralization by cultivation, climatic factors exert more impact on the soil degradation through declining the OM and thus soil N. The natural vegetation has been destroyed and sparse precipitation has made it impossible for plants to grow vigorously to produce large amounts of OM accumulation. After cultivation, the soil also becomes susceptible to serious erosion that results in loss of the topsoil and exposes deeper soil layers. The topsoil contains much more OM and N than the deeper soil layers. Due to the accelerated decomposition of OM and mineralization of organic N, the released N is readily lost in different ways such as erosion, leaching and volatilization although part of it has been taken up by plants and soil organisms. Fertilization and biological N fixation (mainly through planting leguminous crops for rotation or for green manure) help to increase N contents in arable lands, yet their amounts still gradually decrease compared to the natural soil. The degree of decline depends mainly on natural conditions and human activities.

133

Nitrogen in Dryland Soils

Table 2 Nitrogen contents (%) in selected arable lands of China

Soil types

Eolian Loessial

Sierozem Chestnut Dark loessial

Drab soil

Irrigated warping

Locations sampled

North Shaanxi North Shaanxi and west Shanxi South Ningxia and south Gansu Ningxia, and middle Gansu North Shanxi and Inner Mongolia South Ningxia and south Gansu Weibei highland Guanzhong Plain and south Shanxi Yellow Riverirrigated area in Ningxia

Soil samples

Range of total N

Mean of total N

23 605

0.001–0.042 0.010–0.089

0.017 0.042

93

0.010–0.090

0.052

50

0.026–0.117

0.057

56

0.040–0.120

0.072

165

0.044–0.128

0.077

55

0.059–0.178

0.064

Modified from Peng and Peng (1982).

Because of different intensity of the decline, OM varies from one place or one soil to another. As a tendency, N contents decrease from southeast to northwest almost in the same pattern as that under the natural vegetation. In the arid area, extremely low precipitation together with serious wind erosion has made plants growth badly poor and soil OM extremely low: total N contents range from a minimum of 0.01% to a maximum of 0.05%, and even it is too low to be detectable in some lands. In the semiarid areas, the soil N content increases as precipitation increases, plants grow better, and input of fertilizer becomes higher. All of this has led to a relatively high accumulation of OM. Nevertheless, N is still low, varying from 0.02 to 0.12%, and insufficient to meet the crop demand. In subhumid areas, despite the soil OM being increased with precipitation and nutrient input, soil N still lies in a low range between 0.05 and 0.17% with an average around 0.07%. The distribution of N in different areas under natural conditions and human activities can be more clearly demonstrated by soils in northwest China. Located in the temperate zone of the subhumid area, the drab soil formed under deciduous trees and forest-steppe vegetation is fertile and lies along the Fehe River and Weihe River valley plains in the southeast of Shanxi and Shaanxi provinces. By application of organic fertilizer (OF) over thousands of years, the N content of the soil is high, averaging about 0.077%.

134

S. X. Li et al.

Next to this area toward the northwest is a semiarid area with foreststeppe vegetation that includes central Shanxi, north Shaanxi, east Gansu, and south Ningxia. In this area, dark loessial, manural loessial, and loessial soils are widely distributed. The dark loessial soil occurs on relatively flat land with slight water erosion, and thus the soil profile is relatively complete, and N content is high, averaging 0.072%. On the other hand, in the hilly area that has suffered from serious water erosion, top dark loessial soil has been eroded again and again, and newly formed loessial soil has appeared on loess parent material. This soil is very similar to its parent material with little fertilizer being applied, and thus it is low in N content averaging 0.042%. To the northeast of this area, the temperate semiarid area is encountered where the land is flat, but low precipitation (around 400 mm) and low temperature have led to a short growing season for plants, accumulation of OM and N has been limited. Consequently, the dominant chestnut soil contains 0.057% total N. In the arid area with precipitation of 200–400 mm, the shallow sierozem contains 0.052% N due to low OM caused by poor vegetation. In the area furthest to northwest, the situation becomes further serious. Located in the desert-steppe zone with an extreme dry climate and much sparse vegetation, the soils, mainly brown earth and eolian sandy soils, contain only 0.017% total N, the lowest in the arid zone. In some cases, human impacts may play a major role on the N content of soil. A typical example is the Hetao area in the arid desert-steppe zone. The Yellow River water has been used to irrigate the land and careful cultivation and optimum fertilization have been practiced. As a result, a fertile soil (known as an ‘‘irrigated warping soil’’) has been formed by silt deposition from irrigation water. This soil contains 0.064% total N, the highest in the arid zone (Zhao, 1990). Due to such situations as discussed, N deficiency has been found nearly everywhere on the drylands. Deficiencies are much more common than excesses, and improvement of the N supply to plants is essential for raising crop production (Zhao, 1990). However, this does not mean that there are no problems regarding too much N in some soils. In fact, such matters have occurred in some places and for some cash crops, and this has attracted people’s attention to environmental concerns (SAADS, 1979). The nitrate pollution of groundwater and ammonium volatilization to the atmosphere will become more serious with increasing use of N fertilizers.

3. Ways for N Loss and Gain in Dryland Areas With an exception of water and wind erosion that is a great disaster not only causing both mineral and organic N loss (Tang and Kong, 1989) but also the land degradation and even land vanishing, the N loss by itself

Nitrogen in Dryland Soils

135

occurs mainly in a mineral form, and the organic forms can not be lost until it transforms into an inorganic form. For the mineral N, the ammonia N volatilization, nitrate N leaching, and N2O or N2 emission during nitrification and denitrification processes are the major pathways for N loss from soil and fertilizer.

3.1. Mineral N loss by volatilization Although N can lose by volatilization from aboveground plants, the amount is rather small compared to its loss from soil (Li, 1992; Li and Li, 1992; Li et al., 1992a; Tian and Li, 1992; Wang and Li, 2003). Most of the dryland soils are calcareous with high contents of calcium carbonate and high pH, and urea and ammonium-based N fertilizers are the major forms used in China. Consequently, the ammonia volatilization from soils and applied N fertilizers becomes the first problem for N loss compared with other process (Aggarwal and Praveen, 1994; Christianson et al., 1990; Katyal et al., 1987; Roelcke et al., 2002). It is reported that in such soils, ammonia volatilization reached 30–32% of the applied urea N in Chins’ drylands (Zhang et al., 1992) while 55% of the added N being lost in such a way was found in other country (Al-Kanani et al., 1990). Many factors such as climatic conditions, soil properties and fertilizers used, affect ammonia volatilization. It has been proved that the ammonia volatilized was mainly controlled by temperature (Zhao et al., 1983, 1986) and soil properties (pH, CaCO3, CEC, texture, and soil water content) exerted a great effect on its process. The influence of CaCO3 on ammonia volatilization was not caused by CaCO3 itself, but by pH; and the apparent correlation between CaCO3 content and ammonia volatilization was due to the interrelation between CaCO3 and pH (Duan et al., 1990; Li and Liu, 1993). The N volatilized from fertilizers is closely related with applied methods, N forms and rates (Li and Ma, 1993; Li and Wang, 1993b). The clay particle less than 2 mm provides 80% of the negative charges for cation adsorption, and therefore the increase of clay content will correspondingly increase the negative charges as well as ammonium adsorption (Yu, 1976), and thereby will reduce the ammonia volatilization. Jewitt (1942) and Wahhab et al. (1956) found that the loss of ammonia volatilization in dryland soils was partly determined by soil water content. Different forms of N fertilizer have different stability for volatilization. Zhao et al. (1983, 1986) measured the magnitude of ammonia loss on a field 5 days after application of different N fertilizers and found that the loss order was as follows: (NH4)2CO3 (50.9%) > (NH4)2SO4 (17.7%) > NH4NO3 (4.34%) > NH4Cl (2.34%) > Urea (1.74%). Pot experiments by Li (2007) showed that over 31 days, ammonium bicarbonate lost 27.9% of applied N while urea and ammonium sulfate lost 18.6 and 22.4%, respectively. In a field experiment, we found that at the beginning, the ammonia volatilization from urea was small as it was not transformed into ammonium. After a few days when

136

S. X. Li et al.

it was hydrolyzed and transformed into ammonium, the volatilized amount became larger than it was before. As a whole, the long-term measurement shows that the loss of N by volatilization from urea was also high.

3.2. Mineral N loss by denitrification It is estimated that in China, N loss through the emission of N2O by denitrification and nitrification can be as high as 70% of the total emission from N fertilizers in agricultural fields (Xing and Yan, 1999). However, in the dryland areas, low carbon substrate and low rainfall greatly constrain the emission.Ding and Cai (2001) measured N2O emission from original earth column, and concluded that the gaseous loss and N2O emission from fluvo-aquic soils with maize grown were only 3 kg N ha1 yr1 or 2% of the N applied. In the Guanzhong Plain, Liang et al. (2002a,b,c,d, 2003a,b), adopting balanced chamber, close chamber and acetylene blockage techniques, systematically studied the time, placement and amount of N2O from soils derived from loess parent materials, and investigated the factors (water, fertilization, concentration of C and N, and crop managements) affecting the emission of N2O. Their results showed that in such soils, there exited N loss from denitrification. With soil water contents of 70 and 90% of field capacity, the amounts of N2O emission from ammonium N fertilizer were higher than those from nitrate N fertilizer. Both the flux and concentration of N2O in soil were higher after irrigation and rainfall. Water content had no significant effect on N2O emission from ammonium N fertilizer while significantly increased its emission from nitrate N fertilizer. In the maizewinter wheat rotation system, the emission of N2O was higher in the maize grown season than in the wheat-grown season. High rates of N, P, and K together with an application of organic fertilizer significantly promoted denitrification and N2O emission. The amount of N2O emission was higher when soil was changed from wet to dry state than when from dry to wet state. The maximum flux of N2O was found at the layer from 60 to 90 cm where the claying horizon existed. However, in any case, the emitted amount of N2O was small, only 0.8–1.5 kg N ha1 annually. We used closed box to gather the N2O from a wheat field after 1 month of wheat seeding with a large amount of N fertilizer as basal fertilization, and found that the N2O concentration (0.37 mL L1) in the box was almost the same as the ambient air (0.34 mL L1). All this shows that denitrification is not a major way for N loss in the dryland areas.

3.3. Mineral N loss by leaching The nitrate N leaching is a particular problem on cultivated agricultural lands and it is often the most important channel of N loss from field soils. Depending on precipitation, the nitrate N leaching is different from one place to another in the dryland areas. In some subhumid regions with relatively high precipitation or/and with supplemental irrigation, seasonally periodic water infiltration beyond 3 m soil layer was found (Li, 1989), and the water in wells

Nitrogen in Dryland Soils

137

contains a large amount of nitrate N, providing evidence of nitrate N leaching. The summer concentrated precipitation and the vertical small openings of the soils help water downward movement and therefore promote nitrate N leaching. Using 2-m depth as a plant-absorbing N layer criterion, Li et al. (1995a,b,c) demonstrated in fields and lysimeters that the nitrate leaching varied with seasons and years, normally occurring in rainy seasons and wet years with amounts varying from a few to several, even to more than 100 kg nitrate N ha1 yr1, due mainly to N fertilizer rate, crop types and precipitation. However, as a whole, nitrate N leaching is not a serious problem in drylands of China since low precipitation is a limiting factor for such a loss, and in most cases, nitrate N is accumulated in soil profile in a large amount. The accumulated nitrate can be used by the following crops if properly managed, and it also may have a risk to leach and can be gradually leached year by year out of 2 m depth, but the process is very slow.

3.4. Mineral N gains from wet deposition Mineral N can be gained from precipitation. In south China, the amount of N from wet deposition varied from 16.5 to 35.0 kg ha1 yr1 (Gong, 1992; Zhang and Gong, 1987). In dryland areas, Li et al. (1993a) selected a typical dryland-representing location without factories and power stations in Qianxian County, Shaanxi Province to study the mineral N in rainfall for 3 years. Results showed that the mineral N from the wet deposit had a great variation: the amount was 29.7 kg ha1 in 1990, 14.4 kg ha1 in 1991 and 18.5 kg ha1 in 1992. Also, there was a great change in the ratio of nitrate N to ammonium N in the rainfall: it was 0.13 in 1990, 0.39 in 1991, and 0.21 in 1992. Despite the changes in the ratio, the ammonium N was dominant, occupying 3/5–6/7 of the total. From their concentrations in the rainfall, the authors concluded that the wet deposit was a result of dry and wet deposits, the nitrate N from rainfall mainly came from nitrate-bearing soil particles or dust blown by winds while the ammonium N was clearly related with the volatilization of applied fertilizer as well as soil dust particles that contained ammonium N. The sharp decrease of both ammonium and nitrate concentrations with the soil dust content decrease in the deposit provided evidence for such a conclusion. This conclusion has been supported by other investigators (Su et al., 2005a,b) in another location.

4. Rational Application of N Fertilizer to Dryland Soils Since N deficiency occurs widely in the dryland soils and most frequently limits crop production; application of N fertilizer is regarded as the highest priority in nutrient management. Application of N fertilizer has significantly increased crop production above the levels reached by applying

138

S. X. Li et al.

OF and planting leguminous green manure (LGM) or crops. There is no question whether one should apply N fertilizer or not; the question is how one can use it effectively and how much N fertilizer should be applied to different lands for different crops?

4.1. Application of N fertilizer with OF At present, the NUE and NFR are not high, and one important reason for this may be the imbalance of macronutrients in soil, particularly N and P. In some places, the imbalance of macronutrients to micronutrients also exists. For maintaining the soil’s nutrient balance, application of N fertilizer with OF is the most effective and best way to raise soil fertility and provide nutrients to plants (Chen et al., 1989). Organic fertilizer can provide OM and many nutrients, making soil basic properties greatly improved (Zhang, 1984). A rational utilization of OF can raise soil water-holding capacities, and thus the water use efficiency (WUE). ‘‘Crops can grow very well even under drought or waterlogged conditions when more manure is applied.’’ ‘‘Fertile soil is able to bear drought.’’ These agricultural proverbs reflect farmers’ experiences. Organic fertilizer has long been adopted on the drylands for supplying various nutrients that plants need, and played a great role in maintenance of soil nutrient balance and crop production. Ma (1987) reported that application of pig manure at the rate of 15–45 Mg ha1 significantly increased soil OM, water stable aggregates and aggregate porosity, and decreased bulk density. This, in turn, increased waterholding capacity and WUE. Investigations in Huangling County, Shaanxi Province revealed that each Mg of OF increased wheat production by 6–12 kg and maize by 15–29 kg (Li et al., 1987). Since soil is used as bedding material for livestock manure and night soil in dryland areas, the OF is low in OM and plant nutrients. Analytical data for 30 samples from Guanzhong Plain, Shaanxi Province, show that on the basis of air dry weight, the animal manure contains 3.12% OM, 0.15% total N, and 0.092% total P with a wide range, depending on the amount of soil added (Li and Xiao, 1992). In some places where the bedding soil is less, the OF may contain 4–5% OM, 0.27–0.35% total N, 0.07– 0.183% P, and 0.43–0.77% K (Sun, 1957). Cheng et al. (1987) found that wheat yields as well as precipitation use efficiency (PUE) were increased with increased rates of OF. Without addition of OF, wheat yield was 4.02 Mg ha1, and PUE was 0.56 kg m3 water. After application of 19 and 38 Mg ha1 OF, yields were increased to 6.11 and 6.43 Mg ha1, and PUE was raised to 0.85 and 0.89 kg m3 water, respectively (Table 3). Liang et al. (1987) reported that fertilization with animal manure increased the ability of wheat to absorb soil moisture after flowering, and therefore increased WUE. Although OF has so many advantages for use, the nutrients in it are not present in the proportion required for plant uptake. For instance, N in OF is too low in proportion to P for plant growth requirements. The combined

139

Nitrogen in Dryland Soils

Table 3 The effect of applying organic fertilizers (OF) on the increase of precipitation use efficiency (PUE) of wheat

Treatments

Average wheat yields of 4 years (Mg ha1)

Average PUE (kg m3)

Without fertilization With N (50 kg ha1) With N + 19 Mg OF With N + 38 Mg OF With N + 72 Mg OF With N + 124 Mg OF With N + 248 Mg OF

4.02 4.47 6.11 6.43 6.62 6.67 6.83

0.56 0.62 0.85 0.89 0.92 0.95 0.96

Modified from Cheng et al. (1987).

use of organic and chemical fertilizers is the best way to raise soil fertility and maintain nutrient balance (Chen et al., 1989). For increasing fertilizer efficiencies, OF should go hand in hand with N fertilizer, but separated from P fertilizer (Li and Zhao, 1993a,b,c,d,e). Li (2002) demonstrated the effects of the combined use of OF and N fertilizer by field experiments at five sites. Organic fertilizer was not applied to any of the sites for 2–3 years before the experiments. Results showed that due to deficiency of available P, the effect of N was very low. Wheat yields were increased only 1–4 kg for a kg of added N, and there was even no beneficial effect at some sites. In contrast, when OF was combined with N fertilizer, the efficiency of both OF and N fertilizer was raised, and the yields were higher than when applied separately (Table 4). In another experimental field relatively deficient in N and P, three rates of OF and three rates of N fertilizer were combined to form a complete design (Table 5). Calculations from Table 5 show that without OF, the N fertilizer as an average of three rates increased wheat yield by 1094 kg ha1, and each kg N increased yield by 16.1 kg. In contrast, when OF was applied with N fertilizer, the N fertilizer increased wheat yield by 1247 kg, and each kg N increased yield by 18.3 kg. Similarly, without application of N fertilizer, each Mg OF increased wheat yield by 8.9 kg while with addition of N fertilizer, it increased wheat yield by 10.7 kg (Li, 2002).

4.2. Application of N fertilizer with P fertilizer A deficiency of available P is widespread in various places and in many of the dryland areas, although not as widespread as N. Increased crop yields and the common use of N fertilizer with little or no use of P fertilizers have made soil available P decline even further, resulting in a serious imbalance of

140 Table 4

S. X. Li et al.

The effect of application of organic fertilizer on wheat response to N fertilizer Wheat yield with different N rate (kg ha1)

Field site

Olsen-P (mg kg1)

0

34

68

102

Without application of organic fertilizer 1 5.7 1182 1022 1182 1455 2 8.6 1508 2160 2513 2730 3 10.4 1500 1793 1730 1725 4 14.6 1870 2355 2588 2805 5 4.3 1840 1815 1845 2520 Application of organic fertilizer at rate of 75 Mg ha1 1 11.5 1815 3240 3465 3848 2 14.3 2367 2549 2738 2813 3 13.6 1655 1800 3023 3173 4 15.4 2025 3278 3842 4449 5 13.2 2208 2750 3708 4083 Increase of N fertilizer efficiency by addition of organic fertilizer 1 633 2218 2283 2393 2 859 389 225 83 3 155 97 1293 1450 4 155 923 1254 1644 5 368 935 1863 1563

136

1016 2625 2165 2588 2122 4118 2963 3714 4164

3102 338 1549 1576

Modified from Li (2002).

soil N and P. Since N and P nutrition is closely related, P deficiency greatly limits crop response to N fertilizers, and application of N fertilizer combined with P fertilizers significantly increases crop yields and enhances N efficiency. In a field extremely deficient in available P (Olsen-P 5.7 mg kg1), wheat yield was 1125 kg ha1 without application of fertilizer. Following the application of 135 kg N, the yield was decreased to 975 kg. However, when 20 kg P (45 kg P2O5) ha1 was applied, the yield increased to 2775 kg ha1 and when both N and P were combined with same rate as each alone, the yield was increased to 4493 kg ha1 (Table 6), showing clearly the importance of balanced fertilization (Li et al., 1979, 1978). In most cases, the combined use of N and P fertilizers increases N and P efficiency and effectively regulates the imbalance of N and P in the soil. This advantage has been proved by numerous experiments ( Jin, 1989; Lu¨ et al., 1989; Wu, 1989). Combined application of N and P fertilizers also increased WUE. As shown in Table 7, when no fertilizers were added to the soil, each mm precipitation produced 6.6 kg ha1 wheat, but 7.4–10.7 kg ha1 with the

141

Nitrogen in Dryland Soils

Table 5 Effect of application of organic fertilizer together with N fertilizer on wheat yield N rate (kg ha1) Organic fertilizer (Mg ha1)

0

34

Wheat yield (kg ha1) 0 1959 2525 37 2208 2750 74 2834 3459 148 3168 3708 Mean 2513 3111 Yield increase (kg ha1) by N application 0 566 37 542 74 625 148 540 Yield increase (kg ha1) by organic fertilizer 37 249 225 74 875 934 148 1209 1183 Mean 778 781

68

102

Mean

3167 3708 4250 4566 3923

3459 4250 4334 4833 4220

2147 3230 3719 4070

1298 1500 1416 1398

1500 2040 1500 1665

1091 1361 1180 1201

541 1083 1399 1008

791 875 1374 1013

452 942 1291

Modified from Li (2002).

Table 6 Effect of application of N fertilizer together with P fertilizer on wheat yield in soil deficient in P supply (Olsen-P 5.7 mg kg1 soil) N rate (kg ha1)

P fertilizer (P2O5, kg ha1)

0

34

68

102

136

0 17 34 68 102 Yield mean

1662 2289 2698 2782 2800 2446

1800 2409 3032 3155 3515 2782

1729 2480 3085 4493 4738 3305

1738 2965 3618 4404 4764 3498

2163 2822 3804 4520 4778 3617

Mean yield

1819 2593 3247 3971 4119

Modified from Li (2002).

application of various rates of N and P fertilizers. The same trends were observed for other crops (Wang, 1983). Mixing ammonium-based N fertilizer with acid P fertilizer has another advantage. In the dryland areas, calcium superphosphate has been used as a P source for quite a long time, and it has been applied in two ways: separately

142

S. X. Li et al.

Table 7 The effect of inorganic fertilizers (F) on four crop yields and precipitation use efficiency (PUE) Yields (Mg ha1)

PUE (kg m3)

Fertilizers (kg ha1)

Qingke Wheat Barley barley

Qingke Rape Wheat Barley barley

Rape

Without F 112.5 N + 24.5 P 225 N + 49 P 337.5 N + 73.5 P 450 N + 98 P 562.5 N + 112.5 P

3.78 4.22

2.84 4.20

2.66 3.63

1.83 0.66 2.26 0.74

0.50 0.74

0.47 0.64

0.32 0.41

4.89

4.74

4.46

2.38 0.86

0.83

0.78

0.42

5.60

5.21

4.91

2.43 0.98

0.92

0.86

0.43

6.09

5.48

5.51

3.67 1.07

0.96

0.97

0.64

5.67

5.31

5.29

2.42 0.99

0.93

0.93

0.42

Modified from Wang (1983).

applied with N or mixed with N and then applied. The latter can reduce N loss by ammonia volatilization. Li (2007) placed 1 kg soil in closed chambers and applied 0.88 g of ammonium bicarbonate (containing 17% N) as N fertilizer alone and 0.88 g of ammonium bicarbonate mixed with 0.7 g (low amount) and 1.4 g (high amount) of calcium superphosphate (containing 14% P2O5) on the soil surface and then trapped the ammonia volatilized. Results show that when N fertilizer and P fertilizer were mixed and then applied, N loss by volatilization was significantly reduced (Table 8). Without mixing with P fertilizer, the loss was 20%, and it was 17% by mixing with low amount of P fertilizer and 15% by mixing with high amount of P fertilizer.

4.3. Deep application of N fertilizer Placement of fertilizer in different layers has different results. The soil at the top layer in dryland areas is usually dry, but high moisture is generally present and relatively stable in deep layers. For instance, in the Loess Plateau, the moisture in the top 0–10 cm is often lower than wilting point (8.5% dry weight), but in deep layers it is usually above 14–16%. Shallow application of fertilizers cannot be effective because of limited available water, but deep application places fertilizers into wet layers and increases their availability for plant uptake. This is more important for OF. In addition, most of soils on the drylands are calcareous with a high pH that

143

Nitrogen in Dryland Soils

Table 8 Effects of mixing P and N fertilizers on N loss by volatilization N loss (mg pot1 h1)

Day–Month

30–09 01–10 02–10 03–10 04–10 05–10 06–10 07–10 08–10 09–10 10–10 11–10 Mean Total loss % of loss from Fert. a b

Soil temp Air temp ( C) ( C)

Without N

22.3 18.8 17.7 9.2 7.8 7.6 13.8 13.5 12.2 16.7 15.1 16.2 14.2

21 21 21 14 13 15 7 7 12 0 13 7 13 3600

21.7 18.4 17.3 8.8 7.9 7.8 13.6 13.2 11.8 16.0 14.8 16.2 14.0 14.0

N

N mixed N mixed with low with high Pa Pb

823 630 242 203 134 115 16 16 26 29 47 41 34 32 26 25 30 28 2 34 29 19 31 20 120 100 34,560 28,828 20 17

558 172 95 14 26 41 39 35 30 2 17 30 88 25,373 15

Mixing 0.88 g ammonium bicarbonate (containing 17% N) with 0.7 g calcium superphosphate (containing 14% P2O5). Mixing 0.88 g ammonium bicarbonate with 1.4 g calcium superphosphate by broadcast on the surface of each pot that contained 1 kg soil. Modified from Li (2007).

encourages N loss of ammonium-based N fertilizer by volatilization; shallow application encourages such loss while deep application makes it unlikely. Furthermore, nutrient contents are generally higher in the top layers than in deep layers, and the deeper the layer, the fewer the nutrients. In contrast, crop roots on drylands are concentrated in the deeper layers where the soil moisture is better for root growth and nutrient uptake. For this reason, a favorable nutrient status of deeper layers is essential. Shallow application of fertilizers concentrates nutrients in the top layer where there are fewer roots. Such an application only benefits young plants. When plants grow larger, and roots penetrate into deeper layers, they have difficulty obtaining nutrients from the top layer since soluble nutrients from fertilizers are not moved down to deeper layers due to limited rainfall, and P fertilizer can become relatively fixed. Deep application is generally done by broadcasting the fertilizer on the soil surface, and then turning it into deep layer by deep plowing. In this way, most of fertilizer is placed in deep layer while some is mixed with top layer soil, and thus it can meet the

144

S. X. Li et al.

needs of both young and old plants. Because of such advantages, deep placement of fertilizers is regarded as an effective way to increase crop yield, fertilizer efficiency, as well as WUE, and has thus become widely used on the drylands (Li et al., 1976b; Li and Zhao, 1993c). Numerous experiments have proved the effect of deep application of N fertilizer on crop yield and NFE. Rees et al. (1997) conducted a 15N-labeled experiment in the south edge of the Loess Plateau in China to study the effect of placement of N fertilizer. Results showed that the N recovery was only 18% when applied on soil surface, compared to 33% when mixed with 15 cm soil, and 36% when applied in a pit of 15 cm depth. Lu¨ and Li (1987) reported that deep application of N and P fertilizers before sowing wheat met the crop requirement during the whole growth season, and increased fertilizer use efficiency. The loss of ammonium by volatilization was decreased from 39 to 14% for urea, and from 54 to 36% for ammonium bicarbonate. Consequently, wheat yield increased by 20%. Lu¨ and Li (1987) found that the relationship between wheat yields and depths for fertilization was as follows:

Y ¼ 189:6 þ 12:97X  0:89X 2 where Y is wheat yield, and X is applied depth. From this equation and the quadratic curve (Fig. 1), it was found that the suitable depth of applying N to wheat was around 15 cm. Deep application of fertilizers increased N fertilizer efficiency (NFE) and yields compared to shallow application (Li et al., 1976b). Some typical wheat experimental results are presented in Table 9. In these experiments, the fields were deep plowed before application of each fertilizer, and the experiments were designed so that the fertilizer effect by deep application could be separated from the effect of the tillage operation. For example, for

Wheat yield (kg/ha)

3400

3100

Y = 185.9 + 12.97 X –0.891 X n = 11

2800

2500

0

10

20

30

Depth of N added (cm)

Figure 1 Relationship between depth of N dressing and wheat yield. Redrawn with data from L€ u and Li (1987).

145

Nitrogen in Dryland Soils

Table 9 The effects of placements of organic fertilizer (OF) and N fertilizers on wheat yields and fertilizer efficiency

Fertilizer types

Organic (75 Mg ha1)

N (75 kg N ha1)

Fertilizer placements (cm)

Wheat yields (Mg ha1)

Control (35 kg N ha1 in rows) Control + OF (0–8) Control + OF (0–15) Control + OF (0–20) Control (35 kg P) Control + N (0–8) Control + N (0–15) Control + N (0–20)

2.76 2.97 3.03 3.56 3.50 3.87 3.93 4.09

Yield increased by fertilizer (%)

8 10 29 11 12 17

Modified from Li et al. (1976b).

studying the effect of N in different layers, P fertilizer was deeply applied by plowing, while N was mixed with different depths of soil after deep plowing. In this way, the effect of N fertilizer at different depths can be regarded as placement effect, and not as tillage effect induced by placing fertilizer into different depth. Deep application of N fertilizer significantly reduced N loss by ammonia volatilization. We demonstrated in a laboratory experiment that when N fertilizer (ammonium bicarbonate) was applied to the soil surface at 25 C, the N loss by volatilization reached 79% after 2 days. By comparison, the loss was 16% when applied at 5 cm, and 6% when applied at 10 cm. Recently, Li (2007) trapped ammonia volatilized from fertilizer added to soil under a closed chamber. The experiment was carried out in pots with four treatments: no N fertilizer, broadcasting N fertilizer on soil surface (surface), mixing N fertilizer with 0–6 cm topsoil (mixed), and applying N fertilizer to the 6–12 cm depth soil layer (deep). Ammonium bicarbonate was the N fertilizer with a rate of 0.3 g N per pot. Pots for each treatment were placed in a plastic chamber, and ammonia released from the soil was pumped out and absorbed by dilute acid for measurement. The experiment was conducted for 15 days, and the measurements were carried out six times per day, separately at 4, 9, 12, 16, 20, and 24 o’clock each day. Results definitely confirmed the effect of deep application of N fertilizer in reducing ammonia volatilization: N loss was 29% from added fertilizer for surface application, 15% for mixed application, and a negligible loss (1%) for deep application (Table 10). There are two ways for deep application of N fertilizer: one is mixing the N fertilizer with a certain layer of soil such as a 0–20 cm layer of soil, and the other is placed the N fertilizer to a deep depth such as to a 20 deep furrow or

146

S. X. Li et al.

Table 10 Nitrogen (N) loss by volatilization from fertilizer (0.5 g pot1) applied by different methods N volatilized (an average per day (mg d1)) Time (date–month)

Without N

Deep

Mixed

Surface

03–09 04–09 05–09 06–09 07–09 08–09 09–09 10–09 11–09 12–09 13–09 14–09 15–09 16–09 17–09 18–09 Mean Total loss N loss from Fert (mg) N loss from Fert (%)

325 674 65 304 435 183 328 530 806 1150 176 537 0 16 70 272 367 5869 0 0

216 844 185 436 641 420 760 903 1205 730 306 186 12 123 128 126 451 7219 1350 1

8212 4996 1704 1687 2056 1342 1488 1686 1769 1756 731 94 293 26 615 492 1809 28,947 22,880 15

20,793 8528 2039 2079 2650 1934 1528 2378 2078 1849 914 490 402 459 233 431 3049 48,784 42,915 29

Modified from Li (2007).

pit, and then covers the fertilizer with soil. Experiments done in two locations with deferent soil fertilities show that when the soil is poor in N supply for meeting wheat seedling needs, the former obtained highest yield whereas when the soil was fertile and had sufficient N to meet wheat seedling demand, the latter was the best (Wang et al., 1995).

4.4. Timing of N fertilization A large number of experiments have shown that for the most efficient use of N fertilizer on drylands, N fertilizer should be early applied to lands that are unable to be irrigated while split applications should be practiced on those that can be irrigated. The fertilizer effect on crop yields depends on the available water in the soil (van der Paauw, 1962a,b) and fertilizers are more efficient when there is

Nitrogen in Dryland Soils

147

sufficient water in the soil. On drylands where irrigation is not possible, N fertilizer should be applied as early as possible for fall-sown crops that grow during a long dry season. Early application can make full use of precipitation and thereby increase NFE. Early application has several advantages. Nitrate in fertilizers is not leached by rainfall during the long dry spell. When OF is applied earlier to the soil, it can provide available nutrients earlier and keep nutrients available to plants for a longer period so that plants can obtain nutrients at any time. Sometimes the fertilizer is not effective because the soil is too dry, but the effectiveness of the fertilizers is quickly activated when rainfall occurs. Goris and Ludwich (1978) found that early application of N fertilizer could raise crop yield while late application could only improve crop quality. IAEA/FAO (1974) conducted a variety of wheat field experiments in the world, and results showed that in India with abundant precipitation, split application of N fertilizer allowed wheat to take up much more N from N fertilizer than adding N at sowing time, while in Italy where the rainfall was less, application of N fertilizer at sowing time benefited N use by plants. Lu¨ and Li (1987) reported that deep application of the full amounts of N and P fertilizers before sowing wheat met the crop requirement for the whole growth period and increased fertilizer use efficiency. Li (2002) conducted a series of field experiments to demonstrate the effect of N fertilization timing on wheat yield. Two typical experiments are reported here: one was conducted in a field with dryland farming and the other utilized limited irrigation. In both fields, the total amount of N fertilizer was divided into five portions, and different rates were allocated to sowing, winter and spring. As shown in Table 11, wheat yield was the highest when N fertilizer was applied at sowing, and yields were clearly decreased when N fertilizer was delayed and applied at various growth stages. In field 1, wheat yield was 4650, 4350, and 3788 kg ha1 when N was applied at sowing, winter and spring, respectively. The higher the N rate applied at sowing, the higher the yield. The results for field 2 were similar to those for field 1, though there were some differences in yield order. It should be noted that in dryland areas, early application was not only beneficial for N fertilizer, but for OF and P fertilizers as well (Li et al., 1976a). Discussion of organic and P fertilizer is beyond the topic scope of the paper, so we will not go into detail. Results shown above were obtained with winter wheat. For fully irrigated maize, N fertilizer should be applied in increments at different growth stages according to their requirement. A similar experiment of maize was conducted in several sites, and results showed that dividing the full amount of N fertilizer into four portions with one portion applied at sowing, one portion at elongation, and two portions before heading was the best combination (Table 12).

Table 11

Effect of allocation of N rate at different growing-stages on wheat yield

Allocation of N at different growing stage

Experimental field 1 Control (P) N added at sowing N added before winter N ratio of 2:2:1 at sowing, winter and spring N ratio of 2:1:2 at sowing, winter and spring N ratio of 1:2:2 at sowing, winter and spring N ratio of 3:1:1 at sowing, winter and spring N ratio of 1:3:1 at sowing, winter and spring N ratio of 1:1:3 at sowing, winter and spring N ratio of 1:1:1 at sowing, winter and spring Experimental field 2 Control (P) N added at sowing N added before winter N added at early spring N ratio of 2:2:1 at sowing, winter and spring N ratio of 2:1:2 at sowing, winter and spring N ratio of 1:2:2 at sowing, winter and spring N ratio of 3:1:1 at sowing, winter and spring N ratio of 1:3:1 at sowing, winter and spring N ratio of 1:1:3 at sowing, winter and spring N ratio of 1:1:1 at sowing, winter and spring Modified from Li (2002).

Yield (kg ha1)

Yield increase (kg ha1)

Increase rate (%)

Yield increase (kg) by 1 kg N

2648 4650 4350 3788 3030 2940 4125 4163 3435 3188

2002 1702 1140 382 262 1477 1515 787 540

76 64 43 15 11 56 57 30 20

26.7 22.7 15.2 5.1 3.5 19.7 20.2 10.5 7.2

2985 5802 5690 4872 5105 4797 4782 5097 4865 4793 4880

2817 2795 1887 2120 1812 1797 2112 1880 1808 1895

94 91 63 71 61 60 71 62 61 63

37.6 37.3 25.2 28.3 24.2 24.0 28.2 25.1 24.1 25.3

149

Nitrogen in Dryland Soils

Table 12 Effect of N allocation at different growth stages on maize yield

Location

Caoxinzhuang Buwang Xiajiagou Xiajiagou Xiajiagou Caoxinzhuang Buwang Mean Yield increase (kg ha1) Yield increase (%)

N rate (kg ha1)

270 270 338 203 338 135 135

N portion allocated at sowing, elongation and before heading, respectively Without N

1:1:1

1:1:2

2:1:1

2:0:2

1:2:1

4238 4890 4905 4410 4770 4238 4890 4620

4448 7388 5250 5145 5513 3938 6105 5398 778

4748 7390 5490 5550 5988 4650 6630 5778 1158

4373 6330 5228 5250 5468 4298 5962 5273 653

3765 6420 5140 5258 5429 4050 5948 5144 524

4231 5430 5512 5230 5460 4673 6210 5249 629

17

25

14

11

14

Modified from Li (2002).

4.5. Determination of N rate based on soil N-supplying capacity Application of an adequate N rate is a basis not only for economic purposes, but also for environmental concerns. If input of N fertilizer is not enough, it is impossible to obtain a high crop yield. In contrast, when input of N fertilizer is excessive, the crop will not fully use it, and this will result in bioenvironmental issues. Ideally, the rate of N fertilizer applied should ensure high yield while leaving no residual N in the soil after harvest of the crop. Plant growth and crop production depend to a large extent on soil N-supplying capacity (SNSC) (Zhu, 1985). Li et al. (1982) conducted an experiment with 15N and results showed that winter wheat absorbed 75–79% N from soil and only 21–26% from fertilizers. Tang et al. (1986) conducted 5-year studies in high and medium fertility soils of Liaoning Province and revealed that SNSC was negatively correlated with NFR. The dependence of maize on soil N varied from 45 to 70% due to soil fertility difference. On the basis of national experimental results, Guo and Li (1989) found that 45–80% of the N amount needed by high yield crops was provided by the soil. According to field experimental data from eight provinces and municipalities, Zhu (1985) concluded that the dependence of crop on soil N was more than 50% in the majority of soils, and 45–83% in south China. It is clear that high yield crops greatly depend on SNSC. The higher the SNSC, the higher the dependence of crops on SNSC and the lower the NFR. Lu¨ and Li (1987) indicated that on the Loess Plateau

150

S. X. Li et al.

the NFR was 65% when crop yield was 1500 kg ha1 while 43% when crop yield was 3000 kg ha1. Since a crop can take up 45–70% total N needed from the soil, application of N fertilizer according to the SNSC constitutes the prerequisite for raising NFE. Large amounts of N fertilizer without consideration of the SNSC and crop production potential have already caused many problems such as low use efficiency of N fertilizer, crop yield decline, and underground water contamination by nitrate leaching from soils south of the dryland areas where there is relatively high precipitation, or in places where irrigation is available. Great attention has been given to evaluating the SNSC and applying N fertilizer based on its value since the 1960s. Different methods have been used. Of those, however, the most common is soil testing to determine an index of N availability that can be of practical use (Li, 1999; Li et al., 1990a). In the past few decades, concerns on environmental issues rather than economic returns further promoted such research. In China, a variety of work has been done, and many approaches have been investigated and proposed for this purpose, especially for paddy soils (Zhou et al., 1976; Zhu, 1982, 1988, 1990). The laboratory methods used for estimating SNSC can be divided into two types: biological and chemical. They have been extensively reviewed by Bremner (1965), Bundy and Meisinger (1994), Keeney (1982), and Keeney and Bremner (1966). The biological methods generally involve short-term incubation (typically 7–25 days) under either aerobic or anaerobic conditions. These methods have a rational basis because the microbial agents responsible for release of the mineralizable N during incubation are those that make organic N available for crop growth during the growing season. Incubation techniques provide a fairly satisfactory index of the availability of soil N to plants when compared with the results of greenhouse trials, but correlation with field data is typically less satisfactory (Keeney, 1982). One major disadvantage of incubation methods is that they are timeconsuming, and this is a major reason why they have not been widely adopted for routine use. One such incubation method widely used in China in the past was the so-called nitrifying-power incubation (Pegerbyrgckow, 1961). The soil is incubated at 25–28  C with the moisture content maintained at 60% of field capacity for 7–14 days, and the nitrate N accumulated during the incubation is measured. Experimental results have shown that this method had great potential for use (Li, 1965). However, due to time- and labor-consuming constraints, it has not been put into practice. In the past three decades, the aerobic incubation method proposed by Stanford (1982), Stanford and Epstein (1974), and Stanford and Smith (1972) received more attention (Nuske and Richter, 1981; Richter et al., 1980, 1982). This method does not only reflect SNSC, but also can be used to calculate the mineralization potential (N0) of organic N and the mineralization constant (k) (mineralized N amount per unit time).

Nitrogen in Dryland Soils

151

Further work on this approach involved obtaining appropriate corrections of the N0 value for temperature (Stanford et al., 1973a,b) and water (Stanford and Epstein, 1974). The value of k was found to have a Q10 of 2 over the range of 5–35  C, whereas the soil water content, expressed as a percentage of the optimum water content (in the range of field capacity, i.e., 0.3  105 Pa), was directly correlated with a reduced rate of N mineralization (e.g., at a water content of 75% of optimum, only 75% as much N was mineralized). The N0, temperature, and water relationships were utilized to estimate N mineralization during the crop growing period, and a good agreement was obtained between the estimated mineralized N and the crop uptake N plus the mineral N that remained in the soil at various time intervals ( Ju and Li, 1998). Anaerobic incubation has many advantages: determination of ammonium N only, no need to consider the adequate water content and water loss during the incubation period, mineralized N higher than the aerobic incubation in a given period, and higher temperature for incubation and therefore no need to consider the suitable temperature for nitrification as in the aerobic incubation. However, the disadvantage of time constraints still exists, and if needed, nitrate N may be measured separately. Some experiments have shown that the mineralized ammonium N of soils under a waterlogged incubation at 30–40  C for 6–14 days was well correlated with crop uptake N (Li et al., 1990a), and therefore this technique is not only used for paddy soils but also for dryland soils. However, Li et al. (1990a) also showed that the anaerobic incubation was no better than the aerobic incubation for dryland soils. Compared to incubation methods, chemical determination is simple and easier to practice, and thus has attracted more attention. This determination includes soil OM or total N, mineral N initially present in soil, and mineralizable N (Bremner, 1965; Gianello and Bremner, 1986a,b; Keeney, 1982). The OM or total N has some benefits for predicting crop response to added N fertilizer, and it is often well correlated with the crop uptake N, and also with the mineralizable N determined by incubation or extracted by chemical agents. For this reason, it is still a method used in some countries and some states of the USA. However, variability in environmental conditions has often resulted in poor correlations. One of the disadvantages is that either OM or total N amounts are not sufficiently different in a given region, and therefore the sensitivity of neither one is high in reflecting SNSC (Li et al., 1990a). In addition, the relation of OM or total N to the mineralizable N is weak. The mineralized N during incubation is not proportional to OM or total N and the mineralized N produced from the same content of OM or total N may be as different as several folds. This shows that the amount of OM or total N is not the major factor determining the mineralized N amount, but the part that can be mineralized (Li and Li, 2003a,b) during plant growth. For this reason, both are

152

S. X. Li et al.

significant in characterizing the basic fertility of a soil, but are not sufficient for reflecting the SNSC. The extraction and determination of mineral N initially present in soil have been extensively used as a routine laboratory method in many countries and some regions. The well-known Nmin is typically such a type of method (Borst and Mulder, 1971; Soper and Huang, 1962; Wehrmann and Scharpf, 1979). It is known that in regions where leaching and denitrification are not serious before crop sowing or during the plant growth period, the recommendation for N fertilizer application should consider the mineral N that is as effective as N fertilizer. Carter et al. (1974) and Stanford et al. (1977) demonstrated that use of the mineralizable index in combination with nitrate N in soil profile could improve predicting the N requirement. Stanford et al. (1977) have shown that in the case of crops being well managed under a condition of limited precipitation, the amount of available soil N can be more precisely predicted by the residual mineral N in addition to considering the influence of environmental variables (temperature and water) on the mineralization rate of organic N. There is no problem in extraction and determination of nitrate. The key for using this method is sampling depth of soil. Hu and Li (1993a,b,c,d) conducted several field experiments and showed that the amount of nitrate N in the topsoil (0–20 cm) can reflect the soil capacity for supplying N to a certain extent, but the correlation was not high, and therefore the prediction is not precise, but sampling the 0–80 or 0–100 cm profile can give a very satisfactory indication. A determination of soil mineral N only reflects part of SNSC of a soil. The measured usable N that can be directly absorbed by plants provides useful information, but cannot provide information about the mineralizable N. During the growth period, plants absorb not only mineral N already present in soil, but also the mineralized N from OM, the so-called potentially mineralizable N. Utilization of nitrate N as a N availability index is not stable: it is better when the organic N in the soil is small and the mineralization process is slow while poorer when the organic N is large and the process is rapid. The more nitrate N present in the soil, the better the effect for the prediction of SNSC. For this consideration, determinations of the potentially mineralizable N in the soil have been extensively investigated. Apart from incubation methods, there are many methods using adequate chemical reagents to extract the mineralizable N or available N (Boswell et al., 1962; Richard et al., 1960; Truog, 1954). The extracted N forms are mostly the hydrolysable or distillable N, and the chemical reagents used for this purpose include acid (sulfuric acid, hydrochloric acid) (Gracie and Khalil, 1939; Peterson et al., 1960; Tyurin, 1934), alkali (sodium bicarbonate, sodium hydroxide) (Cornfield, 1960), salt (KCl, CaCl2), water (hot water) (Livens, 1959), and acid-oxidizing reagents (various mixture of H3PO4–chromic acid, H2SO4–KMnO4 and others) (Nommik, 1976;

Nitrogen in Dryland Soils

153

Stanford, 1978; Stanford and Smith, 1978). Of these, the so-called mild reagents are more preferred since they may not change the soil properties significantly during extraction, and it is said that they can release the mineralizable N in a selectable way. Since the 1980s, the hydrolysable N extracted by 1 mol L1 NaOH (Cornfield, 1960) has been recommended to use in China. However, the effect of this method is still doubtful by the majority of Chinese agricultural scientists, and no chemical-extracting method is presently considered as satisfactory. Our results from a number of field experiments at various sites of two counties showed that almost all of the methods for measuring the potentially mineralizable N failed to work well in areas where large amounts of N fertilizer had been applied in the past. The loss of their effectiveness was caused by the accumulation of large amounts of residual mineral N in the soil profile. For example, in Yongshou County, Shaanxi Province, a typical dryland area without supplemental irrigation, the mineral N (NO 3– NþNHþ –N) accumulated in soil profile from 0 to 1 m depth averaged 150 4 kg ha1, and in Qishan County, Shaanxi Province, a dryland area with supplemental irrigation, it was as high as 225 kg ha1 for the same layer. Since the mineral N in the soil profile has made a great contribution to crop yields and crop uptake of N, the best soil N availability index is the nitrate N accumulated in the soil profile. The field experiment results showed that at the 25 sites of Yongshou County, the cumulative nitrate in the 1-m layer was very well and significantly correlated with crop yields, plant uptake N, and yield increase. The correlation between N uptake by the aboveground plants of winter wheat with P application and the cumulative nitrate in the 1-m layer was as high as 0.908 with a significance level of 0.01, explaining 81% of the total N uptake by the crop. Due to large amounts of mineral N in soil profile, the potentially mineralizable N, including KCl-extractable N (Gianello and Bremner, 1986a,b; McTaggart and Smith, 1993; ien and Selmer-Olsen, 1980; Selmer-Olsen et al., 1981; Smith and Li, 1993; Whitehead, 1981), the hydrolysable N extracted by 1 mol L1 NaOH– hydrolysis (Cornfield, 1960), the boiling water extracted N (Keeney and Bremner, 1966; Livens, 1959), the alkaline permanganate extracted N (Stanford, 1978) and the acid permanganate extracted N (Stanford and Smith, 1978), and the mineralized N by intermittent leaching aerobic incubation (Stanford and Smith, 1972) and that by waterlogged incubation (Waring and Bremner, 1964), is not well correlated with the field experiment results (Table 13). These results were obtained in soils that, as a whole, contained a large amount of nitrate N in the 1-m soil profile. However, if the residual nitrate N amount was not large, some methods for determining the mineralizable N did effectively reflect SNSC. This could be seen from the following evidence. The first was results from a pot experiment. Ryegrass was planted in pots, and a depleting procedure was used to consume the soil mineral N by

154 Table 13 Correlation coefficients between potentially available N determined by different techniques and wheat yield response to N fertilizer. No P added Method for Determination

Yield

Addition of P

N uptake

Yield

N uptake

Yield increase by N (%)

Boiling KCl-extractable N Whitehead NHþ 4 -N NO 3 -N

0.210 0.485

0.145 0.455

0.247 0.407

0.153 0.472

0.362 0.354

Gianello NHþ 4 -N NO 3 -N

0.204

0.203

0.206

0.169

0.295

0.308

0.306

0.305

0.379

0.274

N released by NaOH hydrolysis NHþ 4 -N

0.039

0.025

0.045

0.057

0.066

N extracted by boiling water NHþ 4 -N Total N Organic N

0.062 0.142 0.147

0.018 0.094 0.119

0.040 0.178 0.197

0.020 0.172 0.198

0.137 0.239 0.235

0.157

0.322

0.288

0.444

N released by alkaline permanganate distillation Ammonium-N produced by NaOH-hydrolysis NHþ 0.145 4 -N

Ammonium-N produced by hydrolysis and oxidation of permanganate NHþ 0.175 0.118 0.251 4 -N Ammonium-N produced by oxidation of permanganate under a alkaline condition NHþ 0.109 0.049 0.110 4 -N N released by acid permanganate extraction Ammonium-N extracted by 1 mol L1 H2SO4 NHþ 0.019 0.001 0.117 4 -N Ammonium-N released by oxidation of permanganate under an acid condition NHþ 0.195 0.174 0.175 4 -N Total extracted N NHþ 0.133 0.128 0.059 4 -N

0.255

0.349

0.128

0.154

0.074

0.216

0.212

0.047

0.113

0.097

Aerobic incubation NO 3 -N

0.202

0.168

0.212

0.257

0.351

Waterlogged incubation NHþ 4 -N

0.156

0.099

0.169

0.178

0.323

Nitrate-N initially present in 1 m layer NO 0.828 3 -N

0.831

0.831

0.908

0.622

Modified from Li (2002).

155

156

S. X. Li et al.

continuous ryegrass planting and cutting to study the relation of crop uptake N to the mineral N and the potentially mineralizable NH4–N extracted by boiling KCl using two procedures, Whitehead (1981) and Gianello and Bremner (1986a,b). Results showed that ryegrass uptake of N at the first cutting had the highest correlation with the nitrate N initially present in soil, and the lowest correlation with the potentially mineralizable NH4–N. However, with cutting number increase, nitrate N in the soil became less and less, and crop uptake N became more and more highly correlated with the mineralizable N (Table 14). This shows that with low nitrate N in soil, determining the mineralizable N plays a significant role in indicating the SNSC. The second is also a pot experiment with 17 soils having varying fertility levels, particularly different amounts of nitrate N in the soil. Half of the pots for each soil were leached with distilled water until there was no nitrate N that could be detected in the leached solution, while the remaining pots were not leached. All soils were irrigated to have same water content and then wheat was sown. Results showed that without leaching, wheat uptake N was well correlated with the initial nitrate N, but not so with the mineralizable N obtained by both the aerobic and anaerobic incubation. However, after leaching, the reverse was the case: N uptake was well correlated with the mineralizable N, but was significantly less correlated with nitrate N amounts. This further gives evidence that nitrate N affects the function of mineralizable N (Table 15). The third was the data from field experiments. The results discussed above were from pot experiments, whether pot experimental results agree with agricultural reality is never known. Therefore, field experiments are needed for verification. As mentioned, a large number of field experiments

Table 14 Correlation between N uptake by ryegrass and nitrate N initially present in soil as well as mineralizable N (ammonium N) extracted by boiling KCl

 Mineralizable NHþ 4 -N and initial NO3 -N extracted by two methods of KCl

Whitehead NHþ 4 -N NO 3 -N Gianello and Bremner NHþ 4 -N NO 3 -N Modified from Li (2002).

N uptake by ryegrass at different harvest time First cut

Second cut

Third cut

0.530 0.735

0.574 0.706

0.747 0.615

0.542 0.815

0.620 0.813

0.795 0.628

157

Nitrogen in Dryland Soils

Table 15 Correlation of N uptake by wheat (aboveground part) with initial nitrate N and with mineralizable N before and after leaching of nitrate N Mineralizable N Nitrate N initially present in soil

Aerobic incubation of 2 weeks

Waterlogged incubation of 1 week

Without leaching initial nitrate N 0.862

0.444

0.119

0.767

0.866

Leaching initial nitrate N 0.557 Modified from Li (2002).

were conducted at 60 sites in two counties. Taking the experimental results from Yongshou County as examples, the experimental data of 25 field sites were obtained. According to the results, the accumulated nitrate N in the 0–1 m soil depth was relatively large with some fields containing more than 200 kg N ha1, and wheat uptake N was well correlated only with the cumulative nitrate levels. However, when sites containing nitrate N amounts over 80 kg ha1 were excluded, the correlation of crop uptake N with the mineralizable N was greatly improved. For example, when determined by the boiling KCl method, the nitrate N effect was significantly decreased (Table 16). Under conditions of small amounts of nitrate N, the potentially mineralizable N indeed showed good results. The field experimental results also show that of the methods for determining the mineralizable N, the incubation, boiling-KCl extracting, and the NaOH–hydrolysis methods show the same trend and are well correlated with the crop uptake N when soils with nitrate N over 80 kg ha1 were excluded. It should be noted that the waterlogged incubation method had a higher correlation with crop uptake N than the aerobic incubation method in the field experiments (Table 16). However, this was only one occasion. A large number of other field and pot experiments have shown that in the majority of cases, aerobic incubation is better than waterlogged incubation in dryland areas (Li et al., 1990a). On the basis of these results, we proposed an index of N availability of nitrate N from 0 to 1 m depth (Table 17), which has shown to be a useful tool for guiding N fertilizer applications. We also suggested that if the nitrate N is adequate according to the index, there is no need to apply N fertilizer and no need to determine the potentially mineralizable N. Otherwise, there is a need to determine the mineralizable N, and according to the determination and the amount of nitrate N in soil profile, the rate and timing of applying N fertilizer can be made.

158

S. X. Li et al.

Table 16 Correlation coefficients between mineralizable N and wheat response to N fertilizer in soils low in nitrate N contents No P addition Boiling KCl extraction

Yield

N uptake

Addition of P Yield

N uptake

Yield increase by N addition (%)

All soils (n ¼ 25) in experiments used for calculation Whitehead NHþ 4 -N NO 3 -N

0.210 0.485

0.145 0.455

0.247 0.407

0.153 0.472

0.362 0.354

Gianello NHþ 4 -N NO 3 -N

0.204 0.308

0.203 0.306

0.206 0.305

0.169 0.379

0.295 0.274

Soils containing nitrate N below 80 kg ha1 in 1 m layer (n ¼ 15) used for calculation Whitehead NHþ 4 -N NO 3 -N

0.731 0.551

0.691 0.529

0.755 0.481

0.719 0.395

0.567 0.189

Gianello NHþ 4 -N NO 3 -N

0.701 0.459

0.747 0.433

0.753 0.057

0.768 0.199

0.511 0.115

Modified from Li (2002).

Table 17 N availability index based on nitrate N contents in dryland soils 1 in 1 m layer) Degree of N deficiency (NO 3 -N, kg ha

Extremely low

Low

Medium

High

63–128

>128

71–168

>168

Drylands without supplemental irrigation <30

30–62

Drylands with supplemental irrigation <30

30–70

Modified from Li (2002).

4.6. Choice of suitable form of N fertilizer for different crops In the past, scientists considered that nitrate and ammonium N had an equivalent nutrient function and the same effect on crop yield on the basis of the same amounts of N. Recently, some results show that this is not always true. Effects of the two forms of N on crop response are determined

Nitrogen in Dryland Soils

159

by many factors, and to a large extent, plant species and medium pH are the most important ones. Due to plant species difference, nitrate N and ammonium N may exhibit different effects. Vegetables prefer nitrate N over ammonium N, and can accumulate large amounts of nitrate. In the field, due to rapid transformation of ammonium N or urea N into nitrate N, the two types of N fertilizers sometimes show no difference on vegetable yields, and neither has shown a detrimental effect. However, in solution culture, nitrate N can ensure good vegetable growth, while ammonium N exhibits a detrimental effect on vegetable growth. In a solution with only nitrate as the N source, the vegetables grew very well while with ammonium as the sole source, the vegetables grew poorly, and even died (Tian et al., 2003). Tobacco is a crop needing nitrate N, and application of nitrate N does not only improve its yield but also its quality (Cao et al., 1991a,b). Rice grows in paddy soil under a waterlogged condition. It was believed in the past that rice preferred ammonium N to nitrate N. However, recent experimental results have shown that rice also does well with nitrate N, and addition of some nitrate N promotes rice growth and raises its yield (He et al., 1998b; Yang and Sun, 1990). With an increase in concentration, the advantages of the addition of nitrate N compared to the disadvantages of the addition of ammonium N become more obvious. A medium pH affects N uptake amounts of the two forms, and therefore their effect on crop yield. For same crops, the uptake amount of nitrate or ammonium N is medium-pH dependent. He et al. (1999) cultured six crops (wheat, maize, tomato, proso millet, Chinese cabbage, and buckwheat) in nutritional solution for the entire growth period and controlled the solution pH at 6.5. Their results showed that wheat, maize, proso millet, and Chinese cabbage absorbed more nitrate N than ammonium N, while buckwheat took up more ammonium N and tomato depended on the duration of fruiting time. However, with pH changes, the results were different. He et al. (1998a) also studied the relation of N uptake to pH by water culture using wheat as a test crop and found that with increasing pH, ammonium N uptake amount was increased, while nitrate N decreased. At pH 6.5, wheat absorbed almost equal amounts of nitrate N and ammonium N, and the total amount N absorbed by wheat was the highest (Table 18). Due to high pH buffering capacity in dryland soils, the effect of nitrate N and ammonium N on crop yield is different in field from that in water culture. Wheat and maize are two major field crops in the drylands of China, and their response to N forms has received attention. Li and Wang (2005) conducted both pot and field experiments to study wheat response to nitrate N and ammonium N. Their results definitely show that wheat preferred nitrate N rather than ammonium N, and application of nitrate N as the sole N source or a combination of higher nitrate with lower ammonium N such as 2:1 ratio of nitrate to ammonium N showed best

160

S. X. Li et al.

Table 18 N uptake by wheat at different pH (mg N per plant) NH4þ-N

pH

5.0 6.5 8.0 a

30.2b 44.7a 47.5a

a

NO 3 -N

Total

49.3a 47.8a 25.6b

79.5b 92.5a 73.1b

Different letters in same column indicate significant at 0.05 level. Modified from He et al. (1998a).

Table 19

Difference of wheat response to nitrate and ammonium nitrogen Field result (kg ha1)

a

Treatment

Grain

Aboveground dry matter

Pot dry matter (g pot1)

Control (without N addition) NHþ 4 -N (NH4Cl) NO 3 -N (NaNO3)  NHþ 4 -N:NO3 -N = 2:1 þ NH4 -N:NO 3 -N = 1:2

2800aa

10,155a

15.5  3.3a

5811b 6654c 6250b 6742c

18,120b 21,480c 19,965b 21,570c

27.9  2.1b 30.4  2.7c 29.1  3.3c 28.2  2.5c

Different letters in the same column indicate significant at 5% level. Modified from Li and Wang (2005).

results in increasing crop yield and improvement of crop growth while with ammonium N as sole N source, wheat yield was the lowest (Table 19). The same trend was found in maize, especially when N rate was low (Li and Wang, 1993a). Why does nitrate N commonly increase yields for wheat and maize on the drylands? Our results show that this may be related to nitrate N accumulation in plants. When a nitrate N fertilizer was applied to soil, or when a soil accumulated a large amount of nitrate N, wheat or maize could take up much more nitrate N than normal and accumulate a large amount of it in the aboveground part. The increase of nitrate uptake and the excessive accumulation of nitrate in vegetative organs are common features for plants and occur at vegetative growth stages. This is true not only for vegetables, but also for other crops. The difference between vegetables and cereal crops is that for vegetables, especially for leafy vegetables, the leaves and stems that accumulate large amounts of nitrate are used for food while for the cereal crops, seeds are used for food, and they contain a little amount or no nitrate (Tables 20 and 21) (Wang et al., 1998). The high nitrate accumulation in cereal crops has several beneficial effects.

161

Nitrogen in Dryland Soils

Table 20 Nitrate N concentration (mg kg1 fresh weight) of cabbage, spinach, and wheat at different growth stages

Crop

Seedlings

Cabbage 1010 Spinach 549 Wheat 807 a

Revival (shooting)a

Flowering (heading)

Grain filling (podding)

Maturity

593 518 204

439 442 334

825 274 257

2723 658 173

Word in the parentheses shows the growth stages of cabbage and spinach, Modified from Wang et al. (1998).

First, high nitrate N concentration in plants promotes the transformation of its absolute amount into ammonium N, and hence increases ammonium N concentration. With an ammonium N increase, amino acid and amide N are also increased. Amide N is a storage form of N, and its high level favors N supply at a critical time when plants urgently need it. For this reason, plants have not only high nitrate N accumulation, but also high ammonium N as well. Furthermore, they contain soluble organic N compounds such as amino acids, amides, and protein (Li and Wang, 2005). Second, high nitrate N accumulation can ensure vigorous growth of plants. During the vegetative growth period, plants with high nitrate accumulation grow vigorously, and their leaves are green to dark green in color during the entire growing period. At the late stage, plants still grow well and the leaves remain green even if N supply from the soil becomes deficient while those with small amount of nitrate N accumulation may grow poorly, and the leaves may become yellow. With continuous growth, the accumulated nitrate N becomes significantly decreased, and at flowering stage, plants with high nitrate N accumulation still contain some nitrate, but those with low nitrate N accumulation have almost no nitrate remaining. As a result, the duration of plant growth period is elongated for the plants with higher nitrate N accumulation. All this creates a favorable condition for plants to have abundant growth (Li and Wang, 2005). Third, as a last result, high nitrate N contents in soil promote plant uptake of more nitrate N, and due to the beneficial effects of nitrate N, plants have higher dry matter (Fig. 2) and high yield. These results have shown that under sufficient nutrient-supplying conditions, plants can have excessive uptake of nitrate N for use at later stages when supply of nitrate N from the medium has declined. Seen from numerous experimental results, we can conclude that the accumulation of nitrate N in plants is a measure of N storage for adaptation to the detrimental environment or conditions that plants may meet. When a large amount of nitrate N is accumulated, the plants can still grow vigorously even if the

162

S. X. Li et al.

Table 21 Nitrate N concentration (mg kg1 fresh weight)a of different organs of cabbage, spinach, and wheat at different growth stages

Plant organ

Chinese cabbage Root Stem Leaf Leaf stalk Leaf blade Flower Fruit Fruit shell Seed Spinach Root Stem Leaf Leaf stalk Leaf blade Flower Fruit Winter wheat Root Stem Leaf Leaf stalk Leaf blade Head Chaff Seed a

Revival (shooting)

Flowering (heading)

923 717 570 758 271

605 498 403 499 252 3

Grain filling (podding)

868 900 622 839 423 197 1259 0

825 664 504 705 233

788 492 335 568 54 0

180 314 97 336 48 41

482 308 87

355 478 252 317 210 9

188 381 190 257 119 0

Maturity

1225 2603 4537

4565 0 524 452 311 589 95 0 114 272 380 214 325 152 0 0 0

Results are weighted averages at each growth stage. Modified from Wang et al. (1998).

nutrient supply from soil is not sufficient at the late stage; and the higher the accumulated nitrate, the better the plant growth. From current research results, nitrate N nutrition seems to offer more safety for maintaining plant growth than ammonium N. The accumulation of nitrate has no harmful effect on plant growth compared to ammonium N, and this may be another reason why nitrate is a suitable form for plant storage. Due to crops having different responses to N forms (Cao and Tibbitts, 1993; 1994; Gentry and Below, 1993; Ismunadji and Dijkshoorn, 1971; Kirkby, 1967), enhancing nitrate nutrition or enhancing ammonium

163

10,000

800 Plant nitrate 700

Dry matter

8,000

600

6,000

500 4,000

400

2,000

300 200 20

30

40

50

Dry matter (kg/ha)

Plant nitrate N concentration (mg/kg)

Nitrogen in Dryland Soils

0 60

Soil nitrate N concentration (mg/kg)

Figure 2 Nitrate N in soil and its relationship to nitrate N in plant at seedling stage and its relationship to dry matter at harvest. Drawn with data from Li and Wang (2005).

nutrition has been proposed for different crops (Dai and Cao, 2000; Dai et al., 1998; Raab and Terry, 1994). For a given crop, the preferred source for supplying N should depend on experimental results. The results shown above were obtained in the dryland soil, so the conclusion cannot be extended to other regions without experimental data. Nowadays, ammonium-based N fertilizer, particularly urea, is the major form produced by industrial processes, so enhancing ammonium nutrition is easily conducted. In comparison, much less nitrate fertilizer is produced, so applying nitrate may be difficult for some countries or some regions. For solving this problem, rapid nitrification of ammonium N may be a solution. It is reported that the population of nitrifies is generally low but increases rapidly after addition of ammonium N (Watanabe et al., 1981). Therefore, a lag period in nitrification ranging from a few days to a few weeks is generally observed following the initial application of fertilizers (Alexander, 1978). However, once the population is increased, it does not decline immediately and stays at a level higher than the initial level for some time. During the period of stabilization, nitrification of added ammonium N proceeds rapidly (Lees and Quastal, 1946). Urea hydrolysis is very rapid from the beginning and results in high local concentrations of NHþ 4 . As a result, ammonia volatilization is also very fast during the first few days following the application of urea (Aggarwal et al., 1987). The high NHþ 4 concentration and vis-a`-vis NH3 volatilization can be reduced by eliminating the lag period in nitrification. This objective could be achieved by pretreating the soil with a small quantity of NHþ 4 –salt, which would in turn increase the population of nitrifying bacteria. Thus, the addition of urea after the lag period of nitrification will result in quicker nitrification of hydrolyzed NHþ 4 . Based on such considerations, Praveen-Kumar and Aggarrwal (1988) added a small amount of (NH4)2SO4 fertilizer to the soil before urea was applied in a

164

S. X. Li et al.

large amount. Results show that the nitrification of added urea increased following the pretreatment. The increase in nitrification rate was directly related to the (NH4)2SO4 amount added as a pretreatment. The results suggested that when urea was applied after sufficient increase in nitrification þ rate, i.e., 3–5 weeks after the pretreatment with NHþ 4 -fertilizers, the NH4 produced by the hydrolysis of urea was quickly nitrified. This measure was once used to reduce NH3 volatilization and increase pearl millet yield. However, since nitrate N is produced during the process, it may be used for enhancing nitrate nutrition. Recently, we applied small amount of urea (about 1/10 of the total amount added) 20 days earlier before wheat planting as a pretreatment and applied the remaining amount of urea at wheatsowing time. Determination at the 20th day after wheat planting shows that the nitrate N in the soil was two times higher than that without such a pretreatment. This indicates that the ammonium N formed by urea hydrolysis has the same function as addition of ammonium N fertilizer.

5. Strategies for Managements of Soil N on Drylands For sustainable agriculture, the most important and essential means is the maintenance of high soil fertility and productivity. Nutrients are basic properties of soil fertility and basic materials for plant growth as well as for crop production. Although not usually recognized as water-saving measures, the wise input of fertilizer and manure may do more to raise WUE than some other means. In addition, fertilizer can do more in controlling erosion contrasted with some of the more obvious mechanical measures. Zheng et al. (1987) demonstrated that an adequate input of nutrients obviously reduced sheet and rill erosion. The importance of raising soil fertility by nutrient management is not only that the nutrients provide basic elements to ensure high crop production and therefore high WUE, but also that some nutrient elements, especially N, are pollutants that affect environmental quality. Nutrients are generally supplied in the form of fertilizers, either in organic forms or in chemical compounds. For producing the chemical fertilizers, particularly N, a large input of energy is required. Nitrogen is an ionic element, and the atmosphere contains 79% N by volume in the form of inert N gas that cannot be used by the higher order plants. Converting atmospheric N by industrial synthesis into ammonia and other forms that plants can use consumes large amounts of energy. This is also true for biological fixation of N. For saving energy, management of nutrients should include full use of organic waste residues (OWRs) and biological resources, intensifying the nutrient cycles. Only in this way can agriculture be sustainable.

Nitrogen in Dryland Soils

165

To improve soil fertility and eliminate fertilizer pollution of the environment, different strategies have been proposed. Roughly, two ways are noted: agricultural and industrial. Of these, agricultural strategies are fundamental and basic.

5.1. Adequate supply of N fertilizer An adequate supply of nutrients, particularly N, P, and K, and maintenance of proper soil pH are essential to crop growth. The current conventional approach in China is to apply nutrients at levels sufficient to assure maximum yields, and excess N is often supplied. Although crop production is still limited by N deficiency on most of drylands, some areas have N supplies higher than plant needs. This has caused low economic returns and environmental pollution. In Shaanxi Province, too much fertilizer has led to nitrate N accumulation in some water wells to levels in excess of the 10 mg N L1 standard of U.S. EPA for drinking water. For sustainable agriculture, precise applications of fertilizer and utilization of banded or split application of N fertilizers to raise their efficiency, and development of methods that can precisely predict the amounts of N existing in soil and those that could be mineralized during the plant growth period are urgently needed. A correct decision for input of N fertilizers can only be made on the basis of the SNSC. On the basis of several years of study, we found that use of the cumulative nitrate N as a main index in considering the mineralizable N estimated by aerobic incubation method has reduced the N fertilizer rate, raised NUE, and avoided N contamination. This is just an example, however. For putting a precise prediction into practice, more work needs to be done, and most importantly, more effort is needed to extend these research results into practice.

5.2. Crop rotation with legumes Crop rotation systems have been used in China for increasing crop production through three major ways: reducing plant damages by pests such as weeds, diseases, and insects; improvement of soil fertility; and management of cropping sequences in such a way that one crop can promote the production of another. The importance of rotation systems in recovering and raising soil fertility has long been realized and many systems have been used on the drylands in accordance with different conditions (Lu¨ and Li, 1987). Perhaps the most important cropping systems are those that contain legumes in the rotations. This system has been used in China at least since the Han Dynasty, between 770 BC and 256 BC, and it has played a great role in sustainable agriculture (Shih, 1959). The common way is to plant legumes alone as seed crops, such as many kinds of beans, or as forage crops and green manure crops. In this case, the crops following the legumes are mostly grain crops and oil-bearing crops. Another type is intercropping or

166

S. X. Li et al.

interplanting cereal crops with legumes, and the legumes may be used for producing seeds or for green manure. The two ways have long been practiced in China. It is well known that legumes can fix N from the atmosphere, and the rate of N2 fixation varies with the host plant, N-fixing organisms, and environmental conditions. Under dryland conditions, alfalfa fixes about 200–250 kg ha1 N per year, and pea fixes 71–91 kg ha1 N per growth cycle (Li et al., 1990b, 1992b). The direct availability of the fixed N permits the host plants to grow in soils deficient in N, and at the same time, reduce losses by denitrification, volatilization, and leaching, thus improving the sustainability of an agricultural system. Use of leguminous crops for dinitrogen fixation is likely to become even more important in the future as population increases in many developing countries necessitate sharply increased crop production, while pollution, energy, and cost concerns limit significant increases in the use of fertilizer N. In some areas of the Cornbelt in USA, the fertile soil plus the rotation of maize alternated with soybean has made a very high NUE: per kg added N produced 47.5 kg maize grain in 1990 and 60.4 kg in 2005 (The Fertilizer Institute, 2008). This shows from another aspect the importance of improvement of soil fertility and use of leguminous crops in crop sequence. In China, the importance of planting leguminous crops on improvement of soil fertility is well known to farmers. The problem is that with the increase of chemical fertilizer use, farmers have reduced the planting of leguminous crops and have become more and more dependent on chemical fertilizers. For sustainable agriculture, attention has to be given to recovering the planting of legumes.

5.3. Full use of biological materials as nutrient sources For the consideration of environmental protection, economic returns, and energy saving, a benign approach to nutrient management is to reduce the need for fertilizers through more efficient management of nutrient cycles. Application of OWRs from animals and crops can derive many benefits to agriculture. The amounts of OWRs, although low in nutrient contents compared with chemical fertilizers, are large and contain almost all of nutrients plants need. When adding to soil, OWRs supplies not only nutrients such as N, P, and K that are of great importance but also OM that can improve the soil in many ways. Organic wastes are also pollutant sources that have the potential for contaminating the environment if not properly used. In China, lack of high-grade rock P and K compounds has limited the production and supply of these commercial fertilizers, and applying OWRs to soil may be the cheapest and most effective way for solving this problem. This is especially true for potash fertilizer. On most of the drylands, the total and plant available K in soils exists at a high or adequate level, and therefore its deficiency does not appear likely for most

Nitrogen in Dryland Soils

167

crops grown on these lands. However, some investigations have shown that lands in some locations have depleted K levels, and some scientists have predicted that more deficiencies of K will appear in the near future ( Jin et al., 1989; Zhang et al., 1991). The principal cause for the depletion is because a portion of the plant uptake K is not returned to the soil. With the increase of N and P fertilizer use and crop yields, more K has been taken up by the bumper harvests of grain crops or forage crops. Of these, forage crops such as alfalfa (Medicago sativa) and clovers (Melilotus) absorb large amounts of this element, and after harvest of hay or fodder (silage), this element will be removed from soil, leading to a rapid depletion of the soil’s readily available K. Grain crops deplete K from the soil less rapidly, provided only the grain is removed. People have historically consumed most of the grain while the forage or the residues of grain crops have been used for animal production or returned to the soil as OWRs. Most K ingested by animals passes through the intestinal and urinary tracts. If all of the manure, night soil, and OWRs are conserved and uniformly redistributed to the land, little additional K will be needed for soils that initially contain adequate amounts (National Research Council, 1989) such as the dryland soils in China. However, the current situation is not promising. There are three reasons responsible for this situation. First, although most or all the animal manure in dryland areas has been returned to the land, nutrients are often inefficiently recycled because of poor storage and application practices. Runoff, volatilization, and leaching losses of plant nutrients in stored animal manure may be so high that only a fraction of the original nutrients remains to be applied to the cropland. Poor manure hauling and spreading practices add to these losses. However, practices that increase the efficient use of nutrients can be expensive. The cost of proper application, for example, can exceed the value of the increase in available nutrients when compared to inefficient application methods. Second, people are much more concerned today than before about their economic income. Therefore, fewer farmers are concerned with returning OWRs and manure to the soil to save time for working at another job that can provide more income. Third, incorporating the crop’s residues directly into soil is expensive when using big machines, and results obtained from the use of OWRs may not be as good as applying chemical fertilizers. For the correct strategies for utilizing these residues, simple but useful methods that can store more nutrients and increase nutrient use efficiency should be developed. For achieving this purpose, government investments to farmers can provide a basis for implementation of the effective measures.

5.4. Improving crop health for better use of nutrients The efficiency of N use by plants depends on many factors, and the crops themselves may be essential. Growing a crop well requires efficient uptake and transformation of N into economic products. As a direct result, NUE is increased.

168

S. X. Li et al.

The opposite is true for growing the same crop poorly. Thus, the management of crops so that they grow well is as important as the management of nutrients. Improving crop health includes various measures, such as soil improvement, water management, seed quality, and control of weeds, insects, and diseases. Of these, water management may be the most important component on the drylands. Shortage of water supply is the major factor limiting crop production on drylands, and conservation of precipitation may be the only way for minimizing this constraint. In this respect, mulch tillage shows real promise. Two kinds of mulch tillage have been widely used on drylands in China: plastic sheet mulching and straw mulching. Plastic sheet mulching is excellent with regards to conserving soil moisture, preventing drought, raising soil temperature, as well as protecting N fertilizer loss against volatilization and promoting N availability. Thus, it has great potential for increasing crop yields and raising NUE. Having these advantages, this technique has been practiced in recent years on the drylands, not only for vegetables, cotton, and tobacco, but also for grain crops. Plastic sheet mulching in Gansu Province has increased wheat yield 1500 kg ha1 or so, and it is now widely extended to other provinces. The problem for using this technique lies in the fact that the plastic sheets are difficult to decompose so they become pollutants. Scientists have estimated that if more than 100 kg ha1 of these plastic sheets are accumulated, they will cause some pollution to the soil. The use of readily decomposable materials to make sheets for mulching instead of plastic sheets is an urgent need for investigation. Straw mulching has the same advantages as plastic sheet mulching except that it may decrease the soil temperature and delay the germination and maturity of some crops, especially spring-sowed crops. However, since more soil moisture, the major factor affecting plant growth, is retained, crop yields and nutrient use efficiency have been significantly increased by such mulching. Combined with minimum or no tillage, this technique will play an increasing important role in agriculture sustainability. For wide extension of this technique on the drylands, research should be encouraged for solving the competing needs of using straw for fuel or forage, and for deciding the optimal rate of straw, mulched crops, and tillage methods. For both plastic and straw tillage systems, attention must be given to soil N management. Our results have shown that with plastic mulch, plants require much more N than conventional tillage due to higher yield. Furthermore, the soil OM is decomposed much faster, resulting in a rapid decline of OM. In contrast, with straw mulch, the decomposition process of OM is slower because of lower soil temperature, and yield reduction in some cases is caused by deficiency of nutrients, especially N. Since the two tillage systems have only been used for a short period of time, demonstration of N management is urgently needed to obtain more definitive information.

Nitrogen in Dryland Soils

169

6. Conclusions Due to poor plant growth induced by shortage of water supplies, serious wind and water erosion, and low input of fertilizes by farmers, the arable dryland soils in China are low in OM and total N. Thus, N deficiency in most dryland areas is a major nutritional constraint for crop production. Nitrogen loss from dryland soils and fertilizers added to soil is mainly caused by ammonia volatilization, and nitrate N accumulation in soil profile in and beyond root zones while denitrification loss is negligible. Application of N fertilizers has greatly increased crop yield and quality. However, excessive addition of N fertilizer that has occurred in some places has resulted in great attention to the rational management of soil N. For improvement of NFR and NUE, the following points should be kept in mind: 1. Application of N fertilizer with OF increases the NUE, and at the same time significantly increases crop yield and WUE. Since N in OF is slowly mineralized, P is too high for plant requirements, OF should be applied together with N fertilizers, but separately from P fertilizer. 2. In most dryland areas, P deficiency has limited crop production and N fertilizer efficiency, and in some lands even prevented crops from responding to N fertilizer. Combining the use of N fertilizer with P fertilizer can increase NFR and NUE as well as WUE. If N fertilizer is mixed with acid P fertilizer, N loss by volatilization can be reduced. 3. Deep application can place N fertilizer in the layer where more water is available, nutrients are deficient, and more roots are present for efficiently using N from the fertilizer applied and the moisture from the soil in addition to reduction of N loss by volatilization, and thus can increase crop yield, and fertilizer use efficiency. Deep application can be conducted with deep plowing so that N fertilizer can be placed in a suitable layer for plant use. 4. In areas without supplemental irrigation, early application of N fertilizer to wheat and other autumn-sown crops should be encouraged while for maize with full irrigation, N fertilizer should be divided into four portions with one portion applied at sowing, one portion at elongation, and two portions before heading. 5. A crop can take up 45–70% of its total N from the soil. Therefore, N fertilizer applications should be made according to the SNSC. Several biological and chemical procedures have been used for evaluating the SNSC, yet none of the methods has proved suitable for agricultural practice. A large number of field results demonstrated that the cumulative amount of nitrate from the 0 to 1 m soil layer was very well and significantly correlated to crop uptake N with a correlation coefficient of

170

S. X. Li et al.

0.908, giving a very satisfactory index of soil availability. Due to high nitrate N accumulated in soil profile, the potentially mineralizable N, estimated by either incubation or chemical reagent extraction, did not show a good correlation. In contrast, in soils with low amounts of nitrate N accumulated in soil profile, some methods for determining mineralizable N exert a certain role in reflecting the SNSC, having certain potentials for use. 6. On drylands, nitrate N is the major form taken up by plants, and the major crops, wheat and maize, respond better to nitrate N than to ammonium N. 7. Applying adequate amounts of N fertilizer, rotating legumes in cropping sequences, using organic materials in combination with chemical fertilizers, and improving crop health for better use of nutrients are some important aspects for consideration in the future.

ACKNOWLEDGMENTS This work was part of the projects (30230230, 40671107, 49070041, 39070526, 39470409, 39770425, 49890330, and 30070429) supported by the National Natural Science Foundation of China (NSFC) and part of the program (IRT0749) supported by the Ministry of Education of China. The authors would like to take the opportunity to express their sincere thanks to the NSFC, for its kindness of supporting these projects in succession, and also to the Ministry of Education of China for its kindness of supporting such program.

REFERENCES Addiscott, T. M., and Benjamin, N. (2004). Nitrate and human health. Soil Use Manage. 20, 1–12. Aggarwal, R. K., Kaul, P., and Praveen-Kumar, P. (1987). Ammonia volatilization losses from urea and their possible management for increasing nitrogen use efficiency in an arid region. J. Arid Environ. 13, 163–168. Aggarwal, R. K., and Praveen, K. (1994). Availability and management of nitrogen in soils of arid ecosystem. Ann Arid Zone 33, 1–18. Alexander, M. (1978). ‘‘Introduction to Soil Microbiology’’. Wiley Eastern India Limited, New Delhi. Al-Kanani, T., Mackenzie, A. F., and Blenkhorn, H. (1990). The influence of formula modifications and additives on ammonia loss from surface-applied urea ammonium nitrate solutions. Fertil Res. 22, 49–59. Bacon, P. E. (1995). ‘‘Nitrogen Fertilization in the Environment’’, pp. 1–25. Marcel Dekker, New York. Bartnicki, J., and Alcamo, J. (1989). Calculating nitrogen deposition in Europe. Water Air Soil Pollut. 47, 101–123. Bergkvist, B., and Felkeson, L. (1992). Soil acidification and element fluxes of a Fagus Sytvatica forest as influenced by simulated nitrogen deposition. Water Air Soil Pollut. 65, 111–133.

Nitrogen in Dryland Soils

171

Bock, B. R., and Hergert, G. W. (1991). Fertilizer nitrogen management. In ‘‘Managing Nitrogen for Groundwater Quality and for Profitability’’ (R. F. Folletti, Dr. Keeney, and R. M. Cruse, Eds.), pp. 139–164. Soil Science Society of America, Inc. Publisher, Madison, WI. Borst, N. P., and Mulder, C. (1971). Nitrogen contents, nitrogen fertilizer rates and yield of winter barley on sandy, clay and silty soils in North Holland. Bedryfsonwikkeling 2, 31–36. Boswell, F. C., Richter, A. C., and Casida, L. E. (1962). Available soil nitrogen measurements by microbiological techniques and chemical methods. Soil Sci. Soc. Am. Proc. 26, 254–257. Bremner, J. M. (1965). Nitrogen availability indexes. In ‘‘Methods of Soil Analysis, Part 2’’ (C. A. Black, Ed.), pp. 1324–1345. ASA-SSSA, Inc. Publisher, Madison,WI. Bumb, B. L. (1995). World nitrogen supply and demand: an overview. In ‘‘Nitrogen Fertilization in the Environment’’ (P. E. Bacon, Ed.), pp. 1–40. Marcel Decker, New York. Bundy, L. G., and Meisinger, J. J. (1994). Nitrogen availability indices. In ‘‘Methods of Soil Analysis. Part 2 – Microbiological and Biochemical Properties’’ (R. W. Weaver, J. S. Angle, and P. S. Bottomley, Eds.), pp. 951–984. Soil Science Society of America, Inc. Publisher, Madison,WI.  Cao, W., and Tibbitts, T. W. (1993). Study of various NHþ 4 /NO3 mixtures for enhancing growth of potatoes. J. Plant Nutr. 16, 1691–1704. Cao, W., and Tibbitts, T. W. (1994). Responses of potatoes to solution pH levels with different forms of nitrogen. J. Plant Nutr. 17(1), 109–126. Cao, Z. H., Zhou, X. R., Li, Z. L., Ling, Y. X., Zhu, Y. N., Wang, E. P., and Zhao, Z. S. (1991a). The accumulation of dry matter in tobacco and the effect of soil environment on alkaloid of tobacco. In ‘‘Soil and Fertilization for Production of High Quality Tobacco’’ (Z. H. Cao, Ed.), pp. 3–9. Jiangsu Science and Technology Publishing House, Nanjing, China. Cao, Z. H., Zhou, X. R., Li, Z. L., Ling, Y. X., Zhu, Y. W., Wang, E. P., and Zhao, Z. S. (1991b). The K contents in tobacco leaves in relation with soil environmental conditions in China. In ‘‘Soil and Fertilization for Production of High Quality Tobacco’’ (Z. H. Cao, Ed.), pp. 17–28. Jiangsu Science and Technology Publishing House, Nanjing, China. Carter, J. N., Jensen, M. E., and Bosma, S. M. (1974). Determining fertilizer needs for sugar beets from residual soil nitrate and mineralizable nitrogen. Agron. J. 66, 319–323. Chen, L. Z., Xia, Z. L., and Wu, S. J. (1989). Application of organic and inorganic fertilizers in China. In ‘‘Proceedings of the International Symposium on Balanced Fertilization’’. (Institute of Soil and Fertilizers, China’s Academy of Agriculture Sciences, Ed.), pp. 380–389. China Agriculture Press, Beijing, China. Cheng, S. Y., Qiao, C. F., Han, G. M., and Guo, X. D. (1987). The effect of fertilizer application upon the improvement of soil moisture productive efficiency in wheat on the drylands. Agric. Res. Arid Areas 5(2), 58–65. Choi, B. C. K. (1985). N-nitroso compounds and human cancer: A molecular epidemiological approach. Am, J. Epidem. 121, 737–743. Christianson, C. B., Bationo, A., Henao, J., and Vlek, P. L. G. (1990). Fate and efficiency of N fertilizers applied to pearl millet in Niger. Plant Soil 125, 221–231. Cornfield, A. H. (1960). Ammonia released on treating soils with N sodium hydroxide as a possible means of predicting the nitrogen – supplying power of soils. Nature 187, 260–261. Cuesta-Santos, O., Collazo, A., and Wallo, A. (2001). Deposition of atmospheric nitrogen compounds in humid tropical Cuba. In ‘‘Optimizing Nitrogen Management in Food and Energy Production and Environmental Protection: Proceeding of the 2nd International

172

S. X. Li et al.

Nitrogen Conference on Science and Policy’’ ( J. Galloway, E. Cowling, J. W. Erisman, J. Wisniewski, and C. Jorden, Eds.), Vol. 1 (S2), pp. 238–244. The Scientific World. Dai, T. B., and Cao, W. X. (2000). The effect of enhancement of ammonium nutrition on promotion of early growth of wheat. J. Triticeae Crops 20(4), 42–46. Dai, T. B., Cao, W. X., and Li, C. D. (1998). The physiological effect of enhancement of ammonium nutrition. Commun. Plant Physiol. 34(6), 488–493. Ding, H., and Cai, G. X. (2001). The N2O emission and N loss from nitrification and denitrification in maize-Fluvo-aquic soil system. Sci. Agric. Sin. 34(4), 416–421. Duan, Y. H., Zhou, Y. L., and Wu, S. R. (1990). The effect of soil properties on ammonia volatilization. Chinese J. Soil Sci. 21(3), 131–134. ECCAY (Editing Committee of China Agricultural Yearbook). (1991). In ‘‘China Agricultural Yearbook’’ (Editing Committee of China Agricultural Yearbook, Eds.), China Agriculture Press, Beijing, China. ECCAY (Editing Committee of China Agricultural Yearbook). (1996). In ‘‘China Agricultural Yearbook’’. (Editing Committee of China Agricultural Yearbook, Eds.), China Agriculture Press, Beijing, China. Evans, L. T. (1998). ‘‘Feeding the Ten Billion’’, pp. 196–224. Cambridge University Press, Cambridge. FAO. (2001). ‘‘Statistical Databases’’. Food and Agriculture Organization of the United Nations, Eds.), http://www.fao.org. FAO. (2008). ‘‘Statistical Databases’’. Food and Agriculture Organization of the United Nations, Eds.), http://www.fao.org. FAO. (Food and Agricultural Organization of the United Nations). (2005).‘‘FAO Statistical Yearbook 2004’’. Vol. 1, p. 364. Roma, Italy. Fenn, M. (1998). Atmospheric ammonia and ammonium transport in Europe and critical loads: A review. Nutrient Cycling Agroecosyst. 51, 5–17. Fine, D. H. (Ed.). (1982). N-Nitresocompounds: Occurrences and Biological Effects’’, Vol. 41, p. 37 9IACR Sci. Publ. Galloway, J. N., and Cowling, E. B. (2002). Reactive nitrogen and the world: 200 years of change. Ambio 31(2), 64–71. Gangolli, S. D., Van Den Brandt, P. A., Feren, V. J., Jan-Zowsky, C., Koeman, J. H., Speijers, G. J. A., Walker, R., and Winshnok, J. S. (1994). Nitrate, nitrite, and N-nitroso compounds. Eur. J. Pharm. 292, 1–38. Gentry, L. E., and Below, F. E. (1993). Maize productivity as influenced by form and availability of nitrogen. Crop Sci. 33, 491–497. Gianello, C., and Bremner, J. M. (1986a). A simple chemical extraction method for assessing potentially available organic nitrogen in soil. Comm. Soil Sci. Plant Anal. 17, 195–214. Gianello, C., and Bremner, J. M. (1986b). Comparison of chemical methods of assessing potentially available nitrogen in soil. Comm. Soil Sci. Plant Anal. 17, 215–236. Gong, Z. T. (1992). The geochemistry direction in red earth research. In ‘‘Li Qingkui and China’s Soil Science Development’’ (Nanjing Institute of Soil Science, Ed.), pp. 19–27. Jiangsu Science and Technology Publishing House, Nanjing, Jiangsu, China. Goris, J. E., and Ludwick, A. E. (1978). Nitrogen fertilization of dryland wheat in eastern Colorado. Colorado State Univ. Program Report. No. 12. Gracie, D. S., and Khalil, F. (1939). The quantity, distribution and composition of the organic matter and available nitrogen in Egyptian soils. Egypt. Min. Agr.Tech. Sci. Service Bul. 222. Grennfelt, D., and Hultberg, H. (1986). Effect of nitrogen deposition on the acidification of terrestrial and aquatic ecosystems. Water Air Soil Pollut. 30, 945–963. Guo, J. R., and Li, G. R. (1989). The state of N fertilizer research and its use development. Soil Fertilizer (4), 28–33.

Nitrogen in Dryland Soils

173

Hardy, R. W. F., and Havelka, U. D. (1975). Nitrogen fixation research: A key to world food? Science (Washington D C) 188, 633–643. He, W. S., Li, S. X., and Li, H. T. (1998a). Effect of nutritional solution pH on wheat uptake of ammonium N and nitrate N. Soils 30(3), 143–146124. He, W. S., Li, S. X., and Li, H. T. (1998b). The characteristics of rice in absorption of ammonium N and nitrate N. Chinese J. Rice Sci. 12(4), 249–252. He, W. S., Li, S. X., and Li, H. T. (1999). Characteristics of ammonium N and nitrate N uptake at different growth stages of six crops. Acta Agron. Sin. 25(2), 221–226. Hong, Q. W., and Huang, B. F. (1994). ‘‘Some Matters in Relation with Soil Science and Plant Nutrition in Agricultural Production’’, pp. 110–123. Science Press, Beijing, China. Hu, T. T., and Li, S. X. (1993a). The effectiveness of different biological indices in reflecting soil nitrogen-supplying capacities. Agric. Res. Arid Areas 11(Suppl), 56–61. Hu, T. T., and Li, S. X. (1993b). The Relationships between mineralisable or mineral N determined by different methods and plant uptake nitrogen. Agric. Res. Arid Areas 11 (Suppl.), 62–67. Hu, T. T., and Li, S. X. (1993c). The contribution of mineral nitrogen from 0–100 cm layer to plant uptake nitrogen. Agric. Res. Arid Areas 11(Suppl.), 68–73. Hu, T. T., and Li, S. X. (1993d). A reliable soil N availability index: Initial NO 3 -N in soil profile. Agric. Res. Arid Areas 11(Suppl.), 74–82. IAEA/FAO. (1974). Isotope studies on wheat fertilization. Tech. Report Series 1974, No. 157. Intern Atomic Energy Agency, Vienna. ISF (Institute of Soil and Fertilizer, Chinese Academy of Agricultural Sciences). (1994). Scitechnological strategy studies for high yield, high efficiency, high quality agriculture and high use efficiency of water and fertilizer from ‘‘Nine-Five’’ Planning Year to 2010.In ‘‘Proceeding of National Symposium on Sci-Technological Planning and Strategy Study for High Yield, High Efficiency, High Quality Agriculture from ‘‘Nine-Five’’ Plan Year to 2010 in China’’Beijing China (Education Department, Ministry of Agriculture, China Agricultural Manegement Society, Eds.), pp. 131–135. Ismunadji, M., and Dijkshoorn, W. (1971). Nitrogen nutrition of rice plants measured by growth and nutrient contents in pot experiment. Ionic balance and selective uptake. Neth J. Agric. Sci. 19, 223–236. Jayanty, R. K. M., Simonaitis, R., and Heicklen, J. (1976). The reaction of NH2 with NO and O2. J. Phys. Chem. 80, 381–385. Jewitt, T. N. (1942). Losses of ammonia from ammonium sulfate applied to alkaline soils. Soil Sci. Soc. Am. Proc. 54, 401–409. Jin, S. L. (1989). Balanced fertilization of spring wheat in the Hexi Corridor. In ‘‘Proceedings of the International Symposium on Balanced Fertilization’’(Institute of Soil and Fertilizers, China’s Academy of Agriculture Sciences, Ed.), pp. 179–184. China Agriculture Press, Beijing, China. Jin, J. Y., Gao, G. L., and Wang, Z. L. (1989). K-suppying capacities of some northern soils in China and the developing trend for utilization of K fertilizers. In ‘‘Proceedings of the International Symposium on Balanced Fertilization’’ (Institute of Soil and Fertilizers, China’s Academy of Agriculture Sciences, Ed.), pp. 1–5. China Agriculture Press, Beijing, China. Ju, X. T., and Li, S. X. (1998). The effect of temperature and moisture on nitrogen mineralization in soils. Plant Nutrition Fertilizer Sci. 4(1), 37–42. Katyal, J. C., Singh, B., Vlek, P. L. G., and Buresh, R. J. (1987). Efficient nitrogen use as affected by urea application and irrigation sequence. Soil Sci. Soc. Am. J. 51, 366–370. Keeney, D. R. (1982). Nitrogen – Availability Indices. 2nd edn., Part 2, A. L. Page, pp. 711–734. ASA-SSSA, Inc. Publisher. Madison,WI. Keeney, D. R., and Bremner, J. M. (1966). Comparison and evaluation of laboratory methods of obtaining an index of soil nitrogen availability. Agron. J. 58, 498–503.

174

S. X. Li et al.

Kirkby, E. A. (1967). A note on the utilization of nitrate, urea, and ammonium nitrogen by Chenopodium album. Z. Pflantzenerna¨hr. Bodenkd. 117, 204–209. Lees, A., and Quastel, J. H. (1946). Kinetics and effect of poisons on soil nitrification as studied by soil perfusion techniques. Biochemistry 40, 803–815. L’hirondel, J., and L’ hirondel, J. L. (2001). Nitrate and Man: Toxic, Harmless or Beneficial? CAB International, Wallingford. Li, Y. H. (1965). Effect of the chemical properties of calcareous soils on the response of crop yield to phosphate fertilizer in Guanzhong areas of Shaanxi Province. Acta Pedologica Sin. 13(1), 39–45. Li, Y. S. (1989). Water relations of soil and plants and its regulation. In ‘‘Soil and Agriculture in the Loess Plateau’’ (X. M. Zhu, Ed.), pp. 342–363. China Agriculture Press, Beijing, China. Li, S. X. (1992). Nitrogen loss from plants by volatilization I. Nitrogen loss from aboveground parts of wheat during later period of growth. Acta Universitalis Agriculturalis Boreali-Occidentalis 20(Suppl.), 1–6. Li, S. X. (1999). Management of soil nutrients on drylands in China for sustainable agriculture. Soil Environ. 2(4), 293–316. Li, S. X. (2002). Ways and strategies for increasing fertilizer nitrogen efficiency in dryland soil. Acta Pedologica Sin. 39(Suppl.), 56–75. Li, S. X. (2007). ‘‘Dryland Agriculture in China’’, pp. 400–407;466–470. Science Press, Beijing, China. Li, S. X., Chang, Q., and He, L. (1992a). Nitrogen loss from plants by volatilization II. The relationship among nitrogen reduction from aboveground plants at maturity and soils, crops as well as fertilization. Acta Universitalis Agriculturalis Boreali-Occidentalis 20(Suppl.), 7–11. Li, S. X., Cun, D. G., Gao, Y. J., He, H. X., and Li, S. Q. (1993a). Mineral nitrogen introduced into soil by precipitation on Loess dryland. Agric. Res. Arid Areas 11(Suppl.), 83–92. Li, S. X., Fu, H. F., Yuan, H. L., and Xiao, J. Z. (1990a). Comparison of several methods reflecting the nitrogen supplying power in rainfed land soils. Soils 22, 190–193. Li, S. X., Gao, Y. J., and Li, S. Q. (1995a). The residual effects of N fertilizers. In ‘‘The Principle of Relationship between Fertilizer and Water of Dry Farming and Its Regulation Technology’’ (D. S. Wang, Ed.), pp. 351–355. Agricultural Science and Technology Publishing House of China, Beijing, China. Li, S. X., Gao, Y. J., Wang, X. Q., Li, S. Q., and Du, J. J. (1995b). The effect of water on nutrient movement. In ‘‘The Principle of Relationship Between Fertilizer and Water of Dry Farming and Its Regulation Technology’’ (D. S. Wang, Ed.), pp. 6–11. Agricultural Science and Technology Publishing House of China, Beijing, China. Li, Z. R., and Li, S. X. (1992). Nitrogen loss from plants by volatilization III. Ammonia loss from soybean during its growth. Acta Univ. Agric. Boreali-occidentalis 20(Suppl.), 12–17. Li, J. M., and Li, S. X. (2003a). Relation of mineralizable N to organic N components in dark loessial soils. Pedosphere 13(3), 279–289. Li, J. M., and Li, S. X. (2003b). Relationship between mineralizable N and organic N components. Plant Nutrition Fertilizer Sci. 9(2), 158–164. Li, S. X., Li, S. Q., Gao, Y. J., Wang, X. Q., and Du, J. J. (1995c). The effect of water on fertilizer use efficiency. In ‘‘The Principle of Relationship Between Fertilizer and Water of Dry Farming and Its Regulation Technology’’ (D. S. Wang, Ed.), pp. 74–79. Agricultural Science and Technology Publishing House of China, Beijing, China. Li, J. K., and Lin, B. (1995). Fertilizer resources in China and progress made in fertilization. In ‘‘Advances in China’s Soil Science’’ ( J. K. Li and B. Lin, Eds.), China Agriculture Press, Beijing, China.

Nitrogen in Dryland Soils

175

Li, S. X., and Liu, C. Y. (1993). Ammonia volatilization from calcareous soil. I. Effects of soil properties on N loss by volatilization. Agric Res Arid Areas 11(Suppl.), 125–129. Li, S. X., and Ma, S. J. (1993). Ammonia volatilization from calcareous soil. II. The relationship between ammonia loss from N fertilizers and applied methods. Agric. Res. Arid Areas 11(Suppl.), 130–134. þ Li, S. Q., Pu, T. Y., and Li, S. X. (1993b). Nitrification of NHþ 4 -N and the fixation of NH4 N by clay minerals. Agric. Res. Arid Areas 11(Suppl.), 99–107. Li, S. X., and Wang, H. P. (1993a). Rational application of fertilizers on drylands. IX. The influence of different application rates of ammonium and nitrate N on maize yield. Agric. Res. Arid Areas 11(Suppl.), 45–49. Li, S. X., and Wang, Z. H. (1993b). Ammonia volatilization from calcareous soil. III. Comparison of two methods for determination of volatilized ammonia. Agric. Res. Arid Areas 11(Suppl.), 135–140. Li, S. X., and Wang, Z. H. (2005). Nitrate accumulation in plants and its relation to crop yield. In ‘‘Contributed Papers’’. (Z. L. Zhu, K. Minami, and G. X. Xing, Eds.), 3rd International nitrogen Conference Oct. 12–16, 2004 Nanjing China, pp. 220–225. Science Press, Beijing, China. Li, S. X., and Wang, Z. H. (2006). Dryland in Eastern Asia. In ‘‘Dryland Agriculture’’ (G. A. Peterson, P. W. Unger, and W. A. Payne, Eds.), pp. 671–731. ASA-CSSA-SSSA, Inc. Publisher, Madison, WI. Li, R. G., Wang, S. M., Wang, K. W., Wu, Y. Z., and Wu, W. K. (1982). Studies on the uptake of soil N and fertilizer by winter wheat and N balance. Chinese J. Soil Sci. 13, 21–22. Li, S. X., and Xiao, L. (1992). Distribution and management of drylands in the People’s Republic of China. Adv. Soil Sci. 18, 147–302. Li, S. X., Yuan, H. L., He, H. X., and Li, S. Q. (1992b). Assessing nitrogen fixation efficiency of two legume plants from nitrogen intake in three successively grown crops. Chinese J. Soil Sci. 23(3), 130–133. Li, S. X., and Zhao, B. S. (1993a). Rational application of fertilizers on drylands. II. Application of organic fertilizer rationally combined with chemical fertilizers. Agric. Res. Arid Areas 11(Suppl.), 7–12. Li, S. X., and Zhao, B. S. (1993b). Rational application of fertilizers on drylands. III. The relationship between fertilization and soil water supply. Agric. Res. Arid Areas 11(Suppl.), 13–18. Li, S. X., and Zhao, B. S. (1993c). Rational application of fertilizers on drylands. V. The effectiveness and function of deep application of fertilizers in raising crop production and fertilizer efficiency. Agric. Res. Arid Areas 11(Suppl.), 23–27. Li, S. X., and Zhao, B. S. (1993d). Rational application of fertilizers on drylands. VI. The function and significance of early fertilization in raising crop production and fertilizer efficiency. Agric. Res. Arid Areas 11(Suppl.), 28–34. Li, S. X., and Zhao, B. S. (1993e). Rational fertilization on drylands VI. Fertilization and irrigation. Agric. Res. Arid Areas 11(Suppl.), 14–17. Li, C. W., Zhao, B. S., Hua, T. M., and Li, H. T. (1987). Some matter of fertilization in WEibai dryland region. In ‘‘Proceedings of the International Symposium on Dryland Farming (Sponsored by Agronomy Society of China, Agricultural Academy of China and Northwest Agricultural University (September 18–22, 1987). Yangling, Shaanxi, China, pp. 408–411. Northwest Agricultural University Press, Yangling, Shaanxi, China Vol. 2). Li, S. X., Zhao, B. S., and Li, C. W. (1976a). The effect of fertilizer application time on wheat yields. In ‘‘Selected Works of Shaanxi Agricultural Science and Technology’’ (Shaanxi Agricultural Ministry, Ed.), pp. 78–82. Shaanxi People’s Publishing House, Xi’an, Shaanxi, China.

176

S. X. Li et al.

Li, S. X., Zhao, B. S., and Li, C. W. (1976b). Increasing fertilizer efficiency by deep application. In ‘‘Selected Works of Shaanxi Agricultural Science and Technology’’ (Shaanxi Agricultural Ministry, Ed.), pp. 73–76. Shaanxi People’s Publishing House, Xi’an, Shaanxi, China. Li, S. X., Zhao, B. S., and Li, C. W. (1978). Rational application of phosphate fertilizer to wheat, part I. Acta Collegii Septentrionali Occidentali Agriculturae 6(1), 77–85. Li, S. X., Zhao, B. S., and Li, C. W. (1979). Rational application of phosphate fertilizer to wheat, part II. Acta Collegii Septentrionali Occidentali Agriculturae 7(1), 55–99. Li, S. X., Zhao, B. S., and Li, C. W. (1990b). The effect of application of N, P and organic fertilizers on N-fixing performance of leguminous crops. Chinese J. Soil Sci. 21(1), 13–15. Li, Q. K., Zhu, Z. L., and Yu, T. R. (1997). In ‘‘Fertilizer Matter for China’s Agriculture Development and Sustainability’’ (Q. K. Li, Z. L. Zhu, and T. R. Yu, Eds.), pp. 3–5. Jiangxi Science and Technology Press. Nanchang, China. Liang, W. L., Liu, H. L., and Hu, H. J. (1987). The effect of nitrogen and phosphorus fertilization on water use efficiency of dryland wheat. In ‘‘Proceedings of the International Symposium on Dryland Farming’’ (Sponsored by Agronomy Society of China, Agricultural Academy of China and Northwest Agricultural University, September 18– 22, 1987. Yangling, Shaanxi, China, pp. 412–414. Northwest Agricultural University Press, Yangling, Shaanxi, China, Vol. 2. Liang, D. L., Tong, Y. A., and Emteryd, O. (2003a). The temporal and spatial variation of N2O concentration in the manural loessial soil profile. Acta Ecol. Sin. 23(4), 731–734. Liang, D. L., Tong, Y. A., and Emteryd, O. (2003b). The effects of available C and N on N2O emission. J. Northwest Sci. Technol. University Agric. For. 31(1), 43–48. Liang, D. L., Tong, Y. A., Emteryd, O., Fan, R. X., and Zhang, S. L. (2002a). The effect of alteration of dryness and wetness of soil on gaseous loss of N2O of drylands. Agric. Res. Arid Areas 20(2), 28–31. Liang, D. L., Tong, Y. A., Emteryd, O., Li, S. X., Fan, R. X., and Zhang, S. L. (2002b). N2O emission from drylands under rainfall and irrigation conditions. Plant Nutrition Fertilizer Sci. 8(3), 298–302. Liang, D. L., Tong, Y. A., Emteryd, O., Li, S. X., Zhao, H. B., and Ma, L. Y. (2002c). N2O loss from loess soil profile. Acta Pedologica Sin. 38(6), 872–879. Liang, D. L., Tong, Y. A., Emteryd, O., and Ma, X. L. (2002d). Study on N2O emission with different N rates in vegetable fields. J. Northwest Sci. Technol. University Agric. For. 30 (2), 28–31. Lin, B., Li, J. K., and Jin, J. Y. (1999). Outlook on fertilizer use in China in next century. In ‘‘Research Progress in Plant Protection and Plant Nutrition,’’ pp. 345–351. (China’s Association of Agricultural Science Societies, Ed.). China Agriculture Press, Beijing, China. Livens, J. (1959). Contribution a` l’e´tude de l’azote mineralisable du sol. Agricultura Louvain 7, 27–44. Lu¨, D. Q., and Li, Y. (1987). Enhancement of the rate of nitrogen utilization for increasing wheat production on Loess Plateau. In ‘‘Proceedings of the International Symposium on Dryland Farming’’ (Sponsored by Agronomy Society of China, Agricultural Academy of China and Norwest Agricultural University, September 18–22, 1987 Yangling, Shaanxi, China) Vol. 2. pp. 236–248. Northwest Agricultural University Press, Yangling, Shaanxi, China. Lu¨, D. Q., Li, Y., and Tan, W. L. (1989). Wheat response to N and P balanced fertilization and the factors affecting the response in Shaanxi loess region. ‘‘Proceedings of the International Symposium on Balanced Fertilization’’, In (Institute of Soil and Fertilizers, China’s Academy of Agriculture Sciences, Ed.), pp. 317–324. China Agriculture Press, Beijing, China.

Nitrogen in Dryland Soils

177

Ma, H. P. (1987). Studies on relationship between fertilized black soil and its physical properties. In ‘‘Proceedings of the International Symposium on Dryland Farming’’ (Sponsored by Agronomy Society of China, Agricultural Academy of China and Northwest Agricultural University, September 18–22, 1987 Yangling, Shaanxi, China Vol. 2, pp. 249–257. Northwest Agricultural University Press, Yangling, Shaanxi, China. Ma, L. S. (1992). Nitrogen management for arable lands and environmental quality as well as crop quality. In ‘‘Nitrogen in Soils of China’’ (Z. L. Zhu and Q. X. Wen, Eds.), pp. 267–287. Jiangsu Science and Technology Press, Nanjing, China. Matson, P., Lohse, K. A., and Hall, S. J. (2002). The globalization of nitrogen deposition: Consequences for terrestrial ecosystems. AMBIO 31(2), 113–119. McTaggart, P., and Smith, K. A. (1993). Estimation of potentially mineralizable nitrogen in soil by KCl extraction. II. Comparison with soil N uptake in the field. Plant Soil 157, 175–184. Mengel, K., and Kirkby, E. A. (2001). ‘‘Principles of Plant Nutrition’’, 5th edn. pp. 161–175, 397–434. Kluwer Academic Publisher, Dordrecht. Minotli, P.L (1978). Nitrogen in the Environment. In ‘‘Soil-Plant-Nitrogen Relationship’’ (D. R. Nielsen and J. G. MacDonald, Eds.), Vol. 2, pp. 235–252. Academic Press, New York. Mosier, A., Kroeze, C., Nevison, C., Oenema, O., Seitzinger, S., and van Cleemput, O. (1998). Closing the global N2O budget: Nitrous oxide emission through the agricultural nitrogen cycle. Nutr. Cycl. Agroecosyst. 52, 225–248. Nanjing Institute of Soil Science, Academia Sinica. (1978). ‘‘Soils of China’’ (Nanjing Institute of Soil Science, Eds.), pp. 360–375. Science Press, Beijing, China. National Research Council. (1989) Research and Science; Economic Evaluation of Alternative Farming Systems. Alternative Agriculture’’ (National Research Council, Eds.), pp. 89–195 National Academy Press Washington, D.C. 89–195. Nommik, H. (1976). Predicting the nitrogen-supplying power of acid forest soils from data on the release of CO2 and NH3 on partial oxidation. Commun. Soil Sci. Plant Anal. 7, 569–584. Nuske, A., and Richter, J. (1981). N-mineralization in loess parabowearthes: Incubation experiments. Plant Soil 59, 237–247. OECD. (1998). Agriculture and the Environment: Issues and Policies’’ OECD, Paris. ien, A., and Selmer-Olsen, A. R. (1980). A laborotary method for evaluation of available nitrogen in soil. Acta Agric. Scand. 30, 149–149. Pegerbyrgckow, A. B. (1961). ‘‘Instruction Book for Agrochemistry Laboratory Work’’, pp. 111–115. Agricultural Literature, Journal and Pictorial Press, Moscow. Peng, L., and Peng, X. L. (1982). Regionalization of the soil N contents on the Loess Plateau and ways for raising its amounts. Bull. Water Soil Conserv. 2(4), 30–35. Peterson, L. A., Attoe, O. J., and Ogden, W. B. (1960). Correlation of nitrogen soil tests with nitrogen uptake by the tobacco plant. Soil Sci. Soc. Am. Proc. 24, 205–209. Praveen-Kumar, R. K., and Aggarrwal, R. K. (1988). Reduction of ammonia volatilization from urea by rapid nitrification. Arid Soil Res. Revalidation 2, 131–138. Pryor, S. C., Barthelmie, R. J., and Carreiro, M. (2001). Nitrogen deposition to and cycling in a deciduous forest. ‘‘Optimizing Nitrogen Management in Food and Energy Production and Environmental Protection: Proceeding of the 2nd International Nitrogen Conference on Science and Policy’’ ( J. Galloway, E. Cowling, J. W. Erisman, J. Wisniewski, and C. Jorden, Eds.), Vol. 1 (S2). pp. 245–254. The Scientific World. Raab, K. T., and Terry, N. (1994). Nitrogen source regulation of growth and photosynthesis in Beta vulgaris I. Plant Physiol. 105, 1159–1166. Raun, W. R., and Johnson, V. G. (1999). Improving nitrogen use efficiency for cereal production. Agron. J. 91, 357–363.

178

S. X. Li et al.

Rees, R. M., Roelcle, M., Li, S. X., Wang, X. Q., Li, S. Q., Stockdale, E. A., McTaggart, I. P., Smith, K. A., and Richter, J. (1997). The effect of fertilizer placement on nitrogen uptake and yield of wheat and maize in Chinese loess soil. Nutrient Cycling Agroecosyst. 47, 81–91. Richard, T. A., Attoe, O. J., Moskal, S., and Truog, E. A chemical method for determining available soil N ‘‘Trans. Intern. Congr. Soil Sci. 7th’’ Vol. 2, pp. 28–35. Madison, WI, USA. Richter, J., Nuske, A., and Boehmer, V. (1980). Simulation of nitrogen mineralization and transport in Loess-Parabrownearthes: Plot experiment. Plant Soil 54, 237–247. Richter, J., Nuske, A., and Habenicht, W. (1982). Optimized N-mineralization parameters of loess soils from incubation experiments. Plant Soil 68, 379–388. Rockman, O.C, and Granli, T (1991). Human health aspects of nitrate intake from food and water. In ‘‘Chemistry of Agriculture and Environment’’ (M. L Richardson, Ed.), pp. 373–388. The Royal Society of Chemistry, Cambridge. Roelcke, M., Li, S. X., Tian, X. H., Gao, Y. J., and Richter, J. (2002). In situ comparisons of ammonia volatilization from N fertilizers in Chinese loess soils Nutrient Cycling Agroecosyst. 62, pp. 73–88. SAADS (Shaanxi Academy of Agricultural Design and Survey). (1979). ‘‘The Exploitation and Utilization of the Fertile Water from Wells’’. Shaanxi Academy of Agricultural Design and Survey, Eds.), pp. 19. Shaanxi People’s Publishing House Xi’an, Shaanxi, China. Salahi, A., Geranfar, S., and Korori, S. A. A. (2001) Nitrogen deposition in the Greater Tehran Metropolitan Area. In ‘‘Optimizing Nitrogen Management in Food and Energy Production and Environmental Protection: Proceeding of the 2nd International Nitrogen Conference on Science and Policy’’( J. Galloway, E. Cowling, J. W. Erisman, J. Wisniewski, and C. Jorden, Eds.), Vol. 1 (S2). pp. 261–265. The Scientific World. Santamaria, P., Elia, A., Parente, A., and Serio, F. (1998). Fertilization Strategies for lowering nitrate accumulation in leafy vegetables: Chicory and rocket salad case. J. Plant Nutrition 21(9), 1791–1803. Schwemmer, E. (1990). Nitrat in Gemu¨se. Gemu¨se 3, 172–175. Selmer-Olsen, A. R., ien, A., Baerug, R., and Lyngstad, I. (1981). Evaluation of a KClhydrolysing method for available nitrogen in soil by pot experiment. Acta Agric. Scand. 31, 251–255. Shen, M. X., Zhai, B. J., Dong, H. R., and Li, H. G. (1982). Studies on nitrate accumulation in vegetables. I. Evaluation of nitrate and nitrite in different vegatables. Acta Horticultura Sin. 9(4), 41–48. Shih, S. H. (1959). Cultivation in shallow pits. In ‘‘On Fan Sheng-Chih Shu’’ (Fan ShengChih’s Book)’’ (S. H. Shih, Ed.), pp. 30–31. Science Press, Peking (Beijing), China. Skeffington, R. A. (1990). Accelerated nitrogen inputs: A new problem or a new perceptive? Plant Soil. 128, 1–11. Smith, K. A., and Li, S. X. (1993). Estimation of potentially mineralisable nitrogen in soil by KCl extraction I. Comparison with pot experiments. Plant Soil 157, 167–174. Soper, R. J., and Huang, P. M. (1962). The effect of nitrate nitrogen in the soil profile on the response of barley to fertilizer nitrogen. Can. J. Soil Sci. 43, 350–358. Stanford, G. (1978). Evaluation of ammonium release by alkaline permanganate extraction as an index of soil nitrogen availability. Soil Sci. 126, 244–253. Stanford, G. (1982). Assessment of soil nitrogen availability. In ‘‘Nitrogen in Agricultural Soils’’ (F. G. Stevenson, Ed.), pp. 651–688. ASA-CSSA-SSSA, Inc. Publisher, Madison, WI. Stanford, G., Carter, J. N., Westermann, D. T., and Meisinger, J. J. (1977). Residual nitrate and mineralizable soil nitrogen in relation to nitrogen uptake by irrigated sugar beets. Agron. J. 69, 303–308.

Nitrogen in Dryland Soils

179

Stanford, G., and Epstein, E. (1974). Nitrogen mineralization – water relationships in soils. Soil Sci. Soc Am. Proc. 38, 103–106. Stanford, G., Frere, M. H., and Schwaninger, D. H. (1973a). Temperature coefficient of soil nitrogen mineralization. Soi Sci. 115, 321–323. Stanford, G., Legg, J. O., and Smith, S. J. (1973b). Soil nitrogen availability evaluation based on nitrogen mineralization potentials of soils and uptake of labeled and unlabeled nitrogen by plants. Plant Soil 39, 113–124. Stanford, G., and Smith, S. J. (1972). Nitrogen mineralization potentials of soils. Soil Sci. Soc. Am. Proc. 36, 465–472. Stanford, G., and Smith, S. J. (1978). Oxidative release of potentially mineralizable soil nitrogen by acid permanganate extraction. Soil Sci. 126, 210–218. Su, C. G., Yin, B., Zhu, Z. L., and Shen, Q. R. (2005a). The gaseous loss of N from arable lands and its effect on wet deposit and environments. Soils 37(2), 113–120. Su, C. G., Yin, B., Zhu, Z. L., and Shen, Q. R. (2005b). The ammonia volatilization from rice fields and the wet deposit of N in rice-growing season. Chinese J. App. Ecol. 14(11), 1884–1888. Sun, X. (1957). ‘‘Agrochemistry’’, p. 231. Educational Publishing House, Beijing, China. Tang, K. L., and Kong, X. L. (1989). A study of soil loss and soil degradation on the Loess Plateau in China. In ‘‘Taming the Yellow River: Silt and Floods’’ (L. M. Brush, et al., Eds.), pp. 299–313. Kluwer Academic Publishers, The Netherlands. Tang, Y. X., Zhang, J. H., Xu, X. C., and Dong, G. L. (1986). Soil N supplying capacity, fertilizer N recovery and their use in practice. Chinese J. Soil Sci. 17(5), 204–208. The Fertilizer Institute. (2008). Fertilizer’s role in agriculture. http://www.tfi.org/ publications/farmpolicy.pdfavailable at (verified July 8, 2008). Tian, X. H., and Li, S. X. (1992). Nitrogen loss from plants by volatilization IV. Nitrogen loss from maize during growth. Acta Univ. Agric. Boreali-occidentalis. 20(Suppl.), 12–24. Tian, X. H., Li, S. X., Wang, Z. H., Yin, H. T., and Chen, S. X. (2003). The response of lettuce to N forms. Chinese J. Appl. Ecol. 14(3), 377–381. Truog, E. (1954). Test for available soil nitrogen. Com. Fertilizer 88, 72–73. Tyurin, I. V. (1934). Soil organic matter. Proc. Ural Acad. Sci. 2, 37. van der Paauw, F. (1962a). Effect of winter rainfall on the amount of nitrogen available to crops. Plant Soil 16, 361–380. van der Paauw, F. (1962b). Fertilization with phosphorus. In Extr. Bull. Docum. No. 32 (Intern. Superphosphate Manufacturers’ Association, Ed.). Paris. Vitousek, P. M., Walker, L. R., Whiteaker, L. D., and Matson, P. M. (1993). Nutrient limitations to plant growth during primary succession in Hawaii Volcanoes National Park. Biogeochemistry 23, 197–215. Wahhab, A., Randhawa, M. S., and Alam, S. Q. (1956). Loss of ammonia from ammonium sulfate under different conditions when applied to soils. Soil Sci. 84, 249–255. Walker, R (1990). Nitrate, nitrite and N-nitroso compounds: A review of the occurrence in food and diet and the toxicological implications. Food Add. Cont. 7, 717–768. Wang, L. X. (1983). Studies on the precipitation production potentials in Northwestern Loess Plateau and its exploitation. In ‘‘Selected Works on Arid and Semi-Arid Farming’’ (L. Shan and P. Niu, Eds.), Vol. 2, pp. 54–61. Xi’an Agricultural Information Center, Xi’an, Shaanxi, China. Wang, Z. H., and Li, S. X. (2003). NH3 volatilization from aboveground plants of winter wheat during late growing stages. Agric. Sci. China 2(4), 384–393. Wang, X. Q., Li, S. X., Roelcke, M., Rees, R. M., McTaggar, I., Smith, K. A., and Richter, J. (1995). Effect of N application methods on wheat yield. Agric. Res Arid Areas 13(3), 27–32. Wang, Z. H., Tian, X. H., and Li, S. X. (1998). Differences between vegetable and wheat in nitrate accumulation. Agric. Res. Arid Areas 16(3), 25–30.

180

S. X. Li et al.

Wang, Z. H., Tian, X. H., and Li, S. X. (2001). The Cause of nitrate accumulation in Leafy Vegetables. Acta Ecol. Sin. 21(7), 1136–1141. Waring, S. A., and Bremner, J. M. (1964). Ammonium production in soil under waterlogged conditions as an index of nitrogen availability. Nature 201, 951–952. Watanabe, I., Padre, B. C., and Santiago, S. T. (1981). Quantitative study on nitrification in flooded rice. Soil Sci. Plant Nutrition 22, 378–382. Wehrmann, J., and Scharpf, H. C. (1979). The mineral content of the soil as a measure of the nitrogen fertilizer requirement (Nmin method). Plant Soil 52, 109–126. Whitehead, D. C. (1981). An improved chemical extraction method for predicting the supply of available soil nitrogen. J. Sci. Food. Agric. 32, 359–365. Wright, R. F., and Rasmussen, L. (1998). Introduction to the N ITREX and EXMAN projects. For. Ecol. Manage. 101, 1–7. Wu, J. G. The effects of combination of N, P and B application on increase of wheat production. In ‘‘Proceedings of the International Symposium on Balanced Fertilization’’, (Institute of Soil and Fertilizers, China’s Academy of Agriculture Sciences Ed.), pp. 357– 360. China Agriculture Press, Beijing, China. Xing, G., and Yan, X. (1999). Nitrous oxide emission from agricultural field in China as estimated by the IPCC methodology. Environ. Sci. Policies. 2, 355–361. Yang, X. E., and Sun, X. (1990). The physiological effect of application of NO3-N and NHþ 4 -N on rice at late stages. Chinese J. Soil Sci. 21(3), 110–114. Ye, Q. L., and Rozelle, S. (1994). Fertilizer demand in China’s reforming economy. Can. J. Agric. Econ. 42, 191–207. Yu, T. R. (1976). ‘‘The Soil’s Eelectro-Chemical Properties and the Methods for Their Studies’’, pp. 13–14. Science Press, Beijing, China. Zhang, F. D. (1984). Organic-inorganic fertilizer combination is the developing direction of modern technique for applying fertilizers. Soil Fertilizer(1), 16–19. Zhang, S. L., Cai, G. X., Wang, X. Z., Xu, Y. H., Zhu, Z. L., and Freney, J. R. (1992). Losses of urea-nitrogen applied to maize grown on a calcareous fluvo-aquic soil in North China Plain. Pedosphere 2, 171–178. Zhang, X. N., and Gong, Z. T. (1987). The geochemical properties of rainfall, surface water and groundwater in south of China. Special Issue of Pedology No. 47. Zhang, W. L., Tian, Z. X., Zhang, N., and Li, X. Q. (1995). Investigation on groundwater nitrate pollution induced by N fertilization of arable fields in northern areas of China. Plant Nutrition Fertilizer Sci. 1(2), 80–87. Zhang, Z. Y., Shi, Z. X., and Zhou, Q. (2003). Impacts of agrochemicals on environment and human health and relevant strategies. J. China Agric. Univ. 8(2), 73–77. Zhang, B. L., Zhang, M. Z., and Qiu, W. W. (1991). Potash fertilizer becomes a new limiting factor for grain production in Jilin Province. Liaoning Agric. Sci.(2), 1–5. Zhao, G. S. (1990). Soil nitrogen state on the Loess Plateau. In ‘‘Agriculture on Loess Plateau’’ (X. M. Zhu, Ed.), pp. 263–280. China Agriculture Press, Beijing, China. Zhao, Z. D., Zhang, J. S., and Ren, S. Y. (1983). Studies on raising the use efficiency of N fertilizers. In ‘‘Selected Works of N Pollution to Environment and N Cycle’’ (Y. X. Liu, Ed.), pp. 124–141. China Agriculture Press, Beijing, China. Zhao, Z. D., Zhang, J. S., and Ren, S. Y. (1986). Volatilization of ammonia in dryland soils. In ‘‘The Current Situation of Soil Nitrogen Studies in China and Prospect for the Future’’ (Committee on Agrochemistry and Committee on Soil Biology and Biochemistry, China’s Soil Science Society, Ed.), pp. 46–54. Jiangsu Science and Technology Publishing House, Nanjing, China. Zheng, F. L., Tang, K. L., and Zhou, P. H. (1987). Approach to the genesis and development of rill erosion on slope lands and the way to control it. Acta Conservationis Soli et Aquae Sin. (1), 36–48.

Nitrogen in Dryland Soils

181

Zhou, M. Z., Yu, W. T., and Fang, Z. F. (1976). Methods for determination of available soil nitrogen. Soils 5–6, 316–323. Zhu, Z. L. (1982). Selection of some parameters for estimation of rice and wheat N fertilizer rate. Soils 14, 136–140. Zhu, Z. L. (1985). Advances in soil N supplying capacity and fertilizer N pathway. Soils 17 (1), 1–9. Zhu, Z. L. (1988). Prediction of N-supplying capacity of paddy soil and the use of suitable average N rate. Soils 20, 57–61. Zhu, Z. L. (1990). Pathway of N fertilizers in arable ecosystems and its management. In ‘‘Nitrogen in Soils of China’’ (Z. L. Zhu and Q. X. Wen, Eds.), pp. 213–249. Jiangsu Science and Technology Publishing House, Nanjing, China. Zhu, J. G. (1995). Present situation of nitrate pollution and study prospect. Acta Pedologica Sin. 32(suppl.), 63–65. Zhu, Z. L., Sun, B., Yang, L. Z., and Zhang, L. X. (2005). Policy and countermeasures to control non-point pollution of agriculture in China. Sci. Technol. Rev. 23(4), 47–51.

C H A P T E R

F O U R

The Agronomy and Economy of Some Important Industrial Crops K. P. Prabhakaran Nair Contents 1. Introduction 2. Arecanut (Areca Catechu L.) 2.1. Origin, history, and geographical distribution 2.2. The botany and taxonomy of the areca plant 2.3. The cytogenetics of the areca palm 2.4. Genetic resource program 2.5. Areca crop improvement 3. The Agronomy of Arecanut 3.1. Soil and climatic requirement 3.2. Management aspects 3.3. Irrigation 3.4. Arecanut nutrition 3.5. Arecanut physiology 3.6. Mixed cropping systems in areca 4. Arecanut Pathology 5. Arecanut Entomology 6. Arecanut Nematology 7. Harvesting and Processing of Arecanut 8. A Peep into Arecanut’s Future 9. Cashew Nut (Anacardium Occidentale L.) 9.1. Cashew nut area and production 9.2. The Indian scenario 9.3. World trade in cashew 9.4. The tale of cashew trade in India over the years 9.5. History, evolutionary origin, and distribution of cashew 10. Economic Botany of Cashew 10.1. Taxonomy of the cashew plant 10.2. Cytogenetics of the cashew plant

187 188 189 189 193 195 198 198 198 199 200 202 202 203 204 206 207 207 208 209 211 213 214 216 217 220 220 221

Distinguished Visiting Scientist, Indian Council of Agricultural Research, New Delhi, India Advances in Agronomy, Volume 101 ISSN 0065-2113, DOI: 10.1016/S0065-2113(08)00804-3

#

2009 Elsevier Inc. All rights reserved.

183

184

K. P. Prabhakaran Nair

11.

12.

13. 14. 15.

16.

17.

18. 19.

20. 21. 22.

10.3. Collection, conservation, and cataloging of genetic resources of the cashew plant The Genetic Improvement of the Cashew Plant 11.1. Breeding 11.2. Selection 11.3. Hybridization 11.4. Biotechnology Establishing and Managing a Cashew Orchard 12.1. Soil requirement 12.2. Water requirement 12.3. Manuring a cashew orchard The Relevance of ‘‘The Nutrient Buffer Power Concept’’ in Cashew Nutrition Some Salient Aspects of Raising Soft Wood Grafting Planting Technology 15.1. High density planting 15.2. Cover cropping 15.3. Intercropping Controlling the Pests and Diseases in Cashew Plantations 16.1. Pest control 16.2. Control of foliage and inflorescence pests 16.3. Biological pest control 16.4. Cashew diseases and their control 16.5. Effect of inclemental weather conditions Cashew End Products 17.1. Cashew kernel 17.2. Cashew kernel peel 17.3. Cashew nut shell liquid 17.4. Cashew shell cake 17.5. Value added products Organization of Cashew Research in India and Overseas A Peep into Cashew’s Future 19.1. Genetic resources 19.2. Varietal improvement 19.3. Biotechnological interventions 19.4. Crop management techniques 19.5. Crop protection 19.6. Post-harvest technology Technology Transfer Biodynamic Cashew The Coconut Palm (Cocos Nucifera L.) 22.1. Origin and evolution 22.2. The evolution of coconut along the drifting coastlines 22.3. Development of wind resistance 22.4. The ‘‘swimming’’ coconut fruit

222 224 224 225 226 230 231 231 232 235 238 238 239 240 241 241 243 244 245 246 247 248 249 249 249 250 252 253 254 256 256 256 256 257 257 258 258 258 259 259 259 260 261

Agronomy and Economy of Important Industrial Crops

23.

24.

25. 26.

27. 28.

29. 30. 31. 32.

33. 34. 35. 36. 37. 38. 39. 40. 41. 42.

22.5. True palm traits 22.6. Human influence on coconut evolution Botany of Coconut 23.1. Morphology 23.2. The inflorescence 23.3. The fruit 23.4. The seed and seedling 23.5. Cytogenetics of coconut 23.6. Genetic improvement of coconut 23.7. Fruit component analysis 23.8. Use of molecular markers 23.9. Early breeding work 23.10. Hybrid vigor in coconut Constraints in Coconut Breeding 24.1. Selection and its progress 24.2. Hybrids and their future Commercial Production of Hybrid Seeds 25.1. In vitro propagation Agronomy of Coconut 26.1. Soils 26.2. Soil water 26.3. Plant nutrients 26.4. Tissue analysis Coconut-Based Mixed Cropping Systems and Their Management Seed and Seedling Management 28.1. Germination rate 28.2. Polybag seedlings 28.3. Seedling selection Field Management Productive Palms Adaptation to Biotic Factors The Range of Coconut of Pests 32.1. Insect pests 32.2. Disease pathogens Adaptation in Coconut Palm A Devastating Virus Adapting the Coconut to Market Needs Yield Potential of the Coconut Palm Quality Traits 37.1. The fatty acid mix Coconut as a Food Item Research and Development in Coconut Production Global Coordination National Research Centers Research in India and Sri Lanka

185 262 262 265 265 269 269 270 271 271 272 272 273 274 274 275 276 277 277 278 278 278 280 281 282 283 283 284 284 285 285 286 286 287 287 288 289 289 289 290 290 291 291 292 292 293

186

K. P. Prabhakaran Nair

43. Research in the Second Half of Twentieth Century 44. A Peep into the Future of Coconut 45. Protection of the Production Base 46. Advances in Processing Technology 47. Contact Information for Research Centers and Institutes Acknowledgments References

293 294 295 296 296 300 301

Abstract A range of perennial tropical crops represent the industrial crops of great economic value to the developing world. Among these, arecanut, cardamom, cashew, cinchona, cocoa, coconut, coffee, oil palm, palmyra, rubber, tea, and wattle are the most important ones with much commercial value to the economy of the third world. This chapter describes the agronomy and economy of the most important of these, arecanut, cashew nut, and coconut. All the three are grown to varying extent on the Asian, African, and the Latin American continents, and individually and collectively, contribute much to the economy of the nations in these three continents. And each of them has a checkered history of its own. For instance, cashew nut came from Brazil, the country of its origin, to the coast of Malabar, along the Arabian Sea, in Southern India to the State of Kerala with the Portuguese colonizers and spread from thereon, within the country and to its neighboring regions, after the British colonizers began patronizing it, as a delicious nut that goes very well with the pre-dinner evening Scotch, which is now a common feature in many social functions and is now contributing much to the Indian economy. While arecanut and coconut contribute much to the economy of Asian and Southeast Asian countries, cashew nut has come to occupy an important position in the Asian economy, in particular Indian, and to some extent African. Arecanut is primarily grown for its masticatory nuts, popularly known as betel nut or ‘‘supari,’’ as is popularly known in Northern India and Pakistan. No social or religious function in either of these two countries is complete without the supari being offered to the guest or participant in the festivities. India exports considerable quantity of arecanut to Pakistan. Each section of the chapter devoted to the individual crop deals with several aspects of production, the crop’s place in world economy vis-a`-vis the countries where it is grown, and the ravages of pestilence, such as which is caused by the tea mosquito bug (TMB) in cashew nut, for which there still is no permanent cure which can be devastating to the cashew crop or the root wilt of coconut or the ‘‘koleroga’’ or ‘‘Mahali’’ (fruit rot) of arecanut. The review also includes the relevance of a refreshing new concept, developed by the author, which is now universally known as ‘‘The Nutrient Buffer Power Concept,’’ in the nutrition of these crops, which offers much scope for a breakthrough in productivity.

Agronomy and Economy of Important Industrial Crops

187

1. Introduction Industrial crops substantially contribute to the economy of many developing countries on the Asian, African, and Latin American continents. With the World Trade Organization substantially focusing on agriculture, the commercial aspects of growing these crops assume considerable economic significance. Within the developing world, there are countries whose sole economic sustenance depends on these crops. Even within the geographical boundary of a country, there are States whose economy is exclusively linked to these crops. For instance, within India, arecanut, coconut, and rubber contribute substantially to the economy of the State of Kerala. Within the Asian continent, oil palm contributes substantially to the economy of Malaysia and Indonesia. Palm oil, a cheap source of cooking oil is fast replacing fossil fuel as ‘‘green fuel’’ from which diesel is extracted. Currently while a ton of crude oil costs around US $600 (though the price surge seems unstoppable as this chapter is being written), palm oil is quoted at more than US $800 a ton. The global commercial impact of these developments can well be imagined. With global warming becoming a global issue and fossil fuel considered as the main culprit, there is an ever growing need for green fuel. Palm oil fits the bill. Within the African continent, tea, coffee, and cocoa contribute substantially to the economy of countries like Kenya and The Republic of Cameroon. On the Latin American continent, rubber is a very valuable foreign exchange earner. India grows some of these crops which contribute substantially to the country’s economy. The ministry of commerce deals with several aspects, and there are commodity boards like the Coconut Board, the Rubber Board, and so on to coordinate research, development, and commerce in these crops. The Rubber Board has been able to create a very effective role in research, marketing, and development of natural rubber. Historically, tea, coffee, and rubber were raised as ‘‘plantation crops’’ on the Asian and African continents by the colonial powers. One can see very large estates, running to hundreds of acres, of these crops in the countries on these continents. For instance, in India, one can see huge estates of tea in Northeastern India and of rubber in the State of Kerala down south. And most of these estates were controlled by the colonial powers. With the political changes that took place in these continents starting late 1950s and early 1960s, and the emergence of independence from the colonial powers that followed, the pattern of ownership changed to native hands. Simultaneously a large number of small holders came into existence. This has happened with both arecanut and coconut in Kerala. Arecanut is a masticatory nut with ‘‘Betel Leaf’’ (an annual twiner) and tobacco along with lime (CaO) to give a red color to the saliva—the practice of ‘‘chewing’’—very

188

K. P. Prabhakaran Nair

popular in many parts of India and Pakistan. In fact, an important market for arecanut from India is Pakistan. In addition, the dried nut is processed into a scented end product known locally as ‘‘Supari,’’ which is very popular both in India and Pakistan, having a very huge market. Coconut is known as ‘‘Kalpavriksha,’’ a term derived from the ancient Indian language Sanskrit meaning ‘‘Heaven’s Tree.’’ It provides materials for culinary purposes from its endocarp (the grated pulp) which is a must in all South Indian preparations, especially those in Kerala, edible oil (Most of the cooking in Kerala is done in coconut oil, which has a high percentage of unsaturated fats and which is now considered ‘‘unhealthy’’ by the medical fraternity, though the opinion is very divided.), and industrial lubricant (the oil extracted from the shell). The tender coconut water is a highly nutritive and invaluable health drink. It can be used to culture cells. Tea and coffee are beverage crops. Cashew is turning out to be a very important industrial crop of India. A highly nutritive nut, free of cholesterol, it has a global market and finds its use in bakery, sweet (particularly Oriental type) preparation, and the cashew nut shell oil (CSL) finds its use in many industrial purposes. Of late, its false fruit is increasingly used in the production of ethanol, another green fuel. In rural India, the false fruit goes into the manufacture of illicit alcohol. This chapter is focused on arecanut, cashew nut, and coconut. The details are included all aspects of evolution, adaptation, cultivation, and trade.

2. Arecanut (Areca Catechu L.) The arecanut palm is the source of a widely used masticatory nut, popularly known as arecanut, betel nut, or supari. While supari is a processed and scented nut powder for mastication, highly popular in Northern India and Pakistan, the term betel nut is derived from the fact that arecanut is used along with betel leaf (a twiner) for chewing purposes. Since ancient times the habit of ‘‘chewing’’ is a symbol of friendship, general well-being. Even in Hindu temples, during festivals, betel nut and arecanut are offered to the Deities as materials of worship. No Hindu dinner or lunch during auspicious occasions as marriage, betrothal ceremony is complete without the offering of arecanut and betel leaves to the guests. Arecanut palm is a popular crop in the States of Kerala, Karnataka, and Tamil Nadu in Southern India, Assam, Meghalaya, and West Bengal in Northeastern India. The areca palm is a monocot which belongs to the family Palmae. The commonly cultivated species is A. catechu in most of the countries where it is used for chewing. In Sri Lanka, the fruits of Areca concinna are occasionally chewed. In the world, the area and production is most in India followed by China.

Agronomy and Economy of Important Industrial Crops

189

2.1. Origin, history, and geographical distribution There is no definitive record on the origin of arecanut. No fossil remains of the genus Areca exist, but, the fossil records of closely related genera indicate its presence during the tertiary period. The maximum diversity of species which number 24 and other indicators suggest that the original habitat is in contiguous regions of Borneo, Celebes, and Malaya (Bavappa, 1963; Raghavan, 1957). There are innumerable references to arecanut palm, arecanut, and its various uses in ancient Sanskrit. The antiquity of such references have been shown, the most important being Anjana Charitra (Sisy Mayana 1300 BC), where the reference had been made to groups of arecanut palms full of inflorescence and branches presenting an exquisite appearance (Bhat and Rao, 1962). Although it is not precisely known when arecanut found its way to the Indian sub-continent, innumerable evidences exist on its antiquity (Mohan Rao, 1982). Arecanut is mentioned in various Sanskrit scriptures (650 to 1300 BC) and its medicinal properties were known to the famous Indian scholar Vagbhatta (500 AD). A well-known cave in central India 200 BC to 900 AD has one exquisitely painted arecanut palm providing a backdrop to the Padmapani Buddha. According to Furtado (1960) one of the earliest references to arecanut dates back to 1510 AD The abundant uses of arecanut in chewing and auspicious religious functions of the Hindus of India were indicated even during the times of the Aryans, the early conquerors of India, who were supposed to have migrated from Europe. Area wise India has 57% of the world’s total and production wise 53% (Table 1). China occupies the second place, which is followed by Bangladesh and Mayamnar. In Asia, Indonesia, Malaysia, Thailand, Philippines, and Vietnam also grow arecanut. Productivity is highest (3752 kg/ha) in China followed by Malaysia (1667 kg/ha), Thailand (1611 kg/ha), and India (1189 kg/ha). Details on production and productivity are given in Table 1. In India the crop is grown largely in the Western Ghats and the northeastern regions. Details on area and production, region wise in India, are given in Table 2. As much as 90%of the area and 95% of the production are from three States, namely, Kerala, Karnataka, and Assam. Within India, productivity is highest (3947 kg/ha) in the State of Maharashtra in central India.

2.2. The botany and taxonomy of the areca plant Though unsatisfactory, as they were not based on real affinities, the earliest attempt to restrict the genus Areca was that of Martius (1832–1850). Subsequently, various species grouped under Areca were separated into different genera and limited the genus to close relatives of the type of the genus A. catechu (Blume, 1836). Furtado (1933) described the limits of the genus

190 Table 1

Country-wise area and production of arecanut Area (000 ha)

Country

1961

1971

1981

Bangladesh China India Indonesia Malaysia Maldives Mayamnar Thailand Global

82.60 0.62 135.00 65.00 6.00 0.003 11.13 – 300.55

40.10 1.33 167.30 75.00 2.50 0.003 24.68 – 310.92

36.43 2.83 185.20 90.00 1.30 0.006 26.47 – 342.25

Production (mt) 1991

1998

1961

1971

1981

1991

1998

35.81 26.96 217.00 95.76 2.20 0.030 28.93 8.50 415.20

36.00 46.00 270.00 75.38 2.40 0.030 29.50 9.00 468.31

62.99 3.71 120.00 13.00 6.50 0.001 8.00 0.00 214.21

23.36 10.07 141.00 15.00 3.00 0.001 19.20 0.00 211.64

25.05 24.35 195.90 18.00 2.50 0.005 25.80 0.00 291.62

24.12 111.09 258.50 22.81 4.00 0.016 92.27 13.25 446.15

28.00 172.57 310.00 32.60 4.00 0.016 31.50 14.50 593.29

Source: Food and Agriculture Organisations (FAO) of the United Nations; mt, million tons.

Table 2

State wise area and production of arecanut in India Area (000 ha)

State

Andhra Andaman and Nicobar Assam Goa Daman and Diu Karnataka Kerala Maharashtra Meghalaya Mizoram Puduchery Tamilnadu Tripura West Bengal

Production (000 t)

1966– 1967

1971– 1972

1981– 1982

1991– 1992

1997– 1998

1966– 1967

1971– 1972

1981– 1982

1991– 1992

0.00 0.00

0.20 0.00

0.20 0.00

0.20 0.00

0.20 3.60

0.00 0.00

0.20 0.00

0.20 0.00

0.20 0.00

0.50 5.20

26.20 0.00

25.90 1.40

47.20 1.70

66.00 1.30

74.10 1.50

25.20 1.20

29.20 1.70

48.10 1.30

50.50 1.50

64.00 1.80

34.80 71.20 0.00 0.00 0.00 0.00 0.00 0.00 0.00

43.20 86.80 2.30 6.30 0.10 0.00 5.00 0.00 3.10

55.20 61.20 2.10 6.50 0.40 0.00 4.30 0.70 3.10

65.40 63.51 1.90 8.90 0.00 0.10 2.80 1.20 5.90

88.40 76.10 1.90 9.50 0.00 0.10 2.70 1.80 8.10

52.70 44.30 0.00 0.00 0.00 0.00 0.00 0.00 0.00

56.30 53.40 2.80 4.20 0.00 0.00 5.00 0.00 0.80

80.20 66.00 2.40 4.90 0.00 0.00 2.80 1.00 0.80

96.00 65.14 2.60 8.70 0.10 0.10 3.40 2.30 7.90

128.50 94.00 7.50 12.10 0.10 0.20 4.30 3.50 12.40

Source: Directorate of Economics and Statistics, Government of India.

1997– 1998

191

192

K. P. Prabhakaran Nair

Areca and its sections. A list of Areca species and their geographical distribution is given in Table 3. Detailed morphology, floral biology, and embroyology have been earlier described (Murthy and Pillai, 1982). The arecanut palm is a graceful looking, erect, and unbranched palm sometimes going to reach a height of 20 m. Shorter varieties have been bred lately. The stem has scars of fallen leaves in regular annulated forms. The girth of the stem depends on genetic variation, soil condition, and plant vigor. The palm has an adventitious root system. The crown of an adult palm normally has 7–12 leaves. The apical bud produces leaves in succession once every 2 months and the leaves live up to about 2 years. The leaves are pinnatisect and consist of a sheath, a rachis and leaflets. The leaf sheath completely Table 3

Geographical distribution of the Areca species

Country

Species

India Andaman Islands (India) Sumatra Sri Lanka Malaysia

Areca catechu, Areca triandra Areca catechu, Areca laxa

Borneo

Java Celebes Australia Solomon Islands New Guinea

Philippines

Moluccas Bismarck Islands Laos Cochin China Lingga Islands

Areca catechu, Areca triandra, Areca latiloba Areca catechu, Areca concinna Areca catechu, Areca triandra, Areca latiloba, Areca montana, Areca ridleyana Areca catechu, Areca borneensis, Areca kinabaluensis, Areca arundinacea, Areca bongayensis, Areca amojahi Areca mullettii, Areca minuta, Areca furcata Areca catechu, Areca latiloba Areca celebica, Areca oxicarpa, Areca paniculata, Areca henrici Areca catechu, Areca alicae Areca niga-solu, Areca rechingeriana, Areca torulo, Areca guppyana Areca congesta, Areca jobiensis, Areca ladermaniana Areca macrocalyx, Areca nannospadix Areca warburgiana Areca catechu, Areca hutchinsoniana, Areca vidaliana Areca costulata, Areca macrocarpa, Areca parens Areca caliso, Areca whitfordii, Areca camariensis, Areca ipot Areca glandiformis Areca novo hibernica Areca laosensis Areca triandra Areca hewittii

Agronomy and Economy of Important Industrial Crops

193

encircles the stem. It is about 54 cm in length and 15 cm broad. Average length of leaf is 1.65 m bearing about 70 leaflets. The leaflets are 30–70 cm in length and 5.8–7.0 cm in breadth depending on the position of the leaf. When grown under favorable conditions, flowering occurs once 5 years appearing at the 10th node in the ‘‘South Kanara’’ cultivar (Murthy and Bavappa, 1960b). In another semi-tall cultivar ‘‘Mangala,’’ flowering occurs earlier in about 3 years (Bavappa, 1977). The inflorescence is a spadix produced at the leaf axils. The number of spadices produced depends upon the number of leaves. Mean number of spadices produced is three to four depending on the age of the plant (Murthy and Bavappa, 1960a). The fruit of arecanut is a monocular one-seeded berry that is orange red to scarlet in color when ripe, encircled by a thick fibrous outer layer, akin to a husk. Each branch bears close to 100–250 fruits. Arecanut has a fairly wide root distribution and the distribution pattern has been described by Shama Bhat and Leela (1969). Within a radius of about 50 cm, approximately 70% of the roots are distributed. Almost 80% of the roots do not go beyond a radius of 85 cm. All the cultural operations are confined within this radius. Depth wise, 0–50 cm contained about 70% of the roots and further from 51 to 100 cm about 18–24%. When the palms are planted close, roots penetrated deeper than when planted at wider spacing. Arecanut is a cross-pollinated, monoecious palm and both male and female flowers are present on the same spadix (Bavappa and Ramachander, 1967). Male flowers last 4–7 weeks and the female flowers which are cream colored at flowering turn green within a week. Flowers open between 2 AM and 10 AM and female flowers last up to 10 days only. The stigma remains receptive up to a week (Murthy and Bavappa, 1960a; Shama Bhat et al., 1962). Wind carries the pollen. Fruit development takes place in three stages (Shama Bhat et al., 1962). Increase in size is rapid in the first phase, followed by increase in volume and dry matter accumulation in kernel in the second phase. During this period the embryo becomes macroscopic and develops rapidly. In the third stage the fruit swells and gradually the green color fades giving rise to a bright yellow color. In all, it takes about 9 months to one full year for fruit development to complete for harvest.

2.3. The cytogenetics of the areca palm Venkatasubban (1945) was the first one to report the number of chromosomes of A. catechu L. at 2n = 32. This was later confirmed (Abraham et al., 1961; Bavappa and Raman, 1965; Raghavan and Baruah, 1958; Sharma and Sarkar, 1956). The chromosome number of Areca triandra Roxb. as 2n = 32 reported by Darlington and Janaki Ammal (1945) was subsequently confirmed (Bavappa and Raman, 1965; Sharma and Sarkar, 1956). Nair and Ratnambal (1978) determined the meiotic chromosome number of Areca

194

K. P. Prabhakaran Nair

macrocalyx Becc. at 2n = 16. Meiotic abnormalities such as nondisjunction, lagging chromosomes, univalents, and pentads were reported by Sharma and Sarkar (1956) in A. catechu, Bavappa and Raman (1965) observed in the meiosis of four eco types of A. catechu, abnormalities, such as univalents at diakinesis, and metaphase I, non-synchronization of orientation, clumpting, delayed disjunction, chromosomes bridges, and laggards at anaphase I and II, chromosome mosaics and supernumerary spores. The meiotic division was found to be quite normal in A. triandra, except for the presence of 14 and 18 chromosomes occasionally during metaphase II (Sharma and Sarkar, 1956). Regular meiotic division in the types of A. triandra was also reported by Bavappa and Raman (1965). Intracultivar variation in meiotic behavior of the Areca species was reported by Bavappa (1974) and Bavappa and Nair (1978). While normal bivalent formation was observed in a few plants, hexavalent, octovalent, and even decavalent formations were observed in others. Bridges and laggards, which are nothing but abnormalities, and disorientation of chromosomes at anaphase I and II were reported in this species. Bavappa and Nair (1978) reported intrapalm variation in chromosome number in the pollen mother cells of A. catechu and A. triandra and their hybrids. Cytomixis to an extent of 39% appeared to have contributed to this abnormality. Despite the presence of a high degree of multivalents in A. catechu, pollen fertility was very high. These authors suggested the possibility of the frequency of multivalent formation and disjunction being under genotypic control and being subjected to selection. Partial desynapsis of chromosomes at diakinesis was reported by Bavappa (1974). Bavappa and Nair (1978) reported the same in A. triandra and A. catechu  A. triandra hybrids. Desynapsis observed at diakinesis was followed by an increase in pairing at metaphase I as reflected by the frequency of bivalents in A. triandra and A. catechu  A. triandra hybrids. This was attributed to distributive pairing, a mechanism that has been possibly adopted for ensuring their regular segregation (Bavappa and Nair, 1978). The extent of desynapsis was higher in the F1 hybrids of A. catechu and A. triandra as compared to A. triandra, suggesting that the gene controlling this character may be dominant. The large number of univalents observed in the hybrid as compared to A. triandra parent has been attributed to reduced homology of the parental chromosomes (Bavappa and Nair, 1978). Venkatasubban (1945) observed two pairs of short satellite chromosomes in the somatic chromosome complement of A. catechu, while Sharma and Sarkar (1956) observed three pairs of long chromosomes, six pairs of medium-sized chromosomes and seven pairs of short chromosomes in the same species. These authors categorized the chromosomes into seven groups based on their morphology and relative length. Two pairs of long chromosomes next to the longest were found to have secondary constrictions. The chromosomes of A. triandra were found to be longer than that of

Agronomy and Economy of Important Industrial Crops

195

A. catechu (Sharma and Sarkar, 1956), while Bavappa and Raman (1965) found the chromosomes of A. catechu and A. triandra to be different in size, total chromatin length, position of primary and secondary constrictions, and the number and position of satellites. These authors concluded that A. catechu is more advanced in evolution compared to A. triandra based on the observations of Sharma and Sarkar (1956) which showed that gradual reduction in chromatin matter had taken place in the evolution from primitive to advanced forms of different genera and tribe of Palmae. The pachytene chromosomes of A. catechu were found to be morphologically similar to the somatic chromosomes, though pachytene chromosomes were about 10 times longer than the somatic chromosomes (Bavapa and Raman, 1965). Raghavan (1957) reported the chromosome morphology of some cultivars of A. catechu from Assam in Northeastern India. There were only minor variations in structure and length of individual chromosomes, total length of the complement and the position of constrictions among the types. On the basis of morphology, the author recognized nine groups in the somatic chromosomes of the cultivars. Bavappa (1974) and Bavappa et al. (1975) investigated the karyotypes of eight cultivars of A. catechu and four ecotypes of A. triandra which showed substantial differences in their gross morphological characteristics. The karyotypes of A. triandra ecotypes showed higher frequency of median and sub-median chromosomes compared to A. catechu. A classification of the karyotypes of the two species according to their degree of asymmetry, which recognizes three grades of size differences and four grades of asymmetry in centromere position, was made by Stebbins (1958). This showed that karyotypes 1B, 2A, 2B, and 3B are represented in A. catechu cultivars, while in that of A. triandra only 1A, 2A, and 2 were represented in its ecotypes. Two different types of asymmetry in karyotypes were observed within the same cultivar of A. catechu, whereas no such variations were seen to occur in the ecotypes of A. triandra. This clearly shows that A. triandra has a more symmetrical karyotype compared to A. catechu. This also led to the conclusion that delineating the cultivars of A. catechu on the basis of a standard karyotype is rather difficult compared to that of A. triandra. Obviously, the lesser chromatin matter and an asymmetrical karyotype in A. catechu compared to A. triandra points to the fact that the former is more evolved than the latter.

2.4. Genetic resource program The Regional Research Station located in Vittal, Karnataka State, affiliated to the Central Plantation Crops Research Institute (CPCRI) situated in Kasaragod, Kerala State, under the overall administrative control of the Indian Council of Agricultural Research (ICAR), New Delhi, is mandated to carry out research in arecanut. One of its primary tasks is the collection

196

K. P. Prabhakaran Nair

of germplasms. Several cultivars have been recognized in Karnataka State (Rau, 1915), Philippines (Beccari, 1919), based on kernel and fruit characteristics. On the basis of stomatal characteristics, nut size, leaf shedding pattern, female flowers, and so on, the areca species can be separated (Bavappa, 1966; Bavappa and Pillai, 1976). A collection of five species, namely, A. catechu, A. triandra, Areca macrocalyx, Areca normanbyii, and A. concinna and two genera, Actinorhytis and Pinanga dicksonii are available (Anuradha, 1999). The germplasm collection now holds 113 accessions (Ananda, 1999b). Among these, 23 exotic accessions were introduced from Fiji, Mauritius, China, Sri Lanka, Indonesia, Vietnam, Singapore, Solomon Islands, and Australia. From the different arecanut growing States within India, 90 collections have been sourced. About 39 of these have been described based on the various descriptors. The different cultivars show wide variations in stem height, inter node length, leaf size and shape, and fruit characters. Nuts obtained from Malnad, parts of Shimoga and Chickmagalur districts in the State of Karnataka are small in size, whereas those from North Kanara and Ratnagiri from the same State are bigger (Murthy and Bavappa, 1962). These traits lead to wide variations in yield, earliness in bearing, fruit bunches, quality and dwarfness. The exotic and indigenous collections have been under evaluation since 1957 for morphology, nut characters, and yield attributes (Ananda, 1999b; Bavappa and Nair, 1982). Yield evaluation resulted in the release of four high yielding cultivars, of which three are selections from exotic collections. The characteristics of these varieties have been described (Ananda, 1999a; Ananda and Thampan, 1999). Among the exotic collection, cultivar VTL-3 introduced from China was released and named Mangala (Bavappa, 1977). This cultivar has bearing earliness, greater number of female flowers, high yield, and shorter stem height compared to other accessions. Other varieties released for cultivation are Sumangala and Sreemangala, which are Introductions from Indonesia and Singapore, repectively. One of the indigenous collections from the State of West Bengal was found to be a high yielder and was released with the name Mohitnagar. Table 4 summarizes the characteristics of these varieties. Other promising cultivars are SAS-I, Tirthahalli, and Calicut-17. The morphological, anatomical, and yield characteristics of 13 cultivars of A. catechu and four ecotypes of A. triandra have been recorded by Bavappa (1974). These data were with reference to the years 1963, 1966, and 1972. The analysis of variance of the results obtained in 1963 showed that the differences between the cultivars are highly significant for all the six morphological characters. Combined analysis of the data for the 2 years for the 24 common characters recorded during 1967 and 1972 also revealed significant interactions between cultivars for all the characters. Significant interaction between years and cultivars was seen with regard to plant height, girth, inter nodal distance, bunch number, inflorescence, length and breadth

197

Agronomy and Economy of Important Industrial Crops

Table 4 Yield and nut characteristics of arecanut varieties Dry nut yield (kg/plant)

Growth habit

Shape and size of nut

South Kanara

Tall

Round, Bold

2.00

Mangala

Semi-tall

Round, Small

3.00

Sumangala

Tall

3.20

Sreemangala

Tall

Mohitnagar

Tall

Oval, Medium Round, Bold Round, Medium __

Variety

3.18 3.67

Remarks

Coastal Karnataka and Kerala State Coastal Karnataka and Kerala State Karnataka and Kerala States Karnatraka and Kerala States West Bengal, Kerala and Karnataka States

Source: Ananda (1999a).

of leaf sheath, length and volume of nut, length, breadth, weight and volume of kernel. From D2 studies, Bavappa (1974) concluded that detection of genetic divergence in the early years of the productive phase is of considerable advantage in formulating breeding programs in a perennial crop like arecanut. The investigations showed that mean nut volume and kernel breadth are the traits of primary importance contributing to the overall genetic divergence in areca. For divergence between A. catechu and A. triandra, mean fruit length, nut, and kernel characters are the most important for differentiation within A. catechu cultivars and between A. catechu and A. triandra types. Results obtained from canonical analysis were also in broad agreement with the clustering pattern found from D2 analysis. However, canonical analysis is only of limited utility in view of the fact that the first two canonical roots accounted for only 85% of the variation or less (Bavappa, 1974). The grouping obtained by D2 analysis showed that the three cultivars each from Saigon and Solomon Islands and the two ecotypes of A. triandra from Indonesia were invariably in one cluster each. As against this, close similarity between the cultivars from different countries has also been observed. The cultivar from Singapore was grouped with the three cultivars from Saigon in one cluster. A similar affinity between the two geographically distant cultivars was shown by ‘‘Ceylon-1’’ and ‘‘Indonesia-6,’’ both

198

K. P. Prabhakaran Nair

always coming within the same cluster. The local cultivar was found to be invariably associated with the Singapore cultivar in forming the cluster. One of the cultivars of A. catechu from Sri Lanka, ‘‘Ceylon-2’’ was always forming a separate cluster, indicating its distant nature of divergence. The clustering pattern of cultivars and ecotypes revealed that geographic diversity need not always be related to genetic diversity (Bavappa, 1974).

2.5. Areca crop improvement The crop can be improved through hybridization. By nature, the areca palm is tall, and this is a definite constraint to spraying the plant to control pest attack, in addition to harvest of the nuts. Hence, dwarfing the plant is a primary objective through hybridization. The hybridization program was initiated with the primary objective of exploiting the variability in the Areca germplasm (Bavappa and Nair, 1982). Primary concern was to evolve high yielding varieties with the regular bearing traits, with nuts of superior quality and to evolve semi-tall ideotypes. Interspecific hybrids of A. catechu  A. triandra had only one stem, as in A. catechu, which indicates dominance of this character (Bavappa, 1974). Hybrids mostly equaled parents in inter nodal length and further exhibited hybrid vigor for a number of characters such as male and female flower number, spadix length, and stem girth. As dwarfing is a primary objective, attempts were made to identify such types, and one such, Hirehalli Dwarf (H. Dwarf) was identified (Naidu, 1963). This is a low yielder with poor quality nuts, which are only suitable for ‘‘chewing’’—the widespread practice in India and Pakistan. High yielding varieties were crossed with Hirehalli Dwarf to exploit the dwarfing nature (Ananada, 2000). Maximum dwarfs and intermediates were recovered in crosses of Sumangala  H. Dwarf, Mohitnagar  H. Dwarf, and Mangala  H. Dwarf from among the 12 combinations.

3. The Agronomy of Arecanut 3.1. Soil and climatic requirement A wide spread of soils are used to grow the arecanut palm. The largest area under the crop, however, is in the gravelly laterite soils of red clay in Northern Kerala and coastal belt of Karnataka State (Nambiar, 1949). The deep black fertile clayey loams of the plains of Karnataka State are very suitable for the crop. Sandy, alluvial, brackish or calcareous, and sticky clay soils are unsuitable. Sub-humid tropical climate suits the crop best and it thrives very well in the regions 28  N and 28  S of the equator. Latitude determines the altitude at which the crop grows (Shama Bhat and Abdul Khader, 1982). In the

199

Agronomy and Economy of Important Industrial Crops

northeastern regions of India, the crop is mostly grown in the plains. At higher elevations lower temperature limits the crop growth. An altitide up to 1000 m above mean sea level (msl) is alright, beyond which the quality of the nut is very adversely affected (Nambiar, 1949). The crop thirves in a range of temperature between 14 and 36  C. It can also withstand very low minimum temperature (4  C) as obtained in winter in West Bengal State in India. The crop requires very high rainfall ranging from 300 to 450 cm per annum. However, in the plains of Karnataka and Tamil Nadu States, where rainfall can be as low as 75 cm supplemental irrigations, especially in the summer months, are required to sustain the areca plantation. In coastal India, the climatic conditions obtaining are different. Southern Kerala gets better distributed rainfall than Northern Kerala and coastal Karnataka, which receives most of the rainfall during the months spread between June and September. This is followed by drought. Hence, in these regions, arecanut is grown principally as an irrigated crop.

3.2. Management aspects The areca palm is a seed-propagated plant. In selecting the planting material, both age of mother palm and seed size are important. While selecting seeds, it must be ensured that they are obtained from trees which are already stabilized in their yielding pattern, at least more than 10 years in maturity. Heavy, mature seeds are selected which ensure good germination. Selected seeds are sown 5 cm apart with their stalk ends pointing upward. Regular irrigation, on alternate days, is crucial to maintain healthy seedlings. Threemonth-old saplings can be transferred to a secondary nursery and planted 30 cm apart. Seedlings can also be raised in poly ethylene chloride (PVC) bags of 25  15 cm size, 150 gauge in a mixture of top soil, cattle manure and sand in the ratio of 7:3:2, respectively. Twelve to eighteen months old seedlings are selected for field planting (Table 5). Table 5 Spacing effects on cumulative yield of arecanut (kg/ha)

Spacing (m  m)

Vittal (coastal Karnataka)

Hirehalli (Plains of Karnataka)

Kahikuchi (Assam)

Peechi (Kerala)

1.8  1.8 1.8  2.7 1.8  3.6 2.7  2.7 3.6  3.6

6290 8167 7829 10722 6169

3130 3705 4132 3867 2417

2.00 3.00 3.20 3.18 3.67

1749 1766 1710 1867 1448

Source: Sannamarappa (1990).

200 Table 6

K. P. Prabhakaran Nair

Effect of spacing on cumulative yield of arecanut (kg/ha)

Spacing (m  m)

Vittal (Coastal Karnataka)

Hirehalli (Plains of Karnataka)

Kahikuchi (Assam)

Peechi (Kerala)

1.8  1.8 1.8  2.7 1.8  3.6 2.7  2.7 3.6  3.6

6290 8167 7829 10,722 6169

3130 3705 4132 3867 2417

2.00 3.00 3.20 3.18 3.67

1749 1766 1710 1867 1448

Source: Sannamarappa (1990).

Spacing depends primarily on depth and soil fertility. The effect of spacing on cumulative yield is given in Table 6. Results show that 2.7 m  2.7 m is the best spacing (Shama Bhat and Abdul Khader, 1982). Sixty centimeter cubic pits, filled with soil to a depth of 50 cm, are used for planting seedlings.

3.3. Irrigation Soil water depletion profile is the major deciding factor for the frequency of irrigation. Therefore, rather than using a fixed irrigation schedule, considerable flexibility should be given to accommodate varying crop-evapotranspirational (ET) losses. Irrigation is done mostly by splashing and basin filling. Several researchers have investigated irrigational requirements for the areca crop in various regions. However, all of these were done by taking into account frequency and the quantum of applied water considering the ET demand and ambient temperature. Systematic studies revealed that at irrigation water/cumulativie pan evaporation (V) ratio of 1, that is, 300 mm depth of water resulted in the best yields (Abdul Khader, 1988; Abdul Khader and Havanagi, 1991; Yadukumar et al., 1985). On the basis of a modified Penman model, a net irrigation requirement of 899 mm during post-monsoon season and an irrigation interval of 6–7 days were found sufficient (Sandeep Nayak, 1996). Using a modified Penman model, Mahesha et al. (1990) estimated that the ET rates of arecanut increased from 4.60 mm/day in December to 6.25 mm/day in April, which fell to 5.78 mm/day in May due to pre-monsoon showers. In this connection results of the drip irrigation system need to be looked into. Drip irrigation has the added advantage of fertigation. An investigation carried over 10 years (1996–2006) to evaluate the efficacy of four fertigation levels (25, 50, 75, and 100% of the recommended fertilizer dose, 100:18:117 g N:P:K/palm/year), three frequencies of fertigation (10, 20, and 30 days), and two control treatments (control 1, i.e., drip irrigation without fertilizer

201

Agronomy and Economy of Important Industrial Crops

application and control 2, i.e., drip irrigation with 100% NPK soil application) on productivity and resource use efficiency of arecanut indicated that adoption of fertigation not only increases productivity, but also ensures higher efficiency of use of the two most critical inputs, that is, water and nutrients (Ravi Bhat et al., 2007). Data on comparative merit of drip irrigation over conventional surface irrigation in influencing areca yield are given in Table 7 (Chinnappa and Hippargi, 2005), which clearly indicated an advantage of more than 25% for the former over the latter. The crucial point to be examined is the economics of drip irrigation over surface irrigation. The data reported here refer to the response of 90 arecanut farmers spread in 15 villages in southern transitional zone of Karnataka State. It was observed that installation of drip irrigation with Indian Standards (ISI) materials required Rs. 50,394 (approx. US $1260) per hectare. Adoption of drip irrigation in arecanut gardens has resulted in additional output of 5.03 qtl/ha (1 qtl = 100 kg) worth Rs. 67,972 (approx. US $1700). The maintenance cost of drip-irrigated gardens was found to be lower at Rs. 29,109 (approx. US $730) as compared to surfaceirrigated gardens at Rs. 37,856 (approx. US $950). There is substantial saving in cost of cultivation at Rs. 8837 ha 1 (US $220) in drip-irrigated gardens which is due to increased use of labor and inputs cost in surface-irrigated gardens. These results reported by Chinnappa and Hippargi (2005) clearly establish the economic viability of drip irrigation. Further, there is economy of water use to the tune of 42.18 acre inches. With this saving in water use, it is possible to irrigate an additional area of 2.5 ha. The saving equivalent of more than US $200 ha 1 is substantial on the Indian economic scale compared to that of the US. Balasimha et al. (1996) reported that the photosynthetic parameters and arecanut yield increased with increases in drip irrigation levels from 10 to 30 l/day in a mixed cropping system where cocoa is interplanted with arecanut. The results are given in Table 8. The saved irrigation water can be used for other crops or intercrops, and the ground water depletion can also be checked. Efficiency of water use can be enhanced through a good combination of irrigation and mulching. Mulching is very important in arecanut Table 7 Comparative merit of drip irrigation over surface irrigation on arecanut yield Details

Yield (qtl/ha)

Drip irrigation Surface irrigation Incremental yield (qtl) Percentage increase

24.93 19.90 5.03 25.28

Note: qtl, quintal = 100 kg

202

K. P. Prabhakaran Nair

Table 8 Effect of irrigation levels on photosynthetic characters and dry nut of arecanut

Treatment

Photosynthesis (mmol/m2/s)

Transpiration (mmol/m2/s)

Stomatal conductance (mmol/m2/s)

Yield (kg/ ha)

I 1 (10 l/day) I 2 (20 l/day) I 3 (30 l/day) LSD (5%)

3.98 3.62 4.48 NS

3.46 3.68 4.75 0.96

0.105 0.122 0.168 0.041

1.80 1.96 2.01 NS

Note: 1, 2 and 3 = Irrigation treatments; NS, Not significant; LSD, Least significant difference. Source: Balasimha et al. (1986).

plantation because the highly porous soils in which the crop is planted can lead to water loss through seepage. Different plastic and organic mulches have been found effective (Abdul Khader and Havanagi, 1991; Shama Bhat, 1978).

3.4. Arecanut nutrition Inasmuch as arecanut nutrition is concerned, available information is primarily based on classical field experiments. Results obtained from several field trials indicate that each palm requires N:P2O55:K2O in the ratio of 100 g:40 g:140 g, respectively, clearly showing that of the three principal nutrients, N, P, and K, it is potassium that is required most. The requirements shown above are annual per palm (Shama Bhat and Abdul Khader, 1982). These nutrients are, generally, supplemented by 12 kg of organic manure, either through cattle manure, compost or green leaves like that of Glyricidia maculata, which can grow well on the boundaries of arecanut gardens. Recent fertilizer trials have shown that doubling the above rates can substantially enhance yield (Sujatha et al., 1999). Doubling the rates of nutrients indicated above has not only resulted in higher yields, but increase in net income and ‘‘Benefit-Cost Ratio.’’ Fertilizer nutrient requirements have been standardized for arecanut by Mohapatra and Bhat (1982).

3.5. Arecanut physiology Balasimha (1986) and Balasimha and Subramonian (1984) reported that approximately 30–50% of photosynthetically active radiation (PAR) is transmitted through arecanut canopy. This varies with season and time of the day. In the arecanut canopy, light environment is highly dynamic due to variation in cloud cover, solar angle, and canopy. The pattern of light transmission varies with the spacing of palms. Mid-day light profile shows

Agronomy and Economy of Important Industrial Crops

203

that a spacing of 3.3 m  3.3 m or 1.8 m  5.4 m allowed maximum transmission. However, in the afternoon 3.9 m  3.9 m spacing allowed maximum transmission. On average, arecanut canopy intercepts 70% of incoming radiation. The interception of the remaining radiation depends on the nature of the intercrop canopy and leaf area index (LAI). For instance, cocoa plant with a compact canopy and LAI of 3–5 intercepts nearly 90% of the incident light. Pepper, which is trained on arecanut stems, receives differential light depending upon directional and diurnal effects. A fully grown arecanut palm, on average, bears 8–9 leaves. The canopy area and leaf area are about 11.2 and 22.0 m2, respectively. The arecanut canopy covers a ground area of 9.1 m2 with a LAI of 2.44. Photosynthesis in arecanut leaves ranged from 2.8 to 8.2 mmol CO2/m/s depending on the cultivar and leaf position. The first fully open and third leaves showed the highest rate of photosysnthesis. Total chlorophyll content also varied among the species of Areca, and the highest content was recorded in A. triandra (Yadava and Mathai, 1972).

3.6. Mixed cropping systems in areca The interspace between arecanut palms can be put to good use by planting associate crops. This would bring in additional income to the farmer. The practice has been quite successful (Bavappa, 1961; Sannamarappa and Muralidharan, 1982). The long pre-bearing age of the areca palm has prompted farmers to grow different annual or semi-perennial crops in the interspace to ensure economic sustainability of the areca plantations. The initial period of 5–6 years (the pre-bearing phase) is the ideal time to grow these intercrops, especially short duration ones. In later years of the palm, as the canopy enlarges in height, mixed cropping with other shade tolerant perennial crops can be practiced. The choice of intercrops in arecanut plantations has been quite wide. A number of annual crops, such as, rice, sorghum, beans (Vigna unguiculata), vegetables, and yams, are grown as intercrops (Abdul Khader and Antony, 1968; Abraham, 1974; Shama Bhat, 1974; Shama Bhat and Abdul Khader, 1970; Muralidharan, 1980; Thomas, 1978). When these crops are cultivated, the cultural and nutritional practices followed are used for the pure stands of the respective intercrops (Sannamarappa and Muralidhran, 1982). Leaving 1 m radius around arecanut palm, the interspaces are prepared for cultivation of the intercrops during the pre-monsoon period. Rice, sorghum, corn, beans, groundnut, and sweet potato are sown in beds. For yam, banana, and pineapple pits or trenches are dug. With the exception of banana and beans, biological productivity of the intercrops was found to be lower than when the crops were sown singly. In most studies in different regions, no deleterious effects on main crops due to intercropping were observed (Abraham, 1974; Muralidharan and Nayar, 1979;

204

K. P. Prabhakaran Nair

Sadanandan, 1974). Banana is the most preferred intercrop in all the arecanut gardens (Bavappa, 1961; Brahma, 1974; Shama Bhat, 1974). It also provides good shade during early growth of arecanut palms. Abraham (1974) and Nair (1982) found Black pepper (Piper nigrum) to be an excellent intercrop. The arecanut stems are used as live ‘‘standards’’ (prop on which the pepper twines). Of the pepper cultivars successfully experimentd with, Panniyur-1 and Karimunda, were found to be good. It was observed that arecanut yield was not depressed due to the presence of pepper. Another important intercrop is cocoa. The microclimate, especially shade, soil moisture, and temperature, in the arecanut gardens was found to be ideal for cocoa growth. Shama Bhat and Bavappa (1972) and Shama Bhat and Leela (1968) have found arecanut–cocoa mixture as good. A confounded asymmetrical factorial design, with different spacings and two fertilizer levels showed significant influence on the number and weight of cocoa pods in the normal spacing of arecanut. A spacing of 2.7 m 2.7 m or a spacing of 2.7 m 5.4 m combination could be safely followed although operational advantages are better in the latter spacing (Shama Bhat, 1983, 1988). High density multispices cropping system in arecanut typically comprises of arecanut–black pepper, cocoa, coffee, or banana occupying different vertical air space levels. Bavappa et al. (1986) tried six crop species in a 17-year-old arecanut plantation. A steady increase in arecanut yield was observed and the intercrops started to yield from the third year onward after planting. The economic dry matter yield of intercrops accounted for about 27% of total economic yield. This intercropping system could accommodate 1300 areca palms with pepper, cocoa, clove, banana, and pineapple numbering 3700 in 1 ha. Investigations on the microflora in the rhizosphere of such combinations indicated that the population of bacteria, fungi, actinomycetes, N2 fixers and P solubilizers increased in the areca–cocoa or pineapple combinations than in monocrop system of areca (Bopaiah, 1991). P solubilizers such as vesicular arbuscular mycorrhizae (VAM) were lowest in areca–banana combination. Microbial biomass was also found to be higher in the combinations, in general. The asybiotic N2 fixers isolated from an arecanut-based high density multiple cropping system had an N-fixing capacity in the range of 2.8–11.8 mg N/100 ml of medium (CPCRI, 1988a).

4. Arecanut Pathology A wide range of diseases attack the areca palm. Of these, the yellow leaf disease (YLD), Phytophthora fruit rot, Anabe caused by Ganoderma and inflorescence die back cause considerable economic loss (Rawther

Agronomy and Economy of Important Industrial Crops

205

et al., 1982; Sampath Kumar and Saraswathy, 1994). By far, the most serious disease is the YLD, though it is restricted only to certain areas such as Southern Kerala State and parts of Karnataka State.This is a highly debilitating disease, and an effective control is still elusive. Phytoplasma is found constantly associated with the disease (Nayar and Selsikar, 1978). The plant hopper Proutista moesta acts as vector for the disease (Ponnamma et al., 1997). The diseased palm when treated with tetracycline showed remission of symptoms, which provides additional supporting evidence for phytoplasmic etiology (CPCRI, 1994). Chracteristic symptoms are leaf yellowing followed by necrosis. Comparative physiology of healthy and diseased plants showed significantly higher stomatal resistance with increased water potential and turgor pressure in the latter (Chowdappa et al., 1993, 1995), while photosynthesis and transpiration were lower. Chlorophyll content was also low in diseased plants (Chowdappa and Balasimha, 1992). The altered values of chlorophyll fluorescence indexes reflected in normal arrangement of antennae pigments of photosystem II (PS-II) and reduction in photosynthetic quantum yield (Chowdappa and Balasimha, 1995). Both nuts and roots were also affected. Different methods have been investigated to control the disease and one such is intercropping and multiple cropping in YLD affected areca plantations. In arecanut, fruit rot is a major disease. In Karnataka State, it is locally known as Koleroga or Mahali and occurs in epidemic form, especially in the monsoon season. At first, water-soaked lesions appear on the surface of the nut and infected nuts turn dark green after losing its natural color. A white mycelial mass then covers the entire nut. The causal organism is identified as Phytophthora Mardi, a fungus. The infection can cause up to 15% loss in yield (Coleman, 1910; Sampath Kumar and Saraswathy, 1994). Effective control of the disease is systematic and periodic spraying of 1% Bordeaux mixture on fruit bunches. Covering of the bunches with polyethylene paper gives complete protection (CPCRI, 1983; Chowdappa et al., 1999; Sastry and Hegde, 1985). The fungus can also cause bud rot. The spindle leaf is affected, which subsequently kills the plant. Early detection, removal of affected plant parts, and treatment with 10% Bordeaux paste can save affected plants. Ganoderma lucidum fungus causes the foot rot, commonly known as Anabe disease. This is a soil borne fungus (Sampath Kumar and Nambiar, 1990). Poor drainage and high water table are the predisposing factors for the onset of the disease. Yellowing of the leaves in the outer whorl is the initial symptoms. Stems show dull brown patches. Subsequently gummy exudation starts and fruiting bodies of the fungus appear. To control the disease, checking the garden periodically is a must. Drenching of the root zone with 0.3% Calixin (15–20 l) or root feeding with the same chemical at the rate of 125 ml/palm is recommended.

206

K. P. Prabhakaran Nair

Inflorescence dieback is another serious disease which causes button shed. Yellowing and subsequent drying of inflorescence rachillae are the usual symptoms. The disease can be observed through the year, but most severely in summer months (Saraswathy et al., 1977). The fungus Colletotrichum gloeosporioides causes the disease (Chandramohanan and Kaveriappa, 1985). Spraying Indofil M-45 at the rate of 3 g/l water at the time female flowers open controls the disease. Stem bleeding is a minor disease occurring in some parts of India caused by the fungus Thielaviopsis paradoxa. Small discolored depressions appear in the basal portion of the stem. Subsequently these patches coalesce and cracks appear, leading to the disintegration of fibrous tissues. Exudation of brown liquid out of the cracks follows. Removing the affected parts and applying coal tar or Bordeaux paste controls the disease. Bacterial leaf blight caused by Xanthomonas campestris (Rao and Mohan, 1970; Sampath Kumar, 1981) causes the bacterial leaf blight, and the disease is confined to the ‘‘Maidan Region’’ of the State of Karnataka. Partial to complete blighting of leaves is caused by the infection. Spraying streptocycline or tetracycline (500 ppm) effectively controls the disease.

5. Arecanut Entomology Mites, spindle bugs, inflorescence caterpillars and root grubs are the insect pests of arecanut (Nair and Daniel, 1982). These are either seasonal or persistent pests on the crop. Two major foliage mites, Oligonychus indicus and Raoiella indica, occur abundantly during dry seasons. Dicofol, carbophenothion, or chlorobenzilate spray controls the disease. The spindle bug, Carvalhoia arecae, infests the leaf axils, and its highest population is found from August to September (Nair and Das, 1962). The bugs suck the sap from the spindles and young leaves resulting in the leaflets getting linear necrotic lesions. This results in the drying up of the spindle which fails to open. Spraying Thimet 10G or Sevin 4G at the rate of 10 g/palm controls the disease. Lecopholis burmeisteri and Leucopholis lepidophora are the most common root grubs. These feed voraciously on the roots causing serious damage. The pest damage is more in low lying areas. Leaves turn yellow and stems taper at the top. Rogor (dimethoate) and phorate, which are soil-applied insecticides, control the white grubs effectively (Prem Kumar and Daniel, 1981). Collection and destruction of adult beetles on emergence from soil is an effective management practice. The pentatomid bug, Halyomorpha marmorea, causes tendernut drop (Vidyasagar and Shama Bhat, 1986). The infestation is prominent between March and August. Bugs pierce the nut, suck the sap, the kernel dries up and the nut abscises and drops. Spraying 0.05% endosulfan can effectively

Agronomy and Economy of Important Industrial Crops

207

control the pest. The inflorescence caterpillar Tirathaba mundella damages areca inflorescence, feeding on tender rachillae. Malathion insecticide controls the pest. Leaves, leaf sheaths and nuts are colonized by scale insects (Aonidiella orientalis), which suck the sap from tissues. When these insects feed on nuts, they yellow prematurely and nut quality is lost. These pests are present throughout the year, but, are most abundant between October and February. Although scale insects were considered minor pests earlier, there have been widespread outbreaks of the insect attack in some of the districts in coastal areas of the country. One region is coastal districts of Karnataka State. Control of scale insects is rather difficult, although spraying malathion (0.1%) and fenthion (0.1%) showed partial control. The most effective is biological control, and Chilocorus sp., the coccinellid beetle, has been found to be quite effective in controlling the pest.

6. Arecanut Nematology Radopholus similes, the burrowing nematode is the most commonly found pest of arecanut (Sundarraju and Koshy, 1988). The intercrops found to be infested with nematodes are banana, cardamom, and pepper. Intercultivating these crops in arecanut gardens increases the nematode infestation (Sunderarraju and Koshy, 1988). Cocoa, pineapple, clove, cinnamon, and nutmeg were completely free of nematodes and thus, are ideal species for mixed cropping with arecanut (CPCRI, 1988b). Nematode infestation, unlike that of other pests like diseases and insects, has not been found to seriously impact economy of arecanut production. Nematicides or neem (Azadirachta indica) cake keeps under control the nematode population.

7. Harvesting and Processing of Arecanut It is very important that harvest of the nuts is done at the correct stage and this is determined by market demands. Harvesting at the most appropriate stage ensures nut quality. For instance, ripe nuts are harvested if the objective is to have dry nuts. On the other hand, green nuts of 6–7 months age are harvested for tender nut processing. Dried whole nuts, colloquially called Chali, are the most popular type of arecanut. Following harvest, ripe nuts are sun dried for 40–45 days. Drying is done by spreading the nuts in a single layer on a polyethylene sheet. Only proper drying will prevent fungal attack. Uniformity in drying can be ensured by turning the nuts in the evening each day. Dry nuts are dehusked manually or mechanically and then marketed. Good quality Chali is free from immature nuts which have

208

K. P. Prabhakaran Nair

no surface cracking, sticking husk, and, of course, fungal or pest (insects) infection. If the market requirement is tender processed nuts, then the nuts are harvested at 6–7 months of maturity, when they are green and soft. Processing consists of dehusking, cutting nuts into halves, and boiling them with water or diluted extract of previous boiling. After boiling, arecanut pieces are coated with Kali, which is a concentrated extract after boiling three to four batches of arecanut, to obtain good quality processed nuts. These nuts are then sun dried, although oven drying is also done often. Dried nuts are broken into bits, blended with flavor, and packed for marketing. These dried bits of nuts which are flavored are known as Supari, and the flavor depends on specific demands of the region. Tender processed nuts are widely used in making scented Supari. Spices like cloves, cardamom, or cinnamon are added and sometimes synthetic flavors. Essence of rose and menthol is commonly used additives. The scented Supari is packed in aluminum foil to preserve its flavor and dryness or in butter paper pouches and eventually marketed under brand names. Also, small portable aluminum containers with the brand name are used for marketing. Recently, an arecanut-based product blended with bits of cashew nuts known as Kaju Supari (Kaju meaning cashew nut in Hindi, the most widely spoken language of India) has become quite popular. No North Indian meal, be it on occasions of marriage or any other festivities, is complete without Supari. The habit is taking a firm hold in Southern India as well. It is widespread in Pakistan as well.

8. A Peep into Arecanut’s Future Like most other agricultural crops in India, arecanut also has taken a beating on the economic front ever since the globalization process set in (Proc. Silver Jubilee Symposium on Arecanut Research and Development. CPCRI, Vittal, 1982). Prices have tumbled during the last decade to its lowest value compared to the pre-globalization period. This has become a matter of great concern to the arecanut-based industry and also exporters of this valuable crop, not to speak of the arecanut farmers. There is substantial scope to utilize most of the production within India, which is world’s biggest producer of the crop. This is because of the internal demand. The National Commission on Agriculture, which was constituted by the Government of India in 2000 to look into various aspects, projected the internal requirement of the crop within India at 190,000 tons (Bavappa, 1982). During the last decade of the century past, production was about 330,000 tons, which means only about a third of the internal requirement only is met by internal production. Despite high production, internal price prevailed at Rs. 13,181 (approximately US $300) a quintal (100 kg) in 2000 which has now tumbled to less than Rs. 4000 (approx. US $100 at current

Agronomy and Economy of Important Industrial Crops

209

US $–Indian Rupee exchange rate). This steep price fall has devastated areca farmers, and the situation has been compounded by the onset of many diseases which also pulled down the crop yield. As a consequence of the WTO agreement, 3022 quintals of arecanut were imported from neighboring Asian countries at a price of Rs. 3122 (approx. US $70) a quintal in 2000–2001, which was less than 25% of the price prevalent in 1999–2000. This was followed by a crash in price in 2001–2002 of the local nut at 7893 (approx. US $175 a quintal), which is almost 100% less compared to prices prevalent at earlier times, which ruined the farmers and the industry. Although the price of imported nut was much lower than that of the local nut, the reduction in the local price was only about 60%. Despite the import, internal demand continued to be steady. Import was mainly from Sri Lanka (2426 tons) and Thailand (419 tons). India is the largest producer and consumer of arecanut in the world, accounting for nearly 52% of world production. Other than in India, no other country cultivates the crop in an organized manner. The positive side of the globalization process is that external markets are opened to the country’s crop. But, unless newer competitive marketing strategies coupled with innovative production technology is in place, the crop’s future is, rather, bleak. There are quite a few areas which need attention. One such is production technology with specific reference to areca nutrition. Classical ‘‘text book knowledge’’ still rules in devising fertilizer requirements of the crop. Latest techniques like ‘‘The Nutrient Buffer Power Concept,’’ developed by the author, which has been tried very successfully in other perennial crops, such as Black Pepper, Cardamom, and so on, need to be tried in arecanut as well.

9. Cashew Nut (Anacardium Occidentale L.) Cashew, which is native of Brazil, is widely cultivated throughout the tropics for its very nutritive, free of cholesterol, nuts ( Joubert and Des Thomas, 1965). It is one of the first fruit trees from the New World to be widely distributed in the tropics by the Portuguese and Spanish adventurers (Purseglove, 1988), who had set out across the world on sea route. The cashew plant has a checkered history. In both Asia and Africa, cashew nuts and its false fruit have been used in local small-scale entrepreneurial projects for more than three centuries. Large cashew-based enterprises were unknown until the early part of the twentieth century, when international trade started to show keen interest in the nut. This is the period when export also began from India. The start was rather slow, but subsequently the momentum picked up, and cashew nut plant became an important commercial crop. In Kerala State, in the central part, cashew processing and export became the center point of commerce. It also brought in a lot of

210

K. P. Prabhakaran Nair

revenue to the State through exports and provided a lot of employment avenues to the local populace. Places like the Kollam District in central Kerala became the hub of cashew nut-related activities. In India, use of cashew apples (to distil alcohol) and nuts was adopted by the local population and such accounts can also be observed in Africa, especially Eastern Africa, in countries such as Tanzania. In fact, in recent years Tanzania has turned out to be the biggest competitor to India in cashew export, and India imports large quantities from there to meet the growing domestic demand. Making cashew wine appears to have been a common practice both in Asia and Africa ( Johnson, 1973). The Maconde tribe of Mozambique call cashew the ‘‘Devil’s Nut.’’ It was offered as a token of fertility at weddings, and research carried out at the University of Bologna has shown the presence numerous vitamins including the old age health enhancer and an aphrodisiac Vitamin E (Massari, 1994) in cashew kernel. Lately enterprising Indian industrialists have started to distil alcohol from cashew apples as ‘‘green fuel’’ for automobiles. At the time of the first attempt to colonize India by the Portuguese, the name used by local population (Tupi natives of Brazil) for cashew was acaju (meaning nut in Portuguese), which later changed to caju (that is the name prevalent for cashew nut in North India, Pakistan, and Bangladesh), which became cashew in English. There is an interesting anecdote in Kerala State as to the origin of the word ‘‘cashew nut.’’ When the British colonized India, the British traveling to Kerala was fascinated by the nut for its palatability (it made a very good snack with a Scotch drink) and asked how much it cost. And the locals said ‘‘Eight for pie’’ (pie was the lowest denomination of British currency in India and in the local language it is called ‘‘Kashu.’’ In a nut shell, the statement in the local language Malayalam that one could get eight nuts for a pie—Kashini ettu, literally changed to cashew nut as coined by the Britishers). Hence, the origin of the word cashew nut is in Kerala State. Most of the names for cashew in Indian languages are also derived from the Portuguese name caju ( Johnson, 1973). However, the British-related incident stuck as the name for cashew nut globally. Cashew is a versatile tree nut. It is one of the most precious gifts of nature to mankind. The kernels contain a unique combination of fats, proteins, carbohydrates, minerals, and vitamins. The nut contains 47% fat, but 82% of this fat is unsaturated fatty acids, and thus can be considered not cholesterol enhancing. The unsaturated fat content not only eliminates the possibility of cholesterol enhancement, but also balances or reduces total cholesterol in blood. The nut contains 21% proteins and 22% carbohydrates and the right combination of amino acids, minerals and vitamins, and therefore, nutritionally speaking, it can be considered on par with milk, meat, and eggs. Since the carbohydrate content in the nut is low, which has only 1% of soluble sugar, the consumer of cashew is privileged to get a sweet taste without having to worry about excess calorie intake from sugary substances. The nut checks diabetes and consumption does not lead to obesity. In a

211

Agronomy and Economy of Important Industrial Crops

nutshell, cashew nut is an excellent snack, a good appetizer, an excellent nerve tonic and stimulant. Although the crop has its roots in Brazil, it is India which nourished it and brought it to international eminence. Today India is the largest producer, processor, exporter, and second largest consumer in the world (Nayar, 1998).

9.1. Cashew nut area and production 9.1.1. Global scenario Wordwide, the important producers of cashew are India, Indonesia, Brazil, China, Mozambique, Tanzania, Sri Lanka, and Vietnam. To a smaller extent, other countries on the Asian and African continents also grow the crop. The global scenario on cashew nut production is depicted in Table 9. Global cashew production is about 1.09 million tons (Table 11; Balasubramian, 2000; Bhaskara Rao and Nagaraja, 2000). Between 1980 and 1995 cashew production increased by 108%, from 0.422 million tons to 0.878 million tons. However, between 1995 and 2000, the growth declined to just 24%. Subsequent to 1995, world raw nut production has been around 1.09 million tons. Global raw nut production is given in Table 10, while country-wise raw nut production data are given in Table 11. India’s share of the world raw nut production is 47%, while the share of Southeast Asian countries has ranged from 14 to 16%. From 1980 to 2000, raw nut production in Southeast Asian countries increased by 45.2%. During the last two decades of the twentieth century, Latin American countries registered 114% increase. Global cashew production

Table 9

Indian subcontinent a

India Bangladesh Sri Lanka

a

Major producers.

Southeast Asia

Africa

Latin America

Vietnam Thailanda Indonesiaa

Angola Benin Burkina Faso

Malaysiaa Philippines China

Guinea-Bissau Madagascar Mozambiquea Mali Nigeriaa Kenyaa Senegal Tanzaniaa Togo

Brazila Barbados Dominicon Republic El Salvador Guadeloupe Honduras

a

212

K. P. Prabhakaran Nair

Table 10 Global production of raw nuts (mt)

Year

Indian subcontinent

1980 0.1483(35.1) 1981 0.1651(35.6) 1985 0.2309(46.4) 1990 0.2957(48.2) 1995 0.3820(44.4) 1996 0.4328(39.8) 1997 0.4450(43.8) 1998 0.4450(44.5) 2000 0.5200(47.7)

Southeast Asia

Africa

Latin America

0.0275 (6.5) 0.0296 (6.4) 0.0305 (6.1) 0.0815 (13.3) 0.1570 (17.9) 0.1638 (15.1) 0.1708 (16.8) 0.1485 (14.8) 0.1520 (13.9)

0.6120 (38.4) 0.1856 (40.0) 0.1114 (22.4) 0.1119 (18.3) 0.1397 (15.9) 0.3205 (29.5) 0.2917 (27.7) 0.3690 (36.9) 0.2000 (18.3)

0.0841 (19.9) 0.0835 (17.9) 0.1246 (25.0) 0.1200 (19.6) 0.1993 (22.7) 0.1689 (15.6) 0.1175 (11.6) 0.0380 (3.8) 0.1800 (16.5)

Total

0.4219 0.4638 0.4974 0.6127 0.8780 1.0861 1.0149 1.0005 1.0900a

a Includes 0.38 mt under others. Source: Cashew Export Promotion Council, Government of India, 2003. Figures in parentheses indicate percent of total raw nut production; mt, million tons.

Table 11 Country-wise cashew raw nut production (mt) Country

Production

India Indonesia Vietnam Brazil Mozambique Nigeria Tanzania Others Total

0.520 0.030 0.122 0.180 0.030 0.040 0.130 0.038 1.090

Source: Cashew Export Promotion Council, Government of India. Data refer to the latest available figures (2003).

213

Agronomy and Economy of Important Industrial Crops

9.2. The Indian scenario In India, the crop is mainly grown in the States of Goa, Karnataka, Kerala, and Maharashtra along the west coast of the country, and in the States of Andhra Pradesh, Tamil Nadu, Orissa, and West Bengal along the east coast of the country. In Andaman and Nicobar Islands, and the States of Madhya Pradesh, Manipur, Meghalaya, and Tripura in Northeastern India, the crop is grown to a limited extent. India’s raw nut production has increased from 0.079 million tons in 1955 to 0.52 million tons by 2000, an increase of 558% in half a century, that is more than an annual growth rate of 10%, which is, indeed, a remarkable increase. Data on raw nut production are given in Table 12. In the last decade of the century past, raw nut production had almost doubled. In 1955 the total acreage in India was 0.11 million ha, which, by 2000 had jumped to 0.683 million ha, a remarkable increase of 520%, which is an annual growth of more than 10%. This shows that the jump in production during this period has mainly been on account of increased acreage. During 1970–1980 though the area increased there was a deceleration in production. It is during the last 5 years of the century past, that is, between 1995 and 2000, the increase in both area and production is phenomenal. If India has to maintain its preeminent place in the international market, the productivity has to necessarily increase. Up to 1970, Table 12 Area, production and productivity of cashew raw nuts in India Area

Production

Year

Million hectare

Percentage increase in 5 years

Million tons

Percentage increase in 5 years

Productivity (kg/ha)

1955 1960 1965 1970 1975 1980 1985 1990 1995 1996 1997 1998 1999 2000

0.110 0.176 0.232 0.281 0.358 0.451 0.509 0.531 0.577 0.635 0.650 0.700 0.730 0.683

– 60.0 31.8 21.1 26.7 25.9 11.4 4.1 8.7 – – – – 18.4

0.079 0.110 0.141 0.176 0.166 0.142 0.221 0.286 0.371 0.418 0.430 0.360 0.460 0.520

– 39.2 21.9 24.8 5.7 14.4 55.6 29.4 22.9 – – – – 40.2

720 630 610 630 460 310 430 540 640 720 835 740 800 865

Cashew Export Promotion Council, Government of India.

214

K. P. Prabhakaran Nair

Table 13 State-wise area, production and productivity of cashew in India (1999–2000)

State

Area (000 ha)

Productive area (000 ha)

Production (000 mt)

Productivity (mt/ha)

Maharashtra Andhra Pradesh Kerala Karnataka Goa Tamil Nadu Orissa West Bengal Othersa Total

121.20 100.00 122.20 90.50 54.40 85.20 84.10 9.10 16.70 683.40

85.00 90.00 118.00 86.00 49.00 84.00 65.00 9.00 15.00 601.00

125.00 100.00 100.00 60.00 30.00 45.00 40.00 8.00 12.00 520.00

1.47 1.10 0.85 0.70 0.61 0.54 0.62 0.90 0.80 0.865

a Madhya Pradesh, Manipur, Tripura, Meghalaya, and Andaman and Nicobar Islands. Source: Cashew Export Promotion Council, Government of India.

productivity was around 630 kg/ha. Between 1975 and 1985, productivity was low at 430 kg/ha. Since 1985, productivity has been steadily increasing from 430 to 865 kg/ha in 2000 (Balasubramanian, 2000; Bhaskara Rao and Nagaraja, 2000). This is due mainly to improved technology of production, such as availability of high yielding planting material, institutional research back up in providing nearly 10 million grafts annually and extensive replantation program. Private nurseries also provided good planting material. State-wise area on production and productivity is given in Table 13.

9.3. World trade in cashew For more than half a century India has been in cashew export trade. Over the years, both the quality and quantum of export have been on the rise. The established processing capacity of raw nuts is around 7 lakh tons. However, domestic production is around 5.2 lakh tons. Hence, India has been importing raw nuts from Africa, principally from Tanzania. This has been done to meet the demand of the cashew-processing industries. The export–import scene over the last more than half a century is presented in Table 14. Export earning has been on the increase since 1955. In 2000, India had an all-time export earning of Rs. 2500 crores (approx. US $625 million). Between 1980 and 1985, although export earnings increased, the quantity of cashew kernels exported decreased. The quantity of cashew kernels exported has steadily increased since 1985. It is estimated that the processing industries can absorb up to 10 lakh (1 million) tons of raw nuts for processing (Bhaskara Rao and Nagaraja, 2000).

215

Agronomy and Economy of Important Industrial Crops

Table 14 Import of cashew raw nuts and export of cashew kernels from India

Year

Import of raw nuts (ton)

Export of kernels (ton)

Export earning (Million Rs)

1955 1960 1965 1970 1975 1980 1985 1990 1995 1996 1997 1998 1999 2000

63,000 95,000 191,000 163,000 160,000 24,000 33,000 59,000 222,000 222,819 192,285 224,968 181,009 199,000

31,000 39,000 56,000 60,000 65,000 38,000 32,000 45,000 77,000 70,334 68,663 76,593 75,026 95,000

12.9 16.1 29.0 57.4 118.1 118.0 180.0 3650.7 12,458.0 12,405.0 12,855.0 13,961.0 16,099.0 25,000.0

Source: Cashew Promotion Council of the Government of India.

Table 15 Global cashew nut output (000 tons) scenario vis-a`-vis India

1990

World 732 Vietnam 140 India 286

2005

Percentage growth in 16 years

Percentage annual average growth

2,662 812 526

263 480 84

16.4 30.0 5.3

India, which had been the pioneer and topper in raw cashew nut production, has been pushed to the third place (Table 15). However, India maintains its top position in processing and exports using imported nuts, mainly from Africa. FAO statistics clearly indicate that Vietnam is the biggest producer followed by Nigeria, pushing India to the third position. In 2005, Vietnam produced 9.61 lakh tons (one hundred thousand tons make 1 lakh ton), followed by Nigeria with 5.94 lakh tons, while Indian output stood at 5.44 lakh tons. Indian cashew-processing industry has an installed capacity to process around 1.2 million tons of raw cashew nuts. But, the indigenous raw nut availability remains still around 50% of its annual requirement. As a result, imports have been on the upsurge. As against 249,315 tons valued at Rs. 960.54 crores (approx. US $240 million at

216

K. P. Prabhakaran Nair

current exchange rate) in 2000–2001, imports in 2006–2007 were at 592,604 tons valued at Rs. 191,162 crores (approx. US $485 million). When the country imports to meet the local industrial need, valuable foreign exchange is drained from the country.

9.4. The tale of cashew trade in India over the years The cashew industry earns crores of Indian rupees in foreign exchange and employs around 1 million laborers, mostly women. The nation has also to its credit the privilege of having pioneered cashew exports in 1945. And the revelation that it took the ingenuity of a British butler to develop the ‘‘Vita Packing’’ provides an interesting sidelight. In fact cashew is the poor man’s crop and the rich man’s food. The Keralites are the pioneers of the cashew industry in India. It is believed that cashew was first discovered in Eastern Brazil by the Portuguese travelers. The Brazilians, while ravishly devouring the false fruits, discarded the nuts. The Portuguese brought the cashew to Goa and planted the seeds along the sea coast to check sea erosion. The country saw the processing and trading of cashew kernels take off in Kollam (in central Kerala State), Mangalore (in Southern Karnataka State), and Vettapalem (in Andhra Pradesh). The kernels were first exported in oil cans. This method was not foolproof as the nuts were found to be infested with weevils at the destination. Subsequently, an English butler tried to store the kernels in cans infused with carbon dioxide by commissioning a soda maker and found the method to be successful. The industry is still using this method, which came to be known as the ‘‘Vita Packing.’’ Nuts have been imported from African countries, to be processed and re-exported to various global destinations. This helped develop a flourishing cottage industry around Kollam. The local women became adept at processing the nuts, and till date, they have been able to retain the skill with finesse. But, over a period of time, India lost its frontal position to Vietnam. The Kerala Government through its Cashew Development Corporation is bringing out value-added cashew products. It has tied up with the Central Food Technology Research Institute, the only one of its kind situated in the city of Mysore, Karnataka State, to bring out five value-added products, namely, cashew powder, cashew soups, cashew bits, cashew vita, and cashew kernels. These value-added products will, besides the domestic market, reach the global market as well. The Government of India is supporting the move by providing the Corporation Rs. 1 crore (approx. US $250,000) for international publicity. The target countries would be those in the Gulf region, Russia, and China, which are the non-conventional consumers, in addition to the traditional ones like Japan, European Union, and so on. It is in this context that the setting up of a cashew export development authority (CEDA) becomes relevant to implement the schemes of the Government of India to increase production of raw nuts in

Agronomy and Economy of Important Industrial Crops

217

the country and arrest India’s dependence on imported raw materials at a time when the nation has immense potential to become self-sufficient in production and further venturing into processing and export. Against the background of all these, it is important to realize that of all the edible nuts, cashew nut remains world’s most favorite nut, and as is well known ‘‘A poor man’s crop, but a rich man’s food.’’ If a global perspective has to develop on cashew nut as a ‘‘global food crop,’’ it is imperative that a global effort must follow. It is in this light that world’s important cashew producers, such as Brazil, India, and Vietnam, are coming together on a global alliance—the ‘‘Global Cashew Alliance’’ (GCA)—which is coming into shape with the assistance of the Cashew Export Promotion Council of the Government of India. A memorandum of understanding (MOU) between India, Brazil and Vietnam has already been reached for setting up the GCA. An initial grant of US $1.5–2 million is expected from the Government of India to get the GCA going. A similar contribution is expected from the other two partners. Finally, if the GCA comes into being, as is sure to, it would chart a new course in the history of this favorite nut of the world.

9.5. History, evolutionary origin, and distribution of cashew It is from Southern Honduras to Parana, Brazil, and Eastern Paraguay that Anacardium is distributed naturally (Ohler, 1979). It is not indigenous to South America west of the Andes, except Venezuela, Colombia, and Ecuador, where Anacardium excelsum is prevalent. A. occidentale is cultivated or adventive throughout the Old and New World tropics. The genus has two centers of diversity, Central Amazonia and the Planalto of Brazil. This is illustrated by the occurrence of four species in the vicinity of Manaus and by three species occupying the same habitat in the Distrito Federal, Brazil. The following five distribution patterns are found in Anacardium. 1. A. excelsum is isolated taxonomically and geographically from its congeners by the Andes. The uplift of the Andes was probably the driving force in the early differentiation of A. excelsum from the rest of the genus. 2. Anacardium giganteum and Anacardium spruceanum have Amazonian– Guyanan distributions. 3. A. occidentale, which is the most widespread species in the genus, has disjunct populations in the Planalto of Brazil, the restingas of Eastern Brazil, the savannas of the Amazon basin, and the Ilanos of Colombia and Venezuela. It should be remembered that the natural distribution of this species is obscured by its widespread cultivation in both the Old World and New World. 4. Three closely related species, Anacardium humile, Anacardium nanum, and Anacardium corymbosum, are restricted to the Planalto of central Brazil.

218

K. P. Prabhakaran Nair

5. Two species of Anacardium are narrow endemics. A. corymbosum, which is restricted to south-central Mato Grosso, is an allospecies of A. nanum, and Anacardium fruticosum (a new species) is endemic to the upper Mazaruni River basin in Guyana. It is closely related to the Amazonian Anacardium parvifolium. The eastern portion of the Amazon river figures prominently in distributions of many plants and animals, many of which are found either exclusively to the north or to the south of the river. However, in the case of Anacardium, all the Amazonian species are found on both sides of the Amazon river. The reason for this is probably the ease with which bats, large birds, and water (in the case of Anacardium microsepalum) carry fruits across water barriers (Mitchell and Mori, 1987). A. Occidentale is cultivated and adventive throughout the Old and New World Tropics where the geographical limits of its cultivation are latitudes 27 north and 28 south, respectively (Nambiar, 1977). A. occidentale is a native of tropical America where its natural distribution is unclear because of its long and intimate association with man. The questions concerning its origin and distribution have been investigated by Johnson (1973) who suggested that it originated in the restinga (low vegetation found in the sandy soil along the coast of Eastern and Northeastern Brazil). The author is probably correct in assuming that the cultivated form of A. occidentale came from Brazil, because cashew trees cultivated in the Old and New Worlds are identical in appearance to native trees found in restinga vegetation. In particular, cultivated and wild populations of cashew species from Eastern Brazil share chartaceous leaf blades and long petioles. A. occidentale is probably an indigenous element of the savannahs of Colombia, Venezuela, and the Guyanas. It is clearly a native, and occasionally a dominant feature of the cerrados (savannah-like vegetation) of central and Amazonian Brazil. The cerrado populations of A. occidentale differ from the restinga populations by having undulate, thickly coriaceous leaves with short and stout petioles. The hypocarps (cashew apples) of cerrado trees are usually smaller and sometimes have a more acidic flavor than those of the restinga. The natural distribution of A. occidentale extends from Northern South America south to Sao Paulo, Brazil. It is probably not native to Central America, the West Indies, or South America west of the Andes. It is believed that A. occidentale originally evolved in the cerrados Central Brazil and later colonized the more recent restingas of the coast. Central Brazil is a center of diversity for Anacardium where the distribution of A. occidentale overlaps the ranges of A. humile, A. nanum, and A. corymbosum. A. humile, the closest relative of the cultivated cashew, is closer morphologically to the cerrado ecotype than it is to the restinga and cultivated populations of A. occidentale (Mitchell and Mori, 1987). The earliest reports of cashew are from Brazil coming from the French, Portuguese, and Dutch observers ( Johnson, 1973). The French naturalist and

Agronomy and Economy of Important Industrial Crops

219

monk A. Thevet was the first to describe in 1558 a wild plant extremely common in Brazil: the cashew tree and its fruits. He recounted that the cashew apple and its juice was consumed, the nuts were roasted in fire and kernels eaten by the natives. He also drew the picture of the natives harvesting the apple and squeezing by hand the juice into large jars ( Johnson, 1973; NOMISMA, 1994). Indications that native Tupi Indians used cashew for centuries exist. They took the crop across to the Bazilian northeastern coast during the migration which is indicated by the considerable intraspecific variations (Ascenco, 1986). The entire cashew fruit, nut, and peduncle will float when mature. This explains the coastward dispersal of the species by rivers draining in the northern and eastern directions. Fruit bats also might have accelerated the dispersal process through the ingestion of the nuts. Fruit bats are the most effective seed dispersal agents within the Amazonian forests ( Johnson, 1973). From its origin in Northeastern Brazil, cashew spread into South and Central America (Van Eijnatten, 1991). Human intervention spread cashew to other continents ( Johnson, 1973). The Portuguese discovered cashew first in Brazil and then spread it to Mozambique and later to India between the sixteenth and seventeenth centuries, possibly between 1563 and 1578 (De Castro, 1994). It first arrived in Africa on the east coast in the second half of the sixteenth century and then spread to west coast and finally in the islands (Agnoloni and Giuliani, 1977). Though the possibility of the Portuguese bringing cashew to Africa can be reasonably surmised, there are no records to substantiate the specific dates of Introduction. Dispersal to eastern part of Africa must be due to the elephants known for the love of fruits ( Johnson, 1973). Possibly attracted by the vibrant color of the fruits, the elephants must have been swallowing the entire apple with the nut and since the nut is too difficult for easy digestion, the dung must have carried on this dispersal. This is how the cashew spread to African east coast along the Indian Ocean (Massari, 1994). The spread of cashew within the South American continent was gradual (NOMISMA, 1994). The plant was first found and described along the coast of Malabar in Kochi. Following Introduction to Southwestern India, the spread occurred through bats, birds, and most importantly, through human intervention. Kochi was the focal point from where the cashew spread to other parts of India, as well as to Southeast Asia ( Johnson, 1973). The plant was spread primarily to control soil erosion in coastal areas ( Johnson, 1973). This interpretation, possibly, smacks of a twentieth century concept in soil management, to a sixteenth century phenomenon. It was the Portuguese who realized that the nuts had medicinal value and the juice could be made into a good wine. This also led to the realization of the early Portuguese colonizers of India of the potential economic value of cashew. Following Introduction into India, the plant was taken to Southeast Asia (NOMISMA, 1994). Dispersal in Southeast Asia was aided by monkeys. It is uncertain whether the plant reached Philippines

220

K. P. Prabhakaran Nair

through India. It might have come directly from the New World on the Manila galleons ( Johnson, 1973). Later, it spread to Australia and parts of North America, such as, Florida. The present diffusion is between 31 north (latitude) and 31 south (longitude) both as a cultivated and wild species (NOMISMA, 1994). Presently cashew is cultivated in many tropical countries along the coast (Ascenco, 1986; Van Eijnatten, 1991). It was in the nineteenth century when the idea of growing cashew as a plantation crop took root and the idea spread to Africa, Asia, and Latin America (Massari, 1994).

10. Economic Botany of Cashew Anacardium is one of the most important genera of the Anacardiaceae family. This importance is due to A. occidentale which is the cashew nut of commerce, a major export of the developing world. The false fruit (hypocarp) is both consumed locally and used to distil the ‘‘Feni,’’ the alcoholic beverage. In South America, especially Brazil, the juice from the cashew apple is marketed widely as a popular drink. The cashew nut shell liquid (CNSL) is used for industrial purposes and has medicinal value. Some of the other Anacardium species have economic potential, but they are currently underutilized. A. excelsum is used in construction and also as a shade tree in coffee and cocoa plantations. A. giganteum is a locally important timber in South America and its hypocarps are very much relished by the local people. The spectacular white leaves of the inflorescence of A. spruceanum make it a tree with excellent ornamental potential. A. humile, a subshrub closely related to A. occidentale, possesses edible hypocarps and seeds. Selective breeding for better quality hypocarps and seeds, as well as hybridization with A. occidentale, could yield subshrubs with fruits that could be harvested mechanically. The economic potential of the other two subshrubs, A. nanum and A. corymbosum also should be investigated (Mitchell and Mori, 1987).

10.1. Taxonomy of the cashew plant The cashew plant belongs to the family Anacardiaceae, genus Anacardium and species occidentale. The genus belongs to the Latin American genus of trees, shrubs, and geoxylic subshrubs, the taxonomic treatment of which is provided by Mitchell and Mori (1987). Anacardiaceae is a moderately large family consisting of 74 genera and 600 species. There are five tribes, namely, Anacardieae, Spondiadeae, Semecarpeae, Rhoeae, and Dobineae. The tribe Anacardieae consists of eight genera namely, Androtium, Buchanania, Bouea, Gluta, Swintonia, Mangifera, Fegimanra, and Anacardium (Mitchell and Mori, 1987). Bailey (1958) suggests that Anacardium is a small genus of eight species

Agronomy and Economy of Important Industrial Crops

221

indigenous to South America. However, Agnoloni and Giuliani (1977) recognize 11 species while and Johnson (1973) recognizes 16 species. Valeriano (1972) names five species namely, A. occidentale L., Anacrdium pumilum St. Hilaire, A. giganteum Hanca, Anacardium rhinocarpus, and A. spruceanum Benth. This author suggests recognition of only two species namely, A. nanum and A. giganteum, which can be further sub-divided based on the color (yellow or red) and shape (round, pear-shaped or elongated) of the pseudo-fruit (apple). This author also considers the division into dwarf and giant species to be the only way to classify cashew in a rational and practical manner. His arguments are based on the characteristics of the apple, the pseudo-fruit. However, the description provided by Peixoto (1960) separates recognition of more than two species. It appears from published literature that A. occidentale L. is the only species which has been introduced outside of the New World. As many as 20 species of Anacardium are known to exist within Central and South America (Table 16). Mitchell and Mori (1987) recognize 10 species of the genus Anacardium, one of which, Anacardium fruiticosum, is described as new. The genus has a primary center of diversity in Amazonia and a secondary enter in the Planalto of Brazil. All known species of Anacardium genus can be found in the South American continent; only four of them, namely, Anacardium coracoli, Anacardium encardium, A. excelsum, and A. rhinocarpus are not present in Brazil. There, the presence of the high number of wild species suggests that the northeast coast is the site where the Anacardium genus, namely, A. occidentale L. originated. In fact, here different forms of cashew can be found with high variability in local populations, namely along the coast and the dune. These days, most species belonging to the genus Anacardium can be found anywhere in Brazil (NOMISMA, 1994). Ascenco (1986) reported that A. occidentale L. is the only species in the genus that attained economic importance. The Anacardium genus appeared to have originated in the Amazon region of Brazil and hence speciation followed different geographic patterns.

10.2. Cytogenetics of the cashew plant There is no detailed study of the cytology of A. occidentale L. Only for A. occidentale has been the chromosome number reported. This morphologically polymorphic species also exhibits chromosome polymorphism (Mitchell and Mori, 1987). In the literature, reported chromosome number varies from 2n = 24 (Goldblatt, 1984; Khosla et al., 1973), 2n = 30 (Machado, 1944), and 2n = 40 (Goldblatt, 1984; Simmonds, 1954) to 2n = 42 (Darlington and Janaki Ammal, 1945; Goldblatt, 1984; Khosla et al., 1973; Purseglove, 1988). In many domesticated trees such chromosome polymorphism is well documented (Khosla et al., 1973).

222 Table 16

K. P. Prabhakaran Nair

The different species of Anacardium Linn.

Botanical name

Country of origin

Anacardium brasiliense Barb. Rodr. Anacardium curatellaefolium St.Hil (same as A.subcordatum Presl.) Anacardium encardium Noronha Anacardium giganteum Hancock ex.Engl Anacardium humile St. Hil (Anacardium subterraneum Liais) Anacardium mediterraneum Vell. Fl. Flum Anacardium nanum St. Hil (same as A.humile Engl., A.pumilum Walp) Anacardium occidentale L. (Cashew nut) Anacardium rhinocarpus D.C. Prod. Anacardium spruceaum Benth ex Engl. Anacardium microsepalum Loes. Anacardium corymbosum Barb. Rodr. Anacardium excelsum Skeels (same as Rhinocarpus excelsa) Anacardium parvifolium Ducke Anacardium amilcarianum Machado Anacardium Kuhlmannianum Machado Anacardium negrense Pires and Fro’es Anacardium rondonianum Machado Anacardium tenuifolium Ducke Anacardium microcarpum Ducke

Brazil Brazil Malaysia Brazil Brazil Brazil Brazil Brazil Brazil Brazil Amazon region Brazil Brazil Amazon region Brazil Brazil Brazil Brazil Brazil Amazon region

Source: Index Kewensis 1996, Royal Botanical Gardens, Kew.

10.3. Collection, conservation, and cataloging of genetic resources of the cashew plant Records on the Introduction of the cashew plant into the Malabar coast of Kerala State, India, from where it spread to other parts of the country, are imprecise. However, it is presumed that initially the Introduction was a few trees and because of their hardy nature, they naturally spread to other parts of the country, especially on the coastal region. These Introductions were A. occidentale. Initially, the focus was on establishing plantations of seedlings. As cashew plant is cross-pollinated and heterozygous, considerable segregations have resulted in the cashew population (Bhaskara Rao and Bhat, 1996). Germplasm collection got an impetus following the establishment of the Indian Council of Agricultural Resarch (ICAR) controlled National Research Center for Cashew (NRCC), and State Agricultural University

Agronomy and Economy of Important Industrial Crops

223

(SAU) administered research centers. Segregants/variants were collected as seed from different parts of the country. Wide variability has been observed in the collections (Bhaskara Rao and Swamy, 1994; Table 17). The NCRR established in Puttur, Karnataka State, has the National Cashew Gene Bank (NCGB). These have exclusive clonal accessions. The accessions are collected after a survey map is established during the fruiting season and the scions from the identified mother plant are collected during the propagation season ( June to September). The grafts are produced and the clonal accessions are planted in the NCGB. Conservation field blocks are also established with clonal accessions. The efforts are coordinated by the All India Coordinated Research Project on Cashew (AICRPC). A total of 1490 accessions have been conserved (Bhat et al., 1999). At NCRR a total of 433 clonal accessions have been conserved in the NCGB. According to the International Plant Genetic Resources Institute (IPGRI) descriptors, 255 accessions have been characterized and cataloged after six annual harvest (10 years of planting), and the Catalogue of Minimum Descriptors of Cashew (A. occidentale L.) Germplasm Accessions-I, II, and III have been published (Swamy et al., 1997, 1998, 2000).

Table 17 A selection of cashew germplasm collections from various research centers in India released for commercial cultivation State

Center

Varieties

Andhra Pradesh Goa

Bapatla

BPP-3, BPP-4, BPP-5, BPP -6

ICAR, Research Center, Goa National Research Center for Cashew, Puttur (NRCC) Chintamani Ullal

Goa-1

Karnataka

Kerala

Anakkayam Madakkathara

Maharashtra Orissa Tamil Nadu West Bengal

Vengurla Bhubaneswar Vridhachalam Jhargram

NRCC Selection 1 and 2

Chintamani-1 Ullal-1,Ullal-2, Ullal-3, Ullal-4, UN-50 Anakkayam-1 (BLA 139-1) Madakkathara-1 (BLA 39-4) Madakkathara-2 (NDR 2-1) K-22-1, Sulabha Vengurla-1,Vengurla- 2 Bhubaneswar-1 VR-1, VR-2, VR-3 Jhargram-1

224

K. P. Prabhakaran Nair

11. The Genetic Improvement of the Cashew Plant 11.1. Breeding A number of plant attributes, such as number of inflorescence per unit area, number of nuts per inflorescence, and the mean weight per nut, decide the ultimate plant yield. These yield attributes, either directly or through their interaction with each other, decide the plant yield. Any attempt to improve the plant yield should precede a clear understanding of the various processes governing the physiology, nutrition, and so on of these yield components. The process of differentiation of reproductive shoot from vegetative shoot is an important aspect that needs to be investigated. Though data currently available inadequately explain the differentiation between vegetative and reproductive shoots, indications are that this could be governed by environmental variables, such as nutrition, soil moisture availability, and weather. Understanding the interaction between these will help breeding varieties having higher yield contributing factors, such as number of inflorescence per unit area, nuts per inflorescence, and fruit-to-nut ratio (Bhaskara Rao et al., 1998). Foltan and Ludders (1995) found that there were no significant differences in fruit set following selfing compared to cross-pollination, except in one case where selfing H-3-13 resulted in significantly lower fruit set while the same variety when crossed with Guntur accessions gave maximum fruit set. The reciprocal combination of these parents resulted in lower fruit set indicating the need to understand the cross-pollinated nature between preferential combinations of parents to realize higher yields. Therefore, studies on the compatibility relationship of cashew varieties and designing the models to establish orchards with polyclones to ensure highest compatibility to ensure higher yields in cashew are a priority. An option to realize high yield is to go in for high density planting ranging from normal spacing, that is, 8 m  8 m (156 plants/ha) or 7.5 m  7.5 m (175 plants/a) going up to 625 plants/ha, depending on soil fertility and canopy structure of the variety to be planted. High density planting ranging from 200 plants/ha (10 m  5 m) up to 625 plants/ha (4 m  4 m) will only be possible with dwarf genotypes with compact canopy structure and intensive branching, with high proportion of flowering laterals per unit area. Therefore, breeding strategies must focus on dwarfing nature of the plant as well, an attribute which can be used as root stock or planting types as such, which can be put through hybridization program to include other yield attributes to structure a genotype which can fit into a high density planting program in which case, suitable canopy management should follow, such as proper pruning techniques, either conventional or through using chemicals, such as Paclobutrazol. Among the different

Agronomy and Economy of Important Industrial Crops

225

Anacardium species, it is only A. microcarpum, supposedly a dwarf genotype, which can be used as a root stock to multiply varieties which will result in compact canopies (Bhaskara Rao et al., 1998). A very important aspect in breeding programs is to focus on the dietary aspects of cashew nut. Tree nuts are generally considered highly nutritive and have been placed in the base of the Mediterranian Diet Pyramid developed by the World Health Organization (WHO), which recommends their daily consumption. Differences with respect to neutral lipids and glycolipids without difference in phospholipids in varieties have been reported (Nagaraja, 1987a,b). A quality index was developed (Anonymous, 1994) based on protein, lysine, and sugar content of cashew kernels. However, the recent emphasis is also on low fat content so that the misapprehension that consumption of cashew kernels is deleterious to health is not propagated among consumers. Some of the varieties having more than 35% protein, lysine and more than 50 mg/mg protein, and less than 14% sugar were identified. These can be used in breeding program to develop varieties with better nutritive value for the diet-conscious consumer. One of the major production constraints in India as well as in other cashew growing countries is the severe incidence of the TMB in the flushing and flowering season. Screening of the available germplasms in India showed little promise to encounter any with built-in resistance to TMB. However, in one accession ‘‘Goa’’ 11/6, a phonological evasion has been noticed which enables the accession to escape severe infestation of TMB (Sundararaju, 1999). Hybrids H-3-17, H-8-1, H-8-7, H-8-8, H-15, and H-1600 have shown moderate tolerance to TMB. One must look for varieties whose flowering does not coincide with the onset of peak population in the TMB. Possibility of identifying tolerant types through screening of somaclonal variants is also a line worth pursuing in breeding. Since cashew yield structure is an integration of the different yield attributes enumerated earlier, an insight into breeding for yield enhancement can also be obtained through specific partitioning of these variables either through experimentation or through statistical analysis, such as, multiple regression or path analysis. Through such an approach, high heritability components contributing to yield need to be identified and efforts made to integrate these attributes through hybridization. These approaches in breeding must also be coupled with yield enhancing management practices in the ultimate task of enhancing cashew productivity (Bhaskara Rao et al., 1998).

11.2. Selection Of the 40 cashew varieties released in India for commercial cultivation, 25 are selections from among the germplasm collections available in different cashew research centers in the country (Abdul Salam and Bhaskara Rao, 2001).These 25 varieties were identified and released based on the

226

K. P. Prabhakaran Nair

germplasm evaluation carried out at the different research centers (Table 17). As the cashew crop was initially propagated from ‘‘plus trees’’ for soil conservation and afforestation, there was not much emphasis on the varietal concept described earlier in the section on breeding. This varietal concept is of recent origin. Initially, the focus was only on the total yield obtained per tree. This has resulted in the release of varieties with kernel grades of more than W 320. Important attributes, such as kernel weight, shelling percentage, and recovery percentage of whole kernels, received but little attention. Of late, because of the emphasis on quality of the nut, there is greater focus on identification of varieties with kernel weights over 2 g, which fall in the export grade of W 210 and W 240. To realize higher recovery of whole kernels, standards have been fixed for shelling percentage, which is not less than 30. This demands identification of donor parents which have these attributes which can be transmitted from parent to progeny (Bhaskara Rao et al., 1998).

11.3. Hybridization Cashew hybridization is increasingly getting popular because of its great potential for enhancement of yield and several other desirable traits in the plant. Australia is currently placing great emphasis on hybridization. Parents of wide genetic variability obtained from different countries around the world are used for the hybridization work in Australia (Chacko, 1993; Chacko et al., 1990). This has necessitated the standardization of pollination techniques in cashew that is reliable, fool proof, and, most importantly, simple to follow. A simple technique of pollination in cashew has been developed at the NRCC (Bhat et al., 1998). The new pollination/crossing technique involves the use of butter paper rolls or pantographic paper rolls. It is as follows: 1. Panicles with flower buds which will open the following day are selected on male and female trees. All opened flowers and nuts, if any, are removed from the selected panicles on the female parental tree. 2. Each morning between 8 and 9.30 AM all the opened male flowers from the selected panicles on female parental trees are removed. Then anthers are removed (emasculated) using ordinary pins prior to anther dehiscence from freshly opened hermaphrodite flowers of the panicles. The stigma along with the style is enclosed with a butter paper roll or pantographic paper roll, which is prepared using a small piece of butter paper sheet (2.5 cm  1.5 cm in size) by rolling it with fingers. 3. Freshly opened male flowers with undehisced anthers are collected in a Petri dish from selected male parents between 8 and 9.30 AM and the anthers are allowed to dehisce under partial shade.

Agronomy and Economy of Important Industrial Crops

227

4. The butter paper roll from the emasculated flower is then removed and the sigma is pollinated with the pollen from freshly dehisced anthers of the male parent collected in a Petri dish. 5. The pollinated stigma along with style is re-enclosed in the butter paper roll. 6. Each panicle is labeled indicating the names of the male and female parents of the cross as well as the panicle number. Each panicle is used only for one cross combination. 7. The earlier procedure is repeated until 8–10 hermaphrodite flowers are pollinated in each of the selected panicle. 8. All opened hermaphrodite flowers which are unused for pollination are removed each day. 9. The remaining flower buds are removed from the panicle on the last day of pollination for that specific panicle. 10. Each panicle with developing hybrid nuts is enclosed in a cloth bag to collect the nuts on maturity. Details of the crosses should be clearly written on the cloth bags. 11. The hybrid nuts obtained as per the above procedure are grown in polyethylene bags for subsequent planting in the field. The procedure described above gives higher percentage of hybrid nuts when compared to procedures of pollination currently used on account of the low physical injury to the delicate cashew flowers. Fruit set and retention of fruits: Investigations carried out at Darwin, in Australia on flowering, fruiting, and genotype compatibility (Foltan and Ludders, 1995) indicated that among the five cultivars used in the investigation, only one, namely, H-3-13 performed differently compared to others. No significant differences were observed in fruit set following selfing compared to cross-pollination in all the combinations except in the cultivar named above, where selfing resulted in significantly lower fruit set. When H-3-13 was crossed with Guntur, a fruit set of 51.7% (maximum) was obtained. While in the reciprocal combination of Guntur  H-3-13, only 38% fruit set was obtained. This clearly shows that very careful parental selection is a pre-requisite in cashew to obtain good fruit set (Bhaskara Rao, 1996). In selfed progenies compared to crossed ones, owing to post-zygotic mechanism, responsible for sterility, general yield reduction was observed (Wunnachit et al., 1992). Premature preferential flower shedding of selfed fruits has been noticed in avocado (Degani et al., 1989). These investigations point to the crucial fact that compatibility of genotypes must be clearly understood to contain premature fruit drop, which is a major problem in cashew production (Bhaskara Rao, 1996). Performance of cashew hybrids: The review of performance of the 34 cashew varieties showed that in the Indian States, where the selections

228

K. P. Prabhakaran Nair

and hybrids were released for commercial cultivation, the hybrids performed better than the selections. On account of the fact that cashew plant is amenable for vegetative propagation, it is possible to exploit hybrid vigor in the plant. The technique of softwood grafting has been standardized and is the best method suitable for commercial multiplication of cashew varieties and clones. It was in Kerala State, in Kottarakkara, that hybridization work was first initiated in 1963 and later continued at the Cashew Research Station in Anakkayam in the same State. Currently it is being pursued at Cashew Research Station at Madakkathara in the same State. In the initial breeding programs, three parents with prolific bearing habit (T.No. 12A, 30, and 30A) and three bold nut type parents (T.No. 27, 8A, and Brazil-18) were used in hybridization at the Cashew Research Station in Anakkayam (Damodaran, 1977). The reports on the evaluation of these hybrids indicated marked variation in the progenies derived from the same parental combinations. Where Brazil-18, an exotic bold nut accession, was used in hybridization, the percentage of progenies with high yield (more than 8 kg raw nuts/tree) was higher by 35% compared to those involving the accessions that were collected within the country (9.1% increase). Of the 28 parental combinations evaluated at the Cashew Research Station at Anakkayan (191 hybrid progenies), and at Vellanikkra (114 progenies), two hybrids H-3-17 and H-4-7 were found to be superior than all the other combinations (Damodaran et al., 1978). It must be noted here that for both these hybrids mentioned above, Brazil-18 accession parent was the exotic male parent. Research results on hybridization from other cashew research centers, such as at Vengurla in Maharashtra State and Bapatla in Andhra Pradesh, indicate that when a prolific bearer is crossed with a bold nut type, chances of realizing a hybrid with better nut weight are far greater (Nagabhushanam et al., 1977; Salvi, 1979). Based on these results, varieties with smaller nut size, but high yield, were crossed with bold nut types, namely, Vetore-56 and Brazil-711 at the Cashew Research Stations in Vengurla, Maharashtra State, Bapatla in Andhra Pradesh, and Madakathara in Kerala State. Results indicated that Vetore-56 possessed high transmitability of bold nut type trait to the progenies (Nawale and Salvi, 1990). Seven hybrids, which were released by the Kerala Agricultural University, namely, Dhana, Kanaka, Priyanka, Dhanashree, Amrutha, Akshaya, and Anagha have at least one parent with bold nut trait (Table 18). Dhana is a cross between ALGD-1 and K30/1, and Priyanka is a cross between BLA 139-1 and K30/1, which has a good nut weight of over 8 g/nut. Kanaka is a cross between BLA 139 and H3-13, which itself is a cross between two parents one of which is the bold nut type Brazil-18. Among the 15 cashew hybrids released in India, three hybrids, BPP-1, BPP-2, Vegurla-5 have small nuts (4–5 g/nut) with kernel grade between W 400 and W 450, whereas the remaining 12 have kernel grade between W180 and 240 (Abdul Salam and

Table 18 Cashew hybrids released in India and their salient features

Center

Hybrid

Bapatla

BPP-1 BPP-2 BPP-8 Madakkathara Dhana (H1608) Kanaka (H1598) Priyanka (H1591) Dhanashree (H3-17) Amrutha (H 1597) Akshaya (H 7-6) Anagha (H 8-1) Vengurla Vengurla-3 Vengurla-4 Vengurla-5 Vengurla-6 Vengurla-7

Source: Abdul Salam and Bhaskara Rao (2001).

Parentage

T.No.1  T.No.273 T.No.1  T.No.273 T.No.1  T.No. 39 ALGD-1  K30-1 BLA 139-1  H3-13 BLA 139-1  K-30-1 T30  Brazil BLA 139-1  H3-13 H4-7  K30-1 T 20 x K30-1 Ansur-1  Vetore-56 Midnapore Red  Vetore-56 Ansur Early  Mysore Kotekar-1/61 Vetore-56  Ansur-1 Vengurla-3  M 10/4

Nut weight (g)

Kernel weight (g)

10.0 11.0 14.5 17.5 19.0 16.9 15.0 18.3 11.7 13.7 14.4 17.2

5.0 4.0 8.2 9.5 6.8 10.8 7.8 7.1 11.0 10.0 9.1 7.7

16.6 13.8 18.5

Yield potential (kg/tree)

Shelling (%)

Kernel grade

1.3 1.0 2.3 2.2 2.1 2.8 2.4 2.2 3.1 2.9 2.4 2.4

27.5 25.7 29.0 28.0 31.0 26.5 30.5 31.5 28.3 29.0 27.0 31.0

W-400 W-450 W-210 W-210 W-210 W-180 W-240 W-210 W-180 W-180 W-210 W-210

4.5

1.3

30.0

W-400

8.0 10.0

2.2 2.9

28.0 30.5

W-210 W-180

230

K. P. Prabhakaran Nair

Bhaskara Rao, 2001; Table 18). These 12 hybrids have had at least one of the parents with a bold nut type and so have derived this advantage in the progeny, and this greatly helps in international trade. In addition, a higher shelling percentage in one parent is also, decidedly, an advantage, which will lead to higher kernel output. Among the hybrids released so far, Kanaka and Prinyanka, for which the common parent is BLA-139-1, have short flowering phase (Bhaskara Rao and Bhat, 1996). Future thrust in cashew improvement must not only focus on higher yield, but other important attributes, such as export-grade kernels, higher shelling percentage, and also high nutritive value of kernels. For commercial cultivation, a desirable hybrid can be multiplied by soft wood grafting. Current research strategy is to have high density hybrid progeny planting at closer spacing for preliminary cultivation and subsequently multiply the identified hybrids with desirable attributes, which takes 6–7 years to evaluate, through soft wood grafting for final testing at different locations. This method of evaluation could be modified to reduce the time lag between production of hybrid combinations and final testing (Bhaskara Rao et al., 1998).

11.4. Biotechnology For rapid multiplication of elite lines, biotechnology route can be accessed. Micropropagation is the means. It is also a tool to produce clonal root stocks. The procedure to use seedling explants has been standardized (D’Silva and D’Souza, 1992; Lievens et al., 1989; Thimmappaiah and Shirly, 1996, 1999). Owing to high contamination, browning, slow growth, and poor rooting micro-shoots, regeneration from mature tree explants has been quite difficult. However, micro-grafting in cashew is being attempted to rejuvenate mature cashew tree explants. Micro-grafting as a means for germplasm exchange has earlier been attempted (Mantell et al., 1997). Ramanayake and Kovoor (1999) reported success in micrografting using a scion of seedling origin on in vitro rootstock. Somatic embryogenesis from maternal tissue, such as the nucellus and leaf, is an alternative for micropropagation and development of synthetic seeds. Somatic embryos can be used as target organs for transformation investigation and also as organs to conserve germplasm. Hegde et al. (1993) reported obtaining somatic embryos from cotyledon leaves. Thimmappaiah (1997) observed embryogenesis from both cotyledons and nucelli. But, the germination of embryoids in all cases was far from satisfactory. Somaclonal variation induced though culture can be exploited to select useful variants, but regeneration from callus (direct organogenesis) is yet to be demonstrated in exploiting as a tool for breeding. In vitro multiplication by culturing callus induced at the base of the microcuttings repeatedly over a period of time has been demonstrated by Bessa and Sardinha (1994). Since immature embryos

Agronomy and Economy of Important Industrial Crops

231

can be regenerated to a complete plant (Das et al., 1996), embryo rescue techniques can used to retrieve and regenerate inviable hybrids. Anther culture can be used to produce haploids and dihaploids, which in turn can be used in genetic studies and to produce homozygous lines (inbreds). Cell or protoplast culture is useful in making somatic hybrids to transfer beneficial characters from alien sources. Thimmappaiah (1997) has reported protoplast isolation in cashew. However, protoplast cultures and regeneration are yet to be reported in cashew. Protoplasts can be used as target organs for transformation provided they are made regenerative to a complete plantlet. Employing micropropagation techniques, it is possible to establish international germplasm exchange, clonal propagation of elite lines and in vitro conservation. Molecular markers, such as DNA markers (Random Amplified Polymorphic DNA, RAPD), random fragment length polymorphism (RFLP), amplified fragment length polymorphism (AFLP), and biochemical markers (isozyme, protein), can be employed to characterize germplasm and somaclonal variants. DNA fingerprinting of varieties using RAPD markers is being done at the National Research Center (NRC) for DNA fingerprinting (NRCDNAF) in New Delhi, and Department of Horticulture, University of Agricultural Sciences (UAS), Bangalore, in collaboration with NRCC, Puttur. RAPD profiles of 20 Tanzanian cashew accessions have been reported by Mneney et al. (1997). Also, RAPD profiles of 19 accessions were done at NRCDNAF, New Delhi. DNA fingerprints of 34 released varieties and one TMB-resistant accession were done by Murali Raghavendra Rao (1999). These techniques can be successfully employed to correlate markers with economically important plant attributes, which will aid in marker-assisted selection. Genetic transformation techniques, such as Agrobacterium-mediaed gene transfers, can be used in cashew for transfer of genes for biotic (TMB/CSRB—cashew stem and root borer resistant genes) and abiotic (drought related) stress.

12. Establishing and Managing a Cashew Orchard 12.1. Soil requirement A prevalent myth around the world about cashew cultivation is that it is a most suitable crop to conserve soil, wasteland development, and afforestation. Inasmuch as India is concerned, the plant was first introduced by the Portuguese sea farers, turning later into colonialists, along the Malabar sea coast in the State of Kerala, from where it spread to other coastal areas of the country. This is also the reason why the crop has been relegated to soils of poor productivity, along hilly slopes and bestowing little attention, which

232

K. P. Prabhakaran Nair

has led to poor productivity. This is not only true of India, but also in many other Asian and African countries. Cashew can thrive well in a variety of soils, such as hard degraded laterites, red sandy loam, and coastal sands. A rating chart for land selection has been suggested by Mahopatra and Bhujan (1974, Table 19). The authors suggest that, instead of considering the soil type only, the class of soil with a grading from Class I, Class V should be adopted while selecting a site to develop a cashew orchard. Class I to III types of soil in the medium acidic to neutral range (pH 6.3–7.3), with a slope of 0 to 15 and water table up to 10 m were recommended to be the best for cashew orchards. However, in many countries, other plantation crops such as rubber compete for such soil types, and as such currently Class 1V and Class V types of soil are put to cashew cultivation. However, while Class 1V types require good soil amelioration, Class V types are best avoided, if high production is targeted.

12.2. Water requirement Cashew, principally, is a rainfed crop and irrigation is uncommon. And in most of the cases where cashew plantations exist, surface water sources are nonexistent. However, new plantations are being raised where supplementary irrigation during summer months is possible by tapping underground water sources. Experimental results in India indicate that supplemental irrigation at the rate of 200 l per tree at fortnightly intervals in summer months, from November to March, can enhance fruit retention leading to doubling the yield. This irrigation schedule would require 10 irrigations during these months and this led to 44% fruit retention, while those plots where no irrigation was provided retained only 30% fruit set. Table 20 shows the results (Yadukumar and Mandal, 1994). Such a practice can be followed in homestead gardens, which are quite common in the State of Kerala. The positive effect of supplemental irrigation has been reported by Nawale et al. (1985). In China, supplementary irrigation is only provided during the early stages of the orchard establishment. The monocrop orchards or the adult orchards rarely receive supplementary irrigation, whereas the practice is seen in some gardens where intercrops are also cultivated. Other Asian countries rarely practice supplementary irrigation. Trials have been conducted on the efficacy of drip irrigation coupled with graded doses of nitrogen (250–750 g/tree), phosphorus and potassium (62.5–187.5 g P2O5, K2O pertree) at the National Research Center for Cashew. It was observed that irrigation alone at the rate of 60–80 l/tree without fertilizers increased yield by 60–70% when compared to trees receiving no fertilizers or irrigation. When the same level of irrigation was provided once in 4 days during dry summer months along with the highest dose of fertilizers nitrogen (750 g/tree), phosphorus and potassium (187.5 g each/tree as

Table 19

Guidelines for selection of land to establish cashew orchard

Soil characteristics Soil depth Texture

Soil reaction

Land features (a) Slope (%) (b)Water table (m) (c) Erosion

(d) Drainage

Very Good

Good

Fair

Poor

Unsuitable

Class I

Class II

Class III

Class IV

Class V

>1.5 m Loam

90 cm–1 m Loamy sand

45–90 cm Clay loam

<23 cm Gravelly clay

Sandy loam

Silty loam

Silty clay loam

Coastal sand

Sandy clay loam

23–45 cm Gravelly clay loam Gravelly silty loam Gravelly sandy loam

Very slightly acidic to neutral (pH 6.3–7.3)

Slightly acidic (pH 6–6.3)

Loamy skeletal Medium acidic (pH 5.6–5.9)

<3 2–5 None to slight (e0)

3–5 1.5–2 (coastal belt) Slight (e1) (sheet erosion)

5–15 8–10 Moderate (e2) (rill and sheet erosion)

15–25 10–13 Severe(e3) (gully erosion)

Well drained

Well drained to somewhat excessively drained

Moderately well drained

Excessively and imperfectly drained

Strongly acidic (pH 5.1–5.5) or mildly alkaline (pH 7.4–7.8)

Sandy clay Silty clay Clay Very strongly acidic (pH <5) or alkaline (pH >7.8) >25 >13 Very severe(e4) (gully and ravine erosion) Poorly drained

(continued)

Table 19

(continued)

(e) Physiography

Very Good

Good

Fair

Poor

Unsuitable

Class I

Class II

Class III

Class IV

Class V

Coastal plains

Alluvial plain

Plateaus

Swamps

Delta reaches Shield plains

Natural levees Upland plains

Hills Domes, mounds

Denuded hill slopes with shallow soils Ridges Steeply undulating terrain with severe erosion

Inland lateritic region adjoining coastal plain Climate and Environmental Factors (a) Altitude (m) <20 (b) Rainfall (cm/yr) 150–250 (c) Proximity to sea <80 (km) (d) Temperature (F) 90–100 Max. in summer (e) Min. in winter 60 (f) Humidity (%) 70–80 (g) Occurrence of None (once in 20 frost years) Source: Mahopatra and Bhujan (1974).

Coastal ridges

Valley Bottoms Escarpments

Steeply sloping mountains Creek plain

20–120 130–150 80–160

120–450 110–130 160–240

450–750 90–110 240–320

750 <250 <320

100–103

103–106

106–110

<110

57–60 65–70 None (once in 15 years)

53–56 60–65 Very rare (once in 10 years)

48–52 50–60 Occasional (once in 5yrs)

<48 <50 or>80 Very often to frequent (every year)

235

Agronomy and Economy of Important Industrial Crops

Table 20

Retention of fruits as affected by irrigation treatments

Treatments

Fruit set (Average of five panicles)

Number of fruits harvested

Irrigation once in 15 days at 200 per tree From November to 27 9 January January–March 16 6 November–March 25 11 Control 31 LSD (5%)

Fruit retention (%)

Yield (kg/tree)

33

4.53

37.5 44.0 4.0

4.93 7.32 3.54 1.47

Source: Yadukumar and Mandal (1994).

P2O5 and K2O), nut yield increased up to 117% over the plots which received neither fertilizers nor irrigation (NRCC, 1998; Table 21).

12.3. Manuring a cashew orchard Fertilizer application for cashew varies from one country to another, even so within the same country. This has been the experience in India as well. A cashew tree bearing 24 kg nuts and 155 kg apples removes annually 2.85 kg N, 0.35 kg P2O5, and 1.26 kg K2O annually, through uptake by root, stem, nut, and apple (Mohapatra et al., 1973). Beena et al. (1995) have estimated that every kilogram of nut harvested along with apples requires 64.1 g N, 2.05 g P, 25.7 g K, 4.19 g Ca, and 1.57 g S. Almost all reports on fertilizer use by cashew from different countries indicate a marked response to the application of nitrogen. But, for balanced application, addition of both phosphorus and potassium is required. In China, cashew trees are fertilized twice a year, during July–September. In the second year, when the plants are about 40–50 cm tall, 0.25 kg urea is applied, which is subsequently increased to 0.5 and 1 kg, respectively, in the third and fourth year. In addition, 0.5 kg calcium phosphate, 0.3 kg muriate of potash, or 20–30 kg organic manure is also added. The current recommendation of fertilizer application in India is 500 g N, 125 g each of P2O5 and K2O per tree annually. In the case of high yielding varieties, response to N was noticed up to an application rate of 750 g/tree. In India, fertilizer mixtures are commonly used in plantation crops. Since these mixtures do not conform to the ratios mentioned above, straight fertilizers to supply these quantities of nutrients are used. Fertilizers are only applied when the monsoon ceases into a shallow trench at the drip line of the tree. It is also recommended that fertilizers are applied in split doses during the pre-monsoon phase (May to

Table 21 Effect of drip irrigation and NPK doses on cumulative nut yield (kg/tree) eight years post planting Fertilizers (g/tree) Treatments

M1

Irrigation (l/tree) 0 7.6 20 10.4 40 10.8 60 12.9 80 12.3 Mean 10.8 M1 = 0:0:0

M2

M3

M4

Mean

10.2 12.4 12.4 12.9 14.0 12.3 M2 = 250:62.5:62.5

9.5 14.4 13.3 15.0 16.4 13.7 M3 =500:125:125

9.9 14.5 14.5 16.5 16.6 14.4 M4 = 750:187.5:187:5

9.3 12.9 12.8 14.3 14.8

Source: NRCC (1998). Note: The numbers in M2, M3, and M4 refer to N, P2O5, and K2O, respectively.

237

Agronomy and Economy of Important Industrial Crops

June) and post-monsoon phase (September to October). However, if a single application is preferred, the post-monsoon period is preferred when there is still enough moisture in the soil. But, the application has to, more or less, commence with the cessation of rains and should not be delayed much thereafter when soil moisture will deplete. In the first year one-third of the recommended dose is applied, which is subsequently increased to twothirds and the full dose is applied in the second and third years, respectively. Rainfall in the east coast Indian region is scanty while in the west coast region heavy rainfall is received. In high rainfall regions fertilizer application in circular trenches of about 25 cm width and 15 cm depth at a distance of 1.5 m away from the trunk is recommended. In low rainfall regions, fertilizer is applied at the soil surface and raked into the soil. In Indonesia, the practice is to apply graded doses from the first year through the third (Abdullah, 1994; Table 22). In Myanmar, cashew growing farmers seldom apply chemical fertilizers to their crop. This is primarily because of the paucity of chemical fertilizers in the country, and hence, application of green manures and organic manures is resorted to. Growing subabul, which is the locally available green manure crop, in the interspaces between trees and cutting them and incorporating the biomass into the soil, is the routine practice (Bhaskara Rao, 1994). In Sri Lanka, again, chemical fertilizers are very rarely applied to the cashew crop and only about 3.8% of the crop grown in the country receives chemical fertilizers. Locally available fertilizer mixtures in the ratio of 3:2:1 (N, P, and K, respectively) at the rate of 2.5 kg/ha is applied. Table 23 summarizes fertilizer application schedules to establish a cashew garden is India.

Table 22 General recommendations of fertilizer application in cashew plantations (g per tree) Age of plantation (years)

Nitrogen

Phosphorus

Potassium

1 2 3 4 5 6 7

100 200 400 500 700 900 1000

80 80 120 130 250 250 500

–

Source: Abdullah (1994).

60 120 130 420 420 300

238

K. P. Prabhakaran Nair

Table 23 General recommended fertilizer quantities on an all-India basis to establish cashew orchard (g/per tree)

Years after planting

Urea

Rock phosphate

Muriate of Potash

I Year II Year III Year IV Year V Year and onward

300 660 990 1230 1650

125 250 375 500 625

40 80 120 160 200

13. The Relevance of ‘‘The Nutrient Buffer Power Concept’’ in Cashew Nutrition The fertilizer management of cashew is still carried out on the basis of ‘‘text book knowledge.’’ The soil analysis for nutrient bioavailability continues to be based on routine soil tests. It is in this context that the role of ‘‘The Nutrient Buffer Power Concept’’ has to be examined in cashew nutrition. The fundamental concept has been explained by Nair (1984) and experimental evidence validating the concept provided by Nair and Mengel (1984) and Nair (1996) has presented an extensive review on the concept in Advances in Agronomy, where experimental evidence supporting the concept on the nutrition of black pepper(P. nigrum L.) and cardamom (Elettaria cardamomum Maton.) has also been included. Cashew is a perennial crop like black pepper and cardamom, and there is a very strong case to examine the validity of this globally accepted concept in cashew nutrition.

14. Some Salient Aspects of Raising Soft Wood Grafting The following is the standardized technique of softwood grafting: Step 1: About 40–45-day-old seedling raised in polyethylene bags (25 cm  15 cm, 300 gauge thickness) are utilized as root stocks. Step 2: For a selected variety, lateral shoots of current season’s growth (nonflowered, 35 months old, pencil thick with prominent terminal bud) are selected and pre-cured on the mother tree by clipping of leaf blades, leaving behind petiole stubs. After 10–15 days these pre-cured shoots are collected and utilized as scions for grafting.

Agronomy and Economy of Important Industrial Crops

239

Step 3: The root stock is prepared by removing all the leaves except the two pairs of bottom leaves. Te terminal growth (soft wood portion) at a height of 15 cm from ground level is decapitated, and a cleft 5–6 m deep is made on the stem. Step 4: The pre-cured scion stick is mended into a wedge shape 5–6 cm long, by chopping off the bark and a little portion of wood from the two opposite sides taking care to retain some bark on the remaining two sides. Step 5: The wedge of the scion is inserted carefully into the cleft of the root stock taking care that the cambium layers of both the root stock and scion come in perfect contact with each other. Step 6: Then the graft joint is secured firmly with a polyethylene strip (15 cm wide, 30 cm long of 100 gauge thickness). Step 7: A long and narrow white polyethylene cap (20 cm  4 cm size of 20 gauge thickness) is inserted on the grafted plant. This protects the apical bud from drying up and enhances sprouting. Step 8: The grafting should be done under shade in the nursery shed, and the grafts are to be kept in the shade for about 10–15 days and later on shifted to the open area in the nursery. Alternatively, if silpaulin sheet roofing is made, instead of shifting the grafts, it is convenient to remove the temporary roof to allow all of the sunlight to fall on the grafts. Step 9: Before shifting the grafts to the nursery, or dismantling the silpaulin roof, the polyethylene caps should be removed. These grafts are to be maintained in the nursery until the following planting season. The grafts will be ready for planting 5–6 months following grafting. In a few comparisons that have been made, grafted trees have grown better and fruited earlier than the seedlings of similar age. However, once the soft wood grafts are planted in the field, it is also necessary to provide adequate care to establish a proper orchard to derive benefits of planting vegetatively propagated materials of high yielding varieties.

15. Planting Technology Planting of soft wood grafts is usually done during the monsoon season ( July–August), both in the West and East coast of India. Therefore, land preparation, such as clearing of bushes and other wild growth, and digging pits to plant, should be done during the pre-monsoon season (May–June). A spacing of 7.5 m  7.5 m or 8 m  8 m is recommended for cashew and this will ensure about 156–175 plants per ha. A closer spacing of 4 m  4 m in the beginning and thinning out in stages and thereby maintaining a spacing of 8 m  8 m by the 10th year after planting can be followed. This leads to higher returns during initial years, and, as the trees grow in volume, final thinning is done. However, where the land is flat, it would be

240

K. P. Prabhakaran Nair

advantageous to adopt a spacing of 10 m  5 m accommodating about 200 plants per ha, which would leave enough space for intercrops to be planted, and will yield additional income to the farmer. Normally cashew grafts are planted in pits of 60 cm  60 cm size. The size of the pits can be 1 m  1 m if hard lateritic substratum is encountered in the subsoil. Pits should be dug at least 15–20 days prior to planting, and this would expose the lower soil to sun, which, in essence, is a biological sterilization process. Pits should be completely filled with a mixture of top soil, 5 kg of compost or 2 kg of poultry manure and 200 g of rock phosphate. This will provide a god organic medium to obtain better plant growth. Grafts are best planted in July–August. Normally 5–12 months old grafts are supplied by research stations and also y private nurseries, in polyethylene bags. Since the cashew plant is grown normally along slopes, arresting soil erosion and run off water during monsoon can be achieved by planting on terraces along the contour and opening pits to catch running water at the lower end. Therefore, before the onset of the Southwest monsoon (May–June), terraces of 2 m radius should be made before the pits are dug. This helps in soil and moisture conservation, resulting in good plant growth in the first year of planting itself. Terraces are made first by removing soil from the top of the slope, spreading the soil to the lower side, and then a flat basin of 2 m radius is made. Terraces may be crescent shaped with the terrace slope facing the elevated side of the land so that top soil which is washed off from above due to lashing of the rain is deposited in the basin of the plant. A catch pit across the slope, 200 cm long, 30 cm wide, and 45 cm deep, at the peripheral end of the terrace is made to withhold water during pre- and post-monsoon showers in sloppy areas. A small channel to connect the catch pit sideways is made to drain out excess water during rains (Bhaskara Rao and Swamy, 2000).

15.1. High density planting Conventionally, cashew is planted in a square or triangular system, spaced 7.5–8 m. Field trials with plant density ranging from 156 to 2500 plants/ha have been conducted by the CPCRI, Kasaragod, in Kerala State, also under the administrative control of ICAR and NRCC, Puttur, in Karnataka State. Plant density of 625 plants/ha, spaced 4 m  4 m during the first 11 years and later thinning to obtain 312 plants/ha spaced 8 m  5.7 m  5.7 m gave maximum cumulative yield of nuts. Higher production resulting from high density planting has also been reported from the State of West Bengal from the research center at Jhargram. Two benefits of high density planting are effective soil conservation and checking weed growth, especially in forest lands. Judicial training and pruning in the initial stages after planting is a must (Bhaskara Rao and Swamy, 2000).

Agronomy and Economy of Important Industrial Crops

241

15.2. Cover cropping For cover crops leguminous plants are used. The primary function of cover crops is to protect the surface soil from erosion, check the adverse effects of water run off and smother weed growth. But when leguminous plants are used they have the additional benefit of enriching soil fertility by adding to the soil fixed nitrogen, in addition to building up the carbon base through biomass incorporation. Crops, such as Pueraria javanica, Calapagonium muconoides, and Centrosema pubescens enrich the soil with organic matter, add plant nutrients, check soil erosion, and also help conserve soil moisture. A seed rate of 7 kg/ha is used and sowing must be done at the commencement of the monsoon. Seeds must be soaked for 6 h prior to sowing which are then sown in 30 m  30 cm beds in the interspaces of the cashew plants. Before harvest of the nuts, the cashew basins must be fully cleared of the cover crops to ensure easy harvest to gather all the fallen fruits with nuts intact. In totally degraded laterites, it is very difficult to establish cover crops because of very poor soil structure and absence of soil moisture. In China, natural grass and leguminous crops are usually maintained at the time of land clearance to conserve soil. During initial years after planting, green manure crops are also grown. Creeping cover crops, such as Pueraria phaseoloides and C. pubescens, bush cover crops, such as G. maculata and Leucaena leucocephala, and nitrogen fixing trees, such as Acacia mangium, are the principal cover crops grown in cashew plantations in Sri Lanka.

15.3. Intercropping Until systematic cashew planting started, intercropping received very little attention in cashew production. The practice of intercropping picked up after the realization that as a sole crop, cashew was to be not very remunerative, especially in small homestead gardens and much of interspaces between plants remained vacant, which could be put to profitable use. Intercrop must be established early in the plantation as delay would lead to a smothering effect due to the spreading cashew canopy. Further, heavy leaf fall is not conducive to the growth of any normally grown field crop. Field investigations with fruit crops, such as pineapple, sapota, forest species, such as casuarina and acacia, and green manure and cover crops, such as subabul and mucuna, have shown good promise. Pineapple when grown in trenches across hilly slopes helps check soil and water erosion. Casuarina and acacia were found unsuitable due to adverse effects on soil structure and soil moisture as the former is a heavy depletory of soil moisture and its root spread almost menacing. In pineapple inter planted plantation, cumulative cashew yield was 61.41 kg compared to 37.74 kg in plot where cashew was grown as a monocrop (Table 24). Cashew + casuarina or acacia gave the least cashew yield.

Table 24

Yield of cashew 2 years after the removal of tree species and cumulative yield (kg/plot of 384 square meter area)

Cropping system

Cashew monocrop Cashew + Pineapple Cashew + Casuarina Cashew + Acacia Cashew + Subabul Cashew + Mucuna Cashew + Guava LSD (95%) for treatments

Five years after planting before removal of intercrops (tree crops)

Six years after planting and 1 year after removal of tree crops

5.60 8.80 4.06 2.03 4.12 4.46 5.30

7.78 14.37 6.75 2.15 5.44 8.43 5.94 3.27

Source: NRCC (1995). Note: LSD, Least significant difference.

Seven years after planting and 2 years after removal of tree crops

14.42 28.34 12.12 10.32 13.15 15.32 13.55 SE of mean 2.29

Cumulative yield for the past 6 years

34.74 61.41 26.16 16.23 26.65 35.40 31.07 7.07

Agronomy and Economy of Important Industrial Crops

243

From practical experience, it seems cashew + pineapple is the best combination not only to generate more income, but also because of the ability of the latter to arrest soil erosion. Pineapple should be preferred to tree spices. An alternative to achieve high production is to plant cashew with high density population at spacing of 5 m  5 m in the square system and 4 m  4 m in the hedge grow system and adopting judicious pruning to realize higher yields in the initial years (NRCC, 1998). Medicinal and aromatic plants can also be planted in the interspace. In Indonesia, sweet potato and peanut are popular intercrops. Recently water melon and sweet melon and vegetables (chilli or hot pepper) have also been tried as intercrops. Vegetables can only be grown as intercrops when facilities for supplemental irrigation are available. When melons are cultivated, a lot of biomass after the harvest of the fruits is available for incorporation into the soil which will help build up the organic carbon content through organic matter addition to the soil. In Myanmar, several intercrops, predominantly annuals, such as sweet potato, sesame, peanut, maize, cassava, and pigeon pea and so on., are grown. In Sri Lanka, banana is a popular intercrop. Pineapple, papaya, pomegranate, and coconut are also grown as intercrops. In Sri Lanka, the common annuals grown in cashew plantations are legumes (cowpea, black and green gram), oil crops (sesame, ground nut), and condiments, such as hot pepper and onion. Field investigations conducted at Si Sa Ket Horticultural Research Center in Thailand have indicated that sweet corn, ground nut, and vegetables can be grown profitably as intercrops in the initial years of planting. The principal constraint in growing intercrops is the availability of water for supplemental irrigation, as without it is impossible to sustain intercrops. But, because of the fact that in most of the situations cashew is grown in land where water is very scarce, growing intercrops becomes a problematic issue. Nevertheless, intercropping is essential in small homestead gardens where extra income from intercrops can be profitably utilized to follow improved farming technologies.

16. Controlling the Pests and Diseases in Cashew Plantations Between diseases and insect pests, the latter form the most important constraint to cashew production (Greathead, 1995). In Asia, the pests attack cashew inflorescence and foliage. Many farmers are unaware of the initial symptoms of pest attack and so fail to take remedial measures initially. Subsequently, even if they adopt curative measures, damage that has already been caused cannot be undone. More than 194 species of insects and mites have been listed as pests occurring in different cashew-growing countries in the world (Nair et al., 1979). In China, more than 40 pests have been

244

K. P. Prabhakaran Nair

identified which attack the cashew stems, branches, leaves, tender shoots, flowers, and fruits in the Hainan plantations (Liu Kangde et al., 1998). In India, more than 84 species have been reported to attack the cashew plant (Pillai, 1979), of which 79 are insects and five mites. Another 26 species of pests (17 insects and nine vertebrate species) were added to the list of pests that damage cashew. Sundararaju (1993) compiled a list of 70 species, in addition to the previous reports, which cause damage. In total, 151 insects, eight mites, and 21 vertebrate species damage cashew. In Indonesia, the main attack leading to low yields is caused by Helopeltis sp. and Cricula. In Myanmar, stem borer is the major pest, caused mainly due to poor phytosanitation measures. Sporadic attack by shot-tip caterpillar and leaf webber are also noticed (Maung Maung Lay, 1998). Hence, the current level of infestation is not cause to excessive economic loss in cashew. Control of stem and root borers is essential to save high yielding trees in plantations. In Philippines, the most important pests are termites, leaf miners, shoot, and root borers, and TMB, while in Sri Lanka, major pest attack is by stem and root borers and TMB. Sporadically, leaf miners, and leaf and blossom webbers also cause damage. In Thailand, the major problem is the TMB which causes heavy damage. Thrips (Haplothrips species) also cause heavy loss by attacking inflorescence and shoots causing dieback. Although a large number of pests are reported to attack cashew (Nair et al., 1979; Rai, 1984; Sundararaju, 1993), the most important ones which limit the production are the cashew stem and root borer (CSRB) and TMB in many countries. Leaf miners (Acrocercops syngramma), and leaf and blossom webbers (Lamida moncusalis) are also the major pests in certain area. In addition to these, there are some pests of minor importance, in general, but, in certain endemic areas, they become very serious. Such pests are defoliating caterpillars, leaf beetles, shot-tip caterpillars, foliage thrips, flower thrips, and apple and nut borers.

16.1. Pest control In most of the Asian countries recommendations are available to control insect pests. In China, recommendations include pesticidal spray of 20% Fenvalerate (an insecticidal spray) with a dilution of 1 ml in 200 ml water and a mixture of 40% Dimethoate and 80% Diarotophos (1:2 ratio) with a dilution of 1 ml in 200 ml water, applied as a low volume spray. To control fruit borers, the recommendation is spraying 20% Fenvalerate or 2.5% Deltamethrin (1 ml in 200 ml water dilution). In India, three sprays are recommended to control foliage and inflorescence pests with monocrotophos and carbaryl during flushing, flowering, and fruit setting. A 0.05% spray of endosulfan or monocrotophos for the first and second rounds, and 0.15% carbaryl for the third round is recommended. In Myanmar, plant protection is hardly practiced. In view of the level of infestation currently encountered, a general recommendation of spray is not warranted.

Agronomy and Economy of Important Industrial Crops

245

In Thailand, thrips (Haplothrips sp.) are controlled by spraying 30 ml carbosulfan in 20 l of water or 50 g carbaryl in 20 liters water. TMB is controlled by spraying 20 g carbaryl in 20 l water or cyhalothrin (10 ml in 20 l water). A major constraint in pest control is the general lack of awareness of farmers (Pimentel, 1986).

16.2. Control of foliage and inflorescence pests TMB (Helopeltis sp): By far, the TMB is the most serious cashew pest all over. The adult and immature stages of this mirid bug suck sap from tender shoots, leaves, floral branches, developing nuts and apples. The injury made by the sucking parts of the mouth of the insect causes tender shoots to exude resinous gummy substances. Tissues around this point of entry of the stylets become necrotic and form brown or black scabs, presumably due to the action of the phytotoxin present in the saliva of the insect injected into the plant tissue at the time of feeding. Finally, the adjoining lesions coalesce and the affected portion of shoot/panicle dries up. Severe infestation on the floral branches may also attract fungal infestation, which will result in the inflorescence blight. Immature nuts infested by this pest develop characteristic eruptive spots, and finally shrivel and drop off. Prophylactic sprays, detailed above, at flushing, flowering, and fruiting can minimize losses, though complete eradication is still very elusive. Leaf miner (A. syngramma M.): The prophylactic spray schedule for TMB can also be effective in controlling leaf miner attack. However, if a serious outbreak is noticed, 0.05% spray of phosphomidon, fenitrothion, or monocrotophos has been found effective. Leaf and blossom webber (Lamida moncusalis Walker and Orthaga exvinacea Hamps.): Application of carbaryl (0.15%) has been found effective to control this pest. Shoot-tip caterpillar (Hypatima haligramma M.): The tiny yellowish or greenish brown caterpillars of the moth damage shoot tips and inflorescence. Systemic insecticides, such as monocrotophos (0.05%), have been found to be effective to control the pest. Foliage thrips (Selenothrips rubrocinctus Giard, Rhipiphorothrips cruentatus Hood, and Retithrips syriacus M.) and flower thrips (Rhynchothrips raoensis G., Scirtothrips dorsalis H., Haplothrips ganglabaueri [Schmutz],Thrips hawaiiensis [Morgn], H. ceylonicus Schmutz, and Frankliniella schultzei [Tryborn]. Cashew plantations, especially those raised on grafts that flush continuously, are prone to damage by foliage thrips. There are three species, namely, S. rubrocinctus Giard, R. cruentatus Hood, and R. syriacus M. Flower thrips cause premature flower shedding and scabs on floral branches, apples and nuts. Infestation on developing nuts results in formation of corky layers on the affected part. Malformation of nut and even immature fruit and nut drop is noticed. Endosulfan, monocrotophos, or quinalphos 0.05% spray controls both the foliage and flower thrips.

246

K. P. Prabhakaran Nair

Apple and nut borer (Thylocoptila panrosema M. and Nephopterix sp.): Apple and nut borers cause heavy economic loss (Dharmaraju et al., 1974). The caterpillars attack the fruit at all stages and the infested fruits shrivel and fall off with the nut. Spying 0.1% carbaryl or 0.05% endosulfan effectively controls the pest attack. Stem and root borer: Timely control of the pest is a must to preempt later damage which will lead to the complete succumbing of the plant. The predisposing factor to the attack is the lack of proper phytosanitation in the cashew orchards. Up to 35% loss can be expected especially in plantations raised by forest departments. The primary species infesting the plant is Plocaederus ferrugineus L. Two other species, Plocaederus obesus Gahan and Batocera rufomaculata De G., also infest cashew. Small holes in the collar region of the plant gummosis extrusion of frass through holes, leaf yellowing, twig drying, and finally the entire plant succumbing are the symptoms of the pest attack (Pillai, 1975; Pillai et al., 1976). The adult is a reddish brown medium-sized longicorn beetle, the head and thorax of which are dark brown or almost black. The eggs of the beetle are laid on the crevices of the tree bark as well on the exposed parts of roots When eggs hatch, grubs make irregular tunnels into fresh tissue and bark and feed on the subepidermal tissues and sap wood. This leads to injury of cells, and a resinous material oozes out when the vascular tissues are damaged. The ascent of plant sap is arrested, leaves turn yellow and subsequently are shed. Several pest management techniques incorporating mechanical, chemical, cultural, and biological methods were tried against this pest. Removal of eggs, grub, and pupae from infested trees and swabbing the trunk after removal of the grubs from infested tree with 0.2% carbaryl or lindane, or panting with a mixture of coal tar and kerosene in the ratio of 1:2 will revive the tree. Whenever an infested tree is noticed in the plantation, it should be treated early in the infestation stage, and all the adjoining trees must also be swabbed with coal tar and kerosene as a prophylactic measure.

16.3. Biological pest control Both from Asia and Africa, a number of natural predators have been found against TMB, recorded by Simmonds (1970). Sundararaju (1993) has reported Telenomus sp., and Chaetostricha minor as natural predators of Helopeltis in India. Devasahayam and Radhakrishnan Nair (1986) reported that Erythmelus helopeltidis paratizes on the eggs of Helopeltis antonii. However, efforts to multiply egg parasitoids met only with little success as these are specialized parasitoids. Crematogaster wroughtonii Forel (Formicidae) was reported as a predator of the nymphs of the pest (Ambika and Abraham, 1979). Spiders, Hyllus sp. (Salticidae), Oxyopes schirato, P. hidippes Patch, and Matidia sp. have been found to predate H. antonii (Devasahayam and

Agronomy and Economy of Important Industrial Crops

247

Radhakrishnan Nair, 1986; Sundararaju, 1984). Three species of reduviid bugs (Sycanus collaris [Fab.], S.phadanolastas signatus Dist., and Endochus inornatus Stil.) have also been noticed as predators of TMB (Sundararaju, 1984). Recently, Rickson and Rickson (1998) observed that a number of ants regularly visit the cashew tree and indicated ants as a possible defense against TMB. These authors have surveyed the plantations in Sri Lanka, India, and Malaysia and indicated the possibility that Oceophyllas maragdina is a promising ant species in this connection. These authors have recorded Crematogaster sp., Monomorium latinode, and Tapinoma indicum, as possible predators entering the open flowers and prey upon the flower thrips or mites. These ants did not appear to interfere with other pollinators. About 80% of a cashew tree’s current vegetative growth is destroyed by TMB (Rickson and Rickson, 1998). The outbreak is normally patchy and it is best a tree-by-tree spraying program schedule is undertaken. This schedule demands a keen surveillance to spot localized attack to be followed by pesticide spray. To control stem and root borers, Bacillus thuringiensis, Bacillus popillae, and fungi, such as Metarhizium anisopliae and Beauveria bassiana, have been recommended (Pillai et al., 1976). Presence of nymphal and adult endoparasitoid and mermithid parasite nematodes was detected in adult populations of TMB for the first time in cashew (NRCC, 1998).

16.4. Cashew diseases and their control When one compares the intensity and the consequential economic loss by pest attack on cashew, the ravages of diseases are much less in intensity compared to that caused by the attack of insect pests. More than four dozen fungus attack cashew, but, the intensity of their attack and its consequence is almost negligible. The disease problem is mainly in the nursery. In China, root rot, stem rot, and dieback are observed mainly in the nursery, while gummosis, defoliation, and root rot have been observed in the adult orchard. In India, dieback or the ‘‘pink disease’’ caused by Corticium salmonicolor, damping off of seedlings (Kumararaj and Bhide, 1962) and anthracnose disease (Singh et al., 1967) were found to be of importance. The other diseases are shoot rot and leaf fall (Thankamma, 1974), cashew nut decline (Ramakrishnan, 1955), and yellow leaf spot (Subbaiah et al., 1986). Oidium sp. cause powdery mildew on the west coast of India in Maharashtra State (Phadnis and Elijah, 1968). Gummosis is also a problem in certain endemic areas, but none of the diseases reported in Asia are of major economic concern. Some of the important diseases and their control are describe below. Anthracnose: C. gloeosporioides is the causal fungus in not only cashew, but also many other fruit trees, such as mango, papaya, avocado, citrus, and so on. The pathogen continues to grow on the dead parts of the host tissues and perpetuates itself even in unfavorable conditions. Bordeaux mixture or

248

K. P. Prabhakaran Nair

copper oxychlorate spray effectively controls the disease. Fungal infection is preceded by the TMB attack (Nambiar, 1974). It is desirable to remove the infected plant parts and burn them rather than resort to chemical control. This is especially so in small orchards, while in large ones chemical spray of Bordeaux mixture or copper oxychlorate is preferable. Inflorescence blight: As the term implies, the disease is characterized by drying up of floral branches, and a gummy exudate can be observed at the site where the infection made a lesion by H. antonii. The causal fungi are Gloeosporium mangiferae and Phomopsis anacardii (Nambiar et al., 1973), which are secondary saprophytic colonizers and are not the pathogens. Dieback: Terminal drying of twigs in cashew is caused by several fungi. These diseases are also called ‘‘Pink Disease’’ caused by Pellicularia salmonicolor or Corticum salmonicolor (Anonymous, 1950). A 1% spray of Bordeaux mixture during post-monsoon period is an effective prophylactic measure against the disease (Nambiar, 1974). Leaf spot: There are several leaf spot diseases in cashew. Pestalotia microspora causes the grey blight, Phyllosticta sp. causes the red leaf spot, and brown leaf spot is caused by C. gloeosporioides. Spraying 1% Bordeaux mixture or 0.3% Benlate is recommended to control the disease. For a long time yellow leaf spot was considered a disease of unknown etiology. However, Subbaiah et al. (1986) associated the disease with low (4.5–5.0) soil pH. Also, the affected leaves were found to contain excess Mg and low amounts of Mo. Spraying molybdenum salt was found to control the disease. Powdery mildew: Oidium sp. cause the powdery mildew disease, which is very rare in Asian countries, but very severe in Africa, and said to infect cashew blossoms in Maharashtra State on cloudy days (Phadnis and Elijah, 1968). Sulfur dusting controls the disease.

16.5. Effect of inclemental weather conditions Untimely rains result in late flowering. Many days with bright sunshine hours lead to bud break. Rise in night temperature to about 20  C together with fewer dewy nights, which coincide with the flowering phase, is detrimental to flowering. Excessive cloudiness during flowering impedes opening of hermaphrodite flowers. However, the precise mechanism or any physiological causes for this is yet to be clearly understood. This is particularly so, even when infestation by TMB infection is not an adverse effect during such weather conditions. The only way the impact of adverse weather conditions can be mitigated to a limited extent is through a mix of cultivars with early or differential flowering characteristics and implementing such a strategy in the planting program (Rao et al., 1999).

Agronomy and Economy of Important Industrial Crops

249

17. Cashew End Products 17.1. Cashew kernel Cashew kernel is the most widely used nut in confectionary. There are 33 different grades, of which 26 are commercially available for domestic consumption and export. The Indian Standards provide precise specifications for the various grades. Broadly, the kernels can be classified as, white wholes, scorched wholes, dessert wholes, white pieces, scorched pieces, and dessert pieces. The kernels as of now are mainly used as snacks in the roasted and salted forms. Bakery, confectionary and chocolate industries use the broken kernels. Of late, different recipes with cashew have been developed in different parts of the world, primarily in Southeast and West Asia, where consumption of nut is very popular. In the emerging global food market, quality and not price has come to rule supreme. This is because consumers, especially in the developed world, are highly quality conscious against the backdrop of excessive use of chemicals in modern agriculture. Food safety is the most important criterion. To gain an entry into global markets, cashew has to conform to internationally stipulated standards inasmuch as quality is concerned. In this regard, even the type of packing materials used come under scrutiny. These packing regulations relate to lead-free solder in tin containers, avoiding toxic/ carcinogenic chemicals in preservation and storage use of environmentally friendly and recyclable materials for packaging, storage, and son on (Nayar, 1998).

17.2. Cashew kernel peel Rich in tannins (25%) kernel peels have great industrial use, especially in leather industry (Nair et al., 1979; Nayudamma and Koteswara Rao, 1967) and the peels with adhering pieces of cashew kernel form an excellent poultry feed (Nair et al., 1979). Products from the false fruit: The false fruit, which is also known as the cashew ‘‘apple,’’ is invariably discarded in many countries. Because of its astringent taste, many people do not consume it in Asia, while it is widely consumed in Brazil, the place of origin of the crop. In many small villages in Kerala, poor people cut the fruit into small pieces and consume it by dipping it in salt, along with the locally brewed alcohol. The more enterprising among them, distil the fruits and make alcohol out of it for local sale or domestic consumption. It is a very juicy fruit, rich in vitamin C—a fivefold increases compared to citrus fruit—and contains 10–30% sugar. The apple is sucked and when it is empty of the juice the fibrous mass is left behind.

250

K. P. Prabhakaran Nair

The astringent and/or active acid is primarily due to the tannins in the fruit (0.35%). Steaming the fruit is the most efficient way to remove the astringent and acid principles. Steam pressure varies 2–5 kg and exposure time 5–15 min. This depends on the quality of the fruit and the end product to be made. The astringent principle can also be removed by boiling the fruit in 2% salt solution for 4–5 min. Alternatively, the fruit juice can be treated with gelatin (0.25–0.4%) and pectin (0.35%) or simple lime juice (25%). A number of cashew apple beverages, such as clarified juice, cloudy juice, apple syrup, or juice concentrate, can be made by following the above procedure where the astringent content is removed. Other popular products are cashew vinegar, cashew apple candy, and jam, canned apple, cashew apple chutney, and cashew pickle. As cashew is a seasonal plant and most of these products have only short shelf life, they are yet to become popular on a large scale. Post-harvest technology in the area of fruit preservation in cashew is still in a primitive stage. The most popular of all the above products is the Feni, the alcoholic beverage, produced in the Goa State in India (the original colony of the Portuguese) along the coast line of Maharashtra State, which has turned now as a global touristic hot spot because of its enchanting beach. The drink has become very popular within and outside the State. There is a colonial legacy to the place, because the Portuguese sea farers landed there. They are the ones who also introduced the cashew crop to the island state. Cashew apple reside left behind after extraction of the juice, constituting 30–40% of the fruit, is nutritious. It contains 9% protein, 4% fat, 8% crude fiber, and 10% pectin. This residue can also be made use of in manufacturing various products, such as drink, jam, chutney, or as a preservative ( Joshi et al., 1993). Also it finds its use as cattle feed after drying or can be utilized for the recovery of low methoxy pectin (Nanjundaswamy, 1984). A comparative picture of cashew apple and other tropical fruits inasmuch as their nutritive contents are concerned is given in Table 25. The chemical composition of cashew apple is given in Table 26, while Table 27 summarizes the composition of cashew kernel vis-a`-vis other important nuts.

17.3. Cashew nut shell liquid One of the most important by products of cashew nut processing is the CNSL (Aggarwal, 1973). Motor-driven expellers extract shell oil. Following extraction, the shell liquid is heated and filtered, sealed into metal drums for export. Depending on the shell weight, 33–38% oil is extracted by weight ( Johnson, 1982). CNSL is an excellent raw material for the manufacture of unsaturated phenol. It is a versatile industrial raw material and has innumerable uses in polymer-based industries, such as friction lining, paints, varnishes, laminating resins, rubber compounding resins, cashew cements,

Table 25

Vitamin and mineral contents of various tropical fruits

Content/100 g

Cashew Apple

Pineapple

Avocado

Banana

Lime

Grape fruit

Mandarin

Orange

Thiamine (microgram) Riboflavin (microgram) Vitamin C(mg) Ca (mg) P (mg) Fe (mg)

99 240 41 11 3

90 20 24 16 11 0.3

120 150 16 10 38 0.3

90 60 10 8 28 0.6

10 Traces 45 14 10 0.1

40 20 40 – – –

70 30 31 33 23 0.4

90 30 49 33 23 0.1

252

K. P. Prabhakaran Nair

Table 26

Composition of cashew apples

Moisture (g/100g) Proteins (g/100g) Fat (g/100g) Carbohydrate (g/100g) Fiber (g/100g) Ash (g/100g) Ca (mg/100g) P (mg/100g) Fe (mg/100g) Vitamin B1(Thiamine mg/100g) Vitamin B2 (Riboflavin mg/100g) Niacin (mg/100g) Vitamin C (mg/100g)

86.1 0.8 0.2 12.6 0.6 0.3 0.2 19.0 0.4 0.2 0.2 0.5 200

Table 27 Kernel composition of different tree nuts (%)

Constituents

Almond

Moisture 5.2 Protein 20.8 Fat (ether 59.9 extract) Carbohydrate 10.5 Fiber 1.7 Mineral matter 2.9

Hazelnut

Walnut

Macadamia nut

Cashew nut

– 12.7 60.9

4.5 15.6 64.5

1.5–2.5 9.2 78.2

– 21.0 47.0

17.7 – –

11.0 2.6 1.89

10.0 – –

22.0 1.3 2.4

polyurethane-based polymers, surfactants, epoxtresins, foundary chemicals, and intermediates in chemical industry (Aggarwal, 1954; Anonymous, 1993).

17.4. Cashew shell cake The residual shell cake after extraction of shell liquid is currently used as fuel in the processing factories and in CNSL extraction plants. This oil cake could also serve as a raw material in the manufacture of plastics and container boards ( Johnson, 1982).

Agronomy and Economy of Important Industrial Crops

253

17.5. Value added products Several products are made from raw cashew kernels. Product development leads to diversified uses of the original raw kernel. Value addition can substantially enhance the economy of cashew production as explained below. (i) Cashew Kernel Flour: The low grade kernels, which can neither be exported nor sold in the domestic market, are made into cashew flour which is highly proteinaceous and is easily digested ( Johnson, 1982). It is not used in baking bread or chapathis—the Indian bread—but, if a concerted effort is made, would turn out to be an excellent supplement to the usual wheat flour. (ii) Cashew Kernel Oil (‘‘Caribbean Oil’’): As in the case of cashew kernel flour, lower grade kernels are also used to extract kernel oil, which is a highly nutritious edible oil, and in terms of quality compares quite well with the healthy and nutritious olive oil ( Johnson, 1982). The kernel contains 35–40% oil (Van Eijnatten, 1991). (iii) Cashew Kernel Butter: The residue of kernel after oil extraction is used to produce cashew kernel butter, which is similar to peanut butter (Nair et al., 1979). The oil expelled kernel can also be processed into cashew nut cake, which is an excellent animal feed, especially for milking cows (Van Eijnatten, 1991). The Central Food Technological Research Institute in Mysore, India, under the administrative control of the Council for Scientific and Industrial Research (CSIR), New Delhi, the premier research institute into food processing in the country, has perfected a procedure to extract cashew butter from the cashew kernels for export, and the technique is available for commercial exploitation and has been passed on to the Cashew Export Promotion Council in Kochi, Kerala State, India. (iv) Coated Cashew Kernels: Sugar, honey, and salt-coated cashew ‘‘baby bits’’ are available in the market, and the technology of manufacture has been developed at the NRCC. Baby bits are the lowest grade kernels which are commercially marketed (Bhaskara Rao and Swamy, 2000). (v) Cashew Kernel Milk: Sweetened and flavored cashew milk can be prepared from cashew baby bits, and the procedure has been developed at NRCC (Bhaskara Rao and Swamy, 2000). (vi) Cashew Spread: The NRCC has developed a procedure to prepare cashew spread from baby bits (Bhaskara Rao and Swamy, 2000). There are different varieties, but the most popular is the sweetened and vanilla flavored. The salted spread is also popular. There is great scope to generate value-added products in cashew ( Jain et al., 1954). Following are some of the avenues:

254

K. P. Prabhakaran Nair

1. Commercial exploitation of cashew butter and oil in the cosmetic industry, especially for the production of cold creams 2. The 2–3% of rejected cashew kernels, which are rich in fat, can be used from which oil is extracted and the refined oil can be used to develop various oil-based by products. The cashew kernel is a rich store house of Vitamin E, which is a particularly useful vitamin in retarding ageing process in the elderly and has wide use in pharmaceutical industry. 3. The fibrous cashew apple can be used from which the fiber can be extracted to blend with other food materials to manufacture fiber-rich foods which are particularly useful for persons having constipatory problems. The NRCC has carried out investigations on cashew fiber after extraction of juice from the apples, and characterize its physical and chemical properties. Quite possibly, the cashew fiber could have anti-diabetic properties and these need to be investigated. 4. Cashew kernels are rich in proteins and these proteins are known to contain all the amino acids. At NRCC, attempts have been made to investigate the functional properties of defatted cashew kernel flour and compare the same with other standard flours like wheat and maize flour, and the cashew flour compares quite well. Additionally, it has also been compared with soybean and almond flours. It has also been shown that stable foam could be produced from cashew kernel flour over a wide range of pH. It would be a very good idea to blend cashew flour with cereal and pulse flours to produce a nutritionally rich flour. Fortification of lower grade flours with cashew flour is a very important avenue to enhance value addition 5. The tannin removed kernel testa has been shown to contain considerable quantities of carbohydrates and protein. Efforts are to be made to develop food and animal feed blends from tannin-free testa. 6. An enterprising industrialist in Kerala State, India has succeeded in extracting ethanol, the ‘‘green fuel’’ from cashew apple, which has been tried on a pilot scale to run automobiles. Commercial production holds much promise especially in areas where supply of apples cannot be a constraint.

18. Organization of Cashew Research in India and Overseas It is India that initiated systematic research in cashew in the world, starting 1950, further strengthened by the establishment of the CPCRI in Kasaragod, Kerala State, under the administrative control of the ICAR in New Delhi. In 1986 the independent research organization, the National

255

Agronomy and Economy of Important Industrial Crops

Research Center for Cashew, was established in Puttur, in Karnataka State. In recent years, considerable research has also been carried out in Australia under the administrative control of the Commonwealth Scientific and Industrial Research Organization (CSIRO) in Darwin. There are others, such as Cashew Training Research Center in Binh Duang, Vietnam, National Research Center on Cashew, Fortaleza, Brazil, Tanzanian Agricultural Research Organization controlled Research Institute in Naliendele, Mtwara, Tanzania, and Hainan Cashew High Yield Research Center in Hainan, Peoples Republic of China (Bhaskara Rao, 1996; NOMISMA, 1994). At present in India, there are nine research centers in eight cashew growing states, all of them under the administrative control of ICAR which coordinates the AICRIP on Cashew as shown in Table 28.

Table 28 Details of AICRIP Research Centers on Cashew in India Year established

Institution

Location

State

CRS, Andhra Pradesh Agricultural University ARS, University of Agricultural Sciences CRS, Kerala Agricultural University RARS, Kerala Agricultural University ZARS, Indira Gandhi Krishi Vishwa Vidyalaya RFRS, Konkan Krishi Vidyapeeth CRS, Orissa University for Agriculture and Technology RRS, Tamil Nadu Agricultural University RRS, Bidhan Chandra Krishi Vishwa Vidyalaya

Bapatla Chintamani

Andhra Pradesh Karnataka

1980

Madakkathara

Kerala

1972

Pilicode

Kerala

1993

Jagadalpur

1993

Vengurla

Madhya Pradesh Maharashtra

Bhubaneshwar

Orissa

1975

Vridhachalam

Tamil Nadu

1970

Jhargram

West Bengal

1982

1971

1970

Note: CRS, Cashew Research Station; ARS, Agricultural Research Station; RARS, Regional Agricultural Research Station; ZARS, Zonal Agricultural Research Station; RFRS, Regional Fruit Research Station; RRS, Regional Research Station.

256

K. P. Prabhakaran Nair

19. A Peep into Cashew’s Future As far as cashew research in India is concerned, since the start of systematic research in the crop in 1950, considerable progress has been made in evolving high yielding varieties, calibrating nutrient requirements, standardizing vegetative propagation techniques, chemical control of major insect pests and diseases, utilization of cashew apple, and production of a very large number of saplings for planting purposes and also replanting declining plantations. In view of the global emerging situation, where cashew is most likely to play a major part as an important fruit nut, the research priorities have to be redrawn. Following are the areas that demand focused attention.

19.1. Genetic resources The different germplasms which have been conserved in NCGB at NRCC in Puttur need to be consolidated and the preparation of a district-wise collection map of the cashew germplasms for the country has to be prepared. Collection of germplasms from other cashew research centers, such as ARS Ullal, RRS, Brahmawar, CRS, Anakkayam, and CRS, Kavali, all of which are not affiliated to the AICRIP under the ICAR set up. Collection of germplasms form non-traditional area, such as the hills of Garo in Meghalaya (Northeast India) and Bastar in Chattisgarh State (East India) The accessions in NCGB have to characterized to support the cashew breeding program, focusing on processing quality of raw nuts, cashew apples for better fiber quality, tolerance or resistance to major insect pests and diseases

19.2. Varietal improvement Genetic investigations on dwarfing traits and cluster bearing with a view to have high density plantations, and breeding for bold nut characters through molecular markers and isozyme banding pattern in different cashew genotypes. Investigations on the reciprocal differences in hybrids, polyclonal interactions, pollen viability, incompatibility, fertility, and post-zygotic abortions

19.3. Biotechnological interventions Through DNA finger printing in collaboration with the University of Agricultural Sciences in Bangalore, Karnataka State and the National Research Center for DNA Finger Printing (NRCDNAFP), in New Delhi, the released cashew varieties and accessions should be characterized

Agronomy and Economy of Important Industrial Crops

257

Independent development of DNA fingerprinting facilities at NRCC, Puttur Evaluation of biochemical and physiological basis for variations for observed responses in mature tree plants Comparison of micro-grafts with normal grafts for their performance with regard to various attributes

19.4. Crop management techniques Yield targeting and working out the production requirements with regard to optimum planting time, fertilizer requirements, irrigation if needed, on the field operations like weed, pest control, and so on Soil fertility management taking into consideration land use planning and all other state of the art techniques Evaluation of less vigorous varieties, such as Ullal-1, H-2, BLA39-4, and so on, varieties for high density planting to obtain maximum yield per unit area A critical evaluation of drip irrigation to enhance water use efficiency A complete assessment of micronutrients in the soil, plant, plant parts and defining critical levels in soil, and plant to target high yields Field evaluation of (1) performance of a scion variety on its own root stock and on different root stock varieties and (2) performance of a root stock variety with its own scion and scions from different varieties Basic studies on the use of hormones, growth regulating chemicals, growth retardants and inhibitors, on cashew yield The role of ‘‘The Nutrient Buffer Power Concept’’ in the nutrition of cashew

19.5. Crop protection Standardization of the revival techniques in CSRB-infested trees through induction of bark regrowth and root development Design and development of traps for CSRB-based on kairomones or utilizing infected trees as a bait Approaches for reducing latent or residual CSRB inoculum Developing models to identify the most vulnerable trees for CSRB infestation in the plantation Investigations on the bioecological aspects of the CSRB infestation The role of entomophilic nematodes on CSRB infestation Management of cashew pests’ infestation using least chemicals, including the use of pheromones Biology and bionomics of flower pests (thrips, apple and nut borers, and shoot-tip caterpillars)

258

K. P. Prabhakaran Nair

Monitoring the fauna in the cashew ecosystem. This is especially important when crop is grown in the forest Investigations on the pest complex of nuts during the post-harvest and processing stages Development of a forecasting system of inflorescence pests Investigations on weeds, pesticide residues in soil and water in the cashew plantations and adjoining area

19.6. Post-harvest technology Development of appropriate protocols to extension of shelf life to cashew apple for consuming fresh using proper package and storage methods Development of cost effective end products from cashew apples, specially to augment employment potential for rural men in the neighborhood of cashew plantations

20. Technology Transfer Assessment of the impact of production technology recommended to the cashew farmers Further refinement of the various aspects of cashew production against the back drop of the feedback from farmers and first-hand experience gained from farmers’ fields Recommendations of appropriate varieties depending on the ecological conditions Production of quality planting materials Establishing intimate institutional professional relationship with farmers’ forums for constant update on developments in cashew production technology

21. Biodynamic Cashew Very recently, a group of 31 farmers in Goa, Maharashtra State, have come out with ‘‘Biodynamic Cashew.’’ The production technique is based on the concept of dynamic agriculture propounded by Rudolf Steiner in Germany in 1924 to a small group of farmers in Koberwitz, East Germany. Biodynamic agriculture is a method of farming that aims to treat a farm as a living system that interacts with the environment, to build healthy living soil and to produce food that nourishes and develops humanity. Mr Steiner introduced the practice of making preparations based on cow manure, silica, and various herbal plants to be used to open up the soil to cosmic influence. He advocated discontinuing the use of chemical fertilizer altogether.

Agronomy and Economy of Important Industrial Crops

259

Because of their inherent lack of life (inorganic nature), he felt that chemicals could not maintain life or increase soil fertility. The cashew produced in this fashion has a very high export value

22. The Coconut Palm (Cocos Nucifera L.) 22.1. Origin and evolution Coconut palm is a unique plant and stands apart from all other palms because of its high degree of consistency and continuity in flowering and fruit production, month after month, year after year, for decades. In Sanskrit, the ancient Indian language, it is called ‘‘Kalpavriksha,’’ which provides humans with everything. In India, it is in the State of Kerala, that life of the people is woven around the coconut palm. The endocarp is everyday’s fare in the kitchen-scrapped or ground to extract the milk and used in all of the culinary specialties of the people, the oil is used for both body massage and for the hair, water from the tender coconut is used as a very nourishing drink, leaves are used for thatching the poors’ homes, and when dried is used as firewood. In fact, even in the case of the poorest, there will be a lone coconut palm in front of the home, even if it is a thatched hut. It is at the sight of the coconut palm the poor wake up to a new day. The above mentioned capability of the palm suggests evolution in an environment free from severe seasonal or episodic constraints on growth. Such an environment would most likely have resembled those few isolated niches where coconut presently thrives unaided by human intervention or management. For instance, in Southwest Java, on the island of North keeling in the Indian Ocean, coconut dominates a dense woodland growing on low sand-cays underlain by freshwater. On the east coast of Cape York Peninsula in Australia, where coconut has been introduced since European settlement in the nineteenth century, pockets of ‘‘coconut woodland’’ have also formed. Seeds cannot naturally move landward beyond this zone because of their big size, which makes it impossible for birds and most other mammals to carry them, hence, the only avenue for dissemination is via ocean to other coasts with fringing sand cays. The coconut has been put to great use along the coast, also at great distances inland. The palm thrives well where rainfall is plentiful (>200 cm annually) and well distributed. In summer months the palm needs good irrigation.

22.2. The evolution of coconut along the drifting coastlines The general course of evolution of coconut, the modern prolific palm, which emerged from the prima ancestral palms of Mesozoic Gondwana, remains obscure, and this has been the theme of much speculation and

260

K. P. Prabhakaran Nair

limited molecular exploration by scientists (Lebrun et al., 1999). Remarkably, no fossil records exist, which can be attributed, in part at least, to the instability of ‘‘migration’’ of the coastal fringe environment. Even within the past 120,000 years, there have been no fewer than six cycles of glaciation when sea levels have fallen between a few tens of meters and more than 100 m in one case around 20,000 years ago (Chappell, 1983; Veeh and Veevers, 1970). The corresponding movements of the coastal zone by many kilometers, in some cases, would leave the coconut zone ‘‘high and dry’’ during the phase of falling sea level and progressively inundated during the rising phase. Potential fossil material would be destroyed by oxidation through exposure to aerobic conditions in the first phase and disintegrated by wave action in the second. Quite likely, that the only ‘‘recent’’ fossil to be found for coconut are those where humans have planted the nut adjacent to a swampy environment, where it would not have been established by natural dispersal (Spriggs, 1984). In this case, the fossil remains were found to be slightly more than 5000 years old. The ancestors of coconut possibly began to emerge from the palm branch of the tree of plant evolution around 100 million years ago ( 10 Myr). According to Harries (1990) the ancestral coconut inhabits the ‘‘north’’ coast of Gondwana as that great land mass of the southern hemisphere began to break up around 80 Myr (Leach et al., in Press). Huge crustal plates carrying exposed or partly submerged land surfaces, which now comprise Australia, India, the Arab peninsula, and smaller fragments, such as New Zealand and Madagascar, began to drift northward. The expanse of ocean, between these wandering land masses and associated islands, is referred to as the Tethys Sea (Harries, 1990). This sea is likely to have been warm and stormy, delivering high rainfall and periodic cyclonic wind gales onto the neighboring coastlines. Recent experience of extremes of weather suggests that a warmer ocean spawns more intense cyclones. The suggestion is that natural selection occurred in the palm population for traits resistant to extreme wind, which is described below.

22.3. Development of wind resistance The above described condition believed to have been in the Tethys Sea would favor the emergence, on the shore line, of a tropically adapted palm with a flexible, wind-resistant trunk that could flex rather than break up during episodes of violent wind. The modern palm has a ‘‘tubular’’ trunk structure elated to the density of its cortex and interspersed vascular fibers, forming a thick outer ‘‘wall’’ surrounding a softer core that undergoes compression without fracture when the trunk flexes itself in the wind. The coconut palm further exhibits an adaptation trait to survive strong wind, which is the capacity to progressively shed older fronds. This brings about a reduction of wind pressure, reducing the risk of damage to the heart

Agronomy and Economy of Important Industrial Crops

261

of the crown of the palm. The coconut palm has also the ability to continue to grow even after the trunk has fallen flat on the ground, with new trunk growth resuming a vertical attitude (Marty et al., 1986).

22.4. The ‘‘swimming’’ coconut fruit The ancestor fossil remains of the coconut found in New Zealand include a small nut, less than 50 cm in diameter and most unlikely to contain any endosperm liquid providing nourishment to a long swimming episode (Ashburner, 1994). Dispersal, by means of a hardy floating seed with a thick husk that allowed it to float high in the water, was subsequent adaptation which allowed the nut to move on the ocean and occupy coastal niches throughout the favorable climatic zone. The coconut seed of such wild places is still capable, in our time, of being picked up in large numbers by a tidal surge or in less number by falling from seaward-leaning palms, which survive 3–4 months at sea swim. Such a period is long enough to travel distances up to thousands of kilometers depending on the wind current. This open ‘‘pooling’’ of genetic diversity would have counteracted the natural tendency toward separate paths or pockets of evolution fostered by isolation. Natural diversity in coconut appears to be limited, indeed, in contrast to most land borne species in which genuine physical isolation of subpopulations has occurred. For example, this has given rise, in the case of Macadamia populations, to easily distinguished species, subspecies, and varieties determined by both morphological traits and molecular methods (Aradhya et al., 1998).The paradox of coconut dispersion naturally through its ‘‘swimming’’ trait has been the subject of intense speculation, much of it failing to appreciate the prolonged time frame during which dispersal has taken place. The major tropical land masses and islands of the Indian and Pacific Oceans and the South China Sea have been in place for some million years. Ample time must have been there for the nut to travel far and wide, though frequent glaciation and rising and falling sea levels would have both aided and also disrupted in the sea ‘‘swim.’’ Narrowed ocean barriers between neighboring islands and land masses would at least favor colonization and further mixing of populations at a greater regional level. When sea levels fall, the nut would have become widespread, not only along the coastlines as such, but also across the ‘‘temporary’’ landscape between the coast at the original sea level and the coast at the eventual low extreme. It has the capacity to compete quite strongly with other vegetation in the short term, but would eventually have been overshadowed by tall forest species. When the great northern hemisphere ice sheets receded, sea level rose once again, and the nut in low-lying areas would have succumbed to inundation. A phase of rising sea level would most likely have been conducive to mobilization of seeds onto the ocean. In common with the fringing

262

K. P. Prabhakaran Nair

coral reefs, the fringing vegetation of the strand, including the coconut, would have followed the fluctuating shorelines of the tropics back and forth.

22.5. True palm traits Though the coconut must have ‘‘branched off’’ in its early evolution, it retains the essential physical traits of a true palm (Ashburner, 1994). As with all mature palm trunks, diametrical expansion of the trunk is fixed once the attached fronds have developed, confining trunk growth to axial (length or heightwise) dimension. A ‘‘unit’’ of growth in the mature palm comprises of a trunk section, supporting an attached frond and inflorescence. In a liberally irrigated, warm environment, the coconut sustains a most remarkably uniform and stable growth pattern through time, giving rise to a trunk increment, a new frond, and a new bunch about every 25 days. A frond generally persists for 2–3 years and detaches cleanly from the trunk (except in especially dry atmospheric environments) along with the remnant of the fruit bunch.

22.6. Human influence on coconut evolution As human (proto-Melanesian people) settlement increased along the Southern and Southeast Asian coastlines, coconut provided a great bounty and a most convenient and welcome food source. Evidence for the settlement of the proto-Melanesian people is entirely obscure, although migration as far as Papua New Guinea (PNG) and Australia can be located between 60,000 and 100,000 years ago. These migrants appear to have preferred the refuge of the secure and isolated mountainous hideaways, far from the convenience, but also the danger of the coast. Quite possibly, Homo erectus, the ‘‘Java Man’’ could have made use of the coconut 1.2 million years ago. It is only recently, between 5000 and 10,000 years ago that the sea faring people, known as Austronesians, made use of the coconut palm in a more interdependent manner. By contrast, the earlier hunter–gatherer inhabitant had no knowledge of farming and there emerged the farming people who began to domesticate plants, such as taro and banana as well as coconut. Not only did coconut provide a source of food and drink for day to day living, but, it also became a staple commodity during travel on sea, including long-range exploration as it does to this day among many island communities. Probably, the greatest age in maritime discovery in human history is the age of Polynesian exploration and colonization reaching out from Southeast Asia through Melanesia to Hawaii, the Marquesas, and Easter Island. By canoe, the domesticated coconut reached the Pacific coast of Central America, accompanied or not by surviving people, who might have perished during the tough journey, or following arrival in an new environment or else been absorbed by the local inhabitants without trace. This era of exploration is believed to have begun around 4000–5000 years BC and extended well into

Agronomy and Economy of Important Industrial Crops

263

the second millennium of the modern era, culminating with the arrival of Polynesian people in New Zealand less than a 1000 years ago. The strong linguistic ties between Sumatra, Borneo, and Madagascar indicate that some Austronesians traveled westward. These voyagers might have arrived 3000 years BC taking with them food crops, such as coconut and banana (Simmonds, 1976). The fact that coconut palm was found wherever Polynesian colonization took place is evidence to its ability to provide food and nourishing drink. The domesticated nut had large size, thin husk, and came to be used as highly efficient and convenient water containers with a high shelf life. Such fruits taken during voyage would eventually germinate and sprout, but would remain as a vital source of food and drink. Whenever a voyaging Polynesian party colonized a new place, some of the remaining coconut supplies from the journey would have been available for planting among the native wild coconut palms. Most modern Polynesian communities in the tropics have inherited various forms of domesticated coconut, well preserved through on going selection. Recent RFLP molecular analysis has shown a close affinity of coconut identity from Malaysia to Panama (Lebrun et al., 1999). In India, coconut dates back to post-Vedic period, that is, about 3000 years BC (Thampan, 1982) and Sri Lanka, for about 2300 years. That a distinct population of coconut has developed in South Asian coastal region is attested by the clear differences in DNA detected by RFLP analysis (Lebrun et al., 1999). There appears, however, to be scarce evidence in India of selection for large nuts with thin husks and high water content, although the variety Kappadam has thin husk, which is an exception, perhaps because a good supply of husk has been vital as a source of household fiber and fuel in Southern India since ancient times. The coconut had probably spread naturally to some of the islands of the Indian Ocean, and through trade or migration to Seychelles, Madagascar, and the coast of East Africa. DNA analysis has also revealed that Southeast Asian germplasm entered the Indian Ocean via Madagascar, giving rise to intermediate forms which also extended to East Africa (Lebrun et al., 1999). The spread of coconut from India to the outside world was triggered by the arrival of the Portuguese explorer Vasco da Gama who landed on the Malabar coast at Calicut (now Kozhikode) in 1497/1498, from where the coconut nuts were taken for planting in tropical lands of the Atlantic ocean, beginning at the Cape Verde Islands. From there, it spread to West Africa mainland and also in the mid-sixteenth century to the Caribbean islands, and from thereon to all coasts of Central America and tropical South America (Harries, 1978). Thus, from the obscure and untraceable early origins of the coconut on the coasts of migrating continents and Southeast Asian islands, the coconut distribution finally encircled the globe about four centuries ago. Current distribution is best described by summarizing production data (Table 29).

Table 29

Coconut producing countries grouped into eight categories based on production

Category

Indian Ocean, Southeast Asia, and Pacific

West Africa, Caribbean, and Americas

<1 kt

Angola, Cocos Islands (Australia), Mauritius, Nauru, Niue, Oman, Seychelles, Singapore Somalia, Tokelau, Tutuila (USA), Tuvalu, Wallis and Futuna Caroline Island, Cook Island, Maldives, New Caledonia

Barbados, Benin, Cameroon, Cape Verde, Central African Republic, Gabon, Guadeloupe, Martinique, Puerto Rico, Senegal, St. Kitts, Democratic Republic of Congo Belize, Costa Rica, Cuba, Dominica,

1–5 kt

5–10 kt 10–50 kt

50–100 kt 100–500 kt 500 kt–1 Mt >1 Mt

China, Comoros, Guam, Kenya, Palau, Tonga Bangladesh, Fiji, French Polynesia, Kiribati, Madagascar, Mozambique, Myanmar, Solomon Islands, Vanuatu, Western Samoa Papua New Guinea, Tanzania Malaysia, Sri Lanka, Thailand Vietnam India Indonesia, Philippines

Note: Oil equivalent—1 kt = 1000 tons; 1 Mt = 1,000,000 tons. Source: Burotrop (1992).

Equatorial Guinea, Grenada, Guinea, GuineaBissau, Honduras, Liberia, Nigeria, Panama, Peru, Sao Tome and Principe, Sierra Leone, St. Lucia, St. Vincent, Suriname and Togo Ecuador, Guyana, Haiti, Trinidad and Tobago Colombia, Ghana, Ivory Coast, Jamaica, Nicaragua, Venezuela

Brazil, Mexico

Agronomy and Economy of Important Industrial Crops

265

23. Botany of Coconut In the genus Cocos of the palm family Arecaceae, coconut stands alone, and has no close surviving relatives. In short, the coconut palm is considered to have evolved on the strand of the ever changing coastline of islands and land masses fringing the Tethys Sea as they drifted north from Gondwana (Harries, 1990). The ‘‘melting pot’’ of the ocean provided frequent (in geographical time) opportunities for mixing of the diversifying progeny of many small populations of palms, occupying many contrasting strand environments, which evolved finally into the modern coconut C. nucifera L. Two distinct forms of coconut are recognized, the tall and the dwarf. The primary difference is in the rate of trunk elongation which is at the least twice rapid in the tall form compared to the short. The diameter of the trunk of the tall form is also generally one and a half times to two times more compared to the dwarf form, which gives a cross section area two- to fourfolds more. There is one important subgroup of the dwarf form, Niu Leka or Fiji Dwarf, which, however, has a similar trunk diameter as that of the tall form, but a trunk extension rate even less than that of the other dwarf forms. The frond length of the tall is around 6 m, compared to the 4 m of the dwarf, resulting in a much larger crown of the tall form. Another major difference is that the all form is predominantly mostly self-pollinated and therefore largely homozygous.

23.1. Morphology 23.1.1. The trunk The coconut trunk comprises an outer dense zone or tube surrounding a central ‘‘rod’’ of much lower density, although both zones become more dense with age, with the outer zone reaching a maximum of around 1.1 ton/m3. Except for the zone immediately below the crown, which comprises a low density trunk formed in the last year or two, the trunk is very tough and relatively flexible. The upper portion of a trunk 15 m long is capable of bending almost parallel to the ground, which allows critical relief of wind-induced mechanical shear. The crown adopts a position presenting a streamlined shape to the wind, minimizing pressure and potential damage. The trunk of the tall form typically has a bole (basal region of large diameter tapering from about 1 m height to the standard diameter at 2 m height) that develops between 3 years and the initiation of the first inflorescence. It can be understood in terms of the assimilate produced by the canopy that is available to support development of the ‘‘sink.’’ During this period, prior to any demand by the reproductive system, the early growth of the tall trunk is

266

K. P. Prabhakaran Nair

very well supported by photosynthetic assimilate supply. As the palm diverts assimilates to meet the demand from its developing inflorescence and later the fruit bearing bunches, the trunk diameter gradually tapers. In the dwarf form, on the other hand, flowering is initiated much earlier and there is practically no development of a bole. During early fruiting, trunk of both tall and dwarf form elongates quite rapidly. However, when yield increases to a high degree, competition for assimilate gradually reduces the rate of trunk elongation in the tall form from its early peak of greater than 1 m/year to 50 mm/year at 60 years of age, whereas the diameter diminishes only by 30% over that period (Foale, unpublished data). Restriction of the diameter and trunk growth is triggered by water and nutrient deficiency and the same is restored to the original when these deficiencies no longer persist. Properties of the wood in a mature trunk are such that valuable timber can be milled out for use in construction and manufacture of ornamental materials. Special sawing equipment, with stellite or tungsten teeth, and special technique, such as injection of cold water into the active cutting zone, are adopted to prevent heat damage to the saw blade and help clear fibers that are released by the blade during sawing. The outer zone of the trunk is milled separately from the inner zone, which is lower in density compared to the outer. 23.1.2. The root system As with other monocots, coconut has an adventitious root system. Attached to the base of the trunk, there are many primary roots which radiate in an ideal light textured soil to form a hemispherical root zone. Where the soil is shallow or has a compact B horizon, with water table close to the surface, downward extension of the root is restricted. Roots of 6–10 mm diameter can extend 5–7 m outward from the base of the palm (Thampan, 1982). Vertical extension down the profile is normally 1–15 m, but this is more in sandy soil. In a plantation significant overlap of the root systems exist, which shows that fertilizer applied anywhere across the inter rows can be accessed for absorption. In a soil with 30% clay, cored to a sandy one, root depth is much reduced (Pomier and Bonneau, 1984). Primary roots have first order (major) root branches of 4–5 mm diameter. Second (2 mm) and third (0.5– 1 mm) order branches fill the role as feeder roots. Coconut roots have no root hairs and no nutrient scavenging mycorrhiza. The resilience the coconut palm shows while growing under water and nutrient stress points to the experimentally unestablished fact, yet, that, there, indeed, might be a positive interaction between soil microorganisms and the coconut roots. Extending a few millimeters from the upper surface of primary roots can be found small, whitish, pointed organs known a pneumatophores, which

Agronomy and Economy of Important Industrial Crops

267

evidently evolved to maintain oxygen supply to the root tip during diurnal submersion in the water table. 23.1.3. The frond Like in most plants, in coconut also the spear shaped frond emerges vertically from the single terminal growing point. While tightly packed together, the leaflets lack chlorophyll, but are transformed rapidly becoming green as they unfold. The angle of phyllotaxis (arrangement around the axis of the stem) of the coconut is close to 140 , either clockwise or counter clockwise. The fifth frond above or below any chosen frond subtends at an angle of 20 to 30 with that frond. That is, each frond is only slightly aside from being placed in vertical alignment with fronds that are five positions above or below. This relationship enables very rapid counting of the number of frond in a palm crown, which is convenient in research investigations on coconut. Collectively, the fronds form the crown of the palm. Some important aspects of the dimensions of the frond base and the interval in the height of the trunk between successive fronds influence the shape and behavior of the crown at different stages in the life of the palm. These effects are crucial to forming a realistic expectation of productivity as the palms get older. 23.1.4. The crown There is a sideways overlap between neighboring fronds of the thick wad of tissue, referred to henceforth in this discussion as the ‘‘base pad’’ (Foale et al., 1994). This section of the base of a frond adds mechanical strength to resist the tendency of the long axis of the frond to rotate around the pivotal zone of firm attachment to the trunk. The attached surface is actually wrapped one-third of the way around the circumference of the trunk, providing a powerful ‘‘grip’’ to resist any pivoting action or ‘‘lowering’’ of the frond from its almost vertical position, hugging the trunk. On the lower trunk of the palm, up to 5 m in height, the average vertical spacing (interval between leaf scars) of fronds has a maximum value of 7 cm. The spacing diminishes to around 4 cm in 25 years, causing the upsweeping attitude seen in all the fronds of a young palm. Gradually, over 10–20 years or so, the behavior of the frond changes. The frond, pivoting on its attachment to the trunk, droops or descends more rapidly from its early vertical position. In time, half of the fronds hang at angles below the horizontal plane and the other half are above the horizontal plane, giving the crown a distinctly spherical shape— the classic iconic shape so much liked and loved by the tourists, when they are in a coconut grove. This change can be explained in terms of mechanical pressure exerted between the base pads of neighboring fronds. The rearrangement of the ‘‘angle of repose’’ of the frond is simply a response to internal pressure on its base pad as adjacent younger fronds expand.

268

K. P. Prabhakaran Nair

Angular movement or drooping of the frond axis of the older (outer) frond about its point of attachment relieves this pressure. When the average interval on the trunk between fronds is 3–5 cm, there is firm pressure due to contact between the base pads of any frond and its neighbors two positions up and two positions down from it. The frond has one-third overlap with each of these neighbors. The pressure increases as trunk extension diminishes so that eventually the base pads of fronds 1 and 4, which have a one-half overlap, are also in firm contact. It appears that the resulting sustained outward pressure weakens the attachment of older fronds so that these fronds progressively give way to as shown by their drooping. The base of the inflorescence would also contribute to this overcrowding at the surface of the trunk, generating even more outward pressure. In extremely old coconut palms, the overcrowding is so severe that the frond base is almost wrenched from the trunk, remaining loosely attached by elongated, fibrous-looking, xylem bundles, leaving a sunken ‘‘scar’’ on the trunk. Other palms of economic importance, such as the oil palm and date palm, differ from coconut palm in that the trunk is of greater diameter and with the width of basal frond attachment is much less than that of coconut, so that inter-frond pressure does not come about until the palms are very old. There are two important consequences of this evolution of the shape of coconut crown. Palms of 25 years age have a crown that evolves during that period from an inverted cone shape to hemispherical. This range of shapes achieves maximum light interception and the incident energy is fairly well distributed over all fronds, ensuring high phytosynthetic efficiency (Foale, 1993a). As the crown takes on a spherical shape, however, with a decreasing proportion of the fronds angled upward, light interception falls until when the fronds are ‘‘half up and half down.’’ Only 50% solar radiation is intercepted at a standard plantation density. Progressive reduction in length of the frond with age serves to reduce interception of light even further. The obvious consequence of the declining rate of capture of solar energy is that potential biomass production falls in proportion, following a downward spiral of reducing photosynthetic infrastructure, and also increasing the intensity of competition for resources between developing frond, inflorescence and trunk. The second consequence of loosening of the older frond in the coconut crown is that these fronds can be shed more readily in a devastating wind. The coconut minimizes risk of such devastation by its ability to reduce the size of the crown, which complements its ability to adopt a streamlined position, already mentioned in the above discussed section of this discussion on origin. Young palms are a lot more prone to wind damage because the crown is more robust and also because there is insufficient length of the trunk to bend down on the wind direction. Yet, it may survive. A further consequence of falling light interception is that in a natural coconut woodland, more light reaches the young palms below struggling to capture sufficient energy to become productive.

Agronomy and Economy of Important Industrial Crops

269

23.2. The inflorescence Each mature and healthy frond has an inflorescence in its axil, emerging about 1 year after the frond expands. First, the encasing spathe (a somewhat leathery sheath) appears and extends its full length, then its lower side splits open and the multi branched ‘‘flower bunch’’ is released. Several thousand small male flowers (4–6 cm long) are borne on the 20–40 branches and begin to shed pollen progressively from the distal ends for about 6 weeks. The rare spicata form of coconut has only one or two short branches bearing just a few dozen male flowers. The female flowers, which expand to about 25 mm diameter during the period of opening of male flowers, are borne toward the proximal end of the mid and lower branches, normally singly or in pairs. The male phase starts with pollen shedding on the day the inflorescence opens and continues for about 3 weeks. However, the duration of the female phase generally lasts only 5–8 days, and the individual stigma will remain receptive during 1–3 days. The female flowers of the tall form become receptive for pollination after pollen shedding is complete, although this depends on the genotype and prevalent environmental conditions. However, on the dwarf varieties, pollen is still abundant within the inflorescence during female receptivity. Thus, the tall varieties are generally out breeding and heterozygous (diverse traits between palms), whereas the dwarf verities are generally inbreeding and homozygous (uniform traits). There is some in breeding in the tall varieties, as well, when a favorable environment (which is seasonal when rainfall is plentiful in many coconut regions) speeds up the rate of emergence of fronds and inflorescences. This results in the next youngest inflorescence being released while some female flowers of the older one are still receptive. Some out crossing occurs in the dwarf varieties through pollen from other palms, brought about by wind and insects, which, in fact, turns out to be a competition with the native pollen.

23.3. The fruit The number of female flowers vary from season to season and is subject to the environmental stresses the palm undergoes, and is usually well in excess of the number of fruits developed. A full 12 months elapse between pollination and fruit maturity, less in warm regions, but as much as 15 months at higher altitudes with cold background. At higher altitudes within the tropics, the delay rate in maturity will be in proportion to the drop in mean temperature below the coastal mean of 28  C. After 6 months, the fruit reaches its full size and its vacuole is filled with water, which is the liquid endosperm. Thereafter, the volume of the vacuole diminishes as the 10–15 cm of layer of kernel (endosperm) is formed on the inner surface of the shell. Maturity of the fruit is usually indicated by the entry of air into the

270

K. P. Prabhakaran Nair

vacuole, so that the water ‘‘splashes’’ audibly when physically disturbed. In an environment with a large atmospheric vapor pressure deficit, early loss of nut water takes place, and the splash in the vacuole can be generated before maturity. In that case, one should avoid interpreting the splash as an indicator of maturity. Both seasonal temperature and soil water deficit appear to influence the shape of the shell within the fruit. A common response is the decrease in diameter of the nut, giving it an elongated appearance in contrast to the dominant spherical shape in most varieties. The size of the fruit differs greatly with genotype, and the weight of mature fruit ranges between 1 and 3 kg, although fruits outside this range are encountered. Fruit size varies with the number borne on the palm at a time, and the contrast is especially notable between the bunches carrying few and many fruits, respectively. Coconut fruit is the very largest seed in the plant kingdom after coco de mer (Loidecea seychellarum L.). The seed provides an almost unique long-term source of energy for the emerging seedling, which enables it to cope with both water and nutrient deficiencies during the first year of the life of the palm (Foale, 1968b). This ability was an important contributor to the survival and competitiveness of the coconut in newly colonized habitats and has earned it the ‘‘weed status’’ in some environmental reserves and semi-urban beach environments where it has proliferated and suppressed other strand vegetation (Foale, unpublished).

23.4. The seed and seedling The shell with its contents constitutes the seed where the husk performs the function of protection and dispersal. The entire fruit is normally left intact, except to trim some husk from the region where the shoot is expected to emerge, when selected for seedling production. Following maturity, the single embryo present in the nut begins to develop by first emerging through the germ pore. In domesticated Pacific coconut types, germination is rapid, often taking place prior to fruit fall, whereas in others there may be several weeks’ delay, or the need for added moisture to stimulate the embryo, prior to germination. The presence of the husk prevents the observation of true germination, as there is about two months’ delay before a sprout develops sufficiently to emerge through the husk. By dehusking and storage, unbagged and free of moisture, germination of the nut can be controlled. A nut in this state will not sprout until the atmosphere is vapor saturated by placing under moist mulch or in a sealed bag with enclosed moisture (Foale, 1993b). The embryo expands through the germ pore, developing within 10 days into a rounded mass of soft white tissue. The internal end of the embryo expands to form the haustorium, an enzymesecreting organ that breaks down and absorbs the kernel and progressively fills the vacuole. Absorption of the energy rich tissue of the kernel supports the continuing expansion of seedling tissue and its differentiation, which

Agronomy and Economy of Important Industrial Crops

271

commences after about 14 days, eventually forming a shoot on the upper side and a root initial below. Growth of the coconut seedling could continue for months in the dark as absorption of the kernel proceeds, but, at about 3 months, from true germination the first small leaf appears and photosynthesis begins. Over the next several months, as leaf area expands, a gradual transition from total dependence on the kernel to complete independence occurs around 12 months of age, when uptake of the kernel is complete (Foale, 1968b). During the early months, the endosperm-assisted growth rate of the seedling is very high compared to the small-seeded plants. If microorganisms invade the endosperm within the first 6 months of germination, the growth rate of the seedling falls sharply, usually leading to rejection of the seedling.

23.5. Cytogenetics of coconut There was early interest in descriptive research of the cytology of coconut (Abraham et al., 1961; Nambiar and Swaminathan, 1960; Ninan et al., 1960), but subsequent interest in this field waned. However, Louis and Rethinakumar (1988) presented a useful report entitled ‘‘Genetic Load in Coconut Palm,’’ which identified mechanisms, arising from the heterozygous nature of the coconut palm, which resulted in the elimination of undesirable recessive genes. Developments in unraveling the genetic code and identification of specific genes and linkages have led to diminished interest in the discipline of cytogenetics.

23.6. Genetic improvement of coconut 23.6.1. Source of diversity That coconut improvement has been an important objective over a long period in human history is testified by the fact that the diversity of coconut populations and its association of readily-identifiable variants with specific dispersed ethnic groups, especially across Southeast Asia and the Pacific. Large-fruited populations, for instance, are widespread wherever there are Polynesian people, who have detectable linguistic and cultural links with island people close to Southeast Asian landmass. The large fruit fulfills the primary human objective of gathering a convenient source of food and water for long sea voyages. A variant of the large fruit has especially long fibers, valuable in the manufacture of rigging for sea-going vessels. Since 1950, there has been an industrial initiative for genetic improvement of the coconut, with the primary objective of raising productivity of oil output and economic profitability of the coconut plantations. All genetic improvement is based on the diversity present in the species. However, in order to be useful to plant breeders, this diversity needs to be characterized reliably.

272

K. P. Prabhakaran Nair

23.6.2. Characterization of the genome of the coconut palm On the basis of appearance and utility, coconut populations have been distinguished since antiquity. Variations in fruit size, shape, and color have been seen in tall forms, and humans have shown a preference for larger fruit, especially for water and supply of food at sea, but also for convenience in processing the kernel for domestic use. Dwarf palms were preferred for their ease of harvest to provide drinking water to the household, but their kernel is less palatable. Tall palms, on the other hand, are more difficult to manage because of out breeding and heterozygosity, the consequence of which is that tall forms provide greater challenges in their characterization to establish the fact whether real or not, heritable differences exist (Liyanage and Sakai, 1960).

23.7. Fruit component analysis Pieries (1935) was the first to employ this method as a tool to characterize coconut palm. He confined his measurements to the dehusked nut. Subsequently, Harries (1978) and Whitehead (1966) continued the work. Subsequent researchers used the method to much advantage. Fruit component analysis allowed identification of similarities and differences between populations with a fair degree of confidence, based on the low sensitivity of fruit composition to most changes in the environment. Harries (1978) made use of differences in fruit components in developing a general theory of the evolution and dissemination of coconut around the world. The basic approach is to weigh the whole newly mature fruit and then its dissected components. The fruit is selected only when a trace of the fresh color remains on the husk. Moisture content of the husk would be expected to be similar in fruits from different populations, at around 20%, and water content of both shell and kernel would be fairly stable. Broad categories were established, named domestic and wild, at the extremes, with low and high proportion of husk, respectively. Many populations were of intermediate proportions, and these were described as introgressed. It is impossible to determine true genetic affinity or difference between diverse populations which are identified relying solely on fruit component analysis. Such distinctions, based on molecular parameters, were impossible to be made earlier.

23.8. Use of molecular markers Today a variety of tools is available, based on price labels and efficacy, to characterize the coconut genome. The basic entity in this approach is called a ‘‘molecular marker,’’ which is defined as ‘‘an inherited chemical trait that can be used to distinguish between individuals, groups of individuals, or

Agronomy and Economy of Important Industrial Crops

273

positions on chromosome’’ (Ashburner, 1999). Until early 1990s, attention was focused on ‘‘gene products,’’ such as isozymes, to detect genetic difference, but a direct assay on the DNA itself has proved to be far more effective. A review of the application of molecular markers on coconut improvement was published by Ashburner (1999), but scientific advances in this field are being made very rapidly. A comprehensive investigation of a very wide range of germplasm encompassing global variations in coconut germplasms was undertaken by the Centre de cooperation internationale en recherche´ agronomique pour le development (CIRAD) in France during the 1990s, which has been reported by Lebrun et al. (1999). Their report reveals the groupings of coconut genotypes based on RFLP analysis. The investigation showed, for instance, that all germplasms in populations extending from Southeast Asia eastward right across the Pacific, including the Pacific coast of Central America, could be grouped together as quite distinct from that of Southern Asia and Caribbean. An intermediate group was apparent that included Madagascar and East Coast of Africa. Dwarf types were found to be genetically similar to the tall types of their geographic region. There are a variety of uses in taking recourse to molecular markers. They are specially useful in many resource management applications, including diversity analysis based on at least 20 individuals per population. This will provide data on genetic distance both within and in between populations, genetic finger printing, such as establishing true crosses from inter population hybrids, and out crossing analysis, which can establish with certainty the mating system of a coconut population (Ashburner, 1999). Many interesting hybrid combinations have been tested in coconut, selected in the early days using a mix of both intuition and geographical separation. Molecular characterization of coconut populations will make possible choice of future combinations with a higher probability of significant improvement, although a great deal more research would be needed to validate this strategy.

23.9. Early breeding work While coconut provided food for the inhabitants of the tropics for millennia, it was the oil that was cherished by the Europeans and Americans in late nineteenth 1 and early twentieth centuries, facing little competition from other oil sources. Following Second World War, there was a concerted effort to enhance genetic improvement of coconut as it was found to be an important oil bearing crop. The genetic improvement of the coconut palm has been occurring during thousands of years, rather unknowingly. However, it was during the twentieth century that the crop assumed a plantation status. Though the effort was widespread, the earliest attempt was initiated in India, with the establishment of four research institutes in 1916, of which

274

K. P. Prabhakaran Nair

the coconut research institute at Kasaragod (Kerala State), which subsequently was enlarged to become the leading coconut research institute of Asia entitled Central Plantation Crops Research Institute (CPCRI which also brought under its ambit arecanut) and another at Nileshwar (also in Kerala State), which carried out pioneering work.

23.10. Hybrid vigor in coconut It was Patel (1937) who first discovered the existence of hybrid vigor in coconut, between West Coast Tall and Chawghat Green Dwarf, which was a landmark in the history of genetic improvement of coconut. Since then, a number of hybrids, both D (Dwarf)  T (Tall) and T  D were evolved and released in India. Some of the important hybrids are Lakshaganga (Laccadive Ordinary  Gangabondam), Anandaganga (Andaman Ordinary  Gangabondam), Keraganga (West Coast Tall  Gangabondam), Kerasree (West Coast Tall  Malayan Yellow Dwarf), Kerasawbhagya (West Coast Tall  Straight Settelelemt Apricot), Kerasankara (West Coast Tall  Chawghat Orange Dwarf), Chandrasankara (Chawghat Orange Dwarf  West Coat Tall), Chandralaksha (Laccadive Ordinary  Chawghat Orange Dwarf), VHC-1 (East Coast Tall  Malayan Green Dwarf), and VHC-2 (East Coast Tall  Malayan Yellow Dwarf). Drought tolerance investigations at CPCRI, Kasaragod showed the possibility of identifying the desirable traits of drought tolerant cultivars under field conditions (Rajagopal et al., 1988). The promising drought-tolerant varieties/hybrids were West Coast Tall  West Coast Tall, FMS (Federal Malay State), Java Giant, Fiji, Andaman, and Laccadive ordinary  Chawghat Orange Dwarf. In Fiji, a hybrid was evolved by crossing the indigenous dwarf known as Niu Leka with Red Malayan Dwarf in the 1920s (Marechal, 1928). Worldwide depression and war halted the further progress of this pioneering research. The Coconut Research Institute in what was then Ceylon (established in 1929 as the Coconut Research Scheme, becoming a full-fledged Institute in 1950) contributed much to early investigations (Pieries, 1935). A broad foundation of understanding was developed in those early decades of the opportunities and the constraints presented to those seeking to improve coconut yield.

24. Constraints in Coconut Breeding Compared to most other field crops, constraints in genetic improvement of coconut are formidable. Height is the most important of all the constraints as this renders pollination extremely difficult. Another is the large size of the palm, which requires at least 8 ha to conduct a properly

Agronomy and Economy of Important Industrial Crops

275

laid-out field trial (Bourdeix et al., 1993). Yet another is the long delay in developing a successful technique for harvesting a manageable quantity of pollen from the inflorescence, which was finally tackled in the 1960s (Whitehead, 1963). Additionally, there are biological constraints of few seeds per palm and per bunch, requiring multiple visits for pollination. And the long turn-around time of 6–10 years between generations of tall varieties. Genetic constraints include a high degree of heterozygosity within any population of tall palms and different levels of combining ability between genotypes. There is also the constraint of lack of a clonal propagation method for outstanding selected or bred genotypes, which are otherwise constrained by the low number of seeds generated (Santos, 1999).

24.1. Selection and its progress Tall populations of coconut have been distinguished mostly by fruit characters locally, while globally or regionally by place of origin. The latter approach seemed somewhat arbitrary until collections of different populations were assembled for research. It was observed that the introduced germplasms were attacked by one or more insect pests that were of mild consequence locally or by diseases that were unknown in the area previously. Examples are attack on Malyasian Tall in Solomon Islands by the leaf beetle Brontispa sp., the outbreak on the leaves of Polynesian Tall in Solomon Islands by the fungus Drechslera sp., and the attack on hybrids that had a West African Tall parent, planted in Indonesia and Philippines, by a Phytophthora strain which differed from strains that were tolerated in West Africa. In India the ‘‘root wilt’’ is devastating and elsewhere in Kerala State, the coconut ‘‘decline,’’ of which the exact etiology remains unknown even to date. A most unusual experience in Vanuatu in Sri Lanka is the appearance of foliar decay by a virus attack. Although it was presumed that the virus was introduced along with new varieties, later it transpired that it had existed locally all along in Vanuatu. The indigenous population possessed tolerance, which allowed it to remain symptoms free, while the pathogen when transmitted to the non-tolerant Introductions, the result was, indeed, devastating. These examples point out that many coconut plantations have adopted to local environment with particular pests, diseases, and aberrant climate, as in cold episodes in Hainan Island (Zushum, 1986). This is an aspect of population identity to which molecular markers are beginning to make an important contribution. Breeding for yield improvement or other positive traits, while retaining ‘‘invisible’’ traits, such as pest resistance or disease tolerance, will be feasible only with the aid of molecular tools. Owing to difficulties in demonstrability of positive traits through heritability (Liyanage and Sakai, 1960), and because of a negative correlation between the number of fruits borne and the copra content, researchers were slow to develop confidence in breeding for yield improvement.

276

K. P. Prabhakaran Nair

Beginning 1950, yield improvement became a major objective in breeding in India, Sri Lanka, Philippines, Ivory Coast, and several Pacific countries, whereas in Jamaica, the main objective was to overcome the ravages of the devastating yellowing disease. In India and Sri Lanka, emphasis was at first on mass selection within a population, even though the first report of dwarf  tall hybrid vigor was made in India in 1937. Other researchers concentrated on inter population hybrids, especially in the 1960s.

24.2. Hybrids and their future The enormous diversity in the F2 generation from hybrids produced in the 1920s in Fiji (Marechal, 1928) between Malayan Red Dwarf and Niu Leka (Fijian Dwarf), and in the 1960s in Solomon Islands evoked much research interest. Parallelly, employing hybrids in the Pacific and in Ivory Coast also has yielded rapid progress for some combinations, such as Malayan Yellow Dwarf  West African Tall and the Malayan Red Dwarf  Rennell Tall, which became commercial cultivars in the 1970s. In each case the first generation hybrids produced a yield advantage of 30% over the best performing tall population (Foale, 1993b). Outstanding yield increase has also been demonstrated for Tall  Dwarf hybrids in India (Nair and Nampoothiri, 1993) and in Sri Lanka (Peries, 1993) in the recent past. On account of the risk of a specific parent lacking tolerance to a potential natural enemy in locations far from the source of the parent genotypes, local industries have latterly moved to include at least one local parent in any Tall  Dwarf hybrids tested. Apart from general combining ability between Tall and Dwarf populations, subsequent testing of many combinations in Ivory Coast has revealed that some are significantly more productive than others (Baudouin, 1999). This author has reported that from 15 Rennell Tall (RLT) palms combined with each of the three ‘‘testers’’—West African Tall (WAT), Malayan Red Dwarf (MRD), and Cameroon Red Dwarf (CDD)—produced very different yields of copra ranging from 15 to 27 kg/yr. The mean value (kg) of 15 palms in each group was (by tester): MRD: 19, WAT: 21, and CRD: 24. Other combinations of ‘‘genetically distant’’ tall parents have proved to be very interesting. Geographic separation, supported by isozyme and latterly RFLP markers (Lebrun et al., 1999), has enabled three major groups to be defined within the global coconut population. Crosses between subpopulations from Group I (Southeast Asia/ Pacific) and Group II (Southern Asia/West Africa/Caribbean) gave higher yield than any combinations within these two groups (Baudouin, 1999). An interesting observation was that an intermediate group (Group III) could be identified from Madagascar and East Africa from which the Mozambique Tall hybrid with WAT equaled the best combinations between the geographically more separated Groups I and II. It can be concluded that

Agronomy and Economy of Important Industrial Crops

277

molecular markers will turn out to be the most important tool in identifying outstanding coconut germplasms in the future in hybridization research.

25. Commercial Production of Hybrid Seeds To produce commercially viable seeds inter planting of the two parents is involved. A Dwarf  Tall cross is most readily done with the Dwarf as female parent, requiring emasculation or removal of all male flowers before pollen is shed. The dwarf palms planted in two or three rows per row of the pollen parent must be checked daily. Any inflorescences that have begun to emerge from the opening spathe are dealt with by complete removal entirely from the field so that the only possible source of pollen is the tall parents nearby. Multiple male parents can be included in such seed gardens to provide the possibility of producing different hybrids as required, but only one hybrid is available by open pollination at any given time. Controlled pollination of bagged inflorescences to produce small lots of seed is also possible but obviously more costly and less successful. Where a smaller scale crossing is to be achieved, with few selected parents, the female parent is bagged after emasculation and the chosen pollen blown into the bag during several days in a row. The procedure is expensive, but suitable for research only. Commercially viable hybrid seed gardens have been set up in many countries, for example, 2 million hybrid seeds are produced each year in India and in Sri Lanka, the capacity to produce hybrids is rising rapidly. Nursery managers know that both yellow and red colors of widely used female parents are due to a recessive gene, which if expressed in a progeny from seed garden, is evidence of self-pollination of the mother palm. A seedling which shows yellow or red color on the petiole is therefore rejected.

25.1. In vitro propagation The method has produced only one commercially viable application—the propagation of embryos from makapuno nuts. In vitro propagation is still a challenge to coconut breeders. A makapuno nut is filled with a jelly-like substance which is rich in coconut flavor, but incapable of stimulating and supporting the embryo germination. The raw makapuno product is sufficiently valuable that there is commercial support for culture of embryos to ensure production of makapuno nuts in the progeny. In vitro propagation though considered promising and presented for funding (Santos, 1999), its viability is still elusive (Harries, 1999). There has to be other objectives besides oil yield to evoke commercial interest in in vitro propagation.

278

K. P. Prabhakaran Nair

Limited propagation of outstanding elite palms to form the basis of a seed garden might possibly be justified, although technically not feasible. The long-term possibility that of genetic transformation can be performed on coconut callus, which is then differentiated into propagules, such as has been achieved in many other agricultural crops, still interests researchers in the quest to streamline this technique. No major changes in the present uncertain prospects of in vitro propagated coconut palms contributing to yield or quality improvement can be expected in the near future. To a limited extent, embryo culture is useful in germplasm exchange, although, even here the technique is not yet well advanced for use by technicians of moderate scientific skill.

26. Agronomy of Coconut 26.1. Soils The coconut palm must have been subjected to diverse environmental conditions in the course of its evolution on the tropical edge of the free land masses which drifted from the Gondwana. The basic requirement of the palm for its water could have been met from diverse sources—highly alkaline sands derived from erosion of coral, silica sands, deltaic silts and loams, black and red clays formed from volcanic ash, and highly acidic lava sands. Tall populations adapt best to such diverse soils, which reinforces the idea that a great deal of natural mixing of genotypes between subpopulations from these diverse environments has taken place over geological time. The result of this phenomenon is that the coconut palm has thrived in the tropics far from its shoreline home, wherever its need for a regular supply of available water has been met. The extremes of soil environment are the coral atoll with sand and gravel at pH 8.3, where some critical nutrients are rendered difficult to be absorbed, as for instance, in the organic peat of Sumatra where the pH is lower than 4.5. This decline is because drainage oxidizes the elemental sulfur in the soil to sulfuric acid, releasing Al and some Mn in the process from clay minerals, which turn out to be toxic.

26.2. Soil water The coconut palm accesses the life-giving fresh water ‘‘lens’’ or reservoir found under atoll soil or the water table of larger land masses that commonly flows to the sea under coastal sand dunes. In other situations the soil must be capable of retaining sufficient water for the coconut palm to survive the longest seasonal intervals between significant rainfall events. Deep, wellstructured clay or clay loam soils, which hold up to 250 mm of plant available water could, from a fully saturated starting condition, sustain

Agronomy and Economy of Important Industrial Crops

279

growth for two dry months. Such soils are found on the uplifted coralline ‘‘benches’’ of many South Pacific Islands and on river plains and deltas worldwide. The coconut palm is also planted widely on banks between irrigated fields, where the water level is controlled by the farmers irrigating other crops, thereby providing an accessible water table. Although the coconut palm appears to thrive in the vicinity of the swamp, which often occupies the swale adjacent to a coastal dune, the palm actually is very sensitive to water logging. Where the water level is static, and low in dissolved oxygen, coconut roots become inactive. On the other hand, the sort of diurnal, tide-induced vertical oscillation of the water level in the freshwater lens of an atoll soil or under a coastal dune is ideal. The roots have pneumatophores (a sort of snorkel), attached to short vertical branchlets which supply oxygen to the main root, while its physiologically active root-tip region is temporarily submerged. It has been found necessary to provide good drainage for coconut on heavy-textured lowland plains to avoid water logging during sustained rainfall. While the coconut palm is susceptible to water logging, it has some tolerance to water deficit as well. Whereas in most island environments the coconut palm was rarely moved far from the coast prior to the plantation era of the late nineteenth century, it has evidently been grown in the sub-coastal zone and further inland in India since antiquity. This provided an opportunity for adaptation to more severe episodes of water deficit than would be experienced elsewhere. This development is supported by evidence that the modern West Coast Tall, and related populations in East and West Africa, Caribbean Islands, are more drought tolerant than most other populations from Southeast Asia and the Pacific. An exception may exist in the Pacific on some of the dryer atolls such as those of northern Kiribati, where the level of salinity in the fresh water lens rises sharply toward the end of long periods of low rainfall. The local tall population generally shows less stressful signs (collapsing lower fronds, premature nut fall, failure of inflorescence emergence) than material introduced from better-watered environments. In this connection it would be interesting to know about the effect of drip irrigation. Coconut palm is almost always rainfed and sometimes irrigated. And when it is irrigated, it is invariably basin irrigated. The importance of irrigating coconut for sustained yield has been emphasized. Among the irrigation systems, drip irrigation is gaining importance as it maintains soil moisture availability and air balance in the root zone of coconut near field capacity throughout the dry season and saves irrigation water. The experiment was carried out during a 6-year period (1993–1999) during the summer months, when water requirement is very severe, at the field experiment station of CPCRI at Kasaragod, Kerala State. The results revealed that annual leaf production and leaf nutrient status (N and K) of coconut palm were significantly higher in the irrigated treatments compared to the rainfed control treatment. Female flower production and nut yield with 66% of open

280

K. P. Prabhakaran Nair

Table 30 Nut yield and female flower production (number/palm) in West Coast Tall (WCT) as influenced by irrigation in a laterite soil Pre-experimental period (1991–1993)

Average of (1993–1999)

Treatment

Female flowers

Nut yield

Female flowers

Nut yield

T1 T2 T3 T4 T5 LSD (P = 0.05)

79.9 73.2 66.0 78.6 79.5 NS

28.2 30.1 24.9 31.6 30.8 NS

184.5 214.7 200.8 225.5 157.4 25.6

68.2 96.5 89.8 98.2 52.6 9.5

Note: T1, Drip irrigation at 33% Eo (Open pan evaporation) daily; T2, Drip irrigation at 66% Eo daily; T3, Drip irrigation at 100% Eo daily; T4, Basin irrigation at 100% Eo applied once in 4 days through hose pipe; T5, Rainfed control; NS, Not significant.

pan evaporation (Eo) was on par with 100% Eo through drip irrigation and 100% of Eo through basin irrigation. Drip irrigation equal to 66% of open pan evaporation proved to be an economically efficient method of irrigation with water saving up to 34% compared to 100% Eo through basin and drip methods. Results are summarized in Table 30.

26.3. Plant nutrients Like most other plants, coconut requires all the essential elements, and unlike others, need much chlorine (von Uexkull, 1972). Legends have existed in coconut-based cultures that when grown away from the sea coast, the palm needs to be sprinkled with salt water. This folklore was finally shown to have scientific substance, when substantial need for chlorine was scientifically proved. Chlorine is deficient most commonly beyond the range of cyclic salt, delivered near coastlines in rainfall, and where soil is readily leached during intense seasonal rainfall. Since 1950 extensive research has been carried out to understand why in a given local situation the crop yield does not reach the potential yield and the implication of specific fertilizer nutrients has been identified through classical fertilizer experiments. Numerous case studies of the association of a specific nutrient deficiency with specific soil characters or landscape history have enabled some findings to be extrapolated to identical locations. For example, on atolls it is found that when nitrogen is not limiting, Fe and Mn become important, rendered poorly available in soils of high pH. Another case concerns sulfur, which has been found deficient in many areas of grassland

Agronomy and Economy of Important Industrial Crops

281

plantation. Frequent natural or human-induced burning of the grass has led to a cumulative loss of sulfur to the atmosphere and, in time, exhaustion of the sulfur reserve in the soil (Southern, 1969). N deficiency is most common in drier environments, especially on sandy soil, which leaches readily during the wet seasons. Where rainfall is high and the dry season is short or absent, N is rarely in short supply as the rapid recycling of nutrients from the considerable body of organic residue being returned from the palms and the under growth meets the needs of the entire plant community. Exported N is replaced by the accession of nitrate from rainfall and from any N-fixing herbs or shrubs. In this well-watered environment, K commonly becomes limiting because of the large amount that is exported to the fruit, as well as what is lost through leaching. When rainwater is high in cyclic salt (the mineral salts present in sea water, usually present near the coast), Na displaces much of K from the clay particles, which is easily leached out of root zone, thereby starving the coconut palm of the much needed K. A low available P level is quite common in many coconut growing soils, especially the laterites of Kerala State, where the soluble Fe and Al bind the P into insoluble forms. Hence, care must be taken in applying P fertilizers, and it is best done in bands so as to minimize soil contact and thereby minimizing interaction at the clay colloidal level. Mg has been found limiting in some soils, especially after other limiting factors have been met. On some clay soils situated in uplifted coral benches, low Mg was evident due to loss induced by both high Ca and Na.

26.4. Tissue analysis Manciot et al. (1979a,b, 1980) published a comprehensive review of the mineral nutrition of coconut, where the tissue analysis form an important guide to the characterization of nutrient deficiencies and thereby lead to appropriate fertilizer recommendations. The coconut palm renders itself particularly well to nutrient investigation based on tissue analysis because of the regular production of foliage and fruit throughout the year. The leaf analysis data lead to determining ‘‘critical levels’’ of nutrients which can be calibrated to arrive at appropriate fertilizer recommendations to tap the potential yield of the coconut palm. Leaf analysis is used most commonly as material of a similar stage of maturity, such as from the 14th frond, which can be used as a standard source for sample collection. Table 31 summarizes data on critical levels of important nutrients (de Taffin, 1993; Manciot et al., 1979a,b). The prescription of fertilizer application based on either nutrient diagnosis or fertilizer experiments is an exercise in economics depending on the relativity of input costs to the increased market return from increased yield. Coconut palm is rather slow to respond to fertilizer application. But, its response is relatively quick (in about 6 months from the date of application) to K and Cl, which is reflected in increased kernel per nut

282

K. P. Prabhakaran Nair

Table 31 Critical values for concentration of mineral elements in the leaf tissue (14th, the youngest frond) of adult tall type Major nutrients

Percentage dry matter

N P K Mg Ca Na Cl S

1.8–2.0 0.12 0.8–1.0 0.20–0.24 0.30–0.40 Not essential 0.5–0.6 0.15–0.20

In Tall  Dwarf hybrid—2.2

Trace elements B Mn

Parts per million (ppm) 10 >30

Remarks

Fe Cu Mo Zn Al

50 5–7 0.15 20 >38

Remarks

In Tall  Dwarf hybrid—1.4 Strong inverse sensitivity to extremes of K Some inverse sensitivity to extremes of K Substitutes for K in case of deficiency

Difficult to fix values as very interactive with Fe in strongly alkaline soil; potentially toxic in extreme acid soils Deficient only in strongly alkaline soils Deficiency very rare, uncertain Common value, no response observed, yet Common value, no response observed, yet Non essential element, but, always present; potentially toxic at values well in excess of this common level

Source: de Taffin (1993) and Manciot et al. (1979a,b, 1980).

(Foale, 1965; Manciot et al., 1979b). Clearly, early detection of limiting nutrients is vital to achieving the most economical yield potential in the local environment.

27. Coconut-Based Mixed Cropping Systems and Their Management Coconut plantations are, invariably, dual cropping systems, where in the field with coconut as main crop, a number of others, such as diverse fruit trees, pineapple, cacao, coffee, root crops, banana, pastures, and several others, are also grown (Nair, 1982). The mixed cropping systems help capture better the solar radiation. For reasons outlined in the section on

Agronomy and Economy of Important Industrial Crops

283

coconut botany, the coconut canopy follows an evolutionary trend in its capacity to intercept light. In a healthy crop, with a plant density of 180 palms/ha, light interception ranges from low values at pre-flowering stage to around 90% at 10 years of age, continuing up to 25 years of age (D. Friend, unpublished data), and then gradually declining to around 45% at 50 years of age. The brief pre-production phase is often quite productive for short-term crops, especially if the land has been newly cleared, providing plentiful supply of nutrients released from the great bulk of residue generated by the destruction of the previous vegetative cover. In some cases coconut density may be lowered, either uniformly or by ‘‘hedgegrow’’ planting, to allow continuous intercropping, but the potential yield of the coconut stand will be proportionately reduced. Intercrops under old and so-called ‘‘declining’’ coconut plantations frequently become more productive with any increase in the amount of solar energy which bypasses the coconut canopy.

28. Seed and Seedling Management Both challenges and opportunities are provided by the big coconut seed. There has been much interest in seedling vigor, since recognition in India and Sri Lanka of its correlation with yield potential of the adult palm (Liyanage and Abeywardena, 1958). One challenge is recognizing relative seedling vigor is to provide a clearly defined starting point for a batch of newly germinated seeds. Some populations, particularly in Southeast Asia and the Pacific, have seeds that germinate very quickly after maturity, some tending to sprout well before the fruit falls naturally. On the other hand, most other populations have a brief dormancy after maturity before being ready to germinate when the dry husk is moistened. If a batch of the latter comprises fruit that has been harvested at a comparable stage of maturity, a firm starting point is achieved. On the other hand, the fast germinating seeds may contain a mix of maturity when harvested, with some embryos already emerged from the germ pore, but remaining hidden for many weeks within the husk. Such seeds are subject to the risk of misorientation in the seed bed when the still-hidden shoot is pointed downward. This can lead to protracted delay or even fatal entrapment as the shoot changes its direction of growth upward once more.

28.1. Germination rate The time lag between expansion of the embryo through the germ pore, the index of true germination, to sprouting (emergence of the sprout from the husk), depends on both the vigor of the seedling and on the thickness of the husk. As these traits are quite variable, in a Tall population, the time

284

K. P. Prabhakaran Nair

lapse could be 6–10 weeks (Foale, unpublished data). A reasonable comparison of the vigor, at least within small batches of seedlings, can be achieved by taking all the nuts that sprout within 1 week, for example, and keeping them together for a within-the group comparison in the nursery. Checks done at time intervals thereafter will allow the seedlings to be sorted into fast, medium, and slow growers on the basis of leaf length and area, robustness of the collar and general appearance. Another potential influence on the growth rate of the seedling is the amount of kernel present within the nut. Most of the seedlings continue to draw energy from the kernel for about 12 months and, interestingly, smaller nuts within a population support a slightly higher early growth rate (Foale, 1968b). The haustorium, obviously, makes more rapid and effective contact with the kernel when the vacuole is smaller. As both genetic and environmental factors influence the amount of kernel in the nut, it would appear that the response of the seedling growth rate to nut size would introduce a small bias into early seedling selection.

28.2. Polybag seedlings The ability of the seedlings to recover from ‘‘bare root planting,’’ aided as it is, by the energy supply from the kernel, has resulted in that method being still in use in many traditional coconut cultures. However, raising seedlings in polybags (made of polyethylene, of size 40 cm, height and 25 cm radius, for 8-month planting out, while larger ones for older ones) has become common. While transplanting no damage occurs to the root system and the growth continues unchecked (Foale, 1968a). Widespread use is made of polybag seedlings in many areas, in spite of greater logistic challenge of carrying into the field the seedling in the polybag, weighing 20–25 kg. Comparative health and vigor can be checked when the seedling is 8 months old, but the polybag can hold the seedling up to 12 months. A 50% increase in the size of the polybag used would be advisable in this case, thereby saving a few months of more expensive field maintenance. It is important, however, to spread older seedlings further apart in the nursery to avoid slowing the growth rate due to mutual shading.

28.3. Seedling selection It is best to select the 50–60% of the vigorous seedlings from an openpollinated population, but one would expect to discard very few seedlings from a batch of hybrid seeds. Where a high proportion of seedlings is to be discarded, this must be done quite early, so that selected seedlings can be transferred from an open nursery bed to polythene bags without a severe setback. On the basis of correlation studies, for 1-year-old seedlings, selection criteria in India include the collar girth (10–12 cm), number of leaves

Agronomy and Economy of Important Industrial Crops

285

(6–8), and early splitting of leaves. Nuts which do not germinate, within 6 months of sowing, should be removed from the nursery (Iyer and Damodaran, 1994). To guard against insect pests, rigorous protection of seedlings in the nursery is required employing appropriate chemicals. The genotype chosen determines the plant density, with Talls at 160–180 ha 1 and hybrids at 180–220 ha 1 for sole coconut plantation. While intercropping, different densities and palm arrangements are chosen other than isometric (triangular) planting.

29. Field Management The early management is very important in a seasonally dry environment where it is common to plant the seedlings with the nut slightly below the soil surface. On Atolls, where the water table is 1 m or lower than at the surface, sometimes the seedlings are placed quite deep in order that the roots reach the most soil zone quickly. Weed must be kept in check in the initial stages to preempt competition for water and nutrients. A common practice is to establish a weed-free circle by manual weeding or herbicide application and subsequently enlarging the spread of the circle as the palm grows. A few South Pacific Talls (Rennell Tall, e.g.) flower within 4 years, and their hybrids with Red Malayan Dwarf flower almost a year earlier, especially in the case of polybag seedlings. In Africa and in India, the populations must endure the summer water deficit. Talls tend to flower after about 7 years, but the Tall  Dwarf hybrids flower 2 years earlier.

30. Productive Palms The nature of the end product determines once palms start bearing. Where high human population density occurs, every part of the palm is processed to make value-added products, like, coconut milk, cream, milk powder, desiccated coconut, oil and so on, coconut drink (fresh or bottled), fuel, charcoal, fiber, and coco peat from coir dust, in addition to the number of utensils and also curios made out of coconut shell. On the other hand, in the case of large plantations, the only end product is the oil via the copra pathway. Increasingly, the oil is separated for export from the country of production, and the residual cake is either exported or fed to livestock locally. Control of pests and diseases are done, where a fatal attack from these is expected. For example, phosphoric acid treatment can arrest the incidence of Phytophthora bud rot, and oxytetracycline is used to protect against the lethal yellowing. Bourgoing (1991) published the recommendations to

286

K. P. Prabhakaran Nair

control insect pests, including the hygienic practices, like removing old coconut logs, where pests harbor and breed.

31. Adaptation to Biotic Factors Throughout thee evolution of the coconut palm, it has been exposed to diverse insect and disease pests and this is particularly so in the case of those populations established next to diverse rainforests on the coastline of large islands or continental landmasses. Establishment of the palms along such coasts, such as on the Australian coast, might not have been always successful. An indigenous rat species, the white-tailed rat, Uromyces caudimaculatus, is capable of chewing through the husk and shell to feed on the kernel. In the past millennia, probably in a period of about 60,000 years, colonizing hunter–gatherer human populations would have contributed to failure by the coconut as they assiduously collected for food, any seed or seedlings on the beach. The tribal language of the Cook-town and Lockhart River regions in the far northeast coast of Australia contains separate words for a pail nut (on the beach) and a nut with an haustorium and emerged seedling (Tucker, 1983; Diana Wood, personal communication). The examples mentioned in this section add to those mentioned earlier in this chapter of adaptation in the form of tolerance or resistance to attack or invasion by insects and microorganisms. A range of types of organisms is presented below to convey the apparent broad attractiveness of coconut tissues and organs as food or habitat.

32. The Range of Coconut of Pests Association of coconut with other biota, some rapidly fatal as the infestation with the palm weevil, Rhynchophorus sp., which lives and multiplies within the upper, softer part of the trunk and growing part, consuming soft tissues until the palm perishes. Similarly, the red ring nematode, Rhadinaphelencus cocophilus, enters the phloem of the coconut trunk, causing slow death of the palm caused by clogging the internal tissue. The Polynesisan rat, Rattus exulans, a native of Indochina, is widespread on many coconut islands, feeding on immature nuts that fall, once damaged. In the Western Pacific, a highly specialized coconut crab, Birgus latro, may have evolved along with the coconut. It possesses a very powerful claw capable of tearing off the husk and smashing the shell to feed on the kernel. The termite also attacks the trunk of the palm consuming tissue inward from beneath the outer layer and eventually proving to be fatal to the palm. Among the microorganism pests, causing fatal diseases, there is a range,

Agronomy and Economy of Important Industrial Crops

287

starting from the heart rot caused by trypanasome (Dollet, 1999), through fungal Phytophthora (bud rot, Dollet, 1999), phytoplasma causing lethal yellowing and many related variants (Harrison et al., 1999), and virus (foliar decay, Randles et al., 1999) to a viroid (cadang–cadang, Rodriguez, 1999).

32.1. Insect pests Evidence exists that some populations have adapted and developed resistance to insect attack. The different intensity of the attack of the leaf beetle Bronstispa sp. was discussed earlier in this chapter. This pest is endemic to PNG, Solomon Islands and Vanuatu, and is currently spreading to Northern Australia. Any exotic genotype brought to the first of the three countries mentioned above requires concerted protection, at least during the first 2–3 years in the field after planting. Local populations, on the other hand, while needing protection in the intense environment of the nursery, are rarely attacked in the field. The Papua New Guinea Rhinoceros beetle Scapanes australis also shows a preference for exotic germplasms, but is still inclined to inflict serious damage on indigenous populations, especially when no choice is available. No record exists of resistance to many other insect pests, such Oryctes rhinoceros, which has an almost global distribution, and various locusts and stick insects. Both species of Rhinoceros beetle often disfigure the coconut canopy without fatal results, but their damage commonly opens the door to Rhyncophorus sp., which infests with fatal consequences. There are many other insect pests of coconut, such as white fly, mite, scale insects, locust, stick insect, leaf miner, and hemipterous nut-fall bugs, for which no observed adaptation has been observed.

32.2. Disease pathogens When the coconut palm adapts to a disease, it normally takes the form of tolerance, but the practical situation is quite often hard to evaluate. The Plant Pathologist starts his investigation first by concentrating on the question why the palm is not doing well, often identified by foliage discoloration or inflorescence distortion (Rodriguez, 1999). By the time such symptoms are visible the palm would have been heavily infected by the pathogen, which can be identified positively using appropriate microscopy, and also molecular probing, and the investigation rests there. Other palms in the population might also have been infected, but it might be quite possible that the symptoms are either not showing or the palm has the capacity to withstand the pathogenic onslaught and keep its spread in check, or, the infection is of a recent origin, and so do not show any classical symptoms. When there is actual tolerance to the infection, some loss of vigor of the palm is observed. If such a situation is observed, it is quite possible the effect is environmental, such as the limiting effect of a specific plant nutrient.

288

K. P. Prabhakaran Nair

Molecular tools for the detection of subclinical pathogens would be useful to dispel any uncertainty in cases of non-lethal or even nonexistent reaction to an invading organism (Vandermeer and Andow, 1986). Tolerant individuals are identified, whereas previously, all survivors of disease onslaught would have been lumped together as ‘‘escapes.’’

33. Adaptation in Coconut Palm The question of adaptation in coconut palm is only beginning to be understood scientifically. With the availability of affordable molecular tools, which are widely applied, the uncertainties surrounding disease infestation can be better tackled. At present, the survival of some members of a population otherwise decimated by a phytoplasma-induced disorder or by bud rot, for example, gives rise to speculation. Do the survivors possess tolerance or resistance? Are they simply lucky or robust enough due to favorable environmental conditions which impart some kind of vigorrelated resistance? The instance of phytoplasma-induced lethal yellowing in the Caribbean, Florida, and Central America illustrates some of the above-mentioned issues quite well. In the Red Malayan Dwarf and Panama Tall, which are widely cultivated in Jamaica and elsewhere, there is known and widely exploited tolerance/resistance to Phytoplasma. This is a quantitative trait with high heritability (Ashburner and Been, 1997). Some of the other genotypes also show tolerance, though usually less than either of the two discussed earlier (Zizumbo et al., 1999). However, there are recent reports that the hybrid has succumbed to the disease in some environments, and it is now known that there are many other phytoplasma-induced lethal diseases worldwide. The organism has been shown, through molecular probing, to have a significantly different form in each of those other locations around the globe. In Southeast Asia phytoplasma infected palms have been found to be free of symptoms. Red Malayan Dwarf originates here, which raises the important question, whether this genotype acquired resistance to phytoplasma through natural selection during the evolutionary history of the coconut palm. It then appears that in countries where the coconut palm has grown for about 3000 years, India can be considered its heartland, because of the palm’s adaptation to this major scourge. The adaptation of the organism itself is part of the problem, because, as is common with so many other pathogens, the pathogen has undergone change through mutation and selection. The convergence of adapting host and adapting pathogen may be clarified in the foreseeable future, however, as molecular tools are brought to bear. Such tools would be used to identify markers associated with tolerance on the coconut side and pathogenicity on the side of the pathogen (Cardena et al., 1999).

Agronomy and Economy of Important Industrial Crops

289

However, to achieve this, an understanding of the genetics of resistance and/or tolerance is required.

34. A Devastating Virus The coconut cadang–cadang viroid (CCCVd) of Northern Philippines and a related form in Guam is a very serious pathogen and is relatively less investigated compared to the phytoplasma scourge. This is due to its limited distribution and slow rate of spread. It has proved to be far more recalcitrant during the stage of detection and proof of its pathogenicity because of the molecular minuteness of the causal organism as it is the smallest infectious particle known to biological science, the absence of any identified transmitted agent, and the existence of nonpathogenic, near analogues in coconut populations worldwide (Hodgson and Randles, 1999; Rodriguez, 1999). There is but little or no evidence of tolerance to CCCVd. Hence, attention is focused on early detection of infected palms using molecular probes, and elimination of these palms in the hope that viroid-free zones might be created in the affected regions.

35. Adapting the Coconut to Market Needs Though coconut is a graceful ‘‘icon’’ that beautifies the environment, human interest in the nut centers on the market needs. The amenity added to the environment by the coconut palm enhances its value in the tourist market or the real estate market. The nut is so very highly valued in some places that a safe form is sought by busy tourist centers, which do not produce large and dangerous fruits that require extensive pruning. As the fruitless coconut is diametrically opposite to the fruitful adaptation sought by all the coconut farmers of the world, its development is not likely to be included soon in the program of coconut industry research institutes.

36. Yield Potential of the Coconut Palm The thrust of genetic research in coconut concentrates on high yield and oil content. Related to increased yield expressed in this way is also yield of edible products and coconut water, shell, and fiber. Hence, increased yield covers most of the market interest in bringing about genetic change. Associated characters that have been sought within the broad thrust for increased are the following:

290

K. P. Prabhakaran Nair

Earliness of the onset of fruiting (precocity) Large nuts to facilitate processing by hand Uniformity of nut dimensions to facilitate mechanization of processing Higher oil content, which is often associated with enhanced flavor Ease of harvest, as in the case of Dwarfs for production of nuts for drinking water f. Special aroma or taste characters, for example, edible husk and perfumed water g. Makapuno, having jellied endosperm for edible purposes and so on a. b. c. d. e.

The flavor and quantity of toddy or ‘‘coconut nectar’’ tapped from the inflorescence are also potential breeding objectives, not presently identified.

37. Quality Traits Within specific markets are often specific quality traits which add value to the product. For example, variations exist in quality and length of fiber for use in the vast array of products derived from coir. There is also interest in possible variants in the mix of fatty acids in coconut oil. There has been an ‘‘anti-coconut lobby,’’ especially the saturated oils lobby like that of soybean, which has brought about a ‘‘coconut scare,’’ implying the adverse effect of saturated fats (cholesterol) in coconut oil. This lobby created the label ‘‘artery-clogging saturated oils’’ in coconut and specifically targeted coconut oil, which enjoyed a global market. This was based on some flawed research in which a deficiency of essential fatty acids in a saturated fat oil diet which was supposed to have driven up the blood cholesterol content. However, recent research underlines three great dietary strengths of coconut oil, as it provides a readily digestible source of energy, critical cell wall lipids, and the precursors to monolaurin and monocarpin which have outstanding antimicrobial and anti-viral properties (Enig, 1999, 2000). About 50% of the fatty acid component of coconut oil is lauric acid, which has high market value in the manufacture of detergents and other industrial chemicals. Surprisingly, the huge increase during recent decades in the supply of lauric oil from palm kernels and from genetically modified canola seems to have stabilized this high demand, working to the advantage of coconut oil.

37.1. The fatty acid mix Some variability exists in the relative mix of the six main fatty acid components of coconut oil, but the range is fairly narrow for each one. If a particularly good market for one component were to be found, it might be met by increased overall production rather than seeking increased yield of that component by genetic modification.

Agronomy and Economy of Important Industrial Crops

291

38. Coconut as a Food Item Coconut as a food item has immense possibilities in both domestic markets of the country where it grows and also in the international market. Canned or tetra-packed coconut water of different grades, including tender and intermediate stages up to matured water, and different degrees of dilution and sugar addition, has received wide acceptability, including fresh water from the recently harvested fruit. In China coconut water is labeled ‘‘official drink of the Chinese State banquet’’ and is widely available in the coastal cities. Demand for coconut milk and cream also will continue to rise as its flavor and nutritional merits become more widely known outside the centers of production. Canned coconut nectar is an attractive sweet drink increasingly consumed in Asia. A need exists for concerted argument and tangible efforts against the marketplace opponents of coconut oil products, who seek to capture the market share by dubious means, where making negative assertions about its nutritional qualities is a popular route. The attempt of the soybean oil lobby is one such attempt to derail the coconut oil from the world market, as discussed earlier in this section, about the ‘‘cholesterol scare.’’ A lot of recent research (Enig, 2000) has shown the outstanding quality of coconut oil even in suppressing activity of some viruses. Such additional benefits must be made known to the common man, who might otherwise vacillate about the choice of coconut oil as a cooking medium, and also coconut as a healthy food. In this connection the experience of Kerala State in India needs particular mention. Over centuries, the favorite cooking oil of the people of Kerala has been coconut oil until the early eighties, when the soybean oil lobby began to drum up the ‘‘cholesterol scare’’ and gradually the people of the State began to move away from the use of coconut oil for culinary purposes. A concerted effort by the Coconut Board of India, situated in the State, to dispel the wrong notions about the ‘‘safety’’ of coconut oil is beginning to show dividends and people, who turned skeptical are now coming back to the use of the coconut oil. This is a positive sign. But, more and determined efforts need to be taken on a global scale to win back the lost confidence. It would be a slow process, but worth pursuing. From the point of view of breeding, all efforts are directed to the overall aim of raising the yield level, and any change in quality traits is purely fortuitous.

39. Research and Development in Coconut Production Research and development (R&D) in coconut production focuses mainly on farmers’ interests. There are many regional institutions, which by themselves singly, cannot advance much on R&D, but together can

292

K. P. Prabhakaran Nair

accomplish much. This is particularly true of the Pacific and the Caribbean. In addition, some institutions exist with a global view of coconut improvement, such as the IPGRI, a United Nations agency, and its specialized offshoot the Malaysia-based Coconut Genetic Resources Institute (COGENT). The French research agency, CIRAD, also takes a very broad view of the needs of coconut research globally. Funding bodies have been formed in some donor countries from which financial aid is directed to support coconut research. For instance, in Europe, the Bureau for the development of Research on Tropical perennial Oil Crops (BUROTROP) is an international non-profit organization established in 1989 with such an aim. Its mandate is to assist, strengthen, and further develop research on coconut and oil palm. It helps to transfer research results to the production sector which will benefit the small-scale farmer in the form of improved self-sufficiency and capacity to produce cash crops for local or regional consumption. The BUROTROP Board of Administrators consists of seven representatives from regional organizations and producer countries in Asia, Africa, Latin America and the Pacific, and seven others from European donor countries and international agencies it operates. The financial support comes from the Directorate General (DG) of Research of the European Commission.

40. Global Coordination COGENT has a very specific mandate to facilitate the description and exchange of coconut germplasms, and its use in generating populations with improved performance around the world. COGENT has linkages with a high proportion of all the coconut-growing countries around the world and promotes standardization of morphological descriptors for coconut as well as providing training in germplasm collection, breeding strategy, and technology. It has obtained funding to support regional collections of coconut germplasm in Brazil, representing Latin America, Ivory Coast, representing the African continent, Kerala State, representing the Indian subcontinent, and PNG representing the South Pacific.

41. National Research Centers Several research centers, which have carried out outstanding research in coconut, are scattered around the world, notably in India, since 1916, which is now the CPCRI in Kasaragod in Kerala State. The CPCRI has concentrated mainly on genetic improvement, focusing on drought tolerance and also the devastating root wilt, now known as Kerala coconut

Agronomy and Economy of Important Industrial Crops

293

decline, which has been a scourge since decades, and appropriate soil management.

42. Research in India and Sri Lanka In Sri Lanka, the Coconut Research Institute was established in 1929. The institute carried out outstanding work on factors controlling coconut yield, and grappled with the challenges of breeding to improve a highly heterozygous population. In India and Sri Lanka, much effort was and still remains, directed toward multiple cropping associated with coconut, having achieved significant progress over the decades. Coconut production was beset by many problems in many other countries early in the twentieth century, and among these the coconut decline in the Caribbean, and insect attack on almost every part of the palm in diverse populations around the world are the significant ones. But, attempts to control these problems were short lived. The most recent example was in Kerala, with regard to a mite attack, which became known as the ‘‘Mandari’’ disease. The symptoms are shriveled nuts with black streaks and the nuts fetch only a poor price. The whole state has been a victim of this scourge and no effective control was devised. In fact, there is a comparison between this and the coconut decline. Replacement of the affected palms was the only remedy. In what was then known as the Federated Malay States, higher yield was sought through replacement of Tall varieties with Dwarf ones. The generation of a Dwarf hybrid Fiji helped matters.

43. Research in the Second Half of Twentieth Century As competition from other oilseed crops hotted up from the 1950 onward, interest in coconut improvement began to spread across the countries in the world where the crop is widely grown. The Philippine Coconut Authority (PCA) was formed to oversee research and has an outstanding track record of improving coconut technology and collaboration with other agencies especially in the development of high yielding hybrids. For instance, France, through CIRAD (mentioned earlier in this section, formerly Institute for Research for Oils and Oil Seeds, IRHO), set up a major facility in Ivory Coast on the African continent early in 1950s, which was later extended to Vanuatu, French Polynesis, Cambodia, and Brazil. The British did likewise in Jamaica, as did Australia in PNG. All of this happened in the 1950s. CIRAD continues to play a major role in coconut research with high technology aided research in Montpellier,

294

K. P. Prabhakaran Nair

France, and the staff is seconded on long-term programs in Ivory Coast and Vanuatu, and on short-term programs to other countries where the coconut palm is grown. Unilever, a major private investor, initiated research in 1951 in Solomon Islands, which was subsequently supported by the colonial government, which was later closed down in the 1970s, after releasing a very promising hybrid which was used to replant the island’s entire plantation. GTZ of Germany, the outreach funding agency of Germany has been active in supporting research in coconut, especially in Tanzania. Other Institutes which are playing significant role are Wye College in United Kingdom, pioneers in tissue culture, University of Adelaide, which provides outstanding expertise in viroid and virus research, Max-Planck-Institut in Koln, Germany, which is contributing significantly to molecular expertise and the Center for Scientific Investigation at Yucatan (CICY), in Mexico, which is a leader in research in phytoplasma related coconut diseases.

44. A Peep into the Future of Coconut Coconut, the ‘‘tree of life,’’ holds out much promise to humanity (Persely, 1992). This tree of the people was hijacked by corporate business to meet a desperate shortage in vegetable oil which began in the 1850s which spilled over a century to the 1950s. Other oilseeds were established closer to the foreign markets in mid-twentieth century, and demand for coconut began to decline. Quite ruthlessly, competing oil producers, what many term as an entrenched ‘‘oil lobby,’’ took early, and more often than not, flawed, research into health effects of different vegetable oils, and interpreted them to the detriment of coconut. The ‘‘cholesterol scare’’ is a real case in point. Without a matching promotional effort by the coconut industry to provide balance in the information reaching the consumers, some of its best global markets simply vanished, both in culinary and food sectors. What happened in Kerala State, India, the home of the coconut, in the culinary sector, illustrates this point eloquently. It is great relief for the coconut industry that the superior properties of lauric acid in both soap and detergent manufacture, provided some stabilizing influence on global trade in coconut in recent years. Even here, lauric acid from coconut has found its competitor in the lauric acid from transgenic canola. As a health food, there are many properties related to coconut oil which have been documented by Enig (1900, 2000). But, generating adequate resources to gain broad community recognition of the true value of coconut in the diet may require a prolonged campaign. In a comprehensive review titled ‘‘Coconut: In support of good health in the twenty first century,’’ Enig (1999) reported some outstanding findings with very important long-term implications. The following briefly touches on this point.

Agronomy and Economy of Important Industrial Crops

295

Monolaurin (formed in the body from lauric acid in coconut oil) is the antiviral, anti-bacterial, anti-protozoal monoglyceride used by the human or animal to destroy lipid-coated viruses, such as HIV, herpes, cytomegalovirus, influenza, various bacteria, including Listeria monocytogenes and Helicobacter pylori, and protozoa such as, Giardia lamblia. Many other investigations cited by Enig (1999, 2000) directly contradict the questionable interpretation of other findings from projects funded by rival vegetable oil industries. These provide the hope that coconut will be restored to its rightful place of pride as a valuable food for all. The challenge that faces the coconut industry now is to adequately educate the consumer, rather than spending funds to further research, because that will help recover the lost ground.

45. Protection of the Production Base Once the future of the coconut market is assured, more attention can be diverted to issues concerning production and processing. As earlier described in this chapter, coconut management is well understood and much progress has been made in adaptation of the crop to different environments, especially those with periodic water deficit. The use of molecular markers now offers the possibility of dealing with many disease entities of coconut with increasing confidence and scientific precision. The phytoplasma group of pathogens has been a serious scourge whose geographical span was not realized well until the new molecular technology became available. Global testing is still incomplete, hence, distribution map for phytoplasma is incomplete. Using molecular techniques, phytoplasma affected palms can be identified before the onset of symptoms, which will allow early clearing out of infected palms and molecular markers linked to resistance or tolerance can be used to identify desirable individuals and populations. Much remains to be done to perfect techniques and reduce costs, and the capacity of an entity such as the phytoplasma to evolve into more virulent forms cautions science to sustain its vigilance. Other diseases need to be added to the list for molecular testing, but the outlook is more hopeful on this front than it has ever been. Phytophthora is another widespread disease, called bud rot, where there are diverse strains that might better be dealt with using the same molecular technology, but little is being done about that at present. There are several other serious diseases that are more localized than phytoplasma caused ones or bud rot, where the use of molecular techniques holds out much promise. The list includes CCCVd in Philippines and its variant in Guam, the viral foliar decay of Vanuatu, and the trypnasome heart rot in the Caribbean (Dollet, 1999).

296

K. P. Prabhakaran Nair

46. Advances in Processing Technology There is enormous potential on the processing technology front along the copra pathway where coconut oil extraction is concerned. On a large scale, copra has yielded an oil requiring costly refining, bleaching, and deodorizing to be marketable. There is an exceptional opportunity to extract oil from shredded kernel dried only to 12%. The avoidance of high temperature and pressure provides a far more attractive aromatic oil for food and cosmetic uses (Etherington and Mahendrarajah, 1998; Etherington et al., 1999). In general, manufacturers accept food standards in the production of coconut milk, cream and desiccated products. The Asian Pacific Cocont Community (APCC, 1997) has published codes and standards for aqueous coconut products, which will assist in standardizing the range of products and raising consumer confidence. Concerted promotion of such standards by industry leaders is required to achieve widespread adoption. When quality is maintained, the range of coconut-based products, such as high value coconut nectar, alcoholic and non-alcoholic drinks (derived from toddy), fiber, shell, mats, wood derivatives, food products, such as coconut chips, ornamental products made out of coconut shell, and so on, will have a good market domestically in the country of production and also when exported. It would be the responsibility of science to help grow and protect the palm in environments where it currently thrives. This will enable the farmers and the industry to join hands in sustaining the nut in the global market. Coconut, indeed, is a major component of the vegetation of the tropics and semi-tropics. This is the why its future is bright.

47. Contact Information for Research Centers and Institutes The following list contains the contact information of research centers and institutes across the world and would be useful for those engaged in coconut research and development Bangladesh Senior Scientific Officer, Horticultural Research Center, Pomlogy Division, Bangladesh Agricultural Research Institute (BARI), GPO Box 2235, Joydepur Gazipur—1701, Bangladesh, Tel: 880-2-9800441/ 9332340, Fax: 880-2-841678, email: [email protected] Benin Directeur, Institut National Des Reserches Agricoles Du Benin (INRAB), Station de Recherche´ sur le Cocotier, BP Cotonou, Benin, Tel: 229-240101, Fax: 229-250266, 225-20 226985/21 248872

Agronomy and Economy of Important Industrial Crops

297

Brazil Head of Coconut Germplasm Bank, EMBRAPA/CPATC, Av. BeiraMar, 3250, CEP 49025-040, Aracaju-SE Brazil, Tel: 55-79-2171300, Fax: 55-79-2319145, email: [email protected] China Director, Wenchang Coconut Research Institute, Chinese Academy of Tropical Agriculture Science, Wenchang Erli, Wenchang City, 571321 Hainan Province, China, Tel: 86-898-3222416/3224820, Fax: 86-8983230949/3223918, email: [email protected] Cook Island Ministry of Agriculture, Government of the Cook Islands, PO Box 96 Rarotonga, Cook Islands, Tel: 682-28711, Fax: 682-21881, email: [email protected] Costa Rica Director, Coconut Research, Ministerio de Agricultura y Ganaderia Siquirres, Frente, Sevicentro Siquirres—Limon, Costa Rica, Tel: 506-718 60 92, Fax: 506-718-7191/768-8410 Cote d’Ivoire Directeur, Station de Recherche´ Marc Delorme, Centre National De Recherche´ Agronomique (CNRA), Port Bouet, 07 PO Box 13, Abidjan 07, Cote d’Ivoire, Tel: 225-21 248872/248067 Cuba Deputy Director, Ministerio de La Agricultura, Instituto de Investigaciones de Citricos y Otros Frutales (IICF), Ave. 7ma No: 3005 entre, 30 y 32, Miramar, Playa, Havana 10600, Cuba, Tel: 537-293585/225526/ 246794, Fax: 537-246794/537-335217 (National Citrus Corporation), email: [email protected] Fiji Principal Agronomist, Ministry of Agriculture, Fisheries and Forests, Private Mail Bag, Raiwaqa, Suva, Fiji, Tel: 679-477044 ext.263, Fax: 679-400262, email: [email protected] Ghana Director, Oil Palm Research Institute, PO Box 74, Kade, Ghana, Tel: 031-804-710229 (Director)/710226/710228, Fax: 031-233-46357, email: [email protected] Guyana Program Leader (Roots and Tubers), National Agriculture Research Institute (NARI), Mon Repos, East Coast Demerara, Guyana, Tel: 592202841/42/43, Fax: 592-204481, email: [email protected] Haiti Coordinator, Ministry of Agriculture, Centre de Recherche´ et de Documentation Agricoles (CRDA), Damien, Republique d’Haiti, Tel: 509 (22) 4503, Fax: 509(45) 4034

298

K. P. Prabhakaran Nair

India Director, Central Plantation Crops Research Institute (CPCRI), Indian Council of Agricultural Research, Kasaragod 671 124, Kerala State, India, Tel: 91-4994-430333, Fax: 91-499-430322, email: [email protected] Indonesia Director, Department of Forestry and Estate Crops, Agency for Forestry and Estate Crops Research and Development, Research Institute for Coconut and Palmae, PO Box 1004, Manado 95001, Indonesia, Tel: 62-431812430, Fax: 62-431-812587, email: [email protected] Jamaica Director of Research, Coconut Industry Board, 18 Waterloo Road, PO Box 204, Kingston 10, Jamaica, Tel: 1-876-9261770, Fax: 1-876-9681360, email: [email protected]/[email protected] Kenya Regional Research Centre (RRC), Mtwapa Box 16 Mtwapa, Kenya, Tel: 254-11 485842/39, Fax: 254-11 486207 Kiribati Chief Agriculture Officer, Division of Agriculture, Ministry of Natural Resources and Development, PO Box 267, Bikenibeu, Tarawa, Kiribati, Tel: 686-28-139/108, Fax: 686-28-139/21-120 Malaysia Station manager, Stesen MARDI Kemaman, Batu 11, Jalan Air Putih PO Box 44, 24007, Kemaman Terengganu, Malaysia, Tel: 60 09-8646361/ 148, Fax: 60 09-8646361, email: [email protected] Marshall Island Chief of Agriculture, Ministry of Resources and Development, Agriculture Division, PO Box 1727, Majuro, Marshall Islands, Tel: 692-6253206/0740, Fax: 692-625-3005, email: [email protected] Mexico Researcher, Centro de Investigacion Cientifica de Yucatan, A.C. (CICY), KM7 Antigua Carretera A Progresso, Ex-Hacienda, Xcumpich, Apartado Postal 87, 97310 Cordemex, Merida, Yucatan, Mexico, Tel: 5299-813923/813966, Fax: 52-99-813900/813941, email: [email protected] Mozambique Research Assistant—Entomology, National Agriculture Research Istitute (INIA), Entomologist Crop Protection Section, Box 3658, AV. Das FPLM, INIA-MAPUTO, Mozambique, Tel: 258(1)- 460097, Fax: 258(1) 460074, email: [email protected] Myanmar Acting Director General, Department of Agriculture, Planing Ministry of Agriculture, Thiri Mingalar Lane, Off Kaba Aye Pagoda Road, Yankin PO, Yangon, Myanmar, Tel: 095-1-665750, Fax: 095-1-663984/651184, email: [email protected]

Agronomy and Economy of Important Industrial Crops

299

Nigeria Chief Research Officer, Nigerian Institute for Oil Palm Research (NIFOR), PMB 1030 Benin City, Nigeria, Tel: 52-440130, Fax: 52248549 Pakistan Director Horticulture, Pakistan Agricultural Research Council, Plot No 20, G-5/1, Post Box 1031, 45500 Islamabad, Pakistan, Tel: 92-51-9207402, Fax: 92-51-920-2968/920-240908, email: [email protected]. undp.org/[email protected] Philippines Deputy Administrator, Agricultural Research and Development Branch, Philippine Coconut Authority, Don Mariano, Marcos Avenue, Diliman, Quezon City, Philippines, Tel: 632-920-0415/632-426-1398, Fax: 632-920-0415, email: [email protected] Papua New Guinea Acting Director, PNG Cocoa and Coconut Research Institute, PO Box 1846, Rabaul, East New Britain Province, Rabaul, East New Britain, PNG, Tel: 675 983-9108/983-9131/983-9185, Fax: 675 983-9115, email: [email protected] Samoa Acting Assistant Director of Research, Ministry of Agriculture, Forests, Fisheries and Meteorology, Research Division, PO Box 1587, Apia, Samoa, Tel: 685 23416/20605, Fax: 685 23426/20607/23996, email: [email protected] Seychelles Director General, Crop Development and Promotion Division, Ministry of Agriculture and Marine Resources, Grand Anse, PO Box 166, Victoria Mahe, Seychelles, Tel; 248-378252/378312, Fax: 248-225425, email: [email protected] Solomon Islands Director of Research for Permanent Secretary, Dodo Creek Research Station, Ministry of Agriculture and Fisheries, PO Box G 13, Honiara, Solomon Islands, Tel: 677-31111/31191/31037, Fax: 677-31039/21955/ 31037, email: [email protected] Sri Lanka Director, Coconut Research Institute, Bandirippuwa Estate, Lunuwila, Sri Lanka, Tel No: 94-31-57391/253795/55300, Fax: 94-31-57391, email: [email protected]/[email protected] Tanzania Director, Mikocheni Agricultural Research Institute (MARI), Ministry of Agriculture and Co-operatives, PO Box 6226, Dar es Salaam, Tanzania,

300

K. P. Prabhakaran Nair

Tel: ++255 51-700552 or 74606, Fax: ++255 51-75549 or 116504, Mobile: ++255-812-784031, email: [email protected] Thailand Director, Horticulture Research Institute, Department of Agriculture, Chatuchak, Bangkok 10900 Thailand, Tel: +66(2) 579-0583/579-0508/ 561-4666, Fax: 662-561-4667, email: [email protected] Tonga Head of Research, Ministry of Agriculture and Forestry, Vainani Research Division, Nuku’alofa, Kingdom of Tonga, Tel: 676-23038, Fax: 676-32132/24271/23093 Trinidad and Tobago Ministry of Agriculture, Central Experimentation Centend V 117, Arima PO 52 La Florissante Garden, Dabadie, Trinidad and Tabago, Tel: 1-809-642-8552/642-0718, Fax: 1-809-622-4246 Tuvalu Acting Director of Agriculture, Ministry of National Resources and Environment, Department of Agriculture, Private Mail Bag, Vaiaku, Funafuti, Atoll, Tuvalu, Tel: 688 20-825 or 186, Fax: 688 20-826 Vanuatu Head of Coconut Division, Vanuatu Agricultural Research Centre, PO Box 231, Espiritu Santo, Vanuatu, Tel: 678 36320/36130, fax:678-36355, email: [email protected]/[email protected] Vietnam Coconut Scientist, Oil Plant Institute of Vietnam (OPI), 171-175 Ham Nghi St., District 1, Ho Chi Minh City, Vietnam, Tel: 84-8-8297336/ 8243526, Fax: 848-8243528, email: [email protected]

ACKNOWLEDGMENTS With boundless joy I dedicate this chapter to Heera, our grand daughter, born in Nashua, New England, near Boston, USA, who brought sunshine into our family. This chapter was compiled under the most trying circumstances and I am grateful to my wife Pankajam, our children Kannan, Sreedevi, and Arvind, our grand children Heera and Maya, and not the least our canine fleet, Black and Charlie, who stood by me, smiling, at the most difficult of times. To them I owe my love. The excellent photographs were provided by Dr M.G. Bhat, Director, National Research Center for Cashew, Puttur, Karnataka State, India and Dr George V. Thomas, Director, Central Plantation Crops Research Institute, Kasaragod, Kerala State, India. To them and their staff, I owe a deep debt of gratitude.

Agronomy and Economy of Important Industrial Crops

301

REFERENCES Abdul Khader, K. B. (1988). ‘‘Effect of Drip irrigation on Yield Attributes of Arecanut.’’ National Seminar on Drip Irrigation. Water Technology Centre, Tamil Nadu Agricultural University, Coimbatore, India. Abdul Khader, K. B., and Antony, K. J. (1968). Intercropping: A paying proposition for areca growers – What crops to grow? Indian Farming 18, 14–45. Abdul Khader, K. B., and Havanagi, G. V. (1991). Consumptive use of water in relation to cumulative pan evaporation (CPE) with and without mulching in arecanut. J. Plant. Crops. 18(Suppl), 139–146. Abdullah, A. (1994). ‘‘Technological Packages for Cashew Development.’’ Upland Farmer Development Project, Directorate-General Estate, Ministry of Agriculture, Indonesia. Abdul Salam, M., and Bhaskara Rao, E. V. V. (Eds.). (2001). ‘‘Cashew Varietal Wealth of India.’’ Directorate of Cashew and Cocoa Development, Government of India, Ministry of Agriculture, Cochin, India. Abraham, K. J. (1974). Intercropping in arecanut helps to build up farmer’s economy. Arecanut Spices Bull. 5, 73–75. Abraham, A., Mathew, P. M., and Ninan, C. A. (1961). Cytology of Cocos nucifera L. and Areca catechu L. Cytologia 26, 327–332. Aggarwal, J. S. (1954). The cashewnut and CSNL. Oleagineux 8(8–9), 559–564. Aggarwal, J. S. (1973). Resins, varnishes, and surface coatings from cashewnut shell liquid. Paint Manuf. 43(8), 29–31. Agnoloni, M., and Giuliani, F. (1977). ‘‘Cashew Cultivation.’’ Library of Tropical Agriculture, Ministry of Foreign Affairs, Instituto Agronomico Per L’oltremare, Florence. Ambika, B., and Abraham, C. C. (1979). Bioecology of Helopeltis antonii Sign. (Miridae: Hemiptera) infesting cashew trees. Entomon. 4, 335–342. Ananada, K. (2000). Exploitation of a dwarf mutant in arecanut breeding. In ‘‘Recent Advances in Plantation Crops Research’’ (N. Muraleedharan, and R. Raj Kumar, Eds.), pp. 69–72. Allied Publications Ltd, New Delhi, India. Ananda, K. S. (1999a). ‘‘Arecanut Varieties.’’ Extension publication, CPCRI, Kasaragod, India. Ananda, K. S. (1999b). Genetic improvements in arecanut. In ‘‘Improvement of Plantation Crops’’ (M. J. Ratnambal, P. M. Kumaran, K. Muralidharan, V. Niral, and V. Arunachalam, Eds.), pp. 52–57. Central Plantation Crops Research Institute (CPCRI), Kasaragod, India. Ananda, K. S., and Thampan, C. (1999). Promising cultivars and improved varieties of arecanut (Areca catechu L.). Indian J. Arecanut Spices Med. Plants I, 24–28. Anonymous. (1950). List of common names of Indian plant diseases. Indian J. Agric. Sci. 20(1), 107–104. Anonymous. (1993). Indian cashewnut shell liquid – A versatile industrial raw material. Indian Cashew J. 21(1), 8–9. Anonymous. (1994). “Annual Report 1993–94.” National Research Centre for Cashew, Puttur, India. Anuradha, S. (1999). Genetic resources and conservation strategies in arecanut (Areca catechu). In ‘‘Improvement of Plantation Crops.’’ (M. J. Ratnambal, P. M. Kumaran, K. Muralidharan, V. Niral, and V. Arunachalam, Eds.), pp. 48–51. Central Plantation Crops Research Instituite (CPCRI), Kasaragod, India. APCC (Asia Pacific Coconut Community). (1997). ‘‘APCC Codes and Standard for Aqueous Coconut Products.’’ Asian Pacific Coconut Community, Jakarta, Indonesia. Aradhya, M. K., Yee, L. K., Zee, F. T., and Manshardt, R. M. (1998). Genetic variability in Macadamia. Gnetic Resour. Crop Evol. 45, 19–32. Ascenco, J. C. (1986). Potential of the cashew crop – 1. Agric. Intl. 38(11), 324–327.

302

K. P. Prabhakaran Nair

Ashburner, G. R. (1994). Characterization, collection, and conservation of Cocos nucifera L. in the South Pacific. Ph.D. Thesis, School of Agriculture and Forestry, University of Melbourne. Ashburner, G. R. (1999). The application of molecular markers to coconut genetic improvement. In ‘‘Current Advances in Coconut Biotechnology.’’ (C. Oropeza, J. L. Verdeil, G. R. Ashburner, R. Cardega, and J. M. Santamarma, Eds.), Kluwer Academic Publishers, Boston. Ashburner, G. R., and Been, B. O. (1997). Characerization of resistance to lethal yellowing in Cocos nucifera and implications for genetic improvement of this species in the Caribbean region. In ‘‘Proceedings of International Workshop on Lethal Yellowing-Like Diseases of Coconut’’ (S. J. Eden-Green, and F. Ofori, Eds.), pp. 173–183. NRI Chatham, Kent, UK. Bailey, L. H. (1958). “The Standard Encyclopedia of Horticulture,” Vol. 1, 7th edn. Macmillan, New York. Balasimha, D. (1986). Light penetration patterns through arecanut canopy and leaf physiological characteristics of intercrops. J. Plant. Crops 16(Suppl), 61–67. Balasimha, D., Abdul Khader, K. B., and Bhat, R. (1996). Gas exchange characteristics of cocoa grown under arecanut in response to drip irrigation. All India Seminar on Modern Irrigation Techniques, June 26–27, 1996, pp. 96–101. Balasimha, D., and Subramonian, N. (1984). Nitrate reductase activity and specific leaf weight of cocoa and light profile in areca-cocoa mixed cropping. In “Proceedings of Plantation Crops Symposium VI,” Kottayam, Indian Society of Spice Crops, Kasaragod, India, pp. 83–88 Balasubramanian, P. P. (2000). Cashew – The premier crop of Indian commerce. In ‘‘Selected Articles on Cashew – State Level Seminar on Cashew’’ (K. K. Mohanty, P. C. Lenka, and A. K. Patnaik, Eds.), pp. 3–11. OSCDC, Bhubaneswar, India. Baudouin, L. (1999). Genetic improvement of coconut palm. In ‘‘Currrent Advances in Coconut Biotechnology’’ (C. Oropeza, J. L. Verdeil, G. R. Ashburner, R. Cardega, and J. M. Santanamarma, Eds.), Kluwer Academic Publishers, Boston. Bavappa, K. V. A. (1961). Some common intercrops in an arecanut garden. ICAC Mon. Bull. 2, 16–17. Bavappa, K. V. A. (1963). Morphological and cytological studies in Areca catechu Linn. and Areca triandra Roxb. M.Sc(Ag) Thesis, University of Madras, India. Bavappa, K. V. A. (1966). Morphological and anatomical studies in Areca catechu Linn and Areca triandra Roxb. Phytomorphology 16, 436–443. Bavappa, K. V. A. (1974). Studies in the genus Areca L. (Cytological and genetic diversity of A.catechu L. and A. triandra Roxb). Ph.D. Thesis, University of Mysore. Bavappa, K. V. A. (1977). Mangala – A superior arecanut variety. Arecanut Spices Bull. 9, 55–56. Bavappa, K. V. A. (1982). Future of arecanut. In ‘‘The Arecanut Palm’’ (M. K. Nair and T. Prem Kumar, Eds.), pp. 319–325. CPCRI, Kasaragod, India. Bavappa, K. V. A., Kailasam, C., Abdul Khader, K. B., Biddappa, C. C., Khan, H. H., Kasturi Bai, K. V., Ramadasan, A., Sundararaju, P., Bopaiah, B. M., and Thomas, G. V. (1986). Coconut and arecanut based high density multispecies cropping systems. J. Plant. Crops. 14, 74–87. Bavappa, K. V. A., and Nair, M. K. (1978). Cytogenetics of Areca catechu L., A. triandra Roxb. and their F1 hybrids (Palmae). Genetica 49, 1–8. Bavappa, K. V. A., and Nair, M. K. (1982). Cytogenetics and breeding. In ‘‘The Arecanut Palm’’ (K. V. A. Bavappa, M. K. Nair, and T. Prem Kumar, Eds.), pp. 51–96. CPCRI, Kasaragod. Bavappa, K. V. A., Nair, M. K., and Ratnambal, M. J. (1975). Karyotype studies in Areca catechu L., A. triandra Roxb and A.catechu x A.triandra hybrids. Nucleus 18, 146–151.

Agronomy and Economy of Important Industrial Crops

303

Bavappa, K. V. A., and Pillai, S. S. (1976). Yield and yield component analysis in different exotic cultivars and species of Areca. In ‘‘Improvement of Horticulture, Plantation and Medicinal Plants’’ (K. L. Chadha, Ed.), Vol. 1, pp. 243–246. Today and Tomorrow Printers and Publ., New Delhi, India. Bavappa, K. V. A., and Ramachander, P. R. (1967). Improvement of arecanut palm, Areca catechu L. Indian J. Genet. 27, 93–100. Bavappa, K. V. A., and Raman, V. S. (1965). Cytological studies in Areca catechu Linn. and Areca triandra Roxb. J. Indian Bot. Sci. 44, 495–505. Beccari, O. (0046). The palms of the Philippine Islands. Philipp. J. Sci. 14, 295–362. Beena, B., Abdul Salam, M., and Wahid, P. A. (1995). Nutrient uptake in cashew (Anacardium occidentale L.). The Cashew 9(3), 9–16. Bessa, A. M. S., and Sardinha, R. M. A. (1994). In vitro multiplication of cashew (Anacardium occidentale L.) by callus culture. In ‘‘Proceedings of 8th International Congress on Plant Tissue Culture,’’ Firenze, Italy, June 12–17, pp. 2–37. Bhaskara Rao, E. V. V. (1994). “Cashew Cultivation in Myanmar.” Deputation Report – Project UNDP/MTA/86/018, ARCPC. Plantation Crops Division, Yangon, Myanmar. Bhaskara Rao, E. V. V. (1996). Emerging trends in cashew improvement. In “Crop Productivity and Sustainability – Shaping the Future” (V. L. Chopra, R. B. Singh, and A. Varma, Eds.), pp. 675–682. Second International Crop Science Congress, Oxford and IBH Publishing Co. Pvt. Ltd., New Delhi, India. Bhaskara Rao, E. V. V., and Bhat, M. G. (1996). Cashew breeding – Achievements and priorities. In “Proceedings of Seminar on Crop Breeding in Kerala,” Department of Botany, Kerala University, Trivandrum, India. pp. 47–53. Bhaskara Rao, E. V. V., and Nagaraja, K. V. (2000). Status report on cashew. In “Souvenir Intl. Conf. on Plantation Crops”, Dec 12–15, 2000 (P. Rethinam, Ed.), pp. 1–7. National Research Center for Oil Palm, Pedavegi, India. Bhaskara Rao, E. V. V., and Swamy, K. R. M. (1994). Genetic resources of cashew. In “Advances in Horticulture, Plantation and Spice Crops, Part 1” (K. L. Chadha, and P. Rethinam, Eds.), Vol. 9, pp. 79–97. Malhotra Publishing House, New Delhi, India. Bhaskara Rao, E. V. V., and Swamy, K. R. M. (2000). Cashew research scenario in India. In “The Cashew Industry in India” (P. P. Balasubramanian, Ed.). DCCD, Cochin, India (in press). Bhaskara Rao, E. V. V., Swamy, K. R. M., and Bhat, M. G. (1998). Status of cashew breeding and future priorities. J. Plant. Crops. 9(4), 18–24. Bhat, M. G., Bhaskara Rao, E. V. V., and Swamy, K. R. M. (1999). Genetic resources of cashew and their utilization in crop improvement. In ‘‘Improvement of Plantation Crops’’ (M. J. Ratnambal, P. M. Kumaran, K. Muralidharan, N. Niral, and V. Arunachalam, Eds.), pp. 91–98. CPCRI, Kasaragod, India. Bhat, M. G., Kumaran, P. M., Bhaskara Rao, E. V. V., Mohan, K. V. J., and Thimmappaiah, K. V. J. (1998). Pollination technique in cashew. The Cashew 12(4), 21–26. Cashew Export Promotion Council of India, 2003. Cashew Statistics, CEPCI, Cochin, India. Bhat, P. S. I., and Rao, K. S. N. (1962). On the antiquity of arecanut. Arecanut J. 13, 13–21. Blume, C. L. (1836). ‘‘Rumhiasive Commntations Botanicae de Plantis Indiae Orientalis.’’ Leyden. Bopaiah, B. M. (1991). Soil microflora and VA mycorhizae in areca based high density multispecies cropping and areca monocropping systems. J. Plant. Crops. 18(Suppl), 224– 228. Bourdeix, R., Sangare, A., Le Saint, J. P., Meunier, J., Gascon, J. P., Rognon, F., and Nuce de Lamothe, M. (1993). Coconut breeding at IRHO and its application in seed production. In ‘‘Advances in Coconut Research and Development’’ (M. K. Nair, H. H. Khan,

304

K. P. Prabhakaran Nair

P. Gopalakrishnan, and E. V. V. Bhaskara Rao, Eds.), International Symposium on Coconut Research and Development., Oxford and IBH Publishing Co Pvt. Ltd., New Delhi, India. Bourgoing, R. (1991). In ‘‘Coconut – A Pictorial Technical Guide for Smallholders.’’ (D. R. A. Benigno, Ed.), p. 301. IRHO/CIRAD, Paris, France. Brahma, R. N. (1974). In North Bengal banana is a paying intercrop in areca gardens. Arecanut Species Bull. 5, 80–81. Burotrop. (1992). “Coconut and Oilpalms: World Production and Consumption.” Map. BUROTROP, Montpellier, France. Cardena, R., Ashburner, G. R., and Oropeza, C. (1999). Prospects for marker assisted breeding of lethal yellowing-resistant coconuts. In ‘‘Current Advances in Coconut Biotechnology’’ (C. Oropeza, J. L. Verdeil, G. R. Ashburner, R. Cardena, and J. M. Santamarma, Eds.), Kluwer Academic Publishers, Boston. Central Plantation Crops Research Institute (CPCRI). (1983). Annual Report, Kasaragod, India. Central Plantation Crops Research Institute (CPCRI). (1988a). Annual Report for 1986, Kasaragod, India. Central Plantation Crops Research Institute (CPCRI). (1988b). Annual Report for 1987. Kasaragod, India. Central Plantation Crops Research Institute. (1994). Annual Report. Kasaragod, India. Chacko, E. K. (1993). Genetic improvement of cashew through hybridisation and evaluation of hybrid progenies. A final report prepared for the Rural Industries R&D Corporation (mimeographed) CSIRO Division of Horticulture, Winnellie, Australia. Chacko, E. K., Baker, I., and Downton, J. (1990). Toward a sustainable cashew industry for Australia. J. Aust. Inst. Agric. Sci. 3(5), 39–43. Chandramohanan, R., and Kaveriappa, K. M. (1985). Epidemiological studies on inflorescence dieback of arecanut caused by Colletotrichum gloeosporioides. In ‘‘Proceedings of Silver Jubilee Symposium on Arecanut Research and Development, CPCRI., Vittal,’’ India. 1982, pp. 104–106. Chappell, J. (1983). A revised sea level for the past 300,000 years from Papua New Guinea. Search 14, 99–101. Chinnappa, B., and Hippargi, K. (2005). An economic impact assessment of drip irrigation technology in arecanut plantations: An empirical evidence from Karnataka. J. Plant. Crops. 33(2), 130–134. Chowdappa, P., and Balasimha, D. (1992). Non-stomatal inhibition of photosysnthesis in arecanut palms affected with yellow leaf disease. Indian Phytopath. 45, 312–313. Chowdappa, P., and Balasimha, D. (1995). Chlorophyll fluorescence characteristics of arecanut palms affected with yellow leaf disease. Indian Pytopath. 47, 87–88. Chowdappa, P., Balasimha, D., and Daniel, E. V. (1993). Water relations and net photosynthesis of arecanut palms affected with yellow leaf disease. Indian J. Microbiol Ecol. 3, 19–30. Chowdappa, P., Balasimha, D., Rajagopal, V., and Ravindran, P. S. (1995). Stomatal responses in in arecanut palms affected with yellow leaf disease. J. Plant. Crops. 23, 116–121. Chowdappa, P., Saraswathy, N., Vinaya Gopal, K., and Somala, M. (1999). Control of fruit rot of arecanut through polythene covering of bunches. In ‘‘Proceedings of Symposium on Plant Disease Management for Sustainable Agriculture, Indian Phytopathological Society. CPCRI,’’ Kayamgulam, India. pp. 18–19. Coleman, L. C. (1910). Diseases of the areca palm (Areca catechu L.) I. Koleroga or rot disease. Ann. Mycol. 8, 591–626. Damodaran, V. K. (1977). F1 population variability in cashew. J. Plant. Crops 5(2), 89–91. Damodaran, V. K., Veera Raghavan, P. G., and Vasavan, M. G. (1978). Cashew breeding. Agric. Res. J. Kerala 15(1), 9–13.

Agronomy and Economy of Important Industrial Crops

305

Darlington, C. D., and Janaki Ammal, E. K. (1945). ‘‘Chromosome Atlas of Cultivated Plants.’’ Allen & Unwin, London. Das, S., Jha, T. B., and Jha, S. (1996). In vitro propagation of cashew nut. Plant Cell Rep. 15, 615–619. De Castro, P. (1994). Summary of the study. In ‘‘The World Cashew Economy’’ (A. M. Delogu and G. Haeuster, Eds.), pp. 11–12. NOMISMA, L’Inchiostroblu, Bologna, Italy. Degani, C., Goldring, A., and Gazit, S. (1989). Pollen parent effect on out crossing rate in ‘‘Hay’’ and ‘‘Fuerte’’ Avocado plots during fruit development. J. Am. Soc. Hort. Sci. 114, 106–111. de Taffin, G. (1993). “Le Cocotier (The Coconut).” Maisonneuve et Larose, Paris, France. p. 166 (French language only). Devasahayam, S., and Radhakrishnan Nair, C. P. (1986). The tea mosquito bug Helopeltis antonii Signoret on cashew in India. J. Plant. Crops. 14, 1–10. Dharmaraju, E., Rao, P. A., and Ayyanna, T. (1974). A new record of Nephopteryx sp. as an apple and nut borer on cashew in Andhra Pradesh. J. Res. Andhra Pradesh Agric. Univ. 1(4&5), 198. Dollet, M. (1999). Conventional and molecular approaches for detection and diagnosis of plant diseases: Application to coconut. In ‘‘Current Advances in Coconut Biotechnology’’ (C. Oropeza, J. L. Verdeil, G. R. Ashburner, R. Cardena, and J. M. Santamarma, Eds.), Kluwer Academic Publishers, Boston. D’Silva, L., and D’Souza, L. (1992). In vitro bud proliferation of Anacardium occidentale L. Plant Cell Tissue Organ Cult. 29, 1–6. Enig, M. (1999). Coconut: In support of good health in the 21st century. In ‘‘Promoting Coconut Products in a Competitive Global Market:’’ Proceedings of the Thirty-Sixth COCOTECH Meeting of the Asia Pacific Coconut Community, Pohnpei. APCC Jakarta, Indonesia. p. 366. Enig, M. (2000). ‘‘Know Your Fats: The Complete Primer for Understanding the Nutrition of Fats, Oils, and Cholesterol.’’ Bethseda Press, Silver Spring, MD. Etherington, D. M., and Mahendrarajah, S. (1998). Economic benefits of Direct Micro Expelling coconut oil in the South Pacific. In ‘‘Trees of Life: The Key to Development: Proceedings of International Cashew and Coconut Conference, Dar es Salaam’’ (C. P. Topper, P. D. S. Caligari, A. K. Kullaya, S. H. Shomari, L. J. Kasuga, P. A. L. Masache Khao, and A. A. Mpumami, Eds.), pp. 457–468. Biohybrids International Ltd, Reading, UK. Etherington, D. M., Zegelin, S., and White, I. (1999). Oil extraction from grated coconut: real-time moisture content measurement and its impact on oil production efficiency. Trop. Sci. 38, 10–19. Foale, M. A. (1965). Nutrition du jeune cocotier dans les Iles Russell (Archipel des Salamon). Oleagineux 20, 1–4. Foale, M. A. (1968a). The growth of coconut seedlings. Oleagineux 23, 651–654. Foale, M. A. (1968b). The growth of the young coconut palm (Cocos nucifera L.). 1. The role of the seed and of photosynthesis in seedling growth up to 17 months of age. Aust. J. Agric. Res. 19, 781–789. Foale, M. A. (1993a). Physiological basis for yield in coconut. In ‘‘Advances in Coconut Research and Development’’ (M. K. Nair, H. H. Khan, P. Gopalasundaram, and E. V. V. Bhaskara Rao, Eds.), pp. 181–189. Oxford and IBH Publishing Co. Pvt. Ltd., New Delhi, India. Foale, M. A. (1993b). The effect of exposing the germpore on germination of coconut. In ‘‘Advances in Coconut Research and Development’’ (M. K. Nair, H. H. Kan, P. Gopalasundaram, and E. V. V. Bhaskarao Rao, Eds.), pp. 247–252. Oxford and IBH Publishing Co. Pvt. Ltd., New Delhi, India.

306

K. P. Prabhakaran Nair

Foale, M. A., Ashburner, G. R., and Friend, D. (1994). Canopy development and light interception of coconut. In ‘‘Coconut Improvement in the South Pacific’’ (M. A. Foale and P. W. Lynch, Eds.), pp. 71–73. Australian Centre for International Agricultural Research, Canberra, Australia. Foltan, H., and Ludders, P. (1995). Flowering, fruit set, and genotype compatability in cashew. Angew. Bot. 69, 215–220. Furtado, C. X. (1933). The limits of Areca Linn. and its sections. Fedde’s Repertrium Specierum Novarum. Regnum Veg. 33, 217–239. Furtado, C. X. (1960). The philological origin of Areca catechu. Principles 4, 26–31. Goldblatt, P., Ed. (1984). ‘‘Index to Plant Chromosome Numbers, 1979–1981.’’ Missouri Botanical garden. Greathead, D. J. (1995). Natural enemies in combination with pesticides for integrated pest management. In ‘‘Novel Approaches to Integrated Pest Management’’ (R. Reuveni, Ed.), pp. 183–197. Lewis Publishers, Boca Raton, FL. Harries, H. C. (1978). The evolution, dissemination, and classification of Cocos nucifera L. Bot. Rev. 44, 265–320. Harries, H. C. (1990). Malesian origin for a domestic Cocos nucifera. In ‘‘The Plant Diversity of Malaysia’’ (P. Baas, J. Payme, and A. Saridan Purwanisingh, Eds.), pp. 351–357. Kluwer, Dordrecht, The Netherlands. Harries, H. C. (1999). Does clonal coconut material have a potential use in any agricultural system? In ‘‘Current Advances in Coconut Biotechnology’’ (C. Oropeza, J. L. Verdeil, G. R. Ashburner, R. Cardega, and J. M. Santamarma, Eds.), pp. 431–436. Kluwer Academic Publishers, Boston. Harrison, N., Cordova, I., Richardson, P., and DiBonito, R. (1999). Detection and diagnosis of lethal yellowing. In ‘‘Current Advances in Coconut Biotechnology’’ (C. Oropeza, J. L. Verdeil, G. R. Ashburner, R. Cardena, and J. M. Santamarma, Eds.), pp. 184–196. Kluwer Academic Publishers, Boston. Hegde, M., Kulasekaran, M., Jayasankar, S., and Shanmugavelu, K. G. (1993). In vitro embryogenesis in cashew. (Anacardium occidentale L.) Indian Cashew J. 21(4), 17–25. Hodgson, R. A. J., and Randles, J. W. (1999). Detection of cadang-cadang viroid-like sequences. In ‘‘Current Advances in Coconut Biotechnology’’ (C. Oropeza, J. L. Verdeil, G. R. Ashburner, R. Cardega, and J. M. Santamarma, Eds.), pp. 227– 246. Kluwer Academic Publishers, Boston. Iyer, R. D., and Damodaran, S. (1994). Improvements of coconut. In ‘‘Advances in Horticulture, Volume IX: Plantation and Spice Crops, Part I’’ (K. L. Chadha, and P. Rethinam, Eds.), pp. 217–242. Malhotra Publishing House, New Delhi, India. Jain, N. L., Das, D. P., and Lal, G. (1954). “Proceedings of Symposium on Fruits and Vegetables Preservation Industry, CFTRI,” Mysore. Johnson, D. (1973). The botany, origin, and spread of the cashew Anacardium occidentale. J. Plant. Crops 1, 1–7. Johnson, D. (1982). Cashewnut processing in Brazil. Indian Cashew J. 12(2), 11–13. Joshi, G. D., Prab, B. P., Relekar, P. P., and Magdum, M. B. (1993). Studies on utilization of cashew apple waste. Abstract, Golden Jubilee Symposium on Horticultural Research – Changing Scenario, Bangalore, India. pp. 372. Joubert, A. S., and Des Thomas, S. (1965). The cashew nut. Farming S. Afr. 40, 6–7. Khosla, P. K., Sareen, T. S., and Mehra, P. N. (1973). Cytological studies on Himalayan Anacardiaceae. Nucleus 16, 205–209. Kumararaj, K., and Bhide, V. P. (1962). Damping off of cashewnut (Anacardium occidentale L.) seedlings caused by Phytophthora palmivora Butler in Maharashtra State. Curr. Sci. 31, 23. Leach, B. J., Foale, M. A., and Ashburner, R. A. (in press). The wild-type coconut population of the Cocos (Keeling) Islands, Indian Ocean. Genetic Resour. Crop Evol.

Agronomy and Economy of Important Industrial Crops

307

Lebrun, P., Grivet, L., and Baudouin, L. (1999). Use of RFLP markers to study the diversity of the coconut palm. In ‘‘Current Advances in Coconut Biotechnology’’ (C. Oropeza, J. L. Verdeil, G. R. Ashburner, R. Cardega, and J. M. Santamarma, Eds.), pp. 73–88. Kluwer Academic Publishers, Boston. Lievens, C., Pylyser, M., and Boxus, P. H. (1989). First results about micropropagation of Anacardium occidentale by tissue culture. Fruits 44, 553–557. Liu Kangde, Liang Shibang, and Deng Suisheng (1998). Integrated production practices of cashew in China. In “Integrated Production Practices of Cashew in Asia” (M. K. Papademetriou, and E. M. Herath Eds.). pp. 6–14. Food and Agricultural Organization of the United Nations, Regional Office for Asia and the Pacific, Bangkok, Thailand. Liyanage, D. V., and Abeywardena, V. (1958). ‘‘Correlation between seednut, seedling, and adult palm characters in coconut. Bulletin No. 16’’ Coconut Research Institute, Ceylon. Liyanage, D. V., and Sakai, K. I. (1960). Heritabilities of certain yield characters of the coconut palm. J. Genet. 57, 245–252. Louis, I. H., and Rethinakumar, L. (1988). Genetic load in coconut palm. In “Coconut Breeding and Management: Proceedings of National Symposium on Coconut Breeding and Management.” (E. G. Silas, M. Aravindakshan, and A. J. Jose, Eds.). Director of Extension, Kerala Agricultural University, Trichur, India. Machado, O. (1944). Estudos novos sobre uma planta velha cajueiro (Anacardium occidentale L.). Rodrguesia 8, 19–48. Mahesha, A., Abdul Khader, K. B., and Ranganna, G. (1990). Consumptive use of irrigation requirement of areca palm (Areca catechu L.). Indian J. Agric. Sci. 60, 609–611. Mahopatra, G., and Bhujan, G. (1974). Land selection for cashew plantation – A survey report. Cashew Bull. 11(8), 8–15. Manciot, R., Ollagnier, M., and Ochs, R. (1979a). Nutrition minerale et fertilisation du cocotier dans le monde I: (Mineral nutrition and fertilization of the coconut around the world). Oleagineux 3, 499–515. Manciot, R., Ollagnier, M., and Ochs, R. (1979b). Nutrition minerale et fertilization du cocotier dans le monde II: Etude des differents elements (Mineral nutrition and fertilization of the coconut around the world II: Study of different elements). Oleagineux 3, 563–580. Manciot, R., Ollagnier, M., and Ochs, R. (1980). Nutrition minerale et fertilisation du cocotier dans le monde II: Etude des differents elements – suite. (Mineral nutrition and fertilization of the coconut around the world II. Study of different elements – continued). Oleagineux 35, 13–22. Mantell, S. H., Boggetti, B., Bessa, A. M. S., Lemos, E. P., Abdelhadi, A., and Mneney, E. E. (1997). Micropropagation and micrografting methods suitable for international transfers of cashew In ‘‘Proceedings of International Cashew and Coconut Conference,’’ Dar es Salam, Tanzania, p. 24. Marechal, H. (1928). Observations and preliminary experiments on the coconut palm with a view to developing improved seednuts for Fiji. Fiji Agric. J. 1(2), 16–45. Martius, C. F. P. V. (1832–1850). “Historia Naturalis Palmarum.” Leipzig, Germany. Marty, G., Le Guen, V., and Fournial, T. (1986). Cyclone effects on plantations in Vanuatu. Oleagineux 41, 64–69. Massari, F. (1994). Inroduction. In ‘‘The World Cashew Economy’’ (A. M. Delogu, and G. Haeuster, Eds.), pp. 3–4. NOMISMA, L’Inchiostroblu, Bologna, Italy. Maung, M. L. (1998). Integrated production practices of cashew in Myanmar. In ‘‘Integrated Production Practices of Cashew in Asia’’ (M. K. Papademetriou, and E. M. Herath, Eds.), pp. 33–46. Food and Agricultural Organisation of the United Nations, Regional Office for Asia and the Pacific, Bangkok, Thailand. Mitchell, J. D., and Mori, S. A. (1987). The cashew and its relatives (Anacardium: Anacardiaceae). Mem. NY Bot. Gard. 42, 1–76.

308

K. P. Prabhakaran Nair

Mneney, E. E., Mantell, S. H., Tsoktouridis, G., Amin, S., Bessa, A. M. S., and Thangavelu, M. (1997). RAPD – Profiling of Tanzanian cashew (Anacardium occidentale L.). In ‘‘Proceedings of International Cashew and Coconut Conference,’’ Dar es Salam, Tanzania, p. 24. Mohan Rao, M. (1982). Introduction. In ‘‘The Arecanut Palm’’ (K. V. A. Bavappa, M. K. Nair, and T. Premkumar, Eds.), pp. 1–9. CPCRI, Kasaragod, India. Mohapatra, A. R., and Bhat, N. T. (1982). Crop management. A. Soils and manures. In ‘‘The Arecanut Palm’’ (K. V. A. Bavappa, M. K. Nair, and T. Premkumar, Eds.), pp. 97–104. CPCRI, Kasaragod, India. Mohapatra, A. R., Vijaya Kumar, K., and Bhat, N. T. (1973). A study on nutrient removal by the cashew tree. Indian Cashew J. 9(2), 19–20. Muralidharan, A. (1980). ‘‘Biomass productivity, plant interactions, and economics of intercropping in arecanut,’’ Ph.D. Thesis, University of Agricultural Sciences, Bangalore, India. Muralidharan, A., and Nayar, T. V. R. (1979). Intercropping in arecanut gardens. In ‘‘Multiple Cropping in Coconut and Arecanut Gardens’’ (E. V. Nelliat, and K. Shama Bhat, Eds.), pp. 24–27. CPCRI, Kasaragod, India. Murali Raghavendra Rao, M. K. (1999). Estimation of genetic diversity in cashew (Anacardium occidentale L.) cultivars by using molecular markers. M.Sc. Thesis, University of Agricultural Sciences, Bangalore, India. Murthy, K. N., and Bavappa, K. V. A. (1960a). Floral biology of areca (Areca catechu Linn.). Arecanut J. 11, 51–55. Murthy, K. N., and Bavappa, K. V. A. (1960b). Morphology of arecanut palm – The shoot. Arecanut J. 11, 99–102. Murthy, K. N., and Bavappa, K. V. A. (1962). Species and ecotypes (cultivars) of arecanut. Arecanut J. 13, 59–78. Murthy, K. N., and Pillai, R. S. N. (1982). Botany. In ‘‘The Arecanut Palm’’ (K. V. A. Bavappa, M. K. Nair, and T. Premkumar, Eds.), pp. 11–49. CPCRI, Kasaragod, India. Nagabhushanam, S., Dasaradhi, T. B., and Venkata Rao, P. (1977). Some promising cashew selections and hybrids in Andhra Pradesh. Cashew Bull. 14(7), 1–4. Nagaraja, K. V. (1987a). Lipids of high yielding varieties of cashew. Plant Foods Hum. Nutr. 37, 307–311. Nagaraja, K. V. (1987b). Protein of high yielding varieties of cashew. Plant Foods Hum. Nutr. 37, 69–75. Naidu, G. V. B. (1962). ‘‘Arecanut in Mysore.’’ Department of Agriculture, Bangalore, India. Naidu, C. V. G. (1963). Seen a dwarf areca palm? Indian Farming 12, 16–17. Nair, M. G. (1982). Intercropping with pepper. Indian Farming 32, 17–19. Nair, K. P. P. (1984). Towards a better approach to soil testing based on the buffer power concept. In “Proceedings of 6th International Colloquium for the Optimization of Plant Nutrition,” Vol. 4, 2–8 September (P.-M. Prevel, Ed), pp. 1221–1228. Montpellier, France. Nair, K. P. P. (1996). The buffering power of plant nutrients and effects on availability. Adv. Agon. 57, 237–287. Nair, K. P. P., and Mengel, K. (1984). Importance of phosphate buffer power for phosphate uptake by rye. Soil Sci. Soc. Am. J. 48, 92–95. Nair, M. K., Bhaskara Rao, E. V. V., Nambiar, K. K. N., and Nambiar, M. C. (1979). ‘‘Cashew (Anacardium occidentale L.). Monograph on Plantation Crops-1,’’ p. 169. Central Plantation Crops Research Institute, Kasaragod, India. Nair, C. P. R., and Daniel, M. (1982). Pests. In ‘‘Arecanut Palm’’ (K. V. A. Bavappa, M. K. Nair, and T. Premkumar, Eds.), pp. 151–184. CPCRI, Kasaragod, India.

Agronomy and Economy of Important Industrial Crops

309

Nair, M. R. G. K., and Das, N. M. (1962). On the biology of Carvalhoia arecae Miller and China, a pest of areca palms in Kerala. Indian J. Ent. 24, 86–93. Nair, M. K., and Nampoothiri, K. U. K. (1993). Breeding for high yield in coconut. In ‘‘Advances in Coconut Research and Development’’ (M. K. Nair, H. H. Khan, P. Gopalasundaram, and E. V. V. Bhaskara Rao, Eds.), pp. 61–69. Oxford and IBH Publishing Co Pvt. Ltd., New Delhi, India. Nair, M. K., and Ratnambal, M. J. (1978). Cytology of Areca macrocalyx Becc. Curr. Sci. 47, 172–173. Nambiar, K. K. (1949). ‘‘A Survey of Arecanut Crop in Indian Union.’’ Indian Central Arecanut Committee, Calicut, India. Nambiar, M. C. (1974). Recent trends in cashew research. Indian Cashew J. 9(3), 20–22. Nambiar, M. C. (1977). Cashew. In ‘‘Ecophysiology of Tropical Crops.’’ (P. T. Alvin and T. T. Koslowski, Eds.), pp. 461–478. Academic Press, San Francisco. Nambiar, K. K. N., Sarma, Y. R., and Pillai, G. B. (1973). Inflorescence blight of cashew (Anacardium occidentale L.) J. Plant. Crops. 1, 44–46. Nambiar, M. C., and Swaminathan, M. S. (1960). Chromosome morphology, microsporogenesis, and pollen fertility in some varieties of coconut. Indian J. Genet. 20, 200–211. Nanjundaswamy, A. M. (1984). Economic utilization of cashew apple. Cashew Causerie 6(2), 2–7. National Research Center for Cashew (NRCC). (1995). Annual Report 1994–95. NRCC, Puttur, India. National Research Center for Cashew (NRCC). (1998). Annual Report 1997–98. NRCC, Puttur, India. Nawale, R. N., and Salvi, M. J. (1990). The inheritance of certain characters in F-1 hybrid progenies of cashewnut. The Cashew 4(1), 11–14. Nawale, R. N., Sawke, D. P., Deshmukh, M. T., and Salvi, M. J. (1985). Effect of black polythene mulch and supplementary irrigation on fruit retention in cashewnut. Cashew Causerie (July–September), p. 89. Nayar, K. G. (1998). Cashew: A versatile nut for health. In ‘‘Proceedings of International Cashew and Coconut Conference. Trees for Life – The Key to Development’’ (C. T. Topper, P. D. S. Caligari, A. K. Kulaya, S. H. Shomari, L. H. Kasuga, P. A. L. Masawe, and A. A. Mpunami, Eds.), pp. 195–199. BioHybrids International Ltd, Reading, UK. Nayar, R., and Selsikar, C. E. (1978). Mycoplasma-like organisms associated with yellow leaf disease of Areca catechu. Eur. J. For. Pathol. 8, 125–128. Nayudamma, Y., and Koteswara Rao, C. (1967). Cashew testa – Its use in leather industry. Indian Cashew J. 4(2), 12–13. Ninan, C. A., Pillai, R. V., and Joseph, J. (1960). Cytogenetic studies on the genus Cocos – I. Chromosome number in C. australis and C.nucifera L. vars spicata and androgena. Indian Coconut J. 13, 129–134. NOMISMA. (1994). In “The World Cashew Economy” (A. M. Delogu and G. Haeuster, Eds.). L’Inchiostroblu, Bologna, Italy. Ohler, J. G. (1979). ‘‘Cashew. Communication 71 Department of Agricultural Research.’’ Koniklijk Instituut voor de Tropen, Amsterdam. Patel, J. S. (1937). Coconut breeding. Proc. Assoc. Biol. 5, 1–16. Peixoto, A. (1960). ‘‘Caju. Servico de Informacao Agricola, Ministerio de Agricltura, Rio de Janerio,’’ Brazil. Peries, R. R. A. (1993). Seedling selection in coconut based on juvenile and adult palm correlations. In ‘‘Advances in Coconut Research and Development’’ (M. K. Nair, H. H. Khan, P. Gopalasundaram, and E. V. V. Bhaskara Rao, Eds.), pp. 181–189. Oxford and IBH Publishig Co. Pvt. Ltd., New Delhi, India.

310

K. P. Prabhakaran Nair

Persley, G. J. (1992). ‘‘Replanting the Tree of Life: Toward an International Agenda for Coconut Palm Research.’’ CAB International, Wallingford, UK. Phadnis, A., and Elijah, S. N. (1968). Development on cashewnut in Maharashtra state. Cashew News Teller 1(8, 9 and 10), 7–12. Pieries, W. V. D. (1935). Studies on the coconut palm II: On the relation between the weight of husked-nuts and the weight of copra. Trop. Agric. 85, 208–220. Pillai, G. B. (1975). Pests of cashewnut and how to combat them. Cashew News Teller (October–December), pp. 31–33. Pillai, G. B. (1979). Pests. In ‘‘Cashew Research and Development’’ (E. V. V. Bhaskara Rao and H. Hameed Khan, Eds.), pp. 55–72. Indian Society for Plantation Crops, CPCRI, Kasaragod, India. Pillai, G. B., Dubey, O. P., and Singh, V. (1976). Pests of cashew and their control in India: A review of current status. J. Plant. Crops. 4, 37–50. Pimentel, D. (1986). Acroecology (Sic) and economics. In ‘‘Ecological Theory and Integrated Pest Management Practice’’ (M. Kogan, Ed.), pp. 299–319. John Wiley & Sons, New York. Pomier, M., and Bonneau, X. (1984). The development of the coconut root systems depending on environmental conditions in Cote d’Ivoire. Oleagineux 42(11), 409–421. Ponnamma, K. N., Solomon, J. J., Rajeev, K., Govindankutty, M. P., and Karnavar, G. K. (1997). Evidence for transmission of yellow leaf disease of areca palm, Areca catechu L. by Proutista moesta (West Wood) (Hompoptera: Derbidae). J. Plant. Crops. 25, 197–200. Prem Kumar, T., and Daniel, M. (1981). Studies on the control of soil grubs of arecanut palm. Pesticides 15, 29–30. Proc. Silver Jubilee Symposium on Arecanut Research and Development. CPCRI, Vittal. (1982). pp. 33–37. Purseglove, J. W. (1988). Anacardiaceae. In ‘‘Tropical Crops – Dicotyledons,’’ pp. 18–32. English Language Book Society, Longman, London. Raghavan, V. (1957). On certain aspects of the biology of arecanut (Areca catechu Linn.) and utilization of its by-products in industry. D.Phil. Thesis, Gauhati university, India. Raghavan, V., and Baruah, H. K. (1958). Arecanut: India’s popular masticatory – History, chemistry and utilization. Econ. Bot. 12, 315–345. Rai, P. S. (1984). ‘‘Hand Book on Cashew Pests.’’ Research Publications, Delhi, India. Rajagopal, V., Voleti, S. R., Kasturi Bai, K. V., and Sivashankar, S. (1988). Physiological and bio-chemical criteria for breeding for drought tolerance in coconut. In ‘‘Proceedings of National Symposium on Coconut Breeding and Management’’ (E. G. Silas, M. Aravindakshan, and A. I. Jose, Eds.), pp. 136–143. Director of Extension, Kerala Agricultural University, Thrissur, India. Ramakrishnan, T. S. (1955). Decline in cashewnut. Indian Phytopath. 8, 58–63. Ramanayake, S. M. S. D., and Kovoor, A. (1999). In vitro micrografting of cashew (Anacardium occidentale L.) J. Hortl. Sci. Biotech. 74(2), 265–268. Randles, J. W., Wefels, E., Hanold, D., Miller, D. C., Morin, J. P., and Rohde, W. (1999). Detection and diagnosis of coconut foliar decay disease. In ‘‘Current Advances in Coconut Biotechnology’’ (C. Oropeza, J. L. Verdeil, G. R. Ashburner, R. Cardega, and J. M. Santamarma, Eds.), pp. 247–258. Kluwer Academic Publishers, Boston. Rao, G. S. L. H. V. P., Giridharan, M. P., Naik, B. J., and Gopakumar, C. S. (1999). Weather inflicted damage on cashew production – A remedy. The Cashew 13(4), 36–43. Rao, Y. P., and Mohan, S. K. (1970). A new bacterial stripe disease of arecanut (Areca catechu) in Mysore State. Indian Phytopath. 23, 702–704. Rau, M. K. T. (1915). The sweet arecanut, Areca catechu L. var.deliciosa. J. Bombay Nat. Hist. Soc. 23, 793. Ravi Bhat., Sujatha, S., and Balasimha, D. (2007). Impact of drip fertigation on productivity of arecanut (Areca catechu L.). Agricultural Water Management 90 (2007), 101–111. Science Direct Journal Homepage: www.elsevier.com/locate/agwat.

Agronomy and Economy of Important Industrial Crops

311

Rawther, T. S. S., Nair, R. R., and Saraswathy, N. (1982). Diseases. In ‘‘The Arecanut Palm’’ (K. V. A. Bavappa, M. K. Nair, and T. Premkumar, Eds.), pp. 185–224. CPCRI, Kasaragod, India. Rickson, F. R., and Rickson, M. M. (1998). The cashewnut, Anacardium occidentale (Anacardiaceae), and its perennial association with ant: Extrafloral nectary location and potential for ant defense. Am. J. Bot. 85(6), 835–849. Rodriguez, M. J. B. (1999). Detection and diagnosis of coconut cadang-cadang. In ‘‘Current Advances in Coconut Biotechnology’’ (C. Oropeza, J. L. Verdeil, G. R. Ashburner, R. Cardega, and J. M. Santamarma, Eds.), pp. 221–226. Kluwer Academic Publishers, Boston. Sadanandan, A. K. (1974). Raise intercrops in arecanut plantation for higher returns. Arecanut Spices Bull. 9, 70–72. Salvi, P. V. (1979). Cashew hybrids for increased production. Indian Farming 28(12), 11–12. Sampath Kumar, S. N. (1981). Bacterial leaf stripe disease of arecanut (Areca catechu L.) caused by Xanthomonas campestris pv. arecae. Ph.D. Thesis, Indian Institute of Science, Bangalore, India. Sampath Kumar, S. N., and Nambiar, K. K. (1990). Ganoderma disease of arecanut palm – Isolation, pathogenicity, and control. J. Plant. Crops. 18, 14–18. Sampath Kumar, S. N., and Saraswathy, M. (1994). Diseases of arecanut. In ‘‘Advances in Horticulture’’ (K. L. Chadha, and P. Rethinam, Eds.), Vol. 10, pp. 930–967. Malhotra Publ. House, New Delhi, India. Sandeep Nayak, J. (1996). ‘‘Crop Weather assessment for Arecanut.’’ All India Seminar on Modern Irrigation Techniques, Bangalore, India. Sannamarappa, M. (1990). Spacing trial in arecanut. J. Plant. Crops. 17(Suppl), 51–53. Sannamarappa, M., and Muralidharan, A. (1982). Multiple cropping. In ‘‘The Arecanut Palm’’ (K. V. A. Bavappa, M. K. Nair, and T. Premkumar, Eds.), pp. 133–149. CPCRI, Kasaragod, India. Santos, G. (1999). Potential use of clonal propagation in coconut improvement programs. In ‘‘Current Advances in Coconut Biotechnology’’ (C. Oropeza, J. L. Verdeil, G. R. Ashburner, R. Cardega, and J. M. Santamarma, Eds.), pp. 419–430. Kluwer Academic Publishers, Boston, USA. Saraswathy, N., Koti Reddy, M., and Nair, R. R. (1977). Colletotrichum gloeosporioides causing inflorescence dieback, button shedding, and nut rot of betelnut palm. Plant Disease Reptr. 61, 172–174. Sastry, M. N. L., and Hegde, R. K. (1985). Control of koleroga in arecanut. In ‘‘Proceedings of Silver Jubilee Symposium on Arecanut Research and Development,’’ CPCRI, Vittal, India, pp. 86–91. Shama Bhat, K. (1974). Intensified inter/mixed cropping in areca garden – The need of the day. Arecanut Spices Bull. 5, 67–69. Shama Bhat, K. (1978). Agronomic research in Arecanut – A review. J. Plant. Crops. 6, 67– 80. Shama Bhat, K. (1983). Plant interactions in a mixed crop community of arecanut (Areca catechu L.) and cacao (Theobroma cacao L.). Ph.D Thesis, University of Mysore, Mysore, India. Shama Bhat, K. (1988). Growth and performance of cacao (Theobroma cacao L.) and arecanut (Areca catechu L.) under mixed cropping system. In ‘‘Proceedings of 10th International Cocoa Research Conference.’’ Cocoa Producers’Alliance, Lagos, Nigeria, pp. 15–19. Shama Bhat, K., and Abdul Khader, K. B. (1970). Inter and mixed cropping in arecanut gardens. Indian Farming 20, 35. Shama Bhat, K., and Abdul Khader, K. B. (1982). Crop management – Agronomy. In ‘‘The Arecanut Palm’’ (K. V. A. Bavappa, M. K. Nair, and T. Premkumar, Eds.), pp. 105–131. CPCRI, Kasaragod, India.

312

K. P. Prabhakaran Nair

Shama Bhat, K., and Bavappa, K. V. A. (1972). Cacao under palms. In ‘‘Cacao and Coconuts in Malaysia’’ (R. L. Wastie and D. A. Earp, Eds.), pp. 116–121. Incorporated Soc. Planters, Kuala Lumpur. Shama Bhat, K., and Leela, M. (1968). Cacao and arecanut are good companions for more cash. Indian Farming 18, 19–20. Shama Bhat, K., and Leela, M. (1969). The effect of density of planting on the distribution of arecanut roots. Trop. Agric. 46, 55–61. Shama Bhat, K., Krishna Murthi, S., and Madhava Rao, V. N. (1962). Studies on certain aspects of floral biology of the arecanut. South Indian Hort. 10, 22–34. Sharma, A. K., and Sarkar, S. K. (1956). Cytology of different species of palms and its bearing on the solution of problems of phylogeny and speciation. Genetics 28, 361–488. Simmonds, N. W. (1954). Chromosome behaviour in some tropical plants. Heredity 8, 139. Simmonds, F. J. (1970). ‘‘A memorandum on the possibilities of biological control of cocoainfesting mirids.’’Commonwealth Institute of Biological Control, Bangalore Report (unpublished). Simmonds, N. W. (1976). ‘‘Evolution of Crop Plants.’’ Longman, London. Singh, S., Sehgal, H. S., Pandey, P. C., and Bakshi, B. K. (1967). Anthracnose disease of cashew (Anacardium occidentale L.), its cause, epidemiology and control. Indian Forester 93, 374–376. Southern, P. J. (1969). Sulphur deficiency in coconuts. Oleagineux 24(4), 211–220. Spriggs, R. (1984). Early coconut remains from the South Pacific. Polynesian Soc. J. 93, 71–77. Stebbins, G. L. (1958). Longevity, habitat, and release of genetic variability in the higher plants. Cold Spring Harb. Symp. Quant. Biol. 23, 365–378. Subbaiah, C. C., Manikandan, P., and Joshi, Y. (1986). Yellow leaf spot of cashew: A case of molybdenum deficiency. Plant Soil 94(1), 35–42. Sujatha, S., Ravi, B., Reddy, V. M., and Abdul Haris, A. (1999). Response of high yielding varieties of arecanut to fertilizer levels in coastal Karnataka. J. Plant. Crops. 27, 187–192. Sundararaju, D. (1984). Studies on cashew pests and their natural enemies in Goa. J. Plant. Crops. 12, 38–46. Sundararaju, D. (1993). Compilation of recently recorded and some new pests of cashew in India. The Cashew 8(1), 15–19. Sundararaju, D. (1999). Screening of cashew accessions to tea mosquito bug (Helopeltis antonii L. Sign.). The Cashew 13(4), 20–26. Sundararaju, P., and Koshy, P. K. (1988). Effect of intercrops on occurrence of Radopholus similis in arecanut palms. J. Plant. Crops 18(Suppl), 299–301. Swamy, K. R. M., Bhaskara Rao, E. V. V., and Bhat, M. G. (1997). ‘‘Catalogue of Minimum Descriptors of Cashew (Anacardium occidentale L.) Germplasm Accessions – I.’’ National Research Center for Cashew, Puttur, India. Swamy, K. R. M., Bhaskara Rao, E. V. V., and Bhat, M. G. (1998). ‘‘Catalogue of Minimum Descriptors of Cashew (Anacardium occidentale L.) Germplasm Accessions – II.’’ National Research Center for Cashew, Puttur, India. Swamy, K. R. M., Bhaskara Rao, E. V. V., and Bhat, M. G. (2000). ‘‘Catalogue of Minimum Descriptors of Cashew (Anacardium occidentale L.) Germplasm Accessions III.’’ National Research Center for Cashew, Puttur, India. Thampan, P. K. (1982). ‘‘Handbook on Coconut Palm,’’ p. 311.. Second printing. Mohan Primlani, Oxford and IBH Publishing Co., New Delhi, India. Thankamma, L. (1974). Phytophthora nicotiane var. nicotiane on Anacardium occidentale in South India. Plant Dis. Reptr. 58, 767–768. Thimmappaiah (1997). In vitro studies in cashew (Anacardium occidentale L.), p. 239. Ph.D. Thesis, Mangalore University, Mangalore, India.

Agronomy and Economy of Important Industrial Crops

313

Thimmappaiah., and Shirly, R. S. (1996). “Micropropagation Studies in Cashew (Anacardium occidentale L.),” p. 65. Natl. Symp. on Hort. Biotech. (Souvenir), Bangalore. Thimmappaiah, R. S., and Shirly, R. S. (1999). In vitro regeneration of cashew (Anacardium occidentale L.). Indian J. Exp. Biol. 37, 384–390. Thomas, K. G. (1978). Crops diversification in arecanut gardens. Indian Arecanut Spices Cocoa J. 1, 97–99. Tucker, R. (1983). ‘‘The Palms of Subequatorial Queensland.’’ Palm and Cycad Society of Australia, Milton, Queensland, Australia. Valeriano, C. (1972). O cajueiro. Boletim do Instituto Biologico de Bahia (Brazil). 11(1), 19–58. Van Eijnatten, C. L. M. (1991). Anacardium occidentale L. In ‘‘Plant Resources of South-East Asia, No. 2. Edible Fruits and Nuts’’ (E. W. M. Verheij and R. E. Coronel, Eds.), p. 446. Pudoc-DLO, Wageningen, The Netherlands. Vandermer, J., and Andow, D. A. (1986). Prophylactic and responsive components of an integrated pest management program. J. Econ. Entom. 79, 299–302. Veeh, H. H., and Veevers, J. J. (1970). Sea level at – 175 m off the Great Barrier Reef 13,000 to 17,000 years ago. Nature (London) 226, 536–537. Venkatasubban, K. R. (1945). Cytological studies in Palmae. Part I. Chromosome number in a few species of palms in British India and Ceylon. Proc. Indian Acad. Sci. 22, 193–207. Vidyasagar, P. S. P. V., and Shama Bhat, K. (1986). A pentatomid bug causes tendernut drop in arecanut. Curr. Sci. 55, 1096–1097. Von Uexkull, H. R. (1972). Response of coconuts to potassium chloride in the Philippines. Oleagineux 27(1), 13–19. Whitehead, R. A. (1963). The processing of coconut pollen. Euphytica 19, 267–275. Whitehead, R. A. (1966). Sample survey and collection of coconut germplasm in the Pacific Islands. 30 May – 5 September 1964. Ministry of Overseas Development (Overseas Research Publication No 16), London, p. 78. Wunnachit, W., Pattison, S. J., Giles, L., Millington, A. J., and Sedgley, M. (1992). Pollen tube growth and genotype compatibility in cashew in relation to yield. J. Hortic.Sci. 67(1), 67–75. Yadava, R. B. R., and Mathai, C. K. (1972). Chlorophyll and organic acid contents of arecanut. Madras Agric. J. 59, 306–307. Yadukumar, N., Abdul Khader, K. B., and Shama Bhat, K. (1985). Scheduling irrigation for arecanut with pan evaporation. In ‘‘Arecanut Research and Development: Proc. Silver Jubilee Symposium on Arecanut Research and Development’’. 1982, pp. 33–37. Yadukumar, N., and Mandal, R. C. (1994). Effect of supplementary irrigation on cashew nut yield. In “Water Management for Plantation Crops: Problems and Prospects” (K. V. Satheesan, Ed.), pp. 79–84. CWRDM, Calicut, India. Zizumbo, D., Fernandez, N., Torres, N., and Orpeza, C. (1999). Lethal yellowing resistance in coconut germplasm for mexico. In ‘‘Current Advances in Coconut Biotechnology’’ (C. Oropeza, J. L. Verdeil, G. R. Ashburner, R. Cardega, and J. M. Santamarma, Eds.), pp. 131–144. Kluwer Academci Publishers, Boston. Zushum, M. (1986). An investigation on meteorological indices for coconut cultivation in China. Oleagineux 41, 119–128.

C H A P T E R

F I V E

Legume–Wheat Rotation Effects on Residual Soil Moisture, Nitrogen and Wheat Yield in Tropical Regions Benjamin O. Danga,*,† Josephine P. Ouma,* Isaiah I. C. Wakindiki,* and Asher Bar-Tal†,1 Contents 1. Introduction 2. Residual Soil Moisture 3. Soil Fertility and Yields 3.1. Soil N enrichment 3.2. Cereal grain yield response 4. Residue Decomposition and N Mineralization 5. Legume–Wheat Rotation Effects on Soil and Yield in a Tropical Humid Climate 5.1. Background 5.2. Soil moisture use effects 5.3. Soil N contribution and yield effects 6. Conclusions References

316 317 320 320 323 326 334 334 335 337 342 343

Abstract Grain legumes grown in rotation with annual cereal crops contribute to the total pool of nitrogen in the soil and improve the yields of cereals. However, the anticipated N benefits of the legume may be positive or negative depending on legume species and its interaction with the environment. Such erratic response may result from excessive water use by the legume phase, its symbiotic performance, effects of soil pH on legume growth and biomass returned N, harvest index and immobilization of nitrate during decomposition of legume residues. A review of the effects of legume–wheat rotation on residual moisture exploitation for enhanced soil N productivity of the tropical soils, including

* {

1

Department of Crops, Horticulture and Soils, Egerton University, Njoro, Kenya Department of Soil Chemistry and Plant Nutrition, Institute of Soils, Water and Environmental Sciences, Agricultural Research Organization, The Volcani Center, Bet-Dagan 50250, Israel Corresponding author

Advances in Agronomy, Volume 101 ISSN 0065-2113, DOI: 10.1016/S0065-2113(08)00805-5

#

2009 Elsevier Inc. All rights reserved.

315

316

Benjamin O. Danga et al.

factors affecting the decomposition of incorporated residues are presented. Whereas soil water storage in the soil profile during the fallow period has been an important consideration in dry land agriculture where water is often limiting crop yields, the growing of a short-term legume in rotation with cereal in a humid climate, and its depletion of fallow water does not adversely affect yield of following cereal crop mainly because of adequate rainfall during the main season for wheat. Soil N is enriched by various grain legumes through biological N fixation which subsequently enhances wheat yields. Non-N benefit includes reduction of wheat root rot incidence which enhances added N uptake, wheat leaf disease and pests. The strategy of using legumes in rotation with wheat in the humid tropics for enhanced soil-N supply, and pest, disease, and weedsbreak effects should therefore be encouraged. Its concluded that introduction of legumes such as chickpea, dolichos, field bean, faba beans in wheat-based cropping is a viable strategy for the reduction of inorganic fertilizer use for the resource poor small and medium scale farmers in Africa.

1. Introduction The immediate need to increase food production in the tropics to feed the rapidly increasing population of the Third World requires that crop yields per hectare must be increased without prejudicing the resource base for future generations (Boddey et al., 1997). Biological nitrogen fixation (BNF), especially that associated with legumes has great potential to contribute to productive and sustainable agricultural systems for the tropics, but more research is required to investigate how biologically fixed N, and the increased BNF contributions resulting from research innovations, can be incorporated into viable agricultural systems to increase crop yields and to substitute N fertilizer inputs in tropical soils. N2 fixation by leguminous crops is a relatively low-cost alternative to N fertilizer, and particularly important in developing countries where few farmers use adequate inorganic fertilizers, because of their limited availability, high cost, and low return on investment (Doyle and Leckie, 1992; Njunie et al., 2004). Nevertheless, the incorporation of legumes in cereal rotation has lagged behind as an alternative cropping system, particularly in sub-Saharan Africa, where nutrient depletion has decreased agricultural production. This is mainly due to a lack of low-cost innovative cropping system that incorporates into the continuous cereal monocultures, a grain legume to increase soil N fertility, during the short dry fallow period under rainfed conditions of the humid tropics. Tropical soils are characterized by 1:1 lattice clays or sesquioxides of relatively low capacity to retain nutrients (CEC) and water (WHC), loss of major nutrients through erosion, and crop harvests (Gachene et al., 1997;

Legume–Wheat Rotation in Tropics

317

Palm et al., 1997). Coupled with widespread soil acidity, these factors contribute to reduced biological N2 fixation, low fertility and reduced crop yields. Nevertheless, long-term fertility of such soils can only be sustained by maintaining their soil organic matter through incorporation of crop residues and the selection of suitable crop rotations or fallows (Sanchez et al., 1997). Increased awareness, by many medium and smallholder farmers who constitute up to 95% of farming community in Africa (Reij, 1991), of the role of legumes as sources of food and fodder and for soil fertility improvement has stimulated research on the influence of both grain and herbaceous legumes in various cropping systems (Njunie et al., 2004). The introduction of legumes into tropical cereal production can increase and sustain their productivity, with only modest inputs of inorganic N fertilizers. This has however been constrained by the alternative use of crop residues as livestock feed resource and the preference for small scale farmers to grow legumes for grain harvest rather than incorporate as green manure. Many studies on legume–cereal rotations have been conducted in the temperate or Mediterranean climates but only few of such studies in Africa. Major considerations in legume–cereal cropping systems have been the ability of the legumes to fix and supply sufficient N and their effects on fallow water use and on the yield of subsequent cereal crops. The objectives of this review were (i) to examine the effects of fallow residual moisture exploitation by legume on yield of following wheat crop, (ii) compare response of wheat to different antecedent legumes in cereal rotations, (iii) examine factors influencing the decomposition of incorporated residues into the soil, and (iv) to examine interactions of chickpea–wheat rotation, soil water and soil N fertility in a humid climate in Kenya.

2. Residual Soil Moisture Crop rotation plays an important role in the maintenance of soil fertility, improvement of the soil physical environment, control of pests, diseases, and weeds, and control of soil erosion (Bagayonko et al., 1992). However, water and N contents of the soil are the main factors affected by legume rotation (Papastylianou, 1993) and therefore constitute the major considerations in substituting a cover crop for fallow in wheat rotations (McGuire et al., 1998). Water is the most crucial factor affecting crop yields (Oweis et al., 2004) hence leaving land fallow to increase soil water storage in the profile and to restore soil fertility of depleted soils has been widely practiced in many parts of the world. However, depending on the environment, seasonal rainfall, and soil type, residual moisture available during dry

318

Benjamin O. Danga et al.

fallow periods could be utilized for the establishment of fast growing legumes for rapid replenishment of soil fertility (Sanchez, 1999), the success of which vary according to the type of legume chosen. The fallow period in dry land cropping systems increases storage of soil water (French, 1978a; Schlegel and Havlin, 1997). In a 2-year rotation system in a semiarid environment, Schlegel and Havlin (1997) reported that growth of legume (Hairy vetch) during the fallow period depleted soil water up to 178 mm and consistently reduced subsequent wheat grain yields, for example, by 12–16 kg ha
1 for every mm decrease of soil water at wheat planting. Nielsen and Vigil (2005) observed similar results. These authors thus concluded that even though legume growth during fallow may have enhanced soil N through BNF, the penalty in reduced grain yield resulting from water use overshadowed any benefit from enhanced N fertility and therefore green fallow is too detrimental to subsequent crop yields in those areas. In a related study in dry land farming, French (1978a) reported that fallowing land increased soil water storage from nil to 125 mm with a mean of 28 mm (0–120 cm depth) compared to non-fallow soil. The amount of storage was related to soil depth particularly the 15–30 cm, soil texture, and the season. He noted that the rainfall received prior to or at the start of the fallow period affected residual soil moisture storage and described additional water (mm), Ws, at sowing due to fallowing by the following linear regression equation,

Ws ¼ 1:7 þ 0:86x; p < 0:01

ð1Þ

where x is the clay content (%) in the 15–30 cm layer. In a similar study, French (1978b) found that each millimeter of water stored through fallowing gave an average 8 kg wheat grain per hectare. Overall, wheat grain yield increased by a mean of 335 kg ha
1 due to fallowing. Thus, water use by a cover crop in dry land farming is generally considered the main disadvantage of substituting a green manure for fallow as demonstrated by the work of McGuire et al. (1998) and Schlegel and Havlin (1997) and supported by the work of French (1978b). A humid climate is characterized by two rainfall seasons per year; a long rainfall season (400–600 mm) and a short one (250–350 mm). In Kenya it was reported that fallowing land (e.g., clean fallow) during the relatively short rainfall season conserved moisture storage in the profile and increased wheat yields in the long rainfall season (Njihia, 1988). Working on an Andosol in Kenya, Njihia (1988) observed that despite losing moisture to near or beyond wilting point (
1,500 kPa) in the top 40 cm, clean fallows contained moisture in the lower depths at around field capacity (33 kPa) whereas the plots that were left with weeds lost soil moisture in the entire rooting zone (135 cm) to near wilting point and much more at the surface

Legume–Wheat Rotation in Tropics

319

soil layers. Nevertheless, evaporation losses were substantial in the order of 290 mm for the 6-month fallow period, sufficient moisture to grow a short season, low water-demanding crop. Thus, in the humid regions where plant-available water is sufficient for annual cropping, forage or grain legumes are often inserted into the rotation to improve soil productivity through increased organic matter and N2 fixation. The benefit of legumes is attributed to increased N supply and enhanced soil quality (Green and Biederbeck, 1995). The stored residual moisture, coupled with the low rainfall (250–350 mm) during the short rainfall season when the legume is grown, was sufficient to grow the legume, while there was no significant negative effect of such water use on the performance of following cereal crop (Njihia, 1988) normally grown during the long rainfall season with adequate rainfall (400–600 mm). In drought years, Utomo et al. (1987) observed that legumes in rotation could substantially reduce soil water and subsequent non-legume grain yields even in humid regions. However, Rathore et al. (1996) reported that residual soil moisture (172–203 mm) was sufficient to support post-rainy season crops of chickpea (Cicer arietinum L.) and mustard (Brassica juncia L.). Nevertheless, studies by Nielsen and Vigil (2005) also point out that the extent of soil moisture depletion at wheat planting and consequently wheat grain yield depend on the termination dates of the fallow legumes. In that study, the wheat grain yield declined from 3,016 to 2,271 kg ha
1 when available soil water at wheat planting declined from 320 to 216 mm over four termination dates, each separated by 2 weeks intervals. Marcellos et al. (1998) compared the effect of chickpea–wheat and wheat–wheat sequences on soil water in a similar environment and observed no consistent differences in effects on soil water in either preharvest or after the summer fallow. In North Dakota, Badaruddin and Meyer (1989) observed that legumes in rotation with spring wheat did not significantly reduce soil water content compared with continuous wheat or wheat-fallow. However, McGuire et al. (1998) reported reductions in soil water content of 1.5 and 6.6 cm in wet and dry winter, respectively, compared with soil water after growing a green manure crop during the winter of the fallow year. The studies of McGuire et al. (1998) point out that although depletion of water occurred with cover crop than with fallow, any detrimental effects on the following wheat crop was eliminated by heavy rains during winter growing seasons. Thus under such conditions, yields of unfertilized wheat following a winter legume cover crop were similar to those of fertilized wheat after fallow. In conclusion, water storage in the soil profile during the fallow period is an important consideration in dry land agriculture where water is often limiting crop yields, but not in a humid tropical environment, where good seasonal rainfall can offset the negative effects of any legume utilization of fallow water.

320

Benjamin O. Danga et al.

3. Soil Fertility and Yields In regions where water use during fallow does not affect significantly yields of succeeding crop, it could be profitable to use the residual soil moisture during a short fallow period to grow a legume crop for enhanced soil N fertility and yields of subsequent cereal crop. The incorporation of legumes in cereal rotation has lagged behind as an alternative cropping system, particularly in sub-Saharan Africa, where nutrient depletion has decreased agricultural production. However, the benefits of legumes to soil nitrogen (N) fertility and cereal yields have been widely reported in different environments for a large number of agricultural production systems in which the legumes improved soil N fertility content through BNF (Ahmad et al., 2001; Felton et al., 1998; Lo´pez-Bellido et al., 2004; Marcellos et al., 1998; Peoples et al., 1995; Toomsan et al., 1995; Turpin et al., 2002). Annual crop legumes, mainly pulses, have been widely used to improve yields of the cereals in rotation and contribute to the total pool of N in the soil (Herridge et al., 1995). It has been suggested that legume crops will add N to the soil system via symbiotic dinitrogen fixation if the total quantity of N symbiotically fixed by the legume crop is greater than the quantity of N removed in harvested grain (Doughton et al., 1993).

3.1. Soil N enrichment The benefits associated with the inclusion of a legume in a crop rotation can be partitioned into the N effect and the non-N effect (Bullock, 1992; Stevenson and van Kessel, 1996). However, several studies have shown that increase in the yield of cereals following the legumes is mainly due to the N contribution (Herridge et al., 1995; Lo´pez-Bellido et al., 2004; McGuire et al., 1998; Turpin et al., 2002), attributable to the symbiotic N2 fixation in the legume (the N effect). Peoples and Craswell (1995) compiled estimates of fixed N by different pulse crops as reported by different authors. They showed that in various parts of the world, pulses fixed from 0 to 450 kg N2 ha
1 per annum, thus indicating that some legume crops may not always make a positive contribution to the soil in which they grow. The amount of N contributed to the soil system by the legume crops depends on the rate of symbiotic N2-fixing activity, growth and N harvest index of the legume crops. The rate of N2 fixation varies considerably, depending on type of legume cultivar, method of measurement, the presence of appropriate rhizobia, and certain soil and environmental variables, including soil moisture, NO3 level, soil acidity, and P nutrition (Amanuel et al., 2000; Andrade et al., 2002; Beck, 1992; Doughton et al., 1993 Herridge et al., 1995).

Legume–Wheat Rotation in Tropics

321

Generally, a large proportion of the N accumulated during the growth of legume crops are removed with the harvested seed, and it is commonly concluded that the net return of fixed N to the soil is likely to be small when the amounts of N2 fixed by the legumes have been compared with the amounts removed in the seed. Thus, most calculations have relied on aboveground measures of fixed N. Hence, most of these previous studies overlooked N content of the roots and therefore underestimated N benefit from legumes (Table 1). Such were the studies of, for example, Evans et al. (1991) and Larson et al. (1989) among others, who reported net negative N contribution for some tropical grain legumes when only shoot N but not N contained in the nodulated roots were taken into account in calculating N balances (Table 1). For example, recent studies using 15N labeling suggest that chickpea can fix up to 146–214 kg N ha
1 (Turpin et al., 2002) when roots are included. Earlier reported values were lower often in the range of 20–140 kg ha
1 (Carranca et al., 1999; Kumar and Abbo, 2001; Lo´pez-Bellido et al., 2004; Marcellos et al., 1998; Pilbeam et al., 1998). The soil N balances for chickpea and faba bean increased to 80–135 and 79–157 kg N ha
1, respectively (Turpin et al., 2002), when the contribution of fertilizer N and below ground N was included. Interestingly, in spite of these high values reported, Lo´pez-Bellido et al. (2004) found chickpea crop to be a poor N2 fixer, which also leaves poor above-ground N residues (C:N 0.8–1.4%) due to a high N harvest index (Lo´pez-Bellido et al., 2004). Thus, they concluded that chickpea crop seems incapable of meeting N demands by fixation and does not even supply an equivalent quantity of 50 kg ha
1 of N fertilizer to the following wheat crop. However, as stated above, these authors did not consider root derived N but instead analyzed N content of straw and seed. Further, they did not use the more precise isotopic 15N-based technique to estimate fixed N and hence N balance as prescribed by Chalk (1998). Comparatively, the narrow leaf lupins fixed less N—an average of 129 kg N ha
1 with a net contribution of 70 kg N ha
1 to the soil, whereas values for pea were 104 kg N and 42 kg N ha
1, respectively (Marcellos et al., 1998). The grain legumes with high biomass N, low N harvest index, and high symbiotic dependence have the greatest potential to contribute positively to soil N status (Chalk, 1998). In tropical Africa, where 15N balance studies have lagged behind, Amanuel et al. (2000) reported that, for a wheat rotation in Ethiopia, faba bean was the best N2 fixer and that soil N balances after faba bean were positive and ranged between 12 and 58 kg N ha–1 (Table 1) Similarly, in Kenya, work by ICRISAT (1993) reported that chickpea contributed 30–35 kg N ha
1 to wheat, while TSBF (1994) reported approximately similar contribution of about 40 kg N ha
1 to maize crop in Kenya. The values were lower than reported elsewhere probably because of the technological gap in N2 fixation measurements. One season study by Onwonga

Table 1 Fixed N and N balance of some legumes with and without root N included Fixed N (kg N ha
1)

N balances (kg N ha
1)

Legume

Roots included

Without roots

Roots included

Without roots

Mung bean Lentil

112 (Shah et al., 2003) 42–85 (mean 68) (Shah et al., 2003) 209–275 (Turpin et al., 2002)

74 (Shah et al., 2003) –

+64 (Shah et al., 2003) +27 (Shah et al., 2003)

+9 (Shah et al., 2003) +16 (Shah et al., 2003)

64–96 (Smith et al. 1987)

79–157 (Turpin et al., 2002)

Chickpea

146–214 (Turpin et al., 2002)

20–60 (Lo´pez-Bellido et al., 2004).

80–135 (Turpin et al., 2002)

Narrow leaf lupins Garden Pea.

129 (Marcellos et al., 1998)

—

70 (Marcellos et al., 1998)

104 (Marcellos et al., 1998)

33–126 (Beck et al. 1991)

42 (Marcellos et al., 1998)

12–58 (Amanuel et al., 2000)
20 to +57 (Smith et al., 1987) 30–35 (ICRISAT, 1993); 40 (TSBF, 1994); 50 (Lo´pez-Bellido et al., 2004)
41 to +135 (mean 38) (Evans et al., 1989)
20 to +1 (Beck et al., 1991)
32 to
96 (mean 18) (Evans et al., 1989)

Faba bean.

Parenthesis indicates reference.

Legume–Wheat Rotation in Tropics

323

(1997) in Njoro found a fertilizer replacement value of 38.1 kg N ha
1 after chickpea crop followed by wheat. Studies by Shah et al. (2003) found that average, mungbean fixed 112 kg N ha
1 (+ residues) and 74 kg N ha
1 (
residues), with N balances of +64 kg N ha
1 (+ residues) and +9 kg N ha
1 (
residues) while lentil fixed 42–85 kg N ha
1, with a mean of 68 kg N ha
1. Average N balances for lentil were +27 kg N ha
1 (+ residues) and +16 kg N ha
1 (
residues). Chalk (1998) has reviewed the sources of N benefit and their relative importance. Apart from its symbiotic N2 fixation, the legume may remove lesser inorganic N from the soil compared with the cereal because part of its N requirement is met by N2 fixation. This concept of ‘‘nitrate sparing’’ only applies to situations when N supply exceeds crop N demand (Turpin et al., 2002), described by Chalk (1998), as apparent N sparing effect. In a comparative study by Turpin et al. (2002), high biomass chickpea and faba bean may not spare significant amounts of soil N especially where soil nitrate levels are low. Other studies reported the apparent N effect of 18 kg N ha
1 (Marcellos et al., 1998) and from 2 to 33 kg N ha
1 (Herridge et al., 1995) for chickpea relative to wheat. Release of mineral N from decomposing legume residues is an important source of legume N benefit (Doughton and McKenzie, 1984). However, Wani et al. (1994) observed that these benefits largely depended upon the total plant biomass produced; amount of N2 fixed and amount of N added to the soil through roots, nodules, and leaf fall. Marcellos et al. (1998) studied the potential of chickpea as a rotation crop with wheat and concluded that chickpea did not fix sufficient N2 or mineralize sufficient N from residues either to maintain soil N fertility or to sustain a productive chickpea–wheat rotation without inputs of additional fertilizer N. Such a situation is likely, particularly when the resulting residues are of poor N content (<1.8%) as a consequence of high N harvest index, but could be different in other environments. Hence, crop management that incorporates inorganic fertilizer supplementation to bridge the N gap could be vital to the sustainability of a wheat–legume rotation.

3.2. Cereal grain yield response Reported grain yield responses of cereals due to the tropical grain legumes varied from 0.2 to 3.68 ton ha
1 compared with cereal–cereal monocrop yields, with relative increases reported in the range of 16–353% (Peoples and Herridge, 1990). However, Evans et al. (1991) and Oikeh et al. (1998) reported that yield responses to previous legume crops are mainly in the range of 50–80%. Peoples and Craswell (1992) reported improvement of cereal yield following monocropped legumes to a tune of 30–359% increases. The measured response is dependent on the antecedent pulse crop.

324

Benjamin O. Danga et al.

The type of legume used as a preceding crop determines the quantity of N benefit. The work of Strong et al. (1986) (Table 2) showed that chickpea was superior to faba bean and garden pea, which increased wheat grain yields by 61, 40, and 47%, respectively. In a series of on-farm trials in northern New South Wales, Australia, yields of wheat were increased by 20–144% following chickpea (Herridge et al., 1994). They explained the yield increases in terms of increased availability of N. According to Dalal et al. (1998) chickpea benefit in terms of wheat grain yields vary, from 17 to 61%, with a mean increase in grain yield of 40% (825 kg ha
1) while Lo´pezBellido and Lo´pez-Bellido (2001) working in a Mediterranean climate in Spain reported for wheat an average rotation effect of 644 kg ha
1, compared to 846 and 948 kg ha
1 for fallow and faba bean, respectively. Using mungbean (Vigna radiata) and blackgram (Vigna mungo) as previous legume crops, Ahmad et al. (2001) reported an increase of wheat grain yields of between 0.5 and 1.1 ton ha
1 and an increase of 54% in wheat grain N compared to cereal–cereal sequence. The differences in the wheat yield response attributed to different legumes probably reflect the differences in environmental conditions of climate and soils under which the experiments were done. In a humid climate in south eastern Nigeria, Okpara et al. (2003) reported that pre-maize mucuna (Mucuna pruriens) green manure significantly increased maize grain yields as compared to other legumes such as centrosema (Centrosema pubescens) and pueraria (Pueraria phaseoloides). Higher maize response to mucuna was attributed to the latters’ ability to fix large amounts of atmospheric N (Okpara et al., 2003). The portion of the rotation benefit not associated with the increased availability of N is referred to as the non-N effect. Improvements in soil structure, the breaking of pest and disease cycles, which afflict cereal monoculture, and phytotoxic and allelopathic effects of different crop residues have all been implicated in the yield response (Peoples and Herridge, 1990). Fu (2000) found that common root rot (Cochliobolus sativus) severity in the wheat–wheat rotation was 15% greater than that in the chickpea–wheat rotation, suggesting that the inclusion of chickpea in the crop rotation facilitated a break in the disease cycle. Probably chickpea was not a suitable host plant for these diseases. Likewise, the wheat leaf disease (tan spot: Pyrenophra tritici-repentis) severity was 2 and 3 units greater than in the chickpea–wheat rotation. Martens et al. (1984) also observed that the inclusion of pea in crop rotations decreased the occurrence of wheat pathogens because pea was not a suitable host plant for wheat pathogens. Root rot and leaf disease severity explained 13 and 8% of the total variation in the wheat grain yield and N accumulation, respectively (Fu, 2000). In the northern grain belt of Australia, Felton et al. (1998) reported that crown rot (Fusarium graminearum Schwabe Group 1) incidence for wheat after wheat was 16%, compared with 2% for wheat after chickpea. They noted that such yield-loss figures, even if partly attributable to crown

Table 2

Grain yield (t ha
1) and N yield of grain +straw (kg ha
1) of wheat following legume, cereal, and oilseed crops Yield response of wheat to legumes relative to Yeld

Oats

Linseed

Antecedent crop

Grain

N

Grain

N

Grain

N

Chickpea Faba bean Field pea Lathyrus Lentil Lupin Vetch Oats Linseed

1.48 1.29 1.35 1.54 1.56 1.69 1.27 0.92 1.27

37.5 34.2 43.4 47.0 55.5 41.8 38.7 19.6 30.6

0.56 (61) 0.37 (40) 0.43 (47) 0.62 (67) 0.64 (70) 0.77 (84) 0.35 (38) – –

17.9 (91) 14.6 (74) 23.8 (121) 27.4 (140) 35.9 (185) 22.2 (113) 19.1 (97) – –

0.21 (17) 0.02 (2) 0.06 (5) 0.35 (28) 0.37 (29) 0.50 (39) 0 (0) – –

6.9 (22) 3.6 (12) 12.8 (42) 16.6 (54) 24.9 (81) 11.2 (37) 8.1 (26) – –

After Strong et al. (1986). Data in parentheses are relative increases (%).

326

Benjamin O. Danga et al.

rot and perhaps other diseases, indicate a potentially large economic impact of disease in the wheat–wheat systems. Bullock (1992), in a critical review regarding crop rotation, observed that since not all pests detrimentally influencing crops are recognized, it may be hypothesized that much of the rotation benefit is probably due to the alleviation of unrecognized pests. Research by Chabi-Olaye et al. (2005) indicated that grain losses maize (Zea mays) due to noctuid stemborer Busseola fusca (Fuller) was reduced 1.9–3.1 times when cowpea and soybean were grown prior to maize, while it was 4.5–11 less in rotation with mucuna cover crop. Other unknown factors that may account for increased wheat grain yields may include other non-N nutrients such as P and K (Bullock, 1992) and the release of growth-promoting substances (e.g., triacontanol) from legume residue (Fyson and Oaks, 1990; Ries et al., 1977) were responsible for a portion of the non-N effect. Rovira (1976) demonstrated that reduced disease severity can increase the N content of wheat tissues. Cook (1992) suggested that no single factor would do more for N-use efficiency in wheat production than having a healthy root system that would take advantage of the N applied to the crop. Plant roots play an important role in N uptake and redistribution. Cook (1992) and Rovira (1976) found that the rotation benefit of pea on wheat root disease damage enhanced wheat root exploration of the soil. Stevenson and van Kessel (1996) reported that reduced severity of leaf disease and grassy weed infestation was related to 91% of the yield advantage associated with the pea (i.e., non-N effect). In south eastern Nigeria, Okpara et al. (2003) reported that mucuna (M. pruriens) green manure residues increased maize grain yields as compared to NPK fertilizers when the fallow period under mucuna cover was for 2 years. Probably, the mucuna green manure improved the physical characteristics of the soil and supplied the macro- and trace elements not contained in the inorganic fertilizer ( Jiao, 1983).

4. Residue Decomposition and N Mineralization Biologically fixed N2 in legumes is returned to the soil mainly through decomposition of incorporated residues. Thus, the immediate impact of residue application is on the availability of nitrogen to the subsequent crop, as a consequence of mineralization–immobilization processes. It has been suggested by Stute and Posner (1995) that if legumes have to be effective in supplying N, there has to be synchrony between N released and crop demand. According to Palm (1995) organic residues release about 80% of their nutrients during decomposition but less than 20% is absorbed by crops. This low efficiency can be explained by the lack of synchrony between

Legume–Wheat Rotation in Tropics

327

nutrient release and uptake by the crop. Studies on decomposition and mineralization rates of chickpea residues are therefore necessary to synchronize nutrient release and uptake by wheat. Mineralization occurs when inorganic forms of an element are released during catabolism of organic resources, for example, CO2 from carbohydrates and NH4+ from organic N components. The rate of mineral N release from organic residues incorporated into the soil is an important factor in the process of synchronizing nutrients’ release and plant uptake processes (Budelman, 1987), and for fertilizer application management. Factors, which affect the decomposition rates of residues and green manures, have been identified by several authors. Generally, decomposition and nutrient release patterns are influenced by climatic factors, that is, temperature and humidity; edaphic factors, that is, soil moisture, aeration, temperature, microbial biomass and nutrient status (Swift et al., 1979) and residue quality factors (Heal et al., 1997; Swift et al., 1979). Work by Fu et al. (1987) and Xu et al. (2006) has shown that soil pH affect mineralization of residues and that mineral N accumulation increased under alkaline than acidic soil pH. Ellert and Bettany (1988) suggested that soil moisture and temperature are the most influential factors affecting mineralization rates in soil. Soil moisture influences microbial activity both directly due to a decline in microbial populations under water stress and due to the inability of dissolved organics and mineralized materials to diffuse from microsites, and indirectly by influencing soil aeration status. Stanford and Epstein (1974) evaluated the effects of water content and matric potential on N mineralization. They found that the relationship between amounts of accumulated mineral N and soil water content is almost linear in the range from optimum water content (
0.03) to (
0.01) to
1.5 MPa. With decreasing water content, N mineralization decreased as well and for water above optimum, mineralized N was reduced presumably due to denitrification. According to Beauchamp and Hume (1997), the optimum moisture level for mineralization of N is ranging approximately between 45 and 60% water-filled porosity (WFP). At about 60% WFP, there is a sharp transition to O2-limiting conditions and denitrification begins to increase (Beauchamp and Hume, 1997). MacDonald et al. (1995) investigated the temperature effects on the kinetics of microbial respiration and net N mineralization. They found the cumulative respired C and mineralized N to increase with temperature. Lomander et al. (1998) suggested that
5  C was the minimum temperature for decomposition activity while the maximum decomposition rate has been reported to occur between 30 and 35  C (Stott et al., 1990). Zak et al. (1999) studied the combined effects of soil temperature and matric potential on the kinetics of microbial respiration and net N mineralization. Using first order kinetic models, they were able to account for 96–99% of the accumulated CO2 and inorganic N, with highly significant correlation coefficients. The authors report significant temperature–matric potential

328

Benjamin O. Danga et al.

interaction, based on the greatest decline of substrate pools for microbial respiration and net N mineralization between (
0.01) and (
0.3) MPa at 25 C. They concluded that high rates of microbial activity at warm soil temperature are limited by the diffusion of substrate to metabolically active cells. The influence of soil texture on N mineralization is primarily related to clay content and to some degree, clay mineralogy (Franzluebbers et al., 1996; Scott et al., 1996). Coarse textured soils have a more active microbial population and organic matter is more available for mineralization than soils of finer texture (Breland and Hansen., 1996; Franzluebbers et al., 1996; Scott et al., 1996). Franzluebbers et al. (1996) found that the rate of basal soil respiration per unit of soil microbial carbon was greatest in soils of coarse texture and as clay content increased, basal soil respiration decreased. Soil organic matter in coarse textured soils appears to be more labile, whereas in finer textured soils, the complex soil structure provides greater protection to the soil organic matter, thus reducing the turnover of organic N (Franzluebbers et al., 1996). As clay content increases, soil surface area and organic matter stabilization potential increases (van Veen et al., 1985). Oxygen levels also affect decomposition. The process of immobilization is enhanced under aerobic conditions and impaired under less aerobic conditions. When large amounts of organic materials are added, conditions leading to anaerobiosis may develop, which affect the ability of soil microorganisms to assimilate C and N and seriously impairing N immobilization (Vinten et al., 2002). Residue management, that is, incorporation technique, amount and time of residue incorporation and mechanical treatment of the residue are important factors influencing the decomposition rates. The decomposition rate is positively correlated to a decreasing particle size of the plant material. Pre-treating of the plant material by cutting it into small pieces or fine grinding prior to incorporation to some extent mimic the effects of rapid comminution by microorganisms on decomposition, offering a relatively larger surface and increasing the possibilities of microbial attack and activity ( Jensen, 1994; Tarafdar et al., 2001). Fine grinding plant residues also result in higher levels of soluble organic C and N, and enhance decomposition of materials with high C:N ratio ( Jensen, 1994). In contrast, a large particle size will further delay the decomposition of recalcitrant plant components. The increased availability of soluble components will lead to a more extensive microbial assimilation of both C and N (Ambus and Jensen, 1997; Rovira and Vallejo, 1997), which reduces the amount of N readily lost by leaching or available for plant uptake (Ambus et al., 2001) as a consequence of enhanced N immobilization. Residue decomposition is highly dependent on the contact with the microorganisms and the available mineral source (Garnier et al., 2003; Henriksen and Breland, 2002). Thus incorporation of straw results in faster decomposition because there is more

329

Legume–Wheat Rotation in Tropics

intimate contact with the soil particles and microorganisms, and the residues are maintained in a moist environment that is more favorable for decomposition. The spatial distribution determines the availability of the incorporated plant materials to the decomposers. By combining fine particle size with a thorough incorporation, the rate of N immobilization in soil is maximized. If the plant material is not evenly distributed in the soil, ‘‘cold and hot decomposition spots’’ may occur in which the rate of N immobilization may differ considerably, mainly depending on the level of available N at each spot (Hessels
e et al., 2001). The effect of addition of readily decomposable substances such as plant residues in soil in stimulating the mineralization of native soil organic C is termed ‘‘priming’’ effect. This phenomenon, reported by Kuzyakov et al. (2000), has been attributed to changes in the activity, amount/or composition of microbial biomass. Disregarding this priming effect may sometimes result in overestimation of the decomposition rate of added plant materials (Wang et al., 2004). Nutrient release patterns have mostly been related to the residue quality characteristics of organic materials (Heal et al., 1997). Various biochemical fractions of the residues and their ratios have been used as indexes of biochemical quality. The concentrations of nutrients particularly total N (Frankenberger and Abdelmagid, 1985) or its inverse (Quemada and Cabrera, 1995) have been reported to be the best indexes for residue C and N release rates of legume or grass residues. Other indexes such as soluble C (Oglesby and Fownes, 1992), C:N ratio (Tian et al., 1995), cellulose (Bending et al., 1998), or lignins (Giller and Cadisch, 1997; Mu¨ller et al., 1988) also influence residue decomposition or C and N mineralization rates. Some ratios such as lignin-to-N (Vigil and Kissel, 1991) and soluble polyphenols and polyphenol:N (Oglesby and Fownes, 1992) as well as polyphenol plus lignin-to-N (Constantinides and Fownes, 1994) have also been used as indexes of residue nutrient N release. Using principal componentregression (PCR), Ruffo and Bollero (2003) observed that large concentrations of neutral detergent fiber (NDF) and acid detergent fiber (ADF) are associated with low cover crop biomass decomposition and slow C and N release rates. They reported that availability of C and N rather than their total concentration in the residue plays a critical role in residue decomposition and nutrient release, underlining the importance of labile fractions in nutrient supply. Attempts have also been made to develop a residue quality index that best describes C and N residue release rates; For example Villegas-Pangga et al. (2000) developed a straw quality index (SQI) to describe the decomposition rate of straw as follows:

SQI ¼
56:85 þ ð11:65  %NÞ þ ð1:25  %DOMÞ þð2:59  %ligninÞ; r 2 ¼ 0:81 where DOM is digestible organic matter.

ð2Þ

330

Benjamin O. Danga et al.

Hence, there is still no consensus on the most suitable set of quality parameters to use in assessing nutrient release. Giller and Cadisch (1997) thus concluded that no single index could characterize the quality of plant residues. It was also noted by Hadas et al. (2004) that the suitability of certain properties to predict rates of decomposition of residues and their effect on available N was difficult since a wide range of materials tested resulted in different conclusions. The N concentration and the C:N ratio of the plant material thus still probably serve as the most robust indexes of residue quality when all plant materials are considered (Constantinides and Fownes, 1994). However, when the plant materials contain high concentrations of recalcitrant C or lignin and/or polyphenols, there may be little mineralization of plant N in spite of N being considerably greater than the critical level (Mu¨ller et al., 1988). Thus, not all organic materials with high N values exhibit net N mineralization. Lignin contents >150 g kg
1 slow N release considerably, and polyphenol contents >30–40 g kg
1 can result in net immobilization of N (Palm, 1995). Constantinides and Fownes (1994) reported that lignin and polyphenols are important modifiers of N release for the fresh, nonsenescent leaves of high quality materials. Nitrogen concentration in tissue ranging from 18 to 22 g kg
1 is the critical value for the transition from net immobilization to net mineralization (Mu¨ller et al., 1988). Nitrogen is required for microbial growth and proliferation, and theoretically the optimum C/N ratio of the decomposable substrate is 25 (Heal et al., 1997). Hence, application of organic matter having C:N ratio greater than 25 will result in N immobilization, while at lower C:N ratios (e.g., leguminous green manure) mineralization will be preferred. However, Vigil and Kissel (1991) showed that the break-even point between net N immobilization and mineralization was at a C:N ratio of 40 when net mineralization of residues after extended incubation (medium to long-term experiments) was considered. This critical C:N ratio is however higher than values often reported and therefore may not be useful for estimating short-term mineralization or mineralization kinetics but points to the role and size of more recalcitrant biochemical fractions in influencing later stages of decomposition. Legume residues are considered as high quality materials and have low C:N ratio, low content of lignin, polyphenols, and therefore release nutrients rapidly during decomposition (Handayanto and Giller, 1997). Onwonga (1997) in Njoro, Kenya and Villegas-Pangga et al. (2000) worked with chickpea residues of a C:N ratio of 13 and 13.3, respectively and observed faster mineralization of chickpea straw releasing 55.1 or 60% of its N content in 42 and 90 days, respectively. In another study, Pilbeam et al. (1998) working at ICARDA in Syria found negative net N mineralization. They used chickpea residues (stems) of high C:N ratio (123.3). Thus, the differences in mineralization rates were related to the composition of the chickpea materials used, especially C: N ratio or percentage N that resulted in faster rates of decay.

Legume–Wheat Rotation in Tropics

331

Pilbeam et al. (1998) suggested that immobilization rather than loss processes was responsible for the decline in the mineral N content of the soil, due to the high C:N ratio (i.e., 123.3) of the residues. The authors conclude that decomposition of residues in the field may in the short term reduce rather than increase the availability of N for crop growth. The C:N ratios of crop residues are dependent on the crop genotype, length of growing season, growth medium, and the environmental conditions. In their models of plant residue decomposition, Hadas et al. (2004) and van Veen et al. (1984) assumed that plant residues incorporated into the soils are composed of three fractions differing in their kinetics of decomposition and release of nutrients. The components include (i) easily or rapidly decomposable, soluble carbohydrates and proteins (e.g., sugars and amino acids) with a turnover period of few hours to few days, (ii) intermediately or slowly decomposable materials attributed to structural carbohydrates such as cellulose and hemi cellulose, and (iii) materials that are relatively resistant to decomposition, such as lignin. Hadas et al. (2004) found that soluble components of the residues had a strong short-term effect on available N in soil while the cellulose-like pool was important for both short- and mediumterm effects. As already pointed out, large concentrations of NDF and ADF are associated with low cover crop biomass decomposition and slow C and N release rates (Ruffo and Bollero, 2003). Knowing the decomposability of the different fractions of heterogeneous materials such as chickpea, rather than their total mineral N content could be useful in estimating their N contribution and management. This can be achieved by adopting a modeling approach. Net mineralization of C and N is best described mathematically by using first-order kinetics models (i.e., rates of decomposition are proportional to the concentrations of the decomposing components in soil and to their decomposition rate constants), which specify how mineralization rates change over time. The most widely used rate equation to predict accumulation of N is that suggested by Stanford and Smith (1972),

Nt ¼ N0 ð1
e
kt Þ

ð3Þ

where Nt is the amount of inorganic nitrogen released at time t, N0 is the nitrogen potentially mineralizable at the beginning of the experiment, and k is the time invariant specific rate of mineralization (d
1) and t is the elapsed time (d). Njunie et al. (2004) used nonlinear regression equations to describe the average percentages of original ash-free plant material dry weight, N, P, K, Ca, and Mg remaining. They found that decomposition and nutrient release from foliage of two legume green manure residues (Dolichos and clitoria) was best fitted by an asymptotic model (4) (Fig. 1) indicating that a fraction

332

Benjamin O. Danga et al.

Nitrogen residue remaining,%

120 Dolichos, 2 mo Dolichos, 4 mo

100 80 60 40 20 0 0

2

4

6

8

10

12

14

16

18

120 Clitoria, 6 wk Clitoria, 10 wk

Nitrogen remaining,%

100 80 60 40 20 0 0

2

4

6

8 10 12 Time in field,wk

14

16

18

Figure 1 Percentage of N remaining in dolichos (top) and clitoria (bottom) residue litterbags as a function of time after placement in the field. Symbols correspond to mean values across four cropping systems, and the lines correspond to the prediction by asymptotic models used to describe nutrient release from clitoria and dolichos residues. Each line represents a reduced model for the 1999 and 2000 growing seasons. (From Njunie et al., 2004).

the residue or nutrients will not decompose or release, at least during the time of study (i.e., 16 weeks in this study).

Y ¼ yo þ ð100
yo Þ exp ð
k1 xÞ þ e

ð4Þ

Legume–Wheat Rotation in Tropics

333

where Y is the percentage of original plant material or nutrient remaining at time x; yo represents the fraction of residue or nutrient that will not be decomposed or released during the time of study; k1 is the dry matter disappearance or nutrient release rate constants; and e is the random error for uncontrolled factors. The use of reduced models suggests that the legume residues (clitoria or dolichos in this study) may be cut at any of the growth stages without compromising the decomposition rate. Thus, the timing of cutting can conveniently be targeted to minimize competition with main crops while recycling substantial amounts of nutrients back to the system. Overall, dolichos residue disappeared faster than clitoria. The residue decomposition rates (k) for clitoria and dolichos were 0.2 and 0.5 wk
1, respectively. The N, P, and Mg asymptotic release curves (Fig. 1 for N; P and Mg not shown) indicate that there were some factors limiting the release of the respective nutrients. The authors suggested that nutrient release depended on microbial activity and was thus constrained under moisture-limiting conditions. Alternatively, soluble nutrient compounds from dolichos residues may have precipitated on the residue material from other sources. One possible explanation is that lignin and polyphenolic compounds may have influenced the decomposition and nutrient release rates of these legumes (Vallis and Jones, 1973). In a similar study in tropical Taiwan and the Philippines, Thonnissen et al. (2000) evaluated the decomposition and N release of two legume green manures (GM), soybean [Glycine max (L.) Merr.] and indigofera (Indigofera tinctoria L.) which were applied either as surface mulch or incorporated GM. They found that the N content of 60 to 74 d soybean GM varied between 110 and 140 kg N ha
1 and that of indigofera between 5 and 40 kg N ha
1. Soybean and indigofera decomposed rapidly, losing 30–70% of their biomass within 5 wk after application, depending on GM placement, season (wet vs dry), and location. Soil nitrate contents increased corresponding to GM N release at all locations and seasons, with a maximum increase of 80–100 kg NO3–N ha
1 with incorporated soybean (Fig. 2). In south eastern Nigeria, Okpara et al. (2003) reported that mucuna (M. pruriens) green manure residues gave higher grain yield of succeeding maize attributable to higher N content of the green manures as a result of higher N fixation of the mucuna compared to other legumes. A lot of work has therefore been done to improve our understanding of the decomposition of residues incorporated into the soil. For example, the fallow grain legumes contribute significant quantities of N upon decay of their residues incorporated into the soil (Carranca et al., 1999; Cheruiyot et al., 2001; Lo´pez-Bellido et al., 2004). However, to develop effective legume fallowing techniques that are acceptable to the medium scale and smallholder farmers in these areas, it is necessary to know the decomposition and nutrient release dynamics of the legume residues at the different stages of maturity, to synchronize the period of maximum supply from the

334

Benjamin O. Danga et al.

Indigo incorporation Indigo mulch

Control

AVRDC, wet season

Soybean incorporation Soybean mulch

Mungbean incorporation

Mungbean mulch

AVRDC, dry season

MMSU

160 R

1993

R

1993/94

L

1993/94

1994/95

120 ISD 0.05

NO3-N (kg ha−1)

80 40 0 BRCI 160 L

1993

1995

120 80 40 0 0

2

4

6

8

10

0

2

4

6

8

10 12 14

0

2

4

6

8

10 12 14 16

Weeks after mulch or incorporation

Figure 2 Nitrate contents in soil (0–30 cm) after application of green manure (soybean, indigofera at AVRDC, Taiwan, and at MMSU, Philippines; soybean and mungbean at BRCI, Philippines) in raised (R) and low (L) beds, 1993–1995. Error bars indicate LSD (0.05); asterisk indicates significance at the 0.1 probability level. AVRDC ¼ the Asian Vegetable Research and Development Center in central Taiwan, MMSU ¼ the Mariano Marcos State University in northern Luzon in the Philippines BRCI ¼ the Bukidnon Resources Co. Inc., in northern Mindanao in the Philippines.

decomposing residue with the period of maximum demand by the principal crop (wheat) (Myers et al., 1994). Further, there is little information on their rates of decay and N release in the acidic soils of the humid tropics under which wheat is grown.

5. Legume–Wheat Rotation Effects on Soil and Yield in a Tropical Humid Climate 5.1. Background A major climatic characteristic of the rainfed wheat growing areas in tropical Africa is the presence of a short dry season with limited moisture which can be utilized to grow a quick growing legume to contribute soil N through BNF to subsequent wheat in rotation. This has the potential to improve soil productivity and sustainability of wheat production in small and medium holder farms with minimal inorganic fertilizer use since inorganic fertilizers are expensive. In Africa, wheat is traditionally grown in the cooler highlands

335

Legume–Wheat Rotation in Tropics

with high rainfall. In most wheat growing areas, rainfall distribution is bimodal. The first season receives 300–600 mm between the months of April and August, while the second season receives 250–400 mm between the months of September and November. Thus under rain fed conditions of the humid and sub-humid climate in Africa, wheat production is restricted to the first season when the moisture supply is adequate, and land is left fallow during the second season due to a low and erratic rainfall. The low soil moisture during the second season has been exploited to grow a drought tolerant legume (Danga, 2008; Njihia, 1988) to provide N to the following wheat crop through BNF thereby reduce fertilizer N use. However, the effects of legume water use on the yield of subsequent wheat crop and its ability to grow on limited amount of water are important considerations in this environment. Typical monthly rainfall distribution for Njoro, Kenya is shown in Fig. 3. The effects of legume–cereal rotation on soil productivity have been studied extensively in different climatic and soil types. In the humid tropics, studies by Amanuel et al. (2000), Cheruiyot et al. (2001, 2003), Guto (1997), Njihia (1988), and Njunie et al. (2004) demonstrated the potential benefit of using legume fallows, and/or legume green manures in improving cereal yields in these regions.

5.2. Soil moisture use effects In a study of the effect of soil moisture utilization by four pre-wheat fallow practices in a humid climate at Njoro, Kenya, Danga (2008) observed that managing the fallow period (second season) with chickpea consistently and significantly reduced available soil water content at 60 cm depth at wheat planting, over the 3 year (2003–2006) period (Table 3). The reduction was 300 2003

2004

2005

2006

Rainfall, mm

250 200 150 100 50 0 Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec Month

Figure 3

Monthly rainfall distribution at Njoro, Kenya (2003–2006).

Table 3

Mean seasonal volumetric water content (m3m
3) in different fallow treatments and soil depths at Njoro, Kenya (Danga, 2008) 2003/2004

Fallow treatment

Depth (cm)

FS CHR GM NT 0–20 20–40 40–60

2004/2005

2005/2006

Season 2

Season 1

Season 2

Season 1

Season 2

Season 1

0.391a 0.372b 0.368b 0.383ab 0.404a 0.378b 0.364c

0.294a (0.147a) 0.284b (0.123b) 0.280b (0.129a,b) 0.296a (0.144a,b) 0.297a 0.285b 0.283b

0.276a 0.261b 0.255b 0.256b 0.302a 0.245b 0.239c

0.229b (0.099a) 0.238ab (0.088b) 0.232b (0.1a) 0.242a (0.103a) 0.240a 0.238a 0.229b

0.225a 0.215b 0.219b 0.221ab 0.234a 0.203c 0.221b

0.227a (0.097a) 0.229a (0.083b) 0.228a (0.097a) 0.23a (0.099a) 0.224b 0.226b 0.235a

Means within a column (fallow treatment or soil depth) followed by the same letter are not significantly different at p = 0.05 level of probability. Means in parenthesis show available water at wheat planting (0–60 cm). FS = Bare Fallow Soil; CHR = Chickpea managed fallow with residues incorporated after grain harvest; GM = Chickpea managed fallow with residues incorporated at 50% flowering; NT = Weeds fallow, no-tillage.

Legume–Wheat Rotation in Tropics

337

greatest during the period 2004–2006 when rainfall recorded prior to the wheat season was lowest, suggesting that residual moisture content was an important factor. In that study, available soil water content at 60 cm depth at wheat planting was positively correlated with wheat yield particularly in the 2005–2006 period when the amount of rainfall in the wheat season declined. The finding that the profile water content was dried below the
1,500 kPa of matric potential (0.24–0.27 m3 m
3) showed that chickpea was an effective moisture scavenger as reported by Leach and Beech (1988). The results by Danga (2008) show that the available soil water in the top 60 cm at wheat planting was significantly lower in the fallow plots in which chickpea was grown to maturity compared to the bare (FS) and no-tillage weeds (NT) fallow treatments (Table 3). They found that chickpea green manure (GM) fallow management maintained significantly higher initial water content at the 0–20 cm depth compared to the other treatments. In contrast, a longer growing period for the mature chickpea increased its water use and consequently further reduced soil water at wheat planting. Less water observed at 20–60 cm depth in the CHR and GM treatments during the subsequent wheat season was explained by lack of effective recharge of the soil water at that depth by the season’s rainfall or extraction by wheat roots.

5.3. Soil N contribution and yield effects There is relative paucity of information on rates of N2 fixation by grain legumes on African soils. Giller et al. (1997) observed that tropical grain legumes can fix substantial amounts of N given favorable conditions, but the majority of this N is often harvested in the grain. For example, soybeans (G. max L.) are very efficient at translocating their N into the grain. However, according to Dakora and Keya (1997), grain legumes fix about 15–210 kg N ha
1 seasonally in Africa, and therefore feature prominently in the cropping systems of traditional farmers. Biologically fixed N2 is returned to the soil mainly through decomposition of incorporated residues. Legumes may be incorporated as green manure or returned to the soil as dry residues after grain removal. The stage or age of the organic material determines the quality of the manure, which determines the rates of residue decomposition and N release. For example, soybean residues at harvest are lignified (10% lignin) with C:N ratios 45:1 and these tend to immobilize N when they are added to the soil (Toomsan et al., 1995). By contrast, chickpea and dolichos grown in humid tropics have residues rich in N (Cheruiyot et al., 2001; Danga, 2008; Onwonga, 1997) and contribute net N to the soil. Working on Andosols in Kenya, Cheruiyot et al. (2001, 2003) compared the rotational effects of five grain legumes, which included chickpea (C. arietinum L), field bean (Phaseolus vulgaris L.), soybean (G. max L. cv Merril), garden pea (Pisum sativum L.), and dolichos (Lablab purpureus L. cv

338

Benjamin O. Danga et al.

Sweet) on soil N, and wheat and maize yields. They reported improved soil N status from the legumes, with dolichos giving highest available N. The maize yield after legumes was 24–68% higher than after weed fallow, whereas wheat grain yield increased by 17% compared to the weed fallow. However, dolichos was applied with green foliage and further research that incorporates green manure of the other legumes, rather than mature residues could make their comparisons more valid. Similar studies with cover crops in which the residues were applied as surface mulch resulted in enhanced soil N fertility for improved yields of tropical food crops maize (Zea mays L.) and cassava (Manihot esculenta Crantz) in coastal Kenya (Njunie et al., 2004). These authors reported that field decomposition and nutrient (N, P, and Mg) release curves (Fig. 1 for N) from foliage of legume species clitoria (Clitoria ternatea L.) and dolichos (L. purpureus L.) planted in monoculture and as an intercrop with the tropical food crops became asymptotic shortly after 60% of the nutrients had been released. Clitoria and dolichos showed their potential to become sources of nutrients for associated crops while protecting the soil surface. Dolichos residue decomposed faster than clitoria with rate constants (k) of 0.5 and 0.2 wk
1, respectively. Nitrogen release from dolichos residue agrees with a study conducted in West Africa (Ibewiro et al., 2000) in which 64% of the N was released from dolichos residue within a period of 4 wk. Danga (2008) reported an increase of wheat grain yields from 4.08 in the no-tillage fallow (with weeds) to 5.6 Mg ha
1 (equivalent of 37% increase) when green chickpea at blooming was incorporated prior to wheat. Chickpea fallow with incorporation of mature residues and bare fallow increased grain yield by 27 and 31%, respectively compared to the no-tillage fallow control. These results were higher than those of Cheruiyot et al. (2003) who reported wheat grain increase of just 17% due to the legumes in a similar environment, probably due to grain loss in the latter caused by pests, Helicoverpa amigera and birds. Guto (1997) found that the chickpea–wheat and bean–wheat sequences, with mature legume residues incorporated, increased wheat grain yields by 22 and 14%, respectively as compared to the bare fallow–wheat sequence. However, the wheat grain increments attributed to the legume–wheat cropping sequences were not significant (Guto, 1997). Njihia (1988) observed that a clean fallow treatment conserved soil water and subsequently increased wheat yields on an Andosol at Njoro. Danga (2008) reported that the common practice of fallow land with no weeds control resulted in the lowest wheat yield due to weed growth that reduced the soil water and nutrients at the soil surface. Chickpea fallow, with its residues incorporated into the soil increased soil mineral N significantly, particularly at planting and at 30 days after planting (Table 4), and the amount of N increase depended on the residue age and quality (Danga, 2008). The gross nutrient N returned by incorporating chickpea residues into the soil depended on biomass yield of chickpea

339

Legume–Wheat Rotation in Tropics

Table 4 Effect of fallow method on soil available N (mg kg
1) at wheat planting and after 30 days in 2005 and 2006 at Njoro, Kenya (Danga, 2008) 2005

Depth (cm) 0–20 20–40 40–60
1 At planting (mg kg ) FS 23.2c 25.3c 12.4a CHR 23.4c 28.5b 13.7a GM 29.1a 33.2a 12.6a NT 26.6ab 30.2ab 11.2a
1 At 30 days after planting (mg kg ) FS 21.2c 19.5c 13.2b CHR 30.3a 26.4a 12.9b GM 26.8b 21.7bc 16.2a NT 24.1bc 23.0b 14.8a

2006

0–20

20–40

40–60

20.5b 24.8b 27.3a 23.4b

19.8b 21.6b 22.3b 21.9b

15.7a 17.4a 16.3a 16.3a

18.7d 29.5a 25.1b 21.2c

15.3b 20.0a 14.6b 16.7b

16.7b 16.4b 21.0a 21.5a

Means within a column followed by the same letter are not significantly different at p = 0.05 level of probability. FS = Bare Fallow Soil; CHR = Chickpea managed fallow with residues incorporated after grain harvest; GM = Chickpea managed fallow with residues incorporated at 50% flowering; NT = Weeds fallow, no-tillage

and its nutrient concentration. However, between the years and residue type, the biomass yield of chickpea was the dominant factor. For example, the total biomass yield of green manure (dry basis) ranged from 0.62 to 1.99 Mg ha
1 while mature chickpea straw yields ranged 0.39–1.75 Mg ha
1 over the 3 year period, the lower values being the dry matter yield in 2004 when Ascochyta leaf blight persistently attacked the chickpeas. The chickpea GM residues had a higher N and P contents than mature residues, the N content ranging from 3.73 to 4.2% in green manure and from 2.26 to 2.8% in the mature residue. Consequently, the GM accumulated between 23.1 to 83.6 kg ha
1 N, while the mature residue accumulated between 9.2 and 49.0 kg ha
1 N at the Njoro site. Therefore, incorporating the GM into the soil returned significantly higher N compared to the mature residues (Danga, 2008). Consequently, the GM treatment had higher soil mineral N at wheat planting than CHR but both were significantly higher than the non-chickpea fallow methods (Table 4). These results are comparable to others. In a similar climatic and soil type, Onwonga (1997) found that mature chickpea residue could return 54.1 kg ha
1 N when the DM yield was 1.67 Mg ha
1 while Guto (1997) found fertilizer replacement value to be 39.7 and 24.0 kg N ha
1 for chickpea–wheat and beans–wheat, respectively, when legume dry residue were incorporated after grain harvests. In a wheat rotation in Ethiopia, Amanuel et al. (2000) found that faba bean had a net soil N balances of 12–58 kg N ha–1 relative to the highly negative N balances (
9 to 44 kg N ha–1) following wheat (Triticum aestivum L.),

340

Benjamin O. Danga et al.

highlighting the importance rotation with faba bean in the cereal-based cropping systems of Ethiopia. The contribution of the chickpea residues to soil N in the study of Danga (2008) was related to C:N ratio of the residues which probably influenced their decomposition rates. The chickpea green manure decomposed faster because of a lower C:N ratio. The chickpea fallow treatments improved soil N by symbiotic dinitrogen fixation through decomposition and mineralization of incorporated residues (Wani et al., 1994) and mineralized rhizo-deposits, legume roots and nodules (Turpin et al., 2002; Unkovich and Pate, 2000), and by nitrate conservation or sparing (Chalk, 1998). Danga (2008) suggested that the slow mineralization of N from chickpea mature residues could be an important factor in N synchrony and optimization of inorganic N use by wheat. Similarly, Njunie et al. (2004) observed that green manure residue nutrient release from dolichos, unlike clitoria, was favored by the shorter (2 mo) cutting management strategy releasing nutrients faster as compared to a 4-mo cutting strategy. The slower release of nutrients from older tissue residues may be attributed to significantly lower N concentrations associated with these foliage residues. Nitrogen concentrations in the 4-mo harvests (i.e., 22.7 g N kg
1) of dolichos were 21% lower than in the 2-mo harvests (i.e., 31.6 g N kg
1). Nevertheless, these N concentrations were still above the critical values (18–22 g N kg
1) for the transition between net N immobilization and net mineralization. One possible explanation is that lignin and polyphenolic compounds may have influenced the decomposition and nutrient release rates of these legumes (Vallis and Jones, 1973). Further, the fact that approximately 50% of the N in the clitoria cut at 6 or 10 wk and more than 40% of N in dolichos remained in the residues at the end of the 16-wk study period led Njunie et al. (2004) to conclude that a considerable amount of N could be available the following season hence, further research was needed to characterize the synchronization between residue nutrient availability and crop nutrient demand. In the Danga (2008) study, chickpea fallow treatments with residue incorporation combined with judicious amount of inorganic N fertilizer at 30 kg N ha
1 resulted in significantly higher wheat grain and N yield (Table 5) in all the seasons. However, inorganic N application had the most marked influence on grain N or protein, increasing grain protein from 12.03% in the no-N control to 14.72% protein in the 60 kg ha
1 N. The increase in grain protein content with increasing level of applied N is in agreement with the observations of McDonald (1992), Bar-Tal et al. (2004), and Lo´pez-Bellido et al. (2004). The finding that chickpea GM with 30 kg ha
1 N treatment combination was more effective in improving wheat yields in the Danga (2008) study suggests that under conditions of humid tropics, chickpea is unable to supply sufficient soil N by fixation to meet the requirements for wheat. Working with soybean and indigofera green manures in a similar

Table 5

2004

The effect of fallow method and inorganic N rate (Kg ha
1) on wheat yield components at Njoro, Kenya (from Danga, 2008)

Fallow method

Inorganic N

2005

Fallow method

Inorganic N

2006

Fallow method

Inorganic N

Grain wt Mg ha
1

Straw wt Mg ha
1

Total DM

Grain protein %

CHR GM FS NT 0 30 60

5.18a 5.60a 5.35a 4.08b 4.66b 5.57a 4.93ab

4.99b 6.23a 5.84ab 3.44c 4.20b 5.81a 5.35a

10.17b 11.83a 11.19ab 7.52c 8.86b 11.38a 10.28a

13.8a 14.4a 14.4a 11.0b 12.0b 13.4ab 14.7a

CHR GM FS NT 0 30 60

3.87b 4.27a 3.60b 2.72c 3.11b 3.83a 3.91a

4.02a 3.28ab 3.70ab 2.65b 3.22a 3.57a 3.45a

7.89a 7.55a 7.30a 5.37b 6.33b 7.40a 7.35a

13.6a 14.0a 12.8ab 11.9b 11.7c 13.3b 14.2a

CHR GM FS NT 0 30 60

3.21ab 3.84a 2.80b 3.07ab 3.09a 3.34a 3.11a

4.53ab 5.77a 4.18b 4.62a 4.63a 5.00a 4.69a

7.74b 9.61a 6.98c 7.69b 7.71a 8.32a 7.80a

13.7a 13.7a 11.2b 12.0ab 10.8c 12.6b 14.5a

Means within a column followed by the same letter are not significantly different at p = 0.05 level of probability. FS = Bare Fallow Soil; CHR = Chickpea managed fallow with residues incorporated after grain harvest; GM = Chickpea managed fallow with residues incorporated at 50% flowering; NT = Weeds fallow, no-tillage.

342

Benjamin O. Danga et al.

environment, Thonnissen et al. (2000) concluded similarly that, with respect to season and location, GM N should be supplemented with N fertilizer starting after 8 wk to ensure optimal tomato yields. Under climatic and soil conditions of the humid tropics, the sustainability of the legume–wheat rotation may be enhanced if the following challenges were addressed (i) identification of appropriate grain legumes, adaptable to the cool droughty conditions of the short dry period during which it is grown (Cheruiyot et al., 2001; Guto, 1997; Njihia, 1988). This is because N2 fixation by legumes can be severely constrained by drought. Inoculation with appropriate Rhizobium of some drought tolerant legume species such as chickpea and dolichos would be appropriate, (ii) widespread soil acidity and severe P deficiency (Andrade et al., 2002) which inhibited rhizobia and decreased yields of subsequent cereal crops. Liming and P application measures have been recommended by Amanuel et al. (2000) and Andrade et al. (2002) (iii) lack of resistant chickpea germplasm to Ascochyta leaf blight, prevalent in the cool weather conditions in the African highlands (Danga 2008; Onwonga 1997).

6. Conclusions Exploitation of limited soil water available during the fallow period by a short growing grain legume was found to be an important strategy to improve soil N fertility and wheat yields in a continuous wheat production system in the humid tropics. However, many studies have shown that such a strategy required careful selection of adapted grain legumes that confer substantial N benefit to the succeeding wheat crop, and to yield adequate grain for the farmer. Thus, the negative effects of residual soil moisture use in a season with a low and unreliable rainfall on N2 fixation by legume, for proper legume growth and yield, and on the yield of subsequent cereal crop remained a challenge due to the variability of rainfall and soil conditions. Incorporating grain legumes in rotation with wheat enriche the soil with N mainly due to fixation by root nodule Rhizobium which results in improved wheat yields. The amount of N benefit depends on antecedent legume and its interaction with the environment. Despite environmental constraints of low soil pH, high aluminum and manganese toxicity in some tropical soils, and drought, activity of Rhizobium and biological N fixation was improved by inoculation with appropriate strains, liming and P application. It was found prudent to employ the more accurate isotopic 15N methodology in quantifying fixed legume N and to consider below ground root N in estimating legume N benefit. Such approach is necessary to correct previously obtained lower values that were used inadvertently as

Legume–Wheat Rotation in Tropics

343

criteria for suitability or ability of certain grain legumes as rotation crops in different environments. Most studies involving wheat in African humid environment concluded that dolichos, chickpea, field bean, garden pea, and faba beans could be used for short-term fallow management. However, incorporation could strive to maximize N returned by harvesting grain at physiological maturity when foliage are still green and quality of resulting residues reasonably good. Although chickpea green manure was found to have an advantage in soil N supply and wheat yield, it is unlikely that the African small scale farmers would find it appealing. Wheat yield increased with N input and was highest when the legume GM N was supplemented with some inorganic N fertilizer. Importantly, most grain legumes did not fix sufficient N and therefore did not mineralize sufficient N from residues nor spare enough soil N to sustain high wheat yield without inputs of additional fertilizer N, hence inorganic N fertilizer supplementation was apparently required. It is concluded that introduction of legumes such as chickpea, dolichos, and field beans in wheat-based cropping is a viable strategy for the reduction of inorganic fertilizer use for the resource poor small and medium scale farmers in Africa.

REFERENCES Ahmad, T., Hafeez, F. Y., Mahmood, T., and Malik, K. A. (2001). Residual effect of nitrogen fixed by mungbean (Vigna radiata) and blackgram (Vigna mungo) on subsequent rice and wheat crops. Aust. J. Exp. Agric. 41, 245–248. Amanuel, G., Ku¨hne, R. F., Tanner, D. G., and Vlek, P. L. G. (2000). Biological nitrogen fixation in faba bean (Vicia faba L.) in the Ethiopian highlands as affected by P fertilization and inoculation. Biol. Fertil. Soils. 32, 353–359. Ambus, P., and Jensen, E. S. (1997). Nitrogen mineralization and denitrification as influenced by crop residue particle size. Plant Soil. 197, 261–271. Ambus, P., Jensen, E. S., and Robertson, G. P. (2001). Nitrous oxide and N-leaching losses from agricultural soil: Influence of crop residue particle size, quality and placement. Phyton. 41, 7–15. Andrade, D. S., Murphy, P. J., and Giller, K. E. (2002). Effects of liming and legume/cereal cropping on populations of indigenous rhizobia in an acid Brazilian Oxisol. Soil Biol. Biochem. 34, 477–485. Badaruddin, M., and Meyer, D. W. (1989). Water use by legumes and its effect on soil water status. Crop Sci. 29, 1212–1216. Bagayonko, M., Mason, S. C., and Sabata, R. J. (1992). Effect of previous cropping systems on soil nitrogen and grain sorghum yield. Agron. J. 84, 863–867. Bar-Tal, A., Yermiyahu, U., Beraud, J., Keinan, M., Rosenberg, R., Zohar, D., Rosen, V., and Fine, P. (2004). Nitrogen, phosphorus, and potassium uptake by wheat and their distribution in soil following successive, annual compost applications. J. Environ. Qual. 33, 1855–1865. Beauchamp, E. G., and Hume, D. J. (1997). Agricultural soil manipulation: The use of bacteria, manuring, and plowing. In ‘‘Modern Soil Microbiology’’ (J. D. van Elsas, J. T. Trevors, and E. M. H. Wellington, Eds.), pp. 643–664. Marcel Dekker, New York.

344

Benjamin O. Danga et al.

Beck, D. P. (1992). Yield and nitrogen fixation of chickpea cultivars in response to inoculation with selected rhizobial strains. Agron. J. 84, 510–516. Beck, D. P., Wery, J., Saxena, M. C., and Ayadi, A. (1991). Dinitrogen fixation and nitrogen balance in cool-season food legumes. Agron. J. 83, 334–341. Bending, G. D., Turner, M. K., and Burns, I. G. (1998). Fate of nitrogen from crop residues as affected by biochemical quality and the microbial biomass. Soil Biol. Biochem. 30, 2055–2065. Boddey, R. M., de Moraes sa´, J. C., Alves, B. J., and Urquiaga, S. (1997). The contribution of biological nitrogen fixation for sustainable agricultural systems in the tropics. Soil Bio. Biochem. 29, 787–799. Breland, T. A., and Hansen, S. (1996). Nitrogen mineralization and microbial biomass as affected by soil compaction. Soil Biol. Biochem. 28, 655–663. Budelman, A. (1987). ‘‘The Effect of the Application of Leaf Mulch of Gliricidia sepium on the Early Development and Yield of the Greater Yam (Dioscorea alota cv. Biazo furte).’’ Agricultural University, Wageningen, The Netherlands. Bullock, D. G. (1992). Crop rotation, Crit. Rev. Plant Sci. 11, 309–326. Carranca, C., de Varennes, A., and Rolston, D. (1999). Biological nitrogen fixation by faba bean, pea and chickpea, under field conditions, estimated by the 15N isotope dilution technique. Eur. J. Agron. 10, 49–56. Chabi-Olaye, A., Nolte, C., Schulthess, F., and Borgemeister, C. (2005). Effects of grain legumes and cover crops on maize yield and plant damage by Busseola fusca (Fuller) (Lepidoptera Noctuidae) in the humid forest of southern Cameroon. Agric. Ecosyst. Environ. 108, 17–28. Chalk, P. M. (1998). Dynamics of biologically fixed N in legume-cereal rotations: A review. Aust. J. Agric. Res. 49, 303–16. Cheruiyot, E. K., Mumera, L. M., Nakhone, L. N., and Mwonga, S. M. (2001). Rotational effects of grain legumes on maize performance in the Rift Valley highlands of Kenya. Afr. Crop Sci. J. 9, 667–676. Cheruiyot, E. K., Mumera, L. M., Nakhone, L. N., and Mwonga, S. M. (2003). Effect of legume-managed fallow on weeds and soil nitrogen in following maize (Zea mays L.) and wheat (Triticum aestivum L.) crops in the Rift Valley highlands of Kenya. Aust. J. Exp. Agric. 43, 597–604. Constantinides, M., and Fownes, J. H. (1994). Nitrogen mineralization from leaves and litter of tropical plants: Relationship to nitrogen, lignin and soluble polyphenol concentrations. Soil Biol. Biochem. 26, 49–55. Cook, R. J. (1992). Wheat root health management an environmental concern. Can. J. Plant Pathol. l4, 76–85. Dakora, F. D., and Keya, S. O. (1997). Contribution of legume nitrogen fixation to sustainable agriculture in sub-saharan Africa. Soil Biol. Biochem. 29, 809–817. Dalal, R. C., Strong, W. M., Weston, E. J., Cooper, J. E., Wildermuth, G. B., Lehane, K. J., King, A. J., and Holmes, C. J. (1998). Sustaining productivity of a Vertisol at Warra, Queensland, with fertilizers, no-tillage, or legumes. 5. Wheat yields, nitrogen benefits and water-use efficiency of chickpea–wheat rotation. Aust. J. Exp. Agric. 38, 489–501. Danga, B. O. (2008). Effects of chickpea managed fallow on soil moisture and wheat yield and simulation of nitrogen and carbon turnover, Ph.D. Thesis. Egerton University, Kenya. p. 177. Doughton, J. A., and McKenzie, J. (1984). Comparative effects of black and green gram (Mung beans) and grain sorghum on soil mineral N and subsequent grain sorghum yields on the eastern Darling Downs. Aust. J. Exp. Agric. 24, 244–249. Doughton, J. A., Vallis, I., and Saffigna, P. G. (1993). Nitrogen fixation in chickpea, I. Influence of prior cropping or fallow, nitrogen fertilizer and tillage. Aust. J. Agric. Res. 40, 791–805.

Legume–Wheat Rotation in Tropics

345

Doyle, A. D., and Leckie, C. C. (1992). Recovery of fertilizer nitrogen in wheat grain and its implications for economic fertilizer use. Aust. J. Exp. Agric. 32, 383–387. Ellert, B. H., and Bettany, J. R. (1988). Comparison of kinetic models for describing net sulphur and nitrogen mineralization. Soil Sci. Soc. Am. J. 52, 1692–1702. Evans, J., Fettell, N. A., Coventry, D. R., O’Connor, G. E., Walsgott, D. N., Mahoney, J., and Armstrong, E. L. (1991). Wheat response after temperate crop legumes in southeastern Australia. Aust. J. Agric. Res. 42, 31–43. Evans, J., O’Connor, G. E., Turner, G. L., Coventry, D. R., Fettell, N., Mahoney, J., Armstrong, E. L., and Walsgott, D. N. (1989). N2 fixation and its value to soil N increase in lupin, field pea and other legumes in south-eastern Australia. Aust. J. Agric. Res. 40, 791–805. Felton, W. L., Marcellos, H., Alston, C., Martin, R. J., Backhouse, D., Burgess, L. W., and Herridge, D. F. (1998). Chickpea in wheat-based cropping systems of northern New South Wales II. Influence on biomass, grain yield, and crown rot in the following wheat crop. Aust. J. Agric. Res. 49, 401–407. Frankenberger, W. T. Jr., and Abdelmagid, H. M. (1985). Kinetic parameters of nitrogen mineralization rates of leguminous crops incorporated into soil. Plant Soil. 87, 257–271. Franzluebbers, A. J., Haney, R. L., Hons, F. M., and Zuberer, D. A. (1996). Active fractions of organic matter in soils with different texture. Soil Biol. Biochem. 28, 1367–1372. French, R. J. (1978a). The effect of fallowing on the yield of wheat. I. The effect on soil water storage and nitrate supply. Aust. J. Agric. Res. 29, 653–668. French, R. J. (1978b). The Effect of fallowing on the yield of wheat. II. The effect on grain yield. Aust. J. Agric. Res. 29, 669–684. Fu, G. (2000). Nitrogen dynamics in a Chickpea-wheat rotation in a hummocky field, A Ph. D Thesis, University of Saskatchewan Saskatoon. p. 219. Fu, M. H., Xu, X. C., and Tabatabai, M. A. (1987). Effect of pH on nitrogen mineralization in crop residue treated soils. Biol. Fert. Soils. 5, 115–119. Fyson, A., and Oaks, A. (1990). Growth promotion of maize by legume in soils. Plant Soil. 122, 259–266. Gachene, C. K. K., Jarvis, N. J., Harry, L., and Mbuvi, J. P. (1997). Soil erosion effects on soil properties in a highland area of Central Kenya. Soil Sci. Soc. Am. J. 61, 559–564. Garnier, P., Neel, C., Aita, C., Recous, S., Lafolie, F., and Mary, B. (2003). Modelling carbon and nitrogen dynamics in a bare soil with and without straw incorporation. Eur. J. Soil Sci. 54, 555–568. Giller, K. E., and Cadisch, G. (1997). Driven by nature: A sense of arrival or departure? In ‘‘Driven by Nature: Plant Litter Quality and Decomposition’’ (G. Cadisch and K. E. Giller, Eds.), pp. 393–499. CAB Int., Cambridge, UK. Giller, K. E., Cadisch, G., Ehaliotis, C., Adams, E., Sakala, W. D., and Mafongoya, P. L. Building soil nitrogen capital in Africa. In ‘‘Replenishing Soil Fertility in Africa’’ R. J. Buresh et al (Eds.), pp. 151–192. SSSA Spec. Publ.51 SSSA, Madison W1. Green, B. J., and Biederbeck, V. O. (1995). ‘‘Farm Facts. Soil Improvement with Legumes’’ Sask. Agric. & Food, Regina, SK. Guto, N. S. (1997). Effect of chickpea (Cicer arietinum L. cv kabuli) and common beans (Phaseolus vulgaris L. cv rose coco) on wheat performance and soil fertility in a legumewheat cropping sequence M.Sc. Thesis, Egerton University, Njoro, Kenya. Hadas, A., Kautsky, L., Goek, M., and Kara, E. E. (2004). Rates of decomposition of plant residues and available nitrogen in soil, related to residue composition through simulation of carbon and nitrogen turnover. Soil Biol. Biochem. 36, 255–266. Handayanto, E., and Giller, K. E. (1997). Nitrogen mineralization from legume tree prunings of different quality: Experiments at Almpung. International Workshop on Biological Management of Soil Fertility on acidic upland Soils in the Humid Tropics, Brawijaya University, Malang, Indonesia. pp. 28–31.

346

Benjamin O. Danga et al.

Heal, O. W., Anderson, J. M., and Swift, M. J. (1997). Plant litter quality and decomposition: A historical overview. In ‘‘Driven by Nature: Plant Litter Quality and Decomposition’’ (G. Cadisch and K. E. Giller, Eds.), pp. 3–30. CAB Int., Wallingford, England. Henriksen, T. M., and Breland, T. A. (2002). Carbon mineralisation, fungal and bacterial growth, and enzyme activities as affected by contact between crop residues and soil. Soil Biol. Biochem. 35, 41–48. Herridge, D. F., Marcellos, H., Felton, W. L., Turner, G. L., and Peoples, M. B. (1994). Legume nitrogen fixation: An efficient source of N for cereal production. In ‘‘Nuclear Related Methods in Soil Plant Aspects of Sustainable Agriculture.’’, FAO/FAEA, Vienna. Herridge, D. F., Marcellos, H., Felton, W. L., Turner, G. L., and Peoples, M. B. (1995). Chickpea increase soil-N fertility in cereal systems through nitrate sparing and nitrogen fixation. Soil Biol. Biochem. 27, 545–551. Hessels
e, M., Pedersen, A., Bundgaard, K., Brandt, K. K., and S
rensen, J. (2001). Development of nitrification hot-spots around degrading red clover (Trifolium pratense) leaves in soil. Biol. Fert. Soils. 33, 238–245. Ibewiro, B., Saginga, N., Vanlauwe, B., and Merckx, R. (2000). N contributions from decomposing cover crop residues to maize in a tropical derived savanna. Nutr cycling in agroecosyt. 57, 131–140. ICRISAT. (1993). In Legume programme international crop annual report, 1992. Patenchure, AP502.324 India Research Institute for the Semi-arid Tropics, p. 264. Jensen, E. S. (1994). Mineralization–immobilisation of nitrogen in soil amended with low C:N ratio plant residues with different particle size. Soil Biol. Biochem. 26, 519–521. Jiao, B. (1983). Utilization of green manure for raising soil fertility in China. Soil Sci. 135, 65–69. Kumar, J., and Abbo, S. (2001). Genetics of flowering time in chickpea and its bearing on productivity in semiarid environments. Adv. Agron. 72, 107–138. Kuzyakov, Y., Friedel, J. K., and Stahr, K. (2000). Review of mechanisms and quantification of priming effects. Soil Biol. Biochem. 32, 1485–1498. Larson, K. J., Cassrnan, K. G., and Phillips, D.A (1989). Yield, dinitrogen fixation, and aboveground nitrogen balance of irrigated white lupin in a Mediterranean climate. Agron. J. 81, 538–543. Leach, G. J., and Beech, D. F. (1988). Response of chickpea accessions to row spacing and plant density on a vertisol on the Darling Down, South-eastern Queensland: II. Radiation interception and water use. Aust. J. Exp. Agric. 28, 377–383. Lomander, A., Ka¨tterer, T., and Andre´n, O. (1998). Modeling the effects of temperature and moisture on CO2 evolution from top- and subsoil using a multi-compartment approach. Soil Biol. Biochem. 30, 2023–2030. Lo´pez-Bellido, R. J., and Lo´pez-Bellido, L. (2001). Effects of crop rotation and nitrogen fertilization on soil nitrate and wheat yield under rainfed Mediterranean conditions. Agronomie. 21, 509–516. Lo´pez-Bellido, R. J., Lo´pez-Bellido, L., Castillo, J. E., and Lo´pez-Bellido, F. J. (2004). Chickpea response to tillage and soil residual nitrogen in a continuous rotation with wheat II. Soil nitrate, N uptake and influence on wheat yield. Field Crops Res. 88, 201–210. MacDonald, N. W., Zak, D. R., and Pregitzer, K. S. (1995). Temperature effects on kinetics of microbial respiration and net nitrogen and sulfur mineralization. Soil Sci. Soc. Am. J. 59, 233–240. Marcellos, H., Felton, W. L., and Herridge, D. F. (1998). Chickpea in wheat based cropping systems of northern N.S.W 1. N2 fixation and influence on soil nitrate and water. Aust. J. Agric. Res. 49, 391–400. Martens, J. M., Seaman, W. L., and Atkioson, T. G. (1984). ‘‘Diseases of field crops in Canada’’ Can. Phytopathot. Soc, Harrow, ON.

Legume–Wheat Rotation in Tropics

347

McDonald, G. K. (1992). Effects of nitrogenous fertilizer on the growth, grain protein concentration of wheat. Aust. J. Agric. Res. 43, 949–967. McGuire, A. M., Bryant, D. C., and Denison, R. F. (1998). Wheat yields, Nitrogen, uptake, and soil moisture following winter legume cover crop vs Fallow. Agron. J. 90, 404–410. Mu¨ller, M. M., Sundman, V., Sininvaara, O., and Merilainen, A. (1988). Effect of chemical composition on the release of nitrogen from agricultural plant materials decomposing under field conditions. Biol. Fert. Soils. 6, 78–83. Myers, R. J. K., Campbell, C. A., and Weier, K. L. (1982). Quantitative relationship between net nitrogen mineralization and moisture content of soils. Can. J. Soil Sci. 62, 111–124. Nielsen, D. C., and Vigil, M. F. (2005). Legume green fallow effect on soil water content at wheat planting and wheat yield. Agron. J. 97, 684–689. Njihia, C. M. (1988). Prospects of soil moisture conservation by fallowing in areas of medium agricultural potential in smallholder farming. Agric. Water Manage. 14, 265–275. Njunie, M. N., Wagger, M. G., and Luna-Orea, P. (2004). Residue decomposition and nutrient release dynamics from two tropical forage legumes in a Kenyan environment. Agron. J. 96, 1073–1081. Oglesby, K. A., and Fownes, J. H. (1992). Effects of chemical composition on nitrogen mineralization from green manures of seven tropical leguminous trees. Plant Soil. 143, 127–132. Oikeh, S. O., Chude, V. O., Carsky, R. J., Weber, G. K., and Horst, W. J. (1998). Legume rotation in moist tropical savanna: Managing soil nitrogen dynamics and cereal yields in farmers fields. Exp. Agric. 34, 73–83. Okpara, D. A., Njoku, J. C., and Ikeorgu, J. E. G. (2003). Maize response to green manure under the humid tropical conditions of south-eastern Nigeria. Trop. Agric. (Trinidad). 80, 1–5. Onwonga, R. N. (1997). The Effect of incorporating chickpea residues on soil nutrient status, nitrogen availability and wheat yield in chickpea-wheat rotation M.Sc. Thesis, Egerton University, Kenya. Oweis, T., Hachum, A., and Pala, M. (2004). Water use efficiency of winter-sown chickpea under supplemental irrigation in a Mediterranean Environment. Agric. Water Manage. 66, 163–179. Palm, C. A. (1995). Contribution of agroforestry trees to nutrient requirements of intercropped plants. Agroforestry Syst. 30, 105–124. Palm, C. A., Myers, R. J. K., and Nandwa, S. (1997). Combined use of organic and inorganic nutrient sources for soil fertility maintenance and replenishment. In “Replenishing Soil Fertility in Africa” (R. J. Bureshi, P. A. Sanchez, and F. Calhoun, Eds), pp. 193–218. SSSE Special Publ. No. 51, Madison, Winconsin. Papastylianou, I. (1993). Estimating total N2 fixation by legumes in long-term rotation studies. Eur. J. Agron. 2, 1–10. Peoples, M. B., and Craswell, E. T. (1992). Biological nitrogen fixation: Investments, expectations and actual contributions to agriculture. Plant Soil. 141, 13–39. Peoples, M. B., and Craswell, E. T. (1995). Nitrogen fixation in agriculture. Plant Soil. 174, 22–39. Peoples, M. B., and Herridge, D. F. (1990). Nitrogen fixation by legumes in tropical and sub-tropical agriculture. Adv. Agron. 44, 155–223. Peoples, M. B., Herridge, D. F., and Ladha, J. K. (1995). Biological nitrogen fixation: An efficient source of nitrogen for sustainable agricultural production. Plant Soil. 174, 3–28. Pilbeam, C. J., Wood, M., Harris, H. C., and Tuladhar, J. (1998). Productivity and nitrogen use of three different wheat based rotations in North West Syria. Aust. J. Agric. Res. 49, 451–458. Quemada, M., and Cabrera, M. L. (1995). Carbon and nitrogen mineralization from leaves and stems of four cover crops. Soil Sci. Soc. Am. J. 59, 471–477.

348

Benjamin O. Danga et al.

Rathore, A. L., Pal, A. R., Sahu, R. K., and Chandhary, J. L. (1996). On-farm rainwater and crop management for improving productivity of rain fed areas. Agric. Water Manage. 31, 253–267. Reij, C. (1991). “Indigenous Soil and Water Conservation in Africa.” HED Gatekeeper series No 27, HED, London. Ries, S. K., Wert, V., Sweeley, C. C., and Leavit, R. A. (1977). Triacontanol: A new naturally occurring plant growth regulator. Science. 195, 1339–1341. Rovira, A. D. (1976). Effects of ammonium, nitrate and phosphate in soi1 and on the growth, nutrition and yield of wheat. Soil Biol. Biochem. 8, 241–247. Rovira, P., and Vallejo, V. R. (1997). Organic carbon and nitrogen mineralisation under mediterranean climatic conditions: The effect of incubation depth. Soil Biol. Biochem. 29, 1509–1520. Ruffo, M. L., and Bollero, G. A. (2003). Residue decomposition and prediction of carbon and nitrogen release rates based on biochemical fractions using principal-component regression. Agron. J. 95, 1034–1040. Sanchez, P. A. (1999). Improved fallows come of age in the tropics. Agroforestry Syst. 47, 3–12. Sanchez, P. A., Sepherd, K. D., Soule, M. J., Place, F. M., Buresh, R. J., Izac, A. N., Mokunywe, A. U., Kwesiga, F. R., Ndiritu, C. G., and Woomer, P. L. (1997). Soil fertility replenishment in Africa: An investment in natural resource capital. In “Replenishing Soil Fertility in Africa” (R. J. Buresh, P. A. Sanchez and F. Calhoun, Eds.). SSSA Spec. Public. No. 51. Schlegel, A. J., and Havlin, J. L. (1997). Green fallow for the Central Great Plains. Agron. J. 89, 762–767. Scott, N. A., Cole, C. V., Elliot, E. T., and Huffman, S. A. (1996). Soil textural control on decomposition and soil organic matter dynamics. Soil Sci. Soc. Am. J. 60, 1102–1109. Shah, Z., Shah, S. H., Peoples, M. B., Schwenke, G. D., and Herridge, D. F. (2003). Crop residue and fertiliser N effects on nitrogen fixation and yields of legume–cereal rotations and soil organic fertility. Field Crops Res. 83, 1–11. Smith, S. C., Bezdicek, D. F., Turco, R. F., and Cheng, H. H. (1987). Seasonal N2 fixation by cool-season pulses based on several 15N methods. Plant Soil. 97, 3–13. Stanford, G., and Epstein, E. (1974). Nitrogen mineralization–water relations in soils. Soil Sci. Soc. Am. Proc. 38, 103–107. Stanford, G., and Smith, S. J. (1972). Nitrogen mineralization potentials of soils. Soil Sci. Soc. Am. J. 36, 465–472. Stevenson, F. C., and van Kessel, C. (1996). A landscape-scale assessment of the nitrogen and non-nitrogen rotation benefits of pea. Soil Sci. Soc. Am. J. 60, 1797–1805. Stott, D. E., Stroo, H. F., Elliot, L. F., Papendick, R. I., and Unger, W. (1990). Wheat residue loss from fields under no-till management. Soil Sci. Soc. Am. J. 54, 92–98. Strong, W. M., Harbison, J., Nielsen, R. G. H., Hall, B. D., and Best, E. K. (1986). Nitrogen availability in a Darling Downs. Soil following cereals, Oilseed and grain legume crops: I. Soil nitrogen accumulation. Aust. J. Exp. Agric. 26, 347–351. Stute, J. K., and Posner, J. L. (1995). Synchrony between legume nitrogen release and corn demand in the Upper Midwest. Agron. J. 87, 1063–1069. Swift, M. J., Heal, O. W., and Anderson, J. M. (1979). ‘‘Decomposition in Terrestrial Ecosystems’’ Studies in Ecology 5: University of California Press, Berkeley, CA. Tarafdar, J. C., Meena, S. C., and Kathju, S. (2001). Influence of size on activity and biomass of soil microorganisms during decomposition. Eur. J. Soil Biol. 37, 157–160. Thonnissen, C., Midmore, D. J., Ladha, J. K., Olk, D. C., and Schmidhalter, U. (2000). Legume decomposition and nitrogen release when applied as green manures to tropical vegetable production systems. Agron. J. 92, 253–260.

Legume–Wheat Rotation in Tropics

349

Tian, G., Brussaard, L., and Kang, B. T. (1995). An index for assessing the quality of plant residues and evaluating their effects on soil and crop in the sub-humid tropics. Appl. Soil Ecol. 2, 25–32. Toomsan, B., McDonagh, J. F., Limpinuntana, V., and Giller, K. E. (1995). Nitrogen fixation by groundnut and soybean and residual nitrogen benefits to rice in farmers fields in North-eastern Thailand. Plant Soil. 175, 45–46. Tropical Soil Biology and Fertility (TSBF). (1994). Annual Report, 1995. Tropical Soil Biology and Fertility Programme Nairobi, Kenya. Turpin, J. E., Herridge, D. F., and Robertson, M. J. (2002). Nitrogen fixation and soil nitrate interactions in field-grown chickpea (Cicer arietinum) and faba bean (Vicia faba). Aust. J. Agric. Res. 53, 599–608. Unkovich, M. J., and Pate, J. S. (2000). An appraisal of recent field measurements of symbiotic N2 fixation by annual legumes. Field Crops Res. 65, 211–228. Utomo, M., Blevins, R. L., and Frye, W. W. (1987). Effect of legume cover crops and tillage on soil water, temperature, and organic matter. In ‘‘The Role of Legumes in Conservation Tillage Systems’’ (J. F. Power, Ed.), pp. 5–6. Soil & Water Conser. Soc, Ankeny, IA. Vallis, I., and Jones, R. J. (1973). Net mineralization of nitrogen in leaves and leaf litter of Desmodium intortum and Phaseolus atropurpureus mixed with soil. Soil Biol. Biochem. 5, 391–398. van Veen, J. A., Ladd, J. N., and Amato, M. (1985). Turnover of carbon and nitrogen through the microbial biomass in a sandy loam and a clay soil incubated with [14C(U)] glucose and [15N](NH4)2SO4 under different moisture regimes. Soil Biol. Biochem. 17, 747–756. van Veen, J. A., Ladd, J. N., and Frissel, M. J. (1984). Modelling C and N turnover through the microbial biomass in soil. Plant Soil. 76, 257–274. Vigil, M. F., and Kissel, D. E. (1991). Equations for estimating the amount of nitrogen mineralized from crop residues. Soil Sci. Soc. Am. J. 55, 757–761. Villegas-Pangga, G., Blair, G. J., and Lefroy, R. D. B. (2000). Measurement of decomposition and associated nutrient release from barrel medic (Medicago truncatula) hay and chickpea (Cicer arietinum) straw using an in vitro perfusion system. Aust. J. Agric. Res. 51, 563–8. Vinten, A. J. A., Whitmore, A. P., Bloem, J., Howard, R., and Wright, F. (2002). Factors affecting N immobilsation/mineralisation kinetics for cellulose-, glucose- and strawamended sandy soils. Biol. Fert. Soils. 36, 190–199. Wang, W. J., Baldock, J. A., Dalal, R. C., and Moody, P. W. (2004). Decomposition dynamics of plant materials in relation to nitrogen availability and biochemistry determined by NMR and wet-chemical analysis. Soil Biol. Biochem. 36, 2045–2058. Wani, S. P., Rupela, D. P., and Lee, K. K. (1994). BNF technology for sustainable agriculture. Transactionsof the 15th World Congress of Soil Science, The International Society of Soil Science. Acapulco, Mexico, 10–16th July1994. 245–262. Xu, J. M., Tang, C., and Chen, Z. L. (2006). The role of plant residues in pH change of acid soils differing in initial pH. Soil Biol. Biochem. 38, 709–719. Zak, D. R., Holmes, W. E., MacDonald, N. W., and Pregitzer, K. S. (1999). Soil temperature, matric potential, and the kinetics of microbial respiration and nitrogen mineralization. Soil Sci. Soc. Am. J. 63, 575–584.

C H A P T E R

S I X

Strategies for Producing More Rice with Less Water M. Farooq,*,†,1 N. Kobayashi,† A. Wahid,‡ O. Ito,§ and Shahzad M. A. Basra} Contents 352 353 354 358 362 364 364 372 373 374 375

1. Introduction 2. Genetic Improvement for Water Productivity 2.1. Selection and breeding strategies 2.2. Molecular and biotechnological approaches 2.3. Water-use efficiency and transpiration efficiency 3. Crop Management 3.1. Production systems 3.2. Other management practices 3.3. Physiological implications 4. Future Thrusts References

Abstract Rice is life for more than half of humanity. It is the grain that has shaped the cultures, diets, and economies of billions of people in the world. Food security in the world is challenged by increasing food demand and threatened by declining water availability. More recently, the increase in area under biofuel crops at the cost of food crops is also threatening. Exploring ways to produce more rice with less water is essential for food security. Water-saving rice production systems, such as aerobic rice culture, system of rice intensification (SRI), ground-cover rice production system (GCRPS), raised beds, and alternate wetting and drying (AWD), can drastically cut down the unproductive water outflows and increase water-use efficiency (WUE). However, these technologies can sometimes lead to some yield penalty, if the existing lowland varieties are used.

* { { } } 1

Department of Agronomy, University of Agriculture, Faisalabad 38040, Pakistan International Rice Research Institute (IRRI), Metro Manila, Philippines Department of Botany, University of Agriculture, Faisalabad 38040, Pakistan Japan International Research Center for Agricultural Sciences, Tsukuba, Japan Department of Crop Physiology, University of Agriculture, Faisalabad 38040, Pakistan Corresponding author: [email protected]; [email protected]

Advances in Agronomy, Volume 101 ISSN 0065-2113, DOI: 10.1016/S0065-2113(08)00806-7

#

2009 Elsevier Inc. All rights reserved.

351

352

M. Farooq et al.

Other new approaches are being explored to increase water economy without compromise on yield. These include the incorporation of the C4 photosynthetic pathway into rice to increase rice yield per unit water transpired, the use of molecular biotechnology to develop rice varieties with improved water-use efficiency, transpiration efficiency (TE), drought tolerance, and the development of varieties for aerobic system, to achieve high and sustainable yields in nonflooded soil. Through the adoption of water-saving irrigation technologies, rice land will shift away from being continuously anaerobic to being partly or even completely aerobic. These shifts will produce profound changes in water conservation, soil organic matter turnover, nutrient dynamics, carbon impounding, weed flora, and greenhouse gas emissions. Although some of these changes can be positive, for example, water conservation and decreased methane emission, others might be negative, for example, release of nitrous oxide from the soil and decline in soil organic matter. The challenge will be to develop effective integrated natural–resource–management interventions, which would allow profitable rice cultivation with increased soil aeration, while maintaining the productivity, environmental safety, and sustainability of rice-based ecosystems. This chapter discusses the integrated approaches like genetics, breeding, and resource management to increase rice yield and to reduce water demand for rice production.

1. Introduction Food security depends on the ability to increase production with decreasing availability of water to grow crops. Rice, as a submerged crop, is a prime target for water conservation because it is the most widely grown of all crops under irrigation. To produce 1 kg of grain, farmers have to supply 2–3 times more water in rice fields than other cereals (Barker et al., 1998). In Asia, more than 80% of the developed freshwater resources are used for irrigation purposes; about half of which is used for rice production (Dawe et al., 1998). Rapidly depleting water resources threaten the sustainability of the irrigated rice and hence the food security and livelihood of rice producers and consumers (Tuong et al., 2004). In Asia, 17 million hectare (Mha) of irrigated rice areas may experience ‘‘physical water scarcity’’ and 22 Mha may have ‘‘economic water scarcity’’ by 2025 (Tuong and Bouman, 2002). There is also much evidence that water scarcity already prevails in rice-growing areas, where rice farmers need technologies to cope with water shortage and ways must be sought to grow rice with lesser amount of available water (Tuong and Bouman, 2002). Rice is very sensitive to water stress and attempts to reduce water inputs may tax true yield potential (Tuong et al., 2004). The challenge is to develop novel technologies and production systems that would allow rice production to be maintained or increased at the face of declining water availability.

Strategies for Producing More Rice with Less Water

353

Former requires a possible shift from the traditional system of flooded rice to growing rice aerobically and the latter needs the development of highyielding varieties that thrive under aerobic conditions (Castan˜eda et al., 2003). Several strategies are in vogue to reduce rice water requirements, such as saturated soil culture (Borrell et al., 1997), alternate wetting and drying (AWD; Li, 2001; Tabbal et al., 2002), ground-cover systems (Lin et al., 2003a,b), system of rice intensification (SRI; Stoop et al., 2002), aerobic rice (Bouman, 2003), raised beds (Singh et al., 2003), etc. Development of rice varieties through conventional breeding, marker-assisted selection (MAS), and employing biotechnological tools for water-limited conditions are the areas of current research (Atlin and Lafitte, 2002; Babu et al., 2003; Cattivelli et al., 2008; Ku et al., 2000). This chapter discusses strategies and options to make rice production more water-efficient with integrative use of crop improvement and management tools.

2. Genetic Improvement for Water Productivity Genetic improvement for adaptation to water-limited conditions is addressed through conventional approach by selecting for yield and secondary traits contributing to water saving (Farooq et al., 2009). The effectiveness of selection for secondary traits to improve yield under water-limiting conditions has been demonstrated in maize (Chapman and Edmeades, 1999), wheat (Richards et al., 2000), and sorghum (Tuinstra et al., 1998). Many studies have been undertaken to find genetic variation in traits that are expected to influence the response of rice to water deficit, including deeper and thicker roots (Yadav et al., 1997), root-pulling resistance (Pantuwan et al., 2002), greater root penetration (Ali et al., 2000; Clark et al., 2000; Fukai and Cooper, 1995), osmotic adjustment (OA; Lilley and Ludlow, 1996), and membrane stability (Tripathy et al., 2000). For the newly introduced aerobic rice culture, promising varieties should possess improved lodging resistance and higher harvest index. Medium-stature and moderately drought-tolerant cultivars are preferred for aerobic rice culture (Atlin et al., 2004, 2006). At IRRI, early heading type of a popular variety IR64 is being developed to provide suitable breeding materials for watersaving rice cultivation (Fujita et al., 2007). Development of early maturing and high-yielding rice varieties has substantially increased the average rice yield and reduced crop duration. This has contributed to a threefold increase in water productivity with respect to total water inputs (Bouman et al., 2006). Hybrid rice has 9% greater yield potential than inbred, with comparable growth duration when grown under flood-irrigated conditions in the tropics (Peng et al., 1999). This yield advantage offers another opportunity to increase the water productivity of flood-irrigated lowland

354

M. Farooq et al.

rice (Peng and Bouman, 2007). The power of molecular biology for locating important gene sequences and introgressing QTL, or even selecting for genetically important QTL to develop cultivars more efficiently utilizing water, strongly depends upon our understanding of yield-determining physiological processes (Kirigwi et al., 2007). Improvement of genetic resistance to biotic stress is also important and effective breeding approach to water-saving cultivation of rice. Rice blast disease, one of the most destructive diseases of rice, is important problem under water-limited conditions, because blast is known to tend to occur in irrigated rice in tropical upland areas (Bonman, 1992) and rainfed lowland prone to drought (Mackill and Bonman, 1992). Actually, severe blast damage was observed in aerobic rice field at IRRI (Kobayashi et al., 2006). In Brazil, blast resistance is the most important target trait for breeding program of aerobic rice variety (Breseghelo et al., 2006). In the following lines, use of different selection and breeding strategies, functional genomic approaches, and biotechnological tools to develop the suitable protocols for water-saving cultivation has been presented.

2.1. Selection and breeding strategies Rice breeding over the last decade has increased water productivity by increasing yields together with reducing crop growth duration, and hence reducing seasonal transpiration (Tuong, 1999). Grain yield is characterized by (a) the amount of biomass produced by photosynthesis and (b) the amount of biomass partitioned to grains, usually expressed as harvest index (HI). Most of the increases in rice yield in the last decades were achieved by improvement in HI. Some scientists argue that HI may now be approaching its theoretical limit in major crops (Richards et al., 1993). The photosynthesis process generally governs biomass production per unit water transpired; referred as water-use efficiency (WUE). Although there is little difference in photosynthetic rate among different commonly grown rice varieties, Peng et al. (1998) reported that WUE was some 25–30% higher for tropical japonica than for indica rice. This implies that significant variation exists in rice germplasm for photosynthesis-to-transpiration ratio, and this could be investigated further to enhance water productivity of rice. Conventional breeding has been based on empirical selection for yield (Atlin and Lafitte, 2002). However, this approach is far from being optimal, since yield is a quantitative trait and characterized by a low heritability and a high genotype  environment interaction (Babu et al., 2003). It is strongly believed that understanding of physiological and molecular basis may help target key yield-limiting traits. Such an approach may complement conventional breeding programs and hasten yield improvement (Cattivelli et al., 2008).

Strategies for Producing More Rice with Less Water

355

For a breeder, individual or combination of traits, that would directly or indirectly be associated with enhanced plant survival, are likely to improve economic yield (with or without stability), which may constitute potential target(s) for study and selection (Kirigwi et al., 2007). It is therefore imperative that utility of trait(s) for enhancing water productivity must be manifested as enhanced plant survival and better grain and dry matter yield under conditions of drought stress when assessed at the level of whole plant and crop community. Finally, the magnitude of expression of each trait and its ability to blend with other causal- or causally related traits will contribute toward its utility in plant-breeding programs (Kirigwi et al., 2007). The root characters such as biomass, length, density, and depth are very important in contributing to water saving (Subbarao et al., 1995; Turner et al., 2001). Deep and thick root system is helpful in extracting water from considerable depths (Kavar et al., 2007). Glaucousness or waxy bloom on leaves helps in reducing water losses and the maintenance of high tissue water potential, and is, therefore, considered as a desirable trait for drought tolerance (Ludlow and Muchow, 1990; Richards et al., 1986). Plant growing under water deficit conditions must conserve available water by reducing transpiration while, at the same time, fixing sufficient CO2 to meet the energy needs of the plant. Studies suggest that transpiration losses can also be reduced even in sunny, dry environments by reducing the leaf size. The plant size as expressed mainly in terms of single plant leaf area or leaf area index (LAI) has a major control over water use under stress. Short stature and small leaf area are generally conducive to low productivity while they limit water use. Botanists have long recognized plants bearing small leaves as typical ecotypes of xeric environments. Such plants withstand drought very well although their growth rate and biomass are relatively low (Ball et al., 1994). Henson (1985) used leaf size as a criterion for selection for water saving in rice. Leaf area is product of leaf length and width, and variation in both these has been recorded in rice lines. Leaf pubescence is a xeromorphic trait that helps protect from excessive heat load. Hairy leaves reduce leaf temperature and transpiration (Sandquist and Ehleringer, 2003), whilst interand intraspecific variation exists for the presence of this trait. Under high temperature and radiation stress, hairiness increases the light reflectance and reduces water loss by increasing the boundary layer resistance to water vapor movement away from the leaf surface. Although drought stress also induces the production of trichomes on both sides of wheat leaves, they had no significant influence on boundary layer resistance. The possession of a deep and thick root system allows access to deep water in the soil, and is considered important in determining drought resistance in upland rice (Kavar et al., 2007). Evidence suggests that it is quality (distribution and structure), not quantity, of roots that determines the most efficient strategy for extracting water during the crop-growing

356

M. Farooq et al.

season. Rice, evolved from a semiaquatic ancestor, was domesticated into lowland or aerobic cultivars. These cultivars developed aerenchyma in roots, a superficial root system, and high levels of nonstomatal water loss from leaves (Lafitte and Bennet, 2003). There is a similar tendency to conserve adaptations to excess water at the cellular and molecular levels. For rice to succeed as an aerobic crop, breeders must overcome the legacy of anaerobic adaptation, enabling the crop to tolerate intermittent water deficits, high soil impedance, and low humidity of air. Extensive genetic variation exists within cultivated rice and additional variation exists in wild relatives. Dramatic contrasts are observed among rice cultivars in the response of root growth to soil drying; some cultivars cease root development, some increase root mass in superficial layers, and others show increased and deep root growth. Genetic variation is also observed in the sensitivity of rice leaf area expansion to both soil and atmospheric water deficit, and in the relative reduction in spikelet number and fertility that occurs in aerobic conditions. The processes like drought rhizogenesis, leaf expansion, and sink pruning are expected to reflect differences in signal reception and transduction in rice compared with other crops. Improved understanding of the molecular basis and genetic control of these signaling processes is likely to develop successful aerobic cultivars that would respond to the environment more like other upland crop species (Lafitte and Bennett, 2002). Rice plants completing life cycle in shortened period of time use less amount of water. Crop duration is interactively determined by genotype and the environment, and determines the ability of the crop to complete growth cycle in lesser time (Dingkuhn and Asch, 1999). Time of flowering is a major trait of crop adaptation to environment, particularly when the growing season is restricted by low water availability and high temperatures during later growth stages. Developing short-duration varieties has been an effective strategy for minimizing yield loss from terminal drought, as early maturity helps the crop to avoid the period of stress (Kumar and Abbo, 2001). However, yield is generally correlated with longer crop duration under favorable growing conditions, and any reduction of crop duration below optimum would tax yield (Turner et al., 2001). Aerobic systems are likely to require cultivars that have been selected from early generations under high-input aerobic management to produce genotypes that combine moderate tolerance of moisture stress with high HI and lodging resistance. Cultivars that perform well under aerobic management usually contain germplasm from both traditional upland and eliteirrigated parents, but some cultivars without elite-irrigated high-yielding variety parentage, and some developed for irrigated systems also display high yields in aerobic systems (Atlin and Lafitte, 2003).

Strategies for Producing More Rice with Less Water

357

For the Australian situation, Reinke et al. (1994) argued that reducing duration could save up to 10% of irrigation water, whereas Williams et al. (1999) concluded that reduced duration will always reduce yield potential and hence water productivity. Despite some evidence for the latter argument, varieties with higher yield potential and shorter duration have been developed (Reinke et al., 2004). Short-duration varieties also facilitate increased WUE of the farming system. For example, earlier maturity allows earlier harvest, increasing the chance of timely establishment of a winter crop after rice and making efficient use of stored soil water and winter rainfall instead of losing it as deep and surface drainage or transpiration by weeds. The most efficient strategy for identifying cultivars for near saturation systems is to screen short-duration elite-irrigated varieties under nonstress management to eliminate cultivars vulnerable to soil drying (Atlin and Lafitte, 2003). Several putative traits contributing to water saving and drought resistance in rice have been suggested (Fukai and Cooper, 1995). Root characteristics such as thickness, rooting depth, root density, rootpulling force, and root penetration ability have been associated with drought avoidance in rice (Nguyen et al., 1997). OA capacity is an important, shoot-related component of drought tolerance in crop plants. OA is the active accumulation of solutes during the development of water stress in plants (Blum, 1988), allows maintenance of higher turgor potential at a given leaf water potential. OA delays leaf rolling, tissue death, and leaf senescence under water stress in rice (Hsiao et al., 1984), and has been shown to enhance grain yield under water-limited conditions in several other crops (Zhang et al., 1999). However, a yield benefit due to OA is yet to be demonstrated in rice. Despite our understanding of the role of putative traits in drought resistance, these traits are rarely selected for crop improvement programs because phenotypic selection for most root traits and OA is difficult and labor intensive. Much effort is currently being directed to developing molecular markers for various traits such as maximum rooting depth (Champoux et al., 1995), the capacity of roots to penetrate hard pans (Ray et al., 1996), and ability of the plant to osmotically adjust to water deficit (Lilley and Ludlow, 1996). Considering these limitations to efficient selection, molecular marker technology is a powerful tool for selecting such traits. QTLs have been detected for several root-related traits and OA in rice (Ali et al., 2000; Lilley and Ludlow, 1996; Ray et al., 1996; Yadav et al., 1997; Zhang et al., 2001; Zheng et al., 2000). A significant proportion of the phenotypic variability of several of these putative drought resistance traits is explained by the segregation of relatively few genetic loci, thus leading to the possibility of indirect selection of these complex traits by MAS strategy. Identification of QTLs associated with water saving is an important tool for MAS of desirable plants.

358

M. Farooq et al.

2.2. Molecular and biotechnological approaches Recent advances in genomics, the development of advanced analytical tools at the molecular level, and genetic engineering provide new avenues for improving yield potential and enhancing drought stress tolerance. Currently, slow progress in breeding to improve water productivity may be accelerated by the discovery and subsequent manipulation of regulatory genes underlying the complex physiological and biochemical responses of rice plants to water deficit. Common research tools, tolerance mechanisms, and breeding solutions are emerging across the evolutionary diversity of crops and plants (Tuong and Bouman, 2003). Enormous public- and private-sector investments in genomic analysis of Arabidopsis thaliana, cereals, and other crops are already contributing greatly to these efforts (Bennett, 2001). Many laboratory and field studies have shown that transgenic expression of some of stress-regulated genes results in increased WUE (Table 1). These transgenic approaches are currently the mainstream method to bioengineer crop plants that would require less water (Bahieldina et al., 2005). However, enhanced expression of these genes is frequently associated with retarded growth and thus may limit its practical applications. To identify the less obvious genetic networks that respond to stress, more straightforward and sensitive methods are imperative. The advent of whole genomics and related technologies are providing necessary tools to identify key genes that respond to drought and their adaptive regulation to stress (Bruce et al., 2002). Introducing a single enzyme or even an incomplete portion of the C4 cycle is, of course, unlikely to have a large impact on photosynthesis. However, some evidence suggests that the manipulations have led to the desired redirection of fluxes (Hudspeth et al., 1992). Introduction of the gene of maize PEP carboxylase in rice indicated remarkably higher level of expression (Ku et al., 1999). The activities of PEP carboxylase in leaves of some of these transgenic rice plants were two- to threefolds higher even than those in maize, and the enzyme accounted for up to 12% of the total leaf-soluble protein. It means that increasing the amounts of PEP carboxylase in isolation does not have dramatic effects on photosynthesis, although it may alter stomatal conductance (Ku et al., 2000). Transgenic rice exhibited reduced O2 inhibition of photosynthesis, but this was probably due to effects of phosphate recycling that affect photorespiration (Table 2; Matsuoka et al., 2000). In a study, Suzuki et al. (2000) overexpressed an unregulated phosphoenolpyruvate carboxykinase (PCK) from C4 plant, Urochloa panicoides, in the chloroplasts of rice leaf. In 14CO2-labeling experiment, up to 20% of the radioactivity was incorporated into C4 acids (malate, oxaloacetate, and aspartate) in leaves of transgenic plants, as compared with about 1% in

359

Strategies for Producing More Rice with Less Water

Table 1

Transgenic rice plants reporting effects under water-limited conditions

Gene

Effect

References

p5cr

Faster shoot and root growth was observed in transgenic seedlings than controls under water-limited conditions. Stress-inducible expression of p5cr transgene gave the greatest effect Majority of transgenics survived an episode of acute drought stress. Under cycles of drought/ recovery, the transgenics had higher biomass and were taller than the controls Transgenic lines showed more sustained plant growth, less photo-oxidative damage, and more favorable mineral balance under water-limited conditions, than controls Overexpression of OsCDPK7 had less wilting than controls Transgenic plants maintained higher growth rates than controls under drought Higher leaf RWC and tolerance to water stress by protecting cell membrane After salt and drought treatments, transgenic lines showed increased stress tolerance (cell integrity and growth), compared to the control plants Accumulation of either PMA80 or PMA1959 correlates with increased drought tolerance Transgenic plants with higher expression levels of sHSP17.7 protein recovered the growth after upon rewatering after the stress period Transgenic plants exhibited less membrane injuries than control plants

Su and Wu (2004)

coda

otsA and otsB

OsCDPK7 HVA1 HVA1 HVA1

PMA80 and PMA1959 sHSP17.7

SWPA2

Sawahel (2003)

Garg et al. (2002)

Saijo et al. (2000) Xu et al. (1996) Babu et al. (2004) Rohila et al. (2002)

Cheng et al. (2002) Sato and Yokoya (2007)

Wang et al. (2005)

360 Table 2

M. Farooq et al.

Transgenic C4 rice, the genes, and their effect

Source of genes

Gene

Effect

References

Maize

PPDK

Ji et al. (2004)

Maize

NADP-ME

Maize

PEPC + PPDK

Maize

PEPC

Urochloa panicoides

PCK

Maize

PEPC

Increased net photosynthesis and decreased photorespiration in transgenic plants Photorespiration rate decreased and net photosynthetic rate increased in transgenic plants Increased net photosynthesis and decreased photorespiration in transgenic plants Photorespiration rate decreased and net photosynthetic rate increased in transgenic plants Threefold greater sucrose synthesis was observed in transgenic plants than in control plants Transgenic plants exhibited a higher photosynthetic capacity (up to 35%) than untransformed plants. The increased photosynthetic capacity in these plants was mainly associated with an enhanced stomatal

Ji et al. (2004)

Ji et al. (2004)

Ji et al. (2004)

Suzuki et al. (2000)

Ku et al. (2007)

x

361

Strategies for Producing More Rice with Less Water

Table 2 (continued) Source of genes

Gene

Maize

PPDK

Maize

PEPC

Maize

PEPC

Maize

NADP-ME + PPDK

Effect

conductance and a higher internal CO2 concentration A higher photosynthetic capacity (up to 35%) than untransformed plants Transgenic C4 plants were 30–35% more efficient in photosynthesis Photosynthetic capacity was increased greatly (50%) under high CO2 supply. In CO2-free air, CO2 release in the leaf was less. In addition, transgenic rice was more tolerant to photoinhibition Photosynthetic rate was increased by 50%

References

Ku et al. (2007)

Ku et al. (1999)

Jiao et al. (2005)

Jiao et al. (2002)

excised leaves of control plants. When 14C-malate was fed to excised leaves the extent of incorporation of radioactivity into sucrose was threefold greater in transgenic than control plants and the level of radiolabeled aspartate was significantly lower in transgenic plants. Thus, expression of PCK in rice chloroplasts led to a partial change in carbon flow in mesophyll cells into a C4-like photosynthetic pathway. Overexpression of maize pyruvate, Pi dikinase (Fukayama et al., 2001), has been claimed to display a higher photosynthetic rate, associated with higher stomatal conductance (Ku et al., 2000).

362

M. Farooq et al.

About 20–70-fold increase in maize NADP-malic enzyme in rice leaves (located mainly in the chloroplasts) led to an aberrant chloroplast structure with a granal thylakoid membranes, and an inverse correlation between NADP-malic enzyme activity and chlorophyll and photosystem II activity (Takeuchi et al., 2000). This is particularly interesting in relation to the presence of granal chloroplasts in the bundle sheath of NADP-malic species. However, other studies on rice overexpressing maize NADP-malic enzyme also indicated a reduction in chlorophyll content, enhanced photoinhibition, and reduced growth. This probably resulted from a greater reduction of the NADP pool as a result of a high activity of the overexpressed enzyme in vivo (Takeuchi et al., 2000; Tsuchida et al., 2001).

2.3. Water-use efficiency and transpiration efficiency While transpiration efficiency (TE) is the ratio between photosynthesis and transpiration (Tuong and Bouman, 2003), whole-plant WUE can be expressed as the ratio of total biomass or grain production to the amount of water transpired. The WUE is determined by both photosynthesis and transpiration. Increasing photosynthesis and/or decreasing transpiration would elevate plant WUE (Tuong and Bouman, 2003). Stomata play a crucial role in both these processes, and hence economizing water. Different plant species have evolved different stomata with great variations in size, density, and morphology. This rapid opening and closing strategy can save energy and increase photosynthesis and WUE (Grantz and Assmann, 1991). The physiological and molecular bases of water saving are complex and highly linked to drought tolerance mechanisms (Chaves and Oliveira, 2004). There are two major ways to increase the plant WUE. One is the engineering-based cropping system where modern irrigation techniques play an important role. Much progress has been made to improve WUE by managing irrigation (Giordano et al., 2007; Jones, 2004; Kacira and Ling, 2001; Kang and Zhang, 2004; Sun et al., 2005). The other way is ‘‘biological water saving (BWS),’’ the physiological and ecological bases of water saving by crops in agriculture (Shan, 1991). The concept of BWS was further developed by Shi (1999) and defined as ‘‘more agricultural products output with the same or less water input by exploiting the physiological and genetic potential of organisms themselves.’’ The core of BWS is to increase WUE of plants, which is used as an indicator for plant’s ability to economize water. In contrast to the irrigation method, BWS is a more efficient and economic way to increase WUE for the obvious reason of less input than engineeringbased methods. Blum (2005) pointed out that high yield under water-limited conditions is generally associated with increased WUE mainly because of high

Strategies for Producing More Rice with Less Water

363

water use. Therefore, selection for high WUE in a breeding program will result in smaller or earlier flowering plants that use less water but have low yield potential at the same time (Blum, 2005). Thus, the challenge is to develop water-efficient genotypes that produce higher yields with limited water supply, and equal or greater yields than current varieties under favorable growth conditions without stress. Because photosynthesis and transpiration rates are generally proportional, there is only a small difference in TE among rice varieties at the single-leaf level when grown under flooded and aerobic cultivation (Singh and Sasahara, 1981). However, developing rice varieties with superior performance under water-saving technologies such as AWD and aerobic cultivation could result in a significant improvement in water productivity of irrigated lowland rice (Peng and Bouman, 2007). Peng et al. (1998) reported that improved tropical japonica rice lines had 25–30% higher TE at the single-leaf level than indica varieties when grown under flooded conditions. This was because the indica varieties had a higher transpiration rate than the tropical japonica lines, whereas the differences in photosynthesis between the two types were relatively small and inconsistent across growth stages and years compared with the differences in transpiration rate. In another study, Yeo et al. (1994) observed large differences among Oryza species in TE at the single-leaf level. Oryza australiensis had significantly greater TE than O. sativa at the same photosynthetic rates. The potential for exploiting this trait, however, has not been investigated. Varietal differences in TE at the single-leaf level and whole-plant WUE measured by gravimetric determinations of growth and water loss from individual plants were reported in rice (Flowers et al., 1988). However, a high WUE was associated with the nondwarf habit and, therefore, it may not be useful to incorporate this trait into commercial varieties to increase water productivity. Increase in waxiness of rice leaves was proposed to reduce nonstomatal transpiration but the impact on WUE was not demonstrated (Lafitte and Bennett, 2002). Transforming the C3 rice plant into C4 by genetic engineering of photosynthetic enzymes and required anatomic structures was suggested as another approach to improve TE. High-level expression of maize phosphoenolpyruvate carboxylase (PEPC) and pyruvate, orthophosphate dikinase (PPDK) and NADP-malic enzyme (NADP-ME) in transgenic rice plants has been achieved (Table 2; Agarie et al., 1998). Ku et al. (2000) reported that PEPC and PPDK transgenic rice plants had up to 30–35% higher photosynthesis than untransformed ones. However, increase in photosynthesis was associated with enhanced stomatal conductance, which reduces the potential for increasing TE by the development of C4 rice plants. Nevertheless, development of C4 rice plants seems more ambitious and very cumbersome.

364

M. Farooq et al.

3. Crop Management The management strategies start from the selection of a good genotype, crop planting site; include seedbed preparation, production system, date and method of planting, sowing time, plant protection, nutrient management; and last till the crop harvesting. In rice systems aiming to have high water productivity, soil type (Tripathi, 1996), weed management (Singh et al., 2003), irrigation method (Beecher et al., 2006), and land leveling (Alam et al., 2003; Kahlown et al., 2002) are of premier importance. One obvious measure to improve the water productivity is to reduce the evaporation by shortening the land preparation period (Tuong, 1999). Moreover, early canopy closure can help to reduce evaporation after crop establishment (Tuong et al., 2000). This can be achieved by proper plant density and growing rice varieties with good seedling vigor (Tuong et al., 2000). These measures can also help the rice plants compete better with weeds, thus reducing nonbeneficial transpiration from weeds and increasing yield (Tuong et al., 2000). Amongst different rice production systems, aerobic rice (Bouman et al., 2007), AWD (Cabangon et al., 2001), raised beds ( Jehangir et al., 2002), SRI (Uphoff and Randriamiharisoa, 2003), and ground-cover rice production system (GCRPS; Dittert et al., 2003) have been recognized to possess high water productivity in different agroecological regions. Some physiological strategies including seed priming (Farooq et al., 2006a,b,c, 2007a,b; Harris et al., 2002), use of osmoprotectants (Farooq et al., 2008; Yang et al., 2007), and silicon (Si) nutrition (Ma, 2004) can be employed to further enhance the rice water productivity.

3.1. Production systems Water surfaces have a higher evaporation rate than soil. Evaporative water loss can also be reduced by adopting the production systems and technologies, which shorten the duration that the field is flooded and/or requirement for water application (Bouman et al., 2007). Rice systems such as AWD irrigation, bed planting, aerobic culture, SRI, and GCRPS are very effective in this regard (Fig. 1). In the following lines, literature available on each of these systems is discussed with emphasis on water-saving rice production. 3.1.1. Aerobic rice system Aerobic rice is a new way of production system in which specially developed, input-response rice varieties with aerobic adaptation are grown in well-drained, nonpuddled, and nonsaturated soils without ponded water

365

Strategies for Producing More Rice with Less Water

Yield (t ha−1)

10

SRI

05

Aerobic rice system

Traditional lowland system Alternate wetting and drying system

Traditional upland system

0 Low

High Water availability Aerobic

S FC Soil condition

Flooded

Figure 1 Schematic presentation of yield responds to water availability and soil conditions in different rice production systems. Abbreviations: SRI, system of rice intensification; FC, field capacity; S, saturation point. (Adapted from Tuong et al., 2004 after modification.)

(Bouman et al., 2007). It entails growing rice in aerobic soil, with the use of external inputs such as supplementary irrigation and fertilizers, and aiming at high yields (Bouman and Tuong, 2001). Main driving force behind aerobic rice is the economic water use. A fundamental approach to reduce water inputs in rice is growing like an irrigated upland crop, such as wheat or maize. Instead of trying to reduce water input in lowland paddy fields, the concept of having the field flooded or saturated is abandoned altogether (Bouman and Tuong, 2001). The adoption of aerobic rice is facilitated by the availability of weed management tools and seed-coating technologies. Case studies showed yields to vary from 4.5 to 6.5 t ha 1, which is about double than that of traditional upland varieties and about 20–30% lower than that of lowland varieties grown under flooded conditions. However, the water use was about 60% less than that of lowland rice, total water productivity 1.6–1.9 times higher, and net returns to water use was twofold higher. Aerobic rice requires lesser labor than lowland rice and can be highly mechanized (Huaqi et al., 2003). Input water savings of 35–57% have been reported for dryseeded rice (DSR) sown into nonpuddled soil with the soil kept near

366

M. Farooq et al.

saturation or field capacity compared with continuously flooded (5 cm) transplanted rice (Sharma et al., 2003; Singh et al., 2003). However, yields were reduced by similar amounts due to iron or zinc deficiency and increased incidence of nematodes. Contrary to the results of small plot replicated experiments, participatory trials in farmers’ fields in India and Pakistan suggest a small increase or 10% decline in yield of DSR on the flat compared with puddled transplanted rice, and around 20% reduction in irrigation time or water use (Gupta et al., 2003). In their experiments on a high-yielding lowland rice variety (IR20) like an upland crop under furrow irrigation, De Datta et al. (1973) reported that total water savings were 56% and irrigation water savings 78% compared with growing the crop under flooded conditions. However, the yield was reduced from 7.9 to 3.4 t ha 1. The WUE of the aerobic varieties under aerobic conditions was 164–188% higher than that of a lowland cultivated rice variety. Aerobic rice maximizes water use in terms of yield and is a suitable crop for water-limiting conditions (Xiaoguang et al., 2003). In a study, rice yields under aerobic conditions were 2.4–4.4 t ha 1, which were 14–40% lower than under flooded conditions (Castan˜eda et al., 2003). However, water use decreased relatively more than yield, and water productivity under aerobic cultivation increased by 20–40% (in one case even 80%) over that under flooded conditions. The aerobic rice technology eliminates puddling and flooding, and presents an alternative system in reducing water use and increase water productivity. Aerobic rice saved 73% of irrigation water for land preparation and 56% during the crop growth period (Castan˜eda et al., 2003). In a two year field experiment at Indo-Gangetic plains to evaluate various tillage and crop establishment systems for their efficiency in labor, water and energy use, and economic profitability, the yields of rice in the conventional puddled transplanting and direct-seeding on puddled or nonpuddled (no-tillage) flat bed systems were equal (Bhushana et al., 2007). Nevertheless, decline in yield was observed when aerobic rice was continuously grown and the decline was greater in the dry than in the wet season (Peng et al., 2006). In crux, aerobic rice is an attractive option to the traditional rice production system. Yield penalty and yield stability of aerobic rice have to be considered before promoting this water-saving technology. 3.1.2. Alternative wetting and drying irrigation Most rice in Asia is transplanted into puddled soils. Puddling is done for a range of reasons including weed control, ease of field leveling and transplanting, and to reduce percolation losses. The relative importance ascribed to each of the above reasons varies. For example, Tabbal et al. (2002) consider that puddling in central Luzon, Philippines, is done primarily for weed control, whereas Kukal and Aggarwal (2003) and Gajri et al. (1992) placed more emphasis on its role in reducing percolation losses in

Strategies for Producing More Rice with Less Water

367

northwestern India, where soils are highly permeable. Puddling is not essential for rice growth and yield. Many studies (but not all, e.g., Singh et al., 2001) reported similar yields for transplanted or direct-seeded rice with and without puddling (e.g., Aggarwal et al., 1995; Humphreys et al., 1996; Kukal and Aggarwal, 2003). The high-yielding rice cultural systems of Australia and California (USA) are not puddled. Although it is widely recognized that puddling reduces percolation, there are surprisingly few reports of quantitative field comparisons of percolation losses in puddled and nonpuddled soils. These indicated that the effect of puddling on percolation rate ranges from little to reductions from 30 to 13 mm day 1 on flooded sandy loam soils and from 17 to 3 mm day 1 on flooded clay soils (Humphreys et al., 1996; Kukal and Aggarwal, 2003). Despite reducing percolation losses during the rice crop, puddling does not necessarily reduce the total water input for rice (Tabbal et al., 2002; Tuong et al., 1996). However, there are only a few reports showing the comparison of total water use or percolation losses in puddled and nonpuddled systems that include the whole period from pre-irrigation to harvest, and that use the same water management after planting. An exception was the study of Singh et al. (2001) which compared water use and yield of water-seeded rice with and without puddling on a sandy loam soil in India, with water depth maintained at 5 cm in both treatments. Averaged over 3 years, there was irrigation water saving of only 75 mm with puddling out of a total irrigation water application of 1537 mm. Thus, even on this highly permeable soil, the irrigation water saving with puddling was relatively small in comparison with the total water use. Puddling for rice induces high bulk density, high soil strength, and low permeability in subsurface layers (Aggarwal et al., 1995; Kukal and Aggarwal, 2003), which can restrict root development and water and nutrient use from the soil profile for wheat after rice (Gajri et al., 1992). Continuous flooding had the highest irrigation water inputs, followed by AWD irrigation, saturated soil culture in raised beds, flush irrigation in aerobic soil, and rainfed treatments. Rice yields did not differ significantly among watering treatments (Lu et al., 2003). AWD has been commonly used as a water-saving practice in many parts of the world for more than a decade (Cabangon et al., 2001). In this system, the soil is allowed to dry for a few days within irrigation events depending on plant developmental stages (Cabangon et al., 2001; Shi et al., 2003). Some success has been reported as far as yield and water demand is concerned (Gani et al., 2003; Lu et al., 2003); however, unproductive water losses could not be totally avoided by AWD. Hence, the water consumption is still high in AWD since the soils need to be submerged at least during the irrigation period. Savings in irrigation water in the AWD treatments were 53–87 mm (13–16%) compared with the continuously submerged regime. Rice grain yields ranged from 7.2 to 8.7 t ha 1 and were not markedly affected by the water regimes.

368

M. Farooq et al.

Water productivity was significantly higher in the AWD regime than in the continuously submerged regime (Belder et al., 2003). 3.1.3. System of rice intensification SRI that evolved in the 1980s and 1990s in Madagascar permits resourcelimited farmers to realize paddy yields of up to 15 t ha 1 even on infertile soils, with greatly reduced rates of irrigation and without external additional inputs (Stoop et al., 2002). The main features of this system are transplanting young seedlings singly in a square pattern with wide spacing, using organic fertilizers and hand weeding, and keeping the paddy soil moist during the vegetative growth phase. Significant phenotypic changes occur in plant structure and function and in yield and yield components under SRI cultivation. SRI increased yields substantially (50–100% or more), while requiring only about half as much water as conventional rice (Uphoff and Randriamiharisoa, 2003), whilst not needing the purchase of additional external inputs. SRI is difficult for most farmers to practice because it requires significant additional labor inputs at a time of the year when liquidity to hire labor is low and family labor effort is already high. This poses the challenge to researchers and policymakers concerned with the promotion of watersaving rice technologies. Even though the yields can be increased while saving water, adoption by farmers is still far from assured (Moser and Barrett, 2003). SRI methods are able to enhance yields of any rice variety, but the highest yields have come from improved high-yielding varieties. Factorial trials in Madagascar explain synergistic dynamics among the SRI practices that account for 100–200% increases in yield (Uphoff and Randriamiharisoa, 2003). A large increase in the productivity of irrigation water use with SRI can make water savings more attractive, compensating farmers well for the extra labor or expenditures involved. The returns to land, labor, capital, and water are all increased by the use of SRI practices (Uphoff and Randriamiharisoa, 2003). Lu et al. (2004) evaluated some modifications in traditional SRI, viz. transplanting three separated seedlings in one hill in a triangular pattern with the leaf age extended to 3–4 weeks; application of herbicide before transplanting; mulching the spaces between plants with straw; adding chemical fertilizers to promote plant growth vigorously when needed; making shallow furrows before transplanting in the zero-till fields; and applying the AWD method for water management with midseason drainage to inhibit tillering. With these modifications, grain yield exceeded 12 t ha 1, being 46% greater than in control using field comparison along with water saving. McHugh et al. (2003) conducted a survey of farmers in Madagascar to investigate farmer implementation of AWD as part of SRI and showed that farmers have adapted AWD practices to fit the soil type, availability of water and labor. The primary drawbacks reported by farmers with implementing

Strategies for Producing More Rice with Less Water

369

AWD were the lack of a reliable water source, little water control, and water-use conflicts. They suggested that by combining AWD with SRI, farmers can increase grain yields while reducing irrigation water demand (McHugh et al., 2003). Uphoff and Randriamiharisoa (2003) proposed that continuously flooded soils constrain root growth and contribute to root degeneration. Moreover, soil microbial life is limited to anaerobic populations. This excludes contributions to plant performance from mycorrhizal fungal associations that are of benefit to most plant species. Keeping paddy fields flooded also restricts biological nitrogen fixation to anaerobic processes, forgoing possibilities for aerobic contributions. In another study, Thiyagarajan et al. (2003) reported savings in irrigation water of 56% and 50% using conventional and young seedlings, respectively, without a significant effect on grain yield under SRI system. Twoweek-old seedlings planted one seedling per hill produced significantly higher yield (6.43 t ha 1) than the farmer’s practice of using 21-day-old seedlings (5.96 t ha 1). However, yields were similar for both age groups when the number of seedlings increased to 2 and 4 per hill. The performance of 15-day-old seedlings improved more than that of 21-day-old seedlings with the addition of well-decomposed organic matter and intermittent irrigation (Makarim et al., 2003). In a cement-box experiment in China, production characteristics, water-use efficiency, nitrogen-use efficiency, and major physiological characteristics of three alternative water management practices SRI, GCRPS, and AWD were compared with a conventional flooded rice system. Water supply in SRI and AWD was 46% and 36% lower than in conventional flooded rice system, respectively; whereas their yields were similar or significantly higher (5% for SRI and 8% for AWD), resulting in greater WUE. The higher yields of SRI and AWD compared with conventional flooded rice system were associated with higher harvest indices but not with differences in total biomass production. Water supply and yield in GCRPS were 65% and 62% lower than in conventional flooded rice system (Cao et al., 2003). 3.1.4. The ground-cover rice production system The plastic film or straw mulching rice production systems have been developed since 1990 in China to improve the tolerance to low temperatures (Shen et al., 1997). This is similar to the success in Japan in the 1960s, but now its benefits for water-saving rice production led to the adoption of this system. In plastic film mulching (PFM), also called GCRPS, lowland rice varieties are used and the soil is kept humid by covering materials (Kreye et al., 2007). In GCRPS, soil is irrigated to approximately 80% of water-holding capacity. Nevertheless, the amount of water saved with this system can be as high as 60–85% of the need in the traditional paddy systems with no adverse effects on grain yield (Huang et al., 1999). However, some

370

M. Farooq et al.

researcher reported significant yield reductions under such conditions (Borrell et al., 1997; Castillo et al., 1992). Thereafter, to check evaporation the soil surface is covered by material, such as plastic film, paper, or plant mulch (Lin et al., 2003b). Although benefits of water-saving rice cultivation in water-limited areas have been illustrated (Gani et al., 2003; Huang et al., 1999; Shen et al., 1997), other experimental evidences suggest moderate to severe yield reduction (Borrell et al., 1997; Castillo et al., 1992) of water-saving cultivation compared to paddy. With lower soil water potentials the elongation of internodes, the number of panicles and the crop growth rate reduced in comparison to flooded conditions (Lu et al., 2000). Lin et al. (2003b) recorded up to 60% reduction in water requirements of rice crop in a GCRPS; however, grain yields were up to 10% lower than the traditional lowland rice. This was associated to micronutrient deficiency and difficulties in nitrogen fertilizer management contributed to higher yield penalty in GCRPS. Two GCRPSs using thin plastic film or straw mulch soil cover were compared to traditional paddy rice production. In the submerged rice fields, methane (CH4) emission was dominant, and only during the drainage period before panicle initiation nitrous oxide (N2O) emission was found. In contrast, CH4 emission from GCRPS was negligible but N2O emission generally increased with water-saving GCRPS, and emission events were clearly linked to fertilization (Dittert et al., 2003). In a recent study, GCRPS including soil surface was covered with 14-mm thick plastic film (GCRPS-plastic); mulched with straw (GCRPS-straw) and uncovered (GCRPS-bare) were compared with lowland rice cultivated under traditional paddy conditions (control). Compared to paddy control, only 32–54% of irrigation water was applied in GCRPS treatments. Plants in GCRPS were smaller, developed fewer panicles and had a smaller LAI than paddy control. Yield was significantly less in GCRPS-bare and GCRPS-straw compared to paddy, while yield in GCRPS-plastic was only 8% lesser than the paddy control yield. WUE in GCRPS-plastic was higher than in paddy control (Tao et al., 2006). 3.1.5. Raised beds The use of raised beds for the production of irrigated non-rice crops was pioneered in the heavy clay soils of the rice-growing region in Australia in the late 1970s (Maynard, 1991), and for irrigated wheat in the rice–wheat system of the Indo-Gangetic plains during the 1990s, inspired by the success of beds for wheat–maize systems in Mexico (Meisner et al., 1992; Sayre and Hobbs, 2004). Potential agronomic advantages of beds include improved soil structure due to reduced compaction through controlled trafficking, and reduced waterlogging and timely machinery operations due to better surface drainage. Beds also provide the opportunity for mechanical weed

Strategies for Producing More Rice with Less Water

371

control and improved fertilizer placement. While the potential benefits of beds for wheat production in the Indo-Gangetic plains have been known for some time (Dhillon et al., 2000), evaluation of beds for rice and permanent beds in rice–wheat system systems commenced more recently (Connor et al., 2002). Farmer and researcher trials in the Indo-Gangetic plains suggest irrigation water savings of 12–60% for direct-seeded and transplanted rice on beds, with similar or lower yields for transplanted compared with puddled flooded transplanted rice, and usually slightly lower yields with direct seeded rice (Balasubramanian et al., 2003; Gupta et al., 2003; Hossain et al., 2003; Jehangir et al., 2002). However, many studies in the northwest IndoGangetic plains indicate little effect of rice on beds on water productivity (typically around 0.30–0.35 g kg 1) as the decline in water input was accompanied by a similar decline in yield ( Jehangir et al., 2002; Sharma et al., 2003; Singh et al., 2003). The causes of reduced rice yield included increased weeds and nematodes, suboptimal sowing depth due to lack of precision, and micronutrient (e.g., iron, zinc) deficiencies. Singh et al. (2003) evaluated the yield and water use of rice established by transplanting, wet and dry seeding with subsequent aerobic soil conditions on flatland and on raised beds. Transplanted rice yielded 5.5 t ha 1 and used 360 mm of water for wetland preparation and 1608 mm during crop growth. Compared with transplanted rice, dry-seeded rice on flatland and on raised beds reduced total water input during crop growth by 35–42% when the soil was kept near saturation and by 47% and 51% when the soil dried out to 20 and 40 kPa moisture tension in the root zone, respectively. Most of the water savings were caused by reduced percolation losses. Moreover, no irrigation water was used during land preparation. However, the dry seeding of rice reduced yield by 23–41% on flatland and by 41–54% on raised beds compared with transplanted rice. There was no great difference in water productivity among treatments. There appears to be little scope for saving irrigation water with furrow-irrigated rice on beds on the heavy clay soils of southern Australia. Investigations over four growing seasons showed irrigation water savings of around 10% with saturated soil culture (water continuously in the furrows), with a similar reduction in the grain yield (Thompson et al., 2003). Irrigation water use of rice grown on beds with intermittent irrigation until 2 weeks before panicle initiation, followed by continuous flooding, was similar to water use of dry-seeded rice on the flat surface with continuous flooding commencing about 1 month after sowing (Beecher et al., 2006). This is in contrast with findings on a more permeable soil in semitropical southern Queensland where irrigation water use of rice on beds with saturated soil culture was 32% less than flooded rice on the flat due to considerably reduced percolation losses (Borrell et al., 1997). Studies in the USA have also shown considerable water savings with furrow-irrigated

372

M. Farooq et al.

rice on beds (Tracy et al., 1993; Vories et al., 2002). Beecher et al. (2006) reported no water saving from the raised bed rice cultivation compared with conventional ponded rice grown on a flat layout. When grown on raised beds, a variety needs to be able to compensate for the loss in cropped area (caused by the relatively large row spacing between the beds) by producing more productive tillers (Singh et al., 2003).

3.2. Other management practices Soil type has a large influence on irrigation water requirement due to much higher percolation losses on coarser textured soils. This is particularly true for rice grown under submerged condition for most of the season. Seasonal percolation losses of 57–83% of the total input water are common in the Indo-Gangetic plains, with highest losses (up to 1500 mm) on sandy and sandy-loam soils, and lowest losses on loams and clay-loams (up to 890 mm) (Tripathi, 1996). The extent of laser leveling in South Asia and China is currently extremely small, compared with 50–80% of the rice land in Australian rice-based systems (Humphreys and Bhuiyan, 2001; Lacy and Wilkins, 2003). Land leveling can reduce evaporation and percolation losses by enabling faster irrigation times and by eliminating depressions. It also reduces the depth of water required to cover the highest parts of the field and for ponding for weed control in rice, and therefore percolation losses, more so on more permeable soils. Rickman (2002) found that rice yields in rainfed lowland laser-leveled fields were 24% higher than in nonlaser-leveled fields in Cambodia, and yield increased with the uniformity of leveling. Pressurized irrigation systems (sprinkler, surface, and subsurface drip) have the potential to increase irrigation water use efficiency by providing water to match crop requirements, reducing runoff and deep drainage losses, and generally keeping the soil drier, reducing soil evaporation and increasing the capacity to capture rainfall (Camp, 1998). There are few reports of the evaluation of these technologies in rice–wheat systems. In Australia sprinkler irrigation of rice to replace evaporative loss reduced irrigation water use by 30–70% (Humphreys et al., 1989). Even at frequencies of up to three times per week yield declined by 35–70% (Muirhead et al., 1989). Irrigation water use was reduced by about 200 mm in rice with subsurface drip commencing 2 weeks prior to panicle initiation compared with flooded rice culture. Yields with drip also decreased, although there was no increase in irrigation water productivity (Beecher et al., 2006). Reducing nonbeneficial evaporation direct from the soil or free water lying on the field is true water saving, although it may be countered to some degree by increased transpiration rates as a result of impacts on the microclimate experienced by the plant. The size of this effect has not been

Strategies for Producing More Rice with Less Water

373

established. Evaporation from the free water surface accounted for 40% of the total evaporative loss from continuously flooded water-seeded rice (Simpson et al., 1992). Substantial irrigation water savings (25–30%) can be achieved by delaying transplantation from mid-May to mid-June (Narang and Gulati, 1995). Direct seeding could help overcome the problem of labor availability, although the optimum sowing date may need to be earlier than the optimum transplanting date, which could increase the crop water requirement. It is not clear if changing to direct seeding will increase or reduce the water requirement for rice, and the impact may vary depending on sites and systems (Dawe, 2004). Although delayed rice planting can save water, it can also delay planting of wheat beyond the optimal time, causing yield loss of 1–1.5% per day due to higher temperatures at grain filling (OrtizMonasterio et al., 1994). While delaying transplanting in the Indo-Gangetic plains to the optimum time saves water, bringing forward transplanting in Eastern India enabled more profitable use of rainfall. Here, irrigation water is scarce, and the need for irrigation can be avoided and total system productivity increased by establishing rice with rainfall supplemented by irrigation from groundwater during the premonsoon period, and by raising bund height to 20 cm to capture rainfall (Gupta et al., 2003). There are few reports of evaluation of mulching for rice, apart from those from China, where considerable input water savings of 20–90% occurred with plastic and straw mulches in combination with aerobic culture compared with continuously flooded transplanted rice (Lin et al., 2003a; Pan et al., 2003; Shen and Yangchun, 2003). Much of the water savings was probably due to higher percolation losses in the flooded systems (Lin et al., 2003a,b).

3.3. Physiological implications One of the short term and the most pragmatic approaches to overcome the drought stress effects is seed priming, which involves partial hydration to a point where germination-related metabolic processes begin but radicle emergence does not occur (Farooq et al., 2006a). Primed seeds usually exhibit increased germination rate, greater germination uniformity, and sometimes greater total germination percentage (Farooq et al., 2006b,c; Kaya et al., 2006). This approach has been applied to overcome the drought stress effects in a range of crop species. However, improvement of rice and other crops for growing in water-scant areas is of current interest. In the newly introduced aerobic rice culture, the frequency and intensity of drought may increase manifold. Du and Tuong (2002) while testing the effectiveness of different osmotica to improve the performance of directseeded rice noted that osmopriming with 4% KCl solution and saturated CaHPO4 solution was successful in improving the seedling emergence,

374

M. Farooq et al.

crop stand establishment, and yield under stress. Harris et al. (2002) reported that in drought-prone areas, primed rice seeds germinated well and seedlings emerged faster and more uniformly leading to increased yield. Germination trial of 11 varieties of upland rice under limited water conditions revealed early and synchronized emergence owing to seed priming (Harris and Jones, 1997). Primed rice seeds emerged faster and showed greater growth, dry matter accumulation, yield and harvest index compared to un-treated ones (Farooq et al., 2006a,b). Osmoprotectants are involved in signaling and regulating plant responses to multiple stresses, including reduced growth that may be part of the plant’s adaptation against stress. In plants, the common osmoprotectants are proline, trehalose, fructan, mannitol, glycinebetaine, and others (Zhu, 2002). Osmoprotectants play adaptive roles in mediating osmotic adjustment and protecting subcellular structures in stressed plants. Yang et al. (2007) suggested that for rice, to perform well under drought stress, it should have higher levels of free spermidine/spermine and insoluble-conjugated putrescene. Si is the second most abundant element in soils and a mineral substrate for most of the world’s plant life. Ample evidence is available indicating that when Si is readily available to plants, it plays a significant role in their growth, mineral nutrition, mechanical strength, and resistance to several stresses (Epstein, 1994). Still, it has not been considered an essential element for higher plants, partly because its role in plant biology is poorly understood (Gong et al., 2003). Nevertheless, numerous studies demonstrate that Si is an important element, and plays an important role in tolerance of plants to environmental stresses (Liang et al., 2007; Richmond and Sussman, 2003; Savant et al., 1999). These beneficial effects are attributed to the high accumulation of Si on the tissue surface, although other mechanisms have also been proposed. Genotypes accumulating high amount of Si have more water productivity (Ma, 2004).

4. Future Thrusts A successful change from the traditional flooded to aerobic rice production requires the breeding of special aerobic rice varieties and the development of appropriate water and crop management practices. Although, considerable progress has been made in the improvement of transgenic rice for improved water-use efficiency and productivity; however, the achievements are not satisfactory. Nevertheless, with the study of the functional genomics of plants, considerably more information about the mechanisms by which plants perceive and transduce these stress signals to initiate adaptive responses will be obtained, and with the improvement of

Strategies for Producing More Rice with Less Water

375

the transgenic approach, marker-free transgenic rice will be produced. Therefore, to combine novel regulatory systems for the targeted expression with useful genes, more effective and rational engineering strategies must be provided for the improvement of rice for higher water productivity. Different strategies need to be tested experimentally to genetically improve the water-use efficiency and drought stress tolerance in rice. Different strategies need to be integrated, and the genes representing distinctive approaches be combined to substantially increase rice water productivity. Wide hybridization using hardy wild rice species is another area to be emphasized. Moreover, combining the transgenic with traditional breeding methods may be an effective approach to develop abiotic stress-tolerant rice cultivar. Site-specific packages of production technologies should be developed for different rice production system in various rice production zones across the continents. Because nutrient dynamics (particularly of micronutrients) are altogether different under water-saving rice production systems, future research should also include the crop nutrition with particular reference to micronutrients. Integrated tools should be developed for weed management in different systems. Newly developed rice systems should also be monitored in ecological perspective. Varieties capable of synthesizing osmoprotectants, manifesting quicker OA and capable of synthesizing stress proteins may be introduced. Various physiological tools should be investigated in detail to harvest maximum paddy yield with minimum water input.

REFERENCES Agarie, S., Tsuchida, H., Ku, M., Nomura, M., Matsuoka, M., and Miyao-Tokutomi, M. (1998). High level expression of C4 enzymes in transgenic rice plants. In ‘‘Photosynthesis: Mechanisms and Effects’’ (G. Garab, Ed.), Vol. V, pp. 3423–3426. Proceedings of the 11th International Congress on Photosynthesis, Budapest, Hungary, August, 1998. Kluwer Academic Publishers, Dordrecht. Aggarwal, G. C., Sidhu, A. S., Sekhon, N. K., Sandhu, K. S., and Sur, H. S. (1995). Puddling and N management effects on crop response in a rice–wheat cropping system. Soil Till. Res. 36, 129–139. Alam, M. M., Asim, J. M., and Raza, Z. I. (2003). ‘‘Economic Evaluation of Resource Conservation Technologies in Rice–Wheat Cropping System.’’ MREP Publication No. 255. Water and Power Development Authority, Pakistan. Ali, M. L., Pathan, M. S., Zhang, J., Bai, G., Sarkarung, S., and Nguyen, H. T. (2000). Mapping QTLs for root traits in a recombinant inbred population from two indica ecotypes in rice. Theor. Appl. Genet. 101, 756–766. Atlin, G. N., and Lafitte, H. R. (2002). Marker-assisted breeding versus direct selection for drought tolerance in rice. In ‘‘Field Screening for Drought Tolerance in Crop Plants with Emphasis on Rice’’ (N. P. Saxena and J. C. O’Toole, Eds.). p. 208. Proceedings of an International Workshop on Field Screening for Drought Tolerance in Rice, Patancheru, India, 11–14 December 2000. ICRISAT, Patancheru/The Rockefeller Foundation, India/New York.

376

M. Farooq et al.

Atlin, G. N., and Lafitte, H. R. (2003). Developing and testing rice varieties for water-saving systems in the tropics. In ‘‘Water-Wise Rice Production’’ (B. A. M. Bouman, H. Hengsdijk, B. Hardy, P. S. Bindraban, T. P. Tuong, and J. K. Ladha, Eds.). Proceedings of a Thematic Workshop on Water-Wise Rice Production, 8–11 April 2002 at IRRI Headquarters in Los Ban˜os, Philippines. International Rice Research Institute, Los Ban˜os, Philippines. Atlin, G. N., Laza, M., Amante, M., and Lafitte, H. R. (2004). Agronomic performance of tropical aerobic, irrigated, and traditional upland rice varieties in three hydrological environments at IRRI. In‘‘New Directions for a Diverse Planet’’. Proceedings of the 4th International Crop Science Congress, Brisbane, Australia. Atlin, G. N., Lafitte, H. R., Tao, D., Laza, M., Amante, M., and Courtois, B. (2006). Developing rice cultivars for high-fertility upland systems in the Asian tropics. Field Crops Res. 97, 43–52. Babu, R. C., Nguyen, B. D., Chamarerk, V. P., Shanmugasundaram, P., Chezhian, P., Jeyaprakash, S. K., Ganesh, A., Palchamy, S., Sadasivam, S., Sarkarung, S., Wade, L. J., and Nguyen, H. T. (2003). Genetic analysis of drought resistance in rice by molecular markers. Crop Sci. 43, 1457–1469. Babu, R. C., Zhang, J. X., Blum, A., Ho, T. H. D., Wu, R., and Nguyen, H. T. (2004). HVA1, a LEA gene from barley confers dehydration tolerance in transgenic rice (Oryza sativa L.) via cell membrane protection. Plant Sci. 166, 855–862. Bahieldina, A., Mahfouz, H. T., Eissa, H. F., Saleh, O. M., Ramadan, A. M., Ahmed, I. A., Dyer, W. E., El-Itriby, H. A., and Madkour, M. A. (2005). Field evaluation of transgenic wheat plants stably expressing the HVA1 gene for drought tolerance. Physiol. Plant. 123, 421–427. Balasubramanian, V., Ladha, J. K., Gupta, R. K., Naresh, R. K., Mehla, R. S., BijaySingh, R. S., and Yadvinder-Singh, R. S. (2003). Technology options for rice in the rice–wheat system in South Asia. In ‘‘Improving the Productivity and Sustainability of Rice–Wheat Systems: Issues and Impacts’’ ( J. K. Ladha, J. E. Hill, J. M. Duxbury, R. K. Gupta, and R. J. Buresh, Eds.). pp. 1–25. ASA Special Publication 65. ASAInc., CSSAInc., SSSAInc., Madison, WI. Ball, R. A., Oosterhuis, D. M., and Mauromoustakos, A. (1994). Growth dynamics of the cotton plant during water-deficit stress. Agron. J. 86, 788–795. Barker, R., Dawe, D., Tuong, T. P., Bhuiyan, S. I., and Guerra, L. C. (1998). The outlook for water resources in the year 2020: Challenges for research on water management in rice production. In ‘‘Assessment and Orientation Towards the 21st Century’’, pp. 96–109. Proceedings of 19th Session of the International Rice Commission, Cairo, Egypt, 7–9 September 1998. FAO. Beecher, H. G., Dunn, B. W., Thompson, J. A., Humphreys, E. S., Mathews, K., and Timsina, J. (2006). Effect of raised beds, irrigation and nitrogen management on growth, water use and yield of rice in south-eastern Australia. Aust. J. Exp. Agric. 46, 1363–1372. Belder, P., Bouman, B. A. M., Spiertz, J. H. J., Guoan, L., and Quilang, E. J. P. (2003). Water use of alternately submerged and nonsubmerged irrigated lowland rice. In ‘‘WaterWise Rice Production’’ (B. A. M. Bouman, H. Hengsdijk, B. Hardy, P. S. Bindraban, T. P. Tuong, and J. K. Ladha, Eds.). Proceedings of a Thematic Workshop on Water-Wise Rice Production, 8–11 April 2002 at IRRI Headquarters in Los Ban˜os, Philippines. International Rice Research Institute, Los Ban˜os, Philippines. Bennett, J. (2001). Status of breeding for tolerance of water deficit and prospects for using molecular techniques. Paper Presented at the International Workshop on Water Productivity, 12–13 November 2001, IWMI, Colombo, Sri Lanka.

Strategies for Producing More Rice with Less Water

377

Bhushana, L., Ladha, J. K., Gupta, R. K., Singh, S., Tirol-Padred, A., Saharawata, Y. S., Gathalaa, M., and Pathaka, H. (2007). Saving of water and labor in a rice–wheat system with no-tillage and direct seeding technologies. Agron. J. 99, 1288–1296. Blum, A. (1988). ‘‘Plant Breeding for Stress Environments.’’ CRC Press, Boca Raton, FL. Blum, A. (2005). Drought resistance, water-use efficiency, and yield potential: Are they compatible, dissonant, or mutually exclusive? Aust. J. Agric. Res. 56, 1159–1168. Bonman, J. M. (1992). Durable resistance to rice blast disease—Environmental influences. Euphytica 63, 115–123. Borrell, A. K., Garside, A. L., and Fukai, S. (1997). Improving efficiency of water use for irrigated rice in a semi-arid tropical environment. Field Crops Res. 52, 231–248. Bouman, B. A. M. (2003). Examining the water-shortage problem in rice systems: Watersaving irrigation technologies. In ‘‘Rice Science: Innovations and Impact for Livelihood’’ (T. W. Mew, D. S. Brar, S. Peng, D. Dawe, and B. Hardy, Eds.). pp. 519–535. Proceedings of the International Rice Research Conference, Beijing, 16–19 September 2002. International Rice Research Institute, Chinese Academy of Engineering and Chinese Academy of Agricultural Sciences, Beijing, China. Bouman, B. A. M., and Tuong, T. P. (2001). Field water management to save water and increase its productivity in irrigated lowland rice. Agric. Water Manage. 49, 11–30. Bouman, B. A. M., Yang, X. G., Wang, H., Wang, Z., Zhao, J., and Chen, B. (2006). Performance of aerobic rice varieties under irrigated conditions in North China. Field Crops Res. 97, 53–65. Bouman, B. A. M., Lampayan, R. M., and Tuong, T. P. (2007). ‘‘Water Management in Irrigated Rice—Coping with Water Scarcity.’’ International Rice Research Institute, Los Ban˜os, Philippines. Breseghelo, F., de Morais, O. P., and Ferreira, C. M. (2006). Upland rice in Brazil: Breeding, adoption and management. Paper Presented at Aerobic Rice Workshop held at International Rice Research Institute, Los Ban˜os, Philippines. Bruce, W. B., Edmeades, G. O., and Barker, T. C. (2002). Molecular and physiological approaches to maize improvement for drought tolerance. J. Exp. Bot. 53, 13–25. Cabangon, R. J., Castillo, E. G., Bao, L. X., Lu, J., Wang, G. H., Cui, Y. L., Toung, T. P., Bouman, B. A. M., Li, Y. H., Chen, C. D., and Wang, J. Z. (2001). Impact of alternate wetting and drying irrigation on rice growth and resource-use efficiency. In ‘‘WaterSaving Irrigation for Rice’’ (R. Barker, R. Loeve, Y. H. Li, and T. P. Tuong, Eds.). Proceedings of an International Workshop held in Wuhan, China, 23–25 March 2001. International Water Management Institute Colombo, Sri Lanka. Camp, C. R. (1998). Subsurface drip irrigation: A review. Trans. Am. Soc. Agric. Eng. 45, 1353–1367. Cao, W., Jiang, D., Wang, S., and Tian, Y. (2003). Physiological characterization of rice grown under different water management systems. In ‘‘Water-Wise Rice Production’’ (B. A. M. Bouman, H. Hengsdijk, B. Hardy, P. S. Bindraban, T. P. Tuong, and J. K. Ladha, Eds.). Proceedings of a Thematic Workshop on Water-Wise Rice Production, 8–11 April 2002 at IRRI Headquarters in Los Ban˜os, Philippines. International Rice Research Institute, Los Ban˜os, Philippines. Castan˜eda, A. R., Bouman, B. A. M., Peng, S., and Visperas, R. M. (2003). The potential of aerobic rice to reduce water use in water-scarce irrigated lowlands in the tropics. In ‘‘Water-Wise Rice Production’’ (B. A. M. Bouman, H. Hengsdijk, B. Hardy, P. S. Bindraban, T. P. Tuong, and J. K. Ladha, Eds.). Proceedings of a Thematic Workshop on Water-Wise Rice Production, 8–11 April 2002 at IRRI Headquarters in Los Ban˜os, Philippines. International Rice Research Institute, Los Ban˜os, Philippines. Castillo, E. G., Buresh, R. J., and Ingram, K. T. (1992). Lowland rice yield as affected by timing of water deficit and nitrogen fertilization. Agron. J. 84, 152–159.

378

M. Farooq et al.

Cattivelli, L., Rizza, F., Badeck, F. W., Mazzucotelli, E., Mastrangelo, A. M., Francia, E., Mare, C., Tondelli, A., and Stanca, A. M. (2008). Drought tolerance improvement in crop plants: An integrative view from breeding to genomics. Field Crops Res. 105, 1–14. ˜ Toole, J. C., Huang, N., Champoux, M. C., Wang, G., Sarkarung, S., Mackill, D. J., OO and McCouch, S. R. (1995). Locating genes associated with root morphology and drought avoidance in rice via linkage to molecular markers. Theor. Appl. Genet. 90, 969–981. Chapman, S. C., and Edmeades, G. O. (1999). Selection improves drought tolerance in tropical maize populations. II. Direct and correlated responses among secondary traits. Crop Sci. 39, 1315–1324. Chaves, M. M., and Oliveira, M. M. (2004). Mechanisms underlying plant resilience to water deficits: Prospects for water-saving agriculture. J. Exp. Bot. 55, 2365–2384. Cheng, W.-H., Endo, A., Zhou, L., Penney, J., Chen, H. C., Arroyo, A., Leon, P., Nambara, E., Asami, T., Seo, M., Koshiba, T., and Sheen, J. (2002). A unique shortchain dehydrogenase/reductase in Arabidopsis glucose signaling and abscisic acid biosynthesis and functions. Plant Cell 14, 2732–2743. Clark, L. J., Aphale, S. L., and Barraclough, P. B. (2000). Screening the ability of rice roots to overcome the mechanical impedance of wax layers: Importance of test conditions and measurement criteria. Plant Soil 219, 187–196. Connor, D. J., Timsina, J., and Humphreys, E. (2002). Prospects for permanent beds in the rice–wheat system. In ‘‘Improving the Productivity and Sustainability of Rice–Wheat Systems: Issues and Impacts’’ ( J. K. Ladha, J. E. Hill, J. M. Duxbury, R. K. Gupta, and R. J. Buresh, Eds.). pp. 197–210. ASA Special Publication 65. ASAInc., CSSAInc., SSSAInc., Madison, WI. Dawe, D. (2004). Water productivity in rice-based systems in Asia—Variability in space and time. In ‘‘New Directions for a Diverse Planet.’’ Proceedings of the 4th International Crop Science Congress Brisbane, Australia, 26 September–1 October 2004. Dawe, D., Barker, R., and Seckler, D. (1998). Water supply and research for food security in Asia. In ‘‘Proceedings of the Workshop on Increasing Water Productivity and Efficiency in Rice-Based Systems,’’ July 1998, International Rice Research Institute, Los Ban˜os, Philippines. De Datta, S. K., Krupp, H. K., Alvarez, E. I., and Modgal, S. C. (1973). Water management in flooded rice. In ‘‘Water Management in Philippine Irrigation Systems: Research and Operations’’, pp. 1–18. International Rice Research Institute, Los Ban˜os, Philippines. Dhillon, S. S., Hobbs, P. R., and Samra, J. S. (2000). Investigations on bed planting system as an alternative tillage and crop establishment practice for improving wheat yields sustainably. In ‘‘Proceedings of the 15th Conference of the International Soil Tillage Research Organisation’’, 2–7 July 2000, Fort Worth, Texas (CDROM). Dingkuhn, M., and Asch, F. (1999). Phenological responses of Oryza sativa, O. glaberrima and inter-specific rice cultivars on a toposequence in West Africa. Euphytica 110, 109–126. Dittert, K., Shan, L., Kreye, C., Xunhua, Z., Yangchun, X., Xuejuan, L., Yao, H., Qirong, S., Xiaolin, F., and Sattelmacher, B. (2003). Saving water with ground-cover rice production systems (GCRPS) at the price of increased greenhouse gas emissions. In ‘‘Water-Wise Rice Production’’ (B. A. M. Bouman, H. Hengsdijk, B. Hardy, P. S. Bindraban, T. P. Tuong, and J. K. Ladha, Eds.). Proceedings of a Thematic Workshop on Water-Wise Rice Production, 8–11 April 2002 at IRRI Headquarters in Los Ban˜os, Philippines. International Rice Research Institute, Los Ban˜os, Philippines. Du, L. V., and Tuong, T. P. (2002). Enhancing the performance of dry-seeded rice: Effects of seed priming, seedling rate, and time of seedling. In ‘‘Direct Seeding: Research Strategies and Opportunities’’ (S. Pandey, M. Mortimer, L. Wade, T. P. Tuong,

Strategies for Producing More Rice with Less Water

379

K. Lopes, and B. Hardy, Eds.). pp. 241–256. International Rice Research Institute, Los Ban˜os, Philippines. Epstein, E. (1994). The anomaly of silicon in plant biology. Proc. Natl. Acad. Sci. USA 91, 11–17. Farooq, M., Basra, S. M. A., and Wahid, A. (2006a). Priming of field-sown rice seed enhances germination, seedling establishment, allometry and yield. Plant Growth Regul. 49, 285–294. Farooq, M., Basra, S. M. A., Tabassum, R., and Afzal, I. (2006b). Enhancing the performance of direct seeded fine rice by seed priming. Plant Prod. Sci. 9, 446–456. Farooq, M., Basra, S. M. A., Khalid, M., Tabassum, R., and Mehmood, T. (2006c). Nutrient homeostasis, reserves metabolism and seedling vigor as affected by seed priming in coarse rice. Can. J. Bot. 84, 1196–1202. Farooq, M., Basra, S. M. A., and Khan, M. B. (2007a). Seed priming improves growth of nursery seedlings and yield of transplanted rice. Arch. Agron. Soil Sci. 53, 311–322. Farooq, M., Basra, S. M. A., and Ahmad, N. (2007b). Improving the performance of transplanted rice by seed priming. Plant Growth Regul. 51, 129–137. Farooq, M., Basra, S. M. A., Wahid, A., Cheema, Z. A., Cheema, M. A., and Khaliq, A. (2008). Physiological role of exogenously applied glycinebetaine in improving drought tolerance of fine grain aromatic rice (Oryza sativa L.). J. Agron. Crop Sci. 194, 325–333. Farooq, M., Wahid, A., Kobayashi, N., Fujita, D., and Basra, S. M. A. (2009). Plant drought stress: Effects, mechanisms and management. Agron. Sustain. Dev. 29, 185–212. Flowers, T. J., Salama, F. M., and Yeo, A. R. (1988). Water-use efficiency in rice (Oryza sativa L.) in relation to resistance to salinity. Plant Cell Environ. 11, 453–459. Fujita, D., Santos, R. E., Ebron, L., Yanoria, M. J. T., Kato, H., Kobayashi, S., Uga, Y., Araki, E., Takai, T., Fukuta, Y., and Kobayashi, N. (2007). Genetic and breeding study on near isogenic lines of IR64 for yield-related traits—QTL analysis for days to heading in early heading lines. Breed. Sci. 9, 114. Fukai, S., and Cooper, M. (1995). Review: Development of drought-resistant cultivars using physio-morphological traits in rice. Field Crops Res. 40, 67–86. Fukayama, H., Tsuchida, H., Agarie, S., Nomura, M., Onodera, H., Ono, K., Lee, B.-H., Hirose, S., Toki, S., Ku, M. S. B., Makino, A., Matsuoka, M., et al. (2001). Significant accumulation of C4-specific pyruvate, orthophosphate dikinase in a C3 plant, rice. Plant Physiol. 127, 1136–1146. Gajri, P. R., Arora, V. K., and Prihar, S. S. (1992). Tillage management for efficient water and nitrogen use in wheat following rice. Soil Till. Res. 24, 167–182. Gani, A., Rahman, A., Rustam, D., and Hengsdijk, H. (2003). Synopsis of water management experiments in Indonesia. In ‘‘Water-Wise Rice Production’’ (B. A. M. Bouman, H. Hengsdijk, B. Hardy, P. S. Bindraban, T. P. Tuong, and J. K. Ladha, Eds.). Proceedings of a Thematic Workshop on Water-Wise Rice Production, 8–11 April 2002 at IRRI Headquarters in Los Ban˜os, Philippines. International Rice Research Institute, Los Ban˜os, Philippines. Garg, A. K., Kim, J.-K., Owens, T. G., Ranwala, A. P., Choi, Y. D., Kochian, L. V., and Wu, R. J. (2002). Trehalose accumulation in rice plants confers high tolerance levels to different abiotic stresses. Proc. Natl. Acad. Sci. USA 99, 15898–15903. Giordano, R., Passarella, G., Uricchio, V. F., and Vurro, M. (2007). Integrating conflict analysis and consensus reaching in a decision support system for water resource management. J. Environ. Manage. 84, 213–228. Gong, H., Chen, K., Chen, G., Wang, S., and Zhang, C. (2003). Effects of silicon on growth of wheat under drought. J. Plant Nutr. 26, 1055–1063. Grantz, D. A., and Assmann, S. M. (1991). Stomatal response to blue light: Water use efficiency in sugarcane and soybean. Plant Cell Environ. 14, 683–690.

380

M. Farooq et al.

Gupta, R. K., Naresh, R. K., Hobbs, P. R., and Ladha, J. K. (2003). Adopting conservation agriculture in the rice–wheat system of the Indo-Gangetic Plains: New opportunities for saving water. In ‘‘Water-Wise Rice Production’’ (B. A. M. Bouman, H. Hengsdijk, B. Hardy, P. S. Bindraban, T. P. Tuong, and J. K. Ladha, Eds.). pp. 207–222. Proceedings of a Thematic Workshop on Water-Wise Rice Production, 8–11 April 2002 at IRRI Headquarters in Los Ban˜os, Philippines. International Rice Research Institute, Los Ban˜os, Philippines. Harris, D., and Jones, M. (1997). On-farm seed priming to accelerate germination in rainfed, dry-seeded rice. Int. Rice Res. Notes 22, 30. Harris, D., Tripathi, R. S., and Joshi, A. (2002). On-farm seed priming to improve crop establishment and yield in dry direct-seeded rice. In ‘‘Direct Seeding: Research Strategies and Opportunities’’ (S. Pandey, M. Mortimer, L. Wade, T. P. Tuong, K. Lopes, and B. Hardy, Eds.). pp. 231–240. International Rice Research Institute, Los Ban˜os, Philippines. Henson, I. E. (1985). Modification of leaf size in rice (Oryza sativa L.) and its effects on water stress-induced abscisic acid accumulation. Ann. Bot. 56, 481–487. Hossain, M. I., Sufian, M. A., Hossain, A. B. S., Meisner, C. A., Lauren, J. G., and Duxbury, J. M. (2003). ‘‘Performance of Bed Planting and Nitrogen Fertilizer Under Rice–Wheat–Mungbean Cropping Systems in Bangladesh.’’ Wheat Research Centre Internal Review Reports. Bangladesh Agricultural Research Institute, Dinajpur, Bangladesh. Hsiao, T. C., O’Toole, J. C., Yambao, E. B., and Turner, N. C. (1984). Influence of osmotic adjustment on leaf rolling and tissue death in rice (Oryza sativa L.). Plant Physiol. 75, 338–341. Huang, Y. D., Zhang, Z. L., Wei, F. Z., and Li, J. C. (1999). Ecophysiological effect of drycultivated and plastic film-mulched rice planting. Chin. J. Appl. Ecol. 10, 305–308 (in Chinese). Huaqi, W., Bouman, B. A. M., Zhao, D., Changgui, W., and Moya, P. F. (2003). Aerobic rice in northern China: Opportunities and challenges. In ‘‘Water-Wise Rice Production’’ (B. A. M. Bouman, H. Hengsdijk, B. Hardy, P. S. Bindraban, T. P. Tuong, and J. K. Ladha, Eds.). pp. 207–222. Proceedings of a Thematic Workshop on Water-Wise Rice Production, 8–11 April 2002 at IRRI Headquarters in Los Ban˜os, Philippines. International Rice Research Institute, Los Ban˜os, Philippines. Hudspeth, R. L., Grula, J. W., Dai, Z., Edwards, G. E., and Ku, M. S. B. (1992). Expression of maize phosphoenolpyruvate carboxylase in transgenic tobacco. Effects on biochemistry and physiology. Plant Physiol. 98, 458–464. Humphreys, E., and Bhuiyan, A. M. (2001). What growers think about crops after rice. Farm. Newslett. Large Area 157, 29–31. Humphreys, E., Muirhead, W. A., Melhuish, F. M., White, R. J. G., and Blackwell, J. B. (1989). The growth and nitrogen economy of rice under sprinkler and flood irrigation in south-east Australia. II. Soil moisture and mineral N transformations. Irrig. Sci. 10, 201–213. Humphreys, E., Muirhead, W. A., Fawcett, B. J., Townsend, J. T., and Murray, E. A. (1996). Puddling in mechanised rice culture: Impacts on water use and the productivity of rice and post-rice crops. In ‘‘Management of Clay Soils for Rainfed Lowland RiceBased Cropping Systems’’ (G. Kirchhof and H. B. So, Eds.). pp. 213–218. ACIAR Proceedings No. 70. Australian Centre for International Agricultural Research, Canberra, Australia. Jehangir, W. A., Murray-Rust, H., Masih, I., and Shimizu, K. (2002). Sustaining crop and water productivity in the irrigated rice–wheat systems. Paper Presented at National Workshop on Rice–Wheat Cropping System Management, Pakistan Agricultural

Strategies for Producing More Rice with Less Water

381

Research Council Rice–Wheat Consortium for Indo-Gangetic Plains, 11–12 December 2002, Islamabad, Pakistan. Ji, B. H., Zhu, S. Q., and Jiao, D. M. (2004). Photosynthetic C4-microcycle in transgenic rice plant lines expressing the maize C4-photosynthetic enzymes. Acta Agron. Sin. 30, 536–543. Jiao, D. M., Hang, X. Q., Li, X., Chi, W., Kuang, T. Y., and Zhang, Q. D. (2002). Photosynthetic characteristics and tolerance to photooxidation of transgenic rice expressing C4 photosynthesis enzymes. Photosynth. Res. 72, 85–93. Jiao, D., Li, X., and Ji, B. (2005). Photoprotective effects of high level expression of C4 phosphoenolpyruvate carboxylase in transgenic rice during photoinhibition. Photosynthetica 43, 501–508. Jones, H. G. (2004). Irrigation scheduling: Advantages and pitfalls of plant based methods. J. Exp. Bot. 55, 2427–2436. Kacira, M., and Ling, P. P. (2001). Design and development of an automated and noncontact sensing system for continuous monitoring of plant health and growth. Trans. Am. Soc. Agric. Eng. 44, 989–996. Kahlown, M. A., Gill, M. A., and Ashraf, M. (2002). ‘‘Evaluation of Resource Conservation Technologies in Rice–Wheat System of Pakistan’’. Pakistan Council of Research in Water Resources (PCRWR), Research Report-1. PCRWR, Islamabad, Pakistan. Kang, S., and Zhang, J. (2004). Controlled alternate partial root-zone irrigation: Its physiological consequences and impact on water use efficiency. J. Exp. Bot. 55, 2437–2446. Kavar, T., Maras, M., Kidric, M., Sustar-Vozlic, J., and Meglic, V. (2007). Identification of genes involved in the response of leaves of Phaseolus vulgaris to drought stress. Mol. Breed. 21, 159–172. ¨ . (2006). Seed treatments Kaya, M. D., Okc¸ub, G., Ataka, M., C ¸ ikilic, Y., and Kolsarica, O to overcome salt and drought stress during germination in sunflower (Helianthus annuus L.). Eur. J. Agron. 24, 291–295. Kirigwi, F. M., Van Ginkel, M., Brown-Guedira, G., Gill, B. S., Paulsen, G. M., and Fritz, A. K. (2007). Markers associated with a QTL for grain yield in wheat under drought. Mol. Breed. 20, 401–413. Kobayashi, N., Ebron, L. A., Yanoria, M. J. T., Kato, H., Fukuta, Y., Magbanua, R., and Atlin, G. (2006). Genetic and breeding study on near isogenic lines of IR64 for yieldrelated traits—Resistance to rice blast disease under the water-saving condition in IRRI. Jpn. J. Trop. Agric. 50, 9–10. Kreye, C., Dittert, K., Zheng, X., Zhang, X., Lin, S., Tao, H., and Sattelmacher, B. (2007). Fluxes of methane and nitrous oxide in water-saving rice production in north China. Nutr. Cycl. Agroecosyst. 77, 293–304. Ku, M. S. B., Agarie, S., Nomura, M., Fukayama, H., Tsuchida, H., Ono, K., Hirose, S., Toki, S., Miyao, M., and Matsuoka, M. (1999). High-level expression of maize phosphoenolpyruvate carboxylase in transgenic rice plants. Nat. Biotechnol. 17, 76–80. Ku, M. S. B., Cho, D., Ranade, U., Hsu, T.-P., Li, X., Jiao, D.-M., Ehleringer, J., Miyao, M., and Matsuoka, M. (2000). Photosynthetic performance of transgenic rice plants overexpressing maize C4 photosynthesis enzymes. In ‘‘Redesigning Rice Photosynthesis to Increase Yield’’ ( J. E. Sheehy, P. L. Mitchell, and B. Hardy, Eds.). pp. 193–204. Proceedings of the Workshop on the Quest to Reduce Hunger held in Los Ban˜os, Philippines, 30 November–3 December 1999. Elsevier, Amsterdam (Studies in Plant Science no. 7). Ku, M. S. B., Cho, D., Li, X., Jiao, D. M., Pinto, M., Miyao, M., and Matsuoka, M. (2007). Introduction of genes encoding C4 photosynthesis enzymes into rice plants: Physiological consequences. In ‘‘Rice Biotechnology: Improving Yield, Stress Tolerance and Grain Quality’’ ( J. A. Goode and D. Chadwick, Eds.). Novartis Foundation Symposium 236, Book Series Novartis Foundation Symposia.

382

M. Farooq et al.

Kukal, S. S., and Aggarwal, G. C. (2003). Puddling depth and intensity effects in rice–wheat system on a sandy loam soil. I. Development of subsurface compaction. Soil Till. Res. 72, 1–8. Kumar, J., and Abbo, S. (2001). Genetics of flowering time in chickpea and its bearing on productivity in the semi-arid environments. Adv. Agron. 72, 107–138. Lacy, J., and Wilkins, J. (2003). Increasing yields, water use efficiency and profit from Ricecheck. Farm. Newslett. Large Area 162, 64–66. Lafitte, H. R., and Bennet, J. (2002). Requirements for aerobic rice: Physiological and molecular considerations. In ‘‘Water-Wise Rice Production’’ (B. A. M. Bouman, H. Hengsdijk, B. Hardy, P. S. Bindraban, T. P. Tuong, and J. K. Ladha, Eds.). Proceedings of a Thematic Workshop on Water-Wise Rice Production, 8–11 April 2002 at IRRI Headquarters in Los Ban˜os, Philippines. International Rice Research Institute, Los Ban˜os, Philippines. Lafitte, H. R., and Bennet, J. (2003). Requirements for aerobic rice: Physiological and molecular considerations. In ‘‘Water-Wise Rice Production’’ (B. A. M. Bouman, H. Hengsdijk, B. Hardy, P. S. Bindraban, T. P. Tuong, and J. K. Ladha, Eds.). pp. 259–274. Proceedings of a Thematic Workshop on Water-Wise Rice Production, 8–11 April 2002 at IRRI Headquarters in Los Ban˜os, Philippines. International Rice Research Institute, Los Ban˜os, Philippines. Li, Y. H. (2001). Research and practice of water saving irrigation for rice in China. In ‘‘Water-Saving Irrigation for Rice’’ (R. Barker, R. Loeve, Y. Li, and T. P. Tuong, Eds.). pp. 135–144 Proceedings of an International Workshop, 23–25 March 2001, Wuhan, China. Liang, Y., Sun, W., Zhu, Y.-G., and Christie, P. (2007). Mechanisms of silicon-mediated alleviation of abiotic stresses in higher plants: A review. Environ. Pollut. 147, 422–428. Lilley, J. M., and Ludlow, M. M. (1996). Expression of osmotic adjustment and dehydration tolerance in diverse rice lines. Field Crops Res. 48, 185–197. Lin, S., Tao, H., Dittert, K., Xu, Y., Fan, X., Shen, Q., and Sattelmacher, B. (2003a). Saving water with the ground cover rice production system in China. In ‘‘Technological and Institutional Innovations for Sustainable Rural Development.’’ Conference on International Agricultural Research for Development. Deutscher Tropentag, 8–10 October 2003, Go¨ttingen. Lin, S., Dittert, K., Tao, H., Kreye, C., Xu, Y., Shen, Q., Fan, X., and Sattelmacher, B. (2003b). The ground-cover rice production system (GCRPS): A successful new approach to save water and increase nitrogen fertilizer efficiency. In ‘‘Water-Wise Rice Production’’ (B. A. M. Bouman, H. Hengsdijk, B. Hardy, P. S. Bindraban, T. P. Tuong, and J. K. Ladha, Eds.). Proceedings of a Thematic Workshop on Water-Wise Rice Production, 8–11 April 2002 at IRRI Headquarters in Los Ban˜os, Philippines. International Rice Research Institute, Los Ban˜os, Philippines. Lu, J., Ookawa, T., and Hirasawa, T. (2000). The effects of irrigation regimes on the water use, dry matter production and physiological responses of paddy rice. Plant Soil 223, 207–216. Lu, G., Cabangon, R., Tuong, T. P., Belder, P., Bouman, B. A. M., and Castillo, E. (2003). The effects of irrigation management on yield and water productivity of inbred, hybrid, and aerobic rice varieties. In ‘‘Water-Wise Rice Production’’ (B. A. M. Bouman, H. Hengsdijk, B. Hardy, P. S. Bindraban, T. P. Tuong, and J. K. Ladha, Eds.). Proceedings of a Thematic Workshop on Water-Wise Rice Production, 8–11 April 2002 at IRRI Headquarters in Los Ban˜os, Philippines. International Rice Research Institute, Los Ban˜os, Philippines. Lu, X., Jiang, X., and Tang, Y. (2004). The system of rice intensification (SRI) for superhigh yields of rice in Sichuan Basin Jiaguo Zheng. ‘‘New Directions for a Diverse

Strategies for Producing More Rice with Less Water

383

Planet.’’ Proceedings of the 4th International Crop Science Congress Brisbane, Australia, 26 September–1 October 2004. Ludlow, M. M., and Muchow, R. C. (1990). A critical evaluation of traits for improving crop yields in water-limited environments. Adv. Agron. 43, 107–153. Ma, J. F. (2004). Role of silicon in enhancing the resistance of plants to biotic and abiotic stresses. Soil Sci. Plant Nutr. 50, 11–18. Mackill, D. J., and Bonman, J. M. (1992). Inheritance of blast resistance in near-isogenic lines of rice. Phytopathology 82, 746–749. Makarim, A. K., Balasubramanian, V., Zaini, Z., Syamsiah, I., Diratmadja, I. G. P. A., Arafah, H., Wardana, I. P., and Gani, A. (2003). System of Rice Intensification (SRI): Evaluation of seedling age and selected components in Indonesia. In ‘‘Water-Wise Rice Production’’ (B. A. M. Bouman, H. Hengsdijk, B. Hardy, P. S. Bindraban, T. P. Tuong, and J. K. Ladha, Eds.). Proceedings of a Thematic Workshop on Water-Wise Rice Production, 8–11 April 2002 at IRRI Headquarters in Los Ban˜os, Philippines. International Rice Research Institute, Los Ban˜os, Philippines. Matsuoka, M., Fukayama, H., Tsuchida, H., Nomura, M., Agarie, S., Ku, M. S. B., and Miyao, M. (2000). How to express some C4 photosynthesis genes at high levels in rice. In ‘‘Redesigning Rice Photosynthesis to Increase Yield’’ ( J. E. Sheehy, P. L. Mitchell, and B. Hardy, Eds.). pp. 167–175. International Rice Research Institute/Elsevier, Philippines/Amsterdam. Maynard, M. (1991). Permanent beds—Their potential role in soil management for the future. Farm. Newslett. Large Area 137, 14–18. McHugh, O. V., Steenhuis, T. S., Barison, J., Fernandes, E. C. M., and Uphoff, N. T. (2003). Farmer implementation of alternate wet–dry and nonflooded irrigation practices in the System of Rice Intensification (SRI). In ‘‘Water-Wise Rice Production’’ (B. A. M. Bouman, H. Hengsdijk, B. Hardy, P. S. Bindraban, T. P. Tuong, and J. K. Ladha, Eds.). Proceedings of a Thematic Workshop on Water-Wise Rice Production, 8–11 April 2002 at IRRI Headquarters in Los Ban˜os, Philippines. International Rice Research Institute, Los Ban˜os, Philippines. Meisner, C. A., Acevedo, E., Flores, D., Sayre, K., Ortiz-Monasterio, I., Byerlee, D., and Limon, A. (1992). ‘‘Wheat Production and Grower Practices in the Yaqui Valley, Sonora, Mexico.’’ Wheat Special Report 6. CIMMYT, Mexico. Moser, C. M., and Barrett, C. B. (2003). The disappointing adoption dynamics of a yield increasing, low external input technology: the case of SRI in Madagascar. Agric. Syst. 76, 1085–1100. Muirhead, W. A., Blackwell, J. B., Humphreys, E., and White, R. J. G. (1989). The growth and nitrogen economy of rice under sprinkler and flood irrigation in south-east Australia. I. Crop response and N uptake. Irrig. Sci. 10, 183–199. Narang, R. S., and Gulati, H. S. (1995). On-farm water management. In ‘‘Proceedings of the Symposium on Water Management—Need for Public Awareness’’, pp. 117–129. Punjab Agricultural University, Ludhiana, India. Nguyen, H. T., Babu, R. C., and Blum, A. (1997). Breeding for drought resistance in rice: Physiology and molecular genetics considerations. Crop Sci. 37, 1426–1434. Ortiz-Monasterio, J. I., Dhillon, S. S., and Fischer, R. A. (1994). Date of sowing effects on grain and yield components of irrigated spring wheat cultivars and relationships with radiation and temperature in Ludhiana, India. Field Crops Res. 37, 169–184. Pan, J., Zhang, D., Liu, X., Ai, Y., Lu, S., and Zheng, X. (2003). Effect of water, fertilization and straw management on N recovery and water use efficiency for rice-wheat rotation at southwest China. Paper presented at the FAO/IAEA Coordinated Research Project on Integrated Soil, Water and Nutrient Management for Sustainable Rice-Wheat Cropping System in Asia, IAEA, Vienna, Austria.

384

M. Farooq et al.

Pantuwan, G., Fukai, S., Cooper, M., Rajatasereekul, S., and O’Toole, J. C. (2002). Yield response of rice (Oryza sativa L.) genotypes to different types of drought under rainfed lowlands. 3. Plant factors contributing to drought resistance. Field Crops Res. 73, 181–200. Peng, S., and Bouman, B. A. M. (2007). Prospects for genetic improvement to increase lowland rice yields with less water and nitrogen. In ‘‘Scale and Complexity in Plant Systems Research: Gene–Plant–Crop Relations’’ ( J. H. J. Spiertz, P. C. Struik, and H. H. van Laar, Eds.). pp. 251–266. Springer, New York. Peng, S., Laza, R. C., Khush, G. S., Sanico, A. L., Visperas, R. M., and Garcia, F. V. (1998). Transpiration efficiencies of indica and improved tropical japonica rice grown under irrigated conditions. Euphytica 103, 103–108. Peng, S., Cassman, K. G., Virmani, S. S., Sheehy, J., and Khush, G. S. (1999). Yield potential trends of tropical rice since the release of IR8 and the challenge of increasing rice yield potential. Crop Sci. 39, 1552–1559. Peng, S., Bouman, B. A. M., Visperas, R. M., Castan˜eda, A., Nie, L., and Park, H.-K. (2006). Comparison between aerobic and flooded rice in the tropics: Agronomic performance in an eight-season experiment. Field Crops Res. 96, 252–259. Ray, J. D., Yu, L. X., McCouch, S. R., Champoux, M. C., Wang, G., and Nguyen, H. T. (1996). Mapping quantitative trait loci associated with root penetration ability in rice (Oryza sativa L.). Theor. Appl. Genet. 92, 627–636. Reinke, R. F., Lewin, L. G., and Williams, R. L. (1994). Effect of sowing time and nitrogen on rice cultivars of differing growth duration in New South Wales. 1. Yield and yield components. Aust. J. Exp. Agric. 34, 933–938. Reinke, R., Snell, P., and Fitzgerald, M. (2004). Rice development and improvement. Farm. Newslett. Large Area 165, 43–47. Richards, R. A., Rawson, H. M., and Johnson, D. A. (1986). Glaucousness in wheat: Its development, and effect on water-use efficiency, gas exchange and photosynthetic tissue temperatures. Aust. J. Plant Physiol. 13, 465–473. Richards, R. A., Lopez-Castaneda, C., Gomez-Macpherson, H., and Condon, A. G. (1993). Improving the efficiency of water use by plant breeding and molecular biology. Irrig. Sci. 14, 93–104. Richards, R. A., Rebetzke, G. J., Appels, R., and Condon, A. G. (2000). Physiological traits to improve the yield of rain-fed wheat: Can molecular genetics help. In ‘‘Workshop on Molecular Approaches for the Genetic Improvement of Cereals for Stable Production in Water-Limited Environments’’ ( J. M. Ribaut and D. Poland, Eds.). CIMMYT, Mexico. Richmond, K. E., and Sussman, M. (2003). Got silicon? The non-essential beneficial plant nutrient. Curr. Opin. Plant Biol. 6, 268–272. Rickman, J. F. (2002). Manual for laser land levelling. Rice–Wheat Consortium Technical Bulleting Series 5. RWC–CIMMYT, New Delhi, India. Rohila, J. S., Jain, R. K., and Wu, R. (2002). Genetic improvement of Basmati rice for salt and drought tolerance by regulated expression of a barley Hva1 cDNA. Plant Sci. 163, 525–532. Saijo, Y., Hata, S., Kyozuka, J., Shimamoto, K., and Izui, K. (2000). Over-expression of a single Ca2+-dependent protein kinase confers both cold and salt/drought tolerance on rice plants. Plant J. 23, 319–327. Sandquist, D. R., and Ehleringer, J. R. (2003). Population- and family-level variation of brittlebush (Encelia farinosa, Asteraceae) pubescence: Its relation to drought and implications for selection in variable environments. Am. J. Bot. 90, 1481–1486. Sato, Y., and Yokoya, S. (2007). Enhanced tolerance to drought stress in transgenic rice plants overexpressing a small heat-shock protein, sHSP17.7. Plant Cell Rep. 27, 329–334. Savant, N. K., Korndo¨rfer, G. H., Datnoff, L. E., and Snyder, G. H. (1999). Silicon nutrition and sugarcane production: A review. J. Plant Nutr. 22, 1853–1903.

Strategies for Producing More Rice with Less Water

385

Sawahel, W. (2003). Improved performance of transgenic glycinebetaine-accumulating rice plants under drought stress. Biol. Plant. 47, 39–44. Sayre, K. D., and Hobbs, P. R. (2004). The raised-bed system of cultivation for irrigated production conditions. In ‘‘Sustainable Agriculture and the International Rice–Wheat System’’ (R. Lal, P. R. Hobbs, N. Uphoff, and D. O. Hansen, Eds.). pp. 337–355. Marcel Dekker, New York. Shan, L. (1991). Physiological and ecological base of water saving agriculture. J. Appl. Ecol. 1, 70–76. Sharma, P. K., Bhushan, L., Ladha, J. K., Naresh, R. K., Gupta, R. K., Balasubramanian, B. V., and Bouman, B. A. M. (2003). Crop–water relations in rice– wheat cropping under different tillage systems and water-management practices in a marginally sodic, medium-textured soil. In ‘‘Water-Wise Rice Production’’ (B. A. M. Bouman, H. Hengsdijk, B. Hardy, P. S. Bindraban, T. P. Tuong, and J. K. Ladha, Eds.). pp. 223–235. Proceedings of a Thematic Workshop on Water-Wise Rice Production, 8–11 April 2002 at IRRI Headquarters in Los Ban˜os, Philippines. International Rice Research Institute, Los Ban˜os, Philippines. Shen, Q., and Yangchun, X. (2003). Cropping system N and water management: Aerobic barley in rotation with aerobic and waterlogged rice. Paper Presented at the FAO/IAEA Coordinated Research Project on Integrated Soil, Water and Nutrient Management for Sustainable Rice–Wheat Cropping System in Asia. IAEA, Vienna, Austria. Shen, K. R., Wang, X. C., Liu, J., and Luo, X. S. (1997). Test and demonstration on wetcultivation with film mulching of rice. Hubei Agric. Sci. 5, 18–22 (in Chinese). Shi, Y. C. (1999). Way of exploitation: Biological water saving. Sci. Technol. Rev. 10, 3–5. Shi, Q. H., Zeng, X. C., Li, M. Y., Tan, X. M., and Xu, F. F. (2003). Effects of different water management practices on rice growth. In ‘‘Water-Wise Rice Production’’ (B. A. M. Bouman, H. Hengsdijk, B. Hardy, P. S. Bindraban, T. P. Tuong, and J. K. Ladha, Eds.). Proceedings of a Thematic Workshop on Water-Wise Rice Production, 8–11 April 2002 at IRRI Headquarters in Los Ban˜os, Philippines. International Rice Research Institute, Los Ban˜os, Philippines. Simpson, H. J., Herczeg, A. L., and Meyer, W. S. (1992). Stable isotope ratios in irrigation water can estimate rice crop evaporation. Geophys. Res. Lett. 19, 377–380. Singh, M. K., and Sasahara, T. (1981). Photosynthesis and transpiration in rice as influenced by soil moisture and air humidity. Ann. Bot. 48, 513–517. Singh, S., Sharma, S. N., and Prasad, R. (2001). The effect of seeding and tillage methods on productivity of rice–wheat cropping system. Soil Till. Res. 61, 125–131. Singh, A. K., Choudhury, B. U., and Bouman, B. A. M. (2003). Effects of rice establishment methods on crop performance, water use, and mineral nitrogen. In ‘‘Water-Wise Rice Production’’ (B. A. M. Bouman, H. Hengsdijk, B. Hardy, P. S. Bindraban, T. P. Tuong, and J. K. Ladha, Eds.). pp. 223–235. Proceedings of a Thematic Workshop on WaterWise Rice Production, 8–11 April 2002 at IRRI Headquarters in Los Ban˜os, Philippines. International Rice Research Institute, Los Ban˜os, Philippines. Stoop, W. A., Uphoff, N., and Kassam, A. (2002). A review of agricultural research issues raised by the system of rice intensification (SRI) from Madagascar: Opportunities for improving farming systems for resource-poor farmers. Agric. Syst. 71, 249–274. Su, J., and Wu, R. (2004). Stress-inducible synthesis of proline in transgenic rice confers faster growth under stress conditions than that with constitutive synthesis. Plant Sci. 166, 941–948. Subbarao, G. V., Johansen, C., Slinkard, A. E., Rao, R. C. N., Saxena, N. P., and Chauhan, Y. S. (1995). Strategies and scope for improving drought resistance in grain legumes. Crit. Rev. Plant Sci. 14, 469–523.

386

M. Farooq et al.

Sun, H., Cheng, M., Zheng, D., and Zhang, J. (2005). Developing models on water-saving agriculture through rainwater harvesting for supplemental irrigation in northern China semi-arid region. Ying Yong Sheng Tai Xue Bao 16, 1072–1076 (in Chinese). Suzuki, S., Murai, N., Burnell, J. N., and Arai, M. (2000). Changes in photosynthetic carbon flow in transgenic rice plants that express C4-type phosphoenolpyruvate carboxykinase from Urochloa panicoides. Plant Physiol. 124, 163–172. Tabbal, D. F., Bouman, B. A. M., Bhuiyan, S. I., Sibayan, E. B., and Sattar, M. A. (2002). On-farm strategies for reducing water input in irrigated rice; case studies in the Philippines. Agric. Water Manage. 56, 93–112. Takeuchi, Y., Akagi, H., Kamasawa, N., Osumi, M., and Honda, H. (2000). Aberrant chloroplasts in transgenic rice plants expressing a high level of maize NADP-dependent malic enzyme. Planta 211, 265–274. Tao, H., Brueck, H., Dittert, K., Kreye, C., Lin, S., and Sattelmacher, B. (2006). Growth and yield formation of rice (Oryza sativa L.) in the water-saving ground cover rice production system (GCRPS). Field Crops Res. 95, 1–12. Thiyagarajan, T. M., Velu, V., Ramasamy, S., Durgadevi, D., Govindarajan, K., Priyadarshini, R., Sudhalakshmi, C., Senthilkumar, K., Nisha, P. T., Gayathry, G., Hengsdijk, H., and Bindraban, P. S. (2003). The System of Rice Intensification in practice: Explaining low farmer adoption and high disadoption in Madagascar. In ‘‘Water-Wise Rice Production’’ (B. A. M. Bouman, H. Hengsdijk, B. Hardy, P. S. Bindraban, T. P. Tuong, and J. K. Ladha, Eds.). Proceedings of a Thematic Workshop on Water-Wise Rice Production, 8–11 April 2002 at IRRI Headquarters in Los Ban˜os, Philippines. International Rice Research Institute, Los Ban˜os, Philippines. Thompson, J. A., Griffin, D., and North, S. (2003). ‘‘Improving the Water Use Efficiency of Rice.’’ Final Report Project 1204. CRC for Sustainable Rice Production, Yanco, Australia. Tracy, P., Sims, B. D., Hefner, S. G., and Cairns, J. P. (1993). ‘‘Guidelines for Producing Rice Using Furrow Irrigation’’ Department of Agronomy, University of Missouri, Columbia, MO. Tripathi, R. P. (1996). Water management in rice–wheat system. In ‘‘Rice–Wheat Cropping System’’ (R. K. Pandey, B. S. Dwivedi, and A. K. Sharm, Eds.). pp. 134–147. Project Directorate for Cropping Systems Research, Modipuram, India. Tripathy, J. N., Zhang, J., Robin, S., Nguyen, T. T., and Nguyen, H. T. (2000). QTLs for cell-membrane stability mapped in rice (Oryza sativa L.) under drought stress. Theor. Appl. Genet. 100, 1197–1202. Tsuchida, H., Tamai, T., Fukayama, H., Agarie, S., Nomura, M., Onodera, H., Ono, K., Nishizawa, Y., Lee, B.-H., Hirose, S., Toki, S., Ku, M. S. B., Matsuoka, M., and Miyao, M. (2001). High level expression of C4-specific NADP-malic enzyme in leaves and impairment of photoautotrophic growth in a C3 plant, rice. Plant Cell Physiol. 42, 138–145. Tuong, T. P. (1999). Productive water use in rice production: Opportunities and limitations. J. Crop Prod. 2, 241–264. Tuong, T. P., and Bouman, B. A. M. (2002). Rice production in water-scarce environments. Paper Presented at the Water Productivity Workshop, 12–14 November 2001. Colombo, Sri Lanka. Tuong, T. P., and Bouman, B. A. M. (2003). Rice production in water-scarce environments. In ‘‘Water Productivity in Agriculture: Limits and Opportunities for Improvement’’ ( J. W. Kijne, R. Barker, and D. Molden, Eds.). pp. 53–67. CAB International, Wallingford, UK. Tuong, T. P., Boling, A., Singh, A. K., and Wopereis, M. C. S. (1996). Transpiration of lowland rice in response to drought. In ‘‘Evaporation and Irrigation Scheduling’’

Strategies for Producing More Rice with Less Water

387

(C. R. Camp, E. J. Sadler, and R. E. Yoder, Eds.). pp. 1071–1077. Proceedings of the International Conference, 3–6 November, San Antonio, Texas. American Society of Agricultural Engineers, Michigan. Tuong, T. P., Pablico, P. P., Yamauchi, M., Confesor, R., and Moody, K. (2000). Increasing water productivity and weed suppression of wet-seeded rice: Effect of water management and rice genotypes. J. Exp. Agric. 36, 1–19. Tuong, T. P., Bouman, B. A. M., and Mortimer, M. (2004). More rice, less water— Integrated approaches for increasing water productivity in irrigated rice-based systems in Asia. ‘‘New Directions for a Diverse Planet.’’ Proceedings of the 4th International Crop Science Congress, 26 September–1 October 2004. BrisbaneAustralia (published on CDROM). Tuinstra, M., Ejeta, G., and Goldsbrough, P. (1998). Evaluation of near-isogenic sorghum lines contrasting for QTL markers associated with drought tolerance. Crop Sci. 38, 835–842. Turner, N. C., Wright, G. C., and Siddique, K. H. M. (2001). Adaptation of grain legumes (pulses) to water-limited environments. Adv. Agron. 71, 123–231. Uphoff, N., and Randriamiharisoa, R. (2003). Reducing water use in irrigated rice production with the Madagascar System of Rice Intensification (SRI). In ‘‘Water-Wise Rice Production’’ (B. A. M. Bouman, H. Hengsdijk, B. Hardy, P. S. Bindraban, T. P. Tuong, and J. K. Ladha, Eds.). Proceedings of a Thematic Workshop on Water-Wise Rice Production, 8–11 April 2002 at IRRI Headquarters in Los Ban˜os, Philippines. International Rice Research Institute, Los Ban˜os, Philippines. Vories, E. D., Counce, P. A., and Keisling, T. C. (2002). Comparison of flooded and furrow irrigated rice on clay. Irrig. Sci. 21, 139–144. Wang, F. Z., Wang, Q. B., Kwon, S. Y., Kwak, S. S., and Su, W. A. (2005). Enhanced drought tolerance of transgenic rice plants expressing a pea manganese superoxide dismutase. J. Plant Physiol. 162, 465–472. Williams, R. L., Farrell, T., Hope, M. A., Reinke, R., and Snell, P. (1999). Short duration rice: Implications for water use efficiency in the New South Wales Rice Industry. In ‘‘Rice Water Use Efficiency Workshop’’ (E. Humphreys, Ed.), pp. 41–44. Proceedings of a Workshop at Griffith, 12 March 1999. Cooperative Research Centre for Sustainable Rice Production, Yanco, Australia. Xiaoguang, Y., Huaqi, W., Zhimin, W., Junfang, Z., Bin, C., and Bouman, B. A. M. (2003). Yield of aerobic rice (Han Dao) under different water regimes in North China. In ‘‘Water-Wise Rice Production’’ (B. A. M. Bouman, H. Hengsdijk, B. Hardy, P. S. Bindraban, T. P. Tuong, and J. K. Ladha, Eds.). Proceedings of a Thematic Workshop on Water-Wise Rice Production, 8–11 April 2002 at IRRI Headquarters in Los Ban˜os, Philippines. International Rice Research Institute, Los Ban˜os, Philippines. Xu, D., Duan, X., Wang, B., Hong, B., Ho, T. H. D., and Wu, R. (1996). Expression of a late embryogenesis abundant protein gene, HVA1, from barley confers tolerance to water deficit and salt stress in transgenic rice. Plant Physiol. 110, 249–257. Yadav, R., Courtois, B., Huang, N., and McLaren, G. (1997). Mapping genes controlling root morphology and root distribution in a doubled-haploid population of rice. Theor. Appl. Genet. 94, 619–632. Yang, J., Zhang, J., Liu, K., Wang, Z., and Liu, L. (2007). Involvement of polyamines in the drought resistance of rice. J. Exp. Bot. 58, 1545–1555. Yeo, A. R., Flowers, T. J., and Yeo, M. E. (1994). Photosynthetic and water use efficiencies in the genus Oryza: Prospects of yield enhancement. In ‘‘Compendium of Current Research in Plant Sciences 1993’’ ( J. R. Witcombe and D. Wrigh, Eds.). pp. 10–12. Overseas Development Administration, London, UK. Zhang, J., Nguyen, H. T., and Blum, A. (1999). Genetic analysis of osmotic adjustment in crop plants. J. Exp. Bot. 50, 291–302.

388

M. Farooq et al.

Zhang, J., Zheng, H. G., Aarti, A., Pantuwan, G., Nguyen, T. T., Tripathy, J. N., Sarial, A. K., Robin, S., Babu, R. C., Nguyen, B. D., Sakarung, S., Blum, A., et al. (2001). Locating genomic regions associated with components of drought resistance in rice: Comparative mapping within and across species. Theor. Appl. Genet. 103, 19–29. Zheng, H. G., Babu, R. C., Pathan, M. S., Ali, M. L., Huang, N., Courtois, B., and Nguyen, H. T. (2000). Quantitative trait loci for root penetration ability and root thickness in rice: Comparison of genetic backgrounds. Genome 43, 53–61. Zhu, J. K. (2002). Salt and drought stress signal transduction in plants. Annu. Rev. Plant Biol. 53, 247–273.

Index

A Abiotic stresses, 97, 108 Absisic acid (ABA), 71–72, 98 Acacia mangium, 241 Acid detergent fiber (ADF), 329, 331 Acidification of soil, 128 Acrocercops syngramma, 244 Acrylonitrile, 38 Aerobic dryland soils, 129 Aerobic rice, 87–88 culture, 353–354, 357, 364–366, 373 Agricultural sustainability, importance, 3–4 Agriculture vs. aquaculture, 107 Agroecological zone, 4 Aldehyde dehydrogenase, 98 Alfalfa, 4 All India Coordinated Research Project on Cashew (AICRPC), 223 Alternate wetting and drying, 87, 353, 363, 366–368 Ammonium-based N fertilizer, 163 Ammonium bicarbonate, 145 Ammonium fertilizer, 129 Ammonium nutrition, 162–163 Ammonium sulphate fertilizer, 163–164 Ammonium volatilization, 134–136, 145 Anabe disease, 205 Anacardium occidentale, 209–210 area and production of raw nuts, in India, 213 state-wise area and production, 214 country-wise raw nut production, 212 distribution, origin and history of, 217–220 economic botany, 220 cytology, 221 taxonomy, 220–222 end products cashew kernel, 249, 252 cashew kernel peel, 249–250, 252 cashew nut shell liquid, 250, 252 shell cake, 252 value-added products, 253–254 future research, areas for, 256 crop management techniques, 257 crop protection, 257–258 genetic resources, 256 post-harvest technology, 258 technology transfer, 258 varietal improvement, 256

genetic improvement of biotechnology, 230–231 breeding, 224–225 hybridization, 226–230 selection, 225–226 germplasm collections and catalogue, 222–223 global cashew production, 211 global raw nut production, 212 hybrids, released in India, 228–230 and The Nutrient Buffer Power Concept, 238 nutritional content and value, 210–211 orchard, cultivation and management of land selection, guidelines for, 233–234 manuring, 235, 237–238 soil requirement, 231–232 water requirement, 232, 235–236 pests and diseases, controlling of, 243–244 adverse weather conditions, impact of, 248 biological pest control, 246–247 cashew diseases and control, 247–248 foliage and inflorescence pests, control of, 245–246 pest control, 244–245 planting technology, 239–240 cover cropping, 241 high density planting, 240 intercropping, 241–243 research organization, 254–255 softwood grafting, technique of, 238–239 trade in India, 216–217 world trade in, 214–216 Animal manure, 138 Anther characteristics, affecting dehiscence, 71 Anther dehiscence, 70 Anthesis, 66 Aonidiella orientalis, 207 Aquic Paleudalf, 4 Arabidopsis thaliana, 96, 358 Arable crop. See also Soil organic matter grown continuously and in rotation experiments after 1970s, 29–32 experiments before 1970s, 28–29 long-term experiments and recent data from, 32–37 on loss of soil organic matter, 26–27 short-term leys, interspersed with, 15–21 Arable rotations, 16, 18, 26 Arachis hypogea, 64 Areca catechu. See Arecanut

389

390

Index

Areca concinna, 188 Arecanut, 188 botany and taxonomy of, 189, 192–193 country-wise area and production, 190 cytogenetics of, 193–195 future prospects, 208–209 genetic resource program, 195–198 geographical distribution, of Areca species, 192 harvesting and processing of, 207–208 hybridization program, 198 insect pests of, 206–207 intercropping system, 203 banana, 204 black pepper, 204 cocoa, 204 irrigation, 200–202 drip irrigation system, 200–201 modified Penman model, 200 mulching, 201–202 photosynthetic parameters and yield, 201–202 nematology, 207 nutrition, 202 origin and history of, 189 pathology, 204 Anabe disease, 205 bacterial leaf blight, 206 fruit rot, 205 inflorescence dieback, 206 stem bleeding, 206 yellow leaf disease, 205 physiology, 202–203 seeds sowing and seedlings spacing, 199–200 soil and climatic requirement, 198–199 state wise area and production, in India, 191 Ascochyta leaf blight, 339, 342 Astronomy, 3 AtNHX1overexpression, 96 Austronesians, 262 aus variety N22, rice, 72 Available water capacity (AWC), 45 AWD. See Alternate wetting and drying Azadirachta indica, 207 B Baby bits, 253 Bacterial leaf blight, 206 Barley. See Hordeum vulgare L. Basal dehiscence length, 79 Batcombe series soil, 4 Betel nut. See Arecanut Biodynamic Cashew, 258–259 Biological materials, as nutrient, 166–167 Biological nitrogen fixation, 316, 334–335 Biological water saving (BWS), 362 Biotic stresses, 62 Birgus latro, 286

Blackgram. See Vigna mungo Black pepper. See Piper nigrum BNF. See Biological nitrogen fixation Brahmaputra, 89 Brassica juncia L., 319 Brassica napus, 10 Breeding populations, 86 British Association for Advancement of Science, 3 Broadbalk winter wheat experiment at Rothamsted, 9 nitrogen applied and mean yield of grain, 39 Bureau for the development of Research on Tropical perennial Oil Crops (BUROTROP), 292 Busseola fusca, 326 C Calapagonium muconoides, 241 Calcium (Ca), 45 Cambric Arenosol, 5 Cancer-causing compounds, 130 Canopy photosynthesis rates, 82–83 Carbon dioxide (CO2), 46, 61, 74, 82–83, 93 Caribbean oil, 253 Carvalhoia arecae, 206 Cashew. See Anacardium occidentale Cashew apple, 249–250, 252 Cashew Development Corporation, Kerala, 216 Cashew export development authority (CEDA), 216–217 Cashew Export Promotion Council, 253 Cashew nut shell liquid (CNSL), 220, 250 Cashew nut shell oil (CSL), 188 Cashew stem and root borer (CSRB), 244 Cassava (Manihot esculenta Crantz), 338 C decay curve, 7–8 Cell death, 98 Cellular ion homeostasis, 107 Cellular membrane thermostability (CMT), 64 Center for Scientific Investigation at Yucatan (CICY), 294 Central Food Technological Research Institute, Mysore, 253 Central Plantation Crops Research Institute (CPCRI), 195, 292 Centre de cooperation internationale en recherche´ agronomique pour le development (CIRAD), 273, 292–293 Centrosema pubescens, 241, 324 C3 grass, 102 Chali, 207 Chemical fertilizers, in China, 126–127 Chickpea. See Cicer arietinum L. China’s agriculture, 130 Chlorophyll (Chl), 83 Chromic Luvisol, 4 Cicer arietinum L, 319, 321–325, 327, 335–342

391

Index

Climate change effects of, 62 flexibility for adjusting and coping with, 108–109 Climate induced stresses and adaptation mechanisms. See also Stress physiology, at ontogenetic stages on rice agronomic approaches, to cope with less water, 86–91 breeding rice for warmer world, 76–80 drought, 80–86 high night temperature, 75 high temperature and humidity, 63 salinity, 93–97 submergence, 97–102 temperature and CO2 interaction, 74–75 Clitoria ternatea L., 338 Clover, 4 C:N ratio, 5, 7–8, 22–23, 132 Coastal rice ecosystems, 96 Cochliobolus sativus, 324 Coconut cadang–cadang viroid (CCCVd), 289 Coconut Genetic Resources Institute (COGENT), 292 Coconut oil, and anti-coconut lobby, 290–291 Coconut palm. See Cocos Nucifera L. Coconut Research Institute, Sri Lanka, 293 Coconut water, in China, 291 Coconut woodland, 259 Cocos Nucifera L. adaptation in, 288–289 agronomy nutrients requirement, 280–281 soil, 278 soil water, 278–280 tissue analysis, 281–282 biotic factors, adaptation to, 286 breeding, constraints in, 274–275 hybrids and future, 276–277 selection and progress, 275–276 cytogenetics of, 271 distribution, on production, 264 early breeding work, 273–274 evolution along drifting coastlines, 259–260 human influence on, 262–264 field management, 285 as food item, 291 fruit component analysis, 272 future prospects, 294–295 genetic improvement genome, characterization of, 272 source of diversity, 271 hybrid seeds, commercial production of, 277 in vitro propagation, 277–278 hybrid vigor in, 274 inflorescence, 269 mixed cropping systems, 282–283

molecular markers, use of, 272–273 morphology, 265 crown, 267–268 frond, 267 fruit, 269–270 root system, 266–267 seed and seedlings, 270–271 trunk, 265–266 national research centers, 292–293 origin, 259 pests, 286–287 disease pathogens, 287–288 insect pests, 287 processing technology, advances in, 296 production base, protection of, 295 production, research and development in, 291–292 quality traits fatty acid mix, 290 research centers and institutes, contact information of, 296–300 research in India and Sri Lanka, 293 seed and seedling management, 283 germination rate, 283–284 polybag seedlings, 284 seedling selection, 284–285 swimming coconut fruit, 261–262 traits of true palm, 262 value-added products from, 285–286 wind resistance, development of, 260–261 yield potential of, 289–290 Cold and hot decomposition spots, 329 Colletotrichum gloeosporioides, 206 Commonwealth Scientific and Industrial Research Organization (CSIRO), 255 Compartmentation, 95 Compatible solutes, roles of, 106 Corticum salmonicolor, 247–248 Cottenham series (SSEW), soil, 5 Council for Scientific and Industrial Research (CSIR), 253 Cowpea. See Vigna unguiculata L. C4 photosynthetic pathway, 358, 361 C4 rice plants, 363 Crop growth cycle, of rice, 63–64 Crop health, 167–168 Cropping sequences, effect on percent organic carbon, 17 Crop rotation, 38, 317 with legumes, 165–166 Crop technology, 62 Crown rot. See Fusarium graminearum C turnover model, 51 Cultivar NSG19, adaptation to environments, 86 D Dark loessial soil, 134 Decomposable plant material (DPM), 46

392

Index

Dehiscence, of anther, 70 Deltas of Mekong, 89 Depth of N dressing and wheat yield, relationship, 144 Deteriorating soils, advantages/disadvantages in, 107–108 Detoxification, 98 Devil’s Nut, 210 DNA-based technologies, 62, 108 Dolichos. See Lablab purpureus L. Drip irrigation, 200–201 Drought as catastrophic event, 81 and CO2 on crop yield and physiological responses, 82–84 genetic basis of grain formation failure under, 84–85 genetic enhancement, of drought-stress resistance, 85–86 milder chronic, 82 strategy for drought resistance improvement, 92–93 Drought-induced inhibition of panicle exsertion, 84 Drought-prone rainfed areas, 82 Drought resistance improvement, 92–93 Drought resistance index (DRI), 85 Dryland soils contents and distribution of N in, 131–134 N loss and gain, ways for (see Mineral N) strategies for managements of soil N on, 164–168 biological materials, as nutrient, 166–167 crop rotation, with legumes, 165–166 improving crop health, 167–168 N fertilizer, adequate supply of, 165 E Engineering-based cropping system, to improve WUE, 362 Ethanolic fermentation pathway, 98 Ethylene-responsive transcription factor (ERF) genes, 99–100 European Community directive, on quality of water, 130 F Farmyard manure, 3–4, 7, 10, 13–14, 16, 22, 30, 34, 36, 38, 40, 43, 53 Fehe River, 133 Feni, 220, 250 Fertilizer and manure inputs on soils, 11–15 Field beans. See Phaseolus vulgaris L.; Vicia faba Floral bud development and heat stress, 104 Food production, 4 Food security, 352 Fruit rot, in arecanut, 205

Fusarium graminearum, 324 FYM. See Farmyard manure G Gaeumannomyces graminis, 34 Ganges, 89 Ganoderma lucidum, 205 Garden pea. See Pisum sativum L. Genetic donors, for heat tolerance and avoidance, 77–78 Genetic improvement, for heat tolerance, 76–77 Genetic load in coconut palm report, 271 Gibberellic acid (GA3), 71–72, 98 Global Cashew Alliance (GCA), 217 Global warming, 102, 106 Gloeosporium mangiferae, 248 Glycine betaine, 72 Glycine max L., 333, 337 Glycolysis, 98 Glyricidia maculata, 202 Grain weight heat susceptibility index, 78 Green fuel, 210, 254 Greenhouse gas methane, 60 Green Revolution, 62, 76 Ground-cover rice production system (GCRPS), 369–370 H Half-life for C and N, 7–8 Halyomorpha marmorea, 206 Heat avoiding genotypes, 68–69 Heat stress during anthesis, 66 on rice crop, 102 (see also Stress physiology) Heat tolerance and avoidance in rice, selection indexes, 78–79 into crop plants, 106 genetics of, 79–80 Helicoverpa amigera, 338 High affinity Kþ transporters (HKT), 94 High night temperature, grain yield, 75 High temperature during grain-filling period, 73 Hordeum vulgare L, 83 Horticultural crops, 8 Humidity, 63 Humified organic matter (HUM), 46 Hydrolysis, 163 Hydro-technological infrastructure, 89 I Indian Council of Agricultural Research (ICAR), 195, 222, 254 Indigofera (Indigofera tinctoria L.), 333 Indo-Gangetic plains, wheat production in, 370–371 Indole acetic acid (IAA), 71–72

393

Index

Industrial crops, and economy of developing countries, 187 Inflorescence dieback, 205 Inorganic fertilizers, 142 Inorganic N, 52 International Plant Genetic Resources Institute (IPGRI), 223 International Rice Research Institute (IRRI), 62, 75, 86–87, 101, 108, 353–354 Ionic balance, 94 Irrawaddy, 89 IRRI breeding programs, 79 Irrigated and rainfed rice, in East, South and Southeast Asia, 61 Irrigated warping soil, 134 Irrigation schemes, 88 Irrigation systems in deltaic regions, 89 typology and potential role, 90–91 K Kaju Supari, 208 Kalpavriksha, 188, 259 Karimunda, 204 Kþ/Naþ ratio, 94 Koleroga (Mahali). See Fruit rot, in arecanut Krilium, 38 L Lablab purpureus L., 337–338 Lamida moncusalis, 244 Leaching, 136–137 Leaf area index (LAI), 203, 355 Lecopholis burmeisteri, 206 Legume–cereal cropping systems, 316–317. See also Legume-wheat rotation, in tropical humid climate residual soil moisture in dry land agriculture, 317–318 humid tropical environment and, 318–319 residue decomposition and N mineralization, factors affecting, 326–334 asymptotic models, for nutrient release from clitoria and dolichos residue, 331–333 decomposition of residues, incorporated into soil, 333–334 N concentration and C:N ratio, 330–331 net mineralization of C and N, 331 N released and crop demand, synchrony between, 326–327 nutrient release patterns, 329 oxygen levels, 328 residue management, 328–329 soil moisture and temperature, 327–328 soil texture, influence of, 328 straw quality index, 329 soil fertility and yields, 320

cereal grain yield response, 323–326 soil N enrichment, 320–323 Legume–wheat rotation, in tropical humid climate, 334–335 soil moisture use effects, 335–337 soil N contribution and yield effects, 337–342 Leguminous green manure (LGM), 138 Leucaena leucocephala, 241 Leucopholis lepidophora, 206 Lignin, 329–331, 333, 337, 340 Lucerne. See Alfalfa M Magnesium (Mg), 3 Maize. See Zea mays Mandari disease, 293 Mangala, 196–197 Marker-assisted backcrossing (MAB), 100 Marker-assisted selection (MAS) strategy, 353, 357 Mayan society, 3 Medicago sativa, 167 Melilotus, 167 Mesopotamia, 2–3 Methemoglobinemia, 129 Microbial biomass (BIO), 46 Mineralization, of organic N, 132 Mineral N gains from wet deposition, 137 loss by denitrification, 136 leaching, 136–137 volatilization, 135–136 Mohitnagar, 196–197 Monsanto Chemicals, 38 Mucuna pruriens, 324, 326, 333 Mulch tillage, on drylands in China, 168 Mungbean. See Vigna radiata Mustard. See Brassica juncia L. N NADP-malic enzyme (NADP-ME), in rice leaves, 362–363 N allocation, at different growth stages on maize yield, 149 National Agricultural Advisory Service, 14 National Cashew Gene Bank (NCGB), 223 National Research Center for Cashew (NRCC), 222, 226, 231, 254 National Research Center (NRC) for DNA fingerprinting (NRCDNAF), 231 National Soil Inventory of England and Wales, 11 N availability index, 158 Neutral detergent fiber (NDF), 329, 331 N fertilization, on C sequestration in soil, 9 N fertilizer efficiency (NFE), 144 N fertilizer recovery (NFR), 127, 131 N fertilizers, 52–53, 126, 131

394

Index

N fertilizers (cont.) rational application to dryland soils deep application of, 142–146 determination of N rate, 149–158 with OF, application of, 138–139 with P fertilizer, application of, 139–142 suitable form for different crops, 158–164 timing of, 146–149 N2 fixation, by leguminous crops, 316 NH4þ in terrestrial ecosystems, 128 NH3 volatilization, 164 Nitrate accumulation, in cereal crops, 160–161 Nitrate concentration cabbage, spinach, and wheat at different growth stages, 161 of different organs at different growth stages, 162 Nitrate concentration limits, for vegetables, 130 Nitrate N in groundwater, areas of China, 129 Nitrate N leaching, 136 Nitrate sparing, concept of, 323 Nitrite (NO2þ), 129 Nitrogen contents, in arable lands of China, 133 Nitrogen (N), 3, 127–128. See also Mineral N Broadbalk Winter Wheat experiment and, 39 deficiency, 137 distribution, in different areas, 133 loss by volatilization from, 146 Nitrosamide, 129–130 Nitrosamine, 129–130 Nitrous oxide (N2O) emission, 136, 370 Noctuid stemborer. See Busseola fusca Non-N effect, 324, 326 NPK fertilizers, 10–11 N rate allocation, at growing-stages on wheat yield, 148 N rate based on soil N-supplying capacity, determination. See also N uptake; Soil N-supplying capacity (SNSC) anaerobic incubation, 151 to calculate mineralization potential (N0), 150–151 data from field experiments, 156 to evaluating SNSC and applying N fertilizer, 150 extraction and determination of mineral N, 152 index of N availability of, 157 laboratory methods, 150 results from pot experiment, 154, 156 without leaching and wheat uptake, 156 N-rich crop, 4 N uptake, 153, 156–157, 159–160 N use efficiency (NUE), 131, 168 Nutrient buffer power concept, 209, 238 O OF. See Organic fertilizer Oligonychus indicus, 206 OM. See Organic matter

Organic carbon after 58 years of different cropping sequences, 19 under arable cropping, amount of, 20–21 in sandy loam soil, 6, 18, 23–24 silty clay loam soil, Ley–arable experiment, 20–21 in top 23 cm on Geescroft Wilderness and Highfield Bare Fallow, 49 on Highfield and Fosters Ley–arable, Rothamsted, 50 from three plots growing, 48 Organic compartment (IOM), 46 Organic compatible osmolytes, 95 Organic fertilizer, 133, 138–139, 142, 145 Organic inputs to soils growing arable crops, 22–25 Organic matter, 130–132, 168 Organic matter content, of soils. See also Soil organic matter changes and causes for arable crop rotations on loss of, 26–27 effects of fertilizer and manure inputs, 11–15 increases in content and permanent grass, 27–28 organic inputs to soils growing arable crops, 22–25 short-term leys interspersed and arable crop, 15–21 straw incorporation, effect of, 25–26 Organic nitrogen, 27 Organic waste residues, 164, 166–167 Oryctes rhinoceros, 287 Oryza australiensis, 363 Oryza glaberrima, 66, 69, 78, 86 Oryza sativa, 66, 69, 72, 78 Osmoprotectants, use of, 374 Osmotic adjustment (OA), in crop plants, 357 OWRs. See Organic waste residues Oxidative stress management, 95. See also Salinity stress, mechanisms of Ozone layer, 130 P Paclobutrazol, 224 Palm oil, 187 Panniyur-1, 204 Peanuts. See Arachis hypogea Pellicularia salmonicolor, 248 Penman model, modified, 200 PEP carboxylase, 358 Pestalotia microspora, 248 P fertilizer, 139–142 Phaseolus vulgaris L., 337 Philippine Coconut Authority (PCA), 293 Phomopsis anacardii, 248 Phosphoenolpyruvate carboxykinase (PCK), 358, 360–361

395

Index

Phosphoenolpyruvate carboxylase (PEPC), 363 Phosphorus (P), 3 Photosynthesis, 61, 74, 83–84, 103 Photosynthetically active radiation (PAR), 202 Phytophthora Mardi, 205 Pi dikinase, 361 Pig manure, 138 Pink disease, 247–248 Piper nigrum, 204 Pisum sativum L., 337 Plant architecture, of rice plant, 68 Plantation crops, 187 Plant breeding, 76–77 Plant nutrients, 3 Pneumatophores, 266–267 Pollen shedding, 78 Pollution of groundwater, by nitrate N, 129 Potassium (K), 3 Potatoes, 8, 22 Powdery mildew disease, 248 Precipitation use efficiency (PUE), 138–139 Pressurized irrigation systems, 372 Priming effect, 329 Principal component regression (PCR), 329 Proline, 95 Proutista moesta, 205 Puddling, 366–367 Pueraria javanica, 241 Pueraria phaseoloides, 241, 324 Pyramiding genes/QTLs for salinity tolerance, 96 Q Quantitative electrolyte leakage, 64 Quantitative trait loci (QTL), 77, 79, 92, 96, 99–100, 109 Quartzipsammetric Haplumbrept, 5 R Radopholus similes, 207 Random amplified polymorphic DNA (RAPD), 231 Raoiella indica, 206 Rattus exulans, 286 Reactive oxygen species (ROS), 95 Red River, 88 Resistant plant material (RPM), 46 Rhadinaphelencus cocophilus, 286 Rhizobium, 342 Rice blast disease, 354 Rice cultivation in hot/dry regions, 67–68 Rice ecosystems shift, 108 Rice production, and water availability, 352–353 crop management, 364 other management practices, 372–373 physiological implications, 373–374 production systems (see Rice production systems)

future research areas, 374 genetic improvement, for water productivity, 353–354 molecular and biotechnological approaches, 358–362 selection and breeding strategies, 354–357 water-use efficiency and transpiration efficiency, 362–363 transgenic rice plants and effects under water-limited conditions, 360 genes and their effect, 361–362 Rice production systems, 62, 364 aerobic rice system, 364–366 AWD irrigation, 366–368 ground-cover rice production system, 369–370 raised beds, 370–372 system of rice intensification (SRI), 368–369 Rice productivity in salt-affected areas, 107 Riverina region, of low humidity, 68 Root rot. See Cochliobolus sativus ROTHC-26.3 model, 51 Roth PC-1 model, 51 S Sabita and KDML105, adaptation to environments, 86 Salinity, in coastal areas, 107 Salinity stress, mechanisms compartmentation and organic compatible osmolytes, 95 ionic balance, 94 Salinity tolerance and adaptive mechanisms, 96–97 Salt-affected soils, 107 Salt response signaling, 107 Salt stress, 107 Sandy loam soils, 8, 37–38 Saturated soil culture (SSC), 87 Scapanes australis, 287 Seed priming, 373–374 Silicon (Si) nutrition, 374 Silty clay loam, 21 Single nucleotide polymorphism (SNPs), 99 Si Sa Ket Horticultural Research Center, 243 Sodium transport, 107 Softwood grafting, 228, 238–239 Soil borne pathogen, 34 Soil cultivation—Highfield Bare Fallow, 48 Soil fertility, 3, 164 Soil methane sink, 128 Soil microbial biomass, 4 Soil moisture, 81 Soil N-supplying capacity (SNSC), 149–153, 156 Soil organic carbon, decline in, 7 Soil organic matter, 3–4 amount and C:N ratio, relationship, 6–8 benefits of, 37–46 nitrogen and organic matter effects from, 40

396 Soil organic matter (cont.) nitrogen, crop rotation, and, 38–40 soil phosphorus and potassium availability, 43–45 and soil structure, 37–38, 40–43 and water availability, 45–46 and crop yields, 28–37 disadvantages from increasing, 52–53 equilibrium levels of, 8–11 increases in, 27–28 modeling changes in, 46–51 nature and determination of, 5 Soil phosphorus, availability, 43–45 Soil potassium, availability, 45 Soil sodicity, 93 Soil structure, 37–38 soil organic matter and, 40–43 Soil Survey of England and Wales (SSEW), 4 Soil texture, 9, 21 SOM. See Soil organic matter Soybean. See Glycine max L. Spring barley grain, yields of, 33 Sreemangala, 196–197 Stem bleeding, 206 Straw incorporation, 25–26, 32 Straw mulching, 168 Straw quality index (SQI), 329 Stress physiology, at ontogenetic stages on rice, 63 reproductive phase, 65–73 anther dehiscence and high temperature, 70–72 flowering patterns of, 67 heat avoiding genotypes, 68–69 heat stress during anthesis leads, 66 screening for heat tolerant donors, 72 spikelet fertility, 65 spikelet sterility, 69–70 ripening phase, 73–74 at vegetative phase, 63–64 Stress-sensitive mega varieties, 86 Stress tolerance QTL(s), by marker-assisted backcrossing, 101 Subabul, 237, 241 Sub1A-2 expression, 102 Sub1A gene, 108 Sub1C-1 allele, 99 submergence 1 (Sub1), 97 Submergence tolerance germplasm screening, beyond Sub1, 101–102 molecular breeding, in rice varieties, 100–101 physiology and molecular biology of, 97–100 Sucrose-phosphate synthase (SPS) activity, 83 Sugar beet, 22 Sumangala, 196–197 Sumerian society, 2 Supari, 188, 208 Symbiotic dinitrogen fixation, 320

Index

Syntenic region in Nipponbare, 99 System of rice intensification (SRI), 87, 367–368 T Target population of environments (TPEs), 81–82, 85, 92 Tea mosquito bug, 225, 244–245, 247 Tethys Sea, 260 Thielaviopsis paradoxa, 206 Thyroid gland disease, 129 Tirathaba mundella, 207 TMB. See Tea mosquito bug Total soluble protein (TSP), 83 Transpiration, 68 Transpiration efficiency (TE), 362–363 Triacontanol, 326 Tropical soils, characteristics of, 316–317 Turnips. See Brassica napus Typhoon, 81 U Urea, 129, 163 Urochloa panicoides, 358 Uromyces caudimaculatus, 286 V Vesicular arbuscular mycorrhizae (VAM), 204 Vicia faba, 22, 47 Vigna mungo, 324 Vigna radiata, 324 Vigna unguiculata L., 64 Vita packing, 216 Volatilization, 135 W Warmer climates advantages/disadvantages, 102–106 accelerated reproductive development under, 105 floral bud development, 104 flower development in cowpea, 104–105 reproductive development of crops, 103 Water saving techniques, 86–87 Water shortages, 106 Water-use efficiency, 83, 93, 138, 354, 357–359, 362–363, 366 Weihe River valley plains, 133 Wheat (Triticum aestivum L.), 339 Wheat yields and depths for fertilization, relationship, 144 Whether incorporating heat tolerance, 106 Winter wheat, 22–23 grain, with different cultivars, average yields, 35

397

Index

Woburn Market Garden experiment, 7 Worsening water stress, 106–107 WUE. See Water-use efficiency

Y Yellow leaf disease (YLD), 204–205 Yellow River water, 134

X Xangsane, 81 Xanthomonas campestris, 206

Z Zea mays, 326, 338