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ADVANCES IN

AGRONOMY

VOLUME 41

This Page Intentionally Left Blank

ADVANCES IN

AGRONOMY Prepared in Cooperation with the AMERICAN SOCIETY OF AGRONOMY

VOLUME 41 Edited by

N. C. BRADY

Science and Technology Agency for International Development Deportment of State Washington, DC

ADVISORY BOARD

T. M. STARLING G. H. HEICHEL H. J. GORZE. J. KAMPRATH R. J. KOHEL G. E. HAM

E. L. KLEPPER R. H. FOLLETT D. R. BUXTONE. S. HORNER

J. J. MORTVEM

ACADEMIC PRESS, INC. Harcourt Brace Jovanovich, Publishers

San Diego New York Berkeley Boston London Sydney Tokyo Toronto

COPYRIGHT 0

1987

BY ACADEMIC PRESS,

INC.

ALL RIGHTS RESERVED. NO PART OF THIS PUBLICATION MAY BE REPRODUCED OR TRANSMI'ITED IN ANY FORM OR BY ANY MEANS, ELECTRONIC OR MECHANICAL, INCLUDING PHOTOCOPY. RECORDING. OR

ANY INFORMATION STORAGE AND RETRIEVAL SYSTEM, WITHOUT PERMISSION IN WRITING FROM THE PUBLISHER.

ACADEMIC PRESS, INC. 1250 Sixth Avenue San Diego, California 92101

United Kingdom Edition published by ACADEMIC PRESS INC. (LONDON) LTD. 24-28 Oval Road, London NW1 7DX

LIBRARY OF CONGRESSCATALOGCARDNUMBER:50-5598

ISBN 0-12-OOO741-X (alk. paper)

PRINTED IN THE UNITED STATES OF AMERICA 87 88 89 90

9

8 7 6 5 4 3 2

I

CONTENTS PREFACE ......................................................

ix

SPECIATION. CYTOGENETICS. AND UTILIZATION OF Arachis SPECIES

.

H . T. Stalker and J . P Moss I . Introduction ............................................. I1. Botany and Taxonomy .................................... 111. Plant Collection and Maintenance .......................... I v. Centers of Origin ........................................ V. Cytogenetics of Aruchis Species ............................ v1. Interspecific Hybridization in Arachis ....................... VII . Germplasm Evaluation .................................... VIII . Utilization of Wild Aruchis Species ......................... IX . Successes and Potentials for Utilizing Arachis Germplasm . . . . . X . Conclusions ............................................. References ...............................................

1 3 4 7 8 10 14 22 32 34 35

CEREAL-LEGUME INTERCROPPING SYSTEMS

Francis Ofori and W. R . Stern I . Introduction ............................................. I1 . Background to Intercropping Systems ....................... 111. Competitive Relationships between Component Crops . . . . . . . . . I v. Some Agronomic Factors Influencing Productivity and Efficiency ................................ V. Nitrogen Economy of the System ........................... VI . Summary and Conclusions ................................ References ...............................................

41 42 52 61 72 83 85

GENOTYPIC VARIATION IN CROP PLANT ROOT SYSTEMS

J . C. O’Toole and W. L. Bland 1. Introduction ........................................... 11. Significance of Genotypic Variation in Root Systems ....... V

91 92

vi

CONTENTS Evidence of Genotypic Variation ........................... IV. Utilization in Research and Crop Improvement.. . . . . . . . . . . . . . V. Phenotypic Plasticity ..................................... V1. Conclusions and Research Needs ........................... References ...............................................

111.

94 120 133 139 140

APPLICATION OF CELL AND TISSUE CULTURE TECHNIQUES FOR THE GENETIC IMPROVEMENT OF SORGHUM, Sorghum bicolor (L.) Moench: PROGRESS AND POTENTIAL

S. Kresovich, R. E. McGee, L. Panella, A. A. Reilley, and F. R. Miller Introduction ............................................. Background ............................................. Goals in Breeding ........................................ 1V. Progress in Cell and Tissue Culture Research . . . . . . . . . . . . . . . . V. Potential Applications .................................... VI. Summary ............................................... References ............................................... I.

11. 111.

147 148 150 153 162 166 168

UPTAKE OF MINERAL NUTRIENTS AND CROP GROWTH: THE USE OF FLOWING NUTRIENT SOLUTIONS

A. Wild, L. H. P. Jones, and J. H. Macduff Introduction ............................................. Systems Employing Flowing Nutrient Solutions . . . . . . . . . . . . . . Nutrient Uptake ......................................... I v. Partitioning of Photosynthate between Shoots and Roots. . . . . . V. Conclusions and Summary ................................ References ...............................................

1. 11. 111.

171 172 177 21 1 214 216

MINERAL NUTRITION OF LINSEED AND FIBER FLAX

Peter J. Hocking, Peter J. Randall, and Andrew Pinkerton I. Introduction ............................................. Production, Crop Growth, and Agronomy. . . . . . . . . . . . . . . . . . . Effects of Mineral Nutrition on Yield.. ..................... I v. Nutrient Disorders. ....................................... V. Salinity ................................................. VI. Soil Acidity.. ............................................ VII. Interactions between Genotype and Mineral Nutrition. . . . . . . . . VIII. General Conclusions and Challenges ........................ References ............................................... 11. 111.

22 1 223 225 25 5 27 8 28 1 283 285 287

CONTENTS

vii

THE IMPACT OF SOIL AND FERTILIZER PHOSPHORUS ON THE ENVIRONMENT

A. N. Sharpley and R. G. Menzel I. Introduction ............................................. 11. Impact of Phosphorus on the Terrestrial Environment . . . . . . . . 111. 'Ransport of Phosphorus from the Terrestrial to Aquatic Environments ........................ IV. Impact of Phosphorus on the Aquatic Environment . . . . . . . . . . V. Conclusions ............................................. References ...............................................

297 298 301 311 317 319

BIOTECHNOLOGY APPLICATIONS IN WEED MANAGEMENT: NOW AND IN THE FUTURE

Kriton K. Hatzios Introduction ............................................. Production and Use of Biological Weed Control Agents.. . . . . . Naturally Occurring Herbicides ............................ Genetic Improvement of Crop Tolerance to Herbicides . . . . . . . . Additional Uses of Genetically Engineered Microorganisms in Weed Management ...................... VI. Conclusions and Future Prospects .......................... References ...............................................

I. 11. 111. IV. V.

325 326 332 337 357 364 367

RECLAMATION OF ACIDIC MINED LANDS IN HUMID AREAS

P. Sutton and W. A. Dick I. Introduction ............................................. 11. Properties of Spoil on Acidic Mine Lands.. . . . . . . . . . . . . . . . . . 111. Application of Various Amendments to Ameliorate Acidic Mine Spoil. ............................. IV. Seeding and Management of Amendment-Treated Spoil . . . . . . . V. Changes in Soil Properties as Affected by Addition of Amendments ................................. VI. Summary of Long-Term Results.. .......................... References ...............................................

371 379 383 389 394 40 1 40 1

viii

CONTENTS WATER AND QUALITY LOSS DURING FIELD DRYING OF HAY

Andy D . Macdonald and E . Ann Clark 1 . Introduction ............................................. I1 . Patterns of Water Loss .................................... I11. Factors Influencing Water Loss during Field Drying of Hay . . . IV. Losses during Forage Conservation. Storage, and Handling . . . . V. Preservation of Wet Hay .................................. VI . Conclusions ............................................. References ...............................................

INDEX .........................................................

407 407 409 417 430 432 435

439

PREFACE

The international status of Advances in Agronomy continues to prevail as illustrated by this volume. Authors from five countries have participated in preparing ten review articles. This participation confirms the wide-ranging interest in crop and soil science. There is a variety of topics covered this year, ranging from reviews of research to improve the yield and quality of two crops of worldwide importance, sorghum and groundnuts, to the reclamation of land around abandoned mines. The two crop improvement reviews show evidence of the use of modern biotechnology to alter genetically and to improve these important food crops. They are examples of what will likely become more and more common as new and improved methods of genetic improvement are applied to crop plants. Two articles are concerned with mineral nutrition. The first deals with one specific crop (flax) and focuses on the effects of nutrient stress on vegetative development, yield components, and yield quality. The second summarizes the advances made in understanding the requirement for nutrients through the use of flowing nutrient solutions which the authors have researched in depth. The effects of environmental factors on the genetic variation in root systems are also reviewed, with the literature covering a number of important field crop species. There appears to be considerable variation that could be used in crop improvement programs. Intraplant and whole-plant factors influencing water and quality loss of hay during drying are also considered. An excellent review is that covering the positive and negative effects of soil and fertilizer phosphorus on the terrestrial environment. Attention is given to potentially toxic heavy metals, which are commonly added with phosphate fertilizers, and to means of reducing phosphate levels in lakes. Cereal-legume intercropping systems that have become increasingly more common in the tropics are well reviewed by scientists who have had considerable experience in this research area. Such systems will likely receive more attention in the future, especially where low-input agriculture is being practiced. The management of weeds using biotechnological methods is the focus of another article. Increasing crop tolerance to herbicides promises to be one of the more exciting aspects of applied biotechnology. We will likely read more on this topic in the future. My thanks to the 24 contributors who prepared these articles. Their efforts should be appreciated by their fellow agronomists around the world.

N. C. BRADY ix

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ADVANCES IN AGRONOMY, VOL. 41

SPECIATION, CYTOGENETICS, AND UTILIZATION OF Arachis SPECIES' H. T. Stalker2 and J. P. Moss3. *Department of Crop Science, North Carolina State University Raleigh, North Carolina 27695 'international Crops Research institute for the Semi-Arid Tropics (ICRISAT) Patancheru, P.O., Andhra Pradesh 502 324, India

I.

INTRODUCTION

Utilization of wild species for improvement of cultivated forms has been investigated since Faircloth made the first interspecific hybrids in 1717. Many wild species have been of value in crop improvement for a large number of traits (Harlan, 1976; Hawkes, 1977; Stalker, 1980a; Hadley and Openshaw, 1980). However, in leguminous oilseeds, utilization of species germplasm has proven difficult, in large part because of barriers to interspecific hybridization between species (Smartt, 1979). Further, sterility often restricts introgression from wild to cultivated accessions even when initial hybridization is possible. Interspecific hybridization is also difficult among the peanut species in the genus Arachis, but breeding populations derived from crossing A . hypogaea L. with related species are currently being evaluated for farmer use (Moss, 1985b). Four species of Arachis have been cultivated, including two diploids (2n = 2x = 20: A . villosulicarpa Hoehne and A . repens Handro) and two tetraploids (2n = 4x = 40:A . glabrata Benth. and A . hypogaea). Arachis villosulicarpa has only been cultivated by Indians in the northwestern part of the Brazilian state of Mato Grosso (Gregory et al., 1973). Arachis repens and A . glabrata have been grown in different parts of South America as forages or as ground covers in urban areas. Arachis glabrata has also been selected for forage qualities in Florida, where recent cultivar releases have been made (Prine et al., 1981). However, A . hypogaea is the only species which is cultivated extensively for commercial production of seeds and oil. 'This article is a contribution from paper No. 10726 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, North Carolina 27695, and Paper No. 635 of the Journal Series of the International Crops Research Institute for the Semi-Arid Tropics (ICRISAI), Patancheru, P.O., Andhra Pradesh 502 324, India. 1 Copyright 0 1987 by Academic Press. Inc. All rights of reproduction in any form reserved.

2

H. T. STALKER AND J. P. MOSS

The cultivated peanut, A . hypogaea, is a major crop in most tropical and subtropical areas of the world. As compared to other oilseeds, plants are relatively drought resistant, which makes them especially important in semiarid regions where precipitation exceeds evaporation for only 2-7 months per year (Bunting et al., 1985). Although the crop grows best in sandy, well-drained soils, peanuts are culfivated in a wide range of field conditions from clays to sands and from acidic to alkaline soils. In the United States, the seeds are used mainly for confectionary purposes or for peanut butter. However, approximately two-thirds of the total world peanut crop is used for oil, which cumulatively represents 20% of the world market (Woodroof, 1973). Most peanuts are used as a cash crop and even small farmers may sell their entire harvest. In addition to seeds being of high value, plant residues are extremely important as fodder for cattle in many regions of the world. Shells are also used for fuel, soil conditioners, fodder, chemicals, resin extenders, cork substitutes, and for hardboard (Gibbons, 1980). The peanut is becoming increasingly important as an income source in tree plantations, such as coconut, rubber, or banana, before tree crops mature. In Africa and Asia, many peanuts are intercropped between maize, sorghum, pulses or, in a few areas, between mature coconut trees. While A . hypogaea has the widest distribution of any Arachis species, it also has many pests and diseases which attack all parts of the plant. Disease controls are seldom practiced in less developed countries, and one of the major plant breeding objectives is to improve resistances to plant pests (Wynne and Gregory, 1981). In addition, increased stability of yield and adaptation of the peanut to many environments are important breeding objectives. Several Aruchis species have significantly higher levels of resistance for many important disease and insect pests than are found in cultivated accessions. As in the cases of other crop species, however, attempts to utilize wild species have limitations and obstacles. For example, sterility barriers and genomic incompatibilities restrict utilizing many potentially important genes even after initial interspecific hybids are obtained. However, significant progress has been made toward utilizing germplasm resources of Arachis. Collection, maintenance, and evaluation of peanut germplasm resources have occurred relatively late compared to many other crop species such as tobacco, wheat, or maize. Only during the past 20-25 years have concentrated efforts been undertaken to collect and evaluate the agronomic potential of germplasm resources in the genus. This chapter reviews the current status in the introgression of germplasm from wild to cultivated species of peanut. The successes have been due to a knowledge of the botany, taxonomy, cytogenetics, and genetics of the species related to A . hypogaea and to effective screening of the available germplasm. Therefore, summaries of

SPECIATION, CYTOGENETICS, AND UTILIZATION

3

species variability, genomic and species relationships, and resources of agronomically important characteristics are presented, followed by an account of the methods used and of achievements in Arachis germplasm utilization.

II.

BOTANY AND TAXONOMY

The genus Arachis belongs to the family Leguminosae, tribe Aeschynomeneae, subtribe Stylosanthenae. Species have alternately attached basal and dorsal anthers, flowers in terminal or axillary spikes or small heads, pinnate leaves, and few leaflets without stipules (Taubert, 1894). Most of the Arachis species have tetrafoliate leaves, but two species, A. tuberosa Benth. and A . guaranitica Chod. et Hassl., are trifoliate. Arachis species, along with Trifoliurnsubterraneum L. and Vigna subterraneu (L.) Verdc., flower above ground but produce fruit and seeds below the soil level. Species of peanut are self-pollinating, but outcrossing may also occur in up to 2.5% of the flowers (Norden, 1980). A structure called a peg grows geotropically after fertilization and carries the developing embryo into the soil, after which elongation ceases, the pod expands, and the embryo initiates a rapid growth phase. Both annual and perennial members of the genus are found in nature. The first peanut species described was A. hypogueu by Linnaeus in 1753. This species has two subspecies, each of which has two botanical varieties (Table I). Not until 1841 were wild species described, including A. villosu Benth., A . tuberosa, A. glabrata, and A . pusilla Benth. (Bentham, 1841). Taxonomic treatments of the genus were later completed by Chevalier

Table I Subspecific and Varietal Classificationof A . hypogaea Subspecies

Variety

Type

hypogueu

hypogaea

Virginia

hirsuta

Peruvian runner

fastigiuta

Valencia

vulgaris

Spanish

fastigiuta

Comments No floral axes on main stem; alternating pairs of floral and vegetative axes on branches; branches short, less hairy No floral axes on main stem; alternating pairs of floral and vegetative axes on branches; branches long, more hairy Floral axes on main stem; sequential floral axes on branches; little branched, curved branches Floral axes on main stem; sequential floral axes on branching; more branched, upright branches

H. T. STALKER AND J . P. MOSS

4

(1933, 1934, 1936), Hoehne (1940), and Hermann (1954). Currently, 22 species diagnoses have been published (excluding A . nambyquarae Hoehne, which is a cultivar of A . hypogaea, and A . batizogaea Krap. et Fern., which originated from a man-made hybrid), with another 12 names commonly used in the literature but not described (Table 11). The guidelines outlined by Resslar (1980) will be used in this paper even though proper diagnoses for many of the taxa have not been published. A complete revision of the taxonomy of the genus is greatly needed, and considering the vast number of new accessions collected within the past 10 years, perhaps 50 or more additional species will eventually be described. Compounding the problem of species names are those designations which have been incorrectly given to taxa in the literature. For example, A . nambyquarue Hoehne is not a separate species, but a cultivar of A . hypogaea. The name A . diogoi Hoehne (a species of section Arachis ) was incorrectly used for an unnamed section Erectoides species by Johansen and Smith (1956), and the names A . prostrata Benth. and A . marginata Gard. have been used incorrectly for many different accessions (Gregory et al., 1973). Since germplasm is widely distributed with collection numbers and names, and there is no written description of many species, misidentifications are easily made. Peanut researchers have associated collection numbers with taxa to circumvent partially the problem of nomenclature. Based on morphological comparisons, and to a lesser extent on crosscompatibility and pollen stainability of interspecific hybrids, Krapovickas (1969, 1973) and Gregory et al. (1973) proposed sectional classifications for the Arachis species. The classification suggested by Gregory et al. (1973) is more commonly adopted and will be followed throughout this review (Table 11). Although sectional names remain tentative because proper diagnoses have not been published, the groups are useful for defining general crossing relationships of taxa in the genus.

111.

PLANT COLLECTION AND MAINTENANCE

Taxa in the genus Arachis are widely distributed in South America from the Atlantic Ocean to the foothills of the Andes Mountains and from the mouth of the Amazon River in the North to approximately 34"s in Uruguay. Although the cultivated species usually prefers sandy, welldrained soil, wild species of Arachis are found in many habitats, including rocky areas, heavy soils, marshy areas, and even in running water (Valls et al., 1985). While many species proliferate in shaded areas, others prefer open and sunny environments. Plants are most often found in ecotypes such as rock outcroppings, broken forested areas, forest-grassland margins, or

Table I1 Taxonomic Subdivision of the Genus Arachif‘ Section Arachis nom. nud. Series Annuae Krap. et Greg. nom. nud. (2n = 2x = 20) A. batizocoi Krap. et Greg. A. duranensis Krap. et Greg. norn. nud. A . spegauinii Greg. et Greg. norn. nud. A . stenosperma Greg. et Greg. nom. nud. A . ipaensis Greg. et Greg. nom. nud. A . spinaclava

Series Perennes Krap. et Greg. norn. nud. (2n = 2x = 20) A. helodes Martius ex Krap. et Rig. A . villosa Benth. var. villosa A. villosa var. correntina Burkart [A. correntina (Burk.) Krap. et Greg. norn. nud.] A . diogoi Hoehne A. cardenasii Krap. et Greg. nom. nud. A . chacoense Krap. et Greg. norn. nud. Series Amphiploides Krap. et Greg. nom. nud. (2n = 4x = 40) A. hypogaea L. ( A . nambyquarae Horne) A . monticola Krap. et Rig. A . x batizogaea Krap. et Fern. (of experimental hybrid origin) Section Erectoides Krap. et Greg. norn. nud. (2n = 2x = 20) Series Trifoliolatae Krap. et Greg. norn. nud. A. guaranitica Chod. et Hassl. A . tuberosa Benth. Series Tetrafoliatae Krap. et Greg. norn. nud. A. benthamii Handro A. martii Handro A. paraguariensis Chod. et Hassl. A. oteroi Krap. et Greg. norn. nud. Series Procumbensae Krap. et Greg. norn. nud. A . rigonii Krap. et Greg. A. lignosa (Chod. et Hassl.) Krap. et Greg. norn. nud. Section Caulorhizae Krap. et Greg. nom. nud. (2n = 2x = 20) A . repens Handro A . pintoi Krap. et Greg. nom. nud. Section Rhizomatosae Krap. et Greg. nom. nud. Series Prorhizomatosae Krap. et Greg. nom. nud. (2n = 2x = 20) A. burkartii Handro Series Eurhizomatosae Krap. et Greg. norn. nud. (2n = 4x = 40) A . glabrata Benth. A. hagenbeckii Harms Section Extranervosae Krap. et Greg. nom. nud. (2n = 2x = 20) A . marginata Card. A . lutescens Krap. et Rig. A. villosulicarpa Hoehne A . macedoi Krap. et Greg. norn. nud. A. prostrata Benth. Section Ambinervosae Krap. et Greg. nom. nud. (2n = 2x = 20) (no species names, valid or invalid, have been given to forms in this section) Section Triseminalae Krap. et Greg. norn. nud. (2n = 2x = 20) A . pusilla Benth. Uncertain sectional affinity A . angustifolia (Chod. et Hassl.) Killip “After Gregory et al. (1973) and Resslar (1980).

6

H. T. STALKER AND J. P. MOSS

in disturbed habitats..They grow from sea level to approximately 1600 m in elevation. The largest number of taxa are found in the west central region of Brazil walls et al., 1985), with the second highest concentration found in Bolivia. Extensive genetic diversity exists in the genus for many traits of agronomic importance. Seeds of the cultivated peanut were among the earliest crops introduced to Europe from the New World and species have been periodically collected in South America since its first discovery. However, not until the late 1950s were concentrated efforts made systematically to collect and preserve variability in Arachis. This was largely due to the inaccessibility of many parts of South America and to the wide geographic distributions of peanut species. Twenty-four expeditions were organized between 1958 and 1983, and 639 wild species accessions plus 961 accessions of A . hypogaeu were collected walls et ul., 1985). Nearly a hundred wild species accessions have also been collected since 1983. Table I11 summarizes major germplasm collections of Arachis species. Priorities for future Arachis germplasm collection in South America for both cultivated and wild species of the genus have been established (Valls et al., 1985). The highest priority for collecting Arachis species is in the Brazilian states of Mato Grosso and Mato Grosso do Sul, and the second priority is for Paraguay. Although Bolivia also represents an important area for future collections, expeditions are currently not planned due to inaccessibility of some areas of the country. Germplasm resources of wild Arachis species are difficult to maintain due to specialized adaptations to many environments. For example, many species are adapted to arid climates, while others are found in wet habitats, and these extremes are difficult to duplicate. Many species accessions do not produce Table 111 Wild Arachis Accessions Collected and Conserved between 1936-1983O Conserved (1983) Section

Collected number

USA

Total

Ambinervosae Arachis Caulorhizae Erectoides Extranervosae Rhizomatosae Triseminalae Totals

20 171 17 345 99 209 9 -

8 150 11 77 45 93 5 -

159 17 82 72 110 9 -

870

389

464

“After Valls et at. (1985)

I5

SPECIATION, CYTOGENETICS, AND UTILIZATION

7

seeds when grown in the United States and, therefore, must be maintained as live plants. Many other accessions will produce seeds at one location and not another, so multiple germplasm storage facilities are required for seed increase and maintenance. Initiation of reproductive development in peanut species has not been adequately investigated, but many environmental factors probably influence pegging and pod development, such as photoperiod, heat, endogenous hormone levels, and plant stresses. A general trend in section Arachis species is profuse flowering in long-day photoperiods with a higher rate of peg formation in shortday photoperiods (Stalker and Wynne, 1983). However, several species [such as A. chacoense Krap. et Greg. nom. nud., A. correntina (Berk.) Krap. et Greg. nom. nud., and A. villosa] produce few to no flowers under shortday conditions. Investigations are urgently needed to find methods to induce seed set because of the expense associated with propagating germplasm collections as vegetative plants plus the required duplications at several locations to ensure long-term survival of accessions under cultivation.

IV.

CENTERS OF ORIGIN

The center of orgin for Aruchis species was most likely in central Brazil (Gregory et al., 1980). The geocarpic habit of the plant suggests that longdistance dispersal has been along water courses. Gregory et al. (1973) presented a theory that the most ancient species were found at high elevations and more recent speciation has occurred as seeds were washed down toward the sea and became isolated. To support this view, they noted that many species are adapted to highland conditions by having tuberoid roots, tuberiform hypocotyls, or rhizomes. Further, as seeds moved to lower elevations they became isolated in major river valleys and different sections of the genus evolved in parallel evolution. Although species in different sections of the genus were once believed to be isolated, considerable overlaps in distributions occur, especially for members of the sections Arachis, Erectoides, Extranervosae, and Rhizomatosae (Valls et al., 1985). Since the major sectional groups of the genus have widespread distributions, species most likely diverged early in the evolutionary history of the genus and subsequently distributed along watersheds. The cultivated species A. hypogaea probably originated from a wild allotetraploid species (Smartt and Gregory, 1967). Arachis monticola Krap. et Rig. is the only tetraploid known to be cross-compatible with A. hypogaea and the most likely direct progenitor. Since this species is found only in the southern Bolivia-northern Argentina region, this is the region of the presumed center of origin for the cultivated peanut (Krapovickas,

H. T. STALKER AND J. P. MOSS

8

1968). Although the tetraploid progenitor species is generally considered to be A. monticola, much speculation has centered around designating the diploid species which gave rise to the allotetraploid. Krapovickas et al. (1974) indicated that A. butizocoi Krap. et Greg. is one of the diploid progenitors and the species is now considered to be the donor of the B genome of A. hypogaea (Smartt et al., 1978a,b; Smartt and Stalker, 1982). The donor of the A genome is more elusive, however, and several species have been suggested, including A. villosu (Varisai Muhammad, 1973), A. duranensis Krap. et Greg. nom. nud. (Seetharam et al., 1973; Gregory and Gregory, 1976) and A. curdenusii Krap. et Greg. nom. nud. (Gregory and Gregory, 1976; Smartt et ul., 1978a). Because of distribution patterns and probable centers of origin of the cultivated peanut, diploid species of section Aruchis, now found far from the Bolivia-Argentina region, can most likely be eliminated as possible direct ancestors. However, as many unique taxa have been collected in Bolivia, and many more are probably still to be found, the donor of the A genome may await discovery. In addition to the primary center of origin, five secondary centers of variability exists for the cultivated species in South America (Gregory and Gregory, 1976; Wynne and Coffelt, 1982). Africa represents another center of diversity for the cultivated peanut (Gibbons et al., 1972).

V.

CYTOGENETICS OF Arachis SPECIES

The chromosome number of 2n = 40 was first reported by Kawakami (1930) for A. hypogaea. Husted (1931, 1933, 1936) confirmed the ploidy level and analyzed the meiotic and somatic chromosomes of seven cultivars. The meiotic chromosomes of A. hypogueu pair mostly as 20 bivalents, but a few multivalents have also been observed (Husted, 1936). Hybrids among subspecific accessions have mostly bivalents at metaphase I, but univalents also exist at a low frequency. Husted (1936), Raman (1976), and Stalker (1980b) concluded that chromosome structural differences exist between the subspecies hypogueu and fmtigiuta. Further, Gregory et al. (1980) observed reduced fertility in hybrids between subspecies, and genetic differences have been reported between the subspecies hypogaea and fustigiuta (Krapovickas, 1973; Wynne, 1974). The somatic chromosomes of A. hypogueu are small and most have a median centromere. Husted (1933, 1936) analyzed somatic chromosomes of several cultivars and distinguished a pair of small chromosomes, which he termed “A” chromosomes, and one pair with a secondary constriction, which he termed “B” chromosomes. Babu (1955) reported several types of secondary constrictions in A. hypogueu, and cultivars can be distinguished

SPECIATION, CYTOGENETICS, AND UTILIZATION

9

based on karyotypic differences (D’Cruz and Tankasale, 1961; Stalker and Dalmacio, 1986). At least I5 of the 20 chromosome pairs have been distinguished and, based on arm ratios and chromosome lengths, Stalker and Dalmacio (1986) were able to separate members of different botanical varieties based on somatic chromosome morphology. Analyses of somatic chromosomes support previous investigations with meiotic chromosomes of A . hypogaea, which illustrated cytological variation between subspecies. Aneuploidy was first observed in A . hypogaea by Husted (1936), who observed a plant with 41-chromosomes plus a chromosome fragment. Other naturally occurring aneuploids were observed by Spielman et al. (1979) and Stalker (198%) after observing somatic chromosomes of plants propagated from small seeds. Eight different trisomics or double trisomics (2n + 1 + 1) were cytologically verified by Stalker (198%). Chemical treatments (Ashri et al., 1977) or ionizing radiation (Patil and Bora, 1961; Patil, 1968; Madhava Menon et al., 1970) have also produced aneuploid plants. In addition, aneuploids are commonly observed after interspecific A . hypogaea hybrids are colchicine-treated (Smartt and Gregory, 1967; Spielman et al., 1979; Company et al., 1982). Davis and Simpson (1976) reported chromosome numbers ranging from 32 to 48 in derivatives of a 6x (A. hypogaea x A . cardenasii) hybrid. Wild species of Arachis were not analyzed cytologically until the late 1940s. A tetraploid (2n = 40)species, A . glabrata, was reported by Gregory (1946), and Mendes (1947) later observed four diploid species in the genus. Only 26 of 33 named species have chromosome numbers confirmed in the literature (Smartt and Stalker, 1982). Published information on the group is highly inadequate; however, judging from unpublished work in several laboratories and inferences from Gregory and Gregory (1979), most species in the genus are diploid (2n = 20). Polyploid (2n = 40) species are also found in sections Arachis and Rhizomatosae, and Smartt and Stalker (1982) concluded that polyploidy evolved independently in the two groups. Analyses of pollen mother cells (PMCs) indicate that chromosomes of diploid species pair mostly as bivalents (Raman, 1976; Resslar and Gregory, 1979; Smartt et al., 1978a,b; Stalker and Wynne, 1979; Singh and Moss, 1982), but quadrivalents have also been observed at a low frequency in the diploid species A . villosa and A . spegazzinii Greg. et Greg. nom. nud. (Singh and Moss, 1982). In polyploids of a section Rhizomatosae species, Raman (1976) reported up to four quadrivalents in PMCs. A second accession reported by Stalker (1985b) averaged 19.92 bivalents and only 0.04 quadrivalents per PMC. In addition to analyses of chromosome pairing at meiosis, Kirti et al., (1983) and Jahnavi and Murty (1985a,b) analyzed the pachytene chromosomes of species in sections Arachis, Erectoides, Extranervosae, Rhizomatosae, and Triseminafaeand distinguished chromosome pairs. Although chromosomes

H.T.STALKER AND J. P.MOSS

10

did not stain well, Jahnavi and Murty (1985b) concluded that six different chromosomes, three specialized chromosomes, and one nucleolus organizer chromosome are present in species of different groups. Most karyological analyses in Aruchis have used species in section Aruchis, in which nine species have been analyzed (Stalker and Dalmacio, 1981; Singh and Moss, 1982; Stalker, 1985a). Genomes of most species are symmetrical with median chromosomes. Although somatic chromosomes are small, ranging from 1.4 to 3.9 mm in length, species can be identified based on karyological differences. By using arm ratios as variables, species of section Aruchis can be divided into clusters in which species with an A genome group together and species with chromosomes typical of the species A. butizocoi (B genome) separate into a second cluster (Singh and Moss, 1982). Further, A. spinucluvu has a highly asymmetrical karyotype with subtelocentric chromosomes not found in other species of the group (Stalker, 1985a). Smartt (1964) reported that the distinctively small chromosome pair found in most section Aruchis species was not present in a section Erectoides species, A. puruguuriensis Chod. et Hassl., which implies that the karyotype of section Erectoides species may be differentiated from chromosomes of most section Aruchis species.

VI.

INTERSPECIFIC HYBRIDIZATION IN Arachis

Interspecific hybrids in the genus were first attempted by Hull and Carver (1938) when they tried to cross A. hypogueu and A . glubrutu, a species now known to be distantly related to the cultivated peanut. The first successful hybrid reported in the genus was between A. hypogueu and the diploid species A. villosu var. correntinu in 1951 (Krapovickas and Rigoni, 1951). The cultivated species has since been hybridized with at least 12 and possibly as many as 18 species of the genus (Kumar et ul., 1957; Smartt and Gregory, 1967; Gregory and Gregory, 1979; Singh, 1985; Singh and Moss, 1984b; Pompeu, 1977; Stalker, unpublished data). To date, the only hybrids between the cultivated peanut and wild species have been with members of section Aruchis. Since many accessions have been introduced recently from South America, this conclusion must be verified using a wide range of variability in the genus, but results thus far have generally coincided with the crossing relationships established by Gregory and Gregory (1979) for sectional groups. Although there are reports of crosses between A. hypogueu and members of other sections, these crosses have not been repeated and thus will not be discussed further in this chapter. Raman and Kesavan (1962) reported the first hybrids among wild species in the genus between A. durunensis and A . villosu var. correntinu. Hybrids

I

SPECIATION, CYTOGENETICS, AND UTILIZATION

11

were fertile and had regular chromosome pairing during meiosis. Since the first hybrid was reported, hundreds of interspecific crosses have been produced to determine the biosystematic relationships among species or to introgress germplasm to cultivated peanut. The most extensive single hybridization program conducted thus far was by Gregory and Gregory (1979), who reported cross-compatibility relationships among 91 accessions of Aruchis species. They showed that intrasectional hybrids are much easier to produce than intersectional ones, but low frequencies of success are still observed for many hybrid combinations within groups. Gregory and Gregory (1979) determined relationships among taxa based on both crossability and pollen stainability data. They found that pollen stainability of intrasectional hybrids of section Arachis averaged 30.2% when crosses were made among species at the same ploidy level. Intrasectional hybrids among species within other groups ranged from a low of 0.2% in section Extrunervosue to a high of 86.8% in section Cuulorhizae. All intersectional hybrids were completely female-sterile and averaged only 1.9% pollen stainability (Gregory and Gregory, 1979). Since A . hypogaeu belongs to section Aruchis, researchers have concentrated efforts within this section. Most interspecific hybrids between species with an A genome have 10 bivalents during meiosis (Resslar and Gregory, 1979; Smartt et a/., 1978a,b; Stalker and Wynne, 1979; Singh and Moss, 1984a). Perennial species of the group generally hybridize more easily as male rather than as female parents. Although meiosis is regular, pollen stainability ranges between 20 and 85% and seed production is limited for several hybrid combinations. In contrast to hybrids between A genome species, when crosses are made between A. batizocoi (B genome) and other members of section Aruchis, all hybrids are sterile and have irregular meioses with a range of 4.6-8.6 bivalents per PMC (Gibbons and Turley, 1967; Smartt et a/., 1978a,b; Stalker and Wynne, 1979; Singh and Moss, 1984a). When the species A . spinucluva (D genome) is hybridized with either A or B genome species, all hybrids are sterile and meiotically irregular (Stalker, 1985a). Many other recently collected taxa must also be analyzed cytologically and, based of fertility data of F, hybrids, additional unique genomes may be found in the group. Because of high levels of sterility in intersectional hybrids between diploid species, crosses have been attempted after raising the ploidy level of species or their hybrids. All attempted crosses between amphidiploids of section Arachis species and amphidiploids or natural tetraploids of species in other sections (Erectoides or Rhizomatosae) have failed. Hybridization at the tetraploid level is more difficult than between diploids and tetraploids for at least some groups of the genus. For example, the two diploid (2n = 20) section Arachis species A . durunensis and A. stenosperma have been hybridized with the 40-chromosome amphidiploids (A. rigonii x A . sp. coll. GKP

12

H.T.STALKER AND J. P. MOSS

9841, PI 262278) of the section Erectoides (Stalker, 1981). A high frequency of bivalents was observed and Stalker concluded that chromosome homologies exist among members of sections Arachis and Erectoides. Complex hybrids between sections Erectoides and Rhizomatosae have also been cytologically analyzed and chromosome homologies reported for at least one hybrid combination (Stalker, 1985b). Also, plants of one 40-chromosome intersectional Erectoides x Rhizomatosae hybrid combination were male-fertile and produced selfed seeds. Several triploid hybrids between section Arachis (2x) and (Erectoides x Rhizomatosae)(4x) have also been made but all hybrids failed to flower even though they had been propagated for several years (Stalker, 1985b). Based on the cumulative cross-compatibility data of interspecific hybrids by many investigators, a series of genomes for Arachis species were proposed by Smartt and Stalker (1982) and Stalker (1985b) as follows: A: B: D: Am: C: E: Ex: T: R:

section Arachis, perennials and most annuals section Arachis (A. batizocor) section Arachis (A. spinaclava) section Ambinervosae section Caulorhizae section Erectoides section Extranervosae section Triseminalae section Rhizomatosae, series Prorhizomatosae

Arachis hypogaea and A . monticola have an AB genome, while the genomes of tetraploid species in section Rhizomatosae may be similar to the A genome of section Arachis and the E genome of section Erectoides. Only the A, By and D genome of section Arachis have been studied intensively, and other genomic designations in the genus remain to be verified cytologically. However, even in section Arachis there are unanswered questions, such as the real differentiation between designated A and B genomes. Based on cytological analyses, only two to four chromosome pairs are differentiated between A . batizocoi and A genome species (Stalker and Wynne, 1979; Singh and Moss, 1984a). Triploid hybrids between A . hypogaea and diploid species have a few trivalents, but hexaploids obtained after colchicine treatment average six or more univalents and may have as many as 20 unpaired chromosomes (Company et al., 1982; Singh, 1985). Pairing mechanisms, or lack thereof, are apparently under genetic control. Further, after backcrossing hexaploids with A . hypogaea, pentaploids are produced with the expected 20 bivalent plus 10 univalents, but after one generation of self-pollination, 25 bivalents have been observed in some progenies (Stalker, unpublished data). This indicates that considerable homology exists among the A and B genomes.

SPECIATION, CYTOGENETICS, AND UTILIZATION

13

Genomic designations outside of section Aruchis are based mostly on cross-compatibilities. Since incompatibilities may result from single genes, cytoplasmic effects, or other factors, there may be considerable homology among the genomes which have been designated as unique. Several problems also remain unanswered for groups of species. For example, why will diploid Prorhizomutosue not hybridize with tetraploid members of the same section when taxa from other sections will hybridize with the tetraploid rhizomatous species? A D genome has been designated for a species which is morphologically identified with members of section Aruchis, but the taxa may be genomically more similar to species in other sections. Regardless, sectional names are useful for communication concerning groups of species and, from present knowledge, potentials for utilizing species in the genus can be determined. Germplasm pools can be also designated for establishing potentials for introgression to A . hypogueu. The primary gene pool comprises A. hypogueu accessions and genetic stocks plus the closely related tetraploid species A . monticolu. Large collections of the cultivated species exist in the United States (cu. 4000 accessions) and at the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), which has more than 8000 lines (Wynne and Coffelt, 1982). Although several accessions of A . monticolu have been cataloged in germplasm lists, they all represent collections from at most two sites in South America. Aruchis hypogueu will hybridize with A . monticolu and produce fertile hybrids which have normal meiosis (Krapovickas and Rigoni, 1957; Raman, 1958). Analyses of somatic chromosomes have confirmed the close relationship between the species, indicating that they belong to the same biological species. The secondary gene pool is represented by diploid members of section Aruchis which have an A or B genome. Hybrids between the diploid and tetraploid species of the group are sterile, but fertility can be restored by manipulating ploidy levels and good evidence exists for homology between the chromosomes of wild and cultivated species (Singh, 1985; Stalker, 1985b). Although A . butizocoi is the most likely representative donor of the B genome in A . hypogueu, identification of the A genome donor species has not been made. However, enough similarities exist between A . hypogueu and most of the A genome species of section Aruchis that gene transfer can occur from wild taxa to the cultivated peanut. The tertiary gene pool includes all taxa outside section Aruchis plus species of section Arachis which do not have an A or B genome (for example, A. spinucluvu). Hybrids between A . hypogaeu and these species have not been produced and specialized techniques will be required to produce hybrids. However, F, generation plants are expected to be completely sterile and methods to introgress small chromosome segments will be necessary for utilization of these germplasm resources.

H. T. STALKER AND J. P. MOSS

14

VII.

GERMPLASM EVALUATION

Identification of desirable traits, especially for disease and insect resistances, in Arachis species must precede utilization of germplasm resources. Disease and insect resistances have had the highest rates for successful introgression from wild species to many crop plants (Watson, 1970; Knott and Dvorak, 1976). Likewise, the most commonly investigated agronomic traits in wild species are pest resistances. The peanut is plagued by a large number of pests, many of which are now worldwide in distribution. Because of the agronomic importance and impact of diseases and pests on yield and quality, introgression of disease resistance from wild species to cultivars has been a high priority in many breeding programs. A.

DISEASE RESISTANCES

The three most important diseases of A . hypogaea worldwide are Cercospora arachidicola Hori (early leafspot), Cercosporidium personatum (Berk. et Curt.) Deighton (late leafspot) and Puccinia arachidis Speg. (peanut rust). Subrahmanyam et al. (1984) estimated that in India, which is one of the largest producers of peanuts, yield losses due to rust and leafspots are approximately 70% annually. Gibbons (1980) estimated that production in locations where fungicides are not used, largely because of high chemical costs, have yield decreases of approximately 50%, and even when chemicals are applied yield may be decreased by 10% (Jackson and Bell, 1969). In addition to actual production losses directly due to the diseases are costs of chemicals, application expenses, and plant damage incurred during applications. Although only one of the leafspots may be common at a particular location during the year, the disease populations may change over years as cultivars are replaced (Smith and Littrell, 1980). Many Arachis species have been evaluated for resistance to the C. arachidicola pathogen (Table IV). The three species A . glahratu, A. hagenheckii, and A . repens have high levels of resistance to this pathogen (Gibbons and Bailey, 1967). Abdou et al. (1974) screened 94 species accessionsin the greenhouse and found members of sections Arachis (A. chacoense GKP 10602), Caulorhizae(A. repens GKP 10538), Extranervosae (A. villosulicarpa, three accessions), and Rhizomatosae (A. sp. GKP 10596) to be immune to C. arachidicola. Melouk and Banks (1978) confirmed the immune reaction of A . chacoense, but Foster et al. (1981) and Company et al. (1982) observed small lesions on leaves of field-grown plants. Kolawole (1976) reported high levels of resistance in a second section Arachis species which Sharief et al. (1978) concluded was the A . stenosperma Greg. et Greg. nom. nud., collection HLK410. Because evaluations of both cultivated and wild species at different locations

SPECIATION, CYTOGENETICS, AND UTILIZATION

15

had been done with different techniques and disease pressures, Foster et a/. (1981) compared 9 section Aruchis species with 14 reportedly resistant cultivated genotypes. They confirmed a high level of resistance in A . stenospermu (HLK 410) and found that A . chucoense had significantly fewer lesions per leaf than any other speciestested. Sharief et a/. (1978)reportedthat resistance in Aruchis species is multigenic, while Company et a/. (1982) found triploid interspecific hybrids between A . hypogueu and the Aruchis speciesA . curdenusii and A . chucoense to be resistant to early leafspot in the field. This indicated at least partial dominance for resistance. Cercosporu uruchidicoluresistance appears to have been introgressed from the wild diploid speciesA . curdenusiito A . hypogueu (Stalker, 1984). Late leafspot (C. personutum) is the most severe peanut disease in many production areas. Abdou et a/. (1974) screened the same 94 accessions mentioned previously for this pathogen in the greenhouse. They reported high levels of resistancein several taxa of sectionsAruchis, Cuulorhizue,Extranervosue, and Rhizomutosue. Aruchis curdenusii was the only speciesin the accessions which is both crosscompatible with A. hypogueu and immune; Subrahmanyam eta/. (1980) confirmed the reactions in separate experiments. Kolawole (1976) reported high levels of resistance in a second species, now believed to be A . stenospermu. Resistance has recently been selected in 40-chromosome interspecific hybrid derivativesof A . hypogueu x sectionAruchis species (Moss, 1985b).

Because populations of leafspotscan change, breeding for only one pathogen and not the other may be futile. Fortunately, high levels of resistance have been found in several genotypes to both early and late leafspots (Abdou et a/., 1974; Kolawole, 1976). In addition, A . curdenasii (which is reported as resistant to C. personutum but susceptible to C. uruchidicolu) has at least moderate levels of resistance to the early leafspot pathogen for lesion number and lesion size, but not for genes conditioning defoliation (Foster eta/., 1981). In addition to fungal pathogens, peanuts are invaded by many viruses. Several of these cause severe damage and yield loss such as bud necrosis and peanut stunt, while others expressonly mild symptomswhich may have little effect on yield. A comprehensivereview of peanut diseases was made by Porter et a/. (1982), who discussed descriptions of causal organisms, symptoms, disease cycles, and controls. This chapter will thus be restricted to reports directly related to wild Aruchis species. Groundnut rosette virus is restricted to Africa south of the Sahara and may be the most destructive virus disease of peanut (Porter et a/., 1982). While resistancehas been found in the AruchisspeciesA . glubrutu and A . repens (Gibbons, 1969), the species A . glubrutu and A . prostrutu Benth. (identified as A . hugenheckii and A . repens by Gibbons, 1969) were reported as being symptomless carriers of the virus by Klesser (1967). Because resistance was found only in species which will not hybridize with A . hypogueu, and at about the same time adequate levels of resistance were also found in the cultivated accessions,

Table IV Pest Resistnnce in Wild Arachh Species" _

_

Collection

e a\

_

~

COllectOP

Section Ambinervosae 12943 GK 129445 GK Section Arachb 408 HLK 410 HLK 7264 K 7830 K 7897 K 7988 K 9484 K 9530 GKP 953 1 GKP 9548 GKP 9901 10017 10038 10602 22585 30006 3001 1 3003 1 30035 30063 30085

A . correntina Manfredi #5 Manfredi #8 Manfredi #36 A. villosa

GKP

GKP GKP GKP Bu GK GK GK GK GKBSPSc GKBSPScZ

-

Species

PI 338452 338454

A . sp. A . sp.

338279 338280 219824 261871 262873

A . stenosperma A . stenosperma A . monticola A . correntina A . correntina A . duranensb A . batizocoi A . correntina A . correntina A . correntina A . sp. A . cardenasii A . spegauinii A . chacoense A , villosa A . sp. A . sp. A . helodes A . sp. A . sp. A . sp. A . correntina A . correntina-villosa A . correntina-villosa A . correntina-villosa A . villosa

-

298639 262808 262809 262839 262270 262141 262133 276235 298636 468 150 468 154

-

468168 468199 468331

-

-

C.a

C.p

R

PSV

Th

PHL

CEW

SM

LCSB

HR HR

I HR

HR HR

S

S S

HR HR

HR HR

S S S S

MR MR

-

-

HR

HR

HR HR

-

R HR

-

-

R R I R R

-

S

-

s

-

-

MR

MR

MR S

S

s

HR MR HR R

HR

s

I I I I

-

-

I I I

-

HR R -

-

-

R

s

-

S

R S

-

I

I I HR HR

-

s

-

I I

R I R R S

-

I

S

I HR

-

I HR HR

-

-

MR

HR HR HR

-

-

-

HR R HR R

HR HR HR HR

HR HR HR HR

S

S

MR

S S S

S

MR MR

MR MR R R

Section Caulorhizae 10538 12787

Section Erectoides

e

4

565-66 9646 9764 9769 9788 9795 9812 9820 9825 9835 9837 9841 9990 9993 10002 10034 10541 10543 10573 10574 10576 10580 10582 10585 10588 11462 11488 14444

GKP GK

GKP GKP GKP GKP GKP GKP GKP GKP GKP

GKP GKP GKP

GKP GKP GKP GKP GK GK GKP GK GK GK GK GK KFC

276199 338447 338297 262842 262859 262862 262790 262863 26279 1 263 105 262278 261877 261878

-

262142 276208 276209 276225 276226 276228 276229 276230 27623 1 276232

KC KHe

338320

A . repens A . pintoi

s -

s -

-

I

I

s

-

HR

-

HR

-

-

S

-

A. paraguariensis A . benthamii

A . benthamii A . sp. A. sp. A. sp. A. sp. A. A. A. A.

sp. sp. sp. sp. A . sp.

A . sp. A. sp. A. rigonii A. oteroi A . sp. A. sp.

A. sp. A. sp. A. sp. A. sp. A. paraguariensis A. sp. A . paraguariensk A. paraguariensis A. sp.

R R MR MR H R R MR MR

-

I I

-

-

s s

s s

-

R R

MR R

-

s

-

-

-

HR HR

S S

R R

HR HR I

HR HR HR

S S

-

-

-

-

R R

I

MR MR

-

MR s

HR R

-

I H R - HR - HR S HR

-

-

-

-

I R S I

-

- MR s - - s R s

-

-

-

-

-

-

I

-

-

R

S

-

R

S

-

-

I

HR

HR

I

I

HR

MR -

-

-

-

HR HR

-

-

-

R

-

-

-

-

I

HR

I

S

-

-

MR

-

S

S

MR

R

R

-

(continued)

Table IV (Continued) Collection

L

00

Section Extranervosae 9906 10127 10406 A. villosulicarpa 1960 #3 1968 #lo0 9553 9562 9564 9566A 9566B 9567 9568 9569 9570 957 1 9572 9574 9575 9576 9578 9580 9587 9591 9592 9610 %1OB %I8 %29 %34 %42

CoUector”

PI

GKP GKP GKP

262272 276203 276198 336985

GKP GKP GKP GKP GKP GKP GKP GKP GKP GKP GKP GKP GKP GKP GKP GKP GKP GKP GKP GKP GKP GKP GKP GKP GKP

-

262801 26281 1 262812 262813 262814 2628 15 2628 16 262817 2628 18 262819 262820 262821 262822 262824 262825 262826 262827 262828 262832 262832

-

262834 262836 262839

A. A. A. A. A. A. A. A. A. A. A. A. A. A. A. A. A. A. A. A. A. A. A. A. A. A. A. A.

Species

C.a

C.p

R

PSV

Th

lutescens macedoi margenata villosulicarpa

R R

HR

-

-

-

S

S

-

-

-

sp. sp. sp. sp. sp. sp. sp. sp. sp. sp. sp. sp. sp. sp. sp. sp. sp. sp. sp. sp. sp. sp. sp. sp. A. sp. A. sp. A. sp.

HR -

-

-

HR -

HR R

MR HR

S S S

S S S

HR HR MR HR MR HR

HR HR MR R MR MR

S

S

HR R

R HR

S S

S S

R

MR

MR HR HR

MR MR MR

S

S MR R

-

-

MR MR

-

I

MR -

I

-

HR

-

I I I

-

-

I

-

-

I I

-

-

I

-

I

-

I

I

I -

-

I I I I I

I I

-

PHL

R HR I

s I

R I I I I I I

s

I

HR

-

-

-

R

-

HR HR -

HR -

-

-

HR HR

-

-

CEW

SM

LCSB

L

rD

9644 9645 9649 9664 9667 9797 9806 9813 9815 9822 9827 9830 9834 9882 9893 992 1 9922 9925 9935

9%6 10105 10120 10550 10559 10566 105% A. glabrata-B,

GKP GKP GKP GKP GKP GKP GKP GKP GKP GKP GKP GKP GKP GKP GKP GKP GKP GKP GKP GKP GKP GKP GK GKP GK GK

-

262840 262841 262844 262847 262848 262807 262792 262793 262794 262795 2627% 262797 262798 262286 262287 2622% 262297 262299 262301 262306 276200 276202

-

276217 276223 276233

-

sp. sp. sp. sp. sp. sp. sp. sp. sp. sp. glabrata glabrata sp. sp. sp. sp. sp. A. sp. A . sp. A. sp. A . sp. A. sp. A. sp. A. sp. A. sp. A. sp. A. glabrata A. A. A. A. A. A. A. A. A. A. A. A. A. A. A. A. A.

Section Trbeminalae 12922

GKP

338449

A. pusilla

HR HR

MR MR R

S S

MR

-

-

S

MR HR S S S

s

I MR I

-

-

HR HR

HR HR

I I I I

R

HR

I

I

1

-

-

I I

I I R

I I

-

-

MR

s

-

HR R HR HR HR MR MR

S S HR R R MR MR R MR

-

S

s

-

MR

MR

-

-

S S

s

-

-

HR MR

-

-

I I I I

S

R S

-

1

I I I I I I

-

-

I I I

-

I

-

-

-

HR -

I I

I I

HR HR

I HR HR HR

-

-

HR I HR HR

HR HR HR HR

-

-

HR HR HR

HR I

R HR

HR

HR R HR

-

-

-

I

I

S HR MR

-

MR HR

-

I I I

-

-

-

-

HR HR

-

I I -

I

I

-

R R R R R MR R MR

-

HR

MR

MR

%sect or disease: C.a, Cercospora arachidicola; C.p, Cercosporidiumpersonatum; R, rust (Pucciniaarachidis);PSV, peanut stunt virus; Th, thrips (Frankliniellafusca);PHL,potato leafhopper (Empoascafabae); CEW, corn earworm (Heliothb zea); SM. spider mite (Tetranychus urticae); LCSB, lesser cornstalk borer (Elasmopalpuslignosellus).Rating: S , susceptible,MR, moderately resistant; R, resistant, HR, highly resistant; I, immunity (based on authors’ interpretation of literature cited in text). bCollectors: B, Banks; Bu, Burkart; C, Cristobal; F, Fugarazzo; G, Gregory; H, Hammons; He, Hemsy; K, Krapovickas; L, Langford; P, Pietrarelli; S, Simpson; Sc, Schinini; Z. Zurita.

20

H.T. STALKER AND J. P. MOSS

little screening or attempted utilization of wild species has occurred. Highyielding rosette-resistant cultivars have been released for grower use in Africa (Gillier, 1978). Tomato spotted wilt virus, the causal organism for bud necrosis disease, is widespread in many peanut production areas and may cause up to 90% yield loss (Saint-Smith et al., 1972). Ghanekar (1980) screened approximately 7000 A . hypogaea accessions and did not find field resistance to the disease. Subrahmanyam et al. (1985) inoculated 42 Arachis species in the greenhouse and field. The species A . pusilla GK 12922, A . correntina GKP 9530, and A . cardenasii GKP 10017 became infected in the greenhouse but expressed no symptoms in the field. Arachis chacoense GKP 10602 showed infection neither after mechanical injection of the virus nor after infection by thrips (although virus was detected after grafting), and this species may represent the best source of resistance to bud necrosis. Since A . chacoense has been hybridized with A. hypogaea, it should serve as a usable source for resistance in a peanut breeding program. Peanut stunt virus was reported in Virginia during 1964 (Miller and Troutman, 1966), and epidemics occurred in the following years. Since then, stunt virus has been found in other production regions in the United States, Japan, and Africa (Porter et al., 1982). Hebert and Stalker (1981) screened approximately 4OOO cultivated accessions and all were susceptible to the virus. However, after evaluating 90 Arachis species accesions, they found high levels of resistance in species of sections Arachis, Caulorhizae, Erectoides, and Rhizomatosae (Table IV). Again, the most accessible species were the highly resistant sources A . duranensis K 7988, A . villosa B 22585, and an A . correntina-villosa genotype (Manfredi #8), and several others with high tolerance levels [A. correntina K 7897 and GKP 9548 and two A . correntina-villosagenotypes (Manfredi #5 and #36)] found in section Arachis. Hebert and Stalker (1981) also reported that resistance is not conditioned by a single dominant gene, so introgression may be difficult from wild to cultivated species. Because peanut stunt virus has effectively been controlled through cultural practices and incidence has been insignificant during the past 10 years, no concentrated efforts have been made to transfer genes conferring stunt virus resistance from the wild to cultivated species. Peanut mottle virus is found worldwide and can infect almost every plant in a peanut field. Demski and Sowell (1981) concluded that economic losses due to the virus are second only to leafspots in the southeastern United States. Kuhn et al. (1968) screened more than 450 cultivated accessions but did not find usable levels of resistance. Later, Kuhn et al. (1978) identified two tolerant A . hypogaea accessions. Demski and Sowell (1981) evaluated seven species accessions of section Rhizornatosae and six of these were immune to peanut mottle virus. Subrahmanyam et al. (1985) screened an additional 50 Arachis accessions and found no infection after mechanical or airbrush inoculations in the species A . pusilla GK 12922, A . chacoense GKP 10602, A .

SPECIATION, CYTOGENETICS, AND UTILIZATION

21

cardenasii GKP 10017, and A . correntina GKP 9530. In addition, A . pusilla and A . chacoense were not infected after grafting infected scions onto their stems; therefore, these two species accessions may have true immunity. Fitzner et al. (1985) evaluated 14 species of section Arachis for the soilborne disease Cylindrocladium black rot, caused by Cylindrocladium crotalariae (Loos) Bell and Sobers. Resistance was reported only for the species A . monticola GKBSPSc 30062. They indicated that a valuable source of resistance for developing cultivars may have been found because A . monticola produces fertile hybrids with A . hypogaea. Another soil-borne problem for peanut production in many areas is nematode infestations. In some regions of the world, peanuts cannot be grown without nematode controls (Porter et al., 1982). Nematodes may also be associated with high levels of aflatoxin and with other soil-borne diseases (see Porter et al., 1982). Meloidogyne hapla Chitwood (northern root knot nematodes) are the most important species which attack peanut. Of 33 Aruchis accessions evaluated by Banks (1969), only accession PI 262286 (of section Rhizomatosae) had moderate levels of resistance. Castillo et al. (1973) evaluated 12 wild species accessions and, in addition to confirming resistance in PI 262286, reported three additional PIS, 262841, 262814, and 262844, as being more resistant than the cultivated controls. B.

INSECTRESISTANCES

Insects can cause severe yield losses in peanut by feeding on all plant parts. In addition, insect specieshave been shown to bevectors for viruses (see Smith and Barfield, 1982). Surveys have not been taken to establish the economically important pests in many production areas, but Smith and Barfield (1982) listed 360 insect species which attack peanuts. The lesser cornstalk borer [Elasmopalpus lignosellus (Zener)] is the most severe subterranean insect pest in the United States, and the southern corn rootworm (Diabrotica undecimpunctata howardii Barber) also has a wide distribution. Aboveground foliage insects-including tobacco thrips (Frankliniellafusca Hinds), potato leafhoppers, (Empoasca fabae Harris), corn earworms (Heliothis zea Bodie), and fall armyworms (Spedopterafragiperda J. E. Smith) are among the most severe insect pests of peanuts in the United States. Many insects move into peanut fields in successive waves. Thrips arrive early in the growing season, followed by corn earworm invasion at peak bloom and then by potato leafhopper migrations (Campbelland Wynne, 1980). In the semiarid tropics, the predominant species of insect pests include the groundnut aphid (Aphis craccivora Koch), thrips [Scirtothripsdorsalis Hood, Caliothrips indicus Bagnall, Frankliniella schultzei (Trybom), F. fusca, and Ennoethripsflavens Moulton], jassids (Empoascasp.), armyworms (Spodoptera sp.), and termites (Microtermes sp. and Odontotermes sp.) (Amin, 1985).

22

H. T. STALKER AND J . P. MOSS

A large number of Aruchis accessions have been screened for insect resistances including resistance to thrips, leafhoppers, and corn earworms (Stalker and Campbell, 1983);armyworms (Lynch et ul., 1981);lesser cornstalk borers (Stalker et ul., 1984); and spider mites (Leuck and Hammons, 1968;Johnson et ul., 1977). High levels of resistance have been found for the first four insect pests listed above, whereas lower (but still high) levels of resistance have been reported for the lesser cornstalk borer and spider mites (Table IV). Most importantly, resistances are found in species which will hybridize with A. hypogueu. Stalker and Campbell (1983)indicated that the mechanism of resistance to H. zeu in Aruchh species was antibiosis. In attempts to utilize the resistances for insect pests, 40-chromosome interspecific hybrid derivatives with A. hypogueu were evaluated and high levels of resistance for several insect pests were found (Stalker and Campbell, 1983;ICRISAT, 1985).

C. OTHERTRAITS Aruchis species are potentially valuable germplasm resources for traits other than disease and insect resistances. For example, peanut seed protein is usually low in the sulfercontaining amino-acids and tryptophan. Aruchis villosulicurpu is high in tryptophan (range of 1.44-1.66Vo)as compared to the highest A. hypogueu line (1.41%) tested by Amaya et al. (1977). Many of the Aruchis species are extremely drought resistant and research in the area of physiological traits needs to be conducted. In addition to seeds and vegetative plants collected on exploration trips in South America since 1976,nodules have also been obtained from many accessions. Brudyrhizobium have been isolated and maintained from both cultivated and wild Aruchis species (Elkan et ul., 1981). Many strains have been evaluated for plant-Rrudyrhizobium interactions; and one strain, NC 92, increased peanut yields under field conditions when applied to the cultivar Robut 33-1 in India (ICRISAT, 1983).

VIII.

UTILIZATION OF WILD Arachis SPECIES

Wild species of Aruchis are a valuable source of desirable characters for cultivar improvement. Especially when the wild species are the only source of resistance to diseases (for example, to peanut clump virus), attempts need to be made to combine the genomes of cultivated and wild species to develop genotypes with stable resistance. Results from the few studies so far conducted indicate that genes for resistance in wild peanut species may be different from those in cultivated peanut (for example, peanut rust) (Singh et ul., 1984). However, difficulties of gene transfer from wild species have precluded widespread use of potential germplasm and have also limited our knowledge of the nature and genetics of resistance in the wild species.

SPECIATION, CYTOGENETICS, AND UTILIZATION

A.

23

INCOMPATIBILITIES RESTRICTING GENE TRANSFER

The major restriction for study and use of wild species in Arachis is crossincompatibilities between most species and the cultivated peanut. With the exception of A. monticofa, all wild species in section Arachis that can be hybridized with the cultivated peanut are diploids. Thus, hybrids are sterile. Even among these cross-compatible species the success rate for interspecific hybrid production may be low. Reasons for this include incompatibilities among species, especially when the wild species is used as a female parent; hybrid sterility due to the polyploidy; genomic differences among species; irregular meiosis in colchicine-treated hybrids; and difficulties encountered during backcross generations when sterile aneuploid or pentaploid plants are obtained. Even when hybrids can be obtained, the problem of eliminating undesirable wild species characters still exists. When genomes are common to both cultivated and wild species, this may not be a problem. If there are no chromosome homologies, it may be necessary to induce translocations for gene introgression. Even where genomes are homologous or homoeologous and pairing occurs, linkage may restrict recombination between desired and undesirable genes and prevent the production of A. hypogaeu-like lines with the desired agronomic characters (Stalker et af., 1979). Before a program can be designed to circumvent interspecific hybridization barriers between A. hypogaea and other species, reproductive ontogeny and isolation barriers must be understood. Approximately 12 hr elapse between pollination and fertilization (Smith, 1956). About a week after fertilization, an intercalary meristem of the peg located in the ovary proximal to the ovules begins a rapid geotropic elongation (Jacobs, 1947). Gibberellic acid has a significant stimulatory effect on peg elongation (Amir, 1969), while auxin inhibits peg elongation and is associated with fruit enlargement (Jacobs, 195 1). Several days after elongation initiates, the peg penetrates the soil, ceases to grow, and expands into a pod (Smith, 1950, 1956; Yasuda, 1943). Ziv (1981) reported that light is necessary for peg elongation. When pegs fail to reach the soil, they remain viable for several days and then wither. Moisture in the absence of light appears to be the most important factor governing pod development (Yasuda, 1943). Although the peg follows a sigmoidal growth pattern, the embryo and endosperm initially divide and then become quiescent; the embryo contains between 5 and 27 cells at the time the peg penetrates the soil (Schenk, 1961). After the peg is underground, the embryo then maintains a rapid growth phase. Both the root apex and cotyledons are initiated in the globular embryo stage, and by the heart-shaped embryo stage the cotyledons appear as a projection (Pallai and Raju, 1975). Schenk (1961) reported physiological changes associated with peg development, and Brennan (1969), Gregory et af. (1973), and Periasamy and Sampoornam (1984) reviewed in more detail the reproductive development of the cultivated peanut. Halward and Stalker (1985, 1987a) reported differences in development of reproductive tissues of wild and cultivated peanuts.

24

H. T. STALKER AND J. P. MOSS

Although many hybridization failures of interspecific crosses have been attributed to embryo abortion, only a few investigations have been reported detailing mechanisms of incompatibility. Johanson and Smith (1956) attributed the failure of A . hypogueu x A . diogoi (not true diogoi vide Gregory and Gregory, 1979) to slow growth and degeneration of the embryo accompanied by hypertrophy of integuments. In A . hypogueu x A . glubrutu (section Rhizomutosae) crosses, Murty et ul. (1980) observed up to a 48-hr delay in fertilization and early embryo abortion. In A . monticolu X A . sp. (section Rhizomutosae) crosses, Sastri and Moss (1982) observed large callus plugs along pollen tubes; however, a few pollen tubes were observed in the ovary. They further reported that gibberellic acid and kinetin treatments stimulated peg production in incompatible crosses. In the more closely related species of section Arachis incompatibility among species can be caused by the failure of pollen to germinate on the stigma, restriction in fertilization, and/or embryo abortion (Halward and Stalker, 1984). Further, interspecific hybrids can abort a early as 6 days after pollination (for example, in diploid x hexaploid hybrids) or remain viable but undeveloped until the time of normal maturity (for example, hexaploid x diploid hybrids) (Halward and Stalker, 1985, 1987b). Future recovery of interspecific hybrids will thus depend on developing techniques to initiate peg development, promote embryo growth on the plant, and to recover viable, but small, embryos in vitro. Promoting embryo development can either be done through ovule culture or, in the case of peanuts, by applying hormones to developing tissues in vivo (Mallikarjuna and Sastri, 1985a). Application of embryo culture techniques will then be necessary to recover small reproductive tissues after embryos have reached the heart-shaped embryo stage of development.

B. DIRECT HYBRIDIZATION WITHIN SECTION Aruchis I . Hybrids between Tetruploid Species

Aruchis monticolu is a tetraploid in series Amphiploides of section Aruchis and is the only wild Aruchis taxon which can be readily crossed with A . hypogueu to produce fertile progeny. The species has been given specific status, but it is a member of the same biological species as A . hypogueu and logically could be considered as a subspecies of the cultivated peanut. Aruchis rnonticolu was used by Hammons (1970) in the pedigree of Spancross and probably by Simpson and Smith (1974) to develop Tamnut 74. For all practical purposes, A. rnonticolu can be considered a wild form of peanut which does not need species manipulations for its utilization in breeding programs.

SPECIATION, CYTOGENETICS, AND UTILIZATION

25

Table V Number of Pods Produced per 100 Pollinations in Successive Backcrosses of Hexaploids to A. hypogaetf Backcross Wild species used in production of hexaploid cardenasii chacoense stenosperma correntina villosa batizocoi Mean

A. A. A. A. A. A.

~

~~~

BC,

BC,

BC,

BC,

BC,

9 6

18 10 16

12 11 8

26

-

I

16

14 25

~~

~

7 10

-

-

9

16

11

5

-

-

-

-

10

16

4

Number of fertile derivatives selected 9 9

2

-

~

“From ICRISAT (1981-1982).

2. Hybrids between A. hypogae and Diploid Species

a. Triploids All attempts at crossing A. hypogaea with diploid species with an A or B genome have produced hybrids, although the crossing success varies depending on the species used and the direction of the cross (Table V). The interspecific hybrids are triploids, usually vigorous, flower profusely, and are mostly sterile (Smartt and Gregory, 1967; Raman, 1976; Gregory and Gregory, 1979; Seetharam et al., 1973). There were an average of 8.8 univalents, 9.1 bivalents, and 1.0 trivalents per PMC in the triploids between A. hypogaea and eight diploid species (Singh and Moss, 1984b). Segregation was irregular and the percentage of stainable pollen grains varied in size. Sterility has been overcome by colchicine treatment to produce hexaploids for many hybrid combinations (Smartt and Gregory, 1967; Spielman et al., 1979; Spielman and Moss, 1976; Company et al., 1982). Although tripolids are usually sterile, seeds were produced on several hybrids of different cross-combinations of interspecific A. hypogaea hybrids (Simpson and Davis, 1983; Singh and Moss, 1984b). Eighty-two percent of the progeny derived from selfing triploids were hexaploid, indicating the formation of competent unreduced gametes (Singh and Moss, 1984b). Progenies other than hexaploids had chromosome numbers ranging from 2n = 20 to 59, indicating that gametes with fewer than 30 chromosomes can be functional. The hexaploids are of special interest as they have been obtained without the need for colchicine treatment, and, unlike colchiploids they have arisen from postmeiotic cells and thus pairing between wild and cultivated chromosomes has occurred. This is also true for the tetraploids; although produced at lower frequencies (8%) than the hexaploids (82Oro), they have the advantage of being at the same ploidy level as

26

H. T. STALKER AND J. P. MOSS

the cultivated peanut. Whereas colchiploids have identical homologies and chromosomes will not segregate (except as a result of homologous pairing), the progenies arising from selfed triploids will be unique because chromosome segregation has occurred. b. Hexaploids. Hexaploids, whether produced by colchicine treatment or from selfing of partially fertile triploids, have many undesirable characters associated with wild species, and none have been seriously considered suitable as the basis for developing the hexaploid peanut as a crop. Hexaploids are, therefore, an intermediate stage in a hybridization and selection process. The chromosome number must be reduced to the teraploid level and then undesireable wild characters eliminated. Tetraploidy can theoretically be achieved in one step by crossing hexaploids with diploid wild species. Although this will achieve the desired ploidy level and provide an opportunity to incorporate additional characters to A. hypogaea if a nonparental wild species is used, it also reduces the proportion of cultivated chromosomes in tetraploid derivatives. Hybrids between hexaploids and diploids have been difficult to produce due to embryo abortion (Halward and Stalker, 1984), and no tetraploid populations have been developed from these crosses. Hexaploids vary in meiotic regularity and in fertility but can be crossed with A. hypogaea. Pods per 100 pollinations of these crosses varies from 7 to 25 (Table VI). From three different hexaploids backcrossed to BCI or to BC, generation, 6775 pollinations produced 894 pods (13010), but only 20 fertile plants with regular meiosis were selected from pregenies (ICRISAT, 1982). c. Alteration of Ploidy Levels. Other ploidy manipulations are available to bypass the sterility of triploids and difficulties of backcrossing hexaploids. These all involve producing tetraploid derivatives of the wild species which can then be crossed with A . hypogaea. Further, the crosses Table V1 Number of Pods Produced per 100 Pollinations in Successive Backcrosses of A. sp. Autotetraploids to A. hypogae& Autotetraploid

BC,

BC,

batizocoi villosa correntina stenosperma spegazzinii Mean

6 5

7

13

6

-

2 3 11

20 14

-

-

-

1

5

8

12

6 (Total)

A. A. A. A. A.

"From ICRISAT (1981-1982).

BC,

Fertile stable derivatives 3 1 1

4

SPECIATION, CYTOGENETICS, AND UTILIZATION

27

have the advantage of producing wild x cultivated hybrids with a range of genome formulas (AABB, AAAB, or ABBB) which encourage intergenomic AB pairing, which Smartt et al. (1978a,b) predicted would be a problem in transferring wild species characters into A . hypogaea. The known genomes in section Arachis are the A and B of A . hypogaea: the B genome in wild A . batizocoi and the A genome of all other wild species except A. spinuclava, which has a D genome (Stalker, 1985a). There are differences in chromosome morphology and degree of pairing in hybrids between the wild A genome species (Singh and Moss, 1984a). Thus, the possible ploidy manipulations are to produce autotetraploids of the A, B, and D genomes and AB, AD, and BD amphiploids. In addition, amphiploids can be produced by crossing two different A genome species and doubling the chromosome number of the hybrid (ICRISAT, 1981; Gardner and Stalker, 1983). A wild range of crosses have been made and tetraploid derivatives produced. i. Autotetraploids. Autotetraploids of A . villosa, A correntina, A . stenosperma, A . duranensis,A . spegazzinii, A . chacoense,A . cardenasii, and A . batizocoi have been produced (Singh, 1986a). Multivalents are frequent in autotetraploids; mean quadrivalent number in different autotetraploids ranges from 2.4 to 4.8 per PMC (Singh, 1986a), and pollen stainability ranges from 8 to l6%, except for autotetraploid A . batizocoi, which has 37% stainable pollen. Although vegetatively vigorous, autotetraploids are difficult to maintain due to reduced seed fertility; when crossed with A . hypogaea, from 2 to 11 pods per hundred pollinations (mean 5%) were produced, but with successive backcrosses to A . hypogaea the crossability increased as hybrids became more fertile (Table VI). The first cross of an autotetraploid to A . hypogaea results in plants with genomic constitution AAAB or ABBB, and the absence of homologs for one genome could lead to increased homoeologous (A-B) pairing, which would increase the range of recombinants produced. Forty-chromosome, meiotically regular, and fertile plants can be obtained from autotetraploids within three generations of crossing to A . hypogaea (ICRISAT, 1982). For example, of five autotetraploids backcrossed to A . hypogaea, 3368 pollinations produced 249 pods (7.4070), from which six fertile derivatives were obtained. Fertile, stable derivatives can be obtained after one cross. Thus, although autotetraploids provide opportunity for homoeologous pairing when backcrossed with A . hypogaea, the number of resulting desirable recombinants is low. ii. Amphidiploids. Presently there are 12 named diploid species in section Arachis, although as recent collections are studied this number will likely increase to 15-20 species in the group. Currently, there are only three genomes known, so only the genome combinations AABB, AADD, and BBDD can be produced. A sum of 10 AABB, 10 AADD, and 1 BBDD

H.T. STALKER AND J. P. MOSS

28

species combinations are thus possible among named species. However, amphiploids can also be produced from two A genome species, so the total number of amphiploids which can be produced among all described taxa is 132; this number includes reciprocal hybrids to account for possible differences in cytoplasmic effects. The amphidiploids represent an important gene pool for peanut improvement , and each one potentially incorporates genes from two species at the same ploidy level as cultivated peanuts. Further, each amphidiploid has at least one genome in common with A. hypogaea, which should promote gene transfer. Many diploid interspecific hybrids have been produced between section Arachis species. Pairing and fertility in AA hybrids are reasonably good, with 8.96-9.8 bivalents per PMC and 40-85'70 pollen stainability. However, in AB, AD, and BD hybrids, paring and fertility are reduced to a mean number of 4.7-8.6 bivalents per PMC and pollen stainability ranging from 3 to 7% for AB, AD, and BD hybrids (Stalker and Wynne, 1979; Singh and Moss, 1982, 1984a; Stalker, 1985a). Comparisons of AA and AB hybrids with the corresponding amphiploids show that pairing is more regular and pollen stainability higher in the AABB amphiploids, but the reverse is true for the AAAA amphiploids (Table VII). When A. hypogaea was hybridized with the four amphiploid genotypes, little difference in frequencies of trivalents (mean = 0.34) or quadrivalents (mean = 0.07) per PMC are observed (Gardner and Stalker, 1983). AABB (A. hypogaea x amphidiploid) hybrids form fewer bivalents (mean = 14) and have more univalents (mean = 6) than would be expected from their genomic formula (ICRISAT, 1982; Singh, 1986b). Ten univalents in the AAAB hybrids were also observed which could easily be assigned to the B genome, but this assumption is not justified, due to the high univalent frequency in AABB hybrids. This indicates that in A. hypogaea x amphidiploid hybrids, both AB and A (wild)-A (Cultivated) pairing is highly probable. Pollen stainability and plant fertility in amphiploids and their hybrids with A. hypogaea is high enough to conduct a backcrossing program. Three fertile and stable derivatives were obtained from a total of 321 pods obtained after backcrossing A. hypogaea with AABB wild species amphiploids, and seven were obtained from 527 pods for AAAA amphiploids (ICRISAT, 1982). These frequencies of about 1% show that both types of amphiploids are a practical means of introgressing genes from wild species to A. hypogaea.

c.

INTERSECTIONAL HYBRIDIZATION FOR GENEINTROGRESSION

Successful intersectional hybridization is rare in Arachis. Of 42 possible combinations, including reciprocals, attempted by Gregory and Gregory (1979), only eight hybrids were produced and none of these involved A.

P

Table VII Frequency of Chromosome Pairing and Pollen Stainability in Diploid Hybrids, Amphiploids, and A . h y p o g w I

I1

Mean

Range

AB hybrid

9.1

7.2-10.6

AABB amphiploid AABB A. hypogaea x amphiploid

3.7

1.5-6.3

6.2

4.5-7.9

AA hybrid AAAA

0.3 2.4 10.6

Mean

IV

111

Range

Mean

5.2

4.7-5.5

0.1

0-0.4

16.6

14.9-18.2

0.4

0.3-0.5

0.4

0.2-0.7

14.1

13.3-14.9

1.1

0.7-1.5

0.6

0.3-0.9

0-0.8 0.9-3.5

9.6 13.6

9.4-9.1 13.0-14.5

0 0.4

0.2-0.8

0.1 2.2

9.7-11.8

11.1

7.2-13.0

1.1

0-2.4

1.0

0

Amphiploids"

Pollen stainabilityb

Range

-

Mean

X

Range

Mean

Range

nc

4

3 4

4

(32)

8-60

4

(63)

-

2

0-0.2 1.4-3.0

77 (41)

74-81

4

-

4

0.3-1.4

(41)

35-51

7

-

amphiploid AAAB A. hypogaea x amphiploid

%om ICRISAT (1981). bFigures in parentheses indicate that data were not available for all hybrids or amphiploids. 'AB-AABB and AA-AAAA comparisons are for the same species combinations, but comparisons of amphiploids and hybrids with A. hypogaea involve different species combinations.

30

H.T. STALKER AND J . P. MOSS

hypogaea. Direct intersectional hybridization with A .hypogaea is currently not a possible means of introgression from wild to the cultivated species. Either hormone treatment and/or embryo rescue will be necessary to produce hybrids, or highly crossable genotypes of Arachis must be found before germplasm from most species of the genus can be transferred to A . hypogaea. Hybrids have been produced between annual diploids (with either an A or B genome) of section Aruchis and a number of section Rhizomatosae accessions, and between section Arachis species and two accessions of section Erectoides (Gregory and Gregory, 1979). This suggests that diploid section Arachis species may be useful for introgressing genes from species distantly related to the cultivated peanut. Gregory and Gregory (1979) suggested elements in common between sections on the basis of crossability, even though the hybrids were all highly sterile. Stalker (1985b) studied a number of intersectional crosses including a 40-chromosome (Erectoides x Erectoides) x (Erectoides x Rhizomatosae) hybrid. The mean number of univalents was less than three, indicating a considerable degree of pairing between members of sections Erectoides and Rhizomatosae chromosomes. A common genome to taxa of both these sections is likely to exist. The intersectional hybrids have potential for making additional crosses and for ploidy manipulation to introgress genes into A . hypogaea, but there have not been concentrated efforts to do this. FOR INTERSPECIFIC HYBRIDIZATION D. In Vitro TECHNIQUES

I.

Cell and Protoplast Culture

Callus is generally easy to produce in Arachis but plant regeneration is difficult. Peanut cotyledons have been used to define biochemical parameters for callus growth in cultivars (Verma and van Huystee, 1971; Verma and Marcus, 1974; Russo and Varnell, 1978; Guy et al., 1978). However, regeneration of plants from cotyledon callus of peanuts has not been obtained. Plants were recovered from tissue segments obtained by freeze-shattering cotyledons and growing the segments on moist filter paper (Illingworth, 1968). Mroginski et al. (1981) and Pittman et al. (1983) were able to regenerate plants in vitro from 3- to 5-day-old immature leaves. Isolation of protoplasts in peanuts was first obtained by Jullian (1970), who mechanically broke the cells. Verma and van Huystee (1971) observed that the cell suspension cultures were heterogenous in size and balanced growth was unlikely. They then developed a method to obtain large numbers of uniform cells in suspension. By using mannitol solutions of different molarity, Holden and Hildebrandt (1972) were able to obtain protoplasts without mechanically injuring the cells. Callus and roots have

SPECIATION, CYTOGENETICS, AND UTILIZATION

31

subsequently been produced from single cells of peanuts (Yung-ru and YuHung, 1978). In summary, callus and protoplast culture techniques could greatly facilitate introgression to A . hypogueu for producing both initial F, hybrids and for manipulating polyploid levels to recover 40-chromosome hybrid derivatives. However, in Aruchis the basic work to regenerate plants from single cells or callus is needed before techniques will be useful as a plant breeding tool. 2. Ovule and Embryo Culture

In vitro culture of ovules or embryos has successfully been used to produce interspecific hybrids is many genera (Narayanaswami and Norstog, 1964; Raghavan, 1980; Collins et ul., 1984). Numerous reviews have also been published describing media requirements and technical aspects of tissue preparation (North, 1976; Raghavan, 1977,1980;Williams et a/., 1982; Collins et ul., 1984). Embryo rescue in peanuts has had a long but sporadic history. Harvey and Schulz (1943) and then Nuchowiz (1955) initiated studies on regenerating peanut embryos. Martin (1970) regenerated peanut ovules only 0.3 mm in length to produce viable plants. However, in an attempt to duplicate Martin’s results, Sastri et ul. (1980), using the same media, could only produce callus, which became necrotic. Further, Johnson (198 1) concluded that in vitro culture of small embryos required a two-step process in which ovules could be cultured until they became large enough to dissect embryos, and then embryos could be cultured to generate plants. Embryo culture in peanuts is difficult for some genotypes, while easy for others. Ziv and Zamski (1975) produced callus and mature seeds from pegs grown on the plant which were allowed to elongate into media until the pod enlarged and embryos reached the heart stage. They then cultured the heart stage embryo in vitro. Ziv and Sager (1984)further found that embryos would develop into young seedlings under red, blue, or far-red subsaturated flux densities, but pod formation was inhibited. Moss et a/. (1985) cultured 1- to 4-day-old peg tips of A . hypogueu and reported ovule growth in many of the cultures. Reports of rescuing interspecific hybrid embryos are less frequent than reports of in vitro culture of A . hypogueu. Bajaj et al. (1982) cultured 30-dayold F, embryos of A . hypogueu x A . villosu. However, this is a hybrid combination which can relatively easily be obtained without the aid of in vitro techniques. Bajaj (1984) reviewed the tissue culture literature and concluded that peanut self and hybrid embryos can be cultured in vitro, but application of techniques are yet to be realized. A series of media varying in auxins, cytokinins, and gibberellic acid produced no significant breakthroughs in ovule culture (Sastri et ul., 1982). Mallikarjuna and Sastri (1985b) and Stalker et ul.

H. T. STALKER AND J. P. MOSS

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(1985) found genotypic differences for responses to gibberellic acid treatments to stimulate peg elongation in the interspecific crosses. Pod and seed development have further been stimulated by applying hormones to peg tissues 10 days after pollination (Sastri and Moss, 1982; Mallikarjuna and Sastri, 1985a). Although hybrid ovules have been stimulated to expand and shoots and roots to develop, fewer than five intersectional hybrids have been recovered (Sastri and Moss, 1982). A major obstacle to hybrid recovery is establishing cultured plants in the greenhouse.

IX.

SUCCESSES AND POTENTIALS FOR UTILIZING Arachis GERMPLASM

Thousands of interspecifichybrids have been produced in many genera between wild species or between wild species and cultivars, but introgression of genes from wild species to genotypes with agronomic potential and subsequent cultivar release are rare. The major constraint to efficient utilization of interspecific hybrids is low fertility, which results in small populations and the inability to select desirable recombinants. Not only must the desirable genetic material be transferred into the cultivated genome, but yield and quality standards must also be met before wild species can be utilized by the grower. Arachis monticola has been used for two cultivar releases. This situation is analagous to the use OfAvenasteralisL. for improvementofA. sativaL. (Frey, 1976) or Hordeum spontaneum C. Kosch for improving H. vulgare L. (Rodgers, 1982), in which cases wild species which are completely crosscompatible with the cultigen were hybridized and selected. Reports of utilization of species germplasm from taxa in which the first generation hybrids are sterile are much more difficult to find in the literature (Stalker, 198Oa). Whole genomes may be added to a cultivar (such as in the case of sugarcane), or, when haploidy techniques are available (such as in the Solanaceae genera Solanum), diploids can readily be produced to select progenies with desired traits and tetraploids then be regenerated. However, few wild species in legume genera have been utilized for crop improvement. In peanut, Stalker et al. (1979) described a highly variable interspecifichybrid population derived from an A. hypogaea x A. cardenasiicrosswhich was selfed for at least eight generations and hybrid derivatives having 40chromosomes recovered. Selections were made from the population for C. arachidicola (Stalker, 1984), C. personatum (ICRISAT, 1984), and P.arachidis resistances (ICRISAT, 1984) and for resistance to several insects (Stalker and Campbell, 1983). Several interspecific hybrid populations are in the final stages of testing for release as general breeding materials, and some are already being used in several breeding programs. Making selections for high yields is as important as developingcultivarswith increased diseaseresistances,and a recurrent selection

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program has resulted in high-yielding lines from the same A. hypogueu x A. curdenusii population which have potential for cultivar release (Guok et ul., 1986). Several of the lines are now being tested in advanced yield trials in North Carolina and Virginia. Lines with high yields and C. personuturn and P. armhidis resistances are also being tested in the national Indian yield trial. Not only are seed yields high in these lines, but there is little defoliation making the vegetative material an extremely valuable source for cattle feed. Efforts to utilize species other than A. curdenusii are also being made to improve peanut production. Many fertile interspecific hybrid populations resulting from triploids or amphidiploids of Aruchis species are currently being evaluated in several peanut breeding programs. Several of these hybrid derivatives have potential as cultivar releases. In most crop species many cultivars have been released which have disease and insect resistances. However, in peanut only the cultivar NC 6 has been released for insect (southern corn rootworm) resistance (Wynne et al., 1977). In addition, several cultivar releases have been made for disease resistances, including NC 8C, selected for Cylindrocladium black rot resistance (Wynne and Beute, 1983); Va 81B, selected for resistance to Sclerotina blight (Coffelt et al., 1984); Southern Runner, resistant to late leafspot (Gorbet et ul., 1986); and several rosette-resistant cultivars in Africa (Gillier, 1978). Obviously missing from this list are cultivars resistant to the important diseases caused by early leafspot (C. urachidicolu) and peanut rust (P. armhidis). Further, cultivars used in the United States have been selected for rather narrow environments and only one genotype for each disease has been released. This is the result of several interrelated factors. First, during the early days of peanut production relatively few diseases were economically important. In the United States, diseases have been controlled with chemicals, and there has been the common belief until recently that no variability was available in cultivated accessions. This last point concerning lack of variability resulted in large efforts by botanists, cytogeneticists, and plant breeders in cooperation with pathologists and entomologists to collect, evaluate, and attempt to utilize the wild species of the genus. In many other crop species the plant breeder has been responsible for acquiring, evaluating, and utilizing germplasm resources of related wild species along with those of the crop species. In Aruchis, efforts have been more diversified, with teams of cytogeneticists working with plant breeders to identify and manipulate useful germplasm. Fortunately, many of the most important traits are found in species which are cross-compatible with the cultivated peanut. In the future, selection for leafspot and rust resistances have a high probability of making a significant contribution to peanut production, especially in tropical and semitropical areas where chemicals are infrequently used by the grower. Significant progress toward exploiting the variability in the genus Aruchis is being made. However, the most productive pathway to obtain 40-chromosome hybrid derivatives has not been determined, and perhaps

34

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no one method will prove to be the best for all traits. Interspecific hybrid derivatives which are cross-compatible with A. hypogaea have been selected for fertility. Most desired traits in Arachis are multigenic and large populations must be derived to utilize the germplasm resources. Like other crop species, hybridization and fertility restoration processes are not easily accomplished, but significant progress is being made toward utilizing wild species of Arachis.

X.

CONCLUSIONS

Utilization of wild species requires a collective effort by botanists, geneticists, cytogeneticists, and plant breeders. In Armhis there exists a large reservoir of germplasm with agronomic potential in numerous taxa. A taxonomic treatment of the genus is urgently needed to avoid confusion and to open communication channels among scientists. However, efforts have been made to evaluate available accessions for the economically important disease and insect pests of peanuts. Arachis hypogaea is an allotetraploid species and will only hybridize with members of section Arachis. In this group are 15-20 species which have genomic similarities to the cultivated peanut. In addition, species accessions have been identified which are highly resistant to the most severe pests of cultivated peanuts. Utilization of these germplasm resources is hindered by sterility of interspecific hybrids due to ploidy and, to a lesser extent, genomic incompatibilities. Attempts to introgress genes conferring resistances to different diseases and insects have proceeded through direct hybridization and through ploidy manipulations. Traits can be transferred to the A. hypogaea genome using either pathway, but eliminating undesirable traits is a major problem which must be solved before growers will actually use the available germplasm. The methodology to utilize species outside section Arachis is proceeding in the area of embryo rescue and recovery of hybrids via growth regulator applications to reproductive structures. However, the cytogenetic similarities among genomes of taxa in different sections remain unclear. Because hybrid embryos can be obtained for wide crosses in the genus, specialized techniques such as protoplast fusion may be unnecessary to utilize the germplasm resources. Regardless, efforts to utilize distantly related species should only be conducted when desired traits are not found in the cultivated species or in species closely related to A . hypogaea. Wild peanut species have been used to create highly variable plant populations which contain early and late leafspot, rust, and insect resistances. Further, transgressive segregates for yield have resulted from

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crossing an unadapted A. hypogueu accession with a small-seeded diploid species. High yields were unexpected, but results of introgression greatly enhanced the potential exploitation of Aruchis species germplasm. Perhaps the most valuable use of these hybrid derivatives will be in tropical and semitropical areas where chemicals are infrequently used, but even small increases in disease resistance will result in significant yield increases. In addition, fodder production is an important consideration in many production areas outside the United States. Yield and disease trials are now being conducted with advanced breeding lines derived from wild diploid species of A ruchis.

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Smartt, J., Gregory, W. C., and Gregory, M. P. 1978a. Euphyticu 27, 665-675. Smartt, J., Gregory, W. C., and Gregory, M. P. 1978b. Euphyticu 27, 677-680. Smith, 9. W. 1950. Am. J. Eot. 37, 802-815. Smith, B. W. 1956. Am. J. Bot. 43, 81-89, 233-240. Smith, D. H . , and Littrell, R. H. 1980. PIanr Dis. Rep. 64, 356-361. Smith, J. W., Jr., and Barfield, C. S. 1982. In “Peanut Science and Technology” (H. E. Pattee and C. T. Young, eds.), pp. 250-325. Am. Peanut Res. Educ. SOC.,Yoakum, TX. Spielman, I. V., and Moss, J. P. 1976. Oleugineux 31, 491-494. Spielman, I. V.. Burge, A. P., and Moss, J. P . 1979. 2. Pfunzenzuecht. 83, 236-250. Stalker, H. T. 1980a. Adv. Agron. 33, 111-147. Stalker, H. T. 1980b. In “Proceedings of an International Workshop on Groundnuts,” pp. 73-81. ICRISAT, Patancheru, A.P., India. Stalker, H. T. 1981. Crop Sci. 21, 359-362. Stalker, H. T. 1984. Euphyticu 33, 529-538. Stalker, H. T. 1985a. Proc. Am. Peanut Res. Educ. Soc. 17, 23 (Abstr.). Stalker, H. T. 1985b. In “Proceedings of an International Workshop on Cytogenetics of Aruchis,” pp. 65-79. ICRISAT, Patancheru, A.P., India. Stalker, H. T. 1985~.In “Proceedings of an International Workshop on Cytogenetics of Aruchis,” pp. 119-123. ICRISAT, Patancheru, A.P., India. Stalker, H. T., and Campbell, W. V. 1983. Peanut Sci. 10, 30-33. Stalker, H. T., and Dalmacio, R. D. 1981. J. Hered. 72, 403-408. Stalker, H. T., and Dalmacio, R. D. 1986. Curyologia, 51, 617-629. Stalker, H. T., and Wynne, J. C. 1979. Peanut Sci. 6, 110-114. Stalker, H. T., and Wynne, J. C. 1983. Peanut Sci. 10, 59-62. Stalker, H . T., Wynne, J. C., and Company, M. 1979. Euphyticu 28, 675-684. Stalker, H. T., Campbell, W. V., and Wynne, J. C. 1984. J. Econ. Ent.. 7, 53-57. Stalker, H. T., Moss, J. P., and Seitz, M. H. 1985. Agron. Abstr. 1985, 72, (Abstr.). Subrahmanyam, P., Mehan, V. K., Nevill, D. J., and McDonald, D. 1980. In “Proceedings of an International Workshop on Groundnuts,” pp. 193-198. ICRISAT, Patancheru, A.P., India. Subrahmanyam, P., Williams, J. H., McDonald, D., and Gibbons, R. W. 1984. Ann. Appl. Biol. 104,467-476. Sunrahmanyam, P., Ghanekar, A. M., Knolt, 9. L., Reddy, C. V. R., and McDonald, D. 1985. In “Proceedings of an International Workshop on Cytogenetics of Aruchis,” pp. 49-55. ICRISAT, Patancheru, A.P., India. Taubert, P. 1894. In “Die natuerlichen Pflanzenfamilien 111 Tiel” (A. Engler and K. Prantl, eds.), pp. 322, 324-325, Part 3. Valls, J. F. M., Rao, V. R., Simpson, C. E., and Krapovickas, A. 1985. In “Proceedings of an International Workshop on Cytogenetics of Aruchis,” pp. 15-33. ICRISAT, Patancheru, A.P., India. Varisai Muhammad, S. 1973. Madras Agric. J. 60, 1428-1432. Verma, D. P. S., and Marcus, A. 1974. J. CeN Sci. 41, 331-337. Verma, D. P. S., and van Huystee, R. 9. 1971. Exp. Cell Res. 6 9 , 4 0 2 4 8 . Watson, I. A. 1970. In “Genetic Resources in Plants-Their Exploration and Conservation” (0.H. Frankel and E. Bennett, eds.), pp. 441-457. Davis, Philadelphia. Williams, E. G.,Verry, I. M.,and Williams, W. M. 1982. In “Plant Improvement and Somatic Cell Genetics” (I. K. Vasil, W. R. Scowcroft, and K. J. Frey, eds.), pp. 119-128. Academic Press, New York. Woodroof, J. G. 1973. In “Peanuts-Production, Processing, Products,” 2nd Ed. Westport, CT . Wynne, J. C. 1974. Ph.D. thesis, N.C. State Univ., Raleigh. Wynne, J. C., and Beute, M.K. 1983. Crop Sci. 23, 184.

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H.T. STALKER AND J. P. MOSS

Wynne, J. C., and Coffelt, T. A. 1982. I n “Peanut Science and Technology” (H. E. Pattee and C. T. Young, eds.), pp. 50-94. Am. Peanut Res. Educ. SOC.,Yoakum, TX. Wynne, J. C., and Gregory, W. C. 1981. Adv. Agron. 34, 39-72. Wynne, J. C., Campbell, W. V., Emery, D. A,, and Mozingo, R. W. 1977. N.C. Agric. Exp. Sta. Bull. 458. Yasuda, S . 1943. Jpn. J. Bot. 13, 243-253. Yung-ru, S., and Yu-hung, L. 1978. Proc. Symp. Plant Tissue Cult., Peking p. 529. Ziv, M. 1981. Ann. Bof. 48,353-359. Ziv, M., and Sagar, J. C. 1984. Plant Sci. Lett. 34, 211-218. Ziv, M., and Zamski, E. 1975. Ann. Bot. 39, 579-583.

ADVANCES IN AGRONOMY, VOL. 41

CEREAL-LEGUME INTERCROPPING SYSTEMS Francis Oforili2and W. R. Stern2 'Plantations Limited, Ghana Cocoa Board, Private Post Bag Accra North, Ghana, West Africa 'Agronomy Group, School of Agriculture The University of Western Australia Nedlands, Western Australia, Australia 6009

1.

INTRODUCTION

In terms of land use, growing crops in mixed stands is regarded as more productive than growing them separately (Andrew and Kassam, 1976; Willey, 1979). Mixed cropping is practiced traditionally in many parts of Africa, Asia, and Latin America (Ahmed et al., 1979) and interest in cereal-legume intercropping is developing in some temperate regions with warm climates such as Australia and the United States (Searle et al., 1981; McCollum, 1982; Allen and Obura, 1983; Chui and Shibles, 1984). This may be due to some of the established and speculated advantages for intercropping systems such as higher grain yields, greater land use efficiency per unit land area, and improvement of soil fertility through the addition of nitrogen by fixation and excretion from the component legume (Agboola and Fayemi, 1972; Willey, 1979; Eaglesham et al., 1981). It seems worthwhile to develop cropping systems that have the capacity to maximize crop yields per unit land area while keeping the fertilizer nitrogen requirement to a minimum. The intercropping of legumes with cereals offers scope for developing energy-efficient and sustainable agriculture (Papendick et al., 1976; IAEA, 1980). Since the reviews by Willey (1979) on intercropping and by Trenbath (1974) on mixed cropping, many papers have been published from both temperate and tropical areas that emphasize general agronomic principles and research needs of intercropping systems. The recent book on multiple cropping systems (Francis 1986) summarizes some of that work and also develops some new ideas. The major emphasis in this chapter is on cereal-legume intercrop systems and we review the available information on various crop combinations. We examine cultural factors such as choice of compatible component crops with diverse morphology, crop geometry and 41 Copyright 0 1987 by Academic Press, Inc. All rights of reproduction in any form reserved.

42

FRANCIS OFORI AND W. R. STERN

density, relative time of sowing of component crops, and competitive relations, as well as nitrogen fertilizer application and nitrogen economy.

II. BACKGROUND TO INTERCROPPING SYSTEMS Intercropping is the growing of two or more crop species simutaneously in the same field during a growing season. Other suggested definitions are (a) any form of cropping system in which there is a significant amount of intercrop competition (Willey, 1979); and (b) the growing of two or more crops in different but proximate rows (Ruthenberg, 1980). Common characteristics of different forms of intercropping are that they have the effect of intensifying crop production and exploiting more efficiently environments with limiting or potentially limiting growth resources (Papendick et al., 1976; Trenbath, 1982). Andrews and Kassam (1976) identified four main types of intercropping: 1. Mixed intercropping: growing component crops simultaneously with no distinct row arrangement; this is commonly used in labor-intensive subsistence farming. 2. Row intercropping: growing component crops simultaneously in different rows; this is used in mechanized agriculture and in present research on intercropping, because it permits crop-specific operations. 3. Strip intercropping: growing component crops simultaneously in different strips to permit the independent cultivation of each crop. 4. Relay intercropping: growing component crops in relay, so that growth cycles overlap.

A. CROP COMBINATIONSAND GLOBALDISTRIBUTION Crop combinations differ with geographical location; there may be intercropping of tree crops, intercropping of tree and field crops, or intercropping of field crops (Trenbath, 1982). On the basis of morphology and growth duration, Herrera and Harwood (1973) distinguished the following crop combinations: crops of similar heights and growth durations such as barley and oats; crops of similar morphology but different growth durations, e.g., 6-month sorghum and 3-month millet; annual or biennial crops with those of longer growth durations such as millet and cassava or soybean and sugar cane; annual crops of cereals; and legumes such as sorghum and pigeonpea, maize and cowpea. Combinations of crops are determined primarily by the length of the growing season and the adaptation of crops to particular environments. In areas with annual rainfall of less than 600 mm and a short growing season, such as northern Nigeria, early-maturing and drought-tolerant crops such

CEREAL-LEGUME INTERCROPPING SYSTEMS

43

as millet and sorghum dominate (Andrews, 1972; Baker, 1979). In areas with annual rainfall greater than 600 mm, cereals and legumes of varying maturities are used. In the tropical and subtropical regions, the cereal component is usually maize, sorghum, millet, or, to a lesser extent, rice, and the legume is usually cowpea, groundnut, soybean, chickpea, bean, or pigeonpea. Both early- and slow-maturing crops are combined to ensure efficient utilization of the whole growing season (Baker, 1979). The millet or sorghum and pigeonpea combinations of the Hyderabad area of India are a typical example (Ruthenberg, 1980). Maize seems to dominate as the cereal component and it is combined with many different legumes. In India, short-duration sorghum and millet (90days) are intercropped with pigeonpea that matures 90days later than the cereal (Willey, 1979). In high rainfall areas of West Africa, a common crop combination is maize and cowpea (Okigbo and Greenland, 1976), whereas in South and Central America, maize and different types of beans dominate (Francis et al., 1976). Combinations of rice and other cereals or legumes are also found in high rainfall areas with a single extended rainy season such as Southeast Asia (Ruthenberg, 1980). In some temperate regions with warm climates, intercrop systems consist of wheat, oats, or barley as the cereal component and field bean, vetch, lupin, or soybean as the legume component (Table I). Geographical patterns for different crop combinations are discernable (Table I), and these generally follow general climatic classificationssuch as the one of Koppen (Trewartha, 1954). B.

INDICES FOR EVALUATING PRODUCTIVITY AND EFFICIENCY

Different indices have been suggested for evaluating productivity and efficiency per unit area of land of cereal-legume intercrop systems (Willey, 1979, 1985; Beets, 1982). These include comparisons of absolute yields, protein yields, caloric equivalent, and in economic terms, gross returns from intercrops and sole crops. Evaluation in economic terms is considered inappropriate due to seasonal price fluctuations of inputs and the lack of cash economy in most areas where intercropping is practiced (Beets, 1982). Yields and prevailing prices of crops tend to fluctuate, composition and quality of crop products will vary, and energy contents and growth durations of the component crops differ, so combined yields are of little value (de Wit et al., 1966; Beets, 1982). Van den Bergh (1968) suggested relative yields for comparing performance of crops in intercropping mixtures. Willey (1 979) also suggested standardizing component crop yields in relation to sole crop yields in order to evaluate intercropping efficiency.

I . Relative Yield Total The mixture yield of a component crop expressed as a proportion of its yield as a sole crop from the same replacement series is the relative yield of the

Table I The Distribution of Various Intercrop Systems in Different Climatic Types

Crop combinations Maize intercropped with Beans

Climatic type after Koppen"

Irrigated or rain fedb

Country and location of study

Reference

Af Am Am Aw Aw Aw

R R R R R R

Costa Rica, Turrialba Sri Lanka, Peradeniya Colombia, Palmira Uganda, Kampala Tanzania, Morogoro Brazil, Pernambuco

9.56"N, 7.16"N. 3.33"N, 0.19"N, 6.49"N, 8.14"S,

83.48"W 80.37"E 76.17"W 32.53"E 37.40"E 38.00"W

Hart (1975)

cowpea

Af Af Am Am Aw Aw BS

R R R R R R

5.54"S, 9.56"N, 7.23"N, 7.16'N, 6.49"N, 8.14"N, 5.33"N,

76.07 "W 83.48"W 3.56"W 80.37"E 37.40"E 38.00"W 0.1S"W

Wade and Sanchez (1984) Chang and Shibles (1985a) Remison (1978) Gunasena et al. (1979) Enyi (1973) Mafra el of. (1979) Haizel (1974)

Groundnut

Am Aw Aw Ca Am Am Aw Ca Da Da Da

R R R R R R R

Peru, Yurimaguas Costa Rica, Turrialba Nigeria, Ibadan Sri Lanka, Peradeniya Tanzania, Morogoro Brazil, Pernambuco Ghana, Accra Sri Lanka, Peradeniya Tanzania, Morogoro India, Hyderabad India, Meerut Trinidad, St Augustine Sri Lanka, Peradeniya Zimbabwe, Harare Australia, Camden U.S.A., Minnesota U.S.A., Ames U.S.A., Amherst

7.16 ON, 80.37 "E 6.49"N, 37.40"E 17.22"N, 78.26"E 29.00"N, 77.42"E 3.38"N, 61.31"W 7.16"N, 80.37"E 17.43"S, 31.OS"E 34.04"S, 150.40"E 40.06"N. 91.46"W 42.02"N. 93.39"W 42.23"N, 72.31"W

Gunasena et af. (1979) Evans (1960) Nambiar et al. (1983) Gangwar and Karla (1982) Dallal (1974) Gunasena et a/. (1979) Beets (1977) Searle et al. (1981) Crookston and Hill (1979) Chui and Shibles (1984) Putnam et a/. (1985)

%

Soybean

1, R

1, R

I I I

Gunasena et al. (1979) Francis et al. (1982a) Willey and Osiru (1972) Enyi (1973) Mafra et of. (1979)

Blackgram Greengram

Ca Am

R

India, Meerut Nigeria, Ibadan

29.00"N, 7.23"N.

Clover Rice

Ca Af

I R

U.S.A., Urbana Phillipines, Manila

40.07'N, 88.12W 14.37"N, 120.58"E

Millet

Aw AW

R

Sorghum

R

Nigeria, Samaru Nigeria, Samaru

11.1l0N, 11.1l0N,

7.38"E 7.38"E

Cassava

Af

R

Costa Rica, Turrialba

9.56"N,

83.48"W

Aw Aw Aw

R R R

Uganda, Kampala Tanzania, Morogoro Brazil, Pernambuco

Cowpea

Aw Aw Ca BS

R R R I

Tanzania, Morogoro Brazil, Pernambuco India, New Delhi U.S. A., Riverside

0.19"N, 32.35"E 6.49"S, 37.40"E 8.149, 38.00"W 6.49"s. 37.40"E 8.14"S, 38.00"W 28.37"N, 77.13"E 33.59"N, 117.22"W

Groundnut

Aw Ca

R R

Tanzania, Morogoro India, New Delhi

6.49"S, 28.37"N,

37.40"E 77.13"E

Soybean

Aw Ca Da

R R I

Blackgram

Ca Aw

R R

Puerto Rico, Isabela India, New Delhi U.S.A., Urbana India, New Delhi Tanzania, Morogoro

18.30°N, 28.37"N, 40.07ON, 28.37"N, 6.49"S,

67.20"W 77.13"E 88.12OW 77.13"E 37.40"E

Ca

R

India, New Delhi

28.37"N.

77.13"E

Singh (1981)

Aw Aw Ca

R R R

Tanzania, Morogoro Brazil, Pernambuco India, New Delhi

6.49"N. 8.14"S, 28.37"N,

37.40"E 38.00"W 77.13"E

Enyi (1973) Mafra et at. (1979) Singh (1981)

R

77.42"E 3.56"E

Gangwar and Karla (1982) Agboola and Fayemi (1972) Kurtz et a/. (1952) IRRI (1974) Andrews (1972) Baker (1979)

Hart (1975)

Sorghum intercropped with Beans

111 P

Chickpea Greengram Pigeonpea

Osiru and Willey (1972) Enyi (1973) Mafra et 01. (1979) Enyi (1973) Mafra et 01. (1979) Singh (1981) Shackel and Hall (1984) Evans (1960) Wagmar and Singh (1984) Elmore and Jacobs (1984) Singh and Jain (1984) Wahua and Miller (1978) Singh (1981) Chowhury and Misangu (1981)

(Continued)

Table I (Continued)

Crop combinations

Climatic type after KoppenO

Irrigated or rain fedb

Country and location of study

Reference

Sorghum intercropped with (cont.) Cotton

Aw

11.1l0N,

7.38"W

Baker (1979)

Aw Aw

R R R

Nigeria, Samaru

Sorghum Millet

Nigeria, Samaru Nigeria, Samaru

11.11"N, 11.11"N,

7.38"W 7.38"W

Norman (1974) Andrews (1974)

Cowpea

Aw

R

Nigeria, Samaru

11.11 ON,

7.38"W

Andrews (1974)

Groundnut

Aw

R

India, Hyderabad

17.22"N,

78.26"E

Reddy and Willey (1981)

Greengram

Aw

R

Tanzania, Morogoro

6.49"S,

37.40"E

May (1982)

Cotton

Aw

R

Nigeria, Samaru

11.1l0N,

7.38"W

Cowpea

Af

R

Phillipines, Manila

14.37"N,

120.58"E

Harwood and Price (1976)

Mungbean

Af

R

Phillipines, Manila

14.37"N, 120.58"E

Harwood and Price (1976)

Peas

BS

R

Cyprus, Prastio

35.10"N.

33.45 "E

Hadjichristodoulou (1973)

Vetch Soybean

BS Ca

R R

Cyprus, Prastio U.S.A., Oklahoma

35.10°N, 35.28"N,

33.45 "E 97.33"W

Hadjichristodoulou (1973) Crabtree and Rupp (1980)

Lupins

Da

R

Greece, Thessaloniki

40.38"N,

22.58"E

Field beans

Da

R

U.K., Reading

51.28"N,

0.59"W

Millet intercropped with

Baker (1979)

Rice intercropped with

Wheat intercropped with

Barley intercropped with

'Koppen's classification as in Trewartha (1954). bR, rain-fed, I, irrigated.

Papadakis (1941) Martin and Snaydon (1982)

CEREAL-LEGUME INTERCROPPING SYSTEMS

41

crop (de Wit and van den Bergh, 1965; van den Bergh, 1968). The sum of the relative yields of component crops is called the Relative Yield Total and is denoted by RYT (de Wit and van den Bergh, 1965; Harper, 1977). When the RYT is equal to or less than 1, there is no advantage to intercropping. Although the calculation of RYT was originally based on the replacement series in competition studies, where proportions of the components in binary mixtures are varied but the overall crop densities remain constant (de Wit, 1960; de Wit and van den Bergh, 1965), the calculation can in fact be applied to any density situation in intercropping. However, Mead and Riley (1981) have argued against its use because the objectives of intercropping are essentially agronomic, that is to find the best ways of growing crops together. 2. Land Equivalent Ratio Willey and Osiru (1972) proposed the concept of the Land Equivalent Ratio (LER) as an index of combined yield for evaluating the effectiveness of all forms of intercropping. LER is defined as the total land area required under sole cropping to give the yields obtained in the intercropping mixture. It is expressed as: LER =

(Yjj/Y,i)

+ (Y,i/qj)

where Y is the yield per unit area, Yi and Yj are sole crop yields of the component crops i and j, and yij and Tiare intercrop yields (Mead and Willey, 1980). The partial LER values, Li and L j , represent the ratios of the yields of crops i and j when grown as intercrops, relative to sole crops. Thus, Li

=

(xj/Yi) and Lj = (Yi/qj)

LER is the sum of the two partial land equivalent ratios so that LER

=

L,

+ Lj

When LER = 1, there is no advantage to intercropping in comparison with sole cropping. When LER > 1, a larger area of land is needed to produce the same yield of sole crop of each component than with an intercropping mixture. For example, when LER = 1.25, 25% more land is needed to produce the same yield from the components as sole crops.

3. Area Time Equivalent Ratio The Area Time Equivalent Ratio (ATER) was proposed by Hiebsch (1980)

and McCollum (1982) as a modification of the LER. This takes into account

48

FRANCIS OFORI AND W. R. STERN

the duration of the crop, i.e., the time it occupies from planting to harvesting; it also permits an evaluation of crops on a yield-per-day basis (Hiebsch and McCollum, 1987). It does not appear to have been adopted widely. It is calculated as: ATER = (Liti

+ Ljtj)/T

where Li and Lj are relative yields or partial LERs of component crops i and j, ti and tj are the durations (days) for crops i and j , and T is the duration (days) of the whole intercrop system. 4. Staple Land Equivalent Ratio

In situations where the primary objective is to produce a fixed yield of one component (staple) crop, usually the cereal, and some yield of the legume, Reddy and Chetty (1984) proposed the concept of the Staple Land Equivalent Ratio (SLER) as an extension of the LER. It is based on the assumption of a basic requirement for minimum supply of a major staple crop such as the cereal. It is estimated as: SLER =

(q/qi) + Pij ( Y j i / Y j j )

where &/yiis “the desired standardized yield” of staple i, Pijis the proportion of land devoted to intercropping, and Yji/yjj is the relative yield of crop j. This index is peculiar to India and does not appear to have been used widely there either.

5.

Comparison of RYT, LER, ATER, and SLER

Examples are given in Table I1 of some RYT, LER, ATER, and SLER values calculated from two experiments that examined maize intercropped with either pigeonpea or soybean in replacement series (Dallal, 1974, 1977). For both intercrop systems, all four functions indicate higher biological efficiency in the maize-pigeonpea combination because components are of contrasting maturities. The lower efficiency of the maize-soybean combination, however, could be due to competitive factors because with similar maturities, the components would be exploiting the same environment. The values of RYT and LER were similar in both intercrop systems but as has already been mentioned, RYT is generally based on the replacement series (de Wit, 1960; de Wit and van den Bergh, 1965). Besides the RYT, both ATER and SLER are restricted to specific intercrop situations. ATER

Table I1 Comparison of Calculated Values of Four Indices for Evaluating Cereal-Legume Intercropping Efficiene Maize-pigeonpea (Dallal, 1974)

Crop duration (days)b Sole crop yield (kg/ha) Intercrop yield (kg/ha) Relative yield total (RYT) Land equivalent ratio (LER) Area time equivalent ratio (ATER) Staple land equivalent ratio (SLER)

Maize-soybean (Dallal, 1977)

Maize

Pigeonpea

Maize

Soybean

110 3130 2979

170 1871 1481

110 5353 5118

110 1634 517

1.74 1.74 1.41 1.35

‘Derived from the data of Dallal (1974, 1977). %ese values have been rounded off to the nearest 10.

1.28 1.28 1.28 1.28

50

FRANCIS OFORI AND W.

R. STERN

is only appropriate in systems with component crops of contrasting maturities such as 110-daymaize and 170-daypigeonpea (Dallal, 1974);when components are of similar growth durations, ATER values are similar to RYT and LER (Table 11) (Hiebsch and McCollum, 1987). SLER is only applicable when it is desired to attain a specificyield of a staple cereal crop and yield from the legume component is a bonus (Willey, 1979). SLER therefore cannot be used to evaluate intercropping efficiency in situations where it is desired to produce yield from equally acceptable component crops, and this is the most common goal of cereal-legume intercrop systems (Willey, 1979). From this, LER is considered to be the most appropriate general function to determine the efficiency of cereal-legume intercrop systems and could be applied to any form of intercropping. However, when the difference between growth durations of component crops is substantial, time becomes an important element and ATER is considered to be a more appropriate index of efficiency of the system. In this chapter, except where otherwise stated, intercropping efficiency or yield advantage due to intercropping will be estimated by the LER.

c. THELANDEQUIVALENT RATIO: ITSADVANTAGES AND DISADVANTAGES The Land Equivalent Ratio (LER) is the most frequently used index to determine the effectiveness of intercropping relative to growing crops separately (Willey, 1985). Generally, the value of LER is determined by several factors including density and competitive abilities of the component crops in the mixture, crop morphology and duration, and management variables that affect individual crop species (Enyi, 1973; Natarajan and Willey, 1980a; Fawusi et al., 1982). It has been suggested that in density studies of cereal-legume intercrop systems, the sole crop yields used as standardization factors for estimating LER should be at the optimum densities of the crops (IRRI, 1974; Huxley and Maingu, 1978). This avoids the confounding of beneficial interactions between components with a response to change in density (Trenbath, 1976). The values of LER follow the density of the legume component rather than that of the cereal (Ofori and Stern, 1987). Differences in competitive ability affect the relative performance of component crops and thus the LER values of different cereal-legume intercrop systems. In maize-bean combinations, Davis and Garcia (1983) found that yield reductions were related to plant height. Maize yields were reduced by 17% when intercropped with the most competitive bean cultivars (Group IVB, the aggressive climbers), by 14% with the intermediate cultivars (Group IVA), and by 8% with the weak climbing cultivars (Group IIIB). The LER values calculated using the mean yield of 114 cultivars of beans as a standardization factor were 1.72 for maize cultivar ICA intercropped with bean Group IVA,

CEREAL-LEGUME INTERCROPPING SYSTEMS

51

and 1.30 with bean Group IIIB. In another study, which compared three contrasting maize cultivars intercropped with beans, LER values for the short maize cultivar, La Posta, and the medium height cultivers, Suwan-1, were 1.04, and with the taller cultivar, ICA H-210, it was 0.94. Using sorghum and soybean, Wahua and Miller (1978a) obtained an LER value of 1.11 with tall and 1.09 with short sorghum, not a significant difference. Differences in growth durations of component crops affect the magnitude of the LER. The LER values in crops with similar maturities are usually less in crop combinations with contrasting maturities (Table 11) (Trenbath, 1976; Willey, 1979). Enyi (1973) studied maize or sorghum intercropped with either cowpea or beans of similar growth durations and found that productivity was less when compared to intercropping of these cereals with 240-day pigeonpea. The estimated partial LER of maize was 0.72 with pigeonpea, 0.64 with beans, and 0.50 with cowpea. With sorghum, a partial LER of 1.65 was obtained with pigeonpea, 1.01 with beans, and 0.93 with cowpea. The almost similar and lower partial LER values for the associations with beans and cowpea may be due to competition for growth-limiting factors, because peak demands on the environment by these crops might have coincided with those of the cereals. The higher partial LERs obtained with sorghum compared to maize indicates that it was offered less competition from the associated legumes and therefore was able to maintain higher plant yields (Andrews, 1974). The availability of water also appears to influence the LER. In maizebean (Fisher, 1977) and sorghum-cowpea intercrop systems (Mafra et al., 1981; Rees, 1986), LER values increased with the availability of water and diminished when water was limiting. However, Natarajan and Willey (1980b) found that LER increased under limited water situations. 1. Advantages for LER

As an index of combined yield, LER provides a quantitative evaluation of the yield advantage due to intercropping (Willey, 1979). Although component crops may give greatly different yields, the estimate of relative yields with sole crops at optimum or recommended densities as references gives comparable scales for both components, permitting comparisons of various crop combinations. LER could be used either as an index of biological efficiency to evaluate the effects of various agronomic variables (e.g., fertility levels, density and spacing, comparison of cultivar performance, relative time of sowing, and crop combinations) on an intercrop system in a locality or as an index of productivity across geographical locations to compare a variety of intercrop systems (Chetty and Reddy, 1984).

52

FRANCIS OFORI AND W. R. STERN

LER is identical to the RYT of de Wit and van den Bergh (1965), and can be used for any set of intercropping treatments (Willey, 1979). The partial LER values give an indication of the relative competitive abilities of the components of intercrop systems. In the intercropping mixture, the species with higher partial LER is considered to be more competitive for growth-limiting factors than the species with lower partial LER (Willey, 1979). Partial LER is more applicable to intercropping experiments than the relative crowding coefficient, k, used in measuring competitive ability in competition studies (de Wit, 1960; Hall, 1974a,b). 2. Disadvantages of LER

LER is based on land area only and does not take the duration of component crops into consideration. However, crop production is a function of both crop duration (time) and land area because land occupancy by a given intercrop system is frequently of longer duration than for sole crops. In these situations, the concept of Area Time Equivalent Ratio (ATER) developed by Hiebsch (1980) is appropriate; a detailed analysis of its applicability is given by Hiebsch and McCollum (1987). Several methods have been suggested in the literature for calculating LER using different sole crop values as standardization factors. These include averaging all the sole yields in each block or replication (Fisher, 1977); using the average sole crop yields in the entire experiment (Mead and Stern, 1980; Oyejola and Mead, 1982); using the average sole crop yield at each treatment level in studies that involve graded levels of a factor such as fertilizer or herbicide (Mead and Willey, 1980); and using the yield of the best sole crop treatment of each crop (Huxley and Maingu, 1978; Mead and Willey, 1980). The choice of sole crop yield for standardizing mixture yield in the estimation of LER is not clear and a generalization is not possible. The method used will depend on the aim of the experiment. As an index of biological efficiency, LER is based on harvested products and not on desired yield proportions of the component crops predetermined at sowing (Mead and Stern, 1980; Mead and Willey, 1980; Riley, 1984). This is overcome by the “effective LER” proposed by Mead and Stern (1980), and again later by Mead and Willey (1980) as an alternative to LER for evaluating the biological efficiency of a given required proportion of component crops in an intercrop system.

111. COMPETITIVE RELATIONSHIPS BETWEEN COMPONENT CROPS In plant populations, competition is defined as the situation in which each of two or more plants growing together in the same area seek the same

CEREAL-LEGUME INTERCROPPING SYSTEMS

53

growth factor, which is below their combined demands (Clements et al., 1929; Donald, 1963). Willey (1979) pointed out that the efficiency of production in cereal-legume intercrop systems could be improved by minimizing interspecific competition between the component crops for growth-limiting factors. Growing component crops with contrasting maturities so that they complement rather than compete for the same resources at the same time is one way of achieving this. It is substantiated by the yield advantages reported from various studies: 85-day bean and 120-day sorghum gave a 55% total yield increase (Willey and Osiru, 1972; Reddy et al., 1980) obtained a 31% yield advantage with 82-day millet and 105-day groundnut; and Natarajar) and Willey (1980a) found a 62% yield advantage with 82-day sorghum and 173-day pigeonpea. In contrast, lesser advantages have been reported in crop combinations in which interspecific competition is evident due to similar or almost overlapping growth durations. In this category, Wahua and Miller (1978a) obtained an 11Vo advantage with a sorghum-soybean combination; 11Yo was obtained with 112-day maize and 116-day soybean (Dallal, 1977); and 8% with a maize-cowpea system (Wanki et al., 1982). No yield advantages were found in maize-cowpea (Haizel, 1974) and sorghum-cowpea (Andrews, 1972; Rees, 1986) intercrop systems in which components were of similar growth durations. Competition between component crops for growth-limiting factors is regulated by basic morpho-physiological differences and agronomic factors such as the proportion of crops in the mixture, fertilizer applications, and relative time of sowing (Harper, 1961; Trenbath, 1976). Where component crops are arranged in defined rows, the degree of competition is determined by the relative growth rates, growth durations, and proximity of roots of the different crops. The cereal component, with relatively higher growth rate, height advantage, and a more extensive rooting system, is favored in the competition with the associated legume. The cereal is described as the dominant component and the legume as the dominated component (Huxley and Maingu, 1978). In considering the relative yields of cereals and legumes in intercropping systems, a survey of 40 published papers shows that the yield of the legume component declined, on average, by about 52% of the sole crop yield, whereas the cereal yield was reduced by only 11% (Table 111). Thus the general observation is that yields of legume components are significantly depressed by cereal components in intercropping. A. THELIGHTFACTOR The rate of dry matter production in crops depends on the efficiency of the interception of photosynthetically active radiation (PAR) (Biscoe and Gallagher, 1977; Monteith, 1977).

54

FRANCIS OFORl AND W. R. STERN

Table I11 Yields of Component Crops and Relative Yield Loss Due to Intercropping in Various Cereal-Legume Intercrop Systems Sole crop yield (kglha) Crop combination Maize-beans

Maize-cowpea

Maize-soybean

Maize-groundnut Maize-calopo Maize-greengram Maize-pigeonpea Sorghum-cowpea

Sorghum-soybean Sorghum-groundnut Sorghum-greengram Sorghum-pigeonpea

Sorghum-blackgram Sorghum-chickpea Millet-groundnut Millet-pigeonpea

Cereal

Legume

7320 7290 4126 5591 2080 1004 3860 6500 2678 7408 8455 9800 5353 8189 41 14 3467 7200 1987 8189 2080 2080 3170 2891 2579 3568 4860 3340 3680 4670 2794 8255 4467 3208 2853 3603 4325 2302 2407 2795 2354

1620 1958 1493 2986 1159 130

1176 2035 584 1500 3430 2100 1634 2677 1824 2290 3278 2441 1742 1159 I210 1195 997 584 676 2741 986 889 1430 704 2304 1017

Cereal

7 10

8 30 +I1 31 + 45 33 25 18 12 46 9 13

14 4 2

Legume

Fisher (1977) Cordero and McCollum (1979) Francis et a/. (1982b) Davis and Garcia (1983)

40

Agboola and Fayemi (1971) Haizel (1974) Remison (1978) Wanki et a/. (1982) Faris et a/. (1983) Ofori and Stern (1986)

83 12 49 41 46 59 44 80 80 46 72 87

1

18

74

3

16 +5

17 6

1446

11

14 9 0 +2

52 48 7

Reference

60 69 39 74

3 +4 +9 4 31 35 +2 47 4

1380 1084

852 849 2463 1235 1244

“Percentage of sole crop yield.

Yield loss due to intercropping ( V O ) ~

43 48 68 72 33 40

Betse (1976) Beets (1977) Dallal (1977) Searle et a/. (1981) Ahmed and Rao (1982) Chetty and Reddy (1984) Chui and Shibles (1984) Baker (1978) Searle el a/. (1981) Agboola and Fayemi (1971) Agboola and Fayemi (1971) Yadav (1982) Andrews (1972) Faris et a/. (1983) Singh and Jain (1984)

42 47 65 77 44 39 32 43 40 36

Wahua and Miller (1978a) Singh and Jain (1984)

60 87 17 73 22

Singh and Jain (1984)

Singh and Jain (1984) Chetty and Reddy (1984) Singh and Jain (1984) Freyman and Venkateswarlu (1977) Natarajan and Willey (1980a) Rao and Willey (1980) Rego (1981) Singh and Jain (1984) Chowdhury and Misangu (1981) Reddy et a/. (1980) Willey and Reddy (1981) Rao and Willey (1983)

CEREAL-LEGUME INTERCROPPING SYSTEMS

55

1. Light Interception

The amount of light intercepted by the component crops in an intercrop system depends on the geometry of the crops and foliage architecture (Trenbath, 1982; Tsay, 1985). The generally taller cereal shades the legume, and at high densities causes reduced growth and yield of the companion legume. Gardiner and Cracker (1981), maintaining a constant bean density of 220,000 plantdha, found that varying maize density from 18,000 to 55,000 plantdha progressively reduced the amount of light available to the beans. At the low maize density (18,000 plantdha), bean received 50% of the incident light, compared to 20% at the highest maize density (55,000 plantdha). At the highest maize density, yield of the intercrop bean was only 30% that of the sole bean. Several studies at the International Crops Research Institute for the SemiArid Tropics (ICRISAT), India, have measured percentage light intercepted using short-duration cereal (e.g., maize, sorghum, or millet) and longduration legume (e.g., pigeonpea or groundnut) both as sole crops and as intercropping mixtures (Reddy and Willey, 1981; Sivakumar and Virmani, 1980, 1984; Marshall and Willey, 1983). In a sorghum-pigeonpea combination, the amount of light intercepted relative to incoming incident radiation at 55 days was 84% in sole sorghum, 65% in sole pigeonpea, and 80% in the intercropping mixture (Natarajan and Willey, 1980b). The data of Sivakumar and Virmani (1980) from a maize-pigeonpea intercrop system shows that light interception was low with the initial slow increase in leaf area index (LAI), and above 80% when LA1 reached about 3. Although light interception in the sole crops and the intercrops were almost similar, the foliage canopy of the intercrop was more effective in capturing the light. The intercrop system attained an LA1 of 3 in 45 days, compared to 50 days in the sole maize and 115 days in the sole pigeonpea. Although the ICRISAT studies do not report the quantity of light reaching the top of the associated legume canopy, the light intercepted by the similarly spaced sole cereal crops suggests significant reductions in light available to the companion legume. Assuming a peak LA1 of 2 for the intercrop cereal in these studies, the amount of light incident on the companion legume canopy would be 30% or less of the total incoming radiation. The reduction in light reaching the legume canopy when intercropped with a taller component crop has been clearly demonstrated in a cassava-soybean intercrop system in southern Queensland by Tsay (1985). He found that PAR transmission was closely related to the distribution of leaf area in the cassava canopy, and isopleths of PAR relative to full sunlight decreased towards the top of the legume canopy (Fig. 1). When intercropped soybean rows were 45-90 cm from cassava rows with an LA1 of about

56

FRANCIS OFORI AND W. R. STERN

3, the amount of light reaching the soybean canopy was about 25% of the total incoming radiation. 2. Efficiency of Conversion of PAR into Dry Matter

The higher productivity of intercrop systems compared to the sole crops may be attributed to better light utilization by a crop canopy composed of plants with different foliage distributions (Willey and Roberts, 1976; Willey, 1979). In a study of maize and pigeonpea, Sivakumar and Virmani (1980) found that dry matter production per unit of PAR absorbed was higher in the mixture than in the sole crops (Table IV). The higher PAR conversion efficiencies of these systems relative to the sole crops may be due to spread of light over greater leaf area, and more efficient distribution of light in the intercrop canopies during early stages of growth. For example, using a pearl millet-groundnut system, Reddy and Willey (1981) obtained energy conversion efficiencies at 68 days of 1.70 g/MJ in sole millet, 1.07 g/MJ in sole groundnut, and 1.95 g/MJ in the mixture. At maximum green leaf area index (61 days after sowing), Marshall and Willey (1983) observed energy conversion efficiencies of 4.1 g/MJ in sole millet, 2.5 g/MJ in sole groundnut, and 4.3 g/MJ in the intercropping mixture. The similarity in efficiency of dry matter production in the sole millet and the mixture was attributed to the greater proportion (60%) of intercrop millet in the total intercrop. 90cm x 90cm

180cm x 45cm

. = 0.15

LA1

LA1 = 3.69

LA1 = 2.8 1

3535d . . ... ..

I: - I.:

.-

.:

I.:.

I

I . . .

..

:.

,

I

-

I"

0

45

0

1

0

I

I

1

45

90

45

J

0

Distance from cassava row (cm)

FIG. 1. Isopleths of relative percentages of available radiation (75%, m-m; A-A;

50%,

of full sunlight) within intercrop cassava canopies in the density of 1.23 plants/m* with interrow spacings of 90 and 180 cm at 98 days after cassava planting. (From Tsay, 1985.) 25%, 0-

0

57

CEREAL-LEGUME INTERCROPPING SYSTEMS

Table IV Total Dry Matter, Grain Yield, Leaf Area Index, Light Interception (@lo), and Efficiency of Dry Matter Production of a Maize-Pigeonpea Intercrop System"

Cropping system

Total dry matter yield (kg/ha)

Grain yield (kglha)

Peak LA1

Sole maize Sole pigeonpea Intercrop total

8,130 7,870 15,290

3,500 1,833 5,038

3.5 3.2 6.1

Light interception at peak LA1

VO)

Dry matter production efficiency WMJ)

90 85 83

3.1 1.2 4.3

%om data of Sivakumar and Virmani (1980).

B. THESIGNIFICANCE O F WATER Water is a most important soil factor in semiarid and subtropical regions, where intercropping is extensively practiced in dryland farming systems and inadequate rainfall may frequently limit crop production (Baker and Norman, 1975). The differences in root systems, depth of rooting, lateral root spread, and root density are factors that affect competition for water between component crops (Babalola, 1980; Haynes, 1980). The use of different parts of the soil profile by root systems of different crop species minimizes the degree of competition for water (Haynes, 1980). When component crops compete for available water, the cereal, with its higher growth rate and more extensive root system, is generally favored. In India, Natarajan and Willey (1980b) found that the amount of water transpired in a sorghum-pigeonpea intercrop system depended on growth durations of the crops. The total water use by sole pigeonpea at the end of the growing period (173 days after sowing) was 584 mm and in the mixture 585 mm; in sole sorghum at the final harvest (82 days after sowing) it was 434 mm. Using 82-day millet intercropped with 105-day groundnut, Reddy and Willey (1981) obtained a total water use of 406 mm in the mixture, compared to 303 mm in sole millet and 368 mm in sole groundnut. Shackel and Hall (1984) in the United States, studying plant water deficits in sorghum intercropped with cowpea in terms of xylem pressure potential, and osmotic potential under irrigated and water-limited conditions, found that they were not substantially affected by intercropping. However, shading by the associated sorghum slightly increased midday xylem pressure potential and osmotic potential of intercropped cowpea leaves. The patterns of cumulative water depletion from the top 195 cm of soil in the intercropped treatments were similar to values obtained in the sole crops. Sorghum

58

FRANCIS OFORI AND W. R. STERN

and cowpea appeared to be competing equally for soil water as evidenced by absence of substantial effects of intercropping on the water relation of these crops. From the humid tropics of Nigeria, Hulugalle and Lal (1986) reported that water use efficiency (WUE) in maize-cowpea intercrop was higher than in the sole crops when soil water was not limiting; however, under drought conditions, WUE in the intercrop was lower compared to the sole maize. For the favorable moisture regimes, WUE (kg grain per mm per ha) of the intercrop (alternate row arrangement) was 3.6 compared to 2.1 in either of the sole crops, and for the droughty conditions, 1.6 for the intercrop, 2.2 for the sole maize, and 0.5 for the sole cowpea. From the above, it may be concluded that cereal and legume intercrops use water equally, and that competition for water may not be an important factor in determining the efficiency of intercrop systems, except under favorable soil moisture conditions.

c.

THEROLEOF SOIL NUTRIENTS

The major soil nutrients for which component crops compete when in limited supply are nitrogen, phosphorus, and potassium. The cereal component, with a faster-growing or more extensive root system, generally has a competitive advantage over the associated legume (Trenbath, 1976). The inability of the legume to compete for these nutrients are attributed to lesser ramification of their root systems (Rabotnov, 1977). Competition for nutrients is important and could begin early in the growth of the component crops in cereal-legume intercropping systems (Wahua, 1983).

I. Nitrogen The mobility and high demand for nitrogen by most crops, particularly nonlegume, leads to severe competition for it in nonlegume-legume associations (Allison, 1973; Beets, 1982). In these associations, Evans (1977) pointed out that the absorption of nitrogen is controlled by the roots of component crops. In cereal-legume intercropping, the legume component is capable of fixing atmospheric N,under favorable conditions and this is thought to reduce competition for N with the cereal component (Trenbath, 1976). In the absence of an effective N-fixing system, both cereal and intercrop legume compete for available soil N (Ofori et al., 1987). Ibrahim and Kabesh (1971) found that horsebean reduced N uptake of wheat by 26% whereas wheat reduced that of horsebean by 44%. It is apparent that dry matter yield and N uptake of the component crops were

CEREAL-LEGUME INTERCROPPING SYSTEMS

59

severely affected in the mixed stands, as shown by the partial LER values:

Dry matter yield N uptake

Wheat

Horsebean

0.63 0.74

0.63 0.56

In a maize-cowpea intercrop system, Wahua (1983) found that at 105 kg N/ha, the crops were in competition for N and that this occurred before anthesis or flowering. The severity of the competition for N was greater for cowpea and occurred at 40 days and was evident in the associated maize 10 days later. Nitrogen uptake by intercrop cowpea was 64 kg/ha compared to 88 kg/ha in the sole cowpea. Nitrogen uptake of intercrop maize was reduced by 17% compared to sole maize. Without applied N, Chang and Shibles (1985a) and Ofori and Stern (1986) reported strong competition for soil N by intercrop maize and cowpea. This was particularly evident between 49 and 63 days when both crop species were at the reproductive stage and required substantial amounts of N. 2. Phosphorus Phosphorus is a major nutrient that determines the production potential of most grain legumes usually intercropped with cereals (Williams, 1936). Legumes are poorer competitors for P when intercropped with grasses or cereals, this being attributed to differences in root morphology (Donald, 1963; Jackman and Mouat, 1972; Evans, 1977). Lai and Lawton (1962) evaluated root competition for P between corn and intercrop field because using 32P-labeledfertilizer placed at different depths. They found that corn was more vigorous in the uptake of P than beans as a result of its more extensive roots. Using a replacement series design, Dallal (1974) observed that intercropped maize and pigeonpea were competing for P from 28 days onward right up to the maize final harvest (112 days after sowing) and that maize was more competitive. At 112 days, intercropping reduced dry matter yield of maize by 32% and pigeonpea by 66% compared to the sole crops; total P uptake was reduced by 25% in maize and 70% in pigeonpea. Wahua (1983) found that maize and cowpea were competing for P and this was evident at anthesis or flowering (Table V). In the absence of applied P, maize was more competitive than cowpea in the initial stages. However, at high rates of applied P, P uptake of intercrop maize was reduced by 30%, indicating competition for P from cowpea. Competition was clearly expressed in the observation that intercrop cowpea took up only 50% of the sole cowpea P uptake in the absence of applied P, while at a high level of P,

60

FRANCIS OFORI AND W. R. STERN Table V Yield and P Uptake of Maize Intercropped with Pigeonpea or Cowpea ~~~~

P uptake (kg/ha)

Dry matter yield (kg/ha) Source of reference and cropping system

28 DAY

112 DAS"

28 DAY

112 DAY ~

Dallal (1974) Sole maize Intercrop maize Sole pigeonpea Intercrop pigeonpea

880 449 197 44

6408 4361 822 28 1

3.6 2.0 1.1 0.2

13.2 10.0 1 .o 0.3

Grain yield (kg/ha) P rate (kg/ha)

Remision (1978) Sole maize Intercrop maize Sole cowpea Intercrop cowpea

3243 5591 1254 1060

3544 5042 1206 1138

P uptake at 50 DAS P rate (kg/ha)

Wahua (1983) Sole maize Intercrop maize Sole cowpea Intercrop cowpea

3.5 4.6 1.4 0.7

5 .O 3.5 1.7 1.1

uDAS, Days after sowing.

65% was taken up. Remison (1978) concluded, however, from grain yield data that intercropped maize and cowpea grown at two levels of P did not compete for P, because there were no significant differences in yields of the sole crops and the intercrops (Table V).

3. Potassium Studies by Drake et al. (1951) showed that cation exchange capacities (CECs) of roots of legumes are approximately double those of cereals. The relatively high CEC of legumes indicates that on soils with low levels

CEREAL-LEGUME INTERCROPPING SYSTEMS

61

of exchangeable K, the legume would be deficient in K because larger amounts of divalent cations would be adsorbed by the roots. The level of K in many soils decreases as the growing season progresses; consequently K uptake in competition with cereal becomes increasingly difficult for the legume (Drake et al., 1951). Dallal (1 974) found that intercropped maize and pigeonpea were competing for K at different stages of growth: Reduction of K uptake due to companion crop (Olo) 42 days

Maize Pigeonpea

112 days

33

16

52

63

In an 82-day sorghum and 173-day pigeonpea intercrop system, Natarajan and Willey (1980b) found sorghum to be more aggressive for K than pigeonpea, and this severely affected the early growth of pigeonpea. At 74 days, intercropping markedly affected K uptake of sorghum; the sole sorghum absorbed 45 kg/ha of K and the intercrop 165 kg/ha. In pigeonpea, K uptake was 28.6 kg/ha in the sole crop and 3 kg/ha in the intercrop, a reduction of 87.5070. In a maize-cowpea combination, Wahua (1983) found maize to be more competitive for K than cowpea, particularly when N was high. At 50 days, application of 115 kg/ha of N caused reductions of 3 1070 in uptake of K in the intercrop maize, and 50% in the intercrop cowpea, compared to the respective sole crops.

IV.

SOME AGRONOMIC FACTORS INFLUENCING PRODUCTIVITY AND EFFICIENCY

The productivity and efficiency of cereal-legume intercrop systems are affected by various agronomic variables that affect crop yields. In this section, the influence of variables such as component crop density, plant spacing and arrangement, relative time of sowing of component crops, and the effect of applied nitrogen will be discussed. A.

COMPONENT CROP DENSITY

The overall mixture densities and the relative proportions of component crops are important in determining yields and production efficiency of cereal-legume intercrop systems (Willey and Osiru, 1972; Lakhani, 1976). When the components are present in approximately equal numbers, productivity and efficiency appear to be determined by the more aggressive

62

FRANCIS OFORI AND W. R. STERN

crop, usually the cereal (Osiru and Willey, 1972;Lakhani, 1976). This can be illustrated with 5O:SO mixture data from a replacement series experiment in a sorghum-bean intercrop (Osiru and Willey, 1972). The optimal density of either crop of 200,000plants/ha gave an LER of 1.41;this was due to increase in intercrop sorghum yield, because the intercrop bean yield remained unchanged. In a study of maize intercropped with cowpea at densities ranging between 10,000and 40,000plants/ha for either crop and planted in the same hill, maize (cv. TZPB composite) was more competitive than cowpea (cv. TVu-1209)(Fawusi et af., 1982). The response of intercrop maize to increasing component density was similar to that of sole maize. At the lowest mixture density, the intercrop maize yield was 2300 kg/ha, 15% less than the sole maize, and increased to 4600 kg/ha (8% less than sole maize) at 40,000 plants/ha. Pod yield of intercrop cowpea with the lowest density of maize was 941 kg/ha, a reduction of 41(70 of sole cowpea yield at optimum density. At the highest overall density, intercrop cowpea yield was 700 kg/ha, i.e., a yield reduction of 66%. When sole crop yields of maize and cowpea in the optimum density treatment were used to standardize mixture yields, LER values rose with increasing mixture density. From the lowest to the highest density, the LER values were 0.91, 1.14, 1.20,and 1.26. The growth and yield of legume component is reduced markedly when intercropped with high densities of the cereal component. In a maize-bean intercrop system, increasing maize density three-fold, from 18,000to 55,000 plant/ha, caused reductions of 24% in leaf area index and 70% in seed yield of the associated bean (Gardiner and Craker, 1981). Using repacement series designs in a maize-cowpea intercrop system, Chang and Shibles (1985b)showed that the level of the maize population generally imposed a limit on the yield of the intercrop cowpea, and that there was no effect of increasing cowpea density. Even though the cereal component usually contributes a greater proportion of the mixture yield, the magnitude of intercropping advantage or efficiency seems to be determined by the legume component (Ofori and Stern, 1986, 1987). Fisher (1977)studied maize-bean intercrop systems at varying densities which at harvesting were 13,700,27,000,and 47,700plants/ha of maize combined with 23,300, 56,300, and 121,000 plants/ha of beans, respectively, designated as low, medium, and high densities. At each density, the yields of intercrop maize did not differ from those of the sole maize. However, intercrop bean yield significantly increased with a rise in bean density. The seed yields of beans were 320 kg/ha, 650 kg/ha, and 940 kg/ha from the lowest to the highest density. Although maize contributed more than 80% of the mixture yield at each density, the LER values followed the trends in intercrop bean yields: 0.57,0.92,and 1.39 from the lowest to the highest density. These observations are consistent with the data of Francis

CEREAL-LEGUME INTERCROPPING SYSTEMS

63

et al. (1982a,b) on maize intercropped with bush or climbing beans at different component crop densities. Results from two separate studies using sorghum and pigeonpea also show that the cereal contributes a greater proportion to mixture yield, but that the efficiency of the system measured in terms of LER follows the trends in the intercrop legume yields (Freyman and Venkateswarlu, 1977; Natarajan and Willey, 1980a). When intercrop sorghum densities were varied from 55,000 to 220,000 plants/ha and combined with constant pigeonpea density of 37,000 plantdha, the intercrop sorghum yield reponse was linear. In contrast, the intercrop pigeonpea yield decreased with rising sorghum density (Freyman and Venkateswarlu, 1977). The highest LER value was obtained at the lowest sorghum density and decreased with rising sorghum density (Table VI). It seems that density of the cereal component determines the level of combined mixture yield, but that the efficiency of cereal-legume intercropping systems, measured in terms of LER, follows the trend of the legume component. B. PLANT ARRANGEMENT AND SPACING Row arrangements, in contrast to arrangements of component crops within rows, improve the amount of light transmitted to the lower legume canopy. Such arrangements can enhance legume yields and efficiency in cereal-legume intercrop systems (Mohta and De, 1980). In a maize-groundnut intercrop system, Evans (1960) obtained LERs of 1.09 in the same row arrangement compared to 1.30 in alternate rows and 1.38 in a hill arrangement (Table VI). In two separate studies involving pigeonpea and soybean, maize yield was not significantly reduced when planted in alternate rows with the legume in comparison to planting in the same rows (Dallal, 1974, 1977). From Table VI, it may be seen that biological efficiency measured as LER was higher when component crops were sown in alternate rows rather than in the same row. In the maize-pigeonpea system, maize yield was not affected in the alternate row arrangement, but this was reduced by 20% when the pigeonpea was in the same row (Dallal, 1974). From these studies, it was found that arrangement of component crops in alternate rows is more beneficial than in the same rows. In contrast to these observations, Agboola and Fayemi (1971) did not observe any difference whether maize and cowpea were planted in the same or alternate rows (Table VI). The use of double rather than single alternate row arrangements of component crops improve the yield and light penetration to the canopy of the

Table VI Effects of Plant Arrangement and Spacing of Component Crops on Productivity and Efficiency of Various Cereal-Legume Intercrop Systems Yield’ (kg/ha) Crop combination and treatment Maize with Groundnut Same row Alternate row In hills (Sole crop) Cowpea Same row Alternate row (Sole crop) Pigeonpea Same row Alternate row (Sole crop) Soybean Same row Alternate row Alternate 2-rows (Sole crop) Soybean Alternate rows Alternate 2-rows (Sole crop) Soybeanb (alternate rows) 70 x 30cm 70 x 60cm 70 x 90cm (Sole crop) Sorghum with Pigeonpea Same row (60 cm)‘ Alternate 2-rows (90 Alternate 2-rows (135 cm)‘ (Sole crop) Soybean Alternate rows Alternate 2-rows (Sole crop) Groundnut Alternate 2-rows (60 Alternate 2-rows (90 Cowpea Alternate 2-rows (60 Alternate 2-rows (90

Intercrop cereal

Intercrop legume

2,851 2,554 2,580 (3,330)

74 1 744 851 (1,422)

LER

Reference

1.09 1.30 1.38

Evans (1960)

593

1.54

Agboola and Fayemi (1971)

590

1.55

1,915 1,950 (L990)

(1.033)

2,4% 2,979 (3,130)

1,239 1,481 (1,871)

1.46

4,480 5,118 5,051 (5,353)

170 517 303 (1,634)

0.94 I .27 1.13

Dallal (1977)

3,530 3,700 (3,344)

550 720 (2,783)

1.25 1.37

Mohta and De (1980)

6,980 7,095 6,910 (7Jm

460 650 820 (3,405)

1.05 1.12 1.14

Chui and Shibles (1984)

8,380

1,040

1.24

Freyman and Venkateswarlu (1977)

8,840

1,070

1.29

9,110

1,150

1.35

(10,483)

(1,634)

1,540 1,500 (1,760)

1,247 (2,783)

990

Dallal (1974)

1.74

1.23 1.30

Mohta and De (1980)

Singh (1981)

3,055

323

I .47

3,185

386

1.52

3,605

418

1.87

3,635

716

2.19

Singh (1981)

‘Figures in parentheses are sole crop yields for calculating LER. bData of 1981 experiment. ‘Denotes cereal row spacing.

CEREAL-LEGUME INTERCROPPING SYSTEMS

65

legume component. In maize-soybean and sorghum-soybean studies conducted by Mohta and De (1980), the yields of the cereals were not affected by intercropping with soybeans when arranged in either single or double alternate rows. In the maize-soybean combination, there was a 3 1070yield increase in the intercropped soybeans when components were arranged in double alternate rows relative to single alternate rows. The LER was 1.37 in the double alternate row and 1.25 in the single alternate row arrangement. With sorghum, intercrop soybean yield in the double alternate rows was 1247 kg/ha, an increase of 26% over the single alternate row arrangement. The LER was 1.30 in the double alternate row arrangement and 1.23 in the single alternate row (Table VI). When alternating pairs of sorghum rows 90 cm apart with two rows of an associated legume, Singh (1981) found that LER was greater compared to sorghum at 60 cm between rows with two rows of the legume in between (Table VI). In a parallel study, Wagmare et al. (1982) found that light penetration was markedly increased in the wider row arrangement. At 60 days, light incident on the intercrop legume canopy with sorghum rows spaced at 60 cm was 52% of the incoming radiation, and 70% when sorghum was spaced at 90 cm. Widening interrow spacing of the cereal component to accommodate more rows of the legume component improves legume yield and efficiency of the intercrop system (Table VI). In a study of sorghum at 220,000 plantslha intercropped with pigeonpea at 37,000 plants/ha, 90-cm interrow spacing of sorghum improved the yield of intercrop sorghum by 5% and that of associated pigeonpea by 3% when compared to 60cm rows (Freyman and Venkateswarlu, 1977). Further widening of sorghum interrows to 135 cm gave a 9% increase in sorghum yield over the 60-cm spacing, and pigeonpea yield was increased by 11070. The LER values for the three spacingswere 1.20 for 60cm, 1.29 for 90cm, and 1.35 for 135 cm. The yield of intercrop soybean increased by 78% when spacing between hills with associated maize was adjusted from 30 to 90cm; the LER for the 30-cm spacing was 1.05 and 1.14 for the 90 cm (Chui and Shibles, 1984).

From several studies, it would appear that the yield of the cereal component is usually less affected by component densities and manipulation of spacing between component crops. Intercrop legume yield usually is reduced significantly, however, depending on the proximity of the cereal component. This could be due to the intensity of shading at the top of the legume canopy, as observed recently in a cassava-soybean combination by Tsay (1985) (see Fig. 1).

c.

RELATIVE TIMEOF SOWING OF COMPONENT CROPS

The relative time of sowing of component crops is an important management variable manipulated in cereal-legume intercrop systems but has not been extensively studied. Andrews (1972) pointed out that differential sowing improves productivity and minimizes competition for growth-limiting

66

FRANCIS OFORI AND W. R. STERN

factors in intercropping. Willey (1979)also pointed out that sowing component crops at different times ensures full utilization of growth factors because crops occupy the land throughout the growing season. Francis et al. (1976)found that sowing maize and beans 5-15 days apart reduced yields of the intercrops compared to sole crops (Table VII). In contrast to simultaneous sowing, maize sown 5-15 days earlier than beans increased maize yields by 13-43'70,and the associated bean yields were rcduced by 20-27%. On average, intercropping efficiency measured as LER was 39% higher when beans were sown 5-15 days before maize. In studies on maize intercropped with four contrasting bean cultivars sown 5-10 days apart, results suggest that near-simultaneous sowing of component crops is optimal to attain the highest combined yields and intercropping efficiencies (Francis, 1978;Francis et al., 1982a). In Columbia, Francis et al. (1982b)varied dates of sowing maize and indeterminate beans (types I1 and 111) and found maize to be more competitive than beans at all sowing dates, except when beans were sown 10 days earlier. Simultaneous sowing resulted in a bean yield reduction of 51 070 and maize yield reduction of 3 1 Yo. Sowing maize 10 days before beans reduced bean yield by 69% and maize by only 7%. Beans sown 10 days earlier reduced maize yield by 53% and bean yield by 21 %. In terms of LER, similar results were obtained by staggered and simultaneous sowings. However, the LER followed the trends in bean yields rather than the maize yield, because sowing maize 5 and 10 days before beans gave relatively lower LER values

Table VII Yield and LER of Component Cereal and Legume Sown Less Than 1 Week Apart Yield (kg/ha) Sowing dates

Cereal

Legume

LER

Reference

Maize-bean combination Sole crop Maize, 5-15 days before beans Simultaneous sowing Beans, 5-15 days before maize

7270 5040 5710 69106

939 394 500 483

-

Francis et at. (1976)O

1.11 1.32 1.46

Millet-greengram combination Sole crop Millet, 7 days before greengram Simultaneous sowing Greengram, 7 days before millet

2288 2115 1825 1528

349 26 87 229

1 .o 1.05 1.32

-

May (1982)

OBean variety Pijoo planted at 300,000 plants/ha and maize H-207 at 40,OOOplantslha. *Seems high but verified against the original data.

67

CEREAL-LEGUME INTERCROPPING SYSTEMS

of 1.30 and 1.23 whereas earlier sowing of beans gave LER values of 1.36 and 1.27, respectively. In Nigeria, Remison (1982) did not find any advantage of staggered over simultaneous sowing of maize and cowpea. Intercropping reduced cowpea yield by 57% and maize yield by 35% when sown simultaneously. Sowing cowpea 14 days earlier than maize gave a LER value of 0.88 whereas maize sown 14 days earlier gave a LER value of 1.20. LER values were higher when maize was sown earlier than cowpea but results of this study do not demonstrate consistent LER trends relative to yields of either of the component crops. In Western Australia, Ofori and Stern (1987) concluded from a maize-cowpea intercrop system that staggered sowing of component crops at intervals of 10 or 21 days were of no advantage over sowing them simultaneously. However, they found that the LER followed the trends in cowpea yield rather than maize (Fig. 2) Cowpea sowing relative to maize (days)

'2 1 10

-

w

I,.,

c

ooc -

'10 I

I

0

-10

I

I

-2 1 I

Sole maize

-0-

Intercrop maize

eooa

Y Q) v

.-*

'

600C

D 0)

4000

C

2000

-A-

I

.

J 1.0

Sole cowpea

'

-/intercrop cowpea -2 1

-10

0

'10

+21

Maize sowing relative to cowpea (days]

FIG. 2. Seed yields of maize and cowpea and LER of the intercrop system when component crops are sown differentially. Sole crops were established only at simultaneous sowing, i.e., 0 days. I, Least significant difference (LSD) @ < 0.05) to compare sole crops and intercrops of maize (M) and cowpea (C). (From Ofori and Stern, 1987.)

68

FRANCIS OFORI AND W.R. STERN

Manipulating the time intervals between growth durations of component crops influences efficiency of cereal-legume intercrop systems. In an 85-day bean and 120-day maize combination, a yield advantage of 20% was removed by sowing beans 28 days after maize (Osiru and Willey, 1976). In another study, May (1982) found that a yield advantage of 32% completely disappeared when greengram was sown one week after bulrush millet (Table VII). From these studies, it may be concluded that staggered or differential sowing of component crops is of no advantage over simultaneous sowing. In staggered sowing, the earlier sown component has an initial advantage over the later sown component. Component crops are unable to compensate fully for yield loss due to earlier or later association with the other component. However, the efficiencies of cereal-legume intercrop systems measured in terms of LER follow trends in the legume yields. It also appears that light is an important factor in determining the magnitude of the LER. D. EFFECTOF APPLIED NITROGEN The responses to applied nitrogen of component crops differ in different cereal-legume combinations 'thus affecting the efficiencies of various intercrop systems (Ahmed and Rao, 1982; Boucher, 1986). I . Maize as the Cereal Component Most studies on the effects of applied N on intercropping systems are based on maize, and these indicate similar grain yield responses to applied N for sole and intercrop maize (Dallal, 1977; Rao et al., 1979; Searle et al., 1981; Chui and Shibles, 1984; Ofori and Stern, 1986). In a maize-soybean intercrop system, Dallal(l977) reported that intercropping drastically reduced the seed yield of soybean, and seed yield showed no response to applied nitrogen (Table VIII). The yields of intercrop soybeans relative to the sole crops were reduced by 80% either with or without applied nitrogen. The efficiency of intercropping measured by the LER was 1.15 without applied nitrogen and 1.09 with 100 kg N/ha. In a maize-groundnut combination with N applied at 0, 25, 50, and 100 kg/ha, Searle et al. (1981) found significant yield reductions ranging from 70 to 88% for intercrop groundnut compared to the sole groundnut. The LER values were 1.36 at nil N, 1.20 at 50 kg N/ha, and 1.24 at 100 kg N/ha (Table VIII). From the United States, Chui and Shibles (1984) reported that maize grain yield responded to applied nitrogen at 135 kg N/ha both as a sole crop and as an intercrop of soybean. Seed yield of the sole soybean at 135 kg N/ha was

69

CEREAL-LEGUME INTERCROPPING SYSTEMS Table VIII Yields (kg/ha) and LER of Various Cereal-Legume Intercrop Systems as Influenced by Applied Nitrogen N rate (To of maximum applied) Source of data and treatments Dallal (1977) Maize Sole Intercrop Soybean Sole Intercrop LER Searle et al. (1981) Maize Sole Intercrop Groundnut Sole Intercrop LER Rego (1981) Sorghum Sole Intercrop Pigeonpea Sole Intercrop LER Chui and Shibles (1984)" Maize Sole Intercrop Soybean Sole Intercrop LER

0

50

5082 4666

-

1478 334 1.15

-

6680 7080 1741 530 1.36

930 1240 1390 800 1.91

6160 6120 3400 570 1.16

-

-

8604 8237

418 1.20

2890 2410

820 1.42

-

100

5623 5099 1789 326 1.09

8941 9178

378 1.24

4590 3710

870 1.43

-

9160 8550

-

3410 420 1.05

"Data for 1981 experiment.

13% more than the yield obtained without nitrogen, but intercrop soybean yield was 585 kg/ha compared to 730 kg/ha without N (Table VIII). The LER value at nil N was 1.28 and 1.13 at 135 kg N/ha. A multilocation appraisal of various maize-legume intercrop systems was conducted under the 5-year INPUTS (Increased Productivity Under Tight

70

FRANCIS OFORI AND W.R. STERN

Supplies) program coordinated in Hawaii, with locations in the tropics and temperate regions (Ahmed et al., 1979). Maize-cowpea and maize-groundnut systems were studied at three locations, maize-mungbean in four locations, and maize-soybeans in seven locations. Levels of applied N in the experimental treatments at each location were specified as 0,25,50, and 100% of the local recommended rate. Table IX shows fitted regression models for sole and intercrop maize grain yields and the intercrop legume seed yields, averaged over all locations for each crop combination in response to applied N. For the maize-groundnut system, only the data from Australia are shown because data from the other locations were incomplete. From these studies, maize grain yield responses to applied nitrogen were positive and behaved similarly to those of cowpea, groundnut, mungbean, or soybean (Fig. 3). However, in the sole maize, there was a lack of response to applied N in the

Table IX Fitted Regression Equations for Component Crop Yields in Response to Applied Nitrogen in Various Maize-Legume Intercrop Systems' Cropping system

Number of locations

Maize-cowpea Sole maize Intercrop maize Intercrop cowpea

3

Maize-groundnutc Sole maize Intercrop maize Intercrop groundnut

1

Maize-mungbean Sole maize Intercrop maize Intercrop mungbean

4

Maize-soybean Sole maize Intercrop maize Intercrop soybean

7

Equationb

R'

y = 2066 y = 1605 y = 1499

+ 29.6~

y = 6830 y = 6964 y = 526

+ 6 2 . 4 ~- 0 . 4 2 ~ ' + 22.3~ + 3 . 2 ~+ 0.02~'

0.91 0.96 0.98

y

= 2257 y = 2044 y = 418

+ + +

19.2~ 15.2~ 4 . 7 ~- 0.04~'

0.95 0.83 0.99

y = 3019 y = 2835 y = 954

+ + +

1806X 11.7~ 5.3x - 0.04xI

0.88 0.96 0.90

+ 22.9~

+

6 . b - 0.04~~

'From the data of Rao eta/. (1979), data averaged over all locations for each system. by is grain or seed yield (kglha); x is N as percentage of recommended dose in kg/ha. 'Data from Australia only.

0.91 0.89 0.99

71

CEREAL-LEGUME INTERCROPPING SYSTEMS

L 0

I

25

I 50

I 100

0

I

I

25

50

I 100

d

------.)--

It--

.

‘.

J1.40

looo

t

RG.3. Mean responses of component crops and land equivalent ratio to applied nitrogen in various maize-legume intercrop systems. ( 0 ), Intercrop maize; (m), intercrop legume; (A), LER estimated from the fitted yield curves. (Drawn from the data of Rao et al., 1979.)

maize-groundnut system that was attributed to high soil N status from previous cropping (Searle et al., 1981). The LER values estimated from the regression models at each nitrogen level ranged from 1.30 to 1.36 in the maize-cowpea system, 1.43-1.55 in the maize-mungbean system, 1.21-1.25 in the maize-groundnut system, and 1.42-1.47 in the maize-soybean system. Intercrop maize increased progressively in response to applied N, heavy applications of N reducing yields of the associated legumes. In a recent study of maize-cowpea intercrop systems, Ofori and Stern (1986) also found declining trends in LER in response to applied nitrogen. They used two maize cultivars of contrasting heights; LER values declined from 1.67 to 1.42 with the short maize XL66 and from 1.40 to 1.32 with the tall maize SR99 as applied N was varied from 0 to 100 kg N/ha.

72

FRANCIS OFORI AND W. R. STERN

The results of these studies show that LER varies with nitrogen status of the soil and in general, where the inherent fertility of the soil is low, there appears to be a greater advantage of intercropping maize with legumes. 2.

Other Cereal Components

Using a 45-cm alternate row arrangement in a sorghum-pigeonpea system with N applied at 0, 60,and 120 kg N/ha at ICRISAT, India, Rego (1981) obtained a progressive increase in grain yields of both sole and intercrop sorghum (Table VIII). At nil N, intercrop sorghum yield was 33% higher than sole sorghum. At 60 kg N/ha, intercropping reduced sorghum yields by 17% and at 120 kg N/ha, the yield reduction was 20%. Applied N did not significantly affect the yield of intercrop pigeonpea, and yield reductions from intercropping ranged from 37 to 42% compared to sole pigeonpea. The LER value without applied N was 1.91 and declined to 1.42 at 120 kg N/ha. These studies show that the intercrop cereal grain yields increase progressively with applied N, while seed yields of companion legumes decrease or are less affected. It would appear that applying nitrogen does not improve the land equivalent ratio and therefore the efficiency of cereal-legume intercrop systems.

V.

NITROGEN ECONOMY OF THE SYSTEM

In fixing atmospheric N,, legumes contribute to the N content of soil either as sole crops in rotation, or as intercrops (LaRue and Patterson, 1981; Heichel and Vance, 1984). In such systems, legumes may either increase the soil N status through fixation and excretion, or in the absence of an effective N-fixing system, compete for N (Trenbath, 1976). In the literature on cereal-legume intercropping, there are surprisingly few data concerning different parts of the system. In the sections following, the existing literature is reviewed on N, fixation, N transfer, losses, and budgeting. Many of the examples cited are derived from agricultural systems other than cereal-legume intercrop systems because the necessary measurements to understand the processes involved are not always available from intercropping studies. A. NITROGEN FIXATION BY THE LEGUMECOMPONENT

The quantity of N, fixed by the legume component in cereal-legume intercropping depends on the species, morphology, density of legume in the

73

CEREAL-LEGUME INTERCROPPING SYSTEMS

mixture, the type of management, and the competitive abilities of the component crops. Legumes of indeterminate growth are more efficient, in terms of N, fixation, than determinate types. Eaglesham et af. (1982) found that in a growing season, soybean fixed more nitrogen than cowpea, but soybean used a greater amount of the N2 fixed to produce seed. While cowpea fixed less N, however, because it had a lower seed N harvest index it thus accrued more N to soil from residues. They also found that this applied to determinate (det) and indeterminate (ind) cowpea types, but there was a difference in the amount accrued: kg N/ha Cowpea cultivar ER-1 (det) TVu 1190 (ind)

N N uptake fixed ~ 32 50 33 101

Seed N

48 49

Residue N balance N 34 + 2 85 + 52

The literature contains only semiquantitative data on N, fixation of intercrop legume. Graham and Rosas (1978) found that N2 (C,H,) fixation by climbing bean (cv. P590)was essentially unaffected by intercropping with maize. N2 fixation reached a maximum of 20.6 moles/plant/hr in the sole bean at 68 days and was reduced by only 10% when intercropped with maize. Shading by the cereal reduces both the seed yield and the N, fixation potential of the companion legume (Wahua and Miller, 1978a,b). In a sorghum-soybean intercrop system, a tall variety of sorghum reduced soybean yield by 75% and N, fixation at the early pod-fill stage by 99% (Wahua and Miller, 1978b).

Soybean intercropped with Tall sorghum Short sorghum

N fixation (mole C,H,/plant/hr) 0.012 3.17

Seed yield (kdha)

688 2275

With the short sorghum, soybean received more than 90% of the incoming radiation, compared to less than 50% with the tall sorghum. In a sorghum-groundnut intercrop system, partial defoliation of sorghum increased the amount of light for the associated legume and enhanced N,(C,H,) fixation (Nambiar et af., 1983). In the absence of applied N, N,(C,H,) fixation of groundnut intercropped with maize was not influenced by shading, although light reaching the legume was reduced by 33% (Nambiar et af., 1983). When 50% kg/ha of N

74

FRANCIS OFORI AND W. R. STERN

was applied, fixation was reduced by 55% and light reaching the groundnut canopy was 54% of daylight. Heavy applications of N fertilizer (100 and 150 kg N/ha drastically reduced fvtation: 7.0 moles/plant/hr at 100 kg N/ha and 3.5 moles/plant/hr at 150 kg N/ha. Using the "N fertilizer dilution method, estimates of N derived from atmospheric N, by cowpea intercropped with maize were about 60%, corresponding to fixation of 81 kg N/ha, and in the sole cowpea, 56%, corresponding to 83 kg N/ha (Eaglesham et al., 1981). Ofori et al., (1987) evaluated the N economy of a maize-cowpea intercrop system using both "N natural abundance and "N-labeled fertilizer methods. They found that cowpea maintained its ability to fix atmospheric N, when intercropped with maize, but that N, fixation was reduced by N fertilizer applications (Table X). The comparable P values (percentage of N derived from atmospheric N,) of the intercropped cowpea with or without applied N was attributed to greater N uptake by the associated maize, which induced the companion cowpea to be more symbiotic.

Methods of Estimating N, Fixation Both direct and indirect methods are used to estimate N, fixation by legumes in the field. The indirect methods are the ureide and acetylene reduction assays; these provide qualitative and semiquantitative information only on N, fixation (LaRue and Patterson, 1981). The ureide method is limited to legumes that produce and translocate N as ureides, e.g., soybean and cowpea (Atkins, 1982), and will not be discussed further. The three principal ways of measuring N2fixation in cereal-legume intercropping are 1. the acetylene reduction method 2. the total N-difference method 3. the I5N dilution methods a. Acetylene Reduction Method. This method evolved from the observations that the enzyme nitrogenase reduced acetylene (C,H,) to ethylene (C,€&) (Dilworth, 1966, Schollhorn and Bums, 1966). Techniques for field assay include excavation and incubation of freshly excised nodulated roots in a chamber with 1-209'0 C,H, for 30 to 120 min, followed by analysis of the ethylene produced with gas chromatography. The ratio of acetylene reduced to N, fixed is assumed to be 3:1, because the reduction of N, to ammonia uses six electrons while the production of ethylene requires two (Hardy et al., 1973). The advantages of the method are sensitivity, speed, and economy. It has been used to evaluate N, fixation in cereal-legume intercropping (Graham and Rosas, 1978; Nambiar et al., 1983). It gives an instantaneous, semiquantitative measure of fixation rather than permitting the measurement of fixation over a growing season by a crop (LaRue and Patterson, 1981), something that most agronomists would be interested in.

75

CEREAL-LEGUME INTERCROPPING SYSTEMS

Table X Proportional Dependence on N, Fixation (P) and N, Fixed by Sole and Intercrop Cowpea Estimated by "N Natural Abundance (NA) and '"-Labeled Fertilizer (NL) Methods under Field Conditionsa Harvest

Cropping system

P

(070)

N, fixed

No added fertilizer N (NA experiment) kg N/ha 150 days

(straw) 150 days

(seed) 150 days

(seed

+ straw)

Sole crop Intercrop

72.0 61.3

32.4 18.0

Sole crop Intercrop

68.0 67.8

54.7 40.8

Sole crop Intercrop

69.4 65.7

87.1 58.8

Fertilizer N added (25 kg N/ha) (NL experiment) kg N/ha 150 days

(straw) 150 days (seed) 150 days

(seed

+ straw)

Sole crop Intercrop Sole crop Intercrop

47.6 64.9

26.2 25.9

57.1 69.3

55.7 47.1

Sole crop Intercrop

53.4 67.7

81.9 73.0

aFrom Ofori el al. (1987).

b. Total N-Difference Method. This estimates N, fixed by a legume as the difference in total N uptake by legume and a nonfixing control plant such as grass or cereal, expressed as:

N2 fixed by legume = Total NIegume - Total Nnonfixing plant The total N of the nonfixing plant is derived solely from soil, and the difference in values of N uptake between the legume and nonlegume is assumed to be the quantity derived by fixation (Williams et al., 1977). The basic assumption of the method is that both the legume and the nonlegume remove soil N from the same N pool and in the same proportion as the amount of N available (LaRue and Patterson, 1981). In addition to nonlegumes, nonnodulating and uninoculated legume genotypes have been used (Nutman, 1976). Bezdicek et al. (1978) compared inoculated and uninoculated soybeans under field conditions in a 2-year study. They found that uninoculated

76

FRANCIS OFORl AND W. R. STERN

soybeans accumulated an average of 90 kg N/ha, while nodulated soybeans yielded 378 kg N/ha. By difference, the net N, fixation was calculated to be 288 kg N/ha. c. I5NDilution Methods. The use of the stable isotope "N for estimating N2fixation is a direct and definitive method extensively used for checking the validity of other methods (Burris, 1974). The technique is based on the fact that 15Noccurs naturally in an almost constant ratio with I4N. The ratio of I4N: ''N is approximately 272:1, so atmospheric Nz contains 0.3663 at.% "N or 3663 ppm "N. The only exceptions to this general rule are the slight variations in Is N enrichment of natural nitrogenous substances such as soils, rocks, and coal (Hauck and Bremner, 1976). The changes in isotopic composition of samples relative to the atmosphere permit calculation of enrichment of "N in samples for the study of fixation, transfer, and transformations of N in systems. The main factors limiting the widespread use of "N techniques have been the high cost of N compounds and the need for mass spectrometers to analyze for "N abundance (Hauck and Bremner, 1976). With l5 N methods, there are two approaches, namely, "N-labeled fertilizer and "N natural abundance. Both methods are based on the assumptions that the N,-fixing and nonfixing plants have similar rooting patterns and obtain equal proportions of N from the soil, and that there is no transfer of N during the measuring period (Chalk, 1985). Eaglesham et al. (1981) used the "N-labeled fertilizer method to demonstrate N transfer in a maize-cowpea intercrop system, and Ofori et af. (1987) used both I5N natural abundance and "N-labeled fertlizer to quantify N2 fixation and evaluate the N economy of a similar intercrop system (Table X). B. NITROGEN TRANSFER Evidence in the literature suggests that the N, fixed by the intercrop legume may be available to the associated cereal in the current growing season (Agboola and Fayemi, 1972; Remison, 1978; Eaglesham et al., 1981; Pandey and Pendleton, 1986) or as a residual N for the benefit of a succeeding cereal crop (Nair et al., 1979; Searle et af., 1981; Singh, 1983). Both forms of N transfer are considered to be important and could improve the N economy of various legume-based intercrop systems. This has led to the suggestion that both current and residual N benefits should be evaluated in intercrop systems in which legumes are a component (Willey, 1979). Roots and nodules of legumes are thought to be the important sources of N transfer because of their high N contents (Butler and Bathurst, 1956). In cowpea, Minchin et al. (1978) found N from these sources to be only 6% of the total plant N; this may be inadequate to produce any substantial N

CEREAL-LEGUME INTERCROPPING SYSTEMS

77

benefit for a subsequent crop. From pot studies, Peoples et al. (1983) report that N from roots and nodules of cowpea are 13% of the total plant N. The degree to which N from intercrop legume may benefit a cereal crop depends on the quantity and concentration of the legume N, microbial degradation (mineralization) of the legume residues, utilization of these residues, and the amount of N2 fixed by the legume (Henzell and Vallis, 1977; Herridge, 1982). The N in legume residues may be tied up in the soil organic N pool and may not be readily available to the cereal crop (Ladd et al., 1983). The rate of mineralization of organic N, determined by microbial activity, is primarily influenced by the prevailing moisture and temperature regimes (Ladd et al., 1984). Henzell and Vallis (1977) estimated that under tropical conditions, 30% of the N in legume residues could be mineralized and taken up by grass after 24 weeks.

1. Current N Transfer

The idea of possible N tranfer in the current season from legumes to intercropped cereals originated from early studies on legume-nonlegume mixtures in pots under greenhouse conditions (Nicol, 1935; Virtanen et al., 1937). In a greenhouse experiment, Agboola and Fayemi (1972) observed that early-maturing legumes could possibly improve yields and N nutrition of associated maize in the current season, while late-maturing types could have this effect in the following season. They found that greengram fixed 224 kg/ha of N2in 49 days and improved intercropped maize yield by 72%. Although cowpea fixed 450 kg/ha of N, in 98 days and calopo fixed 354 kg/ha of N, in 84 days, the N2 fixed by these legumes did not influence the yields of the associated maize. Eaglesham et al. (1981) presented evidence from the field of transfer of N from legume to an intercrop cereal, using the "N-labeled fertilizer method. It was evident that at the low N rates, i.e., No and Nzs, N concentrations (percentages) of the intercrop maize were higher in the presence of cowpea than in the sole maize. The transfer of N was confirmed by the significant dilution of IsN in the intercrop maize compared to sole maize at NZs.This was because fixed N would have an enrichment close to natural abundance and its transfer would result in the dilution of "N enrichment in the intercrop maize derived from the fertilizer. Using replacement series designs, Patra et al. (1986) have reported substantial transfer of N from the legume component to the associated cereal in wheat-gram and maize-cowpea intercrop systems in both greenhouse and field studies. However, they did not provide "N data to demonstrate transfer of legume N that would have obviously resulted in

78

FRANCIS OFORI AND W. R. STERN

lower 15Nenrichment of the intercrop cereal compared to sole crop. In a maize-soybean intercrop system, Pandey and Pendleton (1986) reported that soybean green manure provided 28 kg/ha of N to maize when no N fertilizer was applied. The transfer of N from cowpea to the associated maize was not evident from either the field or the greenhouse pot studies by Ofori et al. (1987), because similar I5Nenrichments were obtained in the sole and intercropped maize. They concluded that cowpea and associated maize were competing for applied N and that the N, fixed by cowpea ended up in the seed and was harvested from the system. These findings are consistent with those reported by Danso et al. (1987)using fava beans and barley. In a maize-cowpea combination planted without N fertilizer, Remison (1978)attributed a 72% increase in intercrop maize grain yield over that of sole maize to the transfer of N from cowpea to the associated maize. Unfortunately, no crop N analysis data were provided to justify the conclusion of current N transfer claimed by the author. Based on yield and N contents, Waghmare et al. (1982) found that sorghum benefited from greengram, groundnut, soybean, and fodder and grain cowpea intercrops. Intercrop sorghum grain yield increases ranged from 3 to 16%, grain protein from 10 to 33V0, and N uptake from 16 to 49%. 2. Residual N Transfer The intercrop legume may accrue to N the soil and this may not become available until after the growing season, improving soil fertility to benefit a subsequent cereal crop. Table XI shows results from various studies that used wheat to measure residual fertility of various cereal-legume intercrop systems. Nair et al. (1979)found a mean wheat yield increase of about 30% after a maize-soybean intercrop, and after maize-cowpea the yield increase was 34% when compared to wheat after sole maize. De (1980)evaluated the residual N value of various legume-based intercrop systems and found that blackgram intercropped with either maize or sorghum improved succeeding wheat yield in all N treatments. Searle et al. (1981)found N uptake of wheat following maize-groundnut and maize-soybean intercrop systems to be higher than after maize alone. At 60 days, N uptake by wheat was about 18 kg/ha when preceded by either intercrop systems, and this was similar to N uptake of wheat supplied with 100 kg/ha. Singh (1983) estimated the N benefits to wheat of various preceding legume intercrops. When comparing wheat after sole sorghum with wheat

79

CEREAL-LEGUME INTERCROPPING SYSTEMS

Table XI Effects of Preceding Cereal-Legume Intercrop System and Applied Nitrogen on Grain Yield and Nitrogen Uptake of Succeeding Wheat N applied (kg N/ha) Parameter and treatment

0

50

100

Reference

Grain yield (kg/ha) After sole maize After maize-soybean After maize-cowpea After sole maize After maize-blackgram After sole sorghum After sorghum-blackgram

940 1240 1260 2520 2900 1610 2280

920 1150 1250 4270 5080 41 10 4540

900

Nair et al. (1979)"

1120 1190 5450 5690 5250 5910

Nitrogen uptake (kg N/ha) After sole maize After maize-soybean After maize-groundnut

11.6 17.5 17.8

15.6 21.8 21.2

17.5 21.8 21.2

De (1980)b

Searle et al. (1981)"

"Derived data; N was applied only to the preceding treatment. bN applied to the wheat crop.

after intercrops, he obtained N fertilizer equivalents of 3 kg/ha with soybean, 31 kg/ha with greengram, 46 kg/ha with grain cowpea and with groundnut, and 54 kg/ha with fodder cowpea. It seems that indeterminate legumes with lower seed yield potentials benefit the associated cereals in terms of N in the current season or as residual N for succeeding crops.

C. NITROGENLOSSES Nitrogen is lost from cropping systems through harvested crop products, gaseous N, and leaching of nitrates from soil beyond the root zone (Greenland, 1977). The complexity of the processes involved in gaseous N losses, namely, denitrification and volatilization, and the paucity of direct measurements that vary widely with environmental conditions, especially temperature and water content, has generally limited an accurate assessment of the magnitude of N losses by these means under field conditions (Vlek et al., 1981). Efforts to quantify N gaseous losses are a recent development (Simpson and Steele, 1983), and earlier estimates were made by inference from

80

FRANCIS OFORI AND W. R. STERN

measurements in nitrogen balance studies (Hauck, 1971). The losses of N in cereal-legume intercropping through denitrification and volatilization have not been studied and the examples cited in this section are mainly from other cropping systems. 1. N Harvested in Crop Products

Nitrogen harvested from crops as seed is the largest source of N loss from any cropping system. Assuming N concentration of 1-3% in cereal grain and 3-6% in legume seed, a cereal yield of 3000 kg/ha of grain removes 30-90 kg/ha of N from the soil, and 800 kg/ha of legume seed removes 24-48 kg/ha of N. 2. Gaseous Losses

The important pathways of gaseous N losses from cropping systems are through denitrification, the reduction of NO, to N 2 0 and N, by microorganisms, and volatilization of NH,. a. Denitrifeation. Denitrification results in an array of gaseous N products that renders it difficult to quantify satisfactorily the N loss via this pathway. Hauck (1971) reviewed the literature pertaining to N balances in plant-soil systems and concluded that N losses via denitrification could be of the order of 10-30070 of the N applied, and that this commonly occurs in soils wet for prolonged periods, with low CO, concentrations (Freney et al., 1978).

b. NH, Volatilization. The review by Chalk and Smith (1983) shows that N losses through NH, volatilization are usually small and that these are generally less than 2% of the total N applied. However, on a calcareous soil, Smith and Chalk (1980) measured significant losses of N, (10% of applied NH, N) and N,O (6% of applied NH, N). Simpson (1968) showed up to 60% loss of urea N applied as topdressings between 67 and 112 kg N/ha and considered NH, volatilization to be important. NH, losses from applied N depend on the ammonium source of the fertilizer and on the soil pH (Zamyatina et al., 1968). The rate loss from urea is more than from either ammonium sulfate or ammonium nitrate. Vlek et al. (1981) presented data from various studies in which different forms of N had been applied ranging between 100 and 500 kg N/ha. Assuming mean N applied to be 300 kg N/ha, the N loss via volatilization was 16.7% of N applied from urea, 1 1.4% from ammonium sulfate, and 5% from ammonium nitrate. A review on NH, volatilization by Freney et al. (1983) has shown that N losses increase with rising pH. This effect was demonstrated by Jewitt

81

CEREAL-LEGUME INTERCROPPING SYSTEMS

(1942) who found NH, losses equivalent to 0, 13, and 87% of N when ammonium sulfate was applied to soils of pH 7.0, 8.6, and 10.5, respectively.

3. Leaching Wetselaar (1962) demonstrated that rainfall is the most important factor affecting leaching of nitrates in soils. He estimated that for each millimeter of rain, the mean movement of nitrates down free-drained soils was 1.075 mm. Although severe leaching of soils of the humid tropics is regarded as a major constraint to their productivity, actual quantitative data on removal of nitrogen from soils remain sparse (Greenland, 1977). The only documentation we could find regarding N loss in cereal-legume intercrop systems was concerned with leaching of soil nitrates. Singh et af. (1978) found in a 180-cm soil profile that maize intercropped with mungbean reduced NO, N loss by 60% and intercropped with blackgram, by 41% when compared with sole maize (Table XII). The total N uptake of the maize-mungbean and maize-blackgram intercrop systems were 36.8 and 23.3 kg N/ha, respectively, over and above the N uptake by sole maize. In an 83-day maize and 155-day pigeonpea intercrop system, Yadav (1982) found that the soil profile (0-105 cm) contained larger amounts of NO, N in either the sole pigeonpea or the sole maize systems than in the mixture. The NO, N content after the mixture was 240 kg/ha, compared to 619 kg/ha after the sole maize and 419 kg/ha after the sole pigeonpea. The higher soil nitrate N in sole crops may be due to insufficient early spread of roots to absorb nitrates from all the interrow spaces, resulting in nitrate N leaching down and so enriching the lower soil profiles. From what has been said so far, it is apparent that a lack of quantitative data does not permit a satisfactory partitioning of N losses via the three main pathways, viz., denitrification, leaching, and volatilization. It can be surmised that N losses by each of these pathways could limit the productivity and

Table XI1 Proportion of Applied NO, N in 1804111Soil Profiles and Total N Uptake of Maize, Maize-Mungbean, and Maize-Blackgram Intercrop System@ NO, N loss as @lo of quantity applied

Total N uptake *g/ha)

N recovery

Treatment Sole maize Maize-mungbean Maize-blackgram

56.8 22.7 33.6

61.9 98.7 85.2

48.3

“Derived from Singh et at. (1978).

(@lo)

78.2 67.5

82

FRANCIS OFORI AND W. R. STERN

efficiency of cereal-legume intercrop systems. However, it appears that leaching losses of nitrates in intercropping systems may be less than in sole crops.

D. NITROGEN BUDGETING There are three main sources of nitrogen in cereal-legume intercrop systems: these are N fixed by the legume component from the atmosphere, from fertilizer, and from soil. The only published data that offer some scope for illustrating N budgeting are those of Eaglesham et al. (1981) in Western Nigeria and Ofori et al. (1987) in Western Australia, both studies with maize and cowpea. Using equations suggested by Rennie et al. (1982) to calculate N from fixation, from fertilizer, and from soil, a N balance sheet was constructed for such a system with the data of Eaglesham el al. (1981) (Table XIII).The densities of component crops as sole crops were 60,OOOplantdha of maize and 110,OOO plantslha of cowpea; the intercrop density was half of each sole crop density.

Table XIII N Balances (kg N/ha) of Sole Crops and Intercrops of Maize and Cowpea Determined by "N-Labeled Fertilizer Method with N Applied at 25 kg/hau Cropping system Sole crop Components Sources of N Soil (available) Seed at sowing Fertilizer Atmosphere Total Crop N Harvested seed Residues Total N lossesb Soil N balance after return of residues'

Maize

Intercrop

Cowpea

23.9

60.4

0.5

2.0

4.0 28.4

4.0 83.2 149.6

25.6 2.8 28.4

53.6 96.0 149.6

-

21.0 -21.1

Maize 28.7 0.3 1.6

Cowpea 24.6 1.O

Total 53.3 1.3 3.3

-

1.7 40.7

40.7

30.6

68.0

98.6

28.0 2.6 30.6

24.5 43.5 68.0

52.4 46.1 98.6

21 .o

4.7

4.6

9.3

+ 35.9

- 26.1

+ 18.9

-1.2

'Derived from data of Eaglesham el at. (1981). bLosses of applied N through unknown mechanisms. 1v in residues minus soil N available to crop.

CEREAL-LEGUME INTERCROPPING SYSTEMS

83

The N contributed by seeds of maize and cowpea at sowing was less than 2 kg/ha, fixed N, by intercrop cowpea was about 41 kg/ha, and N from fertilizer N was 3 kg/ha. The total N in the crops was about 99 kg/ha, consisting of N from seeds, fertilizer, N, fixation, and 53 kg/ha from the soil. Assuming a seed N harvest index of 36% for cowpea and 90% for maize, the quantity of N removed in the intercrop system was about 52 kg/ha, 28

kg/ha from maize and 24 kg/ha from cowpea. The N remaining in residues was 46 kg/ha. If the N losses in the study were due to denitrification, leaching, and volatilization, these accounted for 74% of the N applied. NH, volatilization could account for a large proportion of the N loss in the maize-cowpea study if the N fertilizer was not well incorporated in the soil. Denitrification and leaching could also be important; however, the use of ammonium fertilizer suggests that loss through leaching may be low because of the possible sorption of ammonium ions on clay minerals and organic matter (Freney et al., 1983). The resultant net change in soil N after the grain harvests and the return of residues may be calculated as N = N (residues) - N (uptake from soil)

The maize-cowpea intercrop would result in a loss of 14 kg N/ha to the soil, compared to a loss of 21 kg N/ha after sole cropping of maize and a gain of 36 kg N/ha after sole cowpea. The data of Eaglesham et al. (1981) indicate that growing maize and cowpea together does not deplete N excessively from the soil. Sole cropping of cowpea may enhance soil nitrogen fertility and could be beneficial in cereal and legume rotations if stover of high N concentrations are returned to the soil, whereas sole cropping of maize depletes soil nitrogen. The findings of Ofori et al. (1987) are consistent with these conclusions.

VI.

SUMMARY AND CONCLUSIONS A. AGRONOMIC CONSIDERATIONS

It is apparent that light and nitrogen are the main factors influencing the production efficiency of cereal-legume intercropping systems, as determined by the land equivalent ratio (LER). Various studies conducted at ICRISAT, India, and evidence from the cassava-soybean intercrop study by Tsay (1985) show that the taller component suppresses the companion legume through shading, and this is accentuated by application of nitrogen fertilizer.

84

FRANCIS OFORI AND W.R. STERN

The cereal determines the levels of dry matter, grain, and seed protein yields of the intercropping mixture. The trends in the LER, an index of intercropping efficiency, are, however, associated with the yields of legume rather than that of the cereal. In mixed swards, Harper (1977) proposed that when the components compete for growth-limiting factors, the weaker competitor determines the yield efficiency of the mixture. This theory seems to hold in maize-cowpea intercropping, because cowpea, the weaker competitor for N in the various associations with maize, accounts for more of the variations in the LER than maize (Ofori and Stern, 1986, 1987). IN THE SYSTEM B. THEROLEOF NITROGEN

The observations that the efficiency of cereal-legume intercropping relative to growing crops separately is greatest at low levels of N, suggest that at low N, the intercrop legume has a greater dependence on atmospheric N, and, presumably, competition from the associated cereal is minimized. "N dilution methods, that is, 15Nnatural abundance and "N-labeled fertilizer, provide reliable information on the amounts of N, fixed from the atmosphere by the legume as a sole crop or as an intercrop with cereals. Because in the intercropping situation, the cereal is more effective than the legume in taking up nitrogen, the legume's dependence on the atmosphere for N is enhanced and it fixes N, more efficiently than as a sole crop (Danso et al., 1987). With the increasing availability of N from other sources, such as soil and fertilizer, this dependence on atmospheric N, by the legume decreases (Ofori et al., 1987). Although 15Nmethods provide quantitative data showing the partitioning of N in intercrop systems and thus help to interpret mechanisms of nitrogen movements and transfer, the high cost involved in the analyses using a mass spectrometer precludes a much wider adoption of these methods. With the natural abundance method, the low "N abundance measured as parts per thousand units requires special analytical procedures (Bergersen, 1980). In addition, the "N-labeled fertilizer method also suffers from nonuniform labeling and a decline with time of the enrichment of 15Nin the soil N pool (Witty, 1983). To conclude, it appears that the production efficiency of cereal-legume intercropping could be enhanced through the use of more effective strains of rhizobia and low rates of fertilizer N so as to maximize N, fixation of the intercrop legume. This will partially eliminate competition for N between cereals and legumes as intercrops. To meet the high N requirements of the intercrop cereal while at the same time promoting N, fixation of the companion legume, slow release fertilizers might be used. This might render the N available to the cereal at about the peak vegetative stage, presumably after the N-fixing system of the legume has become well established.

CEREAL-LEGUME INTERCROPPING SYSTEMS

c.

SUGGESTIONS FOR

85

FUTURERESEARCH

Considerable efforts have been devoted to agronomic factors affecting the efficiencies of various cereal-legume intercropping systems. However, there do not seem to be any general proposals for maximizing the production efficiencies of different systems. Some suggestions for future research follow. Research is needed on: 1. The pathways of N losses from cereal-legume intercrop systems to maximize the utilization of N fertilizers and so reduce wastage of an expensiveinput. 2. The application of low rates of N fertilizer early in order to encourage N2 fixation of the intercrop legume, together with later application of N during the peak vegetative stage of the cereal, in order to minimize competition for N. 3. The effects of applied N on N2 fixation of the intercrop legume. 4. The effectiveness of slow-release N fertilizers to establish whether they minimize the losses of applied N. 5 . The amounts of fixed N in belowground parts of component crops to permit accurate estimates of N balances in cereal-legume intercropping systems. 6. The component density combinations that give maximum yields and efficiencies in various cereal-legume intercrop systems. With different crop combinations, the responses may vary with the optimal densities of the different components crops. 7. Finding ways of improving the production efficiency of cereal-legume intercropping systems by giving high priority to maintaining the legume component. This may be achieved through manipulating the interrow spaces of the taller associated cereal, which would also minimize interspecific competition for growth-limiting factors such as light. 8. The efficiencies of specific crop combinations of cereal-legume intercropping systems in different locations. The definitive but expensive ''N methods of quantifying N2 fixation might be used as standards for less expensive indirect approaches such as the total N-difference and the acetylene reduction methods. This requires the cooperative efforts of funding agencies in supporting costs of 15Nfertilizers and the analyses of samples from areas where intercropping remains the most common cropping system. REFERENCES Agboola, A. A., and Fayemi. A. A. 1971. J. Agric. Sci. 219-225. Agboola, A. A., and Fayemi, A. A. 1972 Agron . I 64,409412. . Ahmed, S., and Rao, M. R. 1982. Field Crops Res. 15, 147-161. Ahmed, S., Gunasena, H. P. M., and Yang, Y. H. 1979. Proc. Final INPUTS Rev. Meet. 20-24 Aug., Honolulu, p, 251. Allen, J. R., and Obura, R. K. 1983. Agron. J. 75, 1005-1009. Allison, F. E. 1973. Dev. SoiiSci. 3, 639.

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Andrews, D. J. 1972. Exp. Agric. 8, 139-150. Andrews, D. J. 1974. Exp. Agric. 10, 57-63. Andrews. D. J., and Kassam, A. H. 1976.I n “Multiple Cropping” (R.1. Papendick, P. A. Sanchez, and G. B. Triplett, eds.), pp. 1-10. Spec. Pub. No. 27 Am. Soc. of Agron., Madison, Wisconsin. Atkins, C. A. 1982. I n “Advances in Agricultural Microbiology” (N. S. Subba Rao, ed.), pp. 25-51. Oxford and IBH Publ., New Delhi. Babalola, 0. 1980. Plant Soil 56, 59-69. Baker, E. F. I. 1978. Exp. Agric. 14,293-298. Baker, E. F. I. 1979. Exp. Agric. 15,4148. Baker, E. F. I., and Norman, D. W. 1975.Proc. Cropping Sys. Workshop, pp. 334-361. IRRI, Los Banos, Phillipines. Beets, W. C. 1977. Neth. J. Agric. Sci. 25, 95-102. Beets, W. C. 1982. “Multiple Cropping and Tropical Farming Systems,” p.156. Westview, Boulder, Colorado. Bergersen, F. J. 1980. In “Methods for Evaluating Biological Nitrogen Fixation” (F. J. Bergersen, ed.), pp. 65-110. Wiley, Chichester. Betse, C. E. 1976.Hortscience 11, 236-238. Bezdicek, D. F., Evans, D. W., Adebe, B., and Witters, R. E. (1978).Agron. J. 70, 865-898. Biscoe, P. V., and Gallagher, J. N. 1977.I n “Environmental Effects on Crop Physiology” (J.J. Landsberg and C. V. Cutting, eds.), pp. 75-100. Academic Press, New York. Boucher, D. H. 1986. Field Crops Res. 14, 105-115. Burris, R. H. 1974. I n “The Biology of Nitrogen Fixation (A. Quispe, ed.), pp. 9-31.NorthHolland Publ., Amsterdam. Butler, G.W., and Bathurst, N. 0. 1956. Proc. Int. Grassl. Congr. Palmeston North 7th, pp. 168-178. Chalk, P. M. 1985. Soil Biol. Biochem. 17, 389-410. Chalk, P. M., and Smith, C. J. 1983. Dev. Plant Soil Sci. 9,65-89. Chang, J. F., and Shibles, R. M. 1985a.Field Crops Res. 12, 133-143. Chang, J. F., and Shibles, R. M. 1985b.Field Crops Res. 12, 145-152. Chetty, C. R. K., and Reddy, M. N. 1984. Exp. Agric. 20,31-40. Chowdhury, M. S. and Misangu, R. N. 1981.Proc. Int. Workshop Intercropping (ICRISAT), 10-13 Jan. 1979, Hyderahad, pp. 217-221. Chui, J. A. N., and Shibles, R. 1984. Field Crops Res. 8, 187-198. Clements, F. E., Weaver, J. E., and Hanson, H. C. 1929.Carnegie Inst. WashingronPubl. 398, 202-233. Cordero, A., and McCollum, R.E. 1979.Agron. J. 71, 831-842. Crabtree, R. J., and Rupp, R. N. 1980. Agron. J. 72,445-448. Crookston, R. K., and Hill, D. S. 1979.Agron. J. 71,414. Dallal, R. C. 1974.Exp. Agric. 10,219-224. Dallal, R. C. 1977. Trop. Agric. (Trinidad) 54, 189-191. Danso, S. K. A., Zapata, F., Hardarson, G., and Fried, M. 1987. Soil Biol. Biochem. (submitted). Davis, J. H. C., and Garcia, S. 1983. Field Crops Res. 6, 59-75. De, R. 1980. I n “Nuclear Techniques in the Development of Management Practices for Multiple Cropping Systems,” pp. 73-84. IAEA-TECDOC-235, Vienna. De Wit, C. T. 1960. Versl. Landbouwkd. Onderz. 66, 1-82. De Wit, C. T., and van den Bergh, J. P. 1965. Neth. J. Agric. Sci. 13,212-221. De Wit, C. T., Tow, P. G., and Ennik, G. C. 1966. Versl. Landbouwkd.0nderz. 68, 1-30. Dilworth, M. J. 1966. Biochim. Biophys. Acta 127, 285-294. Donald, C. M. 1%3. Adv. Agron. 15, 1-118. Drake, M., Vengris, J., and Colby, W. C. 1951. Soil Sci. 72, 139-147.

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ADVANCES IN AGRONOMY. VOL. 41

GENOTYPIC VARIATION IN CROP PLANT ROOT SYSTEMS J. C. O’Toole and W. L. Bland Texas Agricultural Experiment Station, Texas A&M University Blackland Research Center, Temple, Texas 76503

I.

INTRODUCTION

Improvement of crop root systems has lagged behind that of aboveground plant characteristics. This disparity may be attributed to the root system’s concealment in the soil and variable nature, both of which enormously complicate observation and experimentation. The root system has traditionally been associated with water and nutrient absorption and anchorage of the plant. More recently increased interest in root biosynthesis of compounds involved in regulating development of the entire plant, interaction of roots with mycorrhizal fungi, and the increasing importance of symbiotic nitrogen fixation have modified the importance of studying plant root systems. The increase in root studies during the 1970s was evidenced by publication of symposia proceedings and authored books covering all aspects of root science (Whittington, 1969; Carson, 1974; Torrey and Clarkson, 1975; Russell, 1977; Harley and Russell, 1979). With the growing interest in root systems came an increased appreciation for interspecies variation in root form. Genetic improvement of crop species, however , requires knowledge of intraspecies variability in significant root parameters and its genetic control. Our review of this subject will emphasize anatomical and morphological characteristics relevant to the spatial and temporal expansion of the root system of a growing crop plant. The nature of this literature ranges from studies in which serendipitous findings revealed differences in dry weight or gross morphology to a few definitive inheritance studies and the even rarer report on genotypic variation of root physiological processes. We will not emphasize genotypic variation in root system response to nutrient concentration and related reports on selective ion transport (see Epstein and Jefferies, 1964; Barley, 1970; Lafever, 1981; Barber and Silberbush, 1984; Gabelman and Gerloff, 1986) but will include relevant morphological findings from that literature. In addition, we address the genetic bases of intraspecies variation in root parameters, although the literature is limited. 91 Copyright 0 1987 by Academic Press, Inc. All rights of reproduction in any form reserved.

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Finally, we discuss variation in root system phenotypes from the viewpoint that such flexibility in root system development is a valuable trait rather than a problematic “genotype x environment” interaction in the anaylsis of variance of experimental results. The last review of genotypic variation in root system traits that we are aware of was presented to a symposium in 1968 (Troughton and Whittington, 1969). Since then a number of limited reviews have been published which treat genotype variation in root parameters from the perspective of the author’s discipline or that of specific crops: tomato and bean (Zobel, 1975); cereals (Hurd, 1974, 1976; Hurd and Spratt, 1975); general crop species (Russell, 1977); plant breeding (Mackey, 1980; Quisenberry, 1982); crop physiology (Sullivan, 1983); C, grasses (Jones, 1985); and soil science (Hamblin, 1985; Taylor and Nguyen, 1987).

II. SIGNIFICANCE OF GENOTYPIC VARIATION IN ROOT SYSTEMS A. ADAPTATION TO DIVERSE ENVIRONMENTS

The soil component of the plant’s environment is more complex and heterogeneous than the aerial component. Soil chemical and physical properties can change severalfold in the space covered by 2 or 3 days’ growth of a single root tip (cu. 10 cm) or in the time it takes for rainwater to infiltrate to a profile location. Ecological studies provide many examples of plant root systems adapted to cope with variability in major edaphic environmental factors such as temperature and water (Ludwig, 1977; Chapin, 1977; Larcher, 1980). For example, Sydes and Grime (1984) demonstrated how four perennial grassland species were adapted to particular edaphic niches in a shallow limestone soil profile. Vertical distribution of root dry matter illustrated that some species were better adapted to rooting in the shallow soil near the surface while others exploited crevices in the underlying limestone. The study also illustrated intrinsic differences in root development among the species when nutrient and surface drought treatments were applied. Collectively the four species demonstrated adaptive traits which optimized utilization of available soil resources. The concept of adaptation to and optimization of edaphic niches appears applicable to agriculture as well. B. EDAPHIC NICHES IN AGRICULTURE

Great diversity exists among the soils of the earth’s tropical and temperate agricultural zones. The ecological analog predicts that root

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systems optimally adapted to them must also exhibit great diversity. However, unlike adaption of wild species, the success of agricultural species is based on the magnitude and stability of the economic product. We now turn to the discussion of edaphic niches in irrigated and rain-fed agriculture.

I . Irrigated High-Input Systems In modern irrigated high-input agricultural systems every effort has been made to eliminate soil physical and chemical constraints to high yield. In such a system high levels of chemical fertilizers, supplied in the root zone several times throughout the crop season, assure luxury levels of nutrient availability in the soil solution. In combination with irrigation, appropriate root zone chemical and physical modifications can potentially result in a growth environment with low levels of root zone stresses (Arkin and Taylor, 1981; Barber, 1984). In this extreme case of high-level technology there may be an opportunity to favorably alter the root morphology of crop species by reducing the dry matter allocation to the root system (Passioura, 1983; Smucker, 1984). For example, a modern semidwarf rice cultivar growing in irrigated and puddled soil with slow release fertilizer placed into the root zone during transplanting may require only a 15-cm rooting depth for its small fibrous root system. (Savant et al., 1982; Yoshida and Hasegawa, 1982). In contrast many landraces of rice adapted to rain-fed culture exhibit substantially larger root systems with some large-diameter roots extending into the plow pan and deeper soil levels (IRRI, 1984; Ekanayake et al., 1985). 2. Rain-Fed Variable-Input Systems

In spite of the promise of irrigated agriculture, most of the world’s cropland is rain-fed and will remain so in the foreseeable future (Boyer, 1982). Thus, the root environment will often be suboptimal. By comparison the root system suited to irrigated high-technology culture may be simpler in form and function than a root system able to cope with temporal and spatial variability in soil characters associated with uncertain soil water status. The amount and frequency of precipitation during the growing season, soil temperature regime, and level of native soil fertility greatly impact the soil environment and root growth and function. Rain-fed agricultural ecosystems range from no-input mixed cropping of shifting cultivators on marginal soils to complex soil, water, and fertility management systems rivaling irrigated areas in production. One avenue of further crop improvement for these diverse rain-fed agricultural complexes lies in optimizing root systems to decrease soilrelated stresses. Success in this endeavor depends on our ability to define

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and understand the edaphic environment while exploiting the range of genetically based adaptability in crop species.

111.

EVIDENCE OF GENOTYPIC VARIATION

Our review of genotypic variation is presented by species, addressing first the monocotyledons and then the dicotyledons. Utilization of genotypic variation in improving crop plants requires knowledge of heritability and genetic control. These topics are also reviewed. We begin with a brief review of relevant aspects of root morphology and anatomy. A.

MORPHO-PHYSIOLOGICALROOTPARAMETERS

I . Monocotyledons

The root morphology and anatomy of monocotyledons differs from that of the dicotyledons in several fundamental respects (Esau, 1977). For the purpose of our discussion the majority of the monocotyledon root system is composed of adventitious or nodal roots arising from a meristem at the base of the stem. Roots initiated in the embryo on the hypocotyl or higher on the axis are termed seminal roots and may be numerous in addition to the embryonic radicle. Some or all seminal roots may die after the adventitious root system begins to develop. The formation of adventitious roots in the Graminae is often associated with tiller development, creating distinct root-tiller relationships. Monocotyledon root anatomy is characterized by a cortex of variable thickness surrounding a central stele with vascular tissues arranged around a central pith. Secondary root growth does not take place in monocotyledons; thus the hydraulic characteristics of the root are established early in its life. In both monocotyledons and dicotyledons lateral roots are initiated in the pericycle and grow through the cortex to emerge at the surface of the parent root. 2. Dicotyledons

Dicotyledon root system morphology varies widely. Most possess a taproot from which lateral roots arise. The taproot produces lateral or branch roots with the youngest located nearest the root apical meristem and the oldest near the base of the plant. Typically the taproot is referred to as the primary root with lateral roots and their branches referred to as secondary and tertiary roots, etc., up to fourth or fifth order branches.

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Lateral roots originate from the pericycle of taproots or other lateral roots. In addition, adventitious roots may arise from nonroot tissues or nonpericycle tissues of older roots. Dicotyledon roots, unlike monocotyledons, are capable of secondary growth arising from the vascular cambium. The anatomy and vascular arrangement in the primary tissue of dicot roots is characterized by the number of xylem poles or vascular bundles arranged around a central point within the stele, i.e., diarch, triarch, tetrarch, or polyarch. As in monocotyledons elongation of individual root axes is by celluar division at the apical meristem and elongation in the zone behind the apex. 3. Pathway of Water Flow

The flux of water through the plant root is driven by gradients in the chemical potential of water from the soil to the leaf mesophyll cell surface, where evaporation takes place. Water entering the root system flows from the soil matrix to the outer wall of the epidermis (includingany root hairs), through the cells of the epidermis, cortex, endodermis, and stelar tissues to the lumen of a xylem vessel. The relative importance of flow along cell walls versus flow through the cells remains a topic of debate (Boyer, 1985). Water movement up to the shoot proceeds via the xylem vessels. The major resistances to water flow in the root can be partitioned into radial (inter- and intracellular from the epidermis to the stelar tissues) and axial resistance (within the xylem vessels) (Taylor and Klepper, 1978). The axial resistance can be viewed primarily as physical and anatomical in nature, dependent on the diameter and number of xylem vessels. However, the radial resistance also exhibits a physiologicalcomponent due to the contribution of cellular membranes (Fiscus, 1983). The pathways, physiology, and kinetics of ion uptake are beyond the scope of our discussion and have been reviewed elsewhere (Epstein, 1972; Barber, 1984). B.

MONOCOTYLEDONS

Genotypic variation has been documented for a wide variety of root parameters in monocotyledonouscrop species. Table I provides a summary of recent literature including a brief indication of the root parameters studied, the proposed value or relevance, and the authors. Not all studies cited in Table I are included in the following discussion due to space limitations. 1. Barley

Although we do not intend to review genotypic variation in ion uptake or transport, variation in root morphology has been revealed by such studies.

Table I

A Summary of Recent Reports on Genotypic Variation in Crop Plant Root Systems of Monocotyledon S p e c i d Crop

Root system parameter

Purpose or value

Barley (Hordeum vulgare L.)

Dry weight, number, and surface area of root members Root diameter, angle Root length and branching

Root system' response to nutrient deficiency Lodging resistance Morphogenetic effects of nitrogen and temperature Drought resistance Optimize water use efficiency

Root depth, number, length Root weight and length or depth Corn (Zea mays L.)

Oats (Avena sp. L.)

Orchardgrass (Dactylisglomerata L.)

Root number, length, dry weight and shootxoot ratio Root weight, pulling resistance Root weight, length per plant Shoot:root ratio, senescence Root pulling resistance, system lateral spread Root length and branching Root elongation rate Number, diameter and angles of nodal root growth Seedling root length Root distribution and fineness Root system length and area Rate of root volume and dry weight increase Root length, dry weight, and volume Number of crown roots, shoot: root ratio Root length to leaf area ratios Root dry weight

Developmental morphology, response to root pruning Lodging resistance Phosphorous uptake Root morphology Tolerance to corn root worms, root rot. and lodging Morphogenetic effects of nitrogen and temperature Nitrogen uptake and translocation Adaptedness to cultural systems

Reference Hackett (1%8) Jezowski (1978) Geisler and Krutzfeldt (1983) Gorny and Patyna (1984) Brown et al. (1987); Cooper et al. (1987) Andrew and Solanki (1966) Nass and Zuber (1971) Nielsen and Barber (1978) Chakraviuty and Karmakar (1980) Jenison et al. (1981); Penny (1981) Geisler and Krutzfeldt (1983) Pan et 01. (1985) Irwin et al. (1985)

Seedling drought tolerance Drought tolerance Nitrogen effects Assessment of genotypic variation

Cutforth et 01. (1986) Aina and Fapohunda (1986) Mackay and Barber (1986) Carrigan and Frey (1980)

Establish range of genetic variation Assessment of genotypic variation Drought resistance Adaptability to soil temperature and moisture

Murphy and Long (1979); Murphy et al. (1982) Heen (1980) Larsson (1982, 1986) Finn and Mack (1964)

Pensacola bahiagrass (Paspalum notatum Fh?e) Rice (Oryza sativa L.)

Rye (Secale cereale L.) Ryegrass (Lolium perenne L.) Ryegrass (Lolium sp.) Sorghum (Sorghum bicolor L. Moench.)

Root diameter

Penetrate compacted soil layers

Elkins and Sickle (1984)

Root diameter, branching, water extraction pattern Root system morphology Root activity at depth Root pulling force Deep root:shoot ratio Rooting depth, density, angle of penetration and general pattern Length, thickness, number, weight Maximum length, thickness Root pulling force Growth rate, cross-sectional area

Drought resistance, adaptation to upland culture Drought resistance Increased soil water use Drought resistance Exploitation of soil water Higher plant water status and yield Drought resistance Drought resistance Drought tolerance Seedling vigor

Nicou et al. (1970)

Armenta-Soto et al. (1983) Ekanayake et al. (1985a) Ekanayake et al. (1985b) Sullivan and Pfahler (1986)

Rate of post-water-stress new root axes development Root weight

Recovery from drought

Troughton (1980)

Relation to nitrate uptake

Goodman (1983)

Rooting depth and morphology

Vegetative drought tolerance

Bhan et al. (1973)

Growth rate, lateral branching Root weight, shoot:root ratio Root activity Weight, length, volume Shoot:root ratios, vertical root length distribution

Morphology, maturity genes Adaptation to semiarid zones Water and nutrient uptake Drought resistance Assessment of genotypic variation, drought resistance

Root length density and root dry weight Root length density and distribution

Increased water extraction at depth during drought Drought resistance, root system response to drought

Blum et al. (1977a) Saint-Clair (1977) Damodar et al. (1978) Nour and Weibel (1978) Jordan et al. (1979); Jordan and Miller (1980); Jordan and Monk (1980) Wright (1981); Wright and Smith (1983) Retta et al. (1982, 1983, 1984)

Chang et al. (1972) Reyniers and Binh (1978) O’Toole and Soemartono (1981) Yoshida and Hasegawa (1982) Mambani and Lal (1983a,b)

(continued)

Table I (Continued) Crop Sugarcane (Saccharurn sp.) Tall fescue (Festuca arundinacea Schreb.) Timothy (Phleumpratense L.) wheat (Triticurn aestivurn L.)

wheatgrass, bluebunch [Agropyron spicatum (Pursh) Scribn., and Smith]

Root system parameter

Purpose or value

Reference

Degree of branching: vertical vs. horizontal orientation Root diameter

Adaptation to soil texture and lodging resistance Root penetration of plowpans

Cited by Jones (1985)

Root diameter or thickness Root weight

Survival and ground cover Response to soil moisture, temperature, and fertility Drought resistance

Williams et al. (1981, 1982) Mack and Finn (1970)

Lodging resistance Water extraction at depth

Pinthus (1%7) Derera et ul. (1%9)

Variety-specific agronomy Descriptive morphology

Katyal and Subbiah (1971) MacKey (1973, 1980)

Effect of semidwarf genes on root system expression Increased depth of soil water extraction Adaptation to rain-fed conditions

Welbank et al. (1973); Lupton et al. (1974) Campbell and Lafever (1977)

Axial root conductance

Cornish (1981) Richards and Passioura (1981a.b); Richards (1987) Sadhu and Bhaduri (1984) Harris and Gwbel(l976)

Vertical growth rate, proportion of root system below 30 cm Spreading angle of root system Maximum depth, number, and weight Horizontal and vertical spread Number, depth, and vertical proportioning of root weight Distribution of root dry weight, length, and activity Seminal root length and growth rate Seminal and nodal root length and extension rate Xylem vessel number and radii Seminal xylem vessel diameter Root length and number Growth at low (2 T)temperature

Traps are listed in alphabetical order by common name.

Increased axial resistance to water

flow Genotypic variation Establishment and adaptedness

Elkins et al. (1977, 1979)

Hurd (1964,1%8)

Sirohi et al. (1978)

GENOTYPIC VARIATION IN PLANT ROOT SYSTEMS

99

The work of Hackett (1968) demonstrated the effect of nutrient level on root morphology. He subjected two barley (Hordeum vulgare L.) cultivars to phosphorus- and potassium-deficient nutrient supplies. In the control plants, the cultivars differed significantly in number and length of seminal, nodal, and lateral roots. Nutrient deficiency dramatically reduced the measured parameters in both cultivars, although to different degrees. Observations of this aspect of genotypic variation, i.e. , differential sensitivity to environmental parameters, are significant and may easily be overlooked. Few studies have compared root growth of a modern cereal cultivar with that of an endemic landrace over the entire crop duration. Brown et al. (1987) monitored growth and distribution of roots of two barley cultivars: Beecher, from California, and Arabic Abiad, a local landrace of Aleppo province, Syria. The study was conducted at two field sites varying in soil type (chromoxeret and calciorthid) and located in different rainfall zones. The landrace exhibited greater total root weight and length below 15-cm soil depth at all growth stages sampled. The vigorous early root and shoot growth and greater root length density at depth exhibited by the landrace resulted in higher evapotranspiration rates throughout the crop season (Cooper et al., 1987). These comprehensive studies illustrated both the temporal and spatial adaptation of the landrace root system. 2. Corn Regardless of the research objective, studies have generally found abundant genotypic variation in root systems among inbred lines of corn (Zea mays L.). Andrew and Solanki (1966) carried out greenhouse observational studies with hydroponically cultured plants. They found significant variation among 14 inbred lines for a number of root morphological parameters. When root parameters were related to field performance, the proportion of seminal roots was highly correlated with early plant height growth and vigor in the field. The role of seminal roots in the latter case may be indirect due to rapid seminal root establishment and the consequent early generation and development of the permanent nodal roots. In addition, when root pruning was performed in the field to simulate stress or mechanical damage to nodal roots, the inbreds with early vigorous seminal root development showed less reduction in height and ear weight. Cutforth et al. (1986) demonstrated variation among corn hybrids for early root system growth. No difference was observed among three hybrids for growth rate as a function of temperature. However, mean root length proved much less sensitive to lowered soil water contents in one of the three hybrids than in the others.

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Other studies have sampled mature field-grown plants in an effort to describe gross morphological structure of the entire root system. Chakravarty and Karmakar (1980) excavated and detailed the threedimensional distribution of root dry weight in four composite cultivars and a local landrace. The horizontal and vertical distribution at 45, 75, and 105 days after sowing revealed that the landrace had the smallest root system and the highest shoot:root ratio. The composite cultivars also varied significantly in total root system dry matter. Two of the composite cultivars continued to show increased root weights up to maturity, contrary to the common perception that cereals suspend root growth after flowering in favor of the ear and developing grains. Irwin et al. (1985) found two distinct phenotypes for nodal root angle in a survey of 11 hybrids. The hybrids had either horizontally spreading or more vertically oriented nodal root systems. Their observational study did not relate the structural differences to function in the plant or crop. Plant breeders have developed a vertical-pull-resistance technique to test entire corn root systems for tolerance to the rootworm complex. Jenison et al. (1981) studied the gross characteristics of root dry weight and nodal root spread in 44 inbreds. Vertical-pull resistance was significantly correlated with both variables and genotypic variation exhibited for all three measurements. In contrast to more detailed root system parameters, these gross characteristics were found to be relatively stable over environments (Jenison et al., 1981; Penny, 1981). Genotypic variation in root morphology has often been exhibited in studies of nutrient uptake. Nielsen and Barber (1978) measured root weights and lengths of 12 inbreds grown in solution culture. They described two- to threefold differences in root weight of seedlings and the relationship of root length to root or shoot dry weight. Although the extension of this work to a field situation did not involve root measurements, shoot uptake of P provided indirect evidence that the variation expressed in the controlled environment of solution culture could be extrapolated to field-grown corn plants. Pan et al. (1985) investigated the differential ability of corn genoppes to take up and translocate nitrate. Lateral root proliferation and specific root length in rapidly growing 5- to 8-day-old seedlings was related to nitrate translocation. Genotypes varied in these morphological parameters through time, leading the authors to conclude that future research on nitrate uptake should also address genotypic variation in root developmental morphology. 3. Forage Grasses

Studies involving the root systems of forage grass species cover a wide range of objectives from perforating plowpans to low soil temperature

GENOTYPIC VARIATION IN PLANT ROOT SYSTEMS

101

tolerance. A number of papers have discussed the use of tall fescue (Festuca arundinacea Schreb.) and Pensacola Bahiagrass (Paspalum notatum Flugge) as means of creating macropores in plowpans. Large root diameter was associated with the desired penetration ability. Succeeding crops are then able to root through the plowpan to exploit additional soil volume for water and nutrients (Elkins and Sickle, 1984; Elkins, 1985). Published accounts of the actual screening are vague, but it appears that there is genotypic variation in root diameter among lines of both Bahiagrass and tall fescue. Although no data are available for Bahiagrass, tall fescue lines exhibited a range of root diameters from 0.5 mm to 3.6 mm (Elkins et a/., 1977). A large-rooted tall fescue line was associated with better stand persistance, percentage plant survival, and sward cover scores. The superior establishment and forage yield of the large-rooted line is presumed to be related to penetration of the compacted plowpan, although measurements of soil water content did not substantiate that presumption (Williams et al., 1981). The combined effects of genotype and environment on root weight of orchardgrass (Dactylis glomerata L.) was investigated by Finn and Mack (1964) and on timothy (Phleum pratense L.) by Mack and Finn (1970). Four cultivars of orchardgrass cultured in a greenhouse showed differential response (changed rank) in total root weight across temperature, moisture, and fertility treatments. Cool soil conditions (10°C) yielded higher root weight than warm (20 "C) temperatures and more important, the cool temperature treatment increased the ability to discriminate genotypes. One entry in the timothy study of Mack and Finn (1970) showed a 25% greater total root dry weight than the other five genotypes when exposed to cool soil temperature (10 "C), whereas at higher soil temperatures (27 and 20 "C) varietal differences were not apparent. These studies illustrate well the complexity of response to be expected when such major factors as water, temperature, and fertilization are varied. They also underline the need to identify the most limiting environmental factor(s) for a particular agroecological region if screening or selection for genotypic differences is to be effective. Striking genotypic variation in growth of roots at low soil temperature was demonstrated by Harris and Goebel (1976) among ecotypes of bluebunch wheatgrass [Agropyron spicatum (Pursh) Scribn. and Smith]. They collected seed at 45 sites from Arizona to Alaska. Figure 1 illustrates the ecotypic variation found for root growth over a 21-day period a 2 "C, ranging from 1.4 m to 11.9 m in length. 4.

Oat'

Carrigan and Frey (1980) compared root and shoot growth of two varieties of cultivated oat (Avena sativa L.) and four lines of a weed species 'See also Section IV.

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J. C. O'TOOLE AND W. L. BLAND

FIG. 1. Root growth of 45 bluebunch wheatgrass genotypes after 21 days at 2°C. (Adapted from Harris and Goebel, 1976.)

(A. sterilis L.) which originated from arid zones around the Mediterranean Sea. The genotypes were grown to maturity in hydroponic culture. Periodically the root volume was estimated by water displacement and dry weight of the entire root system was measured at final harvest. Genotypic variation was found for both root volume and dry weight. One A. sterilis L. line, however, not only produced three to nine times more final root dry weight than the other five genotypes, but continued to expand its root system until maturity. Root volume in the other five genotypes stabilized or increased only slightly after flowering. The exceptional line flowered within 6 days earlier or later than the others and doubled its root volume from flowering to maturity. The authors concluded that the sustained and vigorous root growth of the unusual A. sterilis L. line may provide unique genes for future oat cultivars. 5. RiceZ

The root systems of rice (Oryza sutiva L.) ecotypes or landraces adapted to nonflooded upland conditions have frequently been compared with lowland rices adapted to flooded culture. Hasegawa (1963) reported that Japanese upland cultivars rooted deeper than the lowland types. French workers in West Africa (Nicou et al., 1970) found that Asian lowland * See also Section IV.

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103

cultivars had root systems with small-diameter, highly branched roots. In contrast, upland cultivars from Africa and South America had root systems of greater weight with less branching and larger-diameter main axes compared to those of lowland cultivars. As a part of their drought resistance program in the Ivory Coast, Reyniers and Binh (1977) screened 50 cultivars representing a broad range in geographic and hydrologic orgin. Deeprootedness and root density were ascertained from the uptake of the radioisotope 32P, placed at the 95-cm soil depth. Four well-adapted upland rices from West Africa and South America had activity considerably higher than any others. The authors felt that selection of well-adapted parental lines could be accomplished by this method. Mambani and Lal (1983a), working in Nigeria, conducted field experiments in which they analyzed varietal differences in the vertical and lateral distribution of root length density. Three cultivars, an African upland and two Asian rices, were grown under rain-fed and irrigated conditions. The African cultivar rooted to the depth of measurement, 25 cm, and had greater root mass at any depth or lateral distance from the plant row than did the more shallowly rooted Asian lines. During a period of water stress, soil water matric potential patterns resembled rooting patterns and the African upland rice maintained a higher (wetter) leaf water potential than the Asian cultivars. Relative yield (rain-fed versus irrigated) was also greatest for the African genotype. In a related study Mambani and La1 (1983b) investigated root development, plant water status, and grain yield of 10 rice cultivars. The rices were grown at three locations on a “toposequence” where depth to the water table varied from 100 to 15 cm. The soil water regime did not affect total root dry weight, but depth of root system penetration varied across the water table depth gradient. Genotypes differed in their ability to maintain yield under the more adverse water regimes. As in the previous study, plant water status and relative yield varied greatly among the 10 genotypes and was directly related to rooting depth and density measurements. Due to the fluid nature of flooded and puddled rice soils, traditional soil coring techniques are not always applicable. As an alternative, O’Toole and Soemartono (1981) investigated the association between root parameters and the force required to pull a single rice plant vertically from a flooded paddy. They used correlation and path coefficient analysis to illustrate the relationship of root pulling force and root number, length, weight, branching, root:shoot ratio, and thick root number. Root pulling force is now routinely used in the drought resistance screening program at the International Rice Research Institute (IRRI), Philippines. A significant amount of research, primarily concerned with genotypic variation in rice root systems, has been conducted over the past 19 years at the IRRI. It is beyond the scope of this discussion to adequately review that

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work. However, the reader is referred to Section IV and the annual reports of the IRRI beginning in 1967. 6. Rye

Because rye (Secale cereale L.) is sown in the fall, it must establish itself when seasonal air and soil temperature are declining. Sullivan and Pfahler (1986) measured primary root growth rate (millimeters/day) at 12, 20, and 28”C, as well as a root “bluntness ratio” of several rye genotypes at the seedling stage. The bluntness ratio was a measure of the shape of the primary root tip, which the authors thought may be related to root system ability to penetrate compacted soil. The genotypes were diploid populations and colchicine-derived tetraploids from five adapted cultivars. For root growth rate, they demonstrated significant effects of ploidy level, genotypic background (original cultivar), and temperature as well as numerous significant interactions. The tetraploid group exhibited a higher root elongation rate than the diploid group at all three temperatures. However, it is significant in light of the study objective that the greatest difference between diploid and tetraploid growth rate was at the lowest temperature (12 “C). In addition, the difference between diploids and tetraploids was significantly influenced by the genotypic background. The tetraploids had the lowest bluntness ratios, indicative of a blunter, less tapered root tip. The results indicated the existence of substantial genotypic variation for rye seedling root growth rate over a wide range of temperatures, both within and among cultivars at both ploidy levels.

7. Sorghum As sorghum (Sorghum bicolor L. Moench.) improvement has progressed, many studies of the root system have been undertaken to determine which root parameters are responsible for the crop’s ecological adaptation and drought-resistant character. Due to a naturally occurring drought beginning 17 days after sowing Bhan et al. (1973) observed four resistant and four susceptible sorghum cultivars among entries in a field trial. The cultivars varied greatly in degree of leaf wilting at noon after a 23-day rainless period. Excavation of the root systems revealed that drought-resistant genotypes had more vertically oriented roots, averaging about 15 cm deeper than the susceptible group. The resistant genotypes also had a higher root number, weights, and root:shoot dry weight ratios. Nour and Weibel(l978) compared the root characteristics of 10 sorghum cultivars previously ranked for drought resistance. Three-week-old seedlings were sampled for root weight, length, volume, and dry weight

GENOTYPIC VARIATION IN PLANT ROOT SYSTEMS

105

root:shoot ratio. In general, these root parameters were positively correlated with drought resistance. Damodar et al. (1978) described the rooting pattern at the vegetative and flowering stages for 11 cultivars of field-grown sorghum in India. They used 32Puptake to assess the vertical and horizontal activity of the root systems to the 40-cm depth. The vertical distribution of relative root activity in the vegetative stage varied among genotypes and was related to field performance during previous drought years. In the United States, scientists in Texas and Nebraska have been especially active in research on sorghum root systems. Because these studies were often components of programs concerned with sorghum improvement for drought resistance, diverse genotypes were frequently included in the experiments. In a notably detailed study, Blum et al. (1977a) investigated the effect of maturity genes on the rate of root appearance and growth with isogenic lines. In addition, Blum et al. (1977b) studied the manifestation of heterosis and kinetics of root system development. Hybrids exhibited greater seminal root length, root volume, and rate of adventitious root growth than did their parents. In a succeeding study Jordan et al. (1979) used hydroponic culture to assess the degree of genotypic variation for root characteristics and shoot:root relationships. Their experiment involved 30 sorghum genotypes including several standard parental lines used for hybrid seed production and exotic lines from the USDA-Texas Agricultural Experiment Station Sorghum Conversion Program. The standard lines differed only slightly in shoot:root relations, while lines from the Sorghum Conversion Program exhibited significant variation. Ratios of shoot:root dry weight, root length of nodal axes:root volume, and leaf area:root length of nodal axes ranged from 0.5 to 1.7 among the 30 genotypes. Jordan and Miller (1980) presented frequency distributions of the same ratios for additional sorghum genotypes. They also reported the extension of the hydroponic studies to field-level investigation of 10 genotypes using the minirhizotron technique during the 1977 crop season. Observation of shallow-rooted cultivars in hydroponics coincided with field results. However, hydroponics failed to identify cultivars which rooted deeply in the field in response to drought (Jordan and Monk, 1980). Similar observation of stress-induced root growth patterns were obtained in Nebraska by Retta et al. (1982, 1983) in field studies of root length and soil water extraction patterns among sorghum hybrid genotypes underirrigated and stressed soil water treatments. Genotypes varied in their ability to express stress-induced deep root growth patterns. Wright and Smith (1983) conducted field experiments under rain-fed conditions to determine the morphological and physiological mechanisms responsible for yield differences under water stress conditions in two sorghum genotypes. The higher-yielding genotype E-57 had higher root

106

J. C. O’TOOLE AND W. L. BLAND

length densities than cultivar TX-671, especially below 80 cm, yet used less water than TX-671 prior to booting, due to differences in timing of leaf area development. After the booting stage, when the upper soil profile (0-60cm) water supply was nearly exhausted, water use was reversed, with E-57 extracting more water from below the 80-cm depth. Blum (1972) and Passioura (1972) have shown for sorghum and wheat, respectively, that, given a limited water supply, water use efficiency is increased when the proportion of the total soil water extraction which occurred preanthesis was reduced. In the study of Wright and Smith (1983), E-57 maintained higher water use rates than TX-671 during the reproductive period, resulting in less floral abortion and greater yield in E-57.

8.

Wheat3

Genotypic variation in the root system of wheat (Triticum aestivum L.) has been the subject of numerous studies. Objectives have varied from investigations of genotypic effects on coordinated growth pattern of root and shoot to lodging resistance and the resistance to water flow in the root vascular tissue. Genotypic variation for the degree of horizontal versus vertical orientation of nodal roots has been identified in corn (Irwin et al., 1985) and sorghum (Bhan et al., 1973) and associated with adaptation to cultural practices and drought resistance, respectively. Pinthus (1967) screened wheat cultivars for this gross root system trait. Comparison of lodging scores of 10 wheat genotypes with the spreading angle of the root system indicated that the roots of lodging-resistant varieties had greater horizontal spread than did susceptible genotypes. In Australia Derera et al. (1969) grew diverse wheat cultivars in soil-filled polyethylene tubes ‘in a field environment. Significant genotypic variation was found for maximum depth and rate of penetration, total number of seminal and nodal roots, root weight, and length per unit root weight. Early maturity was the major factor in drought tolerance in their study. In spite of the differences in root development observed, the authors suggested that there was little room for improvement in the exploitation of the soil volume. As in other cereals, the movement of injected radioisotopes has been used to evaluate the rooting pattern of wheat. Katyal and Subbiah (1971) injected 32Pinto wheat plants and after 5 days measured the resultant activity in roots recovered from soil cores. Genotypes varied significantly in vertical and lateral distribution of root activity. The authors speculated that more deeply rooted cultivars would have greater drought resistance and lines with less lateral proliferation could be planted at higher densities.

’ See also Section IV.

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107

A comprehensive root study program was started in 1965 at the Rothamsted Experiment Station, England, to evaluate root systems of modern, semidrawf wheats (Welbank et al., 1973; Lupton et al., 1974). Differences in rooting between the semidrawfs and traditional taller varieties were small; the modern lines may have been more extensively rooted deep in the profile. Welbank et al. (1973) provided a thorough account of methods used. Mackey (1973) reviewed the developmental nature of the seminal and nodal root systems of wheat and presented detailed analyses of shoot and root parameters from spring and winter wheat genotypes and monosomics of a spring wheat. Mackey (1973) argued that a higher number of seminal roots may be a desirable configurations as seminal roots are low in weight per unit length and should penetrate deeply in the soil. This hypothesis led to extensive screening for seed size and associated seminal root number (Mackey, 1980). A positive correlation was found between seed weight and seminal root number in Triticum and Aegilops spp. One wheat cultivar, however, departed appreciably from the others, indicating that it may be possible to combine small seed size with a relatively high (three to five) number of seminal roots. Variation in seminal root xylem vessel diameter in wheat was investigated by Cornish (1981) because of its implication for resistance to water flow (axial root resistance). The five wheat genotypes studied differed significantly in effective xylem radius of the largest xylem vessel, ranging from 34 to 45 pm.

C. DICOTYLEDONS Recent studies of genotypic variation in root parameters of dicotyledonous crop species are summarized in Table 11. The table briefly indicates the type of root parameter(s) studied, summarizes proposed value in crop improvement, and identifies the authors. Due to space limitations, not all studies listed in Table I1 are discussed in the following section. 1. AIfalfa4

Studies of genotypic variation in root parameters of alfalfa (Medicago sativa L.) have addressed tolerance to acid soil, response to soil moisture, and variant anatomy and morphology. Simpson et al. (1979) exposed four alfalfa cutivars to varying treatments of lime application to an acidic subsoil and a range of soil water conditions. the genotypes differed in root fineness (m/g), root length at the 60-85-cm depth, and dry weight shoot:root ratios. ‘See also Section IV

Table Il

A Summary of Recent Reports on Genotypic Variation in Crop Plant Root Systems of Dicotyledon Specid Crop Alfalfa (Medicago safiva L.)

Bean (Phmeolus vulgaris L.)

Root system parameter

Adaptation to cultural practices

Simpson ef al. (1979)

Fibrous vs. taproot habit Root bark development Root length Root system morphology

Winter hardiness Nitrogen and carbohydrate storage Drought avoidance Exploitation of soil volume Lodging resistance

McIntosh and Miller (1980) Brick and Barnes (1982) Carter ef ul. (1982) Barnes (1983) Stoffella ef al. (1979a)

Improved water and nutrient uptake, lodging resistance Vigorous root system Morphogenetic effects of nitrogen and temperature Tolerance to flooding

Stoffella ef al. (1979b, 1981)

Fibrous vs. taproot habit

meter, uprooting resistance Dry weight Root length and branching

Cotton

(Gossypiurn hirsufurnL.)

cowpea [ Vigna unguiculafa (L.1 WalP.1

Reference

Root fineness, length, and weight

Basal and taproot weight and dia-

Chickpea (Cicer ariefinurn L.)

Purpose or value

Several root system components Dry weight, total root length, rates of extension Root:shoot ratios, taproot length, and lateral root number Root length and weight Vascular bundle arrangement, lateral root number and length Total and taproot dry weight, location of laterals Taproot and lateral root dimensions Vertical growth rate

Fawole eta/. (1982) Geisler and Krutzfeldt (1983)

Kahn ef ul. (1985)

Soil water extraction, drought resistance

Vincent and Gregory (1987)

Drought resistance and water use efficiency seedling vigor Decreased axial resistance, drought resistance Drought resistance

Quisenberry ef al. (1981)

Adaptedness to specific soil conditions Exploit soil water at depth

Eissa ef al. (1983) McMichael ef ul. (1985) Cook (1985) Sawyer ef al. (1984) Robertson ef al. (1985)

Measurement of taproot, lateral, and adventitious roots

Descriptive morphology

Kahn and Stoffella (1987)

Root:shoot ratios, root length, and dry weights

Descriptive morphology

Ali-Khan and Snoad (1977)

Primary and lateral root length, volume, and dry weight

Descriptive morphology, effect of growth media

Veitenheimer (1981); Veitenheimer and Gritton

Root weight and length

Establish genetic variation

Jensen (1985)

Peanut (Arachb hypogaea L.)

Taproot length and root number at 1 m depth

Ketring et 01. (1982)

Pepper (Capsicum annuum L.)

Shoot:root dry weight ratio

Water and nutrient uptake deep in the soil profde Lodging resistance

safflower (Carthamus tinctorius L.)

Root length density, taproot vs. fibrous system Root distribution and activity

Breaking up and de-watering heavy soils Variety specific agronomy

Harrigan and Barrs (1984)

Root weight, surface area, length

Water-nutrient uptake

Development rate, dry weight, and pattern Root length density Taproot elongation rate Taproot elongation rate Spatial and temporal development of root length density Vertical root density Angle of lateral root growth Nodule fresh weight, nitrogen content, response to temperature

Adaptation of cultivars to soil environment Irrigation protocol Increased water extraction Sod water extraction Comparison of determinant and indeterminant genotypes Improved crop adaptation Root system configuration Cultivar x Rhizobium strain interactions

Pea (Pisum sativum L. and P.fulvum)

(1984)

Soybean (Glycine may L.)

Kahn (1985)

Sathe et al. (1984) Raper and Barber (1970) Mitchell and Russell (1971) Burch et al. (1978) Taylor ef al. (1978) Kaspar et al. (1978, 1984) Sanders and Brown (1979) Boyer et al. (1980) Kaspar el al. (1981) Munevar and Wollum (1982) (continued)

Table n (Continued) Crop

Root system parameter

Root system permeability

Stone and Taylor (1983) Smucker (1985) Stone (1985) Hernandez and Orioli (1985)

Root fresh and dry weight

Lodging resistance

Aycock and McKee (1975)

Rooting activity, depth, and lateral spread Root weight and length

Drought resistance when grown on stored soil water Study of phenotypic stability

Nagaraj and Gopalachari

Taproot weight and diameter, basal root weight and number

Descriptive morphology

Stoffella (1983)

Number of taproots per plant, percentage fibrous roots Root number, length, dry weight, and vertically penetrating roots Root system length and depth

Study genetic control of root variation Drought resistance, low nutrient tolerance Drought tolerance

Caradus (1977)

Taproot and lateral root extension Root number Extension rate, number of laterals

Sunflower (Helianthus annuus L.) Tobacco (Nicotiana tabacurn L.)

White Clover (Trifolium repens L.)

Reference

Adaptedness to soil temperature Penetration of dense soil layers Adaptation to poor drainage Study of genotypic variation

Soybean (cont.)

Tomato (Lycopersicon esculentum Mill.)

Purpose or value

“Crops are listed in alphabetical order by common name.

(1979)

Gulman and Turner (1978)

Caradus (1981)

Ennos (1985)

GENOTYPIC VARIATION IN PLANT ROOT SYSTEMS

111

Under prolonged soil water deficits at the surface the greater root length below 60 cm of one genotype resulted in significantly greater shoot yield than the other three cultivars. These findings emphasize the potential impact of genotypic variation in evaluation of new agronomic technology. The “root-branching” trait in alfalfa was studied in a descriptive manner by McIntosh and Miller (1980). Using a 1 (taprooted) to 5 (branching rooted) scale they evaluated 100 plants each of three cultivars on each of eight sampling dates. There was no difference in the distribution of branching index values between the cultivars. However, 2-5% of all plants were strongly branched and this trait was positively correlated with total plant dry weight. The authors concluded that the genotypes could be used as a source of germplasm to develop high-yielding, branching-rooted alfalfa lines. Genotypic variation for secondary root growth of alfalfa has been reported by Brick and Barnes (1982). The secondary growth of the root cambium produces “root bark’’ (phloem) toward the exterior and secondary xylem toward the interior. The cross-sectional area of the root attributable to bark among genotypes ranged from 20 to 85%. The authors speculated that extra bark may act as a pool of nonstructural carbohydrates. Two studies addressed the effect of soil moisture on alfalfa root morphology and included genotype as a treatment. Carter et al. (1982) found that genotypic variation in root weight and length exhibited in the control soil water treatment was not identifiable at the two water-stress treatment levels. McIntosh and Miller (1981) found that water stress decreased the level of root branching equally for all genotypes. 2.

Common Bean

Common bean (Phaseolus vulgaris L.) cultivars and breeding lines have been studied in both the field and greenhouse to investigate the relationship between root morphology and lodging resistance. Stoffella et at. (1979a) cultured six dry bean genotypes in the greenhouse and field. They observed genotypic variation for root weight and shoot:root ratio when plants were grown in the field, but greenhouse-grown plants did not differ in root weight. Measurement of uprooting resistance was made in the field as an indirect measure of the root system and cultivars differed significantly. In a related report root dry weight was partitioned into its morphological components (Stoffella et at., 1979b). As in the first study, genotypic differences were inconsistent between greenhouse and field trials. In the field-grown plants the root systems of the six genotypes differed significantly in basal root weight and total diameter in the taproot weights and diameter. The

112

J. C. O’TOOLE AND W. L. BLAND

authors concluded that the larger basal roots are associated with and may contribute to the erect plant type desirable for mechanical harvesting and lodging resistance. 3. Cotton

Cotton (Gossypium hirsutum L.) root morphology has been studied with emphasis on crop establishment and response to soil water deficits. Quisenberry et al. (1981) reported genotypic variation for taproot length and number of laterals at a 1-m depth among 16 greenhouse-grown exotic cotton lines. Variation found in root parameters was significantly correlated with both shoot dry weight and leaf area in plants grown in a field under dryland conditions, but not those grown with irrigation. Eissa et al. (1983) measured two seedling root parameters in 124 cotton genotypes. Root length per plant varied 2.2-fold and specific root length ranged from 0.046to 0.121 g/m. Cook (1985) observed root system traits of 18 commercial cotton cultivars. Genotypes with deeply placed and numerous laterals were characterized as drought resistant. As in wheat, root vascular anatomy of cotton has been studied because of its implications for axial resistance to water flow within the root. McMichael et al. (1985) examined the root vascular bundle arrrangement among eight cotton genotypes. Seven genotypes exhibited tetrarch bundle arrangements (four distinct bundles) while the genotype T-25, a strain native to Central America, exhibited a pentarch arrangement (five distinct bundles). The additional vascular bundle resulted in decreased calculated hydraulic resistance. Additionally, the authors argued that the pentarch arrangement provided another point for lateral root initiation and could increase root length density. Circumstantial evidence collected in the field was presented in support of these conditions. 4.

Cowpea

Robertson et al. (1985) devised a unique screening tool for detecting genotypic variation in rate of vertical root growth of cowpea (Vigna unguiculata L.). The authors wished to select for rapid early root system proliferation associated with growth and yield when the crop was dependent on stored soil water in a semiarid environment. Root development to a particular depth or lateral distance from the plant was indicated by appearance of herbicide damage due to uptake from subsoil bands of the chemical. In a preliminary trial significant variation was found among 33 genotypes for the number of days after seeding to first herbicide symptoms. In a separate field experiment, soil water extraction patterns showed agreement with results from the subsoil herbicide technique.

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Few studies have provided reference information on the cowpea root system. Kahn and Stoffella (1987) examined cowpea root systems of seven genotypes at two field locations. They partitioned root weight into the three traditional components of a dicot root system (taproot, adventitious roots, and lateral roots) and a fourth, “basal” roots. They found significant differences in the total root dry weight and partitioning among genotypes. 5. Pea

Several reports are available on genotypic variation of the pea (Pisum sativum L.) root system. Ali-Khan and Snoad (1977) investigated morphological variation in seven root and shoot parameters of seedlings among 30 diverse pea genotypes. Large variation was found in length of the main root, length to the last lateral root, and number of laterals. Snoad and Arthur (1974) also described seedling root characteristics and Zobel (1974) indicated variability in pea root systems. In a more comprehensive study, Veitenheimer (1981) grew pea genotypes in nutrient solution culture. His initial results showed high within-genotype variability for root parameters, and he concluded that solution culture did not provide sufficiently repeatable results for the study of genotypic differences. In a comparison of 23 genotypes in soil-filled pots, genotypic variation ranged from 12.2 ml to 201.3 ml in root system volume. Several late-maturing lines had root systems twice as large as currently grown cultivars (Veitenheimer, 1981). Jensen (1985) examined the root system of over 400 pea genotypes ranging from landraces to cultivars currently grown in Denmark. Great variation in several root system parameters and high correlations between total root length and shoot height were noted. 6. Peanut

Ketring et al. (1982) screened a total of 149 peanut (Arachis hypogaea L.) genotypes for taproot length and root number at the 30-cm and 100-cm depths. Significant differences in these two characteristics were evident in initial experiments with 23 genotypes grown in greenhouse potting media for 55 days. Mean taproot lengths ranged from 95 cm to 193 cm and mean number of roots at 1 m from 1.0 to 7. Because further experiments were carried out over varying seasonal periods results were normalized by cultivar Starr. Relative to Starr, number of roots at 100 cm and root length for 120 genotypes ranged from 0.1 to 2.0 and from 0.1 to 1.2, respectively. The authors found that shoot dry weight and leaf area were highly correlated with number of roots and taproot length. However, numerous differences

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in configuration of root systems were observed, implying that many novel root system patterns are possible for peanut root systems.

7. Soybean5 In the early 1970s attention was given to the potential for genotypic variation in gross morphology of the soybean [Glycine max (L.) Merr.] root system. The often cited report of Raper and Barber (1970) involved extensive quantification and description of field-grown soybean genotypes. They described root system morphology in detail, including location of the taproot and laterals in single-plant plots of two cultivars, Aoda and Harosoy 63. The gross morphology of Aoda placed 80% of the total root weight in the center soil core containing the taproot and was typical of the 26 genotypes examined. The Harosoy 63 root system was unique, with root surface area twice that of Aoda, smaller-diameter roots, and 1.5 times the root length. However, when the cultivars were studied in row culture two significant changes occurred. First, taproot development diminished and there was a profusion of lateral roots within 15 cm of the soil surface. Second, the lateral roots grew to the interrow center and then sharply turned downward, illustrating the possible role of competition in determining gross morphology of rowcropped soybean root systems in situ. Similar results were reported by Mitchell and Russell (1971) from studies of field-grown soybean root systems. Roots of eight genotypes, including the two examined by Raper and Barber (1970), were sampled on four occasions after planting. Late in the season half of the genotypes increased root weight only slightly while the others continued steady growth. Boyer et al. (1980) measured significant differences in midday leaf water potential between modern and old soybean cultivars. It was proposed that the higher midday leaf water potential of the modern cultivars was due, in part, to a more extensive root system. Smucker (1985) illustrated the varied response of six soybean genotypes to soil compaction. The genotypes varied approximately fourfold in root penetration ratio (number of roots that exit the lower surface of a compacted zone divided by the number of roots that enter the upper surface). His report also demonstrated the interactive effect of soil fertility level on an individual genotype’s root penetration ability. Two of the five genotypes showed a positive effect of increased fertility on the penetration ratio while the other three responded negatively. Sanders and Brown (1979) conducted a field study using the minirhizotron technique to monitor the development of two soybean See also Section IV.

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genotypes, one determinant and one indeterminant in growth habit. For the average root length density from the surface to 70 cm, the genotypes did not differ under the row. Late in the season the determinant line had greater root length density at the interrow sampling location than did the indeterminant line. 8.

Tomato

Mutagenesis has provided an alternative means of realizing genotypic variation in tomato (Lycopersiconesculentum Mill.) root characters. Zobel (1975) reviewed attempts to induce mutations related specifically to root parameters. Zobel (1 972a,b) described induced root mutants that were characterized by a drawf root system (drt), “bushy” root system (brt),and a mutant which required a low exogenous concentration of ethylene for normal root growth (dgt).Mutants of this nature provide experimental material for histological and physiological studies of root system expression. Zobel (1974) investigated the single gene mutant dgt, which is characterized by horizontal growth of both shoots and roots and roots usually lacking lateral branches. In addition the mutant exhibits abnormal curling (hyponasty) of leaves. These abnormalities can be negated by growing the plants in an atmosphere with 5 nl/liter of ethylene. The dgt mutant also exhibits altered secondary vascular differentiation and large secondary xylem vessels do not form. Grafting combinations showed that a normal (VFN8) scion on a dgt mutant root stock resulted in normal root and shoot phenotypes, while the reciprocal yielded plants with a mutant top and normal root system. It was concluded from these results that the normal shoot is supplying a substance to the root system which is absent in the mutant. Stoffella (1983) conducted field comparisons of 23 tomato cultivars and used Zobel’s (1975) root nomenclature to separate roots into taproot, adventitious roots, lateral roots, or basal roots. In Stoffella’s study the basal roots contributed the largest proportion of root dry weight and exhibited two- to threefold differences among genotypes. 9. Other Dicotyledons

Chickpeas (Cicer arietinum L.) grown in Mediterranean areas are often subjected to terminal drought and must rely on stored soil water. Rapid root system development to exploit soil water and nutrients in conjunction with selection for early maturity is a promising approach to higher yields. Vincent and Gregory (1986) assessed genotypic variation for the rate of root extension and other root morphological parameters as a prelude to crop improvement of the chickpea root system. Five genotypes were grown in

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J. C. O’TOOLE AND W. L. BLAND

nutrient solution for 18 days. Genotypes varied in the rate of root elongation and configuration of lateral root length. A Syrian landrace showed high rates of primary root axis extension and lateral root system development. They concluded that the principal attribute of the adapted landrace root system, early rapid vertical development, deserved further study in situ. The ability of the root system to anchor the plant and thus decrease lodging is a highly desirable agronomic trait. Kahn (1985) compared root parameters of lodging-resistant and susceptible cultivars of pepper (Capsicum annuurn L.). Greater total root weights and lower shoot:root ratios were associated with lodging resistance. Recent studies have examined safflower (Carthamus tinctorius L.) germplasm for specific root system traits associated with agronomic practices. Sathe et al. (1984) investigated root system distribution of three safflower genotypes because of its implication for water and nutrient uptake. Significant variation was detected in root distribution using the 32Psoil injection technique. A novel application of variation in root morphology was investigated by Harrigan and Barrs (1984). In order to obtain safflower genotypes well suited for “de-watering” heavy clays in irrigated areas, increased root length at depth was documented. The root permability of three sunflower (Helianthus annuus L.) cultivars was measured by Hernandez and Orioli (1985) by applying hydrostatic pressure on the root system and measuring water flow through the vascular tissue. Root volume, root dry weight, leaf area, and transpiration rate were also determined. Only root permeability was found to differ significantly among the genotypes. One genotype had root permeability threefold that of the other two. Nagaraj and Gopalachari (1979) administered 32Pto five tobacco (Nicotiana tubacum L.) cultivars and after 5 days sampled the soil and roots to determine rooting patterns. Three genotypes had shallow, spreading root systems while others were more vertically oriented and presumably more deep-rooted. White clover (Trifolium repens L.) cultivars vary greatly in the proportion of large taproots to small fibrous roots (Caradus, 1977). Caradus (1981) reported on greenhouse experiments in which five white clover cultivars were monitored for 3 years in glass-fronted boxes 1.3 m deep. The genotypes showed significant variation in number of root tips visable, number of vertically penetrating roots, mean cumulative root length, and root dry weight at termination of the experiment. Clear differentation of root system type did not occur in the seedling year but could be detected after 2 years in the boxes. The root box observations were in agreement with several field-level observations related to water stress tolerance. In another

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study Ennos (1985) selected plants with either long or short root systems from within a single natural population of white clover. Root growth of plant cuttings was measured after 7 days of culture in a calcium nitrate solution. He demonstrated normally distributed variation for root growth ranging from about 10 mm to 110 mm among 272 plants sampled and concluded that selection for root length should be possible.

D. GENETICS The utility of genotypic variation in root parameters depends on its application in developing better-adapted crop cultivars. The majority of reports compiled in Tables I and I1 are of a demonstrative nature. However, information regarding the heritability and genetic control of a trait is necessary to evaluate its potential for successful hybridization and selection. The awareness of genotypic variation in root system parameters in the 1970s and 1980s increased the need for comparable genetic studies. Several of the reports in Tables I and I1 obtained estimates of “broad sense heritability” (BSH), a measure of the degree to which a phenotype is genetically influenced. This is usually accomplished by culturing many genotypes in a common environment and assessing the genetic component of variance. The proportion of observed variability due to the additive effects of genes for a trait is referred to as narrow sense heritability (NSH) and indicates the potential for continued improvement through repeated selection. A small group of studies addressed, in addition to heritability estimates, the genetic control of root parameters. In the following two sections we will describe first the limited information available on the heritability and then the genetic control of root parameters. 1. Heritability

Troughton and Whittington (1969) noted in their review of this subject that few heritability estimates existed for root characters. From their own experience with ryegrass (Loliumperenne L.) they estimated that BSH was 28% for rate of increase in root volume, 25% for rate of increase in root numbers, and 43% for size of individual roots. They concluded that root characters were at least as highly heritable as those of the shoot. Mi-Khan and Snoad (1977), working with 30 genotypes of pea, found BSH of greater than 50% for the number of lateral roots and dry weight of the root system. Tandon and Saini (1977) found significant genotypic variation when root dry weight of wheat in the upper 10 cm and below 30 cm were measured. The estimates of BSH were low but in view of the fact that

118

J. C. O’TOOLE AND W. L. BLAND

the experiment also included soil moisture levels as a treatment, the authors were encouraged as to the possible genetic improvement for deep rooting. In a program to increase the hydraulic resistance of the seminal root system, NSH estimates were used in the decision to select for vessel diameter instead of the number of seminal root axes (Richards and Passioura, 1981b). A NSH of 52% for vessel diameter and 43% for number of seminal root axes was estimated by regressing F3 progeny means on F2 parental values. When Penny (1981) pooled results over four environments of vertical-pull resistance values among corn inbreds they correlated well (r = 0.85) with results from a set of testcrosses, indicating high BSH. Chang et ul. (1982) reported NSH estimates for several root morphological traits of rice derived from regression of F, values on those of the parents. Their results illustrated high NSH values of 61% for maximum root length, 62% for diameter of root base and root tip, 49% for root number, and 43qo for root dry weight. Slightly higher NSH values were reported for shoot parameters and the NSH of the root:shoot ratio was 53%. 2. Genetic Control System

The work of Monyo and Whittington (1970) is often cited and appears to be the only significant work addressing root genetics of cereals for much of the 1970s. They studied growth characteristics of two wheat cultivars and the chromosome substitution lines of one into the other. Development of the root systems was highly correlated with shoot development and conclusions from their respective genetic analyses were similar. Genotypic differences in either root or shoot growth were primarily related to developmental events such as the advent of tillering or the duration of vegetative growth before flowering. Genetic analyses showed that control of the root system was largely by additive gene action. They concluded that variations in root traits were markedly influenced by single genes affecting the duration of vegetative growth and by other polygenic systems. A study addressing the number and length of seminal roots of tall and semidwarf bread wheat seedlings was reported by Virk et ul. (1978). They applied numerous genetic models to the individual characters measured for both root and shoot parameters. The analysis of means and variances indicated considerable heritable variation from crosses between tall and semidwarf height groups. The net directional dominance in general was caused by decreasing dominant alleles. Further, statistical analysis of specific crosses showed the adequacy of simple additive genetic and environmental models, together with indications of dispersed genes in the parents for root number and length. The inheritance of two root characters has been studied in alfalfa. Brick and Barnes (1981) investigated the inheritance of the lobed cambium trait

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119

while Brick and Barnes (1982) studied inheritance of root bark area. In the first study, they used an empirically derived three-class system to rate the degree of lobed cambium visually. Their study revealed that the lobed cambium trait was controlled by a single tetrasomically inherited gene with a triplex dominant effect. Genetic factors affecting another root trait (stubby root) appeared to affect the degree of expression for the lobed cambium trait. Brick and Barnes (1982) selected alfalfa plants in which root bark (phloem cells) ranged from 33 to 85% of the total root cross-sectional area. The data generally supported the hypothesis that root bark composition was controlled by additive gene action. Their study indicated that two tetrasomically inherited nondominant duplicate genes controlled the degree of root bark area development. The inheritance of root system characteristics in rice was first reported by Chang et al. (1982), and additional results of the same study were reported by Armenta-Sota et d(1983). Several root parameters had previously been associated with drought resistance. Their study investigated the inheritance of four root characters in an eight-parent diallel cross and three F, populations. The expression of long roots and high root number were controlled by dominant alleles. Thick root tips were controlled by recessive alleles in three of the parents and by dominant alleles in one parent. Additive and dominance effects were indicated for all four traits. Estimates of NSH were relatively high for maximum root length (6O%), root tip thickness (62Ora), and root:shoot dry weight ratio (52%) while NSH for root number was lower (44Vo). The BSH values of these root parameters were very high, ranging from 83 to 92%. The authors concluded that good progress could be made in selection for these traits using a nonsoil media such as aeroponics. Fawole et al. (1982) studied the heritability and genetic system involved in root dry weight and root:shoot ratio in six crosses of dry bean lines. Broad sense heritabilities for root dry weight among the populations ranged from 69% to 90%. The range of variation found in small F2 and backcross populations suggested that relatively few genes were involved in the variation of root dry weight and that transgressive segregation (progeny exhibiting a degree of trait expression outside the range of the parents) was operating in both directions. The inheritance of cotton seedling root length and relative root weight was investigated by Eissa et al. (1983) as a part of their plant breeding studies for better stand establishment and seedling vigor. They used a fixed model diallel analysis of five strains. The interpretation of the data is for the specific parent used, as the parents were not randomly selected but instead for combinations of the two root characters. This analysis indicated that among the five strains used, a breeder would have difficultly in combining the desired long root trait with high relative root weight (thick roots).

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J. C. O’TOOLE AND W. L. BLAND

Generation mean analysis indicated that considerable additive, dominance, and additive x additive epistatic effects were present for the two traits. The authors concluded that a breeder might be more successful by delaying selection of the F,, thus allowing recombination of epistatic genes, and by using recurrent selection techniques. The inheritance of six rice root characters was investigated by Ekanayake et al. (1985a) in relation to breeding drought-resistant rain-fed cultivars. In general the root parameters exhibited a polygenic system of inheritance. The F, plants demonstrated significant and positive heterosis for thick deep roots. Additive and dominant genetic effects contributed equally to the expression of all six traits. Narrow sense heritability estimates from the regression of F, on F2 and from the F, generation, respectively, were relatively high: maximum root length, 53% and 35%; root thickness, 61% and 80%, number of thick roots, 33% and 53Vo; root volume, 18% and 55%; root length density, 44% and 77%; and root dry weight, 56% and 92%. They concluded that selection should be successful in early segregating generations for these specific characters. In a succeeding study, Ekanayake et al. (1985b) determined the inheritance of the root pulling force trait in rice. Since this is a selection procedure which integrates many individual traits, such as those discussed in Ekanayake et al. (1985a), it is not surprising that transgressive segregation for high root pulling resistance was observed in three crosses. Again, both dominant and additive genes control this composite root system trait. The NSH estimate from regression of F, results on the F2 was relatively low among the three crosses (39, 43, 47%) and F, heterosis was observed. Finally, two recent studies of the genetic nature of variation in root characters have addressed tomato and pea. A tomato mutant described as “cottony root” was studied by Hochmuth et al. (1985). From data of several experiments it was determined that the cottony root trait was controlled by a monogenic, recessive mode of inheritance. Saleh (1986) used generation mean analyses to describe genetic control of root dry weight and root volume in pea. As we have seen in other studies, he found both additive and dominance effects for both parameters in all populations. Broad sense heritability estimates ranged from 77 to 81% for root dry weight and 44 to 77% for root volume.

IV. UTILIZATION IN RESEARCH AND CROP IMPROVEMENT A. CASESTUDIES Over the past 25 years, a few research programs have demonstrated distinct progress in the study of genotypic variation in root system form and

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121

function. In the following section, we discuss six such examples which illustrate the results of productive inquiry and utilization of intraspecies variation for crop plant improvement.

I . Alfalfa in Canada In their review of the significance of genotypic variation in root systems, Troughton and Whittington (1969) noted the difficulty of root studies, particularly in light of the selection demands of a plant breeding program. They concluded that although genotypic variability had been observed in several crop species, many difficulties existed to preclude its utilization in plant breeding. Indeed, they cited only one example, that of the “creeping root habit” of alfalfa, reviewed by Heinrichs (1963). This example is noteworthy and its salient points will be of interest to our discussion. Drought and winter soil heaving are environmental stresses common in the Great Plains of the United States and Canada which jeopardize establishment and maintenance of alfalfa (Medicago sativa L.) plantings. Genotypes used in the 1950s were characterized by a strongly taprooted system which limited lateral growth after drought or winter kill decreased stand establishment. Two potential solutions were “branched root” types and creeping root types. The latter produced shoots at irregular intervals from horizontally spreading roots. The source of creeping rooted genotypes, M. falcata L., had earlier been collected from Europe and Asia. Hybridization between M . sativa and M . falcata and strong selection in situ resulted in incorporation of the creeping root trait into a range of cultivars adapted to specific ecological zones. The key to successful breeding and selection in this case was a rather atypical root- and shoot-related parameter. The desired root phenotype could be visually selected for in the field because the degree of horizontal spread of the root system was indicated by the appearance of new shoots. The distance and pattern of new shoots from the original plant or row provided a remarkably effective selection tool which could be used the year following spring planting of a particular generation. Reports indicated that F3 and F4 generations could be successfully screened for the trait. Although the degree of trait expression varied and the heritability was more complex than initially thought, breeders were able to employ this trait in their efforts to increase stress resistance in alfalfa throughout the world. 2.

Wheat in Canada

In 1961 Hurd and associates began studying the rooting patterns of wheat cultivars in an attempt to incorporate desirable qualities into their breeding program. Hurd (1974) recalled two things which prompted their early studies.

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J. C. O’TOOLE AND W. L. BLAND

First, research in the 1950s on the role of roots in developing more droughtresistant alfalfa cultivars (Heinrichs, 1963) was undertaken at the same research station, Swift Current, Saskatchewan, Canada. This influence coupled with the very dry year of 1961, when wheat cultivars showed obvious differences in field level response to drought, prompted a series of greenhouse experiments during the winter months of the years 1961-1965 (Hurd, 1964, 1968). Initial studies used root growth boxes with sloping glass faces and emphasized measurements of total root length observed at the glass face and dry weight of roots at harvest. The work was ambitious and at once attempted to investigate not only differences among genotypes but the response of genotypes to varying soil moisture and soil type. Unfortunately, it appears that the experimental protocol was not well suited for such a complex and variable experimental design. Hurd (1964) provided evidence that genotype variation in wheat rooting pattern did exist and believed the results also indicated that the genotypic variation was exhibited over varying soil water application treatments. Based on observations over several years, two desirable qualitative characteristics of wheat root systems for the semiarid western Canadian prairies were identified 1. early rapid penetration of the soil profile by numerous primary roots 2. high proportion of total root dry matter at 20 to 30 cm and, when stressed, below 30 cm

The wheat variety Thatcher best illustrated the preferred root system. Further research of a similar nature carried out in the years 1968-1969 and 1972-1973 dealt with numerous cultivars and contributed to the research group’s information base (Hurd, 1974). The research also confirmed the role of rooting pattern in the remarkable drought resistance of wheat cultivar Pitic 62. This cultivar exhibited substantial root mass regardless of water treatment level and a high rate of vertical root growth similar to the cultivar Thatcher. During this period field soil coring was used to augment the greenhouse root box technique. The vertical distribution of root weight in field plots (Fig. 2) agreed well with the relative rating of cultivars gained through root box studies. When viewed in toto these experiments are a landmark in the genetic improvement of root function. Spanning more than 12 years, this work played a significant role in the breeding of new wheat cultivars such as Wascana and Wakooma (Hurd et al., 1972a,b, 1973). 3. Soybeans in the United States

In 1973 a comprehensive research project was initiated at Ames, Iowa, to test the hypothesis that additional soil water could be made available to

GENOTYPIC VARIATION IN PLANT ROOT SYSTEMS WAKOOMA

n

U

WASCANA

HERCULES

U

U

123

A I

I

t

o

:

520 mg PELlSSlER

400

250

PITIC

THATCHER

90

I20

480mg

FIG.2. Bar graphs showing average weight of root recovered from nine cores taken in the field. (From Hurd, 1974.)

soybean plants by modifying their root systems (Taylor, 1983). Taylor (1980) reviewed the careful planning and stepwise approach he and numerous associates took during the 8-10-year program. Initial analysis of hydrologic, climatic, and edaphic data illustrated that on average the deep and fertile soils of Iowa were recharged annually and may have had significant deep percolation losses. However, water deficits often developed in soybean crops during the period July through early September. It appeared that the water resource would be available, but could the soybean root system be modified to capitalize on that resource? The research group used a conceptual model (Taylor and Klepper, 1978) of water uptake by a root system to direct their efforts. Studies began the process of understanding spatial development and function of the soybean root system with emphasis on the yield-sensitive reproductive stages (Kaspar et al., 1978). Taylor (1980) concluded that soybean root length density (cm/cm3) was more than adequate in much of the soil profile with the exception of the upper 15-cm layer and below 1.5-m depth. Root length density deep in the profile was significant given the temporal coincidence of the rooting front arrival at these depths with reproductive growth stages. Utilizing the model to identify significant root parameters, they concluded that neither radial nor axial resistances appeared limiting given the specific ecological conditions set forth in the initial environmental survey (Eavis and Taylor, 1979; Willatt and Taylor, 1978). An increase in vertical growth rate of the root system would access additional water resources and provide a longer period of root activity at depth, potentially eluding stress at the sensitive reproductive stages.

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J. C. O'TOOLE AND W. L. BLAND

-

0.5

t

2.5' 20

I

30

'

'

I

I

so

"

40 50 60 70 80 DAYS AFTER PLANTINO

'

100

FIG. 3. Depths of soybean root penetration at different times throughout the growth season. The IS and 17°C isotherms are shown. (From Taylor, 1983.)

Studies were initiated to determine the extent of genotypic variation for growth rate of the taproot. Taylor et al. (1978) investigated taproot growth rates of 29 soybean cultivars under uniform greenhouse conditions. The rates ranged from 3.1 to 4.3 cm/day over a 27-day period. A 1.2-cm/day increase in vertical growth rate could mean access to an additional 60 cm of soil when flowering begins. During the 1979 crop season the research group conducted a comprehensive field examination of root and shoot growth for a single soybean cultivar, Wayne (Mason et a/., 1982). They found that vertical root growth generally followed the 16°C isotherm (Fig. 3). These results were instrumental in directing further research. Genotypic variation for taproot elongation rate was significant but its interaction with soil temperature became the paramount root-soil interaction upon which any modification of rooting depth would have to be predicated. A series of experiments followed in which genotypes were evaluated at varying temperatures for taproot growth rates and inclination of lateral roots (Kaspar et al., 1981; Stone and Taylor, 1983). The rate of taproot elongation increased with soil temperature up to 17°C and decreased with time. Subsequently, a simulation model to predict rooting depth of soybeans primarily from root zone temperature was developed (Stone et al., 1983). The project illustrated that a comprehensive program must place equal emphasis on investigation of genotypic variation and research to identify soil physical or chemical obstacles to the expression of that variation. The utility of a physically based model in providing direction and assisting in interpretation of results was also demonstrated. 4. Rice in the Philippines

Although the so-called Green Revolution brought about distinctive changes in the aboveground portion of modern high-yielding rice cultivars,

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125

no mention will be found in related literature of the period regarding root systems. However, intensive research on the adaptation of rice to rain-fed and, especially, drought-prone’ rain-fed areas began at the International Rice Research Institute (IRRI) in about 1970. Research aimed at understanding the soil-plant-water relations of rain-fed rice quickly identified the root system as a major factor in adapted cultivars. Although rice is a semiaquatic plant, it is adapted to an extremely diverse range of hydrological conditions which has resulted in great ecophysiological diversity (O’Toole and Chang, 1979). In 1971 scientists at the IRRI began to characterize the root systems of divergent rice ecotypes. Chang et af. (1972) illustrated that rice genotypes from Asian and African upland environments shared common traits when contrasted with lowland ecotypes: greater root length (depth) of main axes, larger root diameter (thickness), and a lower degree of branching. They reported that these traits were not distinct between the upland and lowland adapted rices but rather showed significant overlap across groupings. Chang et af. (1972) also reported that the root system of upland rices exhibited a plastic response to the water regime in which they adjusted rooting depth to maintain water supply. This response appeared to be absent in lowland types. These observations may have been based on qualitative assessment; data presented did not corroborate the statement. Quantitative and photographic evidence of genotypic variation for root length and diameter, collected in 1971 and 1972, were reviewed by Chang and Vergara (1975). O’Toole and Chang (1979) reviewed research on drought resistance in rice germplasm covering the mid-1970s. Work at IRRI during this period, often unpublished, pointed to genotypic variation in rooting depth as a casual mechanism associated with results from field-level drought resistance screening and rain-fed rice yield trials. Through the late 1970s and to the present the ZRRZAnnuaf Reports illustrated quantitatively the high degree of genotypic variation in root systems for root:shoot ratio, deep (below 30 cm) root:shoot ratio (IRRI, 1976), root pulling force (IRRI, 1977, 1984), vertical root density distribution (IRRI, 1978, 1979, 1980), nodal root diameter, 1ength:weight ratio (IRRI, 1978, 1985), root diameter-length relationship (IRRI, 1983), radius of root xylem vessels (IRRI, 1983), and root penetration of compacted soil (IRRI, 1984). Initial studies on the inheritance of root characteristics are found in the annual report for 1980 (IRRI, 1981) Unfortunately, the annual report style is quite brief, focusing primarily on results. Limited published accounts of root research from the IRRI can be found in conference proceedings and journal papers. Yoshida and Hasegawa (1982) reviewed much of their work accomplished in the 1970s. They presented a detailed analysis of tillering and rooting pattern of the rice plant and illustrated dramatic differences among cultivars in root:shoot ratio,

Table In Vertical Distribution of Root Length Density of Seven Rice Varieties Grown in a Dryland Field' Root length density (cm/cm3) at varying depths (cm) Designation

origin

0-10

10-20

20-30

30-40

40-50

50-60

60-70

70-80

IR20 IR2035-117-3 IR442-2-58

Lowland Lowland Lowland Upland Upland Upland Upland

14.4

2.8 5.8 7.1

0.9 0.8 1.2 0.8 0.9 1.9 0.9

0.4 0.1

0.1

-

-

-

22.7 16.8 12.6 11.8 16.2 19.8

0.1 0.1

0.1

-

-

0.8 0.6 0.8 0.9

0.1 0.5

0.8 0.6 0.9

0.1 0.5 0.4

OM Moroberekan Salumpikit 20A

'Trom Yoshida and Hasegawa (1982).

1.4

2.3 5.5 2.6

0.3 0.9 0.8 1.4 0.8

0.5

0.3 0.6

0.2 0.1 0.4

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127

root finess (m/g), and vertical distribution of root length density. The genotypic variation for root length density found between lowland and upland cultivars and breeding lines is demonstrated in Table 111. The tendency for rices of upland origin to be more deeply rooted than lowland lines is clear from these measurements. Yoshida and Hasegawa (1982) also showed a comparison between two isogenic lines which differed markedly in plant height but were almost identical in rooting habit. That experiment indicated there was no apparent immutable relationship between plant height and rooting depth, clearing the way for development of modern semidwarf cultivars with improved rooting depth. In the same conference proceedings Chang et al. (1982) demonstrated genotypic variation in water extraction patterns and confirmed a relationship with midday leaf water potential and rooting depth. This report and that of Armenta-Soto et al. (1983) included a synopsis of the first studies on genetics and heritability of individual traits of the rice system including root:shoot ratio, root weight, root length, root number, and root tip and base diameters. Ekanayake et al. (1985a) demonstrated the linkage among root morphological parameters, their inheritance, and their effect on plant water potential of F, families in field drought resistance trials. These studies of root system morphological characteristics and their function in determining soil water extraction patterns and plant water status span a period of root research at the IRRI of about 15 years. The lack of research associating these root characteristics with yield under drought stress conditions may in part be due to lack of a clearly targeted ecological zone and the relatively wet climatic environment at the IRRI in Los BaRos, Philippines. The former is noted as a critical step in other successful case studies.

5.

Wheat in Australia

The efforts of Passioura and Richards, working in Australia, to modify wheat root anatomy provided a classical study in the application of plant breeding to solving a root-related problem. In the early 1970s research was carried out which illustrated experimentally that wheat plants growing on stored soil water, when forced to rely on a single seminal root, used less water prior to flowering than did normal plants. It was presumed that the single root represented greater resistance to vertical flow of water in the xylem (axial resistance) than that present in normal plants which had three seminal roots (Passioura, 1972). In dry years, nodal root development may be prevented by dry surface soil and the plant must rely on its seminal roots for water. With a higher resistance to flow, the limited water supply is metered out over the season, assuring that sufficient soil water remains for the postanthesis period to maximize yield. Further research summarized by

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Passioura (1977) illustrated two salient points which led to the final working hypothesis on genotypic modification of the wheat root system. First, wheat plants which received water in small doses over time produced a greater yield than those receiving up to four times greater total irrigation but all at sowing as a basal irrigation. There was no relationship in these experiments between yield and total water applied. Second, he found that harvest index was most sensitive to water used after flowering. Plants with high harvest index, 0.4-0.5, were found to have 30% of their total water use after flowering. Conversely, harvest index fell to 0.1 or less when less than 10% of total water use occurred after flowering. If these results were indicative of field-grown wheat, it opened a new and somewhat unorthodox approach to increase rain-fed wheat yields. The application of such root system modification would only be valid in specific agroclimatic zones and should not be viewed as a general principle. Nix (1975) classified the main wheat-growing areas in eastern Australia by the percentage of water commonly used by the crop in the form of soil-stored water. The areas ranged from less than 20% to significant areas where 30-50% of crop water use was from that source and presented an important target environment for field testing this particular root modification. Richards and Passioura (198la) investigated anatomical and morphological means of altering axial root resistance of the seminal root system. They considered decreasing either the number of seminal root axes or the diameter of the main xylem vessel in each seminal root. Based on a survey of over loo0 modern and primitive wheats they chose to select for smaller-diameter xylem vessels because little variability was found in number of seminal roots. In addition xylem vessel diameter demonstrated higher heritability (Richards and Passioura, 1981b). Richards (1982) reviewed the progress of their hybridization and selection efforts. He reported success in manipulation of the trait through several backcrosses and filial generations aimed at incorporating those genes into high-yielding, disease-resistant, and high-quality backgrounds. The entire process has taken perhaps a decade. Richards (1987) presented the first field-level results, which corroborate Passioura’s original hypothesis. Table IV illustrates the results obtained from lines with normal (60 pm) and small (5Opm) diameter xylem vessels. It appears that the line with small vessels performed slightly better in more stressful years, 1981 and 1982, and as expected did not appear to be at a disadvantage in the wet year 1983. In wet years, nodal roots are formed and quickly make the seminal root of little consequence in water uptake. These results must be considered preliminary. Further yield trials in the targeted agroclimatic zones and follow-up field-level research will be desirable to determine if indeed the trait and its mechanistic mode of expression are being realized at the crop level.

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Table IV Mean Grain Yield of Lines Containing Small and Normal Diameter Xylem Vessels in Three Seasons in Southern Australia‘~* Grain yield (tons/ha) Xylem vessel diameter

Condobolin

Wagga Wagga

Wagga Wagga

1981

1982

1983

Small (50 pm) Normal (60pm) LSDC

0.90

0.96 0.88 0.08

3.46 3.35 0.16

0.81 0.07

‘Lines are backcrossl derivatives of the commercial variety Kite. April to October rainfall was 228 mm at Condobolin and 243 and 549 m m at Wagga Wagga in 1982 and 1983, respectively. *From Richards (1987). ‘LSD, Least significant difference (p = 0.05).

6. Oats in Sweden

The careful application of data on genotypic variation in oat root systems to crop improvement was well documented by Larsson (1982, 1986). Larsson’s initial step was to delineate the problem by using climatic data. The drought-prone early summer months in central and eastern Sweden corresponded to specific crop phenological stages, i.e., seedling establishment and early vegetative growth. He then utilized long-term multilocation yield and weather data to identify 15 oat cultivars with divergent field levels of drought resistance and susceptibility. His anaylsis revealed that black oats, which are primarily unimproved landraces, were more resistant than the modern and agronomically superior white oats. Larsson utilized the 15 oat cultivars to develop a simple yet morpho-physiologically based drought resistance index (DRI) relating root length to leaf area of 10-day-old seedlings grown in a controlled environment:

bc DRI= loox a

-

bc

Seger I

where a is the seminal root length, b the combined length of the first and second leaves, and c the width of the first leaf. The analysis of yield and weather records as well as the DRI development used a single cultivar, Seger I, as a reference point. Larsson has used the DRI to evaluate other sources of oat germplasm and the results of numerous crosses aimed at combining high DRI values of

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black oats with the superior grain quality of white oats. In addition, he has illustrated that DRI varies greatly among oat lines derived from mutations. The laboratory test appears quite simple and the resulting rank of entries highly repeatable. However, the breeding effort could not be labeled as successful without field-level testing for yield under early summer drought conditions. Larsson (pers. commun.) has taken that final step by utilizing rain shelters to modify early season rainfall. Illustrated in Fig. 4 are the relative yields from stress and control plots of black oats, white oats, and the results of crosses selected on the basis of the DRI. The progress made in selecting for increased root length: leaf area ratios (DRI) is obvious, as is its relationship to relative yield. The level of DRI may be expected to reach higher levels, as Larsson (1986) illustrated that from one generation to the next (F, to F.,) an improvement of about 20 DRI units could be realized.

B. STRATEGIES FOR CROP IMPROVEMENT Drawing from the experience of the preceding case studies and other reviews related to breeding and selection for root parameters, we shall discuss development of a hypothetical program to improve root function of crop plants genetically.

I . Definition of the Problem

A clear definition of the perceived problem and opportunity for improvement of the agronomic system (stand establishment, drought resistance of a particular crop stage, etc.) is the initial step. Climatic, edaphic, and hydrological data should be collated, and with consulting physical scientists efforts made to define the factor@) most limiting root system function in

Y = 0.267 X + 45.0 r = 0.735 ( P ‘0.0011

BLACK OATS - 0

WHITE El BLACK CROSSES

WHITE OATS I

l

l

l

l

l

l

l

l

l

80 100 120 140 I60 DROUGHT RESISTANCE INDEX ( D R I )

FIG.4. Relationship between relative yield (percentage yield ratio) and drought resistance index (DRI) measured in the laboratory. (S. Larsson, unpublished data.)

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the agronomic system. The coincidence of problematic environmental conditions with specific yield-determining growth stages is of paramount importance and is critical in the following step. 2. Evaluation of Root System Parameters With a defined problem we can now formulate working hypotheses leading to potential solutions. Conceptual and simulation models may be useful as decision aids in this step. The use of a conceptual model is simply an exercise in which potential root system characters are related to solutions of the problems. This process, over many iterations, will assist in identifying significant (in relation to the defined problem) root parameters and may give a first approximation of the magnitude of change necessary. The fact that so little is known about applications of genotypic variation in root parameters to crop improvement should serve as a warning not to follow single-trait solutions. Although conceptual modeling may present what appears to be “the root parameter” for problem solution, mechanistic simulation in the highly interactive soil-plant system serves as a caution against such single-factor solutions. Such mechanistic crop models integrate the many physical and physiological parameters relevant to the crop system, permitting estimation of their relative importance. For example, Barber (1984) discussed sensitivity analyses of a nutrient uptake model. Simulation of an increasingly fine or more branched root system resulted in greater uptake of potassium. The efficacy of increasing the depth of rooting of sorghum and wheat was simulated by Jordan et al. (1983). Simulations were run using 30 years of actual weather data for Lubbock, Texas, Temple, Texas, and Manhattan, Kansas, with typical and more deeply rooted cultivars. The extra water afforded the deeper-rooted lines resulted in frequent yield improvements. However, it should be recognized that present models all contain empiricisms and reflect the creator’s view of how crops normally grow. Thus, alteration of a single trait may have implications beyond those recognized by the model. As Brouwer and de Wit (1968) pointed out, “. . . it is opinion that is really simulated.” Topological analysis (Fitter, 1985) and theoretical considerations such as those of Fowkes and Landsberg (1981) can also provide insight for optimizing root system parameters in relation to the defined problem.

3. Determination of Existence, Level, and Nature of Genetic Variation With the problem well defined and possible solutions related to root system parameters in hand (i.e., ability to root deeper than 1 m, rate of

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vertical root growth at 10°C, root 1ength:leaf area ratio of the seedling), the next step is to quantitatively assess the degree and magnitude of genetic variation for the root character@) of interest. The value of exotic germplasm or landraces adapted to regions characterized by the problem cannot be overemphasized. Without the necessary level or magnitude of genotypic variation plant breeding is no longer a viable solution. The majority of reports in Tables I and I1 illustrate the search for genotypic variation, although problem definition and evaluation are not apparent in many cases. There may be more than one option for attaining a goal, as was the case in reducing the conductance of the seminal root system of wheat in Australia. Clearly experimental design is needed here, given the difficultly of screening for root parameters and the contribution of the environment to phenotype. Rapid seedling tests may be suitable for some traits (e.g., Larrson, 1982), but others such as increased root length density at depth in response to drought (see Section V), require more involved methods. When a significant degree of variation for the trait of interest is found and techniques to measure that trait quantitatively are refined, definitive studies of the genetic nature and degree of heritability should be undertaken.

4.

Hybridization and Selection

From the discussion on heritability and genetic control it appears that root traits may be handled in a crop improvement program in the same way as other quantitatively inherited traits. Of course, the technical difficulties of plant selection must be overcome, and if this is not possible an altered approach, such as screening only for parental lines and lines in advanced yield trials prior to cultivar release, can be adopted. A discussion of breeding systems for root-related solutions to plant stress problems is beyond the scope of this discussion but some insight was provided by Hurd (1969, 1971), Hurd and Spratt (1975), Lafever (1981), Chang et al. (1982), Christiansen and Lewis (1982), Devine (1982), and Barnes (1983). 5. Field Evaluation of Resultant Genotype

From the discussion of case studies it is obvious that few programs have evolved to the point of field evaluation of a resultant new genotype. This evaluation should verify the mechanisms embodied in the working hypotheses of the program. Rain-excluding shelters and/or novel irrigation facilities may be required for timely evaluation of the long-term value of a new root system morphology.

GENOTYPIC VARIATION IN PLANT ROOT SYSTEMS

V.

133

PHENOTYPIC PLASTICITY

Successful genetic improvement of the root function of crop plants requires that the impact of the soil environment on the expression of the phenotype be recognized (Russell, 1977; Taylor, 1983; Hamblin, 1985). While this interaction has traditionally been a problem for plant breeders attempting to evaluate genotypes (Hill, 1975), the responsiveness of roots to the soil environment appears to be of potential value and should be appreciated in its own right. Throughout this chapter we have assumed that crop improvement by altering root parameters will require a mechanistic view of root system form and function. Thus, the effect of specific environments on particular root characters is of interest, rather than only economic yield. The ability of a genotype to alter its phenotypic expression in response to the environment is termed phenotypic plasticity. Ecologists have become increasingly concerned with phenotypic plasticity because of its implications for evolution and adaptation. Full appreciation of its importance by botanists, however, may have been delayed in part due to “growing fascination with the detection and measurement of ‘genetic’ variation, of which plasticity must have seemed the antithesis” (Schlichting, 1986). This observation appears to be well suited to the present state of study of crop plant root systems. Because terminology and concepts surrounding plasticity have multiple meanings and are easily misunderstood, we will elaborate on our usage of the term before reviewing important characteristics and examples of plasticity. A. DESCRIPTION Phenotypic plasticity is the ability of a genotype to express different phenotypes in responseto the physicalenvironment(Bradshaw, 1965). Here we are concerned with the ability of a individual plant (single genotype) to modify the phenotypic expression of a specificcharacter (e.g., root length below 1.O m in the soil, fineness of roots) in response to the environment. The time scale of the environmental variation of interest is within a growing season. The space scale is, for example, the unpredictable variability in soil structure within a field. Adaptation of a species to more predictable variation, such as the longterm climate or soil type, is better handled with the development of a welladapted genotype (Bradshaw, 1965), rather than reliance on plasticity. In this discussion we are assuming resources are available to develop genotypes optimally suited to a defined (though variable) crop environment. Plasticity in individual characteristics may lead to lower variation (stability) in other traits, such as yield. Allard and Bradshaw (1964) stated this in the context of traditional plant breeding:

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...

the stability with which we are concerned does not imply general constancy of phenotype in varying environments. It implies stability in those aspects of phenotype, especially yield and quality, that are important economically. Such stability may in fact depend on holding some aspects of morphology and physiology in steady state and allowing others to vary. Thus the required varieties will show low genotypeenvironments interaction for agriculturally important characters, particularly yield, but not necessarily for other characters. A variety which can adjust its genotypic or phenotypic state in response to transient fluctuations in environment in such ways that it gives high and stable economic return for the place and year can be termed “well-buffered”.6

Allard and Bradshaw (1964) described two types of buffering, populational and individual. We are addressing only the individual case, where only one genotype is present. In populational buffering there is a mix of genotypes present in the population or variety, presumably differentially expressed according to the environment. Definitions of other terms related to phenotypic plasticity were presented in Bradshaw (1965) and Jain (1978). Several important characteristics of phenotypic plasticity relevant to this review are discussed in Section V,B, drawing on the reviews by Bradshaw (1965), Schlichting (1986), and Sultan (1987). B.

CHARACTERISTICS

I.

Specificity

The plasticity that a genotype might exhibit for a character is specific for that character; closely related parameters of the same genotype may show marked differences in plasticity. Additionally, a plastic response should be viewed as specific for a single environmental stimulus and a particular stage of growth. 2. Heritability

The degree of phenotypic plasticity for agiven trait is often an independently inherited characteristic. Plasticity has been associated with heterozygosity in the individual (Allard and Bradshaw, 1964), but there is much evidencethat heterozygosity is not required. This suggests that there must be specific genetic control for many cases of plastic response (Bradshaw, 1965; Sultan, 1987). Perhaps breeding programs that have sought constancy of yield over a range of environments have resulted in unconscious selection for plasticity in root parameters. Reproduced from CROP SCIENCE, Volume 4, No. 5 , September-October 1964, pp. 503-508, by permission of the Crop Science Society of America, Inc.

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135

An example of how the heritability of plasticity can be domonstrated was provided by Jain (1978). Two generations of 20 families of a selfing, highly homogeneous grass (Bromus mollis L.) were grown in a range of environments. Variation in the phenotypes exhibited across the environments was indicative of the plasticity inherent in each family’s genotype. Inheritance of this plasticity was demonstrated by ranking the families for variance in several characteristics and testing for correlation between the rankings for the two generations; significance was obtained for two of the three characters studied. That plasticity may be conferred by specific genes appears to widen the possibilities for creating optimal root systems in crop plants.

3.

Timing and Initiation of Response

For a plastic response to be valuable it must be timely. If the adaptation provided by plasticity does not occur rapidly enough to help ameliorate an imposed stress, the response may not be of value (Bradshaw, 1965). A plastic response may be triggered by the same environmental stimulus at which the response is directed, in which case the response must be relatively rapid to be useful. Alternatively, a timely response could be triggered by a stimulus not ‘directly related to the impending stress. 4. Analysis of Plasticity

Both Schlichting (1986) and Sultan (1987) discussed the analysis of experiments designed to measure plasticity. In the simplest instance, some aspect of the phenotype expressed by two genotypes is compared across several environments. For each genotype, the range of phenotypic expressions exhibited is plotted against environment, yielding the “norm of reaction” of that genotype. Figure 5 , discussed in Section V,C, can be viewed as a norm of reaction for degree of root system proliferation at depth in response to seasonal rainfall. Some expression of variability in the phenotype caused by the environments can be calculated, for example, the variance in the genotype means across the environment. The genotype yielding the highest variance for the characteristic is considered to exhibit the greatest plasticity. However, the norm of reaction curves will not necessarily be similarly shaped, so describing the plasticity with a single number may hide important features of the response of a particular genotype (Sultan, 1987; Schlichting, 1986). Differences in plastic response exhibited by the genotypes would be revealed by a significant genotype x environment interaction in a two-way analysis of variance. If more than two genotypes are included in the experiment,

J. C. O’TOOLE AND W.L. BLAND

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however, further analysis is required to determine the magnitude and direction of each entry’s response (Schlichting 1986). Sultan (1987) emphasized interpretation of the norm of reaction curves. In the analysis of studies of phenotypic plasticity, “It must be stressed that the difference between relatively narrow, invariant phenotypic expression and broad, environmentally reponsive expression reflects different norms of reaction to environment and not ‘environmental’ versus ‘genetic’ means of morphogenetic control . . . individual phenotype is produced jointly by genotype and environment” (Sultan, 1987). In terms of analysis of variance, the relative sizes of the variance terms depends on the environments selected and the genotypes’ norm of reaction curves. Over some range of environments the norms of reaction of the genotypes may be similar, resulting in identical responses. In another set of environments, however, differences in the norms of reaction may be manifested and a large difference between genotypes obtained. Thus, growing plants in a single environment cannot reveal the extent of genetic variation among the entries (Sultan, 1987).

c.

EXPERIMENTAL EVIDENCE IN CROP SPECIES

Expression of phenotypic plasticity in root characteristics of crop plants can be found in the literature, although few experiments have addressed the phenomenon directly. Jordan and Miller (1980) presented an argument for increasing the rooting below 1 m deep by sorghum cultivars grown in central Texas, and field measurements of rooting made in 1977 showed that this trait was available in the sorghum germplasm. Subsequent measurements on the same lines in 1978, 1979, and 1980 indicated that lines exhibiting greater rooting deep in the soil profile apparently did so only in years of low summer rainfall (Jordan and Monk, 1980). Retta et al. (1982) also reported increased rooting by sorghum in response to drought and differences among cultivars in this trait. These observations suggest that it may be possible to add a plastic response for greater deep rooting by sorghum in years when the additional water supply is urgently needed. Figure 5 depicts schematically how a genotype with the desired plasticity would respond to water supply. Jordan and Monk (1980) observed that this trait was not revealed in hydroponic screening. It is difficult to imagine that the required stimulus (whatever it is) could occur in hydroponic culture. In a similar trial with seven spring barley cultivars, Gorny and Patyna (1984) identified one variety, Menuette, that had greater seminal root system growth rate under less frequent irrigations. Semidwarf wheat varieties differed in ability to modify their pattern of root dry matter distribution under drought in experiments reported by Hurd

GENOTYPIC VARIATION IN PLANT ROOT SYSTEMS

137

J ULY R A INFA LL

O

r

LOW PLASTICITY

C

mc. 5. Illustration of plasticity in depth of root proliferation as a function of critical seasonal rainfall. (1968). All six varieties depicted in his Fig. 6 had lower root dry weights in the dry treatment but Pitic, Thatcher, and Yecora appeared to distribute root dry matter more deeply when stressed. Such plasticity in dry matter distribution seems a desirable trait. Differences in plasticity for root parameters in pea cultivars was demonstrated by Veitenheimer (1981) and Veitenheimer and Gritton (1984). In a careful study to check the effects of growth media on variation in root parameters, the media was found to change the entry ranking for root system volume and weight. A soil-based medium gave the same ranking as was observed in field trials. Veitenheimer and Gritton (1984) concluded that studies of pea rooting should be conducted in a soil-based medium if results are to be extrapolated to the field. Similarly, Harrigan and Barrs (1984) reported a reordering of safflower genotypes for root length density when the growth media was changed from sand to clay. The implication of these results for all studies of variability in root systems is that the suitability of model systems must be demonstrated. As discussed previously, root growth in any one environment cannot reveal the extent of genetic variation among the entries. Thus, the concept of a single model environment in which to evaluate genotypic variation is flawed. Sadhu and Bhaduri (1984) discussed such a system and presented data that demonstrated reordering of cultivars for root length in different media. Zobel (1975) described casual observations of soil effects on the root system morphology for different varieties of tomato. A gradation in soil

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J. C. O’TOOLE AND W. L. BLAND

texture from sandy loam to clay across a field resulted in an increase in fine fibrous roots and fewer secondary roots in one of the lines but no change in three other cultivars. Another genotype was observed to respond to water stress by producing many more fibrous roots, while the other did not. Gulmon and Turner (1978) tested Zobel’s (1975) observations in a greenhouse trial. Three genotypes were grown in sand or silt loam soil, with or without added nutrients. Root:shoot ratio was found to vary across environments, but not among the genotypes. The cultivar that expressed the greatest variability in root dry weight across the environments was the line observed to have the greatest plastic response by Zobel (1975). Sirohi et al. (1978) reported varietal differences in the effect of water deficits on several root parameters of wheat. Differences among genotypes in lateral (adventitious) root development were expressed in unirrigated treatments, but not in the irrigated treatment. Also, varietal differences in seminal root length were more pronounced late in the season. Both of these observations suggest that rapid, nonstress screening methods may not be suitable for many important traits. Many other experiments in which root parameters of several genotypes were compared across a range of environments can be interpreted in terms of phenotypic plasticity. Cultivars of dry edible beans demonstrated differing degrees of plasticity in their response to compacted soils. Asady et al. (1985) grew four cultivars of beans in soil compacted to three levels of air-filled porosity. Because the genotypes must be assumed to be inherently different in morphology, we calculated the relative reduction in total root length caused by the treatments. Cultivar NEP-2 was nearly four times as sensitive to compaction, an undesirable plastic response, as was Swan Valley. Alfalfa cultivars were shown to differ in their response to soil wetness by Carter et al. (1982). At low soil water levels the three cultivars tested had similar root growth. As available water was increased only cultivar WL318 responded with greater root weight. Hackett (1968) demonstrated differences between two barley varieties in gross morphological responses to nutrient deficiences. Maize cultivars responded similarly to soil temperature for early root growth but differed in their sensitivity to soil wetness for this character (Cutforth et d., 1986). We have presented phenotypic plasticity as a conceptual framework in which to view the expression of root characteristics. From the ecological literature, it appears that the responsiveness of root phenotype to the environment is a genetically controlled parameter. Such plasticity would be of great value in an “optimal” cultivar in some rain-fed agronomic systems. Utilization of plasticity requires careful definition of the root environment, both in terms of the expected mean and the variability about that mean. Experiments to understand phenotypic plasticity for root traits in crop plants will require carefully formulated hypotheses and good design, using the

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expertise of the quantitative geneticist, plant physiologist, and soil scientist (Hill, 1975).

VI.

CONCLUSIONS AND RESEARCH NEEDS

Our review of the literature indicates there is ample evidence of genotypic variation in the root characteristics of crop species. It also appears that heritabilities of root parameters are relatively high and genetic control systems comparable to those of shoot parameters. The six case studies demonstrated that progress has been made in the solution of clearly stated problems through genetic improvement of plant root function. Certainly other situations exist in which improvement can result from modest changes in root growth parameters. Despite the often cited difficulties inherent in the study of root systems, we feel that progress over the past 15 years, a relatively short period in scientific research and crop improvement terms, gives cause for optimism. As we learn more about this new genetic resource, we can expect continued success in crop improvement by tailoring belowground plant organs to specific well-defined edaphic niches. Specifically, our review has identified genotypic variation in some root parameters for which we believe rapid utilization is possible. In view of the dominant effect of soil temperature dynamics on root system development, especially at temperate latitudes, we reemphasize that several studies detected genotypic differences in root growth at “low” temperatures even when no differences were detected at higher temperatures. This element of genotypic adaptation may have significant ramifications in terms of root systems’ temporal and spatial development and their coincidence with yield-determininggrowth stages. In addition, genotypes of crop species have been identified v;hich continue root growth at normal rates after anthesis during the period when reproductive sinks for photoassimilates were thought to reduce assimilate availability to the root system. Knowledge of this variation may lead to the development of root systems better able to protect sensitive reproductive events from environmental stresses. The ability of plant root systems to make short-term adaptations in response to edaphic conditions has confounded interpretation of many experimental results. However, viewed from an ecological perspective phenotypic plasticity may have a significant positive role in optimizing root systems for rain-fed environments. A quantitative understanding of this phenomenon may allow specification and selection of genotypes capable of altering dry matter allocation in response to local edaphic conditions and the vagaries of weather. However, because of phenotypic plasticity, model

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systems for plant culture or rapid screening must be viewed with skepticism. Adequate resources should be allocated to field verification of such methods. Novel experiments will be required to gain an adequate understanding of the impact of soil environments on phenotype, e.g., the study of phenotype across a continuum of root environments between the endpoints of aeroponic and field soil culture. The significance of numerous interacting factors (temperature, oxygen, strength, water content, etc.) must be recognized and experimental facilities constructed in which as many important physical parameters as possible are controlled. Realization of the full potential for crop root system improvement remains a challenge to the quantitative geneticist, soil physicist, and root physiologist. The traditional bounds of each discipline must be extended until common ground is established and truly interdisciplinary studies result. By comparison with the state of scientific research on plant shoots, root science is in its infancy and may be expected to disclose many new and exciting possibilities to further adapt crop plants to their environment.

REFERENCES Aha, P. O., and Fapohunda, H. 0. 1986. Plant Soil 94, 257-265. Ali-Khan, S. T., and Snoad, B. 1977. Ann. Appl. Biol. 85, 131-136. Allard, R. W., and Bradshaw, A. D. 1964. Crop Sci. 4, 503-508. Andrew, R. H., and Solanki, S. S. 1966. Agron. J. 58, 415418. Arkin, G. F., and Taylor, H. M. 1981. Modifying the root environment to reduce crop stress. Am. SOC.Agric. Eng. Monogr. No. 4. Am. SOC.Agric. Engr., St. Joseph, Michigan. Armenta-Soto, J., Chang, T. T., Loresto, G. C., and O’Toole, J. C. 1983. SABRA0 J. 15, 103-116.

Asady, G. H., Smucker, A. J. M., and Adams, M. W. 1985. Crop Sci. 25, 802-806. Aycock, M. K., and McKee, C. G . 1975. Agron. J. 67, 604-606. Barber, S. A. 1984. “Soil Nutrient Bioavailability.” Wiley (Interscience), New York. Barber, S. A., and Silberbush, M. 1984. In “Roots, Nutrient and Water Influx, and Plant Growth” (S. A. Barber and D. R. Bouldin, eds.), pp. 65-87. ASA Special Publ. No. 49. Am. Soc. Agron., Madison, Wisconsin. Barley, K. P. 1970. Adv. Agron. 22, 159-201. Barnes, D. K. 1983. In “Limitations to Efficient Water Use in Crop Production” (H. M. Taylor, W. R. Jordan, and T. R. Sinclair, eds.), pp. 127-136. Am. SOC.Agron., Madison, Wisconsin. Bhan, S., Singh, H. G., and Singh, A. 1973. Indian J. Agric. Sci. 43, 828-830. Blum, A. 1972. Agron. J. 64, 775-718. Blum, A., Arkin, G. F., and Jordan, W . R. 1977a. Crop Sci. 17, 149-153. Blum, A., Jordan, W. R., and Arkin, G. F. 197713. Crop Sci. 17, 153-157. Blum, A., Mayer, J., and Gozlan, G. 1983. Plant Cell Environ. 6, 219-225. Boyer, J. S. 1982. Science 218, 443-448. Boyer, J. S. 1985. Ann. Rev. Plant Physiol. 36, 473-516. Boyer, J. S., Johnson, R. R., and Saupe, S. G. 1980. Agron. J. 72, 981-986.

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ADVANCES IN AGRONOMY, VOL. 41

APPLICATION OF CELL AND TISSUE CULTURE TECHNIQUES FOR THE GENETIC IMPROVEMENT OF SORGHUM, Sorghum bicolor (L.) Moench PROGRESS AND POTENTIAL’ S. K r e s ~ v i c hR. , ~E. ~ ~McGee,* L. Panella,4 A. A. R e i l l e ~ ,and ~ . ~F. R. Miller4 *Texas Agricultural Experiment Station, The Texas A&M University System Wesiaco, Texas 78596 Department of Soil and Crop Sciences, Texas A&M University College Station, Texas 77843

I.

INTRODUCTION

Having evolved over a great period of time and a broad spectrum of species, plant breeding is a science-based technology directed towards economic objectives (Simmonds, 1983). It is one component of agricultural production and, along with agronomic practices, agricultural chemistry, plant pathology, entomology, and agricultural engineering, has been responsible for past successes associated with increased yields of food, feed, fiber, and fuel at low costs to the consumer. Plant breeding has long since passed from being an “art,” as some of the older plant breeders would lead one to believe, and is now heavily dependent on the disciplines of genetics and biometry. During the past two decades, an integration of plant biochemistry and physiology has allowed for the development of basic techniques, i.e., cell and tissue culture, which have the potential for improving our understanding of plant biology. Early proponents of the cell and tissue culture methodology expounded its advantages and predicted that this array of I This article is a contribution of the Texas Agricultural Experiment Station, College Station, Texas 77843. Approved as Technical Article No. 22187. ’ Present address: United States Department of Agriculture, Agricultural Research Service, Germplasm Resources Unit, NYSAES, Geneva, New York 14456. ’ Present address: DNA Plant Technology Corporation, Watsonville, California 95076.

147 Copyright 0 1987 by Academic Press. Inc. All rights of reproduction in any form reserved.

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of tools might supplant conventional plant breeding. It was suggested that their implementation would cause an agricultural “revolution” which greatly would exceed preceding revolutions initiated through the application of fertilizer and mechanization to the agricultural production system. Fortunately, we all have mellowed a bit from the initial rhetoric associated with cell and tissue culture techniques. Plant scientists now realize that these techniques really cannot supplant plant breeding in all of its many-faceted forms, but rather serve as adjuncts to it. Furthermore, the uniqueness of individual crop species requires researchers to fit new techniques into current frameworks of crop improvement. With these considerations in mind, we attempt to highlight the “state of the discipline” with regard to the status and potential of these techniques for the genetic improvement of sorghum, Sorghum bicolor (L.) Moench. Sorghum is recognized not only as a source of food and feed in marginal production areas, but as a potential “biomass” energy crop because of its productivity, efficiency, and adaptability. Within this article, a framework is developed in which the potential applications of cell and tissue culture techniques to the genetic improvement of sorghum may be viewed and, also, their current limitations may be identified. Specific developments in molecular biology as related to sorghum improvement are considered beyond the scope of this article.

II.

BACKGROUND

The origin of cultivated sorghum is agreed generally to have occurred on the African continent (Mann et al., 1983); however, disagreement exists whether the origin has a monophyletic or a polyphyletic basis. Therefore, the systematics of sorghum are quite complex (de Wet et al., 1970; Doggett, 1970). Sorghum bicolor (L.) Moench includes a diverse collection (Fig. 1) including grass types and cultivated sorghum, all having a diploid chromosome complement of 2n = 20. Harlan and de Wet (1972) have devised a working classification of cultivated sorghum that encompasses five basic races including bicolor, guinea, caudatum, kafir, and durra. These races are identified by mature spikelet and panicle type. In addition, 10 hybrid races are recognized, of which kafir-caudatum is the source of most hybrid grain sorghum grown in the United States (Harlan, 1972). Other important sorghum types grown include sorghum (sorgos) for syrup and sugar, grass types for forages, and specialty types such as popping sorghum and broomcorn. Although sorghum encompasses a great diversity of types, the basic morphology and anatomy is consistent. This is particularly important when considering source tissues for culture establishment.

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FIG. 1. Phenotypic diversity exhibited in Sorghum hicolor (L.) Moench. (Courtesy of L. W . Panella.)

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The seed germinates with the emergence of a coleoptile and coleorhiza. The shoot apex is forced to the soil surface by the expansion of cells in the mesocotyl region, while one to several primary roots may emerge from the embryonic axis. A node forms at the juncture of the mesocotyl and the shoot apex, from which secondary roots and adventitious buds develop. The primary roots and mesocotyl deteriorate at this early stage, and it is the secondary roots that continue to support the sorghum plant. Secondary shoots may form from the adventitious buds at this basal node. The culm is comprised of nodes and internodes. Internode tissue may be thick or pithy, juicy or dry, and either insipid or sweet. Leaves arise from the nodal areas and consist of a sheath region and blade. The sheath base is primarily meristematic tissue which allows the sheath to elongate. In cultivated sorghum, floral initiation may occur 30-75 days or more following germination, with anthesis (flowering) occurring from 15-30 days later. The sorghum inflorescence is a panicle that matures from the apex downward. The rachis branches contain paired spikelets, one sessile and one pedicellate. The sessile spikelet generally contains one fertile and one sterile floret; sometimes both are fertile, resulting in twin seededness. The pedicelled spikelet may contain a floret with functional anthers but is usually without a functional ovary. Anthesis generally occurs during the early morning (around sunrise) when paired lodicules at the base of the spikelet swell, forcing the glumes apart and exposing the stigmas and anthers. This flowering process takes about 10-30 min. The caryopsis reaches physiological maturity about 30-35 days following pollination. The mature seed coat consists of a pericarp and fused testa. The pericarp color may appear as red, yellow, or white. The testa may also contain pigmented compounds, composed mainly of phenolics and tannins (Rooney and Miller, 1982; Oberthur et al., 1983; Doherty et al., 1987). (For more details of growth and development, see Rangaswami Ayyangar and Panduranga Rao, 1936; Artschwager, 1948; Artschwager and McGuire, 1949; Paulson, 1969; Doggett, 1970; Wall and Ross, 1970; Vanderlip and Reeves, 1972).

111.

GOALS IN BREEDING

Regardless of the crop involved, a breeding program must be defined with set goals. In addition, the program must have the means to achieve those goals. Naturally, the screening and selection criteria vary with the crop, location, and goals. A plant breeder may try to address any one or any combination of these goals. The earlier in the improvement program the breeder can identify the trait@)desired, the more quickly progress can be

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made toward the desired goal. However, in some instances expression of a given character may manifest both positive and negative qualities. As an example, phenolic accumulation in sorghum grain imparts a bitter taste which makes it less palatable to birds (Bruggers and Jaeger, 1982). In theory, this quality may help reduce damage caused by birds. This same class of compounds, however, also makes the grain less palatable for humans and less digestible for animals (Butler, 1982; Hahn et al., 1984). The best way to prioritize these criteria is to identify the problems in the area where the crop will be grown. House (1985) suggests a list of general criteria which can be used in selecting phenotypes of grain sorghum. These criteria include general adaptability, pest resistance, and high yield of quality grain. In addition, sorghum produced specifically for energy production may be screened or selected for total biomass yield, high carbohydrate content of the culm, and low lignin content. With self-pollinated species such as sorghum, plant breeding methods are based on the principle that selfing or backcrossing to a homozygous parent leads to homozygosity (Allard, 1960). A new cultivar with better agronomic traits is created and replaces older cultivars or is used as a parent for hybrid seed production. The method of plant breeding most often used in sorghum is the pedigree system. Ideally, a pedigree provides a name for each discrete type of plant which clearly shows its breeding history and its relationship to sister lines (House, 1985). In this system (Table I) a cross is made, the F,hybrid grown, Table I Pedigree Breeding System ~

~~

~

F,

Crosses between parents with desirable traits All seeds are genetically uniform

F,

Maximum variation, heterosis, and recombination Single plants are selected

F1-F6

Increasing homozygosity and uniformity Single plants within families are selected

F,-Flo Uniform lines effectively homozygous Yield testing Testing for combining ability as a hybrid parent F,-F,,

Homozygous cultivars Yield testing Testing for combining ability as a hybrid parent Line purification Seed increase for certification Commercial release

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and seed for the F, generation is bulked. The F, generation exhibits maximum diversity and recombination potential. Each successive generation of self-pollination brings the plants closer to homozygosity. Individual selections are made from the F, population and the seed from selected panicles are grown out as a F3family. Selections within families are grown out this way until the F, or F, generation. The selections then should be homozygous enough to be treated as populations. These populations are grown in small plots and evaluated for their agronomic performance and their combining ability as hybrid parents. This testing continues from 3 to 5 years in a number of locations. By the F,, or F,, generations, final selections have been made, and the seed is purified, increased, and subjected to the certification process prior to release. The duration of some of the steps may vary, but generally a 10- to 16-year period is necessary to develop a new cultivar (for greater detail of crop improvement strategies, see Allard, 1960; Briggs and Knowles, 1967; Simmonds, 1979; House, 1985). Because the time needed to develop a new cultivar is so long, any technique which could shorten this time, such as the potential tools provided by plant biotechnology, would be embraced. The plant breeder is limited in achieving desired goals by the natural variation within the species and its close relatives, i.e., sexually compatible species. The world collection of sorghum currently contains over 21,000 accessions (Mengesha and Prasada Rao, 1982). Most of these are tall, photoperiod-sensitive, tropical accessions unadapted to the temperate areas of the United States. Nonetheless, an increasing amount of this exotic germplasm is finding its way into the U S . breeding programs. This is primarily due to the Sorghum Conversion Project, a joint effort between the US. Department of Agriculture and the Texas Agricultural Experiment Station. In the program, knowledge of the maturity and height genes of sorghum is utilized, allowing tall, tropical cultivars to be converted to short, earlier types adapted to temperate conditions. This is done by backcrossing, in Puerto Rico, exotic cultivars to shorter, temperately adapted types and selecting plants adapted to temperate areas in Texas (Stephens el al., 1967; Johnson et al., 1971; Quinby, 1974; Miller, 1979). Material from this project has introduced new sources of disease resistance (Fredericksen and Rosenow, 1979), insect resistance (Johnson and Teetes, 1979), and improved plant characteristics (Miller, 1979). This infusion of new germplasm is manifest in the rapid gains in sorghum yields and grain quality seen during the last 10 years. Clear goals in sorghum production and breeding have been set, and using conventional techniques, plant breeders are moving to achieve them (Quinby, 1974; Miller and Kebede, 1984).

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IV. PROGRESS IN CELL AND TISSUE CULTURE RESEARCH Inspired by the successes of cell and tissue culture work in sugar cane (Heinz et al., 1977) in the early 1960s, researchers have since given much attention to other forage grasses and cereals (Tomes, 1985; Smith and Bhaskaran, 1986). To this point, successes have been arguably limited in grasses when considering the variability or lack of response of agronomically important genotypes in culture. This is especially true with sorghum despite the effort that has been expended. In sorghum, as with most grasses, primarily very young tissue temporally close to a meristematic state (seedling, immature inflorescence, and embryo) has proven responsive in culture (Tomes, 1985; Smith and Bhaskaran, 1986). Difficulties also are due in part to phenol production by grasses, and sorghum may represent an extreme case (Oberthur et al., 1983; Doherty et al., 1987). Retarded growth has been observed in sorghum cultures producing black and purple pigments (Gamborg et al., 1977; Brettell et al., 1980; Davis and Kidd, 1980). Despite these problems, progress has been reported in the development of sorghum cell and tissue culture techniques. A.

CALLUSCULTURE

Although success with callus induction from aseptically germinated sorghum seedlings has been reported (Strogonov et al., 1968; Marcarenhas et al., 1969, 1975a; Masteller and Holden, 1970; Rogers et al., 1974; Kannangara et al., 1977; Caulkins, 1978; Brar et al., 1979a; Davis and Kidd, 1980; Smith et al., 1983; Kresovich et al., 1986b; Smith and Bhaskaran, 1987), the works of Cure and Mott (1978), Mott and Cure (1978), and King et al. (1978) with maize (Zea mays L.) and other Gramineae suggest this growth (Fig. 2) may be aberrant meristematic tissue and not undifferentiated cells. This growth generally forms at the basal node of the sorghum seedling in response to 2,4-dichlorophenoxyaceticacid (2,4-D), an auxin analog and the growth regulator of choice. Callus formation from immature (10-1 8 days postpollination) sorghum embryos also has been reported (Gamborg et al., 1977; Brar et al., 1979a); however, the cultured tissues described as callus subsequently have been shown to originate from the primary explant by a folding of the scutellar tissue (Fig. 3) (Dunstan et al., 1978, 1979). Dunstan et al. (1978) demonstrated a recalcitrant callus arising in some cultures. Callus also has been induced from cultured unemerged inflorescences (Brettell et al., 1980; Boyes and Vasil, 1984) and immature leaves (Wernicke and Brettell, 1980, 1982; Wernicke et al., 1982).

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FIG.2. Cross section of aberrant meristematicgrowth, mistakenly reported as undifferentiated callus in many sorghum studies. x61. (Courtesy of D. K. Villalon.)

Reports of callus induction from mature somatic tissue are few, and this deficiency in the current technology precludes screening and selection of hybrids and segregating populations through callus culture. Without a routine method of callus induction from field-grown mature plants (Panella et al., 1987), cell and tissue culture techniques are of limited value in early generation screening and selection strategies.

B. ANTHER CULTURE Despite the potential value of anther culture as a source of homozygous germplasm for breeding, progress toward the establishment of callus cultures from microspores of sorghum has been slow (Jinchow Institute of Agriculture, 1978; Liang, 1978). Possibly this void may be associated with the great need for technical support associated with this type of investigation. An initial effort (Caulkins, 1978) directed at the induction of anther cultures of 22 genotypes including 60M and Norghum evaluated the effects of experimental variables such as pretreatment conditions (i.e., cold shock); medium constituents and concentrations (i.e., minerals, auxins, cytokinins, carbohydrates, amino acids, vitimins, and activated charcoal or other antioxidants); and incubation conditions (i.e., temperature and photoperiod length) on culture establishment. Nonetheless, results proved futile. Recent studies by Rose et al. (1986a,b) have demonstrated progress toward the establishment of haploid callus cultures using the sorghum cultivar

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FIG. 3. Somatic embryos derived from scutellar tissue of an immature (14-day-old) embryo of IS362OC. x20. (Courtesy of J. N. Reed.)

Advance. However, despite the establishment of approximately 1200 cultures, only four yielded albino shoots. Clearly much more work will be necessary prior to this application reaching fruition.

c.

REGENERATIONAND SOMATIC EMBRYOGENESIS

For the new gene-splicing and genetic manipulation technology to be applicable in a crop improvement program, it is necessary to have an efficient method of plant regeneration from single cells and/or callus (Fig. 4). Reports of plant regeneration from sorghum tissue cultures are limited,

FIG.4. Germinated somatic embryo of Regular Hegari. x 19. (Courtesy of J . N. Reed.)

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especially when the question of aberrant meristematic growth is considered (Cure and Mott, 1978; Mott and Cure, 1978). Masteller and Holden (1970) reported limited plant regeneration from callus derived from the basal node tissue of sorghum genotypes Norghum and North Dakota 104. Plant regeneration from callus derived from nodal tissue also was highlighted by Thomas et al. (1977). Likewise, Bhaskaran et al. (1983) have reported regeneration of IS3620C (a margaritiferum) from long-term (10-month-old) callus cultures originating from excised nodal tissue. A recent report from the USSR by El’konin et al. (1984) described plant regeneration from callus cultures that had been maintained for a period of 45 months. One of the first reports of plant regeneration of sweet sorghum, from the cultivars Keller and Wray, utilized nodal tissue as the explant source (Kresovich et al., 1986b). Mascarenhas et al. (1975b) reported plant and callus formation from stem segments containing the growth tip; however, no distinction was made as to the exact origin of regenerated tissue as well as the genetics of the explant source, i.e., inbred line or hybrid. Gamborg et al. (1977) observed morphogenesis and plant regeneration from callus of immature embryos (12-18 days postpollination) of the sorghum hybrid X4004A. Prolific shoot formation was reported by Thomas et al. (1977) from immature embryos (10-30 days postpollination) of the sorghum hybrid Oro. Brar et al. (1979a) reported embryogenesis from immature embryos (10-14 days old) of sorghum hybrids GPR-168, NK-300, and X4004A. It must be noted that these efforts employed “immature” embryos collected 10-30 days postpollination from self-pollinated sorghum hybrids as their explant sources. The description of 10- to 30-day-old embryos as immature is inconsistent with the findings of Artschwager and McGuire (1949) and Paulson (1969), which suggest that embryos of sorghum may reach maturity in as little as 12-25 days. Furthermore, a zygotic embryo derived from a selfed hybrid or a heterozygous line will not necessarily have the same genetic constituent as the parent, thereby adding uncontrolled and unwanted factors to the analysis of response. Recent investigations (Ma et al., 1987) with the Chinese inbred C401-1 have highlighted the responsiveness of 9- to 12-day-old embryos. We have found embryos excised 10-14 days after pollination to be a responsive explant source based on our studies of the genotypes (inbreds) RTx430, BTx399, CS3541, MN1500, Regular Hegari, and IS3620C. In our studies, overall embryogenesis (as calculated on a per culture basis) across genotypes ranged from 26 to 69%. The frequency of response for embryos from individual panicles within a genotype reached values as high as 95%. These results suggest that a high degree of phenotypic plasticity exists for in vitro responsiveness. Therefore, potential exists for improving response through intensive selection for this character. Furthermore, Ma et al. (1987) reported that the ability to differentiate plants from callus was heritable, controlled by two gene pairs, and acted as a dominant trait.

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Unlike the results of Ma et af. (1987), we found no correlation between embryo size and responsiveness. As noted in the work of Santos and Torne (1986) with maize, we have identified factors, i.e., greenhouse microclimatology and donor plant preconditioning, that potentially may confound the effects of age and/or size of embryo on explant response. Therefore, one must be sensitive to the role that explant-environment interactions play in affecting in vitro response. Ma et af. (1987) also recognized this point in the interpretation of their data. While documenting the in vitro responsiveness of the immature embryo, many investigators have failed to consider the potential breeding (and interpretational) problems associated with the use of sporophytic (2n) tissue from segregating populations of germplasm, i.e., F, hybrids and selections from early F,-F,, generations. Therefore, many of the interpretations of the response of the immature embryo in culture may have been confounded due to genetic recombination and segregation of the immature embryos. While Thomas et af. (1977) also reported plant regeneration from mature embryos excised from 2-year-old seed of the hybrid Oro, it is unclear as to whether the tissue was derived from a segregating or nonsegregating source. Building on the work of Thomas et al. (1977), Dunstan et al. (1978) examined the development of these embryo-like structures (Fig. 5 ) . Although some embryos were of single-cell origin, no evidence was found that they arose from proliferating callus cells. Instead, the shoots and embryo formed initially from the primary explant (the zygotic embryo). El’konin et al. (1986) recently documented somatic embryogenesis and plant regeneration from callus obtained from mature embryos of the genotypes Norghum 165 and Volzhskoe 2. MacKinnon et af. (1986a) reported somatic embryogenesis and plant regeneration utilizing callus derived from both immature and mature embryos of the sweet sorghum cultivars Keller, Rio, and Wray. Their results demonstrated no major difference in response with either grain or sweet sorghum cultivars; however, they concluded that pigmentation color of the callus differed between grain and sweet types. In our laboratory, we have found the pigmentation response to be correlated with the phenotypic character of secondary plant color (tan, red, or purple), rather than whether the sorghum was either a grain or sweet type. Brettell et af. (1980) described embryogenesis from cultured immature inflorescences of several sorghum cultivars and hybrids including CK-60, Regular Hegari, Plainsman, WAC692, G522DR, FS302, and G83F. All entries except Regular Hegari produced plants, although the hybrid G522DR was superior in culture response. Furthermore, observations of inflorescences from 10 to 50 mm in length indicated that those from 10 to 20 mm yielded the greatest frequency of embryogenic cultures. Studies in our laboratory with the genotypes (33541, MN1500, and Regular Hegari and inflorescence lengths ranging from 16 to 105 mm have suggested a correlation between

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FIG. 5. Longitudinal section of a somatic embryo derived from immature (22 days postpollination) inflorescence tissue of Regular Hegari. x LOO. (Courtesy of J. N. Reed.)

length and embryogenic response. However, the response was not consistent across genotypes. The embryogenic responses of CS3541 and Regular Hegari were negatively correlated with inflorescence length, while MN1500 demonstrated a positive correlation. Also, MN1500 yielded more callus per unit length of inflorescence than CS3541. As with other tissue sources, high variation in response between individual cultured inflorescences was observed. Again, these findings indicate that it may be possible to improve response through selection. Average values of establishment of embryogenic cultures across tested genotypes have ranged from 21 to 75%. Wernicke and Brettell (1980,1982) and Wernicke et al. (1982)reported somatic embryogenesis from immature leaves of G522DR. This group also

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noted the further spatially and temporally, i.e., more mature, leaf tissue was from the apical meristem, the less responsive it became. This finding suggests that competence in vitro, i.e., the ability to regenerate plants, is correlated with continued meristematic activity. These reports indicate that somatic embryogenesis from the primary explant (zygotic embryo, inflorescence, or immature leaf) is possible in a number of sorghum genotypes, although the technique is not yet routine. While these reports are encouraging, plant regeneration of sorghum has been generally of low frequency, limited to a few genotypes, and with great variability (as affected by genetic segregation and donor plant preconditioning) in culture response. Furthermore, as one attempts to integrate cell and tissue culture techniques with plant breeding, practical considerations, i.e., amount of technical support and availability of desired tissue types, may force a balance between ease of explant acquisition and explant responsiveness (Fig. 6). Based on these concerns, complementary studies have been established in our laboratory to identify biochemical differences (markers) discriminating embryogenic from nonembryogenic callus. Discernible biochemical differences ultimately may be utilized as rapid screening tools for the identification of responsive cell types. Preliminary results in our laboratory suggest differences exist in the esterase isozyme patterns between embryogenic and nonembryogenic calli of, RTx430 and Regular Hegari. Similar results have been documented in the maize genotype B73 (Everett et al., 1985). However, routine application of these techniques for identification and/or screening purposes will require further resolution. Nonetheless, isozymic analysis ultimately may prove useful in the study of the genetic regulation of plant development. D.

PROTOPLAST CULTURE

Relatively little research has been directed at sorghum protoplast isolation, culture, and regeneration. This is an important area because many genetic transfer techniques (electroporation, microinjection, etc.) rely on an available source of viable protoplasts (Chin and Scott, 1979). The earliest reported establishment of sorghum protoplast cultures was by Brar et al. (1979b). A cell suspension of the sorghum genotype GPR-168 was initiated from immature embryos, and protoplasts were isolated from 3- to 4day-old suspension cultures. Fusion was reported between sorghum protoplasts and leaf protoplasts of maize. Fusion products underwent several divisions and formed colonies of 8-10 cells. A subsequent study by Karunaratne and Scott (1981) isolated protoplasts from leaves of the genotypk Zulu. While reporting a higher mitotic index than the protoplast suspensions of Brar et al. (1979b), long-term viability of the cultures could not be maintained. Chourey and

FIG.6. Regenerated plants of Regular Hegari ready for transfer to greenhouse. x 1. (Courtesy of R. E. McGee.)

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Sharpe (1 985) described cell suspension establishment and protoplast isolation from callus cultures initiated with immature embryos of a sorghum hybrid and two cultivars, NK-300, M35-1, and SC420, respectively. Protoplasts underwent sustained cell division and callus formation. In some instances, protoplast-derived calli have been put back into suspension, giving rise to “protoclone cell suspension cultures. ” These results indicate that protoplast isolation and subsequent callus formation is possible in sorghum. No observations of plant regeneration from protoplast-derived cultures have been reported.

V.

POTENTIAL APPLICATIONS

Using plant cell and tissue culture techniques as model systems to study and affect plant behavior has been a primary goal of biotechnology. Future applications may include studying cellular and tissue-level mechanisms involved in stress physiology; rapid screening and selection techniques at the cellular and tissue levels; increasing, identifying, and controlling variation; and as adjuncts to genetic studies. A fundamental understanding and control of the in vitro system is necessary to undertake such studies. However, our basic understanding of sorghum cell and tissue culture techniques is limited; nonetheless, several applications have been attempted and potential does exist in these and other areas. A. CELL AND TISSUECULTURE! AS A MODELSYSTEM

Many processes which ultimately affect whole-plant characters occur at the cellular level. By studying these processes using cell and tissue culture techniques free from complicated whole-plant interactions, researchers can gain knowledge into the physiology and biochemistry of the cell. If subsequent correlations can be drawn between cellular and higher organizational level, e.g., whole-plant responses, then these characters may become useful as screening or selection markers. The work of Bhaskaran et al. (1985) exemplifies this type of approach in which proline accumulation in callus cultures of sorghum was measured in response to water stress. These studies have been continued (Newton et al., 1986) to document changes in soluble carbohydrates and organic acids as a function of water stress. Results indicated proline concentration to be an incidental consequence of stress; however, soluble carbohydrate concentrations may be correlated with osmotic adjustment. Research in our laboratory indicating a cellular component of salinity tolerance in sugar cane (Kresovich et al., 1986a; McGee et al., 1986) has en-

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couraged us to begin utilization of cell suspensions of sorghum as a model to elucidate and assess its modes of cellular adaptation to salinity stress. By exploiting cell and tissue cultures of sorghum in this manner, it is hoped that a better understanding of stress physiology and biochemistry will evolve. This model may prove useful in the study of mechanisms such as carbon partitioning and utilization, mineral uptake and compartmentation, and membrane stability. When integrated with an improved understanding of the interactions between tissue types, media, preconditioning effects, and plant regeneration, this approach offers the potential advantage of being better able to isolate and propagate variants with these particular characters. Although there are several well-established model systems (tobacco, carrot, tomato, etc.), it is often difficult to extrapolate results across species. Therefore, the use of sorghum cell and tissue culture techniques to verify and study these phenomena is important.

B.

SCREENING AND SELECTION TECHNIQUES

The phenomenon of callus cultures paralleling whole plant response to salts (Orton, 1980) and heavy metals (Meredith, 1978) in growth media has been observed in several species. Relatively little work has been done with sorghum regarding this application. Smith et af. (1983) tested callus from four sorghum genotypes (BTx3197, BTx623, BTx399, and IS3620C) on media containing 0, 100, 200, and 400 p k f aluminum-EDTA. Differences were reported in growth among the cultivars in response to aluminum. Plants were regenerated from IS3620C after several passages on selection media, but no field test results are yet available. Bhaskaran et af. (1983) reported plant regeneration from IS3620Cderived callus grown on media containing high concentrations (0.1-0.5% w/v) of sodium chloride. Callus obtained by prolonged subculture on this selection medium was better able to grow on a medium containing sodium chloride than donor cells. Regenerated plants grown to maturity exhibited low seed set, initially hindering attempts at progeny analysis. A subsequent study (Bhaskaran et af., 1986) noted differences between selected and nonselected progeny with respect to enhanced tolerance (reported as vegetative growth only) to sodium chloride. However, the control population of the donor plant demonstrated a greater relative tolerance than either tissue culture-selected population; therefore, this finding suggests that the selection methods employed may be of little practical value for sorghum improvement. MacKinnon et af. (1986b) explored four selection methods for the derivation of sodium chloride-tolerant plants of the cultivars Regular Hegari, Keller, and Rio. With regard to frequency of regeneration, many more plants were obtained from selection on solid rather than liquid

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medium. However, results from field tests will be necessary to confirm the frequency of true tolerant or resistant plants versus that of potential escapes. From a crop improvement perspective, one needs to be concerned more with the frequency of obtaining desired variants from a selection technique rather than the total number of plants derived. That is, if field evaluation suggets a great proportion of the regenerated population to be escapes, the in vitro selection technique will be of limited value to the plant breeder. Smith et af. (1985) tested callus from 10 sorghum genotypes on polyethylene glycol-amended screening media (0-20070 w/v). Indications were that preanthesis drought tolerance and performance of cultured tissues might be related. Subsequent studies suggest that extreme variability in callus growth makes discerning differences difficult (Panella, 1986; Reilley, 1986). To our knowledge, no work has been directed at the screening and selection of herbicide-resistant genotypes of sorghum via cell or tissue culture. However, based on promising work with other cereals, e.g., maize (Shaner and Anderson, 1985), this potential screening and selection approach warrants study. The ontogeny of tissue cultures becomes critical when using screening and selection techniques. Dunstan et af. (1978, 1979) and Wernicke et af. (1982), based on their conclusions that regenerated plants arose initially via somatic embryogenesis, albeit from the primary explant only, suggest that variation may be more directed when selection pressure can be applied to single cells. This hypothesis again points to the need for cultures containing synchronized, undifferentiated cells that can be readily induced to differentiate via somatic embryogenesis into new plants. Because relatively little work has been done with sorghum screening and selection for salt, heavy metal, and drought tolerance, it is difficult to predict whether these potential applications of cell and tissue culture techniques can be implemented into traditional breeding programs. In order for these new techniques to be useful, a strong correlation between in vitro and field responses must be established. Furthermore, in vitro screening and selection must become simpler, more reliable, and more cost-effective to be useful in a breeding program. C. SOMACLONAL VARIATION

Somaclonal variation, i.e., the variability expressed in plant phenotype derived from cell and tissue culture, does occur in many, if not all, species. This variation, depending upon how it is expressed, is similar to that caused by a chemical mutagen and is generally detrimental. Somaclonal variation, i.e., variation in leaf morphology and growth habit, was first reported in

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sorghum by Gamborg et af. (1977). Complementary genetic studies suggested that this encountered variation could not be explained on the basis of altered chromosome number. While attempting to link these phenotypic changes with variation in chromosome number and morphology, the investigators overlooked the potential confounding effects of their use of segregating, immature embryos of the sorghum hybrid X4004A. Thus, in this case, normally encountered variation exhibited in the segregating, F2 population may have been misinterpreted as somaclonal variation. Smith et af. (1982) studied variability among regenerated phenotypes derived from IS3620C. Selfed seeds from sorghum plants regenerated from tissue culture were collected, and the progeny of tissue culture-derived plants were compared to the original sorghum line in terms of drought tolerance characters. In two characters, wax load and heat tolerance, the progeny of tissue culture-derived plants were significantly less desirable than the parent population. Desiccation damage, however, was less for the tissue culture-derived progeny. In a related study, Vaughn (1983) examined the F,, F2, and backcross F, progeny of a tissue culture-derived line of IS3620C. No significant differences in chlorophyll content or chromosome number, pairing, homology, or configuration were observed between the tissue culture-derived line and the parent. Observed differences in seed weight could not be separated from inherent genetic variation and inbreeding effects. A recent study conducted by Ma et af. (1987) has documented the presence of heritable variation in plants derived from callus cultures initiated with immature embryos of the Chinese genotype C401-1. Among the phenotypic characteristics observed included height changes, the presence of a waxy midrib, and male and female sterility. A variation in tallness was attributable to one dominant mutant gene. Short stature and male sterility variants appeared to be consequences of recessive mutant genes controlling those traits. In order to determine whether cell and tissue culture techniques are viable tools for creating useful variability in sorghum, it will be necessary to use established, agronomically important lines and evaluate them in terms of agronomic characteristics. One must discriminate between biological (somaclonal variability) and agronomically useful variability. It is important to point out that useful variability may be identified and incorporated more easily in traditional sorghum breeding programs.

D. ADJUNCT APPLICATIONS Recognizing the present status of applications of cell and tissue culture techniques to sorghum improvement, potential still exists for progress in

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several areas. Shoot tip or meristem culture offers the potential of isolating and maintaining desirable, disease-free germplasm. As an adjunct, cultures may be maintained at some initial level of development for either long-term storage or to facilitate international and intranational movement of diseasefree germplasm. Although attempts at using sorghum in wide crosses have been successful in closely related genera such as Saccharurn as well as some allied sorghum species, other efforts have not been as successful. For example, attempts at pollination of maize by sorghum have not produced viable embryos (Heslop-Harrison et al., 1985). Somatic hybridization via protoplast fusion (Brar et al., 1979b) may provide a means of overcoming such prezygotic obstacles, although a regeneration system is not yet in place for such products. Further, postzygotic problems, e.g., embryo-endosperm incompatibilities, may be overcome using embryo rescue techniques. Cell and tissue culture techniques potentially offer adjunct methods useful for genetic studies including the induction, identification, and isolation of monosomics and other valuable genetic markers. Additionally, anther culture coupled with regeneration techniques may result in the production of haploids and subsequent dihaploids. This development could prove useful in genetic studies, and also in the rapid development of homozygous germplasm. With a better understanding of protoplast culture systems, novel concepts such as gene transfer and somatic hybridization may facilitate activities and strengthen the capabilities of sorghum breeders by increasing desirable genetic variability. Competition with current sorghum improvement techniques, i.e., those presently utilized in traditional breeding programs, however, may be hard to supplant. This statement is made in view of the fact that, until the recent work of Chourey and Sharp (1985), long-term maintenance of callus and suspension cultures of sorghum has been suspect. Furthermore, plant regeneration has been limited to a restricted number of genotypes, one of which (IS362OC) has no value in current sorghum improvement activities.

VI.

SUMMARY

During the past two decades, an integration of plant biochemistry and physiology has allowed the development of basic cell and tissue culture techniques, which have the potential of improving our understanding of plant biology. We have attempted to highlight the state of the discipline with regard to the use of these techniques for the genetic improvement of sorghum. Much of what has been accomplished utilizing cell and tissue culture techniques for the genetic improvement of sorghum has been based in empiricism and much of what has been reported may have been misinterpreted

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or misunderstood. Our fundamental understanding of the biology of the sorghum plant is limited, both at the whole-plant and at the cellular level. A critical objective still to be addressed among researchers focusing on applications of cell and tissue culture techniques to the improvement of sorghum involves a demonstration and integration of the following developmental stages: Plant

-. Cell

“Improved” Cell

-

“Improved” Plant

At the present time, only a handful of groups worldwide are actively attempting to integrate cell and tissue culture techniques with the conventional breeding of sorghum. With this limited amount of effort, progress will be slow. Ultimately, the tools that a plant breeder uses are based on (1) the familiarity of the breeder with the tool; (2) the ability of the tool to affect the desired change or recombination of desirable characters; (3) the rate or frequency at which the desired change or recombination is obtained; (4) the cost associated with the use of the tool, in terms of money, personnel, and time; and ( 5 ) the resources available to the breeder. Applications of cell and tissue culture techniques will become common when they are advantageous for the plant breeder to employ. Therefore, the revolution involving plant biotechcology and the genetic improvement of sorghum may be long in coming. However, the rate of “applied evolution” for the genetic improvement of sorghum may increase as a result of the application of cell and tissue culture techniques. In the short term, cell and tissue culture techniques open avenues for an improved understanding of the cellular biology of sorghum. These techniques will prove useful for the study of genetics, developmental biology, and stress physiology. Plant breeders and plant biotechnologists must work cooperatively to find niches for deployment of these tools. This synergism should lead to problem-oriented basic research from which tangible results are envisioned. The potential application of these tools and their subsequent payoffs may be great in the longer term; however, specific targets, defined in the context of current breeding efforts, must be established and efforts directed toward them.

ACKNOWLEDGMENTS The authors thank Janet N. Reed and Robert E. Reed for their research activities. The authors also gratefully acknowledge support of our research from the U.S. Department of Energy through Contact Number 19X-27452C with Martin Marietta Energy Systems, Inc. and from the Gas Research Institute through Contract Number 5083-223-0766. Funds supplied through the Texas Agricultural Experiment Station Expanded Research Program were integral to the establishment of the Plant Cell Culture Laboratory at Weslaco.

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Allard, R. W. 1960. “Principles of Plant Breeding,” 1st Ed. Wiley, New York. Artschwager, E. 1948. U S . Dept. Agric. Tech. Bull. 957. Artschwager, E., and McGuire, R. C. 1949. J. Agric. Res. 78, 659-673. Bhaskaran, S., Smith, R. H., and Schertz, K. F. 1983. 2. Pfanzenphysiol. 112, 4 5 9 4 3 . Bhaskaran, S., Smith, R. H., and Newton, R. J. 1985. Plant Physiol. 79, 266-269. Bhaskaran, S., Smith, R. H., and Schertz, K. F. 1986. J. Plunt Physiol. 122, 205-210. Boyes, C. J., and Vasil, I. K. 1984. Plant Sci. Lett. 35, 153-157. Brar, D. S., Rambold, S., Gamborg, 0. L., and Constabel, F. 1979a. Z. Pfanzenphysiol. 95, 377-388. Brar, D. S., Rambold, S., Gamborg, 0. L., and Constabel, F. 1979b. 2.Pflanzenphysiol. 96, 269-275. Brettell, R. I. S.,Wernicke, W., and Thomas, E. 1980. Protoplasma 104, 141-148. Briggs, F. N., and Knowles, P. F. 1967. “Introduction to Plant Breeding.” 1st Ed. Reinhold, New York. Bruggers, R. L., and Jaeger, M. M. 1982. In “Sorghum in the Eighties: Proceedings of the International Symposium on Sorghum” (J. V. Mertin, ed.), pp. 303-312. ICRISAT, Patancheru, A. P., India. Butler, L. G. 1982. In Proceedings of the International Symposium on Sorghum Grain Quality” (J. V. Mertin, ed.), pp. 294-31 1. ICRISAT, Patancheru, A. P., India. Caulkins, C. D. 1978. M.S. thesis, Texas A&M University, College Station. Chin, J. C., and Scott, K. J. 1979. Ann. Bot. 43, 23. Chourey, P. S., and Sharpe, D. S . 1985. Plant Sci. 39, 171-175. Cure, W. W., and Mott, R. L. 1978. Physiol Plant. 42, 91-%. Davis, M. E., and Kidd, G. H. 1980. Z. Pfanzenphysiol. 98, 79-82. de Wet, J. M. J., Harlan, J. R., and Price, E. G. 1970. Am. J. Bot. 57, 704-707. Doggett, H. 1970. “Sorghum,” 1st. Ed. Longmans, London. Doherty, C. A., Waniska, R. D., Rooney, L. W., Earp, C. F., and Poe, J. H. 1987. Cereal Chem. 64,42-46. Dunstan, D. I., Short, K. C., and Thomas, E. 1978. Protoplasma 97, 251-260. Dunstan, D. I., Short, K. C., Dhaliwal, H., and Thomas, E. 1979. Protoplasma 101, 355-361. EI’Konin, L. A., Tymov, V. S., and Sukhanov, V. M. 1984. Dokl. Vses. Akad. Sel’skokhoz. Nauk. 4, 7. EI’Konin, L. A., Tymov, V. S., Papazyan, N. D., and Ishin, A. G. 1986. Sov. Plant Physiol. 33, 388-395. Everett, N. P., Wach, M. J., and Ashworth, D. J. 1985. Plant Sci. 41, 133-140. Frederiksen, R. A., and Rosenow, D. T. 1979. I n “Biology and Breeding for Resistance to Arthropods and Pathogens in Agricultural Plants” (M. K. Harris, ed.), pp. 137-167. Texas A&M University, University of California, and U.S. Agency for International Development, College Station. Gamborg, 0. L., Shyluk, J. P., Brar, D. S., and Constabel, F. 1977. Plant Sci. Lett. 10,67-74. Hahn, D. H., Rooney, L. W., and Earp, C. F. 1984. Cereal Foods World 29, 777-779. Harlan, J. R. 1972. in “Sorghum in the Seventies” (N. G. P. Rao and L. R. House, eds.), pp. 1-13. Oxford and IBH, New Delhi. Harlan, J. R., and de Wet, J. M. J. 1972. Crop Sci. 12, 172-176. Heinz, D. H., Krishnamurthi, M., Nickell, L. G., and Maretzki, A. 1977. In “Applied and Fundamental Aspects of Plant Cell, Tissue, and Organ Culture” (J. Reinert and Y. P. S. Bajaj, eds.), pp. 3-17. Springer-Verlag. Berlin. Heslop-Harrison, Y., Reger, B. J., and Heslop-Harrison, J. 1985. Theor. Appl. Genet. 70, 252-258.

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House, L. 1985. “A Guide to Sorghum Breeding,” 2nd Ed. ICRISAT, Patancheru, A. P., India. Jinchow Institute of Agriculture. 1978. Acta Genet. Sin. 5, 337-338. Johnson, J. W., and Teetes, G. L. 1979. In “Biology and Breeding For Resistance to Arthropods and Pathogens in Agricultural Plants” (M. K. Harris, ed.), pp. 168-180. Texas A&M University, University of California, and U.S. Agency for International Development, College Station. Johnson, J. W., Rosenow, D. T., Miller, F. R., and Schertz, K. F. 1971. Tex. Agric. Exp. Sta. Prog. Rep. 2942. Kannangara, T., Simpson, G. M., and Armstrong, L. 1977. Sorghum Newslett. 20, 6. Karunaratne, S. M., and Scott, K. J. 1981. Plant Sci. Lett. 23, 11-16. King, P. J., Potrykus, I., and Thomas, E. 1978. Physiol. Veg. 16, 381-399. Kresovich, S., Wadsworth, S. J., Molina, J. J., and McGee, R. E. 1986a. Proc. Int. SOC.Sugar Cane Tech. 19, 245-252. Kresovich, S., McGee, R. E., and Wadsworth, S. J. 1986b. Sorghum Newslett. 29, 94-96. Liang, 0 . H. 1978. Sorghum Newslett. 21, 101-102. Ma, H., Gu, M., and Liang, G. H. 1987. Theor. Appl. Genet. 73, 389-394. McGee, R. E., Wadsworth, S. J., and Kresovich, S. 1986. Proc. Inter-Am. Sugar Cane SOC.6, 232-236. MacKinnon, C., Gunderson, G., and Nabors, M. W. 1986a. Plant Cell Rep. 5, 349-351. MacKinnon, C. Gunderson, G., Petersen, K. M., and Nabors, M. W. 1986b. Life Sci. Adv. Ser. B. Mann, J. A., Kimber, C. T., and Miller, F. R. 1983. Tex. Agric. Exp. Sta. Bull. 1454. Mascarenhas, A. F., Hendre, R. R., Seetharama Rao, B., and Jagannathan, V. 1969. Indian J. Exp. Biol. 1, 65-67. Mascarenhas, A. F., Pathak, M., Hendre, R. R., and Jagannathan, V. 1975a. Indian J. Exp. Biol. 13, 103-107. Marcarenhas, A. F., Pathak, M. Hendre, R. R., Ghugale, D. D., and Jagannathan, V. 1975b. Indian J. Exp. Biol. 13, 116-119. Masteller, V. J., and Holden, D. J. 1970. Plant Physiol. 45, 362-364. Mengesha, M. H., and Prasada Rao, K. E. 1982. In “Sorghum in the Eighties: Proceedings of the International Symposium on Sorghum” (J. V. Mertin, ed.), pp. 323-334. ICRISAT, Patancheru, A. P., India. Meredith, C. P. 1978. Plant Sci. Lett. 12, 25-34. Miller, F. R. 1979. In “Biology and Breeding for Resistance to Arthropods and Pathogens in Agricultural Plants” (M. K. Harris, ed.), pp. 128-136. Texas A&M University, University of California, and U.S Agency for International Development, College Station. Miller, F. R., and Kebede, Y. 1984. In “Genetic Contributions to Yield Gains of Five Major Crop Plants” (W. R. Fehr, ed.), pp. 1-12. Crop Sci. SOC.Am. Special Publ. No. 7, Madison, Wisconsin. Mott, R. L., and Cure, W. W. 1978. Physiol. Plant. 42, 139-145. Newton, R. J., Bhaskaran, S., Puryear, J. D., and Smith, R. H. 1986. Plant Physiol. 81, 626-629. Oberthur, E. E., Nicholson, R. L., and Butler, L. G. 1983. J. Agric. Food Chem. 31, 660-662. Orton, T. J. 1980. Z. Pflanzenphysiol. 98, 105-1 18. Panella, L. W. 1986. M.S. thesis, Texas A&M University, College Station. Panella, L. W., Reilley, A. A., and Smith, R. H. 1987. In Vitro 23, 70A. Paulson, 1. W. 1969. Crop Sci. 9, 97-102. Quinby, J. R. 1974. “Sorghum Improvement and the Genetics of Growth,” 1st Ed. Texas A&M Univ. Press, College Station. Rangaswami Ayyangar, G. N., and Panduranga Rao, V. 1936. Indian J. Agric. Sci. 6, 1299-1 322. Reilley, A. A. 1986. M.S. thesis, Texas A&M University, College Station.

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Rogers, M. S., Gal, H. L., and Homer, H. T., Jr. 1974. In Vitro 9, 463-467. Rooney, L. W., and Miller, F. R. 1982. In “Proceedings of the International Symposium on Sorghum Grain Quality” (J. V. Mertin, ed.), pp. 143-162. ICRISAT, Patancheru, A. P., India. Rose, J. B., Dunwell, J. M., and Sunderland, N. 1986a. Plant Cell Tissue Organ Cult. 6, 15-22. Rose, J. B., Dunwell, J. M., and Sunderland, N. 1986b. Plant Cell Tissue Organ Cult. 6,23-31. Santos, M. A., and Torne, J. M. 1986. J. Plant Physiol. 123, 299-305. Shaner, D. L., and Anderson, P. C. 1985. In “Biotechnology in Plant Science: Relevance to Agriculture in the Eighties” (M. Zaitlin, P. Day, and A. Hollaender, eds.), pp. 287-299. Academic Press, New York. Simmonds, N. W. 1979. “Principles of Crop Improvement,” 1st Ed. Longmans, London. Simmonds, N. W. 1983. In “Genetic Engineering of Plants: An Agricultural Perspective” (T. Kosuge, C. P. Meredith, and A. Hollaender, eds.), pp. 5-25. Plenum, New York. Smith, R. H., and Bhaskaran, S. 1986. In “Biotechnology in Agriculture and Forestry. Vol. 2: Crops” (Y. P. S. Bajaj, ed.), pp. 220-233. Springer-Verlag, Berlin. Smith, R. H., and Bhaskaran, S. 1987. In Vitro 23, 67A. Smith, R. H., Bhaskaran, S., and Schertz, K. F. 1982. In “Plant Tissue Culture 1982” (A. Fujiwara, ed.), pp. 489-490. Japan. Assoc. for Plant Tissue Culture, Tokyo. Smith, R. H., Bhaskaran, S., and Schertz, K. F. 1983. Plant CellRep. 2, 129-132. Smith, R. H., Bhaskaran, S., and Miller, F. R. 1985. In Vitro Cell. Dev.Biol. 21, 541-545. Stephens, J. C., Miller, F. R., and Rosenow, D. T. 1967. Crop Sci. 7 , 3%. Strogonov, B. P., Komizerko, E. I., and Butenko, R. G. 1%8. Sov. Plant Physiol. 15, 173- 177., Thomas, E., King, P. J., and Potrykus, I. 1977. Naturwissenschaften 64, 587. Tomes, D. T. 1985.2ln “Cereal Tissue and Cell Culture” (S. W. J. Bright, and M. G. K. Jones, eds.), pp. 175-203. Martinus NijhoffIDr. W. Junk Publ., The Hague. Vanderlip, R. L., and Reeves, H. L. 1972. Agron. J. 64,13-16. Vaughn, S. F. 1983. M.S. thesis. Texas A&M University, College Station. Wall, J. S., and Ross, W. M. 1970. “Sorghum Production and Utilization,” 1st Ed. Avi, Westport, Connecticut. Wernicke, W., and Brettell, R. 1. S. 1980. Nature (London) 287, 138-139. Wernicke, W., and Brettell, R. I. S. 1982. Protoplasma 111, 19-27. Wernicke, W., Potrykus, I., and Thomas, E. 1982. Protoplasma 111, 53-62.

ADVANCES IN AGRONOMY. VOL. 41

UPTAKE OF MINERAL NUTRIENTS AND CROP GROWTH: THE USE OF FLOWING NUTRIENT SOLUTIONS A. Wild,’ L. H. P. Jones,’ and J. H. Macduff2 ‘ Department of Soil Science, The University of Reading Reading RG1 5AQ, England Department of Plant Nutrition and Growth, Agricultural and Food Research Council (AFRC) lnstltute for Grassland and Anlmal Production Hurley, Maidenhead SL6 5LR, England

I.

INTRODUCTION

Since the first experimentswith fertilizerswere conducted in the middleof the 19thcentury it has been known that some crops are more sensitivethan others to nutrient stress. For example, in early work at Rothamsted Experimental Station, root crops were found to respond more to the application of phosphate fertilizer than cereal crops growing on the same soil (Lawes, 1847). It has also been known for several years that this difference in sensitivity to nutrient stress extends to differences between genotypes of the same crop (Clark, 1983). As the techniques of gene transfer develop and gene expression becomes better understood, the range of sensitivity is likely to increase. It will then become a priority to breed and select crop genotypes which are less sensitive to nutrient stress and which use nutrients more efficiently. If this is to be achieved, these conditions that limit nutrient uptake need to be understood, as do the relations between nutrient uptake, efficiency of utilization of the nutrient within the plant, and plant growth and development. For plants growing in soil, nutrients reach the root surface by convection (mass flow) or diffusion or both (Nye and Tinker, 1977). Nutrients in the soil solution are replenished by desorption from soil surfaces, mineralization of soil organic matter, or by addition of fertilizers. Nutrient uptake from soil is also influenced by conditions in the rhizosphere, by mycorrhizal symbiosis, and, under field conditions, by soil temperature and water supply to the roots. In studies with plants grown in soil it is not possible to keep all these conditions under precise control. Thus although the ultimate objective is to optimize nutrient uptake by soil-grown plants, the process of uptake, and hence the breeding and development of crop genotypes, can only be understood from experiments under more controlled conditions. 171 Copyright 0 1987 by Academic Press, Inc. All rights of reproduction in any form reserved.

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Solution culture has long provided a means of growing plants in a soil-free medium. Flowing nutrient solution is a developmentof this technique. Depending on the sophistication of the culture system, it can provide nutrients to the roots at controlledpH and temperature and at concentrations which can be kept constant or varied. It was first used to answer the question, “What concentrations of nutrients are required external to the root in order to provide the plant with a sufficient supply for its growth and development?” The strength of the technique of flowing solution culture is that it provides precise control of nutrient concentrations at, or very close to, the root surface. When the system is automated the concentrationscan be kept constant and also very low; monitoring provides a continuous record of nutrient addition and therefore of nutrient uptake by roots of whole plants from known nutrient concentration. It provides information on the relationship between external nutrient concentration and growth rates of roots and shoots at concentrations resembling those in soil solutions, under conditions such that nutrient uptake is not limited by rate of transport to the root surface. As the plant roots require little cleaning prior to chemical analysis the distribution of chemical elements between roots and shoots can be assessed unambiguously. This article reviews the relationships between conditions in the external solution that affect plant growth and nutrient uptake under controlled conditions. Because nutrient uptake is required for growth, and the greater the growth the greater the requirement for nutrients, the rates of the two processes are interdependent, that is, there is positive feedback. For clarity the two processes will, where possible, be reviewed separately. Unless stated otherwise experimental results to which reference is made have been obtained using flowing nutrient solution systems. These provide the basis for understanding mineral nutrition of plants under conditions resembling those in soil, but without the complications that arise when the chemistry, physics, and biology of the rhizosphere have to be considered. The results are nevertheless relevant to plants grown in soil because plant growth and development have been found to be similar in the two systems. Many aspects of the uses and limitations of solution culture techniques have been reviewed by Asher and Edwards (1983).

II.

SYSTEMS EMPLOYING FLOWING NUTRIENT SOLUTIONS A. GENERAL

The systems that have been used follow one of two principles: 1. Nonrecirculating nutrient solutions flow past the plant roots and then to waste. In such systems the supply of nutrients is sometimes controlled by

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173

adjusting the flow rate of the solution through the roots, and uptake is measured by analysis of the waste solution and/or plant analysis. They have been used for experiments on nutrient uptake by wheat (Triticurn aestivurn) seedlings (Reisenauer, 1969; Cox and Reisenauer, 1973), by barley (Hordeurn vulgare) seedlings (Bloom and Chapin, 1981), and by different parts of a root (Harrison-Murray and Clarkson, 1973; Russell and Clarkson, 1976). Low-cost, nonrecirculating systems have been used in longer-term experiments using tap water and technical-grade salts (Elliott and Nelson, 1983) or commercial fertilizers (Ganmore-Neumann and Kafkafi, 1980). 2. The nutrient solutions recirculate through the vessels containing plants and concentrations are maintained by monitoring and nutrient additions. In the simplest form of these systems, the nutrients are replenished after manual sampling and analysis of the solution once or twice daily (Asher et al., 1965; Asher and Edwards, 1978; Bhat, 1980; Osmond et al., 1981; Moorby and Nye, 1983; Freijsen and Otten, 1984; Hatch and Canaway, 1984). The pH and temperature of the solutions are sometimes controlled automatically (e.g., Asher et al., 1965; Asher and Edwards, 1978;Tolley-Henry and Raper, 1986). Other systems have also incorporated the means for continuous automatic monitoring and control of some nutrient concentrations (Clement et al., 1974; Deane-Drummond, 1982; Barneix et al., 1984). In the system of mist culture developed by Ingestad and Lund (1979), nutrients are replenished on the basis of manual or automatic measurement of electrical conductivity, or at a rate of addition proportional to plant growth rate. In systems based on both nonrecirculating and recirculating solutions, it is of critical importance to maintain an adequate flow rate in order to minimize nutrient depletion as the solution flows through the roots. Edwards and Asher (1974) showed that the relation between flow rate, F, in liters per vessel per minute and the percentage decrease in concentration of the nutrient between the inlet and outlet, D, is given by

where R is root fresh weight (grams per vessel), u is the unit rate of nutrient uptake (moles per gram of root fresh weight per minute), and, C is inlet concentration, moles per liter. Most recent systems based on recirculating solutions have incorporated constant, high flow rates to ensure that there is adequate nutrient supply at the highest expected growth rates. €3. THEHURLEY SYSTEM

The system of Clement et a1.(1974), hereafter referred to as the “Hurley system,” incorporates the main features of the system of Asher et al. (1965)

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but makes the advance of incorporating automatic monitoring of nutrient concentrations, addition of nutrients to maintain prescribed concentrations in solution, and the recording of these additions. The frequency of monitoring and replenishment may vary between 2 and 20 min. The system comprises eight plant culture units in which the temperature of each solution can be separately controlled in the range 2.5-25°C to within +O.l"C. The culture units are housed in an air-conditioned greenhouse with temperature controlled in the range 15-25°C. For experiments in winter artificial lights provide the plants with total radiation of 16 MJ/m2/day, one-half of which is photosynthetically useful, i.e., within wavelengths 400-700 nm. The monitoring layout is shown in Fig. 1. In the Hurley system each plant culture unit contains 300 liters of nutrient solution which is circulated at a rate of 1.2 liters per min through each of 24 culture vessels of 1.2-liter capacity arranged in parallel. The arrangement of the culture vessels is such that the spatial distribution of the plants simulates

GLASSHOUSE

wall

MONITORING

AN0

CONTROL

LABORATORV

FIG. 1. The Hurley system for studying the uptake of ions by plants from flowing solutions of controlled temperature and composition. Ion-selective electrodes, an NH, probe, and a flame photometer are linked through autotitrator controllers to stock solutions to maintain predetermined concentrations of nutrients and pH; the colorimeter is linked through a computer control to pumps delivering phosphate continuously (P,) and intermittently (Pi). SV, Sampling solenoid valve for delivering aliquots of solution to analysers; Trc, pumps for delivering micro- and other nutrients; W, waste discharge; 4, electrical connection.

UPTAKE OF MINERAL NUTRIENTS AND CROP GROWTH

175

a crop canopy in the field, or a grass sward, thus enabling nutrient uptake and dry matter yield to be calculated on an area basis if this is desired. The four analytical systems shown in Fig. 1 are ion-selective electrodes for H' and NO; (Clement et al., 1974), an ammonia probe for NH: (Hatch et al., 1986), flame photometry for Na' and K' (Woodhouse et al., 1978 describe its use for K'), and colorimetry for phosphate (Breeze et al. 1982). The paper of Clement et al. (1974) described the control of pH by the use of a glass/calomel electrode and autotitrator. Alternatively, microcomputer control can be used (Hatch and Canaway, 1984). The composition of the complete nutrient solution which is generally used is given in Table I (Clement et al., 1978a; Breeze et al., 1984), although the composition is varied according to the purposes of the experiment. For example, NO; was held constant in the range 1.43 pA4 to 143 mM for a study of NO; uptake by perennial ryegrass (Lolium perenne) (Clement et al., 1978a), phosphate was held constant in the range 0.04-32 p M in studying phosphate uptake by the same species (Breeze e t a / . , 1984), and K' was held constant at 1.3-102 pA4 in a study of K' uptake by perennial ryegrass, barley, and fodder radish (Raphanus sativus cv. Slobolt) (Woodhouse et al., 1978). The concentration of K' that was able to support the potential growth rate of four other plant species in flowing solution culture was about 100 times less than required in sand culture (Wild et a/., 1974); the concentration ratio for all nutrients is probably about the same. High ratios of antagonistic ions are avoided; for example, NH: is not included in solutions used to study K' uptake, as in the recent experiments of Pettersson (1986), unless the extent of antagonism is itself under investigation. The co-ions used in the Hurley system are Ca2+in studies of the concentration of anions, e.g., NO;, and S042- for cations, e.g., K',all solutions containing a base concentration of CaS04. The pH is held constant, usually in the range of 4.0-7.0 f 0.1, by the automatic addition of H2S0, or Ca(OH)2 as appropriate. The circulating solution is not sterile but counts have shown bacterial numbers in solution to be very low. The Hurley system has been used to investigate the conditions that determine nutrient uptake rates and the effects of nutrient solution composition on the growth and composition of plants (see Section 111). Using the system high growth rates have been achieved. For example, under optimum conditions of light, temperature, and nutrient supply, relative growth rates (RGR) of 0.31, 0.40, and 0.20/day were achieved with seedlings up to 6 days old of fodder radish, barley, and perennial ryegrass, respectively, with rates of 0.26,O. 17, and 0.19/day, respectively, when the plants were 20 days old (Woodhouse et al., 1978). With older ryegrass plants, dry matter increases of 20 g/m2/day have been obtained (Clement et al., 1978a), which is close to the high rate of 20-25 g/m2/day observed in the field for this species (Alberda and Sibma, 1968).

Table 1 Typical Composition of Flowing Nutrient Solutions as Used with the Hurley System' Nutrient

K Ca Mg

Concentration (pM)

Micronutrients

Concentration (pM)

13-26 230 100

Fe B Mn cu Zn Mo

1.07 4.63 0.91 0.03 0.08 0.05

c1

1.69

"X)

35

S(s0: -) P(H*Po;)

320 50

'Trom Clement et ul. (1978a) and Breeze et ul. (1984). The compositionwill be changed from that in the table according to the purpose of the investigation;the concentrations shown are nonlimiting for growth. See Asher and Edwards (1983) for concentrations used in other systems.

UPTAKE OF MINERAL NUTRIENTS AND CROP GROWTH

Ill. A.

177

NUTRIENT UPTAKE

INTERRELATIONSHIPS WITH PLANT GROWTH

It is now generally accepted that nutrient uptake and plant growth are interdependent (Moorby and Besford, 1983). The relation between these processes can be particularly well studied in flowing culture systems where continuous monitoring of nutrient uptake can be performed in combination with sequential harvesting of plants. A clear demonstration of the rapidity with which cessation of growth affects nutrient uptake is illustrated in Fig. 2 for a sward of perennial ryegrass growing in flowing nutrient solution (Clement et al., 1978b). Within a few minutes of the grass being cut, the uptake of NO; began to decrease and after 2 hr it had fallen to 35% of the rate at the time of cutting. After cutting it was also observed that the CO, flux quickly fell to negative values as respiration exceeded assimilation. The fall in NO; uptake was more gradual than the fall in CO, flux, and the recovery of NO; uptake was also slower than the recovery of COz assimilation. In the same work a close relationship was shown to exist between NO; uptake and CO, flux. The assimilation of CO, was greatest at about 1200 GMT with the maximum rate of NO; uptake commonly occurring 5 or 6 hr later (Fig. 3). This demonstration of the close relation betwen CO, assimilation, level of radiation, and NO; uptake suggests that CO, assimilation, and hence dry matter increase, may stimulate the increase of NO; uptake. It is unclear whether the stimulation is a function of the growth increment, the plant reacting by taking up more

Time (GMT)

FIG. 2. Effect of defoliation on rate of NO; uptake by simulated sward of perennial ryegrass growing under artificial light with a 9-hr photoperiod. Plants were grown in flowing nutrient solution with NO; at 7 p h f and cut at 3 hr after the start of the photoperiod at 5 cm above the base of the shoots. Data from Clement el a/. (1978b).

178

A. WILD E T A L . . U

9.E 20 t

.

- "

I

OL 6r

5

.'..'..

..

0..

0.

..-

FIG. 3. Rates of NO; uptake and of CO, flux by simulated swards of perennial ryegrass during periods of 72 hr in June (experiment 1 ) and in July (experiment 2). Plants were grown in flowing nutrient solutions with NO; at 7 p M , and measurements were made on plants growing in nine adjoining vessels of a plant culture unit. A , 1200 GMT. Data from Clement ef a/. (1 978b).

NO;, or is more a consequence of an increased supply of energy sources in the root which allows increased active uptake. The diurnal fluctuation of NH: and K' uptake and H efflux has also been investigated with perennial ryegrass (Hatch et d.,1986). The lag periods for the peak uptake rates of NH: and K' were about the same as for NO;. The best fit with measured radiation was for a lag period of 5 hr with NH:, 6 hr with K', and 7 hr for H efflux (Fig. 4). The similarity in the lag periods for the uptake of NO;, NH:, and K', and for H' efflux suggests a common mechanism controlling the uptake of the three nutrient ions. The control might be through the supply of a photosynthetic metabolite that is required for the active transport of the ions into roots or into the xylem.

179

UPTAKE OF MINERAL NUTRIENTS AND CROP GROWTH

b

4

I

I

8

12

t

I

I

J

I

1

20

24

4

4

8

12

I

16

16

20

24

Time (GMT)

701

c

24

I1

24

I2

14

I2

24

Time ( G M T )

FIG.4. Diurnal variation of uptake of K' and NH: and efflux of H' by simulated swards of perennial ryegrass growins in flowing nutrient solution under natural light: (a and b) uptake of K' and efflux of H' on 27 June; (c and d) uptake of NH: and solar radiation on 27,28,29 June. Values under peaks are as follows: uptake of N and K as mg/m' for a 24-hr period; H' efflux as mg/m'/day; radiation as MJ/m'/day. Data from Hatch el al. (1986).

180

A. WILD ET AL.

Just as long-term changes in nutrient uptake respond to plant growth, so does plant growth respond to nutrient uptake, or the nutrient status of the plant. The rapidity of growth response to interruption of nutrient supply depends, however, on the period of interruption and on the nutrient concentration within the plant. Clement et al. (1979) found that perennial ryegrass which had been grown for 6 weeks from sowing with NO; at 7 pM showed no reduction in growth rate of their shoots during the following 9-day period when no nitrogen was supplied. During this period the shoots of plants grown with a terminated supply increased in dry weight by 247 f 28 g/m2 and those grown with a continued supply increased by 250 f 28 g/m2. During the 9-day period the dry weight of roots grown with a terminated supply increased by 80k 6.2 g/m2 whereas those with the continued supply increased by only 39 k 6.2 g/m2. A partitioning response of this kind is consistent with early work of Brouwer (1962a,b) and can be predicted from models that define the priorities for shoot and root growth in terms of carbon and nitrogen substrate levels (e.g., Johnson, 1985). The effects of intermittent nutrient supply on growth have been reported in a second experiment by Clement et al. (1979). Perennial ryegrass was grown from sowing for 28 days with NO; at 7 pM. For one set of plants the supply was continued for a further 42 days and for another set it was switched off and on at 3-day intervals during this period. In this experiment an intermittent supply of NO; induced greater transport of photosynthate to the roots (Fig. 5 ) , as in the example above with a terminated supply. Although the total uptake of N was less by plants receiving an intermittent supply, the rate of uptake was about 40% greater during the 3-day periods of supply than by plants on a continuous supply. The explanation suggested for these effects of N supply on plant growth is that for the 6-week-old plants in the first experiment the amount of NO; in the plants provided a sufficient source of N for protein synthesis to continue for at least 9 days without a reduction of growth. In the younger plants in the second experiment, the amount of NO; within the plants was insufficient to maintain growth for 3 days. In both experiments (1) NO; concentrations in roots and shoots decreased when the supply was terminated or intermittent, but did not fall to zero, and (2) NO; reduction was quickly and strongly suppressed when the supply was withdrawn. These observations support the view of Ferrari et al. (1973) that much of the NO; stored in plants, probably in the vacuole, is not readily available for reduction and that the relatively small amount present in the “metabolic pool” requires continuous replenishment via uptake by the roots if maximum rates of reduction are to be maintained.

B. QUANTITATIVE DESCRIPTION IN RELATION TO REQUIREMENTS OF PLANTS Quantitative approaches to the relationships between nutrient uptake, growth rate, and properties of root systems have been described by

UPTAKE OF MINERAL NUTRIENTS AND CROP GROWTH

Aj

5-

OL

181

30

40

50

Days after

60

70

sowing

FIG.5. Cumulative amounts of NO; supplied by nutrient pumps to replace that taken up by simulated swards of perennial ryegrass from flowing nutrient solution with a continuous (0-0) or an intermittent ( 0 - 0 ) supply of NO;. Dry weights of shoots and roots at the end of the experimental period are shown in the insert histograms. Data from Clement ef at. (1979).

Williams (1948), Loneragan (1968), and Brewster and Tinker (1972). These relationships are well discussed in the book by Nye and Tinker (1977), to which the reader is referred for further information. Nutrient uptake rate expressed as a flux (F) (uptake per unit root surface area) is related to growth rate and nutrient concentration in the plant by the following equation (Nye and Tinker, 1969): dx

1

where X = nutrient concentration in plants, W = plant weight, t = time, r = mean root radius, and L = total root length If X is constant, the minimum flux, F, , required to maintain potential growth rates is

182

A. WILD E T A L .

where X,, is the critical, or threshold, concentration required in plant tissues (Woodhouse et a[., 1978). Equation (3) identifies three parameters, Xcrit, relative growth rate, (dW/dt)(l/W) and the ratio of plant weight to root surface area, W/2nrL, each of which contributes to the required flux, F,. It should be noted that F and F, are net fluxes and are averages for the whole root system, as are all values for uptake reported here. Separation into efflux and influx has usually not been made in this form of analysis. Forms of dimensional analysis equivalent to that in Eq. (3) can be derived for minimum inflow (uptake per unit root length) and unit absoption rate (uptake per unit fresh weight). Furthermore, the terms in Eq. (2) and (3) may be stated on a dry weight or fresh weight basis. The latter is to be preferred because nutrient concentrations may change much less with time when expressed on a fresh weight basis (Leigh and Wyn Jones, 1984; Leigh and Johnston, 1985), which gives better justification for the assumption of in Eq. (3). constant Xcrit Measurements of the RGR (Fig. 6) and of the ratio root surface area : plant weight (Fig. 7) have been reported by Woodhouse et a[. (1978) for fodder radish, barley, and perennial ryegrass during their first 3-4 weeks of growth from germination. The response of dry matter to the external concentration of K' in the range 1.3-102 pA4 was radish > barley > ryegrass. For most of the experimental period radish had the highest RGR but the lowest ratio of root surface area to plant weight and therefore required the highest K' flux. The order of response of the three species to the range of K' concentration in the external solution was therefore attributed to differences in potential RGR and ratio of root surface area to plant weight. The changes in these two parameters with the age of the plant (Figs. 6 and 7), and in the internal threshold concentration, may explain why plants are generally most sensitive to the external nutrient concentration during the first 2-3 weeks after germination. Similar observations on the sensitivity of young plants to external phosphate concentration have been made by Breeze et a[. (1984). As the explanation lies in the ontogeny of plants the observations may apply to all nutrients. The ratio F/F, is a measure of the effectiveness of a species in meeting its nutrient requirements (Wild and Breeze, 1982). A ratio of less that 1 indicates that F is less than is required to achieve the potential growth rate, as found with fodder radish and barley during early growth at 1.3 and 6.4 p.M K'. For these two species the relationship between F/F, and F,,, was the same (Fig. 8), which implies that for any required flux, F,, under the experimental conditions, the two species were equally effective in taking up K'. It is therefore suggested that the parameters identified in Eq. (3) can

183

UPTAKE OF MINERAL NUTRIENTS AND CROP GROWTH

0.40r

0 >

3 a

0.30-

T

-e 0,

f 0.20-

s

E01 l aJ .-

5 - 0.100

a

O

0

5

20 25 30 Days after germination

10

15

35

J LO

FIG.6. Relative growth rate, calculated on fresh weight basis, in relation to age of plants of barley (+-+), fodder radish (0-0). and perennial ryegrass (m-m) grown in flowing nutrient solutions with K' at 102 pkf, at which the potential growth rate was achieved; the supply of all other nutrients was adequate. Data from Woodhouse et al. (1979).

10-

f01 .01 3

8-

c

9Q;

- 0

6-

F;

a

g 4-

$ €

2 2

2

3m

2-

o

L

I

I

I

FIG. 7. Change in the ratio root surface area : fresh weight of whole plants in relation to age of three species grown in flowing nutrient solutions with K' at 102 pA4 and all other nutrients in adequate supply. Data from Woodhouse el a/. (1978).

184

A. WILD ET AL. External solution (JJMK+) 1.3 6.4 Radish 0 0 Barley b &

U E

'"1

- 0.5 U

0

0

2

4

8

6

10

12

IL

16

Fin

FIG. 8. Effectiveness of uptake mechanisms of K+ (as F/F,) for two species. F, (mol K+/cm'/sec x lo-'*)is the required flux of K+ into roots to maintain the potential relative growth rate (calculated from Eq. 3). and Fis the measured flux. From Wild and Breeze (1982).

be used as a basis for assessing the effectiveness of different plant species and genotypes in taking up nutrients and using them for growth. There are additional considerations for plants growing in soil where infection by mycorrhiza and Rhizobium and rhizosphere conditions affect uptake (Nye and Tinker, 1977; Barber, 1984).

c.

EFFECTSOF EXTERNAL NUTRIENT CONCENTRATION

The use of flowing nutrient solutions allows nutrient uptake by roots of whole plants to be investigated at low concentrations which can be maintained constant or allowed to vary between prescribed limits. Several workers (e.g., Woodhouse et af., 1978; Wild et al., 1979; Siddiqi and Glass, 1983; Breeze et.al., 1984, 1985) have studied the relationships between external nutrient concentration and uptake and growth from germination onward with sequential harvesting. This enables calculation to be made of average uptake and relative growth rates for intervals between harvests. Continuous monitoring of nutrient addition to maintain constant concentrations (see Section 11) provides a more direct measure of uptake rates over periods from minutes to days. Loneragan and Snowball (1969) employed sequential harvesting to study the relationship between the external concentration of Ca2+and uptake and growth rates for 30 species after transplanting seedlings to the flowing nutrient solutions. Other workers have imposed treatments on transplanted seedlings with harvests only at the beginning and end of the treatment period (e.g., Asher

UPTAKE OF MINERAL NUTRIENTS AND CROP GROWTH

185

and Loneragan, 1967; Asher and Ozanne, 1967; Spear et al., 1978a,b, 1979; Siddiqi and Glass, 1983; Chantkam et al., 1983, 1984). In these studies based on steady-state conditions, and in others in which the external nutrient concentration has been changed (Breeze et al., 1985) or the nutrient supply terminated (Clement et al., 1979), responses in uptake and growth rates have usually been measured over several days. Such work should be distinguished from that in which instantaneous or short-term responses of uptake rates to a range of external nutrient concentrations have been measured, sometimes involving depletion (e.g., Bloom and Chapin, 1981). 1. Steady State Early work in flowing nutrient solutions on the relationship between steady-state external concentration and nutrient uptake has been summarized by Pitman (1975) as shown in Fig. 9. More recent values are given in Table 11. Figure 9 shows the average rates of nutrient uptake by several plant species at RGR up to 0.1-0. W d a y and also shows that these rates can be supported by low external concentrations provided they are maintained. In the original papers the relation between external concentration and uptake was shown to differ between species, and this difference is referred to below.

External concentrat ion (rnrnol/m3 )

FIG.9. Rates of net uptake of five nutrient elements by crops grown in flowing nutrient solutions. Dashed lines show where growth limitations were observed. K' from Asher and Ozanne (1967). assuming RGR of 0.13/day, mean of 14 spp; P from Asher and Loneragan (1967) and Loneragan and Asher (1967), mean of 8 spp; NO; from Clement et al. (1974) for perennial ryegrass; Ca'+ from Loneragan and Snowball (1969), mean of 30 spp; Zn" from Carroll and Loneragan (1968, 1969). mean of 8 spp. Adapted from Pitman (1975).

Table II Concentrations of Nutrient in External Solution Required to Achieve the Potential Relatlve Growth Rate (RGR) Required concentration

Potential RGR (day - '1

Nutrient

Plant species

Days from sowing

NO; H,PO,;

Perennial ryegrass Perennial ryegrass

35-56

14 for 90% PRGR"

0-25 25-40

0.1-0.4 <0.16

0.15-0.20 0.12

K+

Perennial ryegrass

0-14 14-40

<1.3

0.18

< 1.3

0.18

0-14 14-30

6-19

0.24

< 1.3

0.17

0-14 14-30

6-19 <1.3

0.28 0.25

Barley Fodder radish

(N)

Reference Clement et al. (1978a) Breeze et al. (1984) Woodhouse et al. (1978) Woodhouse et al. (1978) Woodhouse el al. (1978)

"Obtained from experiments with NO; concentration held constant in the range 1.43 pM-143 pM over whole growth period of 6 weeks. Later work has shown that the required concentration is <7 pM.

187

UPTAKE OF MINERAL NUTRIENTS AND CROP GROWTH

Woodhouse et al. (1978) investigated the relation between the uptake of

K',expressed as a rate (flux) across unit surface area of root, and the external concentration in the range 1.3-102 f l K', each concentration being maintained constant over the whole period of growth (Fig. 10). In fodder radish at 6 days, the flux increased up to the highest concentration tested, but by 32 days it was constant over the whole concentration range. In perennial ryegrass the effect of concentration over the same range was even less marked and there was no effect on flux after 22 days. The uptake of phosphate from solutions of 0.04-32 f l was investigated with perennial ', the uptake of ryegrass (Fig. 11) (Breeze et al., 1984). As was the case for K phosphate increased less with increasing external concentration as plants aged and it became independent of concentration in this range at a

2ol

Fodder radish

E 2 ln

v)

Otl

I

I

1

I

0

c

a

1;

5 : 0

20

40

60

Concentration of solution

I

I

80

100

I

120

(VM K + I

FIG. 10. Flux of K' into fodder radish and perennial ryegrass plants growing in flowing nutrient solutions under steady-state conditions with K+at 1.3,6.2, 19, and 102 pM. Data from Woodhouse (1977) and Woodhouse et at. (1978).

188

A. WILD ET AL.

low growth rate, but not at a high growth rate, during the experimental period. The two sets of results show that uptake rates of phosphate and K' are increased more by high external concentrations when plants are up to about 3-4 weeks old than when they are older. Figure 11 also shows, as might be expected, that the relation between uptake and the external concentration depends on growth rate. Both effects are discussed more fully below. The relation between flux, F, on a root surface area basis (moles per square centimeter per second) and the external concentration has been simply expressed as F = &,, where a! is a proportionality factor described as the

0 15 days

0.L -

0.2

O L L days

I

0.5

m

1.o

2.0

1.5

2.5

3.0

2,

' -E" Q

0.6-

10 days

.-+4-,0

a

c

ul 0

a

0-0

16 days

0 0 -

33 days

L

0 Y

0

c

a

0 .c'

0

0.5

1 .o

I

1.5

2 .o

6.L

Concentration in solution (JIM P 1

FIG. 11. Rate of uptake of phosphate by perennial ryegrass when grown at two relative growth rates in flowing nutrient solution: (a) 0.18/day at 15 days from sowing, (b) 0.25/day at 16 days from sowing. Data from Breeze et al. (1984).

UPTAKE OF MINERAL NUTRIENTS AND CROP GROWTH

189

root absorption coefficient (Nye and Tinker, 1969) and C, is the concentration (moles per cubic centimeter) at the root surface, that is, in the bulk solution when flowing nutrient solutions are used. The relations shown in Figs. 10 and 11,imply that a! decreases as plants age and is low at low RGR. Christie and Moorby (1975) also showed that a for phosphate uptake decreased with age in plants of three species. In barley supplied with 10 pA4 NH,NOJ, the values of a for NH: increased during treatments at root temperatures between 3 and 25"C, whereas a! for NO; decreased with time at temperatures above 11°C (Macduff and Hopper, 1986). Values of a! obtained from early work have been reported by Nye and Tinker (1977). Recent values are generally higher because of the use of more dilute solutions. For young plants they are up to about lO-%m/sec. It should be noted that a! is, however, no more than a convenient coefficient which has proved useful, for example, when comparing transport processes to roots with uptake rates across root surfaces (see Nye, 1977). In more mechanistic treatments of the relation between I: and C,, the coefficient is replaced by several terms as, for example, in elaborations of the Michaelis-Menten approach (e.g., Siddiqi and Glass, 1986) (see also Section 111,A). With selections of cultivars grown under steady-state conditions, differences in the relation between K' uptake and external K' concentration have been demonstrated with cassava (Manihot esculenta) (Spear et al., 1978a) and barley (Siddiqi and Glass, 1983). In the barley cultivar Fergus, DeaneDrummond (1982) reported that the compensation point for NO; (the concentration at which there was no net uptake) was 10 pA4, whereas Macduff and Hopper (1986) found substantial net uptake of NO; by the cultivar Atem at 10 ph4. In comparing these observations on NO; as with others in the literature, care must, however, be taken because of the different conditions under which the plants were grown. An interesting application of flowing solution culture has been reported by Howeler et al. (1982). They grew rice (Oryza sativa), maize (Zea mays), cowpea ( Vigna unguiculata), green beans (Phaseolus vulgaris), and eight cultivars of cassava in flowing nutrient solutions at 0.1, 1.0, 10, and 100 f l phosphate either with or without inoculation with vesicular arbuscular mycorrhiza. With all species, inoculation resulted in increased growth and phosphate uptake, indicating that the mycorrhiza either provided the roots with a greater absorbing surface or that the fungal surface was more able than the root surface to take up phosphate from low concentrations.

2. Nonsteady State The enhanced uptake of nutrients following a period of deprivation has been observed frequently in stirred solution culture systems (see Lee and

190

A. WILD ET AL.

Rudge, 1986, for a recent example). Flowing culture systems allow the dynamics of the plant response to a change in nutrient concentration to be monitored continuously. It was shown earlier (Fig. 5 ) that when NO; was supplied intermittently at 7 pkfthe uptake rate by perennial ryegrass during the supply periods was higher than that by plants receiving NO; continuously. In another experiment, perennial ryegrass was grown for 29 days at three phosphate concentrations which were held constant (Breeze et al., 1985). In half the culture vessels the concentrations were maintained for a further 14 days and in the other half the concentrations were changed. The uptake of phosphate increased about 4-fold when the external concentration was raised from 0.1 to 6.4 f l and it decreased about 4-fold when the concentration was changed in the opposite direction (Table 111). In treatments in which the external concentration was changed the plants readjusted uptake rates during the first 7 days such that they came close to those of plants held at a constant concentration. A third example from work with flowing solutions is shown in Fig. 12a, which compares perennial ryegrass plants grown at K+ concentrations held constant at 1.3, 6.2, 19, and 102 pkfthroughout the growth period (39 days) with plants grown for the same period at 102 f l ,after which the supply was terminated (Wild et al., 1979). For plants grown at constant concentration the flux of K' just prior to the final harvest was almost constant over the whole range of external concentration, whereas the flux into plants during depletion decreased as concentration decreased. It follows that K, values will be much lower when derived from measurements of flux from steady-state concentrations than from depleting solutions. Using plants of perennial ryegrass grown at constant concentrations of 1.3, 6.2, and 19 f l K' for 39 days and transferred to a solution of 102 Table 111 Rates of Phosphate Uptake (pmol P/g Fresh Weight Root/ Day) by Perennial Ryegrass Grown at Constant and Changed Concentrations of Phosphate in Flowing Solution Cultu& Period (days from sowing) Solution ( f lP)

26-29

29-36

36-43 ~

0.1 0.1 0.4 0.4 6.4 6.4

-

6.4 6.4 0.4

3.23 3.23 8.07 8.07 12.91 12.91

'From Breeze et al. (1984, 1985).

0.32 14.20 3.87 7.75 6.46 2.90

~~~

8.12 3.55 1.75 7.43 4.84

UPTAKE OF MINERAL NUTRIENTS AND CROP GROWTH

9

191

b

8 7 6 A

01

0

20

1

1

60

80

1

100 Concentration of solution +M K+ ) LO

FIG.12. Flux of K' into 39-day-old perennial ryegrass plants under nonsteady conditions: (a) comparison between plants grown under steady-stateconditions (7-V) and plants grown at 102 ph4 K+ and the supply of K' then terminated (0-0); (b) uptake by plants previously grown at 1.3 (A-A), 6.2 (A-A), 19 (W-W), and 102 (0-0)ph4 K', transferred to 102 ph4 K', and supply of K' then terminated. Data from Wild el at. (1979).

192

A, WILD ET AL.

f l ,comparison was made between the flux of K' into plants grown at the three low external concentrations and that into plants grown at 102 pM. No additions of K'were made to the solutions during the period of comparison. When the concentration was raised to 102 pM, the initial flux was about twice as high into plants grown at the three low concentrations as into those grown throughout at 102 pM(Fig. 12b). The difference remained during the period of depletion (about 33 hr). This increased rate of nutrient uptake following transfer to raised concentrations is not a new observation (Pitman, 1976). The two points of interest are first, that in all four solutions the growth rate and K' flux (see Fig. 12a) were independent of concentration before the common concentration of 102 pM was established. In other words, the external concentration was not limiting growth or the K' flux. Second, before establishment of the concentration of 102 p M , the concentrations of K in roots and shoots were almost independent of the external concentration (Table IV). Stimulation of K' flux by raising the external concentration to 102 pM(Fig. 12b) was therefore not related to concentrations in root or shoot. One possibility is that the increased flux resulted from a low concentration of K'in one of the compartments of the root cells, e.g., the cytoplasm, or from low concentrations in the innermost part of the cortex. Alternately, it might be that the number of accessible ion transporters (Clarkson and Deane-Drummond, 1983) in the root cortex or their activity, or both, changed with the external concentration, and that there was a time lag before a new equilibrium rate was achieved. Our hypothesis is that the number of ion transporters and their activity are both involved. We suggest that at low external concentrations the transporters have a high activity but few have access to nutrients, perhaps only those in the epidermal and outer cortical cells of roots (see Section Table IV Composition of Perennial Ryegrass Grown for 39 Days in Solutions of Four K + Concentrations at the Start and End of a Depletion Period from 102 @K+" Concentrations of K+ in external solution ( f l ) ~~~

1.3

6.2

19

102

K in shoots (amol/g fresh weight)

Start End

198 215

203 217

192 203

210 213

K in roots (pmol/g fresh weight)

Start End

78 82

81 75

91 89

91 78

RGR prior to start of depletion (per day)

0.10

0.14

0.11

0.09

"Data are from Table 1 of Wild et a/. (1979), in which the roots are incorrectly identified; root diameter should have been recorded as 0.146 f 0.06 mm for perennial ryegrass.

UPTAKE OF MINERAL NUTRIENTS AND CROP GROWTH

193

III,B,3). At higher concentrations there are more transporters because of the greater penetration by apoplastic transport into the cortex, but these transporters have lower activity. When the concentration is changed the number of transporters with access to nutrient ions is affected quickly and there is a rapid change (increase or decrease) in uptake rate. More slowly, the plant adjusts the activity of its transporters to meet its requirements, perhaps by altering turnover rates or affinities (Glass and Siddiqi, 1984). During the adjustment period uptake exceeds (when the external concentration is increased) or falls below (when the concentration is decreased) the steady-state values. The adjustment is slower to a change of nutrient concentration than to a change in solar radiation (Figs. 3 and 4). It is uncertain whether a hormone or other messenger from the shoots (Clarkson and Hanson, 1980), or changing the concentration of the transported ions close to the transporter, provides the information which, on our hypothesis, would result in a change of transporter activity. Clearly, much more work is needed on the identity, distribution, and activity of transporters across the root and the relations between uptake rate, external concentration, and the concentration gradient across the root. The flowing culture solution system would seem ideally suited to this type of work. 3. Diffusive Pathway into Roots

Between the external solution and absorption sites in the roots there is a thin layer of water on the external surface of roots and there is water-filled “free space” between the cortical cells (macropores) and within their walls (micropores). Before ions pass through a membrane into the symplasm they are in a continuum of solution which extends to the flowing nutrient solution outside the root (Lauchli, 1976). Movement through this continuum is by diffusion and mass flow.From the very dilute solutions to be considered here mass flow makes only a small contribution to the total ion flux, estimated to be up to 1-5070. We can therefore consider movement through the continuum to be by diffusion alone. Assuming planar rather than radial geometry we can, under steady-state conditions, use the equation F = - D(C, - Ci)/x to calculate x, where F is the nutrient flux, D is the diffusion coefficient, C, and Ci are the external and internal nutrient concentrations, and xis the distance between the outer boundary of the layer of water on the roots and the sites of absorption. There is a schematic representation in Fig. 13. Experimentally, F is known for values of C,and to calculate x the following assumptions are made:

-

1. Ci 0 at the absorption sites 2. D is the value in bulk solution 3. x = (xl xJ8) where x1is the thickness of the water film on the root

+

194

A. WILD ET AL. root surface

1

root cortex

site

FIG. 13. Schematic representationof the diffusive pathway for nutrient ions from the external solution through the film of water on the roots (thickness x , ) and the cortical free space (thicknessxr). C,, Concentration in bulk solution; Ci, concentration in solution at sites of absorption.

surface, x2 is the length of the diffusive pathway within the cortex normal to the planar root surface, and 8 is the water-filled pore space (apparent free space) in the roots, expressed as a fractional volume of the fresh roots. The value of 8 is uncertain. Early estimates were 15-25'70 of the root volume but more recently it has been put at about 5 % (Shone and Flood, 1985) and both values are used in Table V. The value of x1is also uncertain. Under the conditions of flowing solution culture, flow is probably partly turbulent and partly laminar. In Table V the values used for x, are 1 pm and 20 pm, which are regarded as extreme values. By measurement, the highest value of flux for K'into perennial ryegrass at the lowest concentration tested (1.3 pkf K') was 13 x lo-'* mol/cm2/sec (Woodhouse et al., 1978). In order to provide this flux at this concentration, calculation shows that x2 must be zero if x , is 20 pm, that is, the K' is absorbed at the epidermis (Table V). If x1is 1 pm, x, is 1 pm for a value of 8 of 0.05 and is 4 pm for a value of 8 of 0.2. In this example K' is removed from solution within one cell length of the epidermis and it must there enter the symplasm if Ci is to remain zero. If C, is not zero, for example, if there is efflux of K', absorption will occur even closer to the epidermis because influx will be greater than net uptake and the concentration gradient will be smaller. Under the conditions described the ryegrass plants were able to maintain potential RGR at 0.19/day. With phosphate the highest flux at the lowest external concentration mol/cm2/sec (Breeze et al., 1984). tested (0.1 pA4) was 1.1 X

195

UPTAKE OF MINERAL NUTRIENTS AND CROP GROWTH

Table V Calculated Distances (x,) of Diffusion of K + and Phosphate into the Root Cortex of Perennial Ryegrass Using Assumed Values of the Diffusion Coefficient (D), Thickness on the Film of Water on the Root (x,), and the Apparent Free Space (0)”.b*c

Potassium: D = 2 x

Phosphate: D = 1 x

cm2/sec

cm2/sec

1

0.05 0.20

20

0.05

0

0.20

0

1

20

1 4

0.05

5

0.20

18 4 14

0.05 0.20

O x 2 is calculated from F = -D(Co-C,)/(xl+xz/O)using values of F = 13 x lo-” mol/cm-'/set from 1.3 x MK’ (CJ, and F = 1 . 1 x lo-” mol/cm2/sec from 1 x lO-’M phosphate (see text). %he average radius of roots is 73 pm. %ee Fig. 13.

Similar calculations to those for K’ show that although phosphate ions moved deeper into the cortex (Table V) absorption occurred in the outer part (the mean root radius was 75 Fm). Under the conditions of the experiment the flux of phosphate was too low to support the potential RGR of 0.20-0.25/day. These calculations of the diffusive pathway require certain assumptions to be made. Values for D are assumed to be those in bulk solution; lower values would lead to the conclusion that absorption occurs closer to the epidermis but, as far as we are aware, there is no evidence to support higher values. Values for x, and 8 (apparent free space) are also uncertain but reasonably extreme values show absorption to occur close to the epidermis. This leads to the question whether diffusion through the cortex limits the rate of transport into the symplasm. Although this has been categorically denied (Anderson, 1975), with the very low concentrations considered here we believe the question to be less simple than at first sight. With a very low concentration gradient, absorption must occur close to the epidermis in order to provide the measured nutrient flux, and the lower the gradient the closer will absorption be to the epidermis. This will limit the number of effective ion transporters. The limitation to absorption under these conditions therefore appears to be through the rate of supply, i.e., diffusion to a reduced number of transporters. If this reasoning is correct, rates of nutrient absorption over a wide range of external concentrations cannot be

196

A. WILD ET AL.

expected to follow the mathematical expressions used in enzyme kinetics, and our experience is that they do not give linear Hofstee or Lineweaver-Burke plots. To improve the fit to Michaelis-Menten equations it might be necessary to express uptake from very low concentrations as a flux (across root surface area), and from higher concentrations as a unit absorption rate (root volume basis), as more transporters across the root become effective in absorption into the symplasm. Our conclusion agrees with that of Wyn Jones (1975) that kinetic constants, e.g., K, are only a useful shorthand and their use should not imply a mechanism for nutrient uptake. OF THE NUTRIENT SOLUTION D. EFFECTOF TEMPERATURE

Flowing solution culture has only recently been employed to investigate the effect of root zone temperature on nutrient uptake and growth. It is particularly suitable because it can provide control of root zone temperature at low and controlled concentrations of nutrients which simulate field conditions, as during spring in temperate climates, when the crop root system is usually at a much lower temperature than the shoots. Work on temperature effects has been reviewed by Richards et ul. (1952), Went (1953), Nielsen and Humphries (1966), Cooper (1973), and Berry and Raison (1981), but this has not included studies conducted in flowing nutrient solutions. Using the Hurley system, the effects of low root temperatures have been investigated with several species, in particular for their relative uptake of NO; and NH:. Clarkson et uL(1986) grew perennial ryegrass from seed for 49 days, lowering solution temperature decrementally from 20°C to acclimatize at 5 "C and then changing it to 3,5,7,9, 11, 13, 17, or 25 "C, with a common air temperature of 25/15 "C dayhight. During the subsequent experimental period of 14 days, total net uptake of N from solutions maintained at 10 ,t"H4NOs increased 6-fold between 3 and 25 "C. The proportion of N taken up as NH: was inversely related to root temperature, increasing from 65% at 25 "C to 93% at 3 "C (Fig. 14). In a similar experiment, the uptake of N by oilseed rape (Brussicu nupus L. cv. Bien venu) increased 3-fold between 3 and 13 "C but showed no further increase above 13°C (Macduff et ul., 1987b); there was little change with root temperature in the ratio NH::NO; taken up (Fig. 14). These two species also differed in the effect of root temperature on dry matter yield, which increased 6-fold between 3 and 25 "C in ryegrass, whereas in rape it showed little response to root temperature. White clover (Trvolium repens), when dependent on both symbiotic fixation of nitrogen and 10 @t NH4N0, in solution, behaved similarly to oilseed rape, with little change in the ratio NH::NO; taken up between 3 and 25 "C (M. J. Hopper et ul., unpublished data), although the total uptake of N and dry matter yield

UPTAKE OF MINERAL NUTRIENTS AND CROP GROWTH

197

i L look "'1 Boriey

?l

2 00

t

3

0

5

7

9

11

White clover

200

c

Root temperature ("C) FIG.14. Effect of steady-state root temperature on the cumulative uptake of NH: and

NO; over 14 days by four species from 10 pA4 NH,NO,; numbers above bars are the percentage of total nitrogen uptake as NI&. Data for perennial ryegrass from Clarkson et al. (1986), for oilseed rape from Macduff et 01. (1987b), for barley and white clover from J. H.Macduff (unpublished data).

198

A. WILD E T A L .

increased throughout this temperature range (Fig. 14). The response by barley was intermediate between perennial ryegrass and the other two species in terms of the ratio NH::NO; taken up (Fig. 14) (Macduff and Hopper, 1986) and showed the greatest increase with temperature in both N uptake and dry matter yield. Comparison among the four species shows that low root zone temperature decreased dry matter yield of perennial ryegrass and barley more than that of white clover and had little effect on rape. A contributing factor to these differences might be that because the meristem is higher in the shoots of rape plants than in perennial ryegrass and barley it will be less influenced by the root zone temperature. Measurements of meristem temperatures with a wider range of species would be needed to test this explanation. The explanation is also uncertain for the effect of temperature on the ratio NH::NO; taken up: of the four species tested the temperature effect was most pronounced with perennial ryegrass. Clarkson and Warner (1979) suggested that NH: and NO; are transported through separate regions of the cell membrane which differ in lipid composition and phase transition temperatures. Alternatively, low root temperatures with NH: treatment may increase NO; efflux, leading to a reduction in net NO; uptake (DeaneDrummond and Glass, 1983). It has also been found that nitrate reductase activity of oilseed rape roots is greater in plants grown at a root temperature of 3 or 7 "C than at 1 I or 17 "C (Macduff and Trim, 1986), and it would be useful to know if rape differs in this respect from other species. As applies generally in the mineral nutrition of plants, an explanation depends on more complete investigation of cell biochemistry, especially the functioning of cell membranes. Using other systems of flowing solution culture, Bloom and Chapin (1981) showed that in the short-term (< 1 day) the uptake by two barley varieties of NH: exceeded that of NO; from a range of 5-200 pA4 NH4N0, at root temperatures of 2.5-25 "C. The ratio NH::NO; taken up was usually lower for the warm-adapted variety (2.1 f 0.2) than for the cold-adapted variety (3.6 & 0.3). Ganmore-Neumann and Kafkafi (1985) reported ratios of NH::NO; taken up by strawberry (Fragaria ananussa) at different root temperatures and with different ratios of NH::NO; in solution. The preference for N source changed with plant age, with NO; preferred during flowering but NH: preferred during later vegetative growth. The response of nutrient uptake to root zone temperature depends on the plant species, the nutrient ion, and the period over which the measurements are made. For example, using the Hurley system, mean daily unit absorption rates were derived from continuous measurements of the uptake of NH:, NO;, and K' by oilseed rape and barley plants growing for 14 days with root zone temperatures in the range 3-25 "C after acclimatization with

UPTAKE OF MINERAL NUTRIENTS AND CROP GROWTH

199

root zone temperature at 5°C for 14 days (Macduff et al., 1987a,b,c). The results suggest that it took 3-5 days for absorption rates in rape, and longer in barley, to adjust upward fully in response to an increase in root zone temperature (Fig. 15). Furthermore, it seems that acclimatization does not always result in steady-state uptake rates that are independent of temperature (Glass and Siddiqi, 1985). It is of interest that Tolley and Raper (1985) observed that the daily rate of NO; uptake per plant oscillated between maxima and minima with a periodicity of 3-5 days in soybean (Glycine mm),irrespective of whether roots were at 14 or 22°C. The lower rate of uptake by plants at 14°C was attributed to a lower rate of uptake per gram of dry root during the first 5-10 days and subsequently to slower root growth. Another factor that might influence nutrient uptake is a temperature gradient in the root system. Using a split root technique with recirculating nutrient solutions, Moorby and Nye (1984) found that phosphate inflow (uptake per unit root length in unit time) by fodder rape (Brassicu nupus cv. Emerald) was independent of temperature in the range 10-23 "C. Inflow into roots at 5 "C was about half that at 23 "C, but there was no evidence that the roots at 23 "C compensated with a higher inflow. The effects of temperature on rate processes, including nutrient uptake, are often related to the Arrhenius equation (Johnson and Thornley, 1985, but see also Raven and Smith, 1978). Values of Qlofor oilseed rape over the range 7-17 "C for the uptake of several ions and the efflux of H+and OHgenerally decreased with time, differences between ions being generally small (Table VI). Temperature effects on uptake rates therefore need longterm experiments if new steady states are to be achieved. The complication is then that root growth and morphology (root length, diameter, and root hair production) change with root zone temperature (Ganmore-Neumann and Kafkafi, 1980, 1983; Moorby and Nye, 1984; Macduff et al., 1986), which necessitates frequent plant harvesting for root measurements or continuous measurement of root length (Moorby and Nye, 1984). Because of the effect of temperature on the ratios root surface area to root weight and root length to root weight, the apparent effect of temperature on uptake rates will depend on whether they are expressed per unit surface area, length or weight of roots. The effect on uptake rates also depends on whether fresh weight or dry weight is used because tissue water content is inversely related to root zone temperature (Macduff et al., 1987a). The very pronounced difference between the effects of temperature on the N content of oilseed rape plants, depending on the weight basis (Fig. 16), gives another reason for expressing results on both a fresh and dry weight basis if a comparison is to be made between experimental results. It is apparent that there is still a lack of understanding of the relative sensitivity to temperature of the different steps in the uptake and transport processes. From the experimental results so far available it seems unlikely that

A. WILD ETAL.

200 Oilseed rape

2o

-6 F

g

t .17

120r NO; uptake

C 0

o

Barley

I

NO; uptake

L-zJ K*uptake

0

2

4

6

8

101214

0

2

4

n

6

8

101214

Time (days)

FIG.15. Mean daily unit absorption rates of N ~NO;. , and K+by oilseed rape and barley in response to root temperature; nutrients supplied at 10p.MNI-LNOsand 2.5 p.MK+,dayhight air temperature 20115 "C for oilseed rape and 25/15 "C for barley, pretreatment period of 14 days at 5 "C.Adapted from Macduff et ul. (1987b) and from J. H.Macduff (unpublished data).

UPTAKE OF MINERAL NUTRIENTS AND CROP GROWTH

201

Table VI Values of QloTemperature Coefficients (7-17OC) for Mean Unit Absorption Rates of Cations and Anions and Efflux Rates of H’ and OH- by Oilseed Rape Grown with 10 pM N as NH; or NO? Q~~valuesb

NH; nutrition

Ion

NO; nutrition

0-5 Days

5-14 Days

0-5 Days

5-14 Days

Ca’+ Mg’+ NH:

3.87 1.24 2.20 2.29

1.41 1.64 1.13

2.16 2.81 2.22

I .46

-

1.27 1.93 1.11

C Cations

2.16

1.46

2.47

1.58

so:-

2.59 3.23

1.19 1.48

-

3.35 4.08 2.26

1.40 1.27 1.36

E Anions

2.65

1.23

2.41

1.36

H+efflux OH- efflux

1.90

1S O

-

-

-

-

1.70

1.01

K‘

HzPOi NO;

-

-

uFrom Macduff et a/. (1987d). bValues are for two intervals in 14-day treatment period following acclimatization with root temperature at 5°C. 0.Xr

a L.2

2 y

3.8 -

-

3.0-

z

2.6-

EE

-

Q32.

-

e 0.28-

3.L -

z 2.2-

P g

-E

0.2L-

--:

0.20-

C

u

J

0.16-

J

3 5 7 9 11 13

17

25

0.14 3

Root temperature

5 7 9

11 13

17

25

(*C 1

FIG.16. Effect of temperature on the mean concentration of total nitrogen in oilseed rape plants expressed on the basis of (a) dry weight and @) fresh weight at days 0, 5, and 14, after and shoots (m) at day 0; roots (A-A) and shoots changing root temperature from 5°C. Roots (0) (A-A) at day 5 , roots (0-0) and shoots ( 0 - 0 ) at day 14. Data from Macduff (198713).

202

A. WILD E T AL.

the differences between species, nutrient ions, and experimental period will have a simple explanation.

E. 1.

RELATION BETWEEN EXTERNAL CONCENTRATION AND PLANT COMPOSITION

Concentrations of Nutrient and Other Elements in Plants

Experiments based on flowing nutrient solutions have shown how, in addition to growth, nutrient contents of plants may be affected by external concentration. Asher and Ozanne (1967)found that the concentration of K in the fresh weight of shoots of 14 species increased, as growth increased, to near maximum with increasing external concentrations from 1 to 24 fl, and that further increases to lo00 f l had very small effects. During the early growth of fodder radish, barley, and perennial ryegrass the K concentration in the fresh weights of shoots and roots depended on the external concentration in the range 1.2-102 pM, but after about 3 weeks, when the RGR had declined to a steady value, the internal concentration became almost independent of the external concentration (Woodhouse et al., 1978). A similar pattern of response to external concentration has been found with P. In seven of the eight species studied by Asher and Loneragan (1967), the P concentration in both fresh and dry weights of shoots and roots increased to a level near the maxium with increasing external concentration further increase to 25 f l had only small effects on infrom 0.04 to 5 ternal concentrations. This pattern of response did not hold for blue lupin (Lupinus digitatus), in which the concentrations in shoots and roots were much higher at 25 than at 5 f l i n the external solution. In two experiments with perennial ryegrass Breeze et al. (1984)found that after early growth the concentration of P in the dry weights of shoots and roots became largely independent of the external concentration, the exceptions being lower tissue concentrations at 0.04 f l in solution in one experiment and at 0.1 and 0.4 f l in the second, in which growth rate was higher. The pattern of response to the external concentration of NO; is shown by the results of experiments with perennial ryegrass grown with NO; as the sole source of N at 1.43 fl-143 mM (Clement et al., 1978a). Except at the extremes of this 10s range, at which the growth of shoots was restricted, the external concentration of NO; had very little effect on the concentration of total N in the dry weights of shoots and roots. Potassium, P, and N are relatively mobile in plants. Studies with the Hurley system have shown that Pb, which is much less mobile, accumulates in the root (Jones et al., 1973;Jarvis et al., 1977), as do Cd (Jarvis et al., 1976; Jarvis and Jones, 1978) and Cu, although to a lesser extent (Jarvis, 1980, 1981, 1984;Jarvis and Robson, 1982).

UPTAKE OF MINERAL NUTRIENTS AND CROP GROWTH

203

The general conclusion from the studies with K', phosphate, and NO; is that, provided growth is not restricted by low external concentrations, the concentrations of K, P, and N in tissues of several plant species are regulated over a very wide range of external concentrations. This is the concept of homeostasis (Clarkson and Hanson, 1980; Glass and Siddiqi, 1984; Clarkson, 1985). Further work is required to determine whether this also holds true for trace elements. Experiments with flowing solution culture have shown the effect of the external concentration of one ion on the concentration of other ions in plant tissues. Increased K' concentrations in the external solution decreased the concentration of Ca in the fresh weight of barley (Johansen et al., 1968). Increased Na' concentrations in solution decreased the concentrations of K in the dry weights of perennial ryegrass and cocksfoot (Dactylis glomerata) but not in timothy grass (Phleum pratense) (Jarvis, 1982). Potassium concentrations in plant tissues are also decreased by NH: in the external solution (see Section III,F,l). Increased concentrations of Ca(NO& in the external solution decreased the concentrations of Mg and S in the dry weight of perennial ryegrass but had no effect on the concentrations of K, Fe, Cu, Mn, or Zn (Clement et al., 1978a). 2. Relative Nutrient Concentrations in Roots and Shoots An observation from experiments in which a wide range of external nutrient concentrations has been studied, is that for K, the ratio of concentration in shoots : concentration in roots increased as the external concentration increased (Asher and Ozanne, 1967; Woodhouse et al., 1978). The concentration ratios of P and N increased much less than in the case of K when the external concentrations of phosphate and NO;, respectively, were increased (Asher and Loneragan, 1967; Clement et al., 1978a; Breeze et al., 1984).

3. Percentage Dry Matter The nutrient concentration in the external solution affects the ratio dry weight : fresh weight, that is, percentage of dry matter of shoots and roots, the highest ratios being found at the lowest external concentrations. For example, with phosphate at 0.04-25 pl4 the percentage dry matter in the shoots of eight annual pasture species decreased with increasing external concentrations from 0.04 to 5 fl,and in the roots it decreased with external concentration up to 1 p M (Asher and Loneragan, 1967). Similarly, in a wide range of 14 cereal, pasture, and weed species the percentage of dry matter decreased as K' in the external solution increased from 1 to 8 f l

204

A. WILD ET AL.

and, in some species, to 24 pit4 (Asher and Ozanne, 1967). It has also been found that the shoots and roots of white clover had higher dry matter percentages when grown at pH 4 than at pH 5, 6, or 7, irrespective of whether they were depending on NO; in solution or fixation of atmospheric nitrogen (V. G. Breeze et af., unpublished data). It is not known whether the effects of external nutrient concentration or pH are due to a changed water content of the vacuoles or the cytoplasm, although the former seems more likely. (See also Section III,D for the effect of root zone temperature on percentage dry matter).

F. EFFECTSOF COMPOSITION OF EXTERNAL SOLUTION 1.

NX

and NO;

Comparison of NH: and NO; as sources of N for uptake and growth has received considerable attention. Even in purely agronomic terms, this is not surprising given the importance of maximizing the efficiency of uptake and utilization of fertilizer N by crop plants and minimizing losses of N from soil-plant-animal systems. The subject has been reviewed comprehensively by Haynes and Goh (1978) and more recently by Haynes (1986). Van Egmond (1978) has discussed the differential effects of NH: and NO; nutrition on the ionic balance of plants, while energy (photon) costs of uptake, assimilation, and the regulation of internal pH have been critically assessed by Raven (1985). The comparative utilization of the two forms of N by crops under field conditions are, however, far more likely to be determined by loss of N (NH, by volatilization, NO; by leaching and denitrification) and the greater mobility of NO; than NH: than by the effects of NO; versus NH: nutrition. The extent to which plant growth is favored by NH:, NO;, or the ratio NH: : NO; (and also the effects on the uptake of other nutrient ions), seems to vary with the plant species, the composition, and the concentrations of ions in solution and time, the latter both in terms of the age of the plant and the time scale of the experiment (see Haynes and Goh, 1978; Haynes, 1986). Flowing solution culture can provide accurate control of NH: and NO; concentrations, and the inclusion of a pH-stat prevents the decrease and increase in acidity with NO; and NH: respectively (Tolley-Henry and Raper, 1986).

In an experiment over 14 days with oilseed rape using the Hurley system (Macduff et af., 1987c), plants receiving 10 @NO; had leaf areas and RGR that were 10% higher than plants receiving 10 pit4NH:. The plants receiving NO; also had lower percentage dry matter than those receiving NH:, suggesting that effects of the two sources of N on growth may arise in part

UPTAKE OF MINERAL NUTRIENTS AND CROP GROWTH

205

from effects on osmoregulation and cell expansion, particularly if NO; is absorbed in excess of assimilatory demands. In contrast, there was no difference in the growth of perennial ryegrass when receiving an adequate or suboptimal supply of NO; or NH: (Jarvis, 1987). Soybean grown for 27 days at pH 5.8 also showed no differences in dry matter or N accumulation between the two sources of N (Rufty et al., 1983) and tomato (Lycopersicon esculentum) showed no differences at pH 6 (Peet et al., 1985). Addition of NH: at 30 pM increased the growth of wheat (Triticum aestivum) seedlings when grown with NO; at 200 pM (Cox and Reisenauer, 1973). A similar result has been obtained when perennial ryegrass was grown with NH: added at 36 f l to a nutrient solution containing NO; at 72 pkf, but higher concentrations of NH: reduced yields (Reisenauer et al., 1982). In studies lasting up to 10 hr with two varieties of barley, Bloom and Finazzo (1986) showed that NH: stimulated NO; uptake in one variety but depressed it in the other. For tomato, a 1:1 ratio of NH::NO; in the external solution gave most growth in treatments over several weeks (GanmoreNeumann and Kafkafi, 1980), but for both tomato and strawberry (Ganmore-Neumann and Kafkafi, 1980, 1983) there were complex interactions between supply ratio, root temperature, and growth. The source of N also affects plant composition (Haynes, 1986). Recent observations are that perennial ryegrass receiving suboptimal supplies of nitrogen as NH: had lower K and higher P concentrations in its dry matter than plants receiving NO; (S. C. Jarvis, unpublished data). With oilseed rape the same observation was made for K but the concentration of P was not affected by the source of N (Macduff et al., 1987d). The reasons for these differences between NH: and NO; as a source of N merit further investigation. Flowing solution systems like that of Barneix et al. (1984), in which Oz/COz exchange between roots and solution can be monitored, may help to elucidate the differences in root metabolism associated with the uptake of the two ions. 2. p H

The conclusion of Arnon and Johnson (1942) using static solution culture was that a pH within the range 4-8 was tolerated by plants provided an adequate supply of nutrients was maintained. Asher and Edwards (1978) pointed out that the data of Arnon and Johnson indicated a narrower range, and the results of Islam et al. (1980) using flowing nutrient solutions support this view. They found greatest growth of six species: ginger (Zingiber officinale), cassava, tomato, bean (Phaseolus vulgaris), wheat, and maize at about pH 5.5. Yields of ginger and tomato were not significantly changed at higher pH values whereas yields of the other four species were depressed, an effect that was partly overcome by foliar spraying

206

A. WILD ET A L .

with FeSO., solution, suggesting a deficiency of Fe. The yields of all species were significantly depressed below pH 5.5, ginger and cassava being affected least. In four cultivars of sunflower (Helianthus unnuus), the yield of tops was greatest at pH 4.5-5.0 with no significant increase at higher pH values, and for one cultivar there was no significant depression of yield at pH 4.0 (Blarney et al., 1982). Breeze et ul. (1987) found that the increase of dry weight of white clover over a 20-day period, whether fixing atmospheric nitrogen or dependent on NO; in solution, was not significantly lower at pH 4 than at pH 5 , 6, or 7. The effect of solution pH on nutrient uptake and plant composition is not yet clear. There has been most interest in the effect on phosphate. Highest uptake rates occurred at pH 5.5 with four cultivars of sunflower but were not significantly less at pH 6.0 (Blarney et al., 1982). With white clover the uptake rate was highest at pH 5 for plants supplied with NO; and highest at pH 7 for plants dependent on symbiotically fixed nitrogen (Breeze et al., 1987). The concentration of P in the shoots and roots of plants grown at pH 4 was lower than at pH 5,6, and 7 whether dependent on symbiotically fixed nitrogen or NO; in solution. There was little, if any, effect of solution pH on the concentration of total N in the shoots or roots of plants dependent on symbiotically fixed nitrogen. In contrast, the shoots of plants grown with NO; at pH 4 had lower concentrations of total N than those grown at pH 5 , 6, or 7. In soybean dependent on NH: as the source of N, pH had a very pronounced effect on growth and N uptake (Tolley-Henry and Raper, 1986). Under pH-stat conditions, the uptake rate of NH: per gram of root over 24 days was 25% lower at pH 5.1 than at 6.1. Within 10 days at pH 4.1 there was cessation of NH: uptake, net dry matter production, and appearance of new leaves. Robson and Loneragan (1970) showed that the rate of MnZ+uptake by two cultivars of subterranean clover (Trifolium subterruneum) from flowing nutrient solutions containing 0.3 p M Mn2+increased by up to 20-fold as solution pH increased from 5.4 to 7.0. The rate of uptake by two Medicago species was similarly increased, but their growth was reduced at pH 7.0 due to Mn toxicity. The sensitivity of the Medicago species to Mn and to the increase in pH was further shown by a 3-fold increase in the concentration of Mn in the shoots; in contrast, the concentration in the shoots of the subterranean clover cultivars increased by less than 2-fold. The effect of solution pH on the uptake of Cd2+has been studied using the Hurley system with perennial ryegrass, cocksfoot (Dactylis glomeratu), lettuce (Lactucu sutivu), and watercress (Rorippa nasturtium-aquaticum). The pH of the nutrient solutions was adjusted to 5.0, 5.5,6.0, and 7.0 when the plants were 4-5 weeks old and CdZ+was added to give 0.018 p M . In a growth period of 6 days, the total uptake of Cd2+by the four species increased by 4- to 10-fold as pH increased from 5.0 to 7.0. The specific uptake rate

UPTAKE OF MINERAL NUTRIENTS AND CROP GROWTH

207

(uptake per day per gram of dry weight of roots) was in the order watercress > cocksfoot > ryegrass > lettuce. The concentration of Cd in the shoots of the four species increased with pH and in the order watercress > cocksfoot > lettuce > ryegrass. The concentration in the roots also increased with pH but in a different order. It is notable that the concentration in the roots was always greater than in the corresponding shoots by 20- to 45-fold for ryegrass and cocksfoot and about two- to sixfold for lettuce and watercress (D. J. Hatch,L. H. P. Jones, and R. G. Burau, unpublished data). With soils the effect of raising pH, as by liming, is generally to reduce the uptake of Cd2' by plants. This may partly be due to a physiological effect of Ca2+but increased retention by soil colloids seems likely to have a bigger effect. 3. H+Efflux

In the work referred to in the previous section (III,F,2), the solution pH was kept constant by titration because net efflux of H' occurs when the net uptake of cations exceeds the net uptake of anions. With NH: as the source of N, Macduff et al., (1987d) showed with oilseed rape grown at 3, 7, 11, and 17°C over a 14-day period that net H' efflux closely balanced cation minus anion uptake (expressed as equivalents), and was equal to the net uptake of NH:. With NO; nutrition under similar conditions, the net efflux of OH- only roughly balanced anion minus cation uptake but was closely balanced by the difference between the amount of NO; that was assimilated and the increase in the carboxylate content of the plants. Net efflux of H+or OH- was calculated from the amounts of Ca(OH)2 or H2S04added to keep the nutrient solution at constant pH of 6.0. A similar method was used to calculate the net efflux of H' from alfalfa (Medicago sativa), red clover (Trifolium pratense), and white clover when dependent on symbiotically fixed nitrogen (when cation uptake exceeds that of anions), again grown in flowing nutrient solutions (Jarvis and Hatch, 1985a). The pattern of H' efflux over a 40-day period (Fig. 17) shows differences between the species at the early stages of growth which became less as efflux became less toward the end of the experimental period. The net efflux was closely correlated with excess cations and with N in the plants. The average ratio of acidity produced to N assimilated (expressed as equivalents) averaged 0.24 and was almost the same for the three species. The production of H' is rapid, as shown in Fig. 18, for a 30 hr period when the pH was allowed to fall. It demonstrates the requirement for frequent pH adjustment; in this experiment it was done automatically at 8-min intervals. Efflux has also been measured from roots of fodder rape plants grown in flowing solutions using a split root technique (Moorby et a[., 1985). There

208

A. WILD E T A L .

OL

c

35

I

L5 55 Days after sowing

65

1

75

FIG. 17. Rates of net H'efflux in solution calculated on basis of shoot weights of alfalfa red clover (0-o),and white clover (m-m) plants grown in flowing nutrient solutions

(0-o),

and dependent on symbiotic fixation of nitrogen. Data from Jarvis and Hatch (1985a).

was apparent efflux of HCO; in the presence of an adequate supply of NO; but when the supply was terminated to part of the root system there was immediate production of H' in that part. Efflux of H' was greater when the supply to the whole root system was terminated. The presence of indicator dye showed that efflux of H' and HCO; was not limited to the root tips but occurred over the whole root surface. ', whether caused by NH: nutrition, biological nitrogen fixaEfflux of H tion, NO; deprivation, or other causes, can significantly increase acidity in the soil rhizosphere. The effect of nutrient availability can be profound, as shown by increased availability of phosphate to rape plants [which received N as NO; in phosphate-deficient soil (Grinstead et al., 1982)], in which the pH of the rhizosphere decreased by 1.5-2 units (Hedley et al., 1982).

209

UPTAKE OF MINERAL NUTRIENTS AND CROP GROWTH

C

0

c

3 0

v

0900

I

1300

1700

2100

0100

0500

0900

1300

1700

Time ( h )

FIG. IS. Changes in pH of flowing nutrient solutions (with no automatic adjustment of pH) over a 30-hr period from 0900 hr on day 47 after sowing alfalfa (0-o), red clover ( 0 - O ) , and white clover (m-m). Data from Jarvis and Hatch (1985a).

4. Aluminum

For plants grown in soil, soluble A1 concentrations are more important that pH per se, it being well established that A1 ions reduce the growth of many species at concentrations above about 10-20 p M . The flowing nutrient solution system has been used to investigate the effect of Al on the growth of white clover when dependent on symbiotically fixed nitrogen. Nitrogenase activity, as measured by acetylene reduction (Fig. 19), and root extension were reduced with Al at 25 pikt initiation of nodules was reduced at 50 plkf but dry matter was not significantly reduced

210

A. WILD ET AL. a

b

0

I’

Fw.19. Effect of aluminum on nitrogenase activity (by acetylene reduction assay) of nodulated white clover roots; plants grown in flowing nutrient solution with A12(S0,), added to give 0 (0-0). 25 ( 0 - 0 ) . 50(0--0), and 1 0 0 ( ~ - ~ ) p M A l(a)Acetylenereductionmeasuredat . intervals during 90-min incubation on day 21 after addition of Al. (b)Acetylene reduction measured after 90-min incubation on days 7, 14, and 21 after addition of Al. Data from Jarvis and Hatch (1985).

UPTAKE OF MINERAL NUTRIENTS AND CROP GROWTH

211

at concentrations up to 100 ph4 (Jarvis and Hatch, 1985b). Under similar conditions, Jarvis and Hatch (1986) reported a rather greater reduction of growth of shoots and roots at 50 and 100 ph4 Al when the clover received NO; than when fixing nitrogen, but the difference between the two sources of nitrogen was small. They also reported that uptake of NO; ceased at 50 and 100 ph4Al and was considerably reduced at 25 ph4. This work is also of interest in showing that Al which had accumulated within the clover roots did not inhibit nodulation when the plants were transferred to an Al-free solution. As found in other work, A1 that accumulated in the roots reduced the transport of phosphate to shoots. There is a considerable literature on the effects of Al on plant growth (Adams, 1981), most of which relates to plants grown in soil or static solution systems. The flowing nutrient solution system is particularly useful for investigating effects of A1 because it allows plants to be grown at low phophate concentrations, and at controlled pH, which is required to prevent the precipitation of Al phosphate. It could be usefully employed in the selection of crop cultivars for growth in acid soils.

IV.

PARTITIONING OF PHOTOSYNTHATE BETWEEN SHOOTS AND ROOTS

Clarkson (1985) stated that “When some nutrients limit plant growth, notably N and P, the roots become relatively stronger sinks for carbohydrate”. This statement is supported by work using flowing solution culture. With NO; as the source of N, the ratio root weight : shoot weight of perennial ryegrass was higher at 1.43 pit4 and decreased over a range of concentrations up to 1.43 mM (Clement et al., 1978a). In another experiment with perennial ryegrass the yield of roots was greater when the supply of NO; was terminated than when it was continued, and again this resulted in a high root : shoot ratio (Clement et al., 1979). In a comparison between sources of N for perennial ryegrass, NO; gave a higher ratio than NH: at the same low external concentration (Jarvis, 1986). With phosphate, the root : shoot ratios for eight species were highest at low external concentration (0.04 ph4) and decreased to a constant value at 1-24 pit4 (Loneragan and Asher, 1967). Breeze et al. (1984) also found highest ratios with perennial ryegrass at low external concentrations of phosphate (0.04 and 0.1 pit4 in two experiments). The results with K are less consistent. Asher and Ozanne (1967) found that the root : shoot ratios of 14 species were generally

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highest at low external concentrations (1 or 8 jd4)and decreased at concentrations up to lo00 pA4 K'. With two species, however, rose clover (Trifoliumhirtum) and veldt grass (Ehrhartalongifolia),there was no effect on the ratio over the whole range of concentration. Similarly, Woodhouse (1977) found no consistent effect of the external concentration of K' (1.3-102 jd4)on the root:shoot ratios of barley, fodder radish, and perennial ryegrass. Spear et al. (1978a), in contrast, found the highest ratios with sunflower, maize, and 12 cultivars of cassava at 0.5-2 pit4 K' and lower ratios at higher concentrations. Root:shoot ratios are also affected by the pH and temperature of the nutrient solution. For example, Breeze et al. (1987) found that for white clover supplied with NO; or dependent on symbiotically fixed nitrogen, the ratio was higher at pH 4 than at pH 5 , 6, or 7. An influence of root temperature has been observed in flowing solution culture by GanmoreNeumann and Kafkafi (1980), Clarkson et al. (1986), and Macduff et al. (1987a). Results from three similar experiments with perennial ryegrass, oilseed rape, and barley are compared in Fig. 20. Plants of each species were acclimatized at a root temperature of 5°C for 14 days and had common shoot temperatures of 20/15 "C to 25/15 "C dayhight. The species differed widely in their partitioning strategy in response to change of root temperature in the range 3-25°C. The root:shoot ratios of perennial ryegrass, for example, were lower at 3 "C than at 25 "C, while the reverse was true for oilseed rape. These effects of the supply of nutrients on root:shoot ratios generally support earlier observations on plants grown in soil or static solutions (Brouwer, 1962a,b). The controlled conditions provided by flowing solution culture show that there are differences between species in their response to K and root temperature, whereas consistently high root:shoot ratios have been found in plants grown at low concentrations of NO; or phosphate. This latter observation differs from that found for localized application of high concentrations of phosphate and N (as NO; or NH:), but not K, which gives vigorous growth of lateral roots in the treated region (Drew, 1975). Except for plants which have large storage organs in their roots for carbohydrate, root dry weight usually accounts for about 10-20% of the total dry weight. This partitioning affects the efficiency with which the crop converts solar radiation into a saleable product. Models have been developed to describe it, e.g., Davidson (1969), Reynolds and Thornley (1982), and Johnson (1985). For plants growing in soil the acquisition of water and nutrients depends very largely on the spatial distribution of roots, but this consideration would take us away from the subject of this chapter.

213

UPTAKE OF MINERAL NUTRIENTS AND CROP GROWTH Oilseed mDe

.-0 c

e

0.20; 0

2

6

L

Days

of

I

I

I

I

8

10

12

1L

treatment

FIG.20. Effect of root temperature on root : shoot ratio (dry weight basis) of three species grown in flowing nutrient solutions, treatment period of 14 days, following acclimatization period of 14 days with root temperature at 5°C. Data for oilseed rape from Macduff ef al. (1987a), for perennial ryegrass from Clarkson et af. (1986), and for barley from J. H. Macduff (unpublished data).

214

A. WILD ET AL.

V.

CONCLUSIONS AND SUMMARY

One outcome of the work on flowing nutrient solutions is an improved understanding of the mechanism of nutrient uptake by plants. This arises from the following observations: 1. Nutrient uptake varies diurnally, the peak rate occurring 5-6 hr after the peak rate of CO,influx, which occurs at about 1200 GMT on a sunny day (see Figs. 3 and 4). Nutrient uptake also decreases quickly after the loss of photosynthetic tissue (see Fig. 2). These observations imply that nutrient uptake responds to metabolic demand resulting from the production of photosynthate. 2. The rate of nutrient uptake depends on the external concentration, but this dependence becomes less as plants age (see Figs. 10 and 11). Under steady-state conditions the nutrient flux becomes constant, or nearly so, down to low concentrations when plants are more than about 3-4 weeks old. At very low concentrations the measured nutrient flux can only be accounted for if absorption occurs close to the epidermis (see Table V). 3. Under nonsteady conditions the nutrient flux changes quickly after the external concentration is changed. There is an increased flux when the external concentration is increased even though the potential growth rate is achieved at the low concentration (see Fig. 13).

We account for the second and third of these observations by proposing that (1) nutrient uptake rate depends partly on the number and activity of ion transporters in the root cortex; (2) the number of transporters receiving nutrient depends on the external concentration and responds quickly to a change of concentration; and (3) the activity of transporters is controlled by the plant, probably in relation to metabolic demand and responds slowly to a change of concentration. This proposal fits our observations if (1) the activity of transporters is high at low external concentrations, though the number is small, and (2) the number is higher but activity is lower at high external concentrations. There is, additionally, the effect of supply of photosynthate, to which nutrient uptake responds, presumably through an effect on the activity of the transporters. The ultimate control of nutrient flux is still uncertain, as is the mechanism through which it acts. Evidence from flowing solution culture suggests that species differ in their response to a range of nutrient concentrations because of differences in potential RGR and the ratio root surface area:plant weight (see Figs. 6 and 7). These factors, and also the concentration of nutrient in plant tissues, explain why young plants (up to about 3-4 weeks old) require higher external nutrient concentrations than older plants.

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The relative response to NHf and NO; has also been investigated in flowing nutrient solutions. With oilseed rape and perennial ryegrass differences were small, but addition of small concentrations of NH: to NO; solutions increased RGR. The relative uptake of NH: and NO; depends on root temperature but the effect differs between species (see Fig. 14). No adequate explanation can yet be given. The effect of root temperature on growth rate also differs between species. There was no effect on oilseed rape in the range 3-25 “C but a large effect on perennial ryegrass and an intermediate effect on barley. The distribution of Cd, Cu, Pb, and Mn between roots and shoots has been measured in plants grown in flowing nutrient solutions at controlled metal concentrations. Lead is strongly held in roots, Cd and Cu less so, and Mn least. There are however, differences between species in uptake of the metals and distribution within plants. Further work is needed if the differences are to be understood. Flowing nutrient solutions incorporating a pH-stat have also been useful in providing net OH- and H’ efflux measurements. A time course, though not necessarily a cause and effect relation with K’ influx, has been demonstrated (see Fig. 4). Studies have also been made of the effects of Al and its interaction with phosphate on the rate of growth of white clover, its symbiotic nitrogen fixation, and its nitrogenase activity (Jarvis and Hatch, 1985b). Flowing nutrient solution culture is particularly useful for this work because in the dilute solutions used precipitation of Al phosphate is minimized. This chapter has been largely devoted to the more fundamental work on the mineral nutrition of whole plants using flowing nutrient solutions. Using plants grown under the controlled conditions that only flowing nutrient solutions can provide, there is, for the future, much potential in examining the mechanism of nutrient uptake, including the control systems within plants, the interrelationship with the supply of photosynthate and the effect of environmental conditions such as pH, temperature, and nutrient concentrations on new genotypes. There is also scope for investigating agronomic problems such as the effects on crop plants of salinity, the acidity “complex,” transfer of micronutrients and other elements within the plant, and the composition of the gaseous phase around roots. Symbiotic nitrogen fixation systems and mycorrhizal associations can be included, and plants can be grown to their reproductive phase. As gene transfer systems for crop plants are improved, gene expression in the growth and nutrition of plants will need to be understood. Flowing nutrient solutions provide the controlled conditions that will make this possible.

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Loneragan, J. F. 1968. Trans. Int. Congr. Soil Sci., 9th, Adelaide 2, 173-182. Loneragan, J. F., and Asher, C. J. 1967. Soil Sci. 103, 31 1-318. Loneragan, J. F., and Snowball, K. 1969. Aust. J. Agric. Res. 20,465-478, 479-490. Loneragan, J. F., Snowball, K., and Simmons, W. J. 1968. Ausf. J. Agric. Res. 19,845-857. Macduff, J. H., and Hopper, M. J. 1986. Plant Soil 91,303-306. Macduff, J. H., and Trim, F. E. 1986. Ann. Bot. (London) 57, 345-352. Macduff, J. H., Wild, A., Hopper, M. J., and Dhanoa, M. S. 1986. Plant Soil 94, 321-332. Macduff, J. H., Hopper, M. J., and Wild, A. 1987a. J. Exp. Bot. 38,42-52. Macduff, J. H., Hopper, M. J., Wild, A. 1987b. J. Exp. Bot. 38, 53-66. Macduff, J. H., Hopper, M. J., Wild, A., and Trim, F. E. 1987c. J. Exp. Bol. 38 (in press). Macduff, J. H., Hopper, M. J., Wild, A., and Trim, F. E. 1987d. J. Exp. Bol. 38 (in press). Moorby, H., and Nye, P. H. 1983. Plant Soil70, 151-154. Moorby, H., and Nye, P. H. 1984. Plant Soil 78, 283-294. Moorby, H., Nye, P. H., and White, R. E. 1985. Plant Soil 84, 403-415. Moorby, J., and Besford, R. T. 1983. In “lnorganic Plant Nutrition” (A. Lauchli and R. L. Bieleski, eds.). Encycl. Plant Physiol. New Ser. 15B,481-527. Nielsen, K. F., and Humphries, E. C. 1966. Soils Fertil. 29, 1-7. Nye, P. H. 1977. Soil Sci. 123,292-297. Nye, P. H., and Tinker, P. B. 1969. J. Appl. Ecol. 6 , 293-300. Nye, P. H., and Tinker, P. B. 1977. “Solute Movement in the Soil-Root System.” Blackwell, Oxford. Osmond, D. L., York, E. K., and Raper, C. D. 1981. Tobacco Sci. 25, 11-12. Peet, M. M., Raper, C. D., Tolley, L. C., and Robarge, W. P. 1985. J. Plant Nutr. 8,787-798. Pettersson, S. 1986. Physiol. Plant. 66, 122-128. Pitman, M. G.1975. In “Ion Transport in Plant Cells and Tissues’’ (D. A. Baker and J. L. Hall, eds.), pp. 267-308. North-Holland Publ., Amsterdam. Pitman, M. G. 1976. In “Transport in Plants IIB. Tissues and Organs” (G.Liittge and M. 0. Pitman, eds.). Encycl. Plant Physiol. New Ser. 2, 95-128. Raven, J. A. 1985. New Phytol. 101,25-77. Raven, J. A., and Smith, F. A. 1978. Plant Cell Environ. 1, 231-238. Reisenauer, H. M. 1969. Soil Sci. 108,350-363. Reisenauer, H . M., Clement, C. R., and Jones, L. H. P. 1982. Proc. Int. Plant Nutr. Colloq., 9th 2, 539-544. Reynolds, J. F., and Thornley, J. H. M. 1982. Ann. Bol. (London) 49, 585-597. Richards, S. J., Hagen, R. M., and McCalla, T. M. 1952. In “Soil Physical Conditions and Plant Growth” (B. T. Shaw, ed.), pp. 303-480. Academic Press, New York. Robson, A. D., and Loneragan, J. F. 1970. Aust. J. Agric. Res. 21, 223-232. Rufty, T. W., Raper, C. D., and Jackson, W. A. 1983. Bot. Gar. 144,466470. Russell, R. S., and Clarkson, D. T. 1976. In “Perspectives in Experimental Biology, Vol. 2: Botany” (N. Sunderland, ed.), pp. 401-41 1. Pergamon, Oxford. Shone, M. G.T., and Flood, A. V. 1985. Plant Cell Environ. 8, 300-315. Siddiqi, M. Y., and Glass, A. D. M. 1983. Can. J. Bot. 61,671-678. Siddiqi, M. Y., and Glass, A. D. M. 1986. Plant Physiol. 81, 1-17. Spear, S. N., Asher, C. J., and Edwards, D. G. 1978a. Field Crops Res. 1, 347-361, 363-373. Spear, S. N., Edwards, D. G., and Asher, C. J. 1978b. Field Crops Res. 1, 375-389. Spear, S. N., Edwards, D. G., and Asher, C. J. 1979. Field Crops Res. 2, 153-168. Tolley, L. C., and Raper, C. D. 1985. Plant Physiol. 78, 320-322. Tolley-Henry, L., and Raper, C. D. 1986. Plant Physiol. 82, 54-60. Van Egmond, F. 1978. In “Nitrogen in the Environment” (D. R. Neilsen and J. G.MacDonald, eds.), Vol. 2, pp. 171-189. Academic Press, New York. Went, F. W., 1953. Annu. Rev. Plant Physiol. 4, 347-362.

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Wild, A., and Breeze, V. G. 1982. In “Physiological Processes limiting Plant Productivity” (C. B. Johnson, ed.), pp. 331-334. Butterworths, England. Wild, A., Skarlou, V., Clement, C. R., and Snaydon, R. W. 1974. J. Appl. Ecol. 11, 801-812. Wild, A., Woodhouse, P. J., and Hopper, M. J. 1979. J. Exp. Bot. 30,697-704. Williams, R. F. 1948. Aust. J. Sci. Res. Ser. B1, 331-361. Woodhouse, P. J. 1977. Ph.D. thesis, Reading University. Woodhouse, P. J., Wild, A., and Clement, C. R. 1978. J. Exp. Bot. 29, 885-894. Wyn Jones, R. G. 1975. In “Ion Transport in Plant Cells and Tissues’’ (D. A. Baker and J. L. Hall, eds.), pp. 193-229. North-Holland Publ., Amsterdam.

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ADVANCES IN AGRONOMY, VOL. 41

MINERAL NUTRITION OF LINSEED AND FIBER FLAX Peter J. Hocking, Peter J. Randall, and Andrew Pinkerton Division of Piant Industry, Commonwealth Scientific and Industrial Research Organization (CSIRO) Black Mountain, Canberra, A.C.T. 2601, Australia

I.

INTRODUCTION

Flax (Linum usitatissimum L.) is one of about 100 species in the genus Linum (family Linaceae). Over many centuries, lines have been selected for fiber production (fiber flax) or for the oil content of their seeds (oilseed flax or linseed). The term linseed will be used in this review for oilseed flax. There are instances, however, where it has not been possible to ascertain from the literature whether linseed or fiber flax is referred to; in these cases we have used the term flax. We have also used flax in a generic sense when information applies to both linseed and fiber flax. It should be pointed out that in the North American literature the term flax refers almost invariably to linseed. The origin of cultivated flax is uncertain, but the crop probably came from the Near East, as it was grown for its fiber in Mesopotamia and Egypt at least 4O00 years ago (Dillman, 1936; Kipps, 1970). Later, appreciation of the high oil content of its seeds led to the selection of oilseed types. In general, fiber flax is tall and single-stemmed with poor seed production, while linseed is shorter and multibranched with high seed yields but poor fiber production and quality (Blackman and Bunting, 1951; Bailey and Soper, 1985). There are dual-purpose fiber and oilseed types, but their yields are lower than those of cultivars selected specifically for fiber or seed production (Martin et af., 1976). Flax fiber has traditionally been spun into linen yarns which are used in the manufacture of threads and twines of various kinds. The yarn is also woven into toweling, clothing fabric, table linen, and other textiles, but the increasing use of synthetic fabrics has eroded these applications somewhat. Fiber flax is used also in the manufacture of high-grade papers. The fiber from linseed is short and often harsh, so it is used in the manufacture of 22 1 Copyright 0 1987 by Academic Press, Inc. All rights of reproduction in any form reserved.

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commodities such as cigarette paper rather than for the production of linen (Martin et al., 1976; Carter, 1984). Approximately 12% of the world's linseed oil supply is obtained as a by-product from seeds of fiber flax (Bailey and Soper, 1985). Linseed is a traditional source of industrial vegetable oil, and it was the mainstay of the oil-based paint, varnish, and linoleum industries (Curteis, 1949; Martin et al., 1976). However, competition from plastics and acrylics has resulted in it becoming a relatively minor industrial oilseed crop (Matheson, 1976). Linseed is an industrial oil because of its high content of the fatty acid linolenic acid, which oxidizes rapidly and imparts a drying characteristic to the oil. The drying quality of the oil is indicated by its iodine value: the higher the value, the better the quality. The oxidation of linolenic acid also produces rancidity, so the oil is generally unsuitable for edible purposes (Green and Marshall, 1981). However, it has been used occasionally as an edible oil after expensive treatment involving conversion of the linolenic acid to linoleic acid. The seed meal which remains after the oil has been extracted is a valuable, high-protein stock feed (Martin et al., 1976).

Recently, experimental lines of high-quality, edible oil linseed have been developed in Australia (Green and Marshall, 1984; Green, 1986a,b). The oil from these lines is high in the desirable unsaturated linoleic acid (%63vo), but is virtually free of linolenic acid (%lVo). The polyunsaturated quality of the oil is better than that of rapeseed and comparable to sunflower oil (Table I). When the edible oil lines are released commercially, considerable Table I Fatty Acid Composition of Oil from Major Edible Oilseeds and a Standard Linseed Cultivar Compared to a Low Linolenic Acid Linseed Mutant Fatty acid composition (Yo) Oilseed

Palmitic

Stearic

Oleic

Linoleic

Linolenic

Safflower' Sunflower' Cottonseed' Peanut' Maize' Soybean' Rapeseed (canola)' Linseed' Linseed Zero mutantb

I 6 22 10 12 10 4 6

3 2 2 3 2 3 1 4 5

12 26 20 60 26 24 60 18 24

78 66 56 22 59 55 25 19 63

1 8 10 53 1

'Anonymous (1982). bGreen (1986b).

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expansion of linseed production is anticipated in Australia and elsewhere, especially in traditional wheat-growing areas with a predominantly winter rainfall which are therefore unsuitable for other oilseed crops such as sunflower. Edible oil linseed is likely to be grown on a wide range of soil types by farmers who have little or no previous experience with the nutritional requirements of the crop. This article reviews the mineral nutrition of linseed and fiber flax. The purposes of our review are to bring together and summarize the scattered information on the crops and to highlight areas for future research. Much of the published research deals with fiber flax, although at present it is not grown as widely as linseed. The following section on fiber flax and linseed production and agronomy is intended to provide background information pertinent to the mineral nutrition of the crops.

II.

PRODUCTION, CROP GROWTH, AND AGRONOMY A.

PRODUCTION

The total world production of linseed and fiber flax has declined steadily since 1945,and is currently about 6 x lo6ha of linseed and 2.0 x lo6ha of fiber flax (Bailey and Soper, 1985). The leading producers of fiber flax are the USSR, France, Belgium, and the Netherlands and of linseed Argentina, Canada, India, the USSR, and the United States of America (Matheson, 1976). The average world yield for dryland fiber flax is about 0.4 tons (metric) fiberha, and for linseed about 0.5 tons seed/ha with 3246% oil and 18-2670 protein. However, outstanding linseed yields in excess of 2.8 tons/ha have been achieved where moisture, fertilizer applications, and management practices (especially weed control) were optimal (Martin et al., 1976). Australia currently does not grow fiber flax. The area of linseed in Australia has expanded and contracted reflecting world prices and the supplies of other manufacturing oils (Scarisbrick et al., 1980). Annual production of linseed in Australia at present is about 5000 tons, and average yields are 0.8 tons seed/ha. B.

CROP GROWTH

Flax is a temperate winter-growing annual ranging in height from 20 cm to over 100 cm depending on the cultivar (Matheson, 1976). Seedling establishment is slow at the low temperatures experienced in winter in the Australian wheat belt, and the small seedling competes poorly with weeds. The plant initially has a distinct main stem, but four to seven axillary stems

224

PETER J. HOCKING E T A L .

(tillers) later arise from the compressed lowest nodes of the main stem. However, when it is sown densely, as for fiber flax production, only the main stem develops (Cutting, 1974). Shortly after establishment, the plant enters a semidormant vegetative stage during which leaf growth is slow, but root growth is active (Martin et al., 1976). After this stage, there is a period of from 45 to 60 days of rapid vegetative growth and tiller elongation. Leaves, though numerous, are small and the leaf area per plant at flowering is among the lowest of any crop species (Matheson, 1976). Flax is fairly shallow rooted, with a short taproot which branches freely and penetrates about 1.2 m down the profile in light-textured soils (Matheson, 1976). Under optimum conditions, flowering is indeterminate and continues until vegetative growth ceases. Under field conditions, however, growth is usually restricted by lack of water or nutrients, so that the plant is semideterminate and flowers over a period of 15-20 days. Flowers are predominantly self-pollinated, and pollination mainly occurs during midmorning. Some linseed cultivars produce flowers and capsules in distinct cycles (Yermanos and Worker, 1964), whereas others have a single continuous flowering period (Chopde and Thakre, 1969). The pattern of flower production in flax appears to be genetically controlled (Davidson and Yermanos, 1965), but environmental conditions may modify the extent and timing of the flowering cycles (Knowles et al., 1959; Dybing, 1964; Hovland and Dybing, 1973). Each flower produces a capsule (termed a boll in North America) which potentially contains 10 seeds (Matheson, 1976), but in only a small proportion of the mature capsules does the full complement of 10 seeds develop (Martin et al., 1976). Commercial maturity of seeds occurs from 30 to 60 days after flowering, and mature seed color is variable: cream, yellow, or dark tan depending on the cultivar (Cutting, 1974). Seed size varies markedly between and within cultivars, depending on seasonal conditions, but seeds of fiber flax are invariably small whereas those of linseed can be either large or small. The growth cycle of the crop takes from 150 to 180 days (Matheson, 1976), and linseed is usually harvested in early to midsummer (December to January) in Australia, depending on locality. In North America, linseed is generally harvested when about 90% of the capsules have turned brown, as delay may result in problems with weeds (J. T. Moraghan, personal communication).

c.

OUTLINE OF AGRONOMY

In Australia, linseed competes with temperate cereals, especially wheat, for a place in rotations based on subterranean clover (Trifolium subterraneum L.) pasture, and it also competes with rapeseed as a winter oilseed crop (Matheson, 1976). Linseed has proved a useful disease break crop in cereal/pasture rotations (Bunting, 1969). Linseed and fiber flax tolerate a

MINERAL NUTRITION OF LINSEED AND FIBER FLAX

225

wide range of soil types but do best on fertile, well-drained, medium- to heavytextured soils, especially silty or clay loams of about pH 6 (Sinha and Saxena, 1965a).They are generally grown where the annual rainfall is 4-75 cm, but also grow well under irrigation. Fiber flax requires high levels of soil moisture as well as cool conditions during the growing season, but from maturity, dry warm weather facilitates harvesting, curing, and drying after retting (Martin et al., 1976). Linseed prefers moderate to cool conditions, particularly during seed filling, and is thus essentially a crop for temperate regions although some cultivars are suitable for the subtropics (Martin et al., 1976). Linseed is more tolerant of moisture stress than many oilseed crops (Bailey and Soper, 1985)but has a greater water requirement than wheat and, because of its shallow root system, depends for moistureon the top 60cm or so of the soil profile (Martinet al., 1976). Linseed is particularly susceptible to water stress at the seedling stage, at flowering, and during early seed development (Martin et al., 1976). Rainfall or irrigation late in the season can result in a flush of new tillers, which cause problems at harvest because the capsules from the late tillering are green and immature when the earlier-formed capsules are mature (Cutting, 1974). Temperatures above 30T during flowering and capsule development reduce seed number and shorten the period of seed filling, thereby reducing seed size, oil content, and oil quality (Price, 1968).Young plants are susceptibleto frost at tiller elongation, and linseed is particularly susceptibleto frost at the green capsule stage (Matheson, 1976). Linseed can tolerate mild waterlogging (Anonymous, 1984), but waterlogging increases its sensitivity to frost (Millikan, 1946a)and may cause or exacerbate nutritional problems (Millikan, 1951a; Olomu and Racz, 1974). Because of the small seed size and poor vigor of flax seedlings, a finely prepared, weed-free, moist seedbed is necessary (Clydesdale and Burns, 1948; Patton, 1965). Linseed is particularly susceptible to competition from weeds, and improved weed control has been a major factor contributingto the yield increases achieved during the past 20 years in North America (J. T. Moraghan, personal communication). Flax seedlings are less able than cereal seedlings to force their way through the surface crust formed after heavy rainfall and subsequent drying (Bailey and Soper, 1985). Sowing rates vary from 20 to 50 k g h a, depending on reliabilityof rainfall and soil conditions. In general, early sowings produce higher linseed yields and better quality oil than late sowings (Matheson, 1976), because of the moister, cooler growing conditions.

111.

EFFECTS OF MINERAL NUTRITION ON YIELD A.

GENERAL CONSIDERATIONS

The nutrition of fiber flax and linseed cannot be considered in isolation from other aspects of their agronomy. Yields vary greatly because both

226

PETER J. HOCKING ET AL.

crops are grown under such a wide range of environments, agronomic practices, and economic circumstances. Fertilizer requirements should therefore be considered in relation to the yield expected from a particular combination of these practices and circumstances. Response to a particular nutrient may be masked by the lack of other nutrients, water or temperature stresses, weeds, or insect pests and diseases. In general, nutrition affects yield quantity to a far greater extent than it affects seed or fiber quality. The nutrient elements to be reviewed are nitrogen (N), phosphorus (P), sulfur (S), potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), and molybdenum (Mo). Sodium (Na) and chlorine (Cl) will be considered because they occur in saline soils and may interact with the uptake and partitioning of other mineral nutrients in plants. Toxicities of Mn and, especially, aluminium (A) will also be considered in relation to soil acidity. Most of the reported research on the nutrition of linseed and fiber flax concerns N and P and, to a lesser extent, K. There is also a considerable, but often restricted, range of information on other nutrients (Millikan, 195la; Ukrainetz e t a / . , 1975). Because flax was used as a test plant in a number of early studies on the roles of minerals in plants, deficiency and toxicity symptoms have been particularly well described for some of the micronutrients (Millikan, 1951a). TO FERTILIZERS B. YIELD RESPONSES

I. Major Nutrients There are many reports of positive yield responses by fiber flax and linseed to applications of N, P, and K fertilizers. However, the results of fertilizer trials are sometimes inconsistent, especially with N (e.g., MacIsaac and McGregor, 1941; Molberg, 1961; Culbertson et al., 1961; Puhr, 1962) or P (Ali-Mohammad, 1930; Granhall and Larsson 1946, 1948; Blackman and Bunting, 1951; Mathur e t a / . , 1958). Much of this variation in response is probably due to the interactive effects on yield of the initial nutrient status of the soil, seasonal factors, and soil moisture availability (Opitz, 1943; Matheson, 1976). According to Milthorpe (1943, 1946), N and P are the most important nutrients for fiber flax yields; the effect of K is not so important, as its influence depends on the ratio of the other nutrients. Cooke and Warren (1959) reviewed an extensive series of fertilizer trials with fiber flax in Britain and reported that overall responses to N fertilizer were small ( ~ 5 V o for shoot yields and 2% for fiber). The responses were more frequent and larger after several years of cropping, when presumably soil N had been depleted, than directly after pasture. Responses to P were frequent at sites

MINERAL NUTRITION OF LINSEED AND FIBER FLAX

227

where the citric acid-soluble P was below 22 mg/kg soil, and responses to K were small and infrequent except at sites where values for acetic acid-soluble K were below 80 mg/kg soil. Linseed, like fiber flax, is not considered particularly sensitive to the P or K status of the soil as yield responses to these fertilizers are often small (Bunting, 1969), nor does it appear to have a high requirement for N (Robinson and Weidemann, 1932; Landon, 1934; Anderson and Tedin, 1945; Blackman and Bunting, 1951). However, many of the soils in Australia suitable for flax production are much lower in N and P than North American or European soils, and consequently, substantial amounts of N and P fertilizers may be required for maximum yields (Milthorpe, 1943). Linseed responds well to high residual soil fertility of N and P but will often give a poor response to direct application of these nutrients in fertilizer (McGregor, 1960). Where N, P, and K fertilizers are required, the rates applied to achieve maximum yields vary greatly between localities and countries. In general, fertilizer recommendations for flax are 20-60 kg N/ha (Opitz, 1943; Mathur et al., 1958; Negi and Kingra, 1959; Bunting, 1969; Gad and ElFarouk, 1978), 10-30 kg P/ha, and 20-30 kg K/ha (Aukema and Friederich, 1959; Khan and Gupta, 1959; Larsen, 1960; Friederich, 1962; Khan et al., 1963; Rodewald and Ulbricht, 1963; Bhan and Singh, 1973; Deineka, 1974; Sin et al., 1975; Andrushkiv et al., 1976; Singh and Singh, 1978). In North America, N fertilizer rates for dryland linseed are based on levels of nitrate N in the upper 60 cm of the soil profile (Ukrainetz et al., 1975). Small applications of N and P to flax on responsive sites usually have a greater effect on straw yield than on seed yield (Opitz, 1941). High N fertilizer applications may give high yields, but often without improving the net economic returns (Table 11) (Gauca and Les, 1976; Gad and El-Farouk, 1978; Les and Popirlan, 1981; Nayital and Singh, 1984). Singh and Singh (1978) showed that applications of up to 40 kg N/ha and 9 kg P/ha increased the yield and water use efficiency of dryland linseed in India (Table 111) and gave the best response in terms of seed produced per kg N or P applied. Higher rates of N depressed oil yields, mainly because of reduced seed yield. Excessively high applications of N fertilizer can cause problems because rank crop growth results in lodging and poor fiber yields in fiber flax (Dempsey, 1975) and poor capsule production in linseed (Elliott, 1959; Larsen, 1960; Andrushkiv el al., 1979). Nevertheless, high rates of N can increase yields of fiber flax without lodging (Les et al., 1977), even under irrigation (Basu and Bose, 1974; Rowland, 1980). Hamdi et al. (1971) recommend 72 kg N/ha, 15 kg P/ha, and 48 kg K/ha for dual-purpose flax under irrigation. In Australia, 80 kg N/ha and 20 kg P/ha are recommended for an irrigated linseed crop which yields about 1.2 tons seed/ha (Wetselaar et al., 1968; Douglas and Wade, 1985a).

228

PETER J. HOCKING ET AL.

Table I1 Effect of Nitrogen Fertilizer Rates on the Yield and Profitability of Irrigated Linseed

N applied (kg/ha)

Seed yield (kglha)

Net return per rupee invested in fertilizer N

(Rs)

Reference

-

0 33.6 67.2 100.8

618 722 848 893

2.51 2.88 2.11

0 30 60 90

628 751 1097 1106

3.35 1.43 4.19

Kinra el al. (1970)'

Nayital and Singh (1984)b

'Tield plot trials at Kanpur, India. Pooled means for three seasons. bField experiments over two seasons, northwestern Himalayas, India.

Table 111 Effects of Nitrogen and Phosphorus Fertilizers on Yield and Water Use Efficiency of Rain-Fed Linseed",' Fertilizer applied (kg/ha)

Seed yield (tondha)

Oil yield (kglha)

0 20 40 60 SEC

1.13 1.25 1.51 1.31 0.0s

467 510 606 521

0 9 18 27

1.20 1.41 1.32 1.26 0.05

482 567 538 515

N

P

SEC

Response (kg grain/ kg nutrient applied)

6.0 9.5 3.0

-

23.3 6.7 2.2

Water use efficiency (kg seed/mm/ha)

5.4 6.1 7.4 6.4

5.8 6.9 6.4 6.1

"Adapted from Singh and Singh (1978). bField experiments near Varanasi, India. Values are means of two seasons. 'SE, Standard error of the mean.

MINERAL NUTRITION OF LINSEED AND FIBER FLAX

229

The form of the N applied seems occasionally to affect yields, but conflicting results have been reported. Derebon and Stetsenko (1974) compared the response of fiber flax to equivalent rates of six forms of N fertilizer and obtained the highest fiber yield when N was applied as liquid ammonia. Although calcium nitrate usually stimulates greater initial growth of fiber flax than equal rates of N as ammonium sulfate (Maddens, 1976b), it may promote such rank growth that the crop later lodges, thereby reducing fiber yield (Lahola, 1976; Maddens, 1976b; Bockstaele, 1977). Ammonium nitrate often gives the best fiber yields (Maddens, 1976b) and seems to be a satisfactory compromise between the ammonium and nitrate forms of N. Dai and Zhang (1981) found that ammonium N gave better linseed yields than nitrate N. Cantar and Pesteanu (1974) found that linseed responded best to urea N in some seasons, but that ammonium N and nitrate N were just as good in other seasons. However, there are many instances in which the form of the applied N had little or no effect on fiber or seed yield (e.g., El-Damaty and El-Kobbia, 1956; Beech and Norman, 1968; Andrushkiv et al., 1976; Maddens, 1976a; Bockstaele, 1977). The yield response of flax to N fertilizer may be modified also by soil pH, as Saxena and Sinha (1966) found that the rate of ammonium nitrate application which was optimum for fiber flax at pH 6.0 depressed growth and yield at pH 8.5-9.0 because of decreased N uptake. Placement of N (Mathur et al., 1958) and P (Neenan and Devereux, 1973) fertilizers below the soil surface generally gives a better yield response than broadcasting them on the surface. However, even moderate levels of fertilizer placed with flax seeds at sowing may result in both delayed and poor seedling emergence. For example, Molberg (1961) found that 20 kg N/ha as ammonium nitrate placed with the seeds reduced seedling emergence by 27%, and by 49% when the level of N was increased to 40 kg/ha. Nyborg (1961) and Mason (1971) showed that germinating linseed was far more susceptible than rapeseed or winter cereals to injury from ammonium nitrate and triple superphosphate placed with the seeds, and the injurious effects were more severe when either soil moisture or soil temperature was low. Placement of N fertilizer about 2.5 cm below or t o one side of the row is recommended to avoid injury to germinating seeds (Table IV) (Nyborg and Hennig, 1969). High rates of granular urea N applied at sowing seem to be particularly detrimental to seedling emergence, especially in sandy soils (Maddens, 1976b; Bockstaele, 1977), whereas the less mobile ammonium N is not as toxic (Lahola, 1976). It is likely that the death of germinating seedlings is due to the toxic effects of free ammonia formed during hydrolysis of the urea (Cooke, 1962) and to the accumulation of toxic levels of nitrite N in the seed zone (Court et al., 1962, 1964; Gasser, 1965). Nyborg (1961) and Mason (1971) have recommended ammonium nitrate as the most suitable

230

PETER J. HOCKING ET AL.

Table IV Effect of Banding Nitrogen and Phosphorus Fertilizers in the Row or below the Row on Plant Establishment and Yield of Linseed'*b*C Fertilizer N

Rate (kg/ha)

Placement

0

In row

34

P

0 7 29

Seed yield (tonslha)

49.5 c 51.0 c

0.68 bc 0.66 b

In row Below In row Below

28.2 b 50.2 c

0.64 b 0.67 b

15.9 a 48.2 c

0.58 a 0.71 c

In row Below In row Below In row Below

54.1 c 51.3 bc

0.97 ab

37.7 b 42.5 b

1.21 b 1.32 b

6.1 a 39.8 b

0.79 a 1.59 c

Below 17

Number of plants per meter of row

1.06 ab

'Adapted from Nyborg and Hennig (1969). bValues are means of four field trials in western Canada. For each character within a nutrient, values followed by the same letter do not differ at p = 0.05. 'Fertilizer placed 2.5 cm below row. N applied as ammonium nitrate; P applied as triple superphosphate.

N fertilizer for linseed at sowing because it is less damaging to germinating seeds than urea, but even so, it should not be placed in the seed furrow. Mono- and diammonium phosphate may reduce linseed emergence if banded with the seeds, and it seems that the N component of these fertilizers is more detrimental than the P (Nyborg and Hennig, 1969; Douglas and Wade, 1985b). Flax seeds are also very sensitive to P fertilizer (Sadler, 1980), and yields are generally reduced by the application of more than 5 kg/ha of watersoluble orthophosphate P fertilizer placed with seeds, because it suppresses germination (Racz, 1967; Ridley and Tayakepisuthe, 1974). Placing P fertilizer 2.0-2.5 cm directly below the seeds gives better yields than when it is banded 1.5-2.5 cm to the side or placed in the row with the seeds (Table IV) (Nyborg and Hennig, 1969; Sadler, 1980) or uniformly mixed in the top 5-10 cm of the soil (Pandey and Sinha, 1969). It appears that placement of P fertilizer at seeding needs to be very precise to obtain maximum uptake and yield responses. For example, Sadler and Bailey (1981) showed that the efficiency of uptake by linseed of P applied to a P-deficient, noncalcareous soil was 22.6% when 10 kg P/ha was placed 2.0 cm below the seeds, but

MINERAL NUTRITION OF LINSEED AND FIBER FLAX

23 1

only 16.1(70 when the P was placed 2.0 cm below the seeds and 1.5 cm to one side of the row. Placement is even more critical in calcareous soils for response to fertilizer P, because of the conversion of plant-available P to less available forms (Sadler, 1980). Root studies have indicated that this requirement for precise placement of P fertilizer to maximize linseed yield responses was largely due to the pattern of early root development. During the first few weeks of seedling growth, the root system is restricted to a small cylinder of soil (about 2 cm in radius) immediately surrounding the tap root (Sadler, 1980). Consequently, correct placement of P in relation to the seeds is critical not only to minimize injury to germinating seeds, but also to maximize crop response to the applied P (Sadler, 1980). Germinating linseed appears to be sensitive to K fertilizer, as Bailey (1967) found that placement of 9.3 kg K/ha as potassium sulfate with the seeds gave a significant yield increase, but 18.6 kg K/ha placed with the seeds resulted in poor plant emergence and yield decreases of as much as 20%. In contrast, McConaghy and McAllister (1964) observed that 35 kg K/ha as potassium chloride drilled with the seed had no detrimental effect on the emergence of fiber flax seedlings. These apparently conflicting results, however, may be due to varietal differences in sensitivity of germinating seeds to K, or to the form of K fertilizer used. According to Gupta et al. (1961), application of N in one dressing or in split dressings seems to have similar effects on flax yields. Maddens (1972) has recommended that N should be applied in a single application at seedbed preparation, but that extra N could be applied as a spray of 5% urea solution if the plants appeared N deficient. However, Pate1 (1977) obtained the highest linseed yields when N and P were applied in three equal dressings, one drilled at sowing and the others as foliar sprays after emergence. Kinra et al. (1970) found that over a 3-year period, average yields of linseed were increased by split applications of 67.2 kg N/ha as ammonium sulfate, half applied at sowing, and the remainder at the first irrigation. Peive and Andrianova (1939) obtained the highest yield of fiber flax when half the N fertilizer was applied just before the stage of rapid shoot elongation and the rest at flowering. Consequently, it seems that on balance, split N applications can improve yields, although a late application to irrigated linseed may cause problems at harvest if it stimulates the production of new tillers which bear green capsules when the bulk of the crop is ready for stripping. However, for long-season (2OO-day), irrigated linseed, application of N fertilizer when the crop is in bloom is recommended to stimulate a second flush of capsules and thus increase yield. Curtailment of irrigation at an appropriate time can be used to control capsule maturity (Knowles et al., 1959). Linseed is somewhat unusual in that there is apparently little change in root:shoot dry weight ratios as the N supply increases from deficient to luxurious. In other crop species, such as barley and corn, the shoot : root ratio

PETER J. HOCKING ET AL.

232

increases substantially with increased N supply (Turner, 1922, 1926). Similarly, concentrations of total N in linseed shoots vary surprisingly little with N supply, although large differences were recorded for N concentrations in barley shoots (Turner, 1922). However, the harvest index of linseed increases with increased availability of major nutrients such as N and P , despite the apparent lack of change in root:shoot ratios. Results of a comparative study showed that the harvest index of linseed and soybean increased with improved P nutrition, whereas that of rapeseed did not change, even at very high rates of applied P (Table V) (Bailey and Soper, 1985).

Where weed control is poor, N and P fertilizers may stimulate weed growth, with deleterious effects on yield (Culbertson et al., 1961), especially if weed competition is severe when the crop is at the seedling stage. Applications of N, P, and K fertilizers at moderate rates can lessen the susceptibility of flax to infection by fusarium wilt (Fusarium Iinii) on acidic and nutrientpoor soils, the beneficial effect of the fertilizers being particularly noticeable in wet seasons (Sadowski, 1968). Responses of fiber flax and linseed to K fertilizer have often been negligible or variable, probably because the crops are generally grown in finetextured soils that have adequate supplies of K (Bailey and Soper, 1985). For example, Shaaban et al. (1982) found that application of K fertilizer to fiber flax did not increase plant height or shoot dry matter, and Lewis (1943) found no effect of K fertilizer on fiber development or yield. Potassium deficiency, however, has been observed in northwestern Europe, and applications of K fertilizer to fiber flax have increased fiber yields in Table V Effect of Phosphorus Fertilizer on the Harvest Index of Selected Oilseed CropsaBb Harvest indexC P applied (kg/ha) 0 20

40 60 120

Linseed

Rapeseed

Soybean

21.7 21.6 19.1 28.0 31.2

21.7 21.7 22.8 21.7 22.1

30.6 31.8 37.5 36.9 41.9

"From Bailey and Soper (1985). bField experiment, Manitoba, Canada. P was placed 2.5 cm directly below the seed; basal N applied at 100 kg/ha. CHarvest index defined as (seed mass x 100)+(total aboveground mass).

MINERAL NUTRITION OF LINSEED AND FIBER FLAX

233

Denmark (Larsen, 1960) and Sweden (Granhall and Larsson, 1945, 1948). Applications of K fertilizer have also increased yields of fiber flax grown on K-deficient derno-podsolic soils in the Moscow region of the USSR (Panitkin et al., 1980). If the soil is low in N and P, however, application of K fertilizer to fiber flax may have a negligible or even negative effect on fiber yield (Milthorpe, 1946; Frederiksen, 1954a). Increased yields of linseed have been reported from western Canada in response to K fertilizer rates of up to 130 kg/ha applied to soils in which the exchangeable K was 115 mg/kg soil or less (Bailey, 1967). There is a good probability that linseed will respond to applications of K fertilizer if the ammonium acetate-exchangeable K' is less than 200 mg/kg soil (Bailey, 1967). Recommended rates of K fertilizer for linseed in western Canada are relatively low, however, and range from 30-60 kg/ha, reflecting the generally modest yield responses achieved (Bailey, 1967). In Britain, yield responses to K fertilizer by flax were found rarely except at sites where the acetic acid-soluble K was below 80 mg/kg soil (Cooke and Warren, 1959). There is remarkably little information on yield responses of flax to the application of Ca, Mg, and particularly S fertilizers. Most of the work with Ca has been related to the application of lime to correct soil acidity and its associated nutritional disorders and imbalances, rather than to correction of Ca deficiency itself. In the USSR, applications of magnesium sulfate to acidic soils low in Mg have increased fiber and seed yields of flax (Anspok, 1975). However, high applications of Mg can depress fiber yields (Milthorpe, 1946). Yield responses of linseed to S fertilizer have been obtained in pot experiments with S-deficient soils in Australia (Anderson and Spencer, 1950b), and India (Tandon, 1984). Some of the soils suitable for growing linseed in Australia are recognized as S-deficient for pastures (Williams and Andrew, 1970), so it is surprising that there are no reports of yield responses by linseed to S in the field. This is probably due to the extensive use in Australia of single superphosphate (containing 11070 S) to correct widespread P deficiency. In the northern hemisphere, the high accessions of S in rainfall (Ryaboshapko, 1983) and the use of fertilizers with adventitious S may provide sufficient S to mask any inherent deficiency in the soil. However, Ukrainetz et al. (1975) state that a yield response can be expected for linseed in Saskatchewan, Canada, when the sulfate-S content in the soil to a depth of 60 cm is less than 25 kg/ha. In practice, N, P, and K fertilizers are often applied in combination, but reported optimum N:P:K ratios vary considerably. For example, Scheel (1938) obtained the highest fiber yields with a combination of lN:lP:O.SK, whereas Kondratowicz (1970a,b) obtained the highest fiber yields and best quality fiber when 40 kg N/ha was combined with P and K in the ratio of lN:0.75P:1.5K. Positive interactions have been reported between N and P

PETER J. HOCKING ET AL.

234

on seed and fiber yields of field-grown dual-purpose flax (Pande et al., 1970), and a positive interaction has been reported between N and S on the dry matter yield of linseed grown in pot culture in soils deficient in both N and S (Anderson and Spencer, 1950b). However, Douglas and Wade (1985a) reported no interaction between N and P, and Dalal and Gill (1966) found no interaction among N, P, and K on the yield of linseed. Little work has been done on the interaction of plant density and nutrient supply on the yield of either fiber flax or linseed. However, field trials with linseed in Britain did not reveal any significant interaction between plant density and N, P, and K fertilizers on yield, when the fertilizers were applied either singly or in combination (Blackman and Bunting, 1951). Gad and ElFarouk (1978) found no interaction between seeding rates and N fertilizer levels on the yield of dual-purpose flax over two seasons in Egypt. The beneficial effects of adequate and balanced fertilizer applications to flax may be manifested in the subsequent generation of plants. Gritsenko and Batenchuk (1975) found that seeds from fiber flax plants which received adequate N, P, and K fertilizer gave slightly higher yields in the next generation than seeds from plants which received no fertilizer. Nutrient-induced changes affecting later generations are discussed further in Section VI1,A. 2. Micronutrients

There are few quantitative data in the literature showing yield responses of flax to applications of micronutrients. In Western Australia, application of 11.2 k g h a of zinc sulfate to fiber flax increased fiber yield by almost 40% (Cass-Smith and Harvey, 1948). In South Australia, aerial applications of aqueous ferrous sulfate to linseed grown on calcareous black clay soils have resulted in marked yield increases @. C. Lewis, personal communication). A few yield responses of fiber flax have been reported in the USSR to ByMo, and Zn, either applied singly or included with N, P, and K fertilizers (Peive and Andrianova, 1939; Barsukov and Leonenko, 1982), and to B and Cu applied to peaty soils in the Ukraine (Lesik and Chepikov, 1966). In northern India, application of Mn as sulfate to calcareous soils has increased linseed yields (Khan and Gupta, 1959). Shekhawat et al. (1971) reported a 70% increase in linseed yield due to Cu (10 kgha) as sulfate applied to a black alkaline soil in Rajasthan, and applications of ByCu, and Mn to alkaline soils in central India have increased linseed yields, although the effects of the applications varied considerably according to soil moisture and seasonal conditions (Dastur and Bhatt, 1965a).

c.

UPTAKE AND DISTRIBUTION OF NUTRIENTS I . Major Nutrients

a. Uptake. Flax can remove appreciable quantities of nutrients from the soil, although removals may not be as great as for some other crops.

235

MINERAL NUTRITION OF LINSEED AND FIBER FLAX

Gulyakin and Deryugin (1975) found that mature fiber flax shoots from adequately fertilized plots contained 84 kg N/ha, 34 kg P/ha, and 86 kg K/ha or a ratio of lN:0.4P:lK. The dual-purpose cultivar Giza4 accumulated 47 kg N/ha, 19 kg P/ha, and 57 kg K/ha to produce 1 ton/ha of fiber and 283 kg oil/ha (Hamdi et al., 1971). Dual-purpose flax removes more nutrients from the soil than linseed because the whole shoot is removed at harvest (Table VI) @astur and Bhatt, 1965b), but with linseed, only the seed capsules are removed and crop residues, if not burnt, return substantial quantities of nutrients to the soil. Thus, approximately 30Vo of the N taken up by a linseed crop is returned to the soil in the crop residue (Harbison et a/., 1986), and although a high-yielding linseed crop may contain up to 150 kg K/ha in its mature shoots, the proportion of this K removed in the seeds at harvest can be as low as 5 % (Bailey and Soper, 1985). It was once widely believed that flax crops depleted the soil of plant nutrients to a much greater extent than cereals, and this belief is still held by some farmers. However, the reputation flax gained as a “robber” crop was probably due to the buildup of diseases and weeds when it was grown in the same field for more than one season (Knowles et al., 1959), rather than to the depletion of soil nutrients, as the quantities of nutrients removed in the economic yield are fairly modest. For example, Lewis (1943) showed that fiber flax removed only slightly more nutrients than a crop of barley, and Clagget et al. (1952) concluded from a study of nutrient removal by average-yielding crops in Minnesota and North Dakota that linseed removed less N, P, K, and S per hectare than grain of wheat, oats, or barley. In Manitoba, Racz et al. (1965) showed that linseed removed similar amounts of N and P to wheat, but less than rapeseed (Fig. 1). Nutrient removals in seeds from their highest yielding treatments were 36, 38, and 82 kg/ha N and 6.1, 6.3, and 14.5 kg/ha P for linseed, wheat, and rapeseed, respectively. Table VI Amounts of Major Nutrients Removed from Soil by Dual-Purpose Flaxueb Amount of nutrient (kg/ha) Nutrient

In mature plants

Returned to soil in fallen leaves

Net loss from soil

Net loss from soil as a percentage of total amount taken up by plants

55.0 11.6 34.8 45.0 9.8

5.4 1.8 9.5 13.6 1.8

49.6 9.8 25.3 31.4 8.0

90 84 73 70 82

~~

N P

K

Ca Mg ~~

~

Trom Dastur and Bhatt (1965b). bPlants grown in field, Madhya Pradesh, India. Values are means of five cultivars.

236

PETER J. HOCKING ET AL.

-

i

m

r

150-

Y m +. n 3

I.-9

18t 12-

(u

Y

100-

/fA/

6-

L/A’

I/ 00

I

30

60

I

90

I

120

/

00

1

30

60

A

0 rapeseed

wheat A

flax

I

90

120

Days after sowing

FIG.1. Uptake of (a) N and (b) P by linseed, rapeseed, and wheat grown on a fallow soil treated with 67.2 kg N/ha and 22.4 kg P/ha. (Adapted from Racz eta/., 1965.)

Although rates of N and P uptake by linseed were lower than by wheat, linseed took up N and P over a longer period of its growth cycle (Fig. 1). Evans et al. (1987), however, showed that dryland linseed ceased to take up N earlier than wheat when no fertilizer N was applied but had a similar period of uptake when ammonium nitrate was added to the soil. In either case, linseed took up less N than wheat. There have been instances, though, in which nutrient removal from the soil by linseed was considerably greater than by wheat or rapeseed. Harbison et af. (1986) assessed N removal by crops on the Darling Downs in Queensland and found that a linseed crop which yielded 2.0 tons grain/ha removed 56 kg N/ha, compared with grain yields of 2.6 and 1.2 tons/ha and N removals of 49 and 40 kg/ha for winter wheat and rapeseed, respectively. Although linseed is grown successfully on neutral to alkaline soils, such as those of the Canadian prairies, it has often shown relatively low phosphate uptake from such soils. In pot experiments, linseed took up less P than rapeseed, soybean, or wheat from alkaline sandy or clay loams low in available P (Table VII) (Kalra and Soper, 1968; Soper and Kalra, 1969; Kalra, 1971). It was particularly poor at recovering added fertilizer P and recovered much less P from fertilizer pellets than wheat, buckwheat, or rapeseed (Kalra, 1973; Strong and Soper, 1973). Sadler (1980) observed that linseed root development was slower and less extensive in a calcareous soil of pH 7.8 than in a noncalcareous soil of pH 6.8 and suggested that this was partly the cause of the low P uptake by linseed from alkaline soils. Soil pH

237

MINERAL NUTRITION OF LINSEED AND FIBER FLAX

Table VII Effect of Phosphorus Fertilizer on the Yield and Phosphorus Concentration of Selected Oilseed Crops and Wheat, and the Efficiency of Phosphorus Uptake from FertilizeP,*J -

Yield (g/pot)

Shoot P concentration ( d k g dry wt)

Fertilizer P uptake by plants

Crop

+P

-P

+P

-P

(%)

Rapeseed Mustard Sunflower Soybean Wheat Linseed LSD~

3.6 4.2 3.6 2.9 1.3 1.3 0.7

1.o 2.6 2.8 2.3 1 .o 1.2 0.5

2.7

1.7 1.8 1.6 2.4 2.7 2.4 0.5

39.8 37.9 14.5 13.1 10.8 7.5 4.3

2.2 1.9 2.8 2.9 3.1 0.4

'From Kalra (1971). bGreenhouse pot experiment. Plants harvested 48 days after sowing. %ach pot contained 2 kg soil. NaHC0,-extractable P of soil was 3.3 mg/kg; soil pH was 7.7. Fertilizer P (20 mg per pot) labeled with NaH, "PO, was applied in solution as a band 1.3 cm below seeds. dLSD, Least significant difference ( p = 0.05).

can also affect N uptake. For example, Sinha and Saxena (1965b) found that uptake of N by linseed was greatest at pH 6.0 and decreased with increasing pH, with a highly significant interaction between pH and N supply on N uptake. It appears that residual soil P markedly depresses uptake of currently applied fertilizer P by linseed (Spratt and Smid, 1978). Long-term trials over 8 years on chernozemic soils (PH7.2-7.4) in the eastern prairies of Canada have shown that linseed has consistently smaller responses to residual P than wheat, and that it recovers only half as much of the original P fertilizer as wheat (Bailey et al., 1977). In view of the work of Bole (1973), who showed that linseed roots can take up more P per unit root length than wheat but considerably less than rapeseed, the above results may indicate a greater root density for wheat than for linseed in these soils. The proportion of fertilizer N taken up by fiber flax is highest when the N is applied as nitrate (Lahola, 1976). Linseed seems to have a restricted capacity to take up either residual or applied ammonium N (Evans et al., 1987). From a study of six linseed cultivars, Dai and Zhang (1981) found that there were two peaks of N uptake, the first between 35-45 days and the second between 52-62 days after emergence. The mean efficiency of uptake by the crop of urea N applied up to 50 days from emergence was 39%, and the highest uptake efficiency of 49% occurred when urea was applied 30 days after emergence. The highest yield was achieved when the N was applied 50 days

PETER J. HOCKING E T A L .

238

after emergence. Dastur and Bhatt (1965b) found that maximum uptake by linseed of N, as well as K, occurred between 45-56 days after sowing. However, maximum uptake of P occurred later, at about 56 days after sowing, and during flowering there was a marked decline in K uptake. The efficiency of P uptake by dual-purpose flax decreased when the rate of application was increased from 15 to 45 kg/ha; the crop took up 45% of the initial 15 kg P applied, 26% of the next 15 kg, but only 9% of the last 15 kg (Hamdi et al., 1971). The period of greatest demand for N, P, and K by fiber flax is from bud formation to peak flowering, with daily rates of uptake being highest during early flowering (Gulyakin and Deryugin, 1975). By the end of flowering, fiber flax plants have accumulated about 60% of their final dry matter and N, P, Ca, and Mg contents, but 80% of their K content (Opitz, 1939). Application of N fertilizer to linseed increases the uptake of other macronutrients such as P and K, presumably because of enhanced growth (El-Damaty and El-Kobbia, 1958). Kyzlink (1939) states that nitrate rather than ammonium favors absorption of K by flax and its movement from roots to aerial parts, as is usual for many species (Kirkby, 1969). However in the field N is usually more limiting than K, so the choice of the form of N fertilizer will depend on cost and on the efficiency of recovery of N by the crop rather than on its effect on other nutrients. Flax is reported to have a large capacity to absorb Ca and, as with many other crop species, high rates of K fertilizer will decrease uptake and tissue concentrations of Ca, Mg, and Na. Linseed apparently has a greater requirement for Ca but lower requirement for K than does fiber flax (Opitz, 1939). Soil temperature has a marked effect on uptake of P and Zn by linseed (Table VIII). At 7 "C, flax shoots contained only 1.9 g P/kg dry weight and Table VIIl Effect of Soil Temperature on Uptake of Phosphorus and Zinc by Linseed from a Calcareous Soilu,' Nutrient accumulation

Soil temperature ( "C)

I 16 24

Zn

P Plant tops (B)

g/kg

mglplant

mg/kg

Fg/plant

1.6 8.2 15.2

1.9 2.1 3.5

3 22 53

16 24 25

25 192 376

uAdapted from Moraghan (1978). 'Data from pot experiment for 9-week-old plants receiving P (120 mg/kg soil), Zn (4 mg/kg), and Fe (0.5 mg/kg).

239

MINERAL NUTRITION OF LINSEED AND FIBER FLAX

showed symptoms of P deficiency. Raising the soil temperature to 16 or 24°C greatly increased the P concentration in shoots. Similar but less marked trends were noted for Zn uptake (Moraghan, 1978, 1980a). b. Concentrations. Concentrations of N, P, and K are highest in the young linseed plant and decrease in all vegetative organs as the plant ages, particularly during flower and capsule development (Fig. 2) (Dastur and Bhatt, 1965b). Concentrations of Mg and Ca in adequately fertilized plants, however, tend to remain fairly constant or to increase in vegetative organs, especially leaves, throughout the life of the crop. In mature flax plants, concentrations of N and P are highest in seeds, whereas K, Ca, Mg, and C1 concentrations are highest in stems and leaves (Table IX) (Lewis, 1943; Dastur and Bhatt, 1965b). Concentrations of N and P in seeds of the dual-purpose cultivar Giza4 ranged from 3.5 to 3.8% and 1.2 to l.8%, respectively, and in straw from 0.34 to 0.65% and 0.17 to 0.24% (Hamdi et a/., 1971). A much lower N concentration (1.75%) has been reported for linseed grain in Australia (Harbison eta/., 1986). In Canada, K concentrations of 0.6-1 .O% have been reported in linseed grain (Bailey and Spratt, 1979; Dybing and Lay, 1981). Artsem'eva (1983) found that concentrations of N in the fiber from 22 flax cultivars ranged from 0.27 to 0.38%. In a comparative study, Dastur and Bhatt (1965b) consistently found higher concentrations of K in a linseed cultivar resistant to fusarium wilt 4-

Potassium

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90

120

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Flc. 2. Changes in concentrations of major nutrients in linseed shoots from the field during the growing season. Seeds included with capsules. (Adapted from Dastur and Bhatt, 1965b.)

240

PETER J. HOCKING ETAL.

than in a wilt-susceptible cultivar, and they suggest that the capacity of some linseed cultivars to accumulate high levels of K is a factor in determining their resistance to the disease. c. Distribution. The patterns of distribution of nutrients between the aboveground organs of flax change as the plant ages. With linseed, leaves initially have the greatest proportion of the total shoot content of N and P, but there are dramatic changes during capsule formation and seed filling, such that by maturity capsules (including seeds) have accumulated 70-80% of these nutrients in the shoot (Fig. 3) (Dastur and Bhatt, 1965b). Although the distribution pattern of K is similar to that of N or P in young plants, it later changes markedly. After flowering, the stems become as important as the capsules as a sink for K, so that at maturity, each plant part contains about 38% of the total K content (Fig. 3). Seeds of dual-purpose and fiber flax contain a lower proportion of the total shoot content of a particular nutrient than seeds of linseed. The capsules themselves usually accumulate much smaller quantities of nutrients than the seeds they contain, so the seeds can accumulate well over 50% of the total amount of N and P in the mature plant. For example, the seeds of mature plants of several dual-purpose flax cultivars had from 41 to 63% of the total N and P contents (Hamdi et a/., 1971), while seeds of adequately fertilized fiber flax had 52% of the N, 50Vo of the P, but only 27% of the K in the whole shoot (Gulyakin and Deryugin, 1975). d. Redistribution. Considerable internal redistribution of some nutrients occurs from vegetative organs of linseed during capsule development and seed filling. Dastur and Bhatt (1965b) found that up to 85% of the N and 75% of the P accumulated by mature capsules was obtained by redistribution of these nutrients from stems and, especially, from the leaves (Fig. 3). Redistribution of K is not as pronounced as that of N and P, and leaves Table IX Concentrations of Nutrients in Field-Grown Linseed at Maturity" ~

~~

Concentration (percentage dry wt) Nutrient

Stems

Leaves

Capsulesb

Whole shoot

N P K Ca Mg

0.68 0.12 1.16 1.17 0.16

1.23 0.42 2.28 3.28 0.42

2.47 0.49 0.73 1.08 0.36

1.70 0.35 1.08 1.39 0.30

Trom Dastur and Bhatt (1965b). bIncludes seeds.

MINERAL NUTRITION OF LINSEED AND FIBER FLAX Nitrogen

24 1

Phosphorus

Potassium

30 45 60 75 90 105 120

30 45 60 7 5 90 105 120

100

n

*

40

20 30 45 60 75 90 105 120

KI

stem

leaves

capsules

Days after sowing

FIG.3. Changes with plant age in the distribution patterns of N, P, and K among shoot organs of field-grown linseed. Seeds included with capsules. (Adapted from Dastur and Bhatt, 1965b.)

appear to be the sole source of redistributed K destined for capsules, as stems continue to accumulate K throughout the life of the plant. The data of Dastur and Bhatt (1965b) indicate that in adequately fertilized linseed there is only limited redistribution of Mg from stems and leaves and that there is negligible redistribution of Ca. The extent to which each of the major nutrients is redistributed from vegetative organs to developing capsules of linseed is in accord with the generally accepted ranking of their mobilities in the phloem (Loneragan et al., 1976). In practical terms, this means that if the supply of major nutrients in the soil becomes depleted or unavailable after flowering, then internal redistribution can provide some of the N, P, and K, but little or none of the Mg and Ca required for capsule development and seed filling. The nutritional status of flax apparently influences the extent to which a particular nutrient is redistributed. For example, increased N nutrition of flax can enhance the redistribution of nitrogenous compounds, as well as carbohydrates, from stems to developing capsules (Kosheleva et al., 1981, 1983a-c). 2. Micron utrients

There is little information available on concentrations, patterns of distribution, or the extent of redistribution of micronutrients in flax. Linseed has the capacity to accumulate high levels of Mn (over 2000 mg/kg) in lower leaves (Moraghan, 1985), and Mn toxicity has been reported for linseed grown in some calcareous soils low in available Fe (Moraghan, 1979). The older leaves of flax have higher concentrations of Mn than young leaves, and Mn accumulates preferentially in the distal portion of the

242

PETER J. HOCKING E T A L .

leaf lamina (Millikan, 1950b). The accumulation of Mn in linseed is greatly influenced by the Fe supply (Wikoff and Moraghan, 1986), and high Fe can result in very low levels of Mn in leaf tissue (Moraghan, 1985). SUPPLY ON YIELD COMPONENTS D. EFFECTSOF NUTRIENT The yield components of fiber flax are technical length (the unbranched section of stem from which the fiber is obtained), stem diameter, and percentage fiber. For linseed, the yield components are number of capsules per plant (the product of the number of tillers per plant, the number of fruiting branches per tiller, and the number of capsules per branch), number of seeds per capsule, single seed weight, and seed oil concentration (Patil e t a / . , 1980). 1. Major Nutrients

There are many conflicting reports on the effects of nutrient applications on yield components of fiber flax and linseed. Scarisbrick et al. (1980) found that rates of N from 0 to as high as 525 kg/ha did not affect yield components of linseed except for a small reduction in seed oil concentration at the highest rates, so presumably their treatment without fertilizer N provided sufficient N for maximum yield. Prashar e t a / . (1968) found that although increased P supply to linseed increased the number of tillers per plant, this did not result in a significant increase in yield because of competition between vegetative and reproductive growth. A number of field and greenhouse studies with fiber flax have shown little or no influence of K on the development of fibers and their yield (Fig. 4) (Lewis, 1943; Milthorpe, 1943, 1946), and application of K fertilizer had no effect on yield components of four linseed cultivars (Shaaban et a/., 1982). There are, however, numerous reports of positive responses of yield components to applied nutrients, especially N. In linseed, increased N supply usually results in an increase in most yield components, especially numbers of tillers and capsules per plant (Tables X, XI) (Dybing, 1964; Prashar et al., 1968; Singh, 1968; Al-Shamma and Jabro, 1972; Moursi and El-Hariri, 1977; Reddy, 1983; Al-Mukhtar and El-Hariri, 1984; Nayital and Singh, 1984). In dual-purpose flax grown under dryland conditions, N applications of up to 30 kg/ha increased the technical length, percentage fiber, the number of capsules, single seed weight, and seed oil concentration (Gad and El-Farouk, 1978). Irrigated dual-purpose flax may require up to 72 kg N/ha before the response of these yield components reaches a plateau (Table XII) (Hamdi et al., 1971). In fiber flax, N and P fertilizers usually increase the fiber yield per plant, the technical length, and the cross-sectional area of

MINERAL NUTRITION OF LINSEED AND FIBER FLAX

243

=130 t

m n

1

r E 90 .-a

> L

0

g U

50

0

Ql

7

X

N NP NPK

N

NP NPK

N NPNPK

Fertilizer applied

FIG.4. Interactions of N, P, and K on yield and fiber quality of fiber flax: (a) dry matter per plant, (b) fiber yield per plant, (c) percentage of fiber (dry weight) in stem, (d) percentage fiber by volume in stem, (e) cross-sectional area of fiber, and (f) number of fiber cells per cross section of stem. Greenhouse experiment. Plants grown in pot culture and supplied with nutrient solutions. N, High N, low P and K; NP, high N and P, low K; NPK, high NPK. (Adapted from Milthorpe, 1946.)

fiber in the stem. However, application of N, especially at high rates, often results in a lower percentage of fiber by weight or volume, as the growth of other stem tissues is stimulated to a greater extent (Fig. 4) (Schmalfuss, 1936; Milthorpe, 1946). Nitrogen deficiency has no effect on the percentage fiber (Opitz, 1940). In contrast, P fertilizer usually increases the percentage fiber (Robinson, 1933; Les et a[., 1977), and P deficiency lowers it. Hussein et al. (1970) showed that N applied to fiber flax as ammonium sulfate significantly increased plant height and thus fiber length, whereas the effects of other forms of N fertilizer were inconsistent. The number of capsules per plant is the most responsive yield component of linseed to fertilizer (Tables X, XI, XII) (Blackman and Bunting, 1954; Dybing, 1964; Singh, 1968; Hamdi et a[., 1971). For example, Blackman and Bunting (1954) in summarizing the results of 25 field trials with linseed in Britain, reported that application of N fertilizer (35 kg/ha) increased mean seed yield by about 12%, and that most of this was derived from an

244

PETER J. HOCKING ET AL. Table X

Effect of Nitrogen Supply on Vegetative Characters, Seed Production, and Seed Quality of Linseed Cultiver A n ~ y " , ~

N supplied (mg/liter) Character measured'

14

28

56

112

224

Days to flowering Plant height (cm) Fresh wt (g/plant) Number of tillers/plant Number of capsules/plant Number of seeds/capsule lo00 seed wt (9) Seed yield (g/plant)d Seed oil concentration(%) Iodine value

57 a 75 a 1.8 a 0.7 a 4.0 a 8.7 a 4.31 a 0.15 40.3 a 185 a

57 a 79 a 2.2 a 0.6 a 4.6 a 9.1 b 4.52 b 0.19 39.5 a 181 b

56 a 91 b 4.1 b 1.9 b 9.3 b 9.0 b 4.57 b 0.38 37.9 b 181 b

51 b 97 b 8.3 c 2.7 c 20.7 c 8.7 a 4.60 b 0.83 38.1 b 178 bc

50 b 90b 13.4 d 3.4 d 24.0 d 8.9 ab 4.46 ab 0.95 36.7 b 175 c

Fatty acids (Yo) Palmitic Stearic Oleic Linoleic Linolenic

6.6 a 2.4 a 22.0 a 15.6 a 53.4 a

6.5 ab 2.2 a 24.9 b 15.6 a 50.8 b

6.5 ab 2.1 a 25.4 bc 15.2 ab 50.8 b

6.3 b 2.2 a 27.3 cd 14.5 bc 49.7 bc

6.4 ab 2.3 a 29.2 d 14.0 c 48.2 c

Trom Dybing (1964). bPlants grown in culture solution in controlled environment. 'For each character, values followed by the same letter do not differ at p = 0.05. h o t given in Dybing (1964), but derived from data in original table.

increased number of capsules per plant rather than from an increased number of seeds per capsule or single seed weight. When Al-Shamma and Jabro (1972)applied 80 kg/ha N fertilizer to three linseed cultivars, seed yield doubled through a 90% increase in the number of capsules per plant and a 26% increase in the number of seeds per capsule (Table XIII), but there was a slight decrease in single seed dry weight. However, Singh (1968) obtained a small increase in single seed weight with increased N application (Table XI). In solution culture experiments, increasing the N (Hocking and Pinkerton, 1987)or S (Pinkerton and Hocking, 1987)supply from deficient to supraoptimal had little effect on the number of seeds per capsule, single seed weight, or seed oil concentration, although plants on a low N or S supply had fewer than a quarter of the capsules of plants on high N or s. The release of lateral buds from apical dominance and the subsequent development of lateral fruiting branches is positively correlated with N supply (McIntyre and Larmour, 1974), such that Ndeficient linseed plants produce few laterals and capsule production is restricted. As mentioned in Section

Table XI Effects of Nitrogen Fertilizer on Yield Components and Yield QonUty of Linseedu** seed oil

Number of

N applied (kg/ha)

Number of capsules/plant ~

0 25 50 75

Wt/capsule (mg)

9.2 10.4 11.1 11.4

(Q4

Oil yield (kgha)

Protein content of seed (percentage dry wt)

Iodine value of oil

44.0 44.1 43.7 42.3

190 330 417 537

25.1 25.8 26.8 27.6

171.6 170.9 169.9 169.1

Seed yield (tonslha)

concentration

capsule

Single seed wt (mg)

7.9 8.5 8.9 9.5

7.8 8.1 8.2 8.5

0.43 0.75 0.95 1.27

seeds/

~~

35.3 58.1 69.3 88.8

"From Singh (1968). bField experiment on soil low in available N near Varanasi, India; values are means of four replicates. Table X n Effect of Nitrogen Application on Growth and Yield Components of Irrigated Dual-PurposeFlaxu,*

N applied (kglha)

Straw yield (tonslha)

Fiber yield (tonslha)

Technical length (cm)

Number of fruiting brancheslplant

Number of capsuleslplant

Seed yield (tonslha)

0 36 72 108 144

4.8 7.0 8.3 8.2 9.1 1.6

1.o 1.4 1.6 1.6 1.7 0.4

42.2 71.2 68.5 72.2 64.9 11.6

4.2 6.7 10.3 9.8 11.2 2.0

6.3 9.9 14.1 13.7 15.0 1.1

0.64 0.90 1.21 1.25 1.18 0.29

LSDC

"From Hamdi et al. (1971).

Seed oil concentration (070)

40.9 41.5 40.0 38.5 38.7

-

bField experiment at Giza, United Arab Republic, on moderately fertile soil which received basal dressing of P and K. 'LSD, Least significant difference ( p = 0.05).

Oil yield (kglha) 262 375 485 482 457 67

246

PETER J. HOCKING ET AL. Table XI11

Effect of Nitrogen Fertilizer on Some Yield Components of Liaseed0sb N applied (kg/ha)

Capsules/plant

Moroccan 10 Moroccan 50 Indian 68 Mean Increase (Vo)

Seedslcapsule

Seed yield (tons/ha)

0

80

0

80

0

80

4.5 5.1 5.1 4.9

9.6 9.1 9.2 9.3

5.1 6.2 6.0 5.8

6.8 7.8 7.2 1.3

0.67 0.71 0.81 0.73

1.42 1.55 1.42 1.46

90

26

100

aFrom Al-Shamma and Jabro (1972). bField experiment near Baghdad, Iraq. Values are means of two seasons.

II,B, linseed has distinct flowering periods separated by brief resting periods, so that capsules are produced in cycles (Yermanos and Worker, 1964). The N and P status of the plant is important in determining the number of cycles and the number of flowers produced in each cycle (Dybing, 1964; Hovland and Dybing, 1973). Plants which receive high rates of N and P have extended flowering as a consequence of at least two flowering flushes, whereas plants which are stressed for N or P have only one flush (Dybing, 1964). Seeds from the second flush of capsules, however, usually have lower dry weights, oil contents, and iodine values than those from the first flush. The effects of other nutrients on cyclic flowering in linseed are not known. Time of flowering may be influenced by the level of a particular nutrient or the balance of nutrients. For example, linseed plants given P alone flowered up to a week earlier than those given a balanced application of N, P, and K fertilizers (Bhatt, 1974), and Opitz (1940, 1941) observed that P fertilizer accelerated the maturity of fiber flax by 15 days and of linseed by 27 days. Dybing (1964) found that very low supplies of N (Table X) or P delayed the onset of flowering in linseed by about 7 days. In general, an increased supply of N, or N and P mixtures, extends the period of vegetative growth, and in this regard the report of Mandy (1968) of increased N supply reducing the period of vegetative growth, as well as the period from flowering to maturity, seems anomalous. Although increased N supply may increase the number of flower buds formed and the number of buds which develop into flowers, it can markedly decrease the proportion of buds which develop into mature capsules (Fig. 5 ) (Sinha and Saxena, 1965b). It has been suggested that this is because vigorous vegetative growth continues in plants receiving a high N supply at

247

MINERAL NUTRITION OF LINSEED AND FIBER FLAX

75r

A

50- d

a

50- e

10

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B

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C

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P supply (rng/liter)

FIG.5. Reproductive characters of linseed as affected by N and P supplies. Effect of N supply (A) and P supply (B) on (a, f) number of flower buds per plant; (b, g) number of flowers per plant; (c, h) number of capsules per plant; (d, i) number of seeds per plant; and (e, j ) the percentage fertility per plant, calculated as (seeds per plant x 100)/(flower buds per plant x 10). There are 10 ovules (potential seeds) per flower bud. Plants grown in nutrient solution. (Adapted from Sinha and Saxena, 1965b.)

the same time as bud and flower formation are at a maximum, so many buds and flowers fail to develop as a consequence of the competition for assimilates and mineral nutrients. Sinha and Saxena (1965b) showed that seeds produced under high-N conditions had lower individual weights than those produced under a moderate N supply, supporting the concept of competition for assimilates. In the experiments of Sinha and Saxena (1965b), P supply had an effect similar to N on the number of flower buds which formed, but did not affect the number of buds which developed into flowers or the proportion of buds which produced sound capsules (Fig. 5 ) (Sinha and Saxena, 1965b).

248

PETER J . HOCKING ET AL.

However, the positive effects of P supply on reproductive characters were not as pronounced as those of N supply (Table XIV). The pH of the rooting medium influences flower and capsule development, probably by effects on the N nutrition of the plants. Interactions between N, P, and pH mainly affect the number of flower buds produced (Table XIV) (Sinha and Saxena, 1965b). Little information exists on when yield components of linseed are determined during growth, or on the sensitivity of a particular yield component to nutrient stress. We are attempting to obtain this information using constant and changed N supplies at defined stages of crop growth. Preliminary results (Fig. 6) (Hocking and Pinkerton, 1987) from comparative work with linseed, rapeseed, and sunflower show that oil yield per plant is depressed to about the same extent in each species by a constant inadequate N supply. However, oil yield of linseed is much more responsive to relief of N stress than is the yield of rapeseed, or especially sunflower, and a reasonable yield can still be obtained by application of N as late as the start of the first flush of flowers on the primary inflorescence. In contrast, capsule production and hence the oil yield of linseed is more vulnerable to the onset of N stress at flowering than is the oil yield of rapeseed or sunflower, which appear to have greater reserves of N available in their vegetative organs for redistribution to developing seeds in the event of inadequate uptake of N from the soil. Experiments with changed supplies of N, P, and K to linseed during flowering have also highlighted the sensitivity of capsule production to the onset of nutrient stress (Table XV) (Dybing, 1964). Nutrient stress early Table XIV Analysis of Variance of the Effects of Nitrogen, Phosphorus, and pH on Reproductive Characters of Linseed'jb Mean squaresC Source of variance

Flower buds

N P PH N x P N x pH P x pH N x P x p H

38381*+ 837** 115944122 1168*+ 134 NS 309+

Flowers 5605**

22 NS

440** 24 NS 332+* 65** 32'.

Capsules

Seeds

Fertilityd

83** 3 NS 17+* 5 NS 11' 2 NS 1 NS

658** 454*+ 198** 203++ 158*+ 6 NS 54+*

10,657*+ 59333 NS 150.. 207 NS 75 NS 117 NS

%om Sinha and Saxena (1965b). bPlants grown in pot culture and provided with nutrient solutions. '"s, Not significant; +,p = 0.05; **, p = 0.01. qertility of plant assessed as seeds per plant/(buds per plant x 10). There are 10 ovules (potential seeds) per flower bud.

r

MINERAL NUTRITION OF LINSEED AND FIBER FLAX 100

50

-Emn

O

L

al

a

100

m

249

Linseed

Rapeseed

0

.-al >. .0

50

5

.-E

z X

O

Sunflower NsupPlv changed a t

Constant

Changed

N supply (rnghiter)

FIG.6. Effects of constant and changed N supplies on oil yield of linseed, rapeseed, and sunflower grown in nutrient solution. Changes in N supply were made at the following growth stages: bud@)visible on primary inflorescence (star stage of sunflower), start of flowering on primary inflorescence (three-row anthesis of sunflower), end of flowering on primary inflorescence (full anthesis of sunflower). Oil yield data for each species have been normalized to a common percentage scale in which a value of 100 designates the oil yield per plant of the constant high-N (210 mglliter) treatment. (Adapted from Steer and Hocking, 1981; Hocking and Pinkerton, unpublished data.)

in flowering severely restricted capsule production from the first flush of flowers, but if the stress was relieved within about 3 weeks from first flowering, then capsule production from the second flush of flowers was not impaired. In these experiments, combined P and K stresses had similar effects to N stress on yield components, and relief of N stress without relief of the combined P and K stresses and vice versa had no effect on yield components. Furthermore, the experiments showed the relative insensitivity of

Table XV Response of Linseed to Nutrient Supply during Capsule Formation4*' Nutrient solution supplied during capsule development ~

No

~

Complete NPK solutiond

Complete NPK solution (control)

Character measuredc

NPK

224 mg N/literd

224 mg N/liter

78 mg K/liter + 62 mg P/liter

Number of capsules/plant (first bloom)

72*

86'

92.

98*

94*

122

Number of capsules/plant (second bloom)

36**

49**

43**

42**

97

88

Number of seeds/capsule (Fist bloom)

7.1

7.3

6.6

6.8

Number of seeddplant (Fist bloom) loo0 seed wt (g) Oil concentration (To) Iodine value

51 1* 4.4 35.6 163

624 4.3 34.5 161

606

662

5.2 490*

6.7 814

4.2

4.6

4.2

4.4

35.1

35.0

34.6

35.1

162

166

163

164

%om Dybing (1964). 'Greenhouse study. '*, **, Significantly different at 5 and 1To levels, respectively, from control treatment receiving complete nutrient solution throughout capsule development. %ants received water only for first 21 days of flowering period; nutrient solution then supplied from 22nd day until 67th day after flowering commenced, when plants were harvested.

MINERAL NUTRITION OF LINSEED AND FIBER FLAX

25 1

single seed weight, seed oil concentration, and the iodine value of the oil to the imposition or relief of N, P , and K stresses during capsule development. Interacting effects on yield components in linseed and fiber flax have been reported between nutrient and water supplies. Positive interactions have been shown between N and irrigation on fiber yield, the number of fruiting branches, and capsules and seed number per plant, as well as on seed oil concentration (Pande et af., 1970; Moursi and El-Hariri, 1977). Positive interactions have also been reported between P supply and irrigation frequency on seed number and fiber yield per plant, single seed dry weight, and seed oil concentration (Pande et al., 1970). Temperature can affect the responses of yield components of flax to nutrient supply. For example, Millikan (1957) found that high P supply increased the number of tillers and fruiting branches when the day temperature was 20 "C but did not when it was 26 "C. 2. Micronutrients Little information exists for flax on the effects of micronutrient supply on specific yield components. There is a report from India (Khan and Gupta, 1959) that application of Mn to linseed grown on a light calcareous soil increased seed oil concentration. OF NUTRIENT SUPPLY ON YIELD QUALITY E. EFFECTS

I . Nitrogen

Responses of yield quality to N fertilizer are often variable. For fiber flax, there are reports of N fertilizer increasing yields without effect on quality (Aukema and Friederich, 1959), but in most cases moderate applications seem to improve fiber quality. There are several ways in which N fertilizer may improve fiber quality. First, it increases the technical length of stem, resulting in longer fibers (see Table XII) (Hamdi et af., 1971). Second, it promotes the development of phloem and xylem elements (Terent'ev et al., 1974, 1976) and, in consequence, a greater cross-sectional area of fiber and more fiber cells per cross-sectional area of stem (Fig. 4) (Milthorpe, 1946). Third, it increases the accumulation of mono- and disaccharides in the stem, resulting in more intense formation of hemicellulose and cellulose in phloem and xylem tissues (Terent'ev et af., 1976). Last, it seems that moderate applications of N reduce lignin deposition, as Mikhailova (1975) found that 15-30 kg N/ha improved fiber quality by decreasing the lignification of stems, as well as by increasing the cellulose content of fiber. Excessively high applications of N fertilizer, however, can decrease fiber quality because of reduced cellulose content (Bakhnova and Kosheleva,

252

PETER J. HOCKING ET AL.

1975) and increased lignification (Mikhailova, 1975). It can also decrease fiber strength (Zajceva, 1962; Velchev and Balevska, 1964; Les and Popirlan, 1975) and fiber fineness (Table XVI) (Gad and El-Farouk, 1978). High N can further reduce fiber quality by causing cleavage of the fiber bundles (Opitz et a/., 1937), resulting in increased fiber losses during retting (Canev, 1962). Application of P fertilizer can counteract to some extent the detrimental effects of high N on fiber quality (Opitz, 1942). Effects of the form of N fertilizer applied to fiber flax are also somewhat inconsistent. Nitrogen applied as calcium ammonium nitrate sometimes produces better quality fiber than N applied as urea (Guleria and Singh, 1983). However, rate of application of calcium ammonium nitrate can also affect quality because, although 50 kg N/ha gave the highest fiber yield, 25 kg N/ha produced the finest fibers with the highest cellulose content (Guleria and Singh, 1983). According to Derebon (1975), application of N in the ammonium form can improve fiber quality because of an increase in the proportion of a-cellulose, in addition to an increase in total cellulose and a decrease in lignin. Nitrate N had the opposite effects and reduced fiber quality. Seed oil concentration usually decreases significantly with increasing N supply (Tables XI, XII, XVI) (Blackman and Bunting, 1951; Gupta et a/., 1961; Beech and Norman, 1964; Dybing, 1964; Saxena and Sinha, 1966; Singh, 1968; Singh et al., 1968; Hamdi et a/., 1971; El-Nekhlawy et a/., 1978; Dai and Zhang, 1981), especially at application rates above 50 kg N/ha (Dybing, 1964; Woodhead and Neilson, 1976; Yayock and Quinn, 1977; Singh and Singh, 1978), as found for other oilseed crops such as sunflower (Steer et a/., 1984). Consequently, although seed yield may

Table XVI Effect of High Nitrogen on Fiber and Seed Quality of Dual-Purpose Flax Cultivar Giz11-4~*~

N applied (kg/ha)

Fiber length (cm)

Fiber fineness index

Seed protein

(YO)

Iodine value of oil

35.7 71.4 107.1 142.8

75.6 a 82.7 b 82.7 b 82.4 b

171 a 147 b 133 b 125 b

21,l a 22.4 b 23.8 c 25.0 d

175.3 a 169.8 b 165.5 c 160.9 d

"Adapted from Gad and El-Farouk (1978). bField experiments at Kafr El-Sheikh, Egypt. Values are means of results over two seasons. Within a column, values followed by the same letter do not differ significantly at p = 0.05.

MINERAL NUTRITION OF LINSEED AND FIBER FLAX

253

increase with increasing N supply, oil yield per hectare may occasionally stay the same or even decline slightly (Mathur et al., 1958). Moderate N applications, however, sometimes increase both linseed yield and seed oil concentration (Gupta et a[., 1961; Khan et al., 1963; Abdul-Kareem, 1966; Pande et al., 1970). The iodine value of the oil usually does not change much (Tables XI, XVI) (Singh, 1968; Singh and Singh, 1978), except at very high rates of N supply (Table X) (Dybing, 1964; Gad and El-Farouk, 1978). The lowering of the iodine value by high N applications can be largely overcome by application of K fertilizer (Opitz and Egglhuber, 1939). The decrease in seed oil concentration (and hence in yield quality) results from an increase in seed protein content as the N supply increases (Tables XI, XVI) (Singh, 1968; Singh et al., 1968; Gad and El-Farouk, 1978; Singh and Singh, 1978). Sometimes however, lowering of the oil concentration is accompanied by a decreased iodine value as a consequence of lowered levels of linolenic and linoleic acids and elevated levels of oleic acid (Table X) (Dybing, 1964; El-Nekhlawy et al., 1978; Singh and Singh, 1978). Gad and El-Farouk (1978) found that increasing N supply up to 45 kg/ha increased linseed oil concentration as well as protein content but reduced the iodine value of the oil. Time of application of N fertilizer (Gupta et al., 1961) and the form of N (El-Damaty and El-Kobbia, 1956) seem to have little effect on either seed oil concentration or on its iodine value. Interactions between components of combined N, P , and K fertilizers on oil quality are complex and often conflicting, so no precise interpretation can be made of the overall effects of combined treatments (Blackman and Bunting, 1951). Large, combined applications of N, P, and K to the dualpurpose cultivar Istru resulted in a low oil concentration and a poor iodine value because of an increase in palmitic, stearic, and oleic acids at the expense of linoleic and linolenic acids (Filipescu and Simota, 1977). Singh and Singh (1978) obtained higher seed protein contents at the expense of seed oil concentration with increased applications of combined N and P fertilizers. On balance, therefore, it seems that with combined N, P, and K fertilizers, the effects of N on yield quality are dominant, as concluded for other oilseed crops (Garner et al., 1914; Fabian, 1928; Schmalfuss, 1937). 2.

Phosphorus

Phosphorus fertilizer usually improves fiber yield and increases fiber quality (Terent’ev et al., 1974) by increasing the percentage of long fibers (Velchev and Balevska, 1964), but excessive P fertilizer (e.g., 70 kg P/ha) may reduce quality by decreasing the percentage of long fibers (Frederiksen, 1954b) and fiber strength (Les and Popirlan, 1975). Milthorpe (1946) found that increased P supply did not increase the technical length or stem

254

PETER J. HOCKING ET A L .

thickness but gave better quality fiber by increasing the number of fiber cells per cross-sectional area of stem (Fig. 4). There are reports (Schmalfuss, 1936; Milthorpe, 1946) that P fertilizer alone either does not affect or reduces the cross-sectional area of fiber cells, but that the same level of P increases the fiber area when applied with N. With P fertilizer, there appears to be a trade-off between fiber yield and quality, as according to Opitz (1940), the best quality fiber is obtained with a P application which is usually below that required for maximum yield. Application of P fertilizer to linseed may increase seed oil concentration (Singh and Singh, 1978; Abdul-Kareem, 1966), but usually it does not alter the iodine value of the oil (Khan and Gupta, 1959). There are, however, some reports of improved iodine values in response to increased P supply (Helgeson etal., 1940; Opitz, 1940,1941; Dolgova, 1969), and areport ofdecreased iodine values when P was applied at 9 kg/ha (Singh and Singh, 1978). 3. Potassium

Much of the older literature on the fertilizer requirements of fiber flax stresses the importance of K for fiber quality (Opitz, 1936a,b; Scheel, 1938; Peive and Andrianova, 1939; Ulbricht, 1939), apparently because it was found to improve the technical length and strength of the fiber (Powers, 1928; Canev, 1962). Potassium fertilizer can also increase fiber quality by increasing the number and size of fiber cells (Scheel, 1938), but it does not increase the size of fiber cells unless N and P levels are high and Na and Ca levels are low (Milthorpe, 1946). Schmalfuss (1936) showed that although moderate K applications did not increase the number of fiber cells per crosssectional area of stem unless N was applied as well, very high K applications did increase the number of fiber cells, irrespective of whether or not N was applied. If a high level of Na is present in the soil, then increased K supply can actually decrease the number of fiber cells per cross-sectional area (Milthorpe, 1946). Some of the reported effects of K fertilizer on yield quality may in fact be due to the accompanying anion rather than to the K itself. For example, application of potassium sulfate gave closely packed fiber cells with cellulose deposition almost completely filling their lumina, whereas the same application of potassium chloride increased the bulk of the fiber but the cell walls remained thin and the lumina large, so quality was poor (Tobler, 1929a,b; Schmalfuss, 1938). This suggests that chloride has a detrimental effect on yield quality. Potassium application has been reported to increase seed oil concentration of linseed and the iodine value of the oil (Shaaban et al., 1982). AbdulKareem (1966) found that, although 25 kg K/ha reduced seed yields, there was no reduction in the concentration of oil or protein in the seeds.

MINERAL NUTRITION OF LINSEED AND FIBER FLAX

255

4. Other Major Nutrients

Combinations of Ca and N fertilizers can increase the degree of lignification of the fiber cells and reduce fiber quality if applied at excessive rates (Scheel, 1938). There appear to be interactions between cations on fiber quality. Increased Mg supply to fiber flax grown in sand culture decreases lignification in the presence of both high Na and high Ca (Milthorpe, 1943). However, if Na is low, the effect of high Mg is reduced.

5. Micronutrients There is little information on the effects of micronutrient deficiencies or excesses on yield quality. High B may reduce the quality of linseed by decreasing the iodine value of the oil (Deo and Ruhal, 1971). Beneficial effects of micronutrients on fiber quality have been reported in the USSR. Applications of B, Cu, Mn, and Zn improved fiber fineness, although only B increased fiber length (Peive, 1938b; Sen’kov, 1974). There is a report from Western Australia that application of zinc sulfate to Zn-deficient fiber flax improved the fiber grade by decreasing the percentage of tow fiber (Cass-Smith and Harvey, 1948).

IV.

NUTRIENT DISORDERS

A.

DEFICIENCIES AND TOXICITIES 1. Boron

a. Deficiency. Historically, flax was used in a number of studies which demonstrated the effects of B deficiency and the essentiality of this element for plant growth (Shkol’nik, 1934). When B deficiency is severe, growth is stunted and the stem thickened and twisted; with mild B deficiency, symptoms may not appear until the plants are flowering. Young leaves become chlorotic and yellow near the growing points, which usually die (Wallace, 1961). There may also be abnormal production of axillary shoots (Laganiere and Sackston, 1967a). Boron-deficient linseed plants may flower but the petals are often small and the capsules characteristically turn yellow and abort or fail to develop at all (Randall and Hocking, unpublished). Boron-deficient flax plants typically have shoot concentrations of about 8 mg B/kg dry weight compared with 20-40 mg/kg in B-sufficient plants (Dzikovich, 1968). In the field, the supply of B to flax plants may vary considerably during the season because of fluctuating soil moisture levels, and this may result in transient B deficiency. It is noteworthy that linseed plants recovering from

256

PETER J. HOCKING ET A L .

an early B deficiency may develop fasciated stems, with leaves forming dense tufts at their apices. Flowers on these stems may also be fasciated, being twice as long as broad, and have a row of stamens and pistils in the center (Laganiere and Sackston, 1967a). Microscopic examination of B-deficient linseed showed that necrosis and disorganization of apical meristems, leaf primordia, and root meristems coincided with early macroscopic symptoms of the deficiency (Laganiere and Sackston, 1967b). Flax, together with wheat, other temperate cereals, and peas, was considered by Berger (1949) to have a low requirement for B (less than 0.1 mg available B/kg soil). In sand culture studies, we have found the external B requirement for maximum linseed yield to be at least 0.3 mg/liter in the nutrient solution, compared with less than 0.03 mg/liter for wheat (Randall and Hocking, unpublished). Shorrocks (undated) considered linseed to be moderately susceptible to B deficiency and recommended application of 0.5-1.5 kg B/ha, either as a soil application or as a foliar spray, to correct the deficiency. However, it is not clear if these recommendations are based on specific experiments with linseed or are merely offered as a guide based on experience with other crops. Peive (1938a,b) has reported yield responses of fiber flax to 0.4-3.0 kg B/ha in the USSR, and it seems that B deficiency there occurs most frequently on limed soils (Millikan, 1951a). Protection from flax rust (Melumpsora line following heavy dressings of borax (67 kg/ha) was reported from a field experiment by Heggeness (1942). The borax caused tip burn of the leaves, but treated linseed plants did not develop rust, while all control plants were infected. Boron-deficient linseed is particularly susceptible to fusarium wilt (Keane and Sackston, 1970). b. Toxicity. The first symptom of B toxicity is a yellowing of the tips of older leaves. This later spreads towards the base and finally develops into a browning and scorching of the whole leaf (Laganiere and Sackston 1967a; Chauhan et al., 1984). In nutrient solution studies, severe B toxicity causes a grayish green transparent appearance of older leaves which then wither. Young leaves are not affected (Millikan, 1949). Underground irrigation water in India and elsewhere may contain up to 10 mg Blliter, which is well above the level of 3.5 mg B/liter in soil solution at which B toxicity symptoms appear in linseed. Seed yields are not reduced, however, until B levels in the soil solution reach about 8 mg/liter (Table XVII) (Chauhan et al., 1984). Mild B toxicity appears to affect seed and straw yields to about the same extent (Table XVII). Plants with a shoot concentration of B greater than about 90 mg/kg dry matter and a Ca:B concentration ratio of less than 50 are likely to develop B toxicity symptoms (Table XVIII) (Chauhan et ul., 1984). A similar threshold value was reported by Dzikovich (1968), who found concentrations of 96-380 mg B/kg in plants with symptoms of B toxicity compared with 20-40 mg B/kg in healthy plants.

MINERAL NUTRITION OF LINSEED AND FIBER FLAX

257

Table XVII Effects of Boron Levels in Soil on Seed and Straw Yield of Irrigated Linseed‘,” Yield (tons/ha) B in saturation extract of soil (mg/liter)

Seed

Straw

0.8 5 .O 6.5 9.6 SEC

1.28 1.32 1.24 1.04 0.06

3.88 3.80 3.64 3.08 0.22

‘Adapted form Chauhan el al. (1984). ’Values are means of field experiments over two seasons at Agra, India. ‘SE,Standard error of the mean.

Table XVIII Effect of Boron Levels in Soil on Boron and Calcium Concentrations in Linseeda*b Whole shoot concentration B in saturation extract of soil (mg/liter)

B (mg/kg)

Ca (g/kg)

0.7 1.8 3.7 5.2 SEd

51.3 66.2 95.5 121.2 5 .O

4.1 3.9 3.4 2.8 0.4

Ca : B concentration ratio

79.9 58.9 35.6c 23.lC

-

“Adapted from Chauhan et al. (1984). %hoots from plants harvested 45 days after sowing. Values are means of field experiments over two seasons at Agra, India. ’Wants with B toxicity symptoms. dSE, Standard error of the mean.

In many semiarid regions of India, underground irrigation water not only contains high levels of B, but is also saline (Mann and Singh, 1977). The salinity of this water is often sufficiently high to reduce yields, but it may also have the effect of decreasing B toxicity symptoms in linseed (Chauhan et al., 1984). Boron toxicity symptoms are delayed, and yields are unaffected in soils with high levels of soluble Ca salts at concentrations of B in the soil solution of up to 12 mglliter (Chauhan et al., 1984), probably

258

PETER J. HOCKING ET AL.

because of the formation of insoluble compounds of B with Ca which are not toxic to plant growth (Werkhoven, 1964). High levels of B can significantly reduce tissue concentrations of Ca in linseed (Table XVIII) (Chauhan et al., 1984).

2. Calcium In Australia, Ca deficiency has occurred in some flax crops growing on heavy, acidic, clay soils in the Gippsland and western districts of Victoria (Millikan, 1944a), in southwestern Western Australia (Elliott, 1959), and in Tasmania (Millikan, 1951a), particularly in wet seasons. The disorder, termed “withertop,” may cause heavy losses in some flax crops. Waterlogging may bring about, or exacerbate, withertop symptoms (Fig. 7) (Millikan, 1944a). Plants are usually not affected by withertop until the period of rapid vegetative growth, when they are 30-40 cm high, and the effects of waterlogging on the development of the disorder are most pronounced at this stage (Millikan, 1951a).

lwr

111111111

I

L

e

5

’5 5 ‘5 UI

60 40 -

c

-nm C

*-

0

20

-

x 1

80

40

Days after sowing

lQc.7. Effects of waterlogging and liming on the incidence of withertop disorder due to Ca deficiency of fiber flax cultivar Lira1 Crown. Horizontal solid bar indicates the period of waterlogging. Plants were grown in soil in pots and were about 35 cm high when waterlogging began. Vertical bars indicate least significant differences ( p = 0.01). Treatments are as follows: (0-o),no lime; (0-o), 2.5 tons lime/ha; (H-B), 5 tons lime/ha; (0-0).10 tons lime/ha. (Adapted from Millikan, 1Wa.)

MINERAL NUTRITION OF LINSEED AND FIBER FLAX

259

The first symptom of withertop is the appearance of a sharp bend in the stem 5-10 cm below the apex. Stem tissue at the bend loses rigidity, collapses, and the top of the shoot dies, so that affected patches of the crop have a brown appearance when viewed from a distance. The stem below the bend remains apparently normal and, within a few days of the collapse of the top of the stem, secondary shoots develop immediately below the necrosis. These shoots develop normally, but they may also develop withertop symptoms, especially if the soil is waterlogged again. Mildly Ca-deficient flax plants often grow normally for about 16 weeks before a slight brownish discoloration appears about 4-5 cm below the shoot apex. This is followed within a few hours by collapse and necrosis of the discolored zone of the stem, although the shoot tip remains green for some time. Droplets of sugary liquid (probably sap from ruptured phloem sieve tubes) often appear on the surface of the stem near the zone of collapse (Hocking, unpublished). Within 2-3 days, the stem above the collapsed zone becomes necrotic in typical withertop development. If Ca deficiency occurs some time after bud formation, only a few flower stalks may collapse. A very mild form of the disorder, involving only a necrosis of the flower sepals from the tip downwards, has been observed in the field (Millikan, 1944a). The roots of mildly Ca-deficient flax plants become brown, and necrosis develops from their tips. Seedlings with severe Ca deficiency do not survive for long. The bottom leaves become abnormally dark green and the tops chlorotic. Necrosis of the upper leaves from their tips follows quickly, while the lower leaves remain green (Pierre and Allaway, 1941; Millikan, 1944a). Shoot apices die back and numerous secondary shoots subsequently develop near the base of the plant; the apices of these soon die, and the process is repeated, with the result that the plant develops a rounded bushy appearance (Roberts and Hocking, unpublished). Roots become necrotic and the plant usually dies. Plants with severe withertop produce short fibers of poor quality. Each fiber usually breaks during scutching at the point where the regrowth joins the main stem, thus producing two fibers of considerably less than acceptable length (Millikan, 1944a). Plants with extreme Ca deficiency may show no fiber cell development at all (Millikan, 1951a). There appears to be considerable genetic diversity among fiber flax and linseed lines in their susceptibility to withertop (Millikan, 1951a), so that the disorder could probably be controlled to a large extent by plant selection and breeding. For example, progeny of a tolerant selection from the cultivar Liral Crown showed only 15% incidence of withertop, whereas the parent Liral Crown line showed 64% incidence when both were grown under conditions conducive to the development of the disorder (Millikan, 1951a). Analysis of flax tops during the period of rapid stem elongation before flowering has indicated that concentrations of Ca below 5.0 g/kg dry weight

260

PETER J. HOCKING ET AL.

may be associated with withertop symptoms (Millikan, 1944a), although this threshold concentration of Ca varies with cultivar and locality (Table XIX). Withertop can be corrected by the application of from 2 to 4 tons lime/ha at sowing (Elliott, 1959), but some soils may require up to 10 tons lime/ha (Fig. 7) (Millikan, 1944a). Where lime is applied to fiber flax, excess should be avoided as it may injure the plants (Truninger, 1927) and cause pocr quality short fibers of low strength (Deterre, 1928; Robinson and Cook, 1931). It appears that seasonal conditions can interact with lime application on the yield quality of fiber flax. According to Kostiuchenko (1938), lime application has a favorable effect on fiber quality when soil moisture is adequate, but the same rate of lime may have detrimental effects in a dry season because of the uptake of excessive amounts of Ca by the plants, although the reason for this is not clear. The detrimental effects of excess Ca may be mitigated by the application of Mg (Robinson and Cook, 1931) or Byboth of which prevent the accumulation of excessive concentrations of Ca in the shoot (Table XVII) (Kostiuchenko, 1938). High rates of liming may induce a chlorosis of flax due to Fe deficiency (Scholz, 1934, 1937), although this can be cured by application of iron sulfate during early growth. Heavy liming can also induce P deficiency (Scholz, 1937) and incipient K deficiency (Magnitskii and Vladimirova, 1968). In addition, liming may decrease concentrations of B and Mn in flax, probably through the combined effects of inhibited uptake and translocation of these nutrients (Loneragan, 1974; Yamauchi et al., 1986). Table XIX Calcium Concentrations In Healthy and Calcium-Deficient Flax Plants from Commercial Crops in Victoria and South Australia',h Ca concentration (g/kg dry wt) Cultivar

Locality

Concurrent Gnarpurt Concurrent Birregurra Concurrent Irrewarra Concurrent Struan Liral Crown Struan Liral Crown Gisborne Liral Crown Diggers Rest Mean (all cultivars and localities)

Healthy plants

Deficient plants with withertop symptoms

5.1 5.6 5.1 6.8 5.5 1.2 6.9 6.0

2.8 4.3 4.3 4.0 5.4 6.3 4.1 4.5

~

'From Millikan (1944a). hPlants about 16 weeks old were cut off 18 cm above the soil and analyzed.

MINERAL NUTRITION OF LINSEED AND FIBER FLAX

26 1

In linseed, very high or low Ca supply may inhibit chlorophyll formation (especially chlorophyll a) by causing unbalanced uptake of other nutrients and by affecting cell membranes (Pal and Laloraya, 1972). Calciumdeficient linseed plants tend to accumulate abnormally high levels of sucrose in leaves, stems, and roots (Srivastava and Ranjan, 1970) and have increased respiration rates (Pal et al., 1973) but decreased protein contents (Pal and Laloraya, 1973).

3.

Copper

Flax has featured in several early studies on Cu deficiency. The essentiality of Cu as a plant nutrient was first demonstrated using linseed (Sommer, 1931), for linseed plants grown without Cu in the nutrient solution died within 7 weeks. Copper deficiency symptoms have been described only for vegetative plants grown in solution culture (Piper, 1942; Millikan, 1944b). Severely deficient plants are stunted, and young leaves are pale green, small, and twisted or puckered, with inrolled margins. Older leaves show a grayish green discoloration and later wither, but the lowest leaves may be unaffected. Secondary shoots formed on tillers are chlorotic and contorted. Mildly Cu-deficient linseed plants may grow normally until flowering, when they produce fewer flowers than normal and fail to set seed (Lipman and McKinney, 1931). As copper deficiency interferes with lignin synthesis and reduces pollen viability (Bussler, 1981), it would seriously impair production of either fiber or seed. Successful linseed crops have, however, been grown on peat soils in northwest Minnesota where wheat and other cereals have failed completely due to Cu deficiency, and this led Wallingford (1975) to conclude that linseed has a much lower Cu requirement than barley, oats, or wheat. The apparent low requirement for copper may explain why there are only a few reports of yield responses of field-grown flax crops to Cu fertilizer. Increases in fiber yields have been claimed for the application of 25-30 kg copper sulfate/ha to fiber flax growing on peat soils in the USSR (Bakhulin, 1934; Zenyuk, 1935; Zelenov, 1940). There is a report that the application of P fertilizer to a calcareous soil reduced concentrations of Cu in linseed shoots to 2.5 mg/kg, a level considered to be deficient for many crop species (Spratt and Smid, 1978). However, as plant Cu levels were raised only marginally by the application of chelated Cu at 3 kg/ha, and as the superphosphate used to supply the P in these experiments contained Mo, it was considered that a Mo-Cu antagonism, as well as an effect of P, could have been implicated in reducing plant Cu concentrations.

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PETER J. HOCKING E T A L .

4.

Iron

Iron deficiency has been reported in flax growing in soils of pH 8 and above in Germany (Scholz, 1934), southern Australia (Millikan, 1945; Cutting, 1974), the northern Great Plains of the United States of America (Moraghan, 1978) and the prairie provinces of Canada (Olomu and Racz, 1974). Mildly Fe-deficient flax plants grow fairly normally, apart from young leaves near shoot apices showing a pale yellowish chlorosis, whereas older lower leaves have a normal green appearance (Wallace, 1961). Young leaves of severely Fe-deficient plants have a bleached white or yellowish appearance, and growth is stunted (Millikan, 1951a). In the work of Moraghan (1978), tissue levels of 30-80 mg Fe/kg were associated with chlorotic symptoms in shoots of 21-day-old linseed plants, whereas normal plants typically had concentrations of 100-200 mg Fe/kg. The deficiency may be cured by a foliar spray of a 1% solution of iron sulfate (Millikan, 1945). Iron deficiency is particularly prevalent in linseed grown on calcareous black clay soils (pH 7-8) in southeastern South Australia (Cutting, 1974), and up to three aerial applications of aqueous iron sulfate may be required to ensure normal growth. In Australia, a soil dressing of 0.25 tons iron sulfate/ha has been effective in curing the deficiency, whereas a lighter soil dressing of 0.025 tons iron sulfate/ha was only partially effective (Millikan, 1945), due presumably to rapid immobilization of Fe in the soil. Much smaller applications of Fe appear to be effective when it is applied in the chelated form, ferric ethylenediaminedi(0-hydroxypheny1)-acetate (FeEDDHA) (Moraghan, 1978). Iron deficiency symptoms in flax are often worse in winter when temperatures are low and waterlogging occurs (Millikan, 1945), probably because these conditions tend to decrease or prevent translocation of Fe to the shoots and increase the Mn:Fe ratio (Olomu and Racz, 1974). Partial recovery of Fedeficient flax may occur in warmer weather (Millikan, 1951a), presumably as a result of increased transpiration and thus uptake of Fe. High levels of available Mn can induce Fe deficiency in flax (Millikan, 1950). Iron deficiency may occur in flax as part of a disorder syndrome associated with certain calcareous soils. In North America, this disorder is known as “chlorotic dieback” (Flor, 1943) and has been shown to be due to a combined deficiency of Fe and Zn (Moraghan, 1978). The symptoms include a chlorosis of younger leaves which is more severe in winter when the soils are cold and wet, a stunting of leaves and rosetting near the terminal bud, and, in severe cases, the death of the main stem and the development of lateral branches. Application of iron chelates alleviates only the chlorosis (Moraghan, 1978). Interactions between Fe and other nutrients have been reported in a number of studies with linseed. Soil application of 2 mg Fe/liter as iron

MINERAL NUTRITION OF LINSEED AND FIBER FLAX

263

chelate prevented the development of chlorosis of younger leaves and spotting and death of older leaves attributed to Mn toxicity in linseed grown on a soil of pH 8.1 which had very low extractable Fe (Moraghan and Freeman, 1978). Manganese concentrations in Fe-treated plants were reduced to 50 mg/kg from the toxic level of 590 mg/kg in untreated plants. However, caution should be exercised when using iron chelates. In a greenhouse study with linseed, Moraghan (1985) found that 2 mg Fe (as FeEDDHA)/kg soil inhibited Mn uptake to such an extent that young vegetative growth developed symptoms of Mn deficiency. Conversely, application of Mn to linseed can decrease its uptake of Fe, and this may result in a mild chlorosis associated with incipient Fe deficiency if the plantavailable Fe in the soil is marginal (Scholz, 1934). There appear to be quite large differences between linseed cultivars in the extent of the Fe-Mn interaction, and soil temperature can also interact with cultivar on Mn and Fe uptake (Table XX) (Wikoff and Moraghan, 1986). Application of iron chelate to linseed may also reduce Zn uptake by the plant and concentrations in most tissues, especially in older leaves (Moraghan, 1979; 1980b). Iron deficiency in many species leads to enhanced excretion of H + ions and chelating substances from roots, and to increased reducing activity in the apical zones of roots. The rate of reduction of Fe in the rhizosphere is increased, leading to greater availability and plant uptake. This Fe-stress response may also increase the rate of Mn reduction in the rhizosphere and its uptake by the plant (Marschner et al., 1986). In some calcareous soils low in available Fe, the increase in Mn uptake by linseed as a result of the Table X X Effects of Soil Temperature and Cultivar on Concentrations of Manganese and Iron in Linseed Shoot&' Concentration (mg/kg)

Soil temperature ("C)

Cultivar

30-Day-old plants

61-Day-old plants

Mn

Mn

Fe

Fe

~

15 24

F values'

Clark Culbert Clark Culbert Temperature Cultivar Temperature x cultivar

29 30 35 17 NS

*** *

84

26

73

160

14

75

35 16

84 61 67

101

* *** ***

**

*** **

*

*

NS

"Adapted from Wikoff and Moraghan (1986). hPlants grown in greenhouse in a Calciaquoll soil treated with 2 mg FeEDDHA-Fe/kg soil. CNS, Not significant; *, p = 0.05; **, p = 0.01; *Ip *= , 0.001.

264

PETER J. HOCKING ET A L .

Fe-stress response is considerable and may even be large enough to cause Mn toxicity (Moraghan and Freeman, 1978; Moraghan, 1979). Treatment of linseed with Fe can alleviate some chlorotic symptoms associated with toxicities of heavy metals such as Zn, Mn, and Ni, (Millikan, 1949), and leaf spotting due to excess accumulation of Mn (Moraghan and Freeman, 1978; Wikoff and Moraghan, 1986), but not the chlorosis of lower leaves produced by Mo toxicity (Millikan, 1949). The application of Fe, however, does not alleviate other symptoms characteristic of these toxicities (Millikan, 1950). 5. Magnesium

Growth of Mg-deficient flax appears normal or slightly stunted. Leaves are a paler green than normal, especially in the interveinal areas, so that the whole plant has a slightly chlorotic appearance. Lower leaves develop a yellowing which starts from their tips, and this later becomes a brown scorching as the leaves die, resulting in premature defoliation. The upper leaves usually appear normal, although they may be slightly chlorotic (Wallace, 1961).

6, Manganese a. Deficiency. Although Mn deficiency is common in a wide variety of crops, particularly in light calcareous soils, and flax is considered relatively susceptible to Mn deficiency (J. T. Moraghan, personal communication), there are few reliable reports of commercial flax crops being affected. Manganese deficiency in fiber flax has been reported from Scandinavia (Lundegardh, 1932) where, as with oats, it was referred to as “grey speck.” In sand culture, Mn-deficient flax plants show a pale yellowish chlorosis of leaves near the tips of shoots (Wallace, 1961). In the field, manganese sulfate applied at 58 kg/ha has corrected the deficiency in linseed growing on soils with low Mn indices (Johansen, 1942). Nitrate N fertilizer may exacerbate the deficiency on soils low in available Mn, and hence N is better applied as ammonium sulfate. Excess liming may increase the likelihood of Mn deficiency in linseed (Lundegardh, 1932). b. Toxicity. Manganese toxicity is commonly associated with acid soils of pH below 5 . 5 , which usually have high levels of plant-available (divalent) Mn. Flax grown on such soils in southern Australia may suffer from Mn toxicity, particularly if the crop follows immediately after pasture in the rotation, when higher concentrations of plant-available Mn in the soil may be expected (Millikan, 1948/1949). The toxicity is often apparent in winter, although the symptoms may disappear in spring.

MINERAL NUTRITION OF LINSEED AND FIBER FLAX

265

Although Mn toxicity is normally associated with acid soils, linseed growing in some soils of high pH may also suffer from Mn toxicity if Fe availability is low (Moraghan and Freeman, 1978). The plants develop an Fe-stress response which solubilizes Mn at a rate sufficient for it to accumulate to toxic levels, as discussed in Section IV,A,4. Symptoms of Mn toxicity in flax have been called “lower leaf scorch” because they begin on the middle to lower leaves as brown necrotic spots, particularly near the tips and margins. The distal half of the leaf becomes necrotic, but a characteristic of Mn toxicity of flax is that the necrosis usually does not spread to the proximal half of the leaf, which remains a normal green color. Growth is stunted as the symptoms appear on progressively younger leaves. Young leaves near the shoot apex may be slightly paler than normal (Millikan, 194811949; Moraghan and Freeman, 1978). The leaf spotting is probably the result of localized accumulations of Mn (Millikan, 1951b), as found for other species (Williams and Vlamis, 1957). Solution culture experiments suggest that the development of Mn toxicity in flax may be worse when N is supplied as nitrate rather than as ammonium, since plants supplied with ammonium N consistently have lower tissue concentrations of Mn (Millikan, 1950). Molybdenum application has been shown to ameliorate Mn toxicity symptoms of flax grown in acid soils (Millikan, 194811949) and in nutrient solution culture (Millikan, 1947b), but not in Fe-responsive calcareous soils (Moraghan and Freeman, 1978), in which Mo is likely to be more available. Tissue levels of Mn at which yield reductions occur in flax are not well defined. Concentrations of Mn in shoots of 7-10-week-old plants showing toxicity symptoms are usually in excess of 500 mg/kg (Millikan, 1948/1949; LaFleur and Craddock, 1973; Moraghan, 1979). However, interpretation of tissue analyses for diagnosis of toxicity must take into account the rapid decline in Mn concentrations with age (Millikan, 1948/1949; Moraghan and Ralowicz, 1979). Moraghan (1980b) found Mn levels in plants showing toxicity symptoms compared to normal plants of 550 versus 34 mg/kg in whole shoots, 2295 versus 133 mg/kg in old leaves, 95 versus 7 mg/kg in old stems, and 42 versus 34 mg/kg in roots. Hence, old leaves are the tissue of choice for diagnosis of Mn toxicity. Adequate sampling for Mn analysis is important because excess Mn accumulates in very localized areas of leaves with incipient necrosis (Millikan, 1951b; Moraghan and Freeman, 1978).

7. Molybdenum

a. Deficiency. Symptoms characteristic of Mo deficiency in flax include stunted growth, fading and withering of old leaves, young leaves turning a yellowish green and later developing light brown necrotic tips, decreased flowering, and wilting of pedicels about 2 cm behind the flower buds

266

PETER J. HOCKING E T A L .

(Hewitt, 1956). In mildly deficient flax, only leaves in the central zone of the tillers may show necrotic symptoms. In more severe cases, most of the lower leaves show symptoms, while in extremely deficient plants all the leaves (including the growing tips) may die and secondary shoots develop from the base of the plant; these shoots are usually chlorotic (Millikan, 1951a). Plants mildly affected by Mo deficiency may grow out of the condition later in the season, but their maturity is delayed (Millikan, 1951a). Molybdenum deficiency has been reported in flax grown on acidic soils (PH4.5-5.2) in Victoria, Australia, as one aspect of “lower leaf scorch,” a disorder related to soil acidity and Mn toxicity (Millikan, 1948/1949). Heavy dressings of manganese sulfate may depress Mo uptake by flax from some soils sufficiently to induce Mo deficiency (Anderson and Spencer, 1950a). The incidence of lower leaf scorch may be increased appreciably by the application of ammonium sulfate and by dry periods in late winter and early spring (Millikan, 1951a). In common with other nonleguminous plants, Mo-deficient flax has a depressed leaf protein content and elevated tissue levels of nitrate (Anderson and Spencer, 1950a) due to low nitrate reductase activity. Monitoring either tissue nitrate levels or nitrate reductase activity, following a test application of Mo, can provide an unequivocal identification of the deficiency (Randall, 1969). b. Toxicity. A golden yellow chlorosis, especially of the older leaves, indicates molybdenum toxicity in flax. The symptoms differ from those of Fe deficiency in that the yellowing is more intense and the chlorosis appears on all the leaves instead of only on the young leaves (Millikan, 1950). The symptoms have been described from greenhouse experiments; it is most unlikely that Mo toxicity would occur in field-grown flax. 8. Nitrogen

Nitrogen-deficient flax plants are stunted, with an upright habit and thin stems. Leaves are small, erect, pale green or yellowish green, and the plant defoliates prematurely from the base of the stem. Few tillers and capsules are produced, and the capsules tend to ripen early (Wallace, 1961). The symptoms are similar to those for P deficiency, but the two deficiencies can be distinguished by the initial darker-than-normal green of P-deficient leaves (Millikan, 1944a). Nitrogen deficiency is probably the most common nutritional disorder of fiber flax and linseed, as attested by the quantities of N fertilizer applied to these crops. It is therefore surprising to find no diagnostic criteria, such as critical N levels, in the literature, despite the numerous greenhouse and field experiments which have investigated yield responses of flax to a wide range of N supply.

MINERAL NUTRITION OF LINSEED AND FIBER FLAX

267

Nitrogen deficiency lowers the concentration of total and nonprotein N in flax, in contrast to S deficiency, which increases the N concentration (Anderson and Spencer, 1950b). Nitrogen-deficient flax does not respond in color or growth to increased S supply (Anderson and Spencer, 1950b).

9. Phosphorus Severely P-deficient flax plants are stunted, but mildly deficient shoots can grow to normal height. Deficient plants are thin and upright in habit, with leaves that are small and pressed upright against the stems (Millikan, 1944a). Leaves are initially a darker green than normal or a dull bluish green, but they become yellow and senesce prematurely from the base of the shoot, resulting in early defoliation. The plant has few tillers and flowering branches, and flower and capsule production are severely curtailed (Wallace, 1961). Although P deficiency is probably second to N deficiency as a nutritional disorder of flax, there are no reliable diagnostic criteria or critical levels for this nutrient in the literature. Phosphorus deficiency in fiber flax can change the allocation of carbon assimilates to the various plant organs (Kosheleva et al., 1983~). Phosphorus-deficient linseed plants have high levels of free amino acids, especially arginine, and low levels of protein-bound amino acids due to lack of P coenzymes (Ranjan and Malaviya, 1962). Similarly, total and nonprotein N concentrations increase in P-deficient flax,and the protein N content of all organs decreases (Pandey, 1968, 1975). Sucrose accumulates in P-deficient flax, but levels of several organic acids, including malate, are reduced (Pandey and Ranjan, 1964; Pandey, 1975). 10. Potassium

Shoot growth of K-deficient flax is slightly stunted, due mainly to a shortening of the internodes. In young plants, a brown scorching develops from the tips of lower leaves so that they eventually wither and fall off (Wallace, 1961). Upper leaves initially have a normal appearance, but they also become scorched and withered as the deficiency worsens (Millikan, 1944a). Symptoms of K deficiency, however, may not appear until just before flowering. The upper part of the shoot then becomes a slightly paler green than normal, and the tips of young leaves become twisted and scorched. Lower leaves develop the characteristic brown, scorched appearance and wither. The upper parts of stems become hollow and collapse if squeezed gently (Roberts and Hocking, unpublished). Later, the upper leaves also

PETER J. HOCKING ETAL.

268

become scorched and withered, and the stem tissue collapses about 5 cm below the shoot apex, the symptom being similar in many respects to that of the withertop disorder due to Ca deficiency. However, shoot collapse as a result of K deficiency can be distinguished from withertop by the immediate brown scorched appearance of the shoot tip, which does not remain green for a few days, and the absence of droplets of sugary sap exuded from the stem near the zone of tissue collapse. Potassium deficiency has little effect on the percentage fiber content of flax stems but reduces the technical length, the number of capsules per plant, and single seed weight (Table XXI) (Frederiksen, 1954a). Higher K concentrations are found in the upper than in the lower parts of the plant, whereas the reverse occurs in plants adequately supplied with K (Magnitskii and Vladimirova, 1968). Potassium deficiency results in a decreased capacity of flax to utilize other mineral nutrients effectively (Bailey and Soper 1985). A number of biochemical changes occur in K-deficient flax shoots, including increased levels of y-aminobutyric acid, alanine, arginine, glycine, serine, and sucrose (Pandey, 1975) but decreased levels of some organic acids (Pandey and Ranjan, 1964). The nonprotein N compound, putresine, usually accumulates in K-deficient plants, but it does not appear to be produced by K-deficient flax (Coleman and Richards, 1956). 11. Suljur

Sulfur-deficient flax plants are a pale yellowish green and are similar in appearance to N-deficient plants (Anderson and Spencer, 1950b). In older, mildly S-deficient plants, only the upper leaves are pale green. Tillering of S-deficient linseed is restricted and few capsules are produced (Pinkerton and Hocking, unpublished). Nitrate accumulates to high levels in S-deficient flax when nitrate is the main source of N, so S deficiency increases the concentrations of nonprotein Table X X I Effects of Potassium Deficiency on Yield and Quality Aspects of Fiber Flax?** K status of plant

Plant height (cm)

Technical length (cm)

~

Adequate Deficient

66 53

Number of capsules/ plant

weight (g)

Fiber content of stem (070)

5.2 4.1

20.5 18.9

loo0 Seed

~~

56 43

3.8

2.5

'From Frederiksen (1954a). bValues are means for plants from four fields of K-adequate fiber flax and four fields of K-deficient plants in Denmark.

MINERAL NUTRITION OF LINSEED AND FIBER FLAX

269

N and total N (Anderson and Spencer, 1950b). Citrulline, which is not normally present in healthy flax plants, is produced in moderate amounts by S-deficient plants. The level of arginine increases substantially (Coleman, 1957). Sucrose accumulates to abnormally high levels in all organs of S-deficient flax (Srivastava and Ranjan, 1970). 12. Zinc

a. Deficiency. Flax is classified as sensitive to Zn deficiency (Viets et al., 1954; Duncan, 1968; Moraghan, 1984), placing it in the same category as castor bean and corn. Other oilseed crops such as mustard, safflower, and sunflower are considered relatively insensitive to Zn deficiency. Zinc deficiency usually occurs within relatively small patches rather than throughout entire fields of linseed (Millikan, 195la; Moraghan, 1983). Symptoms of Zn deficiency of flax have been particularly well described by Millikan (1951a). Early symptoms are a slowing down of growth at the apex of the main stem, with the young leaves becoming stunted and a paler green. Older leaves on the main stem appear normal. This is followed by the appearance of grayish brown collapsed spots on the younger leaves, which subsequently become necrotic. Later-formed leaves near the shoot apex develop bronze spots, and the internodes in this region are short, giving the plant a rosettelike appearance. Growth of the shoot ceases and the top of the plant dies back as early as 3-4 weeks after emergence. Later, secondary shoots may proliferate from the base of the main shoot (Millikan, 1942, 1946b, 1951a; Anonymous, 1970; Moraghan, 1983). Some linseed lines show a higher incidence of wide bidentate leaves in response to Zn deficiency and develop fasciated stems (J. T. Moraghan, personal communication) which are similar to the fasciated stems of plants recovering from early B deficiency (see Section IV,A, 1,a). In Australia, flax crops sown in early to midwinter have shown a greater incidence of Zn deficiency than those sown in early spring (Millikan, 195la). Partial recovery of winter-sown crops from the deficiency may occur, however, as the root system extends and more zinc becomes available to the plant in the warmer weather of spring (Millikan, 1951a), but yield is usually adversely affected (Matheson, 1976). Maturity of the capsules is delayed by 10-14 days when mainstem dieback occurs, as flower production may be confined to the tillers (J. T. Moraghan, personal communication). Zinc deficiency in flax was first identified in Australia in 1942. The disorder, known as “dieback,” was a particular problem on light sandy soils in the south of Western Australia (Elliott, 1959). Zinc deficiency was prevalent in flax grown on black soils of the Wimmera District of Victoria, and has occurred sporadically in the Western District of Victoria (Millikan, 1942,

270

PETER J. HOCKING ET A L .

1944a). It was also a problem on heavy-textured soils of northern New South Wales (Hewitt, 1962), the mid-north of South Australia (Adam and Piper, 1944), and the Darling Downs and central-western region of Queensland (Anonymous, 1970; Douglas and Wade, 1985~).Elsewhere, chlorotic dieback symptoms have been reported in linseed growing in alkaline soils of the northern Great Plains of the United States of America (Flor, 1943; Moraghan, 1978, 1983), and linseed has been described as the crop most susceptible to Zn deficiency in that region (Moraghan, 1984). In Australia, dieback has been largely overcome by the addition of Zn to superphosphate (Elliott, 1959). Soil applications before sowing of 1 kg Zn/ha as zinc sulfate, for light acidic soils (Duncan, 1968), and 11-22 kg Zn/ha for alkaline soils (Cutting, 1974) also have been effective. In the northern Great Plains of the United States of America, an application of 11 kg Zn/ha as zinc sulfate is recommended if the soil has less than 0.6 mg Zn/kg extracted by diethylenetriamine pentaacetic acid (DPTA) (Moraghan, 1983). Zinc sulfate is best incorporated by plowing but if only a shallow incorporation is possible, application of 3.5 kg Zn/ha as zinc chelate is generally superior to zinc sulfate (Moraghan, 1983). Young flax plants grown on heavy alkaline soils are particularly susceptible to Zn deficiency (Cutting, 1974), but their recovery can be achieved by foliar application of a 1% zinc sulfate solution no later than 3 weeks after emergence (Duncan, 1968). Neither the use of seed from a crop well fertilized with Zn nor a seed dressing of zinc sulfate has proved a satisfactory alternative to foliar or soil application to correct the deficiency in linseed (Whitehouse, 1973). Low soil temperatures appear to accentuate the deficiency, probably by reducing the availability of Zn in the soil (Bauer and Lindsay, 1965). For example, Zn uptake by linseed was reduced as the soil temperature fell from 24 to 7 "C (see Table VIII), and the plants showed severe dieback symptoms in the absence of applied Zn. Responses to applied Zn were relatively much greater under the cooler conditions, and conversely, lower rates of Zn application were needed for maximum yield as the soil temperature increased (Moraghan, 1978, 1980a). Zinc deficiency of flax is often exacerbated by a long fallow before sowing (Anonymous, 1970). There is evidence from sand culture experiments that N supplied as nitrate alone can induce Zn deficiency in linseed (McIntyre, 1975), just as it can in subterranean clover (Ozanne, 1955). This effect probably arises because uptake of the nitrate ion by the plants raises the pH of the rhizosphere, rendering Zn unavailable (Sauchelli, 1969). It can be prevented or reduced by providing some of the N in the ammonium form, as preferential absorption of the ammonium ion by linseed lowers the rhizosphere pH and promotes Zn uptake (McIntyre, 1975). A high level of available P in soils increases the likelihood of Zn deficiency in many crops (Murphy and Walsh, 1972), and several instances of Zn

MINERAL NUTRITION OF LINSEED AND FIBER FLAX

27 1

deficiency in linseed induced by application of fertilizer P have been reported in Australia (Millikan, 1946b, 1947c, 1951a; Loneragan, 1951) and North America (Moraghan, 1983). Fertilizer P can induce Zn deficiency in calcareous soils (Millikan, 1946b), as well as in more acid soils (Cass-Smith and Harvey, 1948; Millikan, 1946b; Loneragan, 1951). Spratt and Smid (1978) concluded that linseed crops grown in calcareous soils should be monitored for Zn deficiency when the bicarbonate-extractable P is 20 mg/kg or above. It is likely that at least two effects are involved in P-induced Zn deficiency in flax. First, the application of P to responsive soils may stimulate growth and thus dilute plant Zn concentrations below the critical level (Loneragan, 1951; Moraghan, 1980b). Second, P and Zn may react in roots, resulting in decreased translocation of Zn from roots to the shoot (Loneragan, 1951; Burleson and Page, 1967). Flax roots normally contain a higher concentration of Zn than the shoot, and Spratt and Smid (1978) found that the difference was widened at high P levels. Negative correlations have been reported between P and Zn concentrations in shoots of linseed plants at the early seed filling stage (Fig. 8a) (Spratt and Smid, 1978), and shoots of Zndeficient fiber flax usually have P:Zn ratios greater than 150 (Fig. 9) (Millikan, 1951a; Millikan et al., 1968). A third aspect of the interaction is that current applications of P may have little effect on Zn uptake by linseed plants, but the residual effect of previously applied P on current Zn uptake may be considerable, indicating a long-term influence of P application on Zn availability (Fig. 8b) (Spratt and Smid, 1978). Liming, by raising the pH, may also enhance the susceptibility of flax to Zn deficiency, as there is a report of yield responses to applied Zn as the rate of liming was increased (Seatz et a/., 1959). b. Toxicity. We are not aware of any reports of Zn toxicity in field-grown flax, but Millikan (1947a) reported Zn toxicity in fiber flax grown for research purposes in a galvanized wire netting bird-proof cage. Initially, growth of affected plants was stunted. Later, a chlorosis then necrosis developed from the tips of older leaves, and these symptoms spread progressively up the plant. The top of the plant developed a rosetted appearance due to the reduced length of internodes. In some cases, the main shoot died and the plants produced secondary shoots from the basal node. Shoots showing toxicity symptoms typically contained 700-800 mg Zn/kg dry weight.

B. DIAGNOSTIC CRITERIA There are remarkably few diagnostic criteria for deficiencies of nutrients in fiber flax or linseed, and few correlations between yield and concentrations of

PETER J. HOCKING ET AL.

272

u

0

10

20

30

40

50

Residual P in top 15cm of soil (kg / ha )

.-C

c

N

20-

.c

0 C

.-o

15-

e c C 8

10-

c

E V 5L1

0.5

1

1.0

1.5

I

2.0

2.5

I

3.0

I

3.5

Concentration of Pin shoot ( g / kg )

FIG.8. Residual effects of fertilizer P on P and Zn concentrations in linseed. The fertilizer P was applied in 1965 at 0,100,200, and 400 kg/ha to two soils in Manitoba, Canada. The flax was grown in 1972 and harvested at the early seed-filling stage, 63 days after sowing. (a) Effects of residual P on Zn concentrations (-) and P:Zn concentration ratios (----)in linseed shoots grown on Carol clay ( 0 , m) and Waskada clay loam (0, 0 ) . (b) Relationship between concentrations of P and Zn in linseed shoots grown on Carol clay ( 0 ) and Waskada clay loam (0). (Adapted from Spratt and Smid, 1978.)

MINERAL NUTRITION OF LINSEED AND FIBER FLAX

273

0'

c

> 40

150

L

y= 0 . 3 1 ~ 75.9

;

160

200

r =0.9

I

1 00

1

120

140

180

220

P:Zn concentration ratio in shoots

FIG.9. Relationship between the severity of Zn deficiency symptoms and the P:Zn concentration ratios in shoots of 1 1-week-old linseed plants. Correlation coefficient significant at p = 0.01. Pot trial in soil of pH 8.2. Index of severity of Zn deficiency based on mean of proportion of plants showing symptoms of Zn deficiency and proportion of plants showing dieback symptoms. (Adapted from Millikan, 1951a.)

a nutrient in a particular organ. In developing diagnostic criteria, it is important to bear in mind that concentrations of key nutrients such as N, P, and K in fiber flax and linseed are highest during the early growth stages and decrease with plant age to maturity, irrespective of nutrient supply (Yurshis, 1976). Tserling e t a / . (1975) suggest that concentrations of N, P, and K should be determined in the basal portions of shoots at various stages of plant development for diagnostic purposes in fiber flax, but these authors give neither critical levels for the stages nor reasons for their choice of tissue. Corbean et al. (1979) claim that optimum concentrations (dry weight basis) in fiber flax shoots during the period of rapid vegetative growth before flowering are 3.9% N, 0.38% P, and 2.76% K if fiber flax follows maize or oats, and 2.2% N, 0.30% P, and 0.77% K if the crop follows wheat. The reasons are not clear, however, for the dependence of these concentrations on the preceding crop. According to Magnitskii and Vladimirova (1968), fiber flax plants at bud formation with 1.4 g K/kg and 1.1 g Na/kg fresh weight are likely to be K deficient, whereas those with 1.6 g K/kg and 1.7 g Na/kg fresh weight are adequate for K. Tissue concentrations of Na are apparently an important consideration in assessing if fiber flax is K deficient, because a small amount of Na can substitute for a much larger amount of K (Magnitskii and Vladimirova, 1968). Bailey and Spratt (1979) consider that a K concentration of 20.0 g/kg is required in linseed shoots at flowering for maximum seed yields of crops grown on the Canadian prairies. We have assembled from the literature tissue nutrient levels for flax that may be useful in assessing the nutritional status of the crop (Table XXII).

Table XXII Tissue Nutrient Concentrations for Flax Which May Aid Assessment of the Nutrient Status of the Crop Concentration range Age of

Nutrient Boron (mg/kg)

Calcium (g/kg)

Copper

Iron (mgkg)

Tissue analyzed Shoot Upper shoot Upper shoot Shoot Leaves Shoot Leaves Leaves Shoot Shoot Old leaves Young leaves Shoot Shoot Upper shoot Shoot Shoot Shoot Shoot Shoot Shoot Shoot Shoot Shoot Old leaves Young leaves Shoot

tissue (weeks)

4 10

12 6.5

7 7.5 8 8

9 10

10 10

11 13 16

7.5 9 9 11

3 7.5 9 9 10 10 10 11

Culture Pot Pot Field Field Pot Pot Pot Field Field Pot Pot Pot Field Pot Field Pot Pot Field Field Pot Pot Pot Field Pot Pot Pot Field

Showing deficiency or yield reduced or considered inadequate

Apparently adequate

or normal 25-35 20-40 21 48-70 15 9.6-17.0 12-20 24 9.2-16.2 13.6-14.4 31 M 2 . 6 20.6-21.4 5.5-7.3 32.7-44.5 6.0-7.2 3.5-9.8 4.5-9.3 4.8-7.8 5.1-6.7 97-242 111-240 73-88 112-166 52-84 73-271 80-122 %-I88

Very high or toxic

Reference Dizikovich (1968) Dzikovich (1%8) Dzikovich (1%8) Chauhan ef a/. (1984) Roberts and Hocking (unpublished) Moraghan (1979) Heikal ef a/. (1980) Dastur and Bhatt (1%5b) Spratt and Smid (1978) Moraghan (1980b) Moraghan (1980b) Moraghan (1980b) Spratt and Smid (1978) Moraghan (1979) Millikan (1944a) Moraghan (1979) Spratt and Smid (1978) Spratt and Smid (1978) Spratt and Smid (1978) Moraghan (1978) Moraghan (1979) Spratt and Smid (1978) Spratt and Smid (1978) Moraghan (198Ob) Moraghan (198Ob) Moraghan (1980b) Spratt and Smid (1978)

Shoot Shoot Shoot Shoot Shoot Shoot Old leaves Young leaves Shoot

7-10 9 9 9 10 10 10 11

Pot Pot Pot Pot Pot Field Field Field Pot Pot Pot Pot Pot Field Pot Pot Pot Field

Molybdenum (mg/kg)

Shoot

18

Pot

Nitrogen (g/kg)

Shoot Shoot Leaves Shoot Shoot Shoot Shoot

Phosphorus (g/kg)

Leaves Stem Shoot Leaves Leaves Shoot Shoot Shoot Old leaves Young leaves Shoot Shoot

6 7 8 9 9 9 11 7 7 7.5 8 8 9 9 9 10

Pot Field Field Pot Field Field Field Pot Pot Pot Pot Field Pot Pot Field Pot Pot Pot Field

Magnesium (g/kg)

Manganese (mg/kg)

Leaves Old leaves Young leaves Shoot Leaves Leaves Shoot Shoot Shoot

7 7 7 7.5 8 8 9 I1

13

-

10

10 11

1.6-3.1 2.7-3.5

6

14-18

50

-

108-145 10849 34 133 56 96-188 0.7-4.5

32.3

37.5-41.9 20.8-27.0 38 24.7-30.3 13.9-35.9 15.8-20.0 13.9-18.1

2.1-3.2 1.2-1.3

5.0-7.9 2.8 3.7-6.9 9.0

1.9-2.1

2.7-10.0 1.5-3.0 1.4-3.5 1.7-2.0 4.9-8.0 4.6-8.3 1.8-2.7

-

-

0.54.3

Roberts and Hocking (unpublished) Roberts and Hocking (unpublished) Roberts and Hocking (unpublished) Moraghan (1979) Heikal el u/. (1980) Dastur and Bhatt (I%5b) Spratt and Smid (1978) Spratt and Smid (1978) Bower and Pierre (1944)

5.8 7.2-7.5 5.5-6.3 3.6-6.5 4.0 5.0 3.9-7.8 3.2-4.7 9.5-15.6

5.0

500-2000

-

550 2295 1015

Wallace (I%]) Millikan (1948/1949) Moraghan (1985) Spratt and Smid (1978) Spratt and Smid (1978) Moraghan (1980b) Moraghan (198Ob) Moraghan (198Ob) Spratt and Smid (1978) Anderson and Spencer (195Oa) Turner (1922) Racz et u/. (1%5) Dastur and Bhatt (1%5b) Spratt and Smid (1978) Spratt and Smid (1978) Racz et a/. (1%5) Spratt and Smid (1978) Roberts and Hocking (unpublished) Roberts and Hocking (unpublished) Moraghan (1979) Heikal er o/. (1980) Dastur and Bhatt (1%5b) Moraghan (1978) Spratt and Smid (1978) Spratt and Smid (1978) Moraghan (1980b) Moraghan (1980b) Moraghan (1980b) Spratt and Smid (1978)

(continued)

Table XXII (Continued) Concentration range ~

Nutrient Potassium (B/kg)

Sodium (B/kg)

Sulfur (B/kg) Zinc (mg/kg)

Tissue analyzed Old leaves Young leaves Leaves Shoot Leaves Leaves Shoot Shoot Shoot Young leaves Shoot Shoot Shoot Shoot Shoot LeaVeS Shoot Old leaves Young leaves Shoot Leaves Stem Shoot Shoot Shoot Shoot Shoot Shoot Shoot Shoot

Age of tissue (weeks)

7 7 7 7.5 8 8 9 9 10 10 11

13

I5 6.5 7.5 8 10 10 10 13 7

7 6 7.5 9 9 9 10 10 11

Culture

Pot Pot Pot Pot Pot Field Pot Field Pot Pot Field Pot Field Field Pot Pot Pot Pot Pot Pot Pot Pot Field Pot Pot Pot Field Pot Pot Field

Showing deficiency or yield reduced or considered inadequate

6.8-21.0 19.2-33.2

-

-

15.5

-

2.0-2.3 0.8-0.9

15-18 -

10.4-14.9 18-20 12.7-19.0

-

Apparently adequate or normal

56.2-61.3 42.3-49.4 51.5 25-35 29 29 9.6-23.0 15.1-33.7 26.9-31.0 31.4-38.0 12.1-16.6 25.9

m

10.8-11.5 0.3-3.9 2.0-13.0 3.7-4.5 10.6-13.8 4.9-5.0 4.0-10.1 4.7 1.6

-

19-37 24-3 1 17-35 19.1-34.2 32-83 19.3-38.0 29.0-43.4

Very high or toxic

Reference Roberts and Hocking (unpublished) Roberts and Hocking (unpublished) Roberts and Hocking (unpublished) Moraghan (1979) Heikal er 01. (1980) Dastur and Bhatt (1%5b) Spratt and Smid (1978) Spratt and Smid (1978) Moraghan (198Ob) Moraghan (198Ob) Spratt and Smid (1978) Bower and Pierre (1944) Bailey and Spratt (1979) Chauhan ef ul. (1984) Moraghan (1979) Heikal ef ul. (1980) Moraghan (198Ob) Moraghan (1980b) Moraghan (198Ob) Bower and Pierre (1944) Roberts and Hocking (unpublished) Roberts and Hocking (unpublished) Millikan (I947b) Moraghan (1979) Moraghan (1978) Spratt and Smid (1978) Spratt and Smid (1978) Loneragan (1951) Moraghan (1980) Spratt and Smid (1978)

MINERAL NUTRITION OF LINSEED AND FIBER FLAX

277

We emphasize, however, that the values are presented only as guidelines in relation to tissue testing for diagnosis of nutrient deficiencies and toxicities, and not as critical levels. Not surprisingly, there are instances of overlapping ranges of values for the deficient and adequate categories. In some cases, this may be due to a Piper-Steenbjerg effect (Bates, 1971), in which a plant with a moderately high concentration of a nutrient may be either extremely deficient or have an adequate supply. Data from the linseed P-placement experiment of Sadler (1980) illustrate just how misleading tissue concentrations can be in relation to plant nutrient status and yield. Linseed was grown on a calcareous soil (pH 7.8) low in available P (2 mg/kg bicarbonate-extractable P). The best treatment at harvest 8 weeks after sowing was 20 kg P/ha banded 3 cm below the seeds: this yielded 4.84 tons dry matter/ha in contrast to 1.19 tons/ha for control plants (no P). However, the P concentration in plants from the best P treatment was 2.12 g/kg dry weight, compared with 2.32 g/kg for control plants. It is not possible to ascertain from the data whether this apparent anomaly is due to a PiperSteenbjerg effect or to some other factor. Seed yields of linseed are positively correlated with plant N content (Fig. 10) (Beech and Norman, 1968), especially during vegetative growth, and seed yields are positively correlated with P and K concentrations in the shoot during the budding and flowering stages (Corbean et al., 1981).

r

0

10

20

30

40

N content of shoots (kg/ ha)

FIG.10. The relationship between the N content of linseed tops at maturity and seed yield. Plants grown in the field under irrigation at Kununurra, Western Australia. (Adapted from Beech and Norman, 1968.)

278

PETER J . HOCKING ETAL.

V.

SALINITY

Linseed and fiber flax are classified in some reports as fairly intolerant of NaCl salinity (Grillot, 1956; Knowles et al., 1959), in others as having moderate salt tolerance @uPont, 1924; Hayward and Spurr, 1944; Van Den Berg, 1950a,b; United States Salinity Laboratory Staff, 1954; Hayward, 1956), and in others again as having high tolerance (Kearney and Scofield, 1936; Mitkees et al., 1972; Abo-Elsaod et al., 1974). In the Imperial Valley of California, irrigated linseed is regarded as more sensitive to saline soils than most other field crops and to be unprofitable if the salt content in the soil saturation extract has a conductivity above 5 mmhos/cm (Knowles et al., 1959). In a field study in India, linseed yields started to decline when the conductivity of the irrigation water reached about 6 mmhos/cm (Table XXIII) (Chauhan et al., 1984), although rapeseed grown under similar conditions was not adversely affected until the conductivity reached 17.5 mmhos/cm @eo and Ruhal, 1971). Heikal et al. (1980) found that concentrations of salt (NaCl) greater than 60 mol/m3 reduced the dry matter yield of dual-purpose flax (Table XXIV) and considered the crop as having moderate salt tolerance. High levels of salinity reduce germination, delay seedling emergence, and decrease plant height (Al-Mukhtar and ElHariri, 1984) and straw and seed yields (Chauhan et al., 1984) of flax and may delay flowering, capsule set, and seed maturity of linseed (Hayward and Spurr, 1944; Lehr and Wybenga, 1955). According to Van Den Berg (1950b) fiber percentage and fiber quality decline sharply when soil salinity (NaC1) rises above 0.3% in the saturation extract. A level of 0.8% NaCl

Table XXIII Effects of NaCl Salinity in Irrigation Water on Grain and Straw Yield of Llnseedozb Yield (tons/ha) NaCl salinity level (mmhos/cm) 2 4 8 12

SEC

Seed

Straw

1.36 1.48 1.28 0.88 0.06

4.60 4.48 3.40 2.44 0.22

uAdapted from Chauhan et ul. (1984). bValues are means of field experiments at Agra, India, over two seasons. SE, Standard error of the mean.

279

MINERAL NUTRITION OF LINSEED AND FIBER FLAX

Table XXIV Effects of NaCl Salinity on the Growth and Mineral Composition of Dual-Purpose Flax CuItivar Giza-@ Mineral concentration in leaves (g/kg dry wt) NaCl treatment (mol/m') 0 20

40 60 80 100

LSDC

Dry matter Wplant) 7.2 7.5 8.7 6.1 5.0 3.2 0.3

Na

K

Ca

Mg

P

N

2.0 13.3 20.0 25.6 31.0

41.6 28.6 23.0 17.0 10.0

4.1 5.3 4.0 3.5 3.5

8.9 13.1 18.8 19.8 21.8

-

-

15.7 15.1 14.1 13.7 8.2

-

-

13.0 20.3 12.3 10.0 8.9

1.2

2.6

1.7

0.3

2.2

0.8

-

-

'Adapted from Heikal et al. (1980). *Plants grown in solution culture. Seedlings (20 days old) transferred to NaCl treatments and grown for a further 60 days. CLSD,Least significant difference ( p = 0.05).

salinity in the soil depressed the seed yield of dual-purpose flax by 28% and fiber yield by nearly 40% (Table XXV) (Abo-Elsaod et al., 1974). The quality of the fiber from salt-stressed flax is poor because of a reduction in the proportion of phloem tissue in stems (Abo-Elsaod et al., 1974), and in fiber cell size (Tesu et al., 1979). Single seed weight and seed oil concentration, however, are not affected by up to 0.8% NaCl salinity (Abo-Elsaod et al., 1974) and overall, it appears that fiber yield is more sensitive to salinity than seed yield. Although high salinity may sometimes reduce the seed oil concentration of linseed, it may also improve the iodine value of the oil (Deo and Ruhal, 1971). Uptake of Na increases when fiber flax is grown in K-deficient soil, and Na may substitute for K in K-deficient plants to such an extent that yield is not reduced (Magnitskii and Vladimirova, 1968; Panitkin et a/., 1980). In linseed, tissue concentrations of K and K:Na ratios decrease with increasing NaCl salinity (Chauhan et al., 1984). The critical K:Na concentration ratio range for seed yield in 45-day-old linseed plants is from 1.4 to 2.1 (Chauhan et al., 1984). In solution culture, high salinity (80 mol/m3 NaC1) can reduce concentrations of K, CayMg, and N to 25% of normal but increase P concentrations in leaves and stems of young flax plants (Table XXIV) (Heikal et al., 1980). Tesu et at. (1979) found also that concentrations of P were increased and N were lowered in leaves and stems of salt-stressed flax plants, but that the opposite occurred in seeds. Fiber flax is reported to have a fairly large capacity to take up Na (Lewis, 1943; Bower and Pierre, 1944). According to Bower and Pierre (1944), this

Table X X V Effcet of Salinity on Growth, Yield Components,and Yield Quality of Dud-Purpose Flax Cnltivar Gizp-4a*b Straw components salinityc

added

Straw @/plant)

Fiber yield @/plant) 0.56 0.57 0.52

0.8

3.87 3.65 3.60 3.42 2.55

L s d

0.18

0.09

(To) 0

0.2 0.4 0.6

0.48

0.34

Seed components

Fiber content of stem (To) 14.4 15.6 14.4 13.5 13.3 1.3

Seed yield @/plant) 1.97

2.45 1.96 1.98 1.45 0.35

1OOOSeed weight @) 6.6 6.4 6.3 5.8 4.8

NS

Fiber quality

Seed oil concentration

Phloem in

Fiber length

Fiber fmeness

(To)

stem tissue (To)

(m)

index

Fiber strength (ocular classification)

38.5 38.5 38.2 37.7 37.7

29.8 29.4 28.3 24.6 19.2

45 44 41 37 34 2.4

75.0 70.1 67.1 62.7 58.9 3.2

7.0 6.0 5.0 4.0 3.0 0.5

NS

-

'Adapted from Abo-Elsaod el al. (1974). bPot experiment. 'Salinity mixture was NaCl, CaCI,. and MgSO,, 1:l:l by weight, added as percentage of dry weight of soil. %SD, Least significant difference (p = 0.05).

MINERAL NUTRITION OF LINSEED AND FIBER FLAX

28 1

Na prevents excessive uptake of Ca and Mg in relation to K when fiber flax is grown on K-deficient soils. The generally small response of fiber flax to K application, even for soils low in K, may be due to Na substituting for K in the plant (Bower and Pierre, 1944). Excessive Na, especially in the presence of high K, can induce mild Ca deficiency in fiber flax, resulting in withertop symptoms and low seed yield (Lehr and Wybenga, 1955). Chloride is readily absorbed by fiber flax, and high tissue levels appear to have adverse effects on fiber yields and quality (Schmalfuss, 1938; Brioux and Jouis, 1939; Opitz et al., 1939; Frederiksen, 1954b). Flax apparently takes up chloride more readily than sulfate (Opitz et al., 1939). There are a number of reports, however, which show that Na has a favorable influence on the development and quality of fiber flax straw (Lehr and Wybenga, 1955), in particular by increasing the fiber content of stems and the proportion of long fibers. In contrast, the same level of Na applied to linseed may cause a depression in seed yields. The best compromise level of Na has been stated as 6% of the exchangeable basic ions in the soil solution (Lehr and Wybenga, 1955). Although Na can partially replace K in fiber flax nutrition and in some cases increase fiber content, the N and P nutrition of the crop needs to be high to achieve this (Milthorpe, 1946).

The application of Na to fiber flax, however, is not a practical measure to be recommended. In reviewing 28 field experiments in Britain, Cooke and Warren (1959) concluded that it was undesirable to extend the practice of substituting agricultural salt (NaCl) for K fertilizers from sugar beet to fiber flax. Salt applications of 376 kg NaClha consistently reduced total shoot yields, straw yields, and the percentage of scutched fiber recovered. There appears to be genotypic potential for improving tolerance of fiber flax and linseed to salinity. For example, a study of 735 linseed lines in India showed that there was great variability in their tolerance to salinity, and that there were a number of salt-tolerant genotypes which could be exploited in breeding programs (Rai and Sinha, 1980). Tissue culture has been used to create a salt-tolerant linseed line (McHughen and Swartz, 1984); its performance on saline soils in Canada, however, has not been superior to established lines (Rowland, 1986).

VI.

SOIL ACIDITY

Flax is reported to grow satisfactorily in soils of pH 5 (Chizherskaia, 1927), although Selle (1926) found that fiber yields were highest on limerich soils and lowest on acidic soils. Flax can change the pH of its rhizosphere, and in solution culture Bison linseed plants lowered the pH of

282

PETER J. HOCKING ET A L .

their nutrient solution irrespective of the original composition of the solution or whether the source of N was nitrate or ammonium (Helgeson et al., 1940). We have an incomplete understanding of the reaction of flax to the major factors of the soil acidity complex. There is limited information only on the tolerance of flax, relative to other crops, to high plant-available Mn, high Al saturation, low levels of cations such as Ca and Mg, and low availability of Mo. As discussed in Section IV,A,6,b, Mn toxicity occurs in flax, as in other crops, in acid mineral soils with high levels of divalent , plant-available Mn (Millikan, 1948/1949). However, Mn toxicity has also been reported in soils of high pH which are low in available Fe, when the Fe-deficiency stress response of linseed roots solubilizes excessive Mn, as well as Fe (Moraghan, 1979; Moraghan and Freeman, 1978; Marschner et al., 1986). Hewitt (1952) reported that flax was relatively tolerant of high Mn. This conclusion was supported by Lohnis (1951), who showed that flax was little affected in soils where crops such as lucerne and green beans suffered severe Mn toxicity. Millikan (1949) found linseed less tolerant of excess Mn than oats, which are regarded as fairly tolerant, In sand culture experiments (De Marco, unpublished), linseed was able to withstand about the same concentration of Mn in solution as subterranean clover. In both species there were marked differences between cultivars. Variation between linseed lines in susceptibility to the Fe deficiency and Mn toxicity syndrome (see Section IV,A,4) (Moraghan and Ralowicz, 1979; Wikoff and Moraghan, 1986) may indicate the possibility of improving crop tolerance by selection. However, it is not known if this variation would also be manifest in soils with high divalent Mn levels without the complication of Fe deficiency. The tolerance of flax to Al toxicity appears to be low to moderate. Hewitt (1952) stated that growth of flax was reduced at levels of A1 which severely affected barley but did not injure oats. In solution culture, linseed (cultivar Glenelg), rape (cultivar Wesway), and the Al-sensitive wheat cultivar Egret were much less tolerant of 100 pA4 Al than the Brazilian wheat cultivar Carazinho or sunflower (cultivar Sunfola 683) (Randall and DeMarco, unpublished). Millikan (1951a) reported that 185 pA4 Al reduced root development of fiber flax in solution culture but had no effect on shoot growth. On the evidence available, it might be expected that flax would perform reasonably well on acid soils where Mn toxicity would limit the growth of Mn-sensitive species. In addition, Fe should be relatively available in most acid soils, and its uptake may reduce the incidence of Mn toxicity (Wikoff and Moraghan, 1986). However, on more acidic soils where high soluble Al levels constrain plant growth, flax is likely to perform poorly.

MINERAL NUTRITION OF LINSEED AND FIBER FLAX

283

Where liming is practiced to reduce soil acidity, the adverse effects of high rates on flax can be overcome by adding high rates of K, or by replacing some of the lime with dolomitic material (Avdonin and Ampilogov, 1974). In some regions of the USSR where fiber flax is grown on dernopodzolic acid soils of pH as low as 3.9 (Gritsenko and Batenchuk, 1975), large applications of lime to increase the pH have resulted in increased fiber yields without adversely affecting fiber quality (Shil’nikov et al., 1977).

VII.

INTERACTIONS BETWEEN GENOTYPE AND MINERAL NUTRITION A.

NUTRIENT-INDUCED GENOTROPHS

Durrant (1958, 1962, 1971) found that environmentally induced heritable changes occured in some cultivars of fiber flax grown with various nutrient treatments either in the greenhouse or the field. The progeny of plants of the cultivar Stormont Cirrus from certain N or combined N, P, and K treatments were larger than the parental line, but progeny from combined N and K treatments were smaller than the parental line when grown under uniform conditions. These variants, called genotrophs, were stable for several generations, although some other nutrient treatments gave rise to variants which remained plastic (Durrant, 1958, 1962). In later experiments, large genotrophs were produced by adding N as ammonium sulfate to the potting compost and small genotrophs by adding triple superphosphate to compost of low pH (Durrant, 1962, 1971). Early growth conditions, particularly temperature and pot size, were shown to be important for inducing genotrophs (Durrant, 1962, 1971; Durrant and Jones, 1971). Not all flax cultivars behave in this way. Heritable changes can be induced nutritionally in the fiber flax cultivars Stormont Cirrus and Lyral Prince, which are thus regarded as plastic cultivars, presumably as long as they are maintained in noninducing environments (Durrant, 197 1). Other cultivars are not plastic; for example, the linseed cultivar Royal does not produce genotrophs, at least in the environments tested. As the loss of plasticity is a heritable change, plasticity or lack of it may be a reflection of the ancestral nutritional environment of the variety (Durrant and Timmis, 1973).

These findings have stimulated a considerable amount of research into the nature of the nutritionally induced changes, and their genetic and molecular bases. The work has been reviewed by Cullis (1977) and Durrant (1981). The large genotroph induced in cultivar Stormont Cirrus was taller at maturity and produced up to six times more dry matter than the small genotroph. The morphology of the plants was distinctly different. The large

284

PETER J. HOCKING BTAL.

genotroph had more tillers than the small one, and thus they resembled the oilseed and fiber types, respectively. The original variety and the small genotroph had hairs on their capsules, while the large genotroph had hairless capsules. The large genotroph had 15% more nuclear DNA and a 60% greater number of ribosomal RNA genes than the small genotroph. Ghogain et al. (1982) showed that the large genotroph had considerably greater rates of seedling root growth. This was accompanied by increased accumulation of rRNA, apparently not as a direct consequence of the increased rRNA gene number but due mainly to posttranscriptionalcontrols, as rates of rRNA synthesis were similar. Further work on the molecular basis of the induction of heritable changes in plants in relation to the nutritional conditions under which they are grown should improve our understanding of gene control and may ultimately contribute to plant improvement. Questions concerningthe effect of such nutrient-inducedchanges on subsequent yield responses to fertilizersand on nutrient use efficiency and other nutritional characteristics have yet to be addressed. In the light of our present knowledge of the phenomenon in flax, care should be exercised in the production of seed for farmers, and in the choice of seed for agronomic and nutritional research, as well as in the interpretation of experiments. There are also potential ramifications of the phenomenon with respect to breeding and selection programs for yield improvement in flax. B. GENOTYPIC DIFFERENCES IN NUTRITIONAL CHARACTERISTICS Considerable variation exists within many crop species in the ability to acquire and use nutrients (Clark, 1982) and to tolerate mineral toxicities (Foy et al., 1978; Epstein, 1980). Attempts are being made to exploit this variation in plant breeding programs (Silva, 1976; Fehr, 1982; Graham, 1984). While there is no reason to suspect a lack of nutritional variation within flax, this aspect has been little studied. Johnson (1932) suggested that linseed cultivars responded differently to changes in soil fertility, and Garber (1960) reported that N, P, and K fertilizer application increased oil yields in some but not all of the fiber flax and linseed cultivars tested. According to Matheson (1976), there are substantial differences among linseed cultivars in their response to N, and it seems that linseed lines of European origin are more responsive to N than those of Indian ancestry. Blackman and Bunting (1951), in carefully designed multifactorial field experiments, found that linseed cultivars in some experiments differed in their yield response to N fertilizer. There are indications of similar differences in response to N among the linseed cultivars compared by Yayock and Quinn (1977).

MINERAL NUTRITION OF LINSEED AND FIBER FLAX

285

According to Millikan (1951a), there is considerable genetic variation within flax in susceptibility to the withertop disorder associated with Ca deficiency. This could be exploited in breeding programs aimed at improving the tolerance of linseed to the soil acidity complex, a component of which is a low level of available Ca. Differences in tolerance of the Fe deficiency and Mn toxicity syndrome found in some calcareous soils (see Section IV,A,4) were observed among four linseed lines carrying different flax rust resistance genes in a genetically similar background. The different responses could not be accounted for in terms of the Mn, Fe, or P concentrations in the plants (Moraghan and Ralowicz, 1979). Other work has shown genotypic differences among linseed cultivars in the accumulation of Fe and Mn by shoots, as well as differences in the Fe-Mn interaction with respect to applied FeEDDHA (Wikoff and Moraghan, 1986), which could be exploited to improve the tolerance of linseed to soil acidity. Genotypic differences in tolerance of salinity in linseed (Rai and Sinha, 1980) are discussed in Section V, and the use of tissue culture to produce salt-tolerant variants of linseed somaclonally (McHughen and Swartz, 1984) holds promise for the future, despite initial setbacks (Rowland, 1986).

VIII.

GENERAL CONCLUSIONS AND CHALLENGES

A considerable body of work exists on the responses of flax to fertilizer nutrients. The information relates to widely scattered areas of the world with different soil types, levels of soil fertility, climatic and seasonal conditions, farming practices, and yield expectations. Accordingly, it is not surprising that yield responses are variable and fertilizer recommendations frequently conflicting. There is, however, general agreement that high yields may be obtained with moderate fertilizer applications, at least of the major nutrients such as N and P. It is also clear that the nutritional requirements of linseed are fairly similar to those for fiber flax. Design of improved fertilizer management strategies to maximize yield under different conditions is difficult with the information available at present. The most valuable field trials are those which relate responses of flax due to applied nutrients to soil characteristics, soil test values, and environmental parameters, since they allow for more meaningful interpretation of the results and cautious extrapolation to similar sites. Unfortunately, many reports of fertilizer trials with flax do not include this information and hence are of restricted value. At the agronomic level there is a need for calibration of soil test values against the probability of yield response to fertilizer. This is particularly

286

PETER J. HOCKING ET A L .

important for P, as linseed and fiber flax are often reported to respond poorly to applied P compared with other species. Part of the reason for the poor response is apparently due to placing the fertilizer either too close to the seed, causing early damage (Nyborg and Hennig, 1969), or too far away from the seeds and thus inaccessible to the roots during early growth (Sadler, 1980; Sadler and Bailey, 1981). Flax roots explore only a very limited volume of soil during the first few weeks after germination (Sadler 1980), and knowledge of the appropriate rates and method of application of P fertilizer is thus important. In the longer term, selection for better early root geometry may be worth investigating to improve nutrient uptake and crop performance. At the physiological level, some knowledge of the kinetics of nutrient uptake from the soil solution appears necessary in order to understand better nutrient responses of flax compared with other crop species. To improve the efficiency of fertilizer application, research is required on phenological and physiological responses of flax to nutrient stresses. This should lead to an understanding of the effects of these stresses on yield components and help identify growth stages during which yield components are particularly sensitive to nutrient deprivation. Such an approach will also highlight the crop’s nutrient requirements at different growth stages to achieve maximum yield, thus providing guidelines for the rational application of fertilizer to flax, especially N to irrigated crops. Little attention has been paid to the internal nutrient requirements of flax to produce optimum yields and quality, and there is a paucity of data on which to base diagnostic indices. There is a need to study the distribution patterns of minerals within the flax plant during development to determine how these are modified by nutrient supply and to investigate the extent to which nutrients are redistributed from vegetative organs to developing capsules. Some information is already available for macronutrients but there is little for the micronutrients. This information is essential for the development of soundly based diagnostic criteria. Further, there are remarkably few published critical values for flax, to indicate incipient deficiency of any nutrient, or threshold values, to indicate toxicities. Little work has been reported on the role of S in the production of flax. Some attention to this appears to be warranted for linseed because of the importance of S in maintaining oil yield and meal protein quality in other oilseed crops (Randall and Wrigley, 1986) and the expectation that S deficiency may become more widespread in the future as a consequence of the increasing use of high-analysis P fertilizers which lack the adventitious S of single superphosphate. The ability of flax to tolerate acid soil conditions requires better definition. There are some reports of excess Mn limiting growth of flax in the field, but little is known of the degree of tolerance of commonly used cultivars to toxicities of Mn or Al.

MINERAL NUTRITION OF LINSEED AND FIBER FLAX

287

In many crops, considerable genetic diversity in the efficiency of nutrient uptake or utilization has been revealed, and plant breeding and selection programs have resulted in cultivars with superior nutritional characteristics. For example, cultivars of soybean resistant to the chlorosis associated with Fe deficiency have been released (Fehr, 1982), and Al-tolerant wheat varieties have resulted from selection on acid soils (Silva, 1976). In view of the lack of knowledge of the degree of genetic diversity in nutrient use efficiency and other nutritional characteristics of fiber flax and linseed, it is perhaps premature to suggest a plant breeding approach to overcome some of their more common nutritional problems. However, it should be worthwhile to investigate existing cultivars for variation in resistance to Fe and Zn deficiencies and to Mn and A1 toxicities as a first step in evaluating such a solution.

ACKNOWLEDGMENTS We thank Mrs. M. Onorato for preparing the typescript, and Professor J. T. Moraghan for his helpful comments on a draft of this review.

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ADVANCES IN AGRONOMY, VOL. 41

THE IMPACT OF SOIL AND FERTILIZER PHOSPHORUS ON THE ENVIRONMENT A. N. Sharpley and R. G. Menzel Water Quality and Watershed Research Laboratory United States Department of Agriculture (USDA) Agricultural Research Service (ARS), Durant, Oklahoma 74702

I.

INTRODUCTION

Phosphorus in the form of phosphate (P) is essential for plant growth, and its application to agricultural land often improves crop production. Production per unit area is increased with fertilizer P, The increased plant cover that is possible with proper use of fertilizer can reduce soil erosion from the cultivated area. Addition of phosphorus to fish ponds may also increase fish production. Although P is not toxic, the continued application of P fertilizer can produce detrimental effects in both the terrestrial and aquatic environments. In any discussion of the impact of soil and fertilizer P on the environment, the detrimental effects must be considered along with the benefits. Although there are no direct detrimental effects of P on the terrestrial environment, the continued application of fertilizer P to agricultural land can result in the buildup of natural trace contaminants contained in the fertilizer. There have been concerns, for example, about the accumulation of the contaminants cadmium, uranium, and radium in P-fertilized soils. The transport of P from the terrestrial to aquatic environment in surface and subsurface runoff can result in a deterioration in water quality from accelerated eutrophication. Eutrophication of surface water leads to problems with its use for fisheries, recreation, industry, or drinking, due to the increase in growth of undesirable algae and aquatic weeds. In addition to P, nitrogen (N) and carbon (C) are also commonly associated with these problems. However, control of accelerated eutrophication through limiting C and N inputs is restricted, due to the difficulty in controlling atmospheric exchanges of these elements. Thus, P is often the limiting element and its control is of prime importance in reducing the accelerated eutrophication of surface waters. It should be recognized, however, that even where the most severe eutrophication 297

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problems arise, there is no fundamental threat to the general environment, although fish kills can occur due to oxygen depletion in a water body. As Taylor and Kilmer (1980) pointed out, the problems are due to biological productivity of the aquatic environment when a lower level is desired for aesthetic, recreational, or economic reasons. Only part of the P in the terrestrial and aquatic environment is available for plant growth. While nearly all of the soluble inorganic P (Pi) is readily available, much of that attached to eroded soil or sediment (particulate P) may be unavailable. Soluble organic P (Po) may become available through hydrolysis and phosphatase enzyme mineralization. As exchange between the soluble and particulate forms of P can occur in both the terrestrial and aquatic environments, knowledge of these processes is essential in evaluating the impact of soil and fertilizer P on the environment. This chapter discusses the impacts of soil and fertilizer P on the terrestrial environment, the processes involved in the transport of P from the terrestrial to the aquatic environment, and finally, the impact of P on the aquatic environment.

II. IMPACT OF PHOSPHORUS ON THE TERRESTRIAL ENVIRONMENT A. BENEFICIAL EFFECTS

Fertilizer P use has become an integral and essential part of the food production system. Its use permits adequate food and fiber production for domestic consumption and export demands (Viets, 1975). In addition, fertilizer use permits this production on a reduced acreage, thereby benefiting the environment in several ways. If production is confined to a smaller acreage, less herbicides and insecticides are needed. Finally, if the more erodible land can be kept in grass or forest cover, erosion is minimized. Thus, a larger proportion of the terrestrial environment will have less disturbance, and less sedimentation will occur in aquatic environments. Fertilizer P has been used to establish a vegetative cover on infertile and badly eroded soils, which reduces the transport of water, soil, and nutrients in surface runoff. The fact that fertilizers can have a positive effect as a conservation tool to minimize runoff was recognized in New Zealand 30 years ago (Campbell, 1950). The role of P fertilizers in forest establishment, regeneration, and production on several soils has been demonstrated (Baule, 1973). Thus, from an economical and environmental standpoint, P fertilizers have an important beneficial function.

THE IMPACT OF SOIL AND FERTILIZER PHOSPHORUS

B.

299

DETRIMENTAL EFFECTS

Potentially toxic chemical elements may be introduced into the food chain by adding P fertilizer to the soil (Tremearne and Jacobs, 1941; Bowen, 1966; Lisk, 1972). This results from the fact that several heavy metals, such as arsenic, cadmium, chromium, lead, and vanadium, occur in P rock ore and are not eliminated during the manufacture of P fertilizer. The cadmium (Cd) content of P fertilizers has been studied extensively, due to its common occurrence in P rock, long-term persistence in the soil, uptake and accumulation by plants and animals, and toxicity at low levels (Schroeder and Balassa, 1961; Lagerwerff, 1971; Mortvedt, 1987). The Cd content of P fertilizer has been shown to vary with the source of the P rock and concentration of P in the fertilizer (Mortvedt and Giordano, 1977). Phophorus fertilizers produced from Florida deposits generally have a lower Cd content (10-20 mg Cd/kg) than those from western U.S. deposits (50-200 mg Cdlkg). Williams and David (1973) found that Australian P fertilizers contained from 25 to 50 mg Cd/kg. In addition, they observed that P and Cd contents of the fertilizer material were highly correlated and suggested that most of the Cd in P rock was concentrated in phosphoric acid during the manufacture of high-analysis fertilizers. Several studies have reported an increase in Cd content of soil in the cultivated layer following application of high rates of P fertilizer (Table I). At normally recommended fertilizer P rates, however, little Cd accumulation has been found in crops following long-term applications (>50 years) (Mortvedt, 1987). The accumulations of Cd in soil have resulted in increases in the Cd content of certain plants (Table I). Schroeder et al. (1967), however, found that the Cd concentration in many plant species did not increase and in several species decreased as a result of fertilizer P application. Apparently, differences in Cd uptake occur between plant species. Furthermore, the bioavailability of Cd increases with a reduction in soil pH (CAST, 1976; Williams and David, 1976; Mortvedt et al., 1981) due to a decrease in Cd sorption on the soil (Anderson and Nielson, 1974; Garcia-Miragaya and Page, 1978; Jarvis and Jones, 1980). Consequently, long-term production of Cd-accumulating crops on acid soils (PH 5.5) may require special P fertilizers with low Cd content. No detectable increase in arsenic, chromium, lead, or vanadium concentration in soil was found following the application of 8888 kg/ha of concentrated superphosphate (Goodroad and Caldwell, 1979). It is unlikely that there is any danger of contamination following P fertilization as long as the content of these elements in P fertilizers remains low. Similarly, Mortvedt and Giordano (1977) concluded that the plant uptake of chromium and lead in fertilizer was not significant at the rates of P usually used. In addition to heavy metals, P fertilizers contain radioactive material from the rock source in amounts between 30 and 200 mg/kg uranium (v)

Table I Effect of P FertWzer Application on the Amonnt of Cadmiam in Soils and Plants Cadmium content of cadmium

Reference Andersson and hlahlin (1981)

Description Clay: barley grain

Fertilizer P applied WhdYr)

Unfertilized

(yr)

15 15 15

23 23 23

0.23 0.23 0.23

0.022'

15 45

15 15 15

23 23 23

0.07 0.07

5 15 45

Fine sand: barley

Plant

soid

content of fertilizer (mgW

5

Duration

Unfertilized (mg/kg)

Fertilized (mgks)

0.024'

0.013 0.013 0.013

0.013' 0.013' 0.016

0.06'

-

-

0.07

0.070 0.W

-

-

(mgflrg)

Fertilized (mglkg)

0.023'

Andrews ef of. (1979)

Clay loam: grass/clover

40

30

5

0.02

0.04

0.003

0.005

Mortvedt er of. (1981)

Sit loam: wheat grain straw

50 50

2

0.07

2

153 153

0.07

0.11 0.11

0.03 0.07

0.09 0.12

Mulla ef of. (1980)

Sandy loam: barley Swiss chard

175 175

36 36

174 174

0.7 0.7

1.o 1.o

0.01 0.26

0.01' 1.60

Reuss ef of. (1978)

Silt loam: raddish lettuce peas

127 127 127

1 1 1

174 174 174

-

-

0.40 0.20

3.40 6.30

0.20

0.90

Krasnozem: oats clover lucerne

125 125 125

20 20

50

0.05 0.05 0.05

0.45 0.45 0.45

0.03 0.10 0.09

0.28 1.07 0.46

Williams and David (1973, 1976)

'No significant effect at 5% level.

20

50 50

-

THE IMPACT OF SOIL AND FERTILIZER PHOSPHORUS

301

and 10 mg/kg thorium (Caro, 1964; Barrows, 1966; Menzel, 1968). Thus, the addition of radionuclides with high application rates of superphosphate for a century is similar to amounts occurring naturally in the plow layer. Marsden (1964) reported that topdressing of pasture with superphosphate for 16 years at the high annual rate of 2200 kg/ha resulted in only a 5 % increase in a activity in the soil. In a recent investigation of U accumulation from the long-term application of superphosphate (33 kgP/ha/yr) to a clay loam under pasture at Rothamsted, England, Rothbaum et al. (1979) observed that most of the U applied since 1889 (1.3 kg/ha) was retained, like P, in the plow layer. The radiation hazard which might result from the uptake of radionuclides into food plants from applied fertilizer appears to be negligible (Menzel, 1968; Mays and Mortvedt, 1986). The production of phosphoric acid removes almost all radioactive contaminants. Thus, highanalysis P fertilizers have no radiation hazard. Due to similar strengths of P and U sorption by soil, Menzel(l968) suggested that losses of U from surface soil could occur by erosion and might be similar to the losses of added fertilizer P. In addition, U is generally considered to be mobile in the absence of organic matter (Hostetler and Garrels, 1962; Schultz, 1965) and may, thus, leach from sandy soils containing little organic matter. In fact, a significant increase in the U content of rivers draining intensively fertilized and farmed agricultural land in the southwestern United States was measured by Spalding and Sackett (1972). The U increase was in some cases attributed to the application of P fertilizer. The heavy metal and radionuclide contaminants discussed are generally strongly absorbed by soil, as is P. Consequently, these contaminants may be preferentially transported with finer soil particles during rainfall and erosion and accumulate in deposited sediment.

111. TRANSPORT OF PHOSPHORUS FROM THE TERRESTRIAL TO AQUATIC ENVIRONMENTS The transport of P from terrestrial to aquatic environments in runoff can occur as either soluble or particulate P. The term particulate P includes P sorbed by soil particles and organic matter eroded during runoff. Soil erosion is a selective process in which runoff sediment becomes enriched in finer-sized particles and lighter organic matter. Because P is strongly absorbed on clay particles (Syers et al., 1973a; Barrow, 1978; Parfitt, 1978; Sibbesen, 1981) and organic matter contains relatively high levels of P, the major proportion of P transported to the aquatic environment from cultivated land is usually in the particulate form (Burwell et al., 1977;

302

A. N. SHARPLEY AND

R. G. MENZEL

Logan et al., 1979; Nelson et al., 1979; Sharpley and Syers, 1979). In runoff from grassland or forest soils, which carries little suspended soil, most of the P may be transported in the soluble form (Burwell et al., 1975; Singer and Rust, 1975). Most soluble P forms found in runoff are biologically available, but the bioavailability of particulate P from various sources differs greatly (Syers et al., 1973b; Porter, 1975; Lee et al., 1978; McCallister and Logan, 1978; Logan et al., 1979). In addition, transformations between the two P forms can occur during transport (Carter et al., 1971; Kunishi et al., 1972; Sharpley et al., 1981~).Consequently, knowledge of the mechanisms involved in the extraction and detachment of soluble and particulate P during runoff, in addition to knowledge of the nature of the particulate matter in runoff and the various sources and amounts of P, is important in evaluating the impact of soil and fertilizer P on the aquatic environment. A.

AMOUNTS TRANSPORTED FROM TERRESTRIAL ENVIRONMENTS

Increases in the amounts of soluble and particulate P transported in surface runoff have been measured after the application of fertilizer P (Table 11). These increases result from an increase in the available P content of surface soil (Barrow and Shaw, 1975; Elrashidi and Larsen, 1978; Fukely, 1978; Barber, 1979) and total P content of eroded soil material, respectively, compared to unfertilized soil. The losses of fertilizer P are influenced by the rate, time, and method of fertilizer application; form of fertilizer; amount and time of rainfall after application; and vegetative cover. Detailed reviews of the effect of fertilizer P on the amounts of P transported from agricultural land have been presented previously (Ryden et al., 1973; Viets, 1975; Timmons and Holt, 1980). Though it is difficult to distinguish between losses of fertilizer P and native soil P, the losses of fertilizer P are generally less than 1% of that applied. The losses of P in subsurface drainage are small, with applications of fertilizer at recommended rates normally having no significant effect on P losses. Phosphorus losses in surface runoff may be reduced by incorporating fertilizer material into the surface soil away from the zone of extraction and detachment and by using conservation or minimum tillage methods to reduce soil erosion. The two main consequences of conservation tillage are the increase in amount of residues on the surface and the reduction in mechanical manipulation and mixing of the soil. Although this may result in decreased runoff volumes (Burwell and Kramer et al., 1983; Langdale et al., 1983; McDowell and McGregor, 1984; Moldenhauer et al., 1983; Wendt and Burwell, 1985), P can build up in the surface 0-3 cm of soil (McDowell and McGregor, 1984; Randall, 1980; Wells, 1985). Consequently, the interaction between runoff water and surface soil and subsequent transport of

THE IMPACT OF SOIL AND FERTILIZER PHOSPHORUS

303

soluble and particulate P in runoff is affected. The loss of total P in runoff from conservation tillage practices is lower than from conventional practices, due to reduced erosion rates and runoff volumes with the former (Andraski et al., 1985; Langdale et al., 1985; McDowell and McGregor, 1984). In contrast, slight increases in soluble P loss from no-till land with unincorporated residues compared to incorporated residues have been reported (Romkens et al., 1973; Barsisas et al., 1978; Reddy et al., 1978; Langdale et al., 1985). For example, McDowell and McGregor (1984) found that conservation tillage (no-till) reduced total P losses nine-fold compared to conventional practices for corn (for silage) in Mississippi. However, an eight-fold increase in soluble P loss was measured for the no-till compared to conventional tillage. Conservation tillage practices have been shown to increase the proportion of clay-sized particles transported in runoff and, thus, increase the P:sediment ratio (Logan and Adams, 1981). In a recent study, Andraski et al. (1985) reported a 63% reduction in algal available P (resin extractable) loss in runoff from no-till compared to conventionally tilled corn. This highlights the need to consider the algal availability (or bioavailability) of particulate P transported in runoff when evaluating management decisions aimed at reducing the impact of P on the trophic status of a waterbody. In an attempt to improve the trophic status of Lake Erie, The U.S. Army Corps of Engineers (1982) showed that nonpoint-source particulate P load needed to be reduced by about 26% to reduce eutrophication significantly. From field experimentation and calculation, Forster et al. (1985) concluded that accelerated implementation of conservation tillage, including no-till, on soils suited to those practices on the United States side of Lake Erie basin can achieve the required particulate P load reduction in 20 years. Even though the amounts of P transported in runoff may be small compared with amounts applied, it is evident that P concentrations of both surface and subsurface runoff are greater than the critical values (0.01 and 0.02 mg/liter for soluble and total P, respectively) suggested by Sawyer (1947) and Vollenweider (1968), above which biological growth can be stimulated. This is also true for several unfertilized watersheds (Table 11). In addition, P levels in rainfall may exceed the critical values (Schindler and Nighswander, 1970; Murphy and Doskey, 1975; Sharpley et al., 1985b; Tabatabai et al., 1981) and can result in natural eutrophication (Schindler, 1977; Lee, 1973). Consequently, the critical P level approach should not be used as the sole criterion in quantifying permissible tolerance levels of P in surface runoff as a result of differing management practices (Sharpley et al., 1985a). B.

PHOSPHORUS DESORPTION

The first step in the transport of soluble P is the desorption and dissolution of P from soil. The desorption of P from soil material in relation to

Table Il Effect of P Fertilization on the Concentration and Amounts of P Trnnsported in Surface Runoff and Subsurfnce Drainage Concentration Reference and location

Land use

P applied (kg/ha/yr)

Soluble P (mghter)

Amount

Particulate P (mg/liter)

Soluble P (kg/ha/yr)

Particulate P (kg/ha/yr)

33.15 0.02 18.19

-

0.10 0.39 0.25 0.15 0.14

1.27 0.71 0.40 1.14

0.15 0.12 0.16 0.10

0.76 0.45 0.08 0.20

Surface runoff Burwell et u/. (1975). Minnesota

Burwell el a/. (1977), Iowa

McColl et ul. (1977), New Zealand McDowell et a/. (1980), Mississippi

Fallow Hay Contour corn Rotation corn Rotation oats

0 0 29 29 30

8.43 5.01

Contour corn Contour corn Grazed bromegrass Terraced corn

41 67

0.25 0.19 0.80 0.57

Native forest Pasture

0 75

0.01 0.03

0.06 0.14

0.01 0.04

0.20 0.29

Corn grain Corn silage

30 30

0.11 0.05

4.5 4.4

4.3 0.2

0.02 0.02

66

40

Menzel et 01. (1978). Oklahoma

0 6.5 25

0.02 0.28 0.72

0.6 0.75 1.00

0.04 0.30 1.10

0.50 1.90

0 54 0 56 113

0.3 3.7 0.07 0.24 0.44

1.8 7.4 -

0.20 1.20

1.40 2.90

Pasture

0 50

0.20 0.98

0.24 9.61

0.50 2.80

0.67 2.74

Alfalfa (tile drainage) Continuous corn Continuous corn Grazed bromegrass Terraced corn

0 29

0.180 0.210

-

0.12 0.19

66 40 41 67

0.009

-

0.007 0.005 0.028

-

0.04 0.03 0.03 0.17

Hanway and Laflen (1974), Iowa

Corn (tile drainage)

38

0.018

96

O.Oo0

100

0.004

Sharpley and Syers (1979), New Zealand

Pasture

0 50 0 50

0.020 0.033

-

-

Nicholaichuk and Read (1978), western Canada Romkens and Nelson (1974), Indiana Sharpley and Syers (1979), New Zealand w v 0 ,

Rotation grazing Wheat Cotton Wheat/ summerfallow Fallow

-

-

5.60

-

Subsurface drainage Bolton el 01. (1970), Canada Burwell el al. (1977), Iowa

Pasture (tile drainage)

0.064 0.190

0.005 O.Oo0 0.004 0.004

0.12 0.08

0.44

-

306

A. N. SHARPLEY AND R. G. MENZEL

plant availability and water quality has been studied using various extraction mediums and so1ution:soil ratios (Table 111). Few studies have used filtered runoff (Wang, 1974) or lake water (Bahnick, 1977) as the support medium, due to the technical problems involved in preparing large volumes of filtrate of constant chemical composition. Bahnick (1977) reported an increase in P desorption from clay deposits from Lake Superior to deionized water compared to filtered Lake Superior water, which was attributed to a lower pH of the deionized water. As the amount of P desorbed depends upon the ionic strength and cationic species in extracting medium (Ryden and Syers, 1977b) and so1ution:soil ratio used (Hope and Syers, 1976; Barrow and Shaw, 1979; Sharpley et al., 1981b), a need for the standardization of methodologies used to relate P desorbed to the potential availability of P to plants and transport in runoff is indicated. These conditions should be related to the soil solution composition, simulating field conditions as closely as possible (Wendt and Alberts, 1984). Experimental conditions should Table I11 Methods Used to Determine Desorbable P Extractant 0.1 M NaCl

0.01 M CaCI,

Anion exchange resin (Dowex I-X4) (Dowex I-XB) (Dowex bX4) (Dowex 21K) Distilled water

j*P + distilled water j2P + 0.1 M NaCl I*P + 0.1 M NaCl jrP + stream water Filtered lake water (Lake Superior)

Solution: soil ratio

References

100: 1 50 : 1 50 : 1

Li et at. (1972) Ryden et at. (1972) Romkens and Nelson (1974)

10: 1 10: 1 10: 1 5: 1 25 : 1 6 : 1-300 : 1 50: 1

30 : 1 100: 1 25 : 1 50 : 1

White and Beckett (1964) Taylor and Kunishi (1971) Gardner and Jones (1973) Elrashidi and Larsen (1978) Green el a/. (1978) Barrow (1979) Oloya and Logan (1980) Ballaux and Peaslee (1975) Evans and Jurinak (1976) Vig ef at. (1979) Bache and Ireland (1980)

100: 1 - 1 m : I

Sharpley et al. (1981b) Bahnick (1977) Baker (1964) Li el a/. (1972) Ryden and Syers (1977a) Schreiber et a/. (1977)

2000: 1-4OOo: 1

Bahnick (1977)

10 : 1-1000 : 1 2000 : 1-4000 : 1

1:l 100: 1 40: 1

THE IMPACT OF SOIL AND FERTILIZER PHOSPHORUS

307

be governed by soil type and processes simulated, be it release to soil solution (narrow so1ution:soil ratio) or runoff (wide so1ution:soil ratio). The desorption of soil P is rapid (Kunishi et al., 1972; Ryden and Syers, 1977b; Oloya and Logan, 1980). Evans and Jurinak (1976) reported that 50% of P desorbed from a desert soil in 50 hr occurred in the first hour of reaction, and Sharpley et al. (1981b) found that approximately 75% of the P desorbed in 4 hr from several soils occurred in the initial 30 min. Consequently, P can be desorbed from surface soil by short rainfall and runoff events. In fact, a highly significant linear relationship between the amount of desorbable P in the surface soil and the soluble P concentration of surface runoff has been found (Hanway and Laflen, 1974; Romkens and Nelson, 1974; Sharpley et al., 1978, and 1981a). In the case of soils containing low amounts of readily desorbed P (10-90 mg P/kg), Oloya and Logan (1980) observed that the pool of desorbable P was large compared to the amount that readily desorbed (5-8Vo of the desorbable P in 24 hr). Consequently, these soils may release low levels of P over a long period of time. The desorption of soil P by rainfall runoff water is brought about by interaction with a thin layer of surface soil (1-3 mm) (Donigian et al., 1977; Ahuja et al., 1981; Sharp’ et al., 1981a; Sharpley, 1985a). If the surface water percolates through the il profile, sorption of P by P-deficient subsoils generally results in low concentrations of soluble P in subsurface flow (Ryden et a/., 1973; Baker et al., 1975; Burwell et al., 1977; Sawhney, 1977; Sharpley and Syers, 1979). Exceptions may occur in organic or peaty soils, where organic matter may accelerate the downward movement of P together with organic acids and Fe and A1 (Fox and Kamprath, 1971; Hortensteine and Forbes, 1972; Singh and Jones, 1976; Duxbury and Peverly, 1978; Miller, 1979). Similarly, P is more susceptible to movement through sandy soils with low P sorption capacities (Ozanne et al., 1961; Adriano et al., 1975; Sawhney, 1977) and in soils which have become waterlogged, where a decrease in Fe (111) content occurs (Ponnamperuma, 1972; Gotoh and Patrick, 1974; Khalid et al., 1977). ~

c.

PHOSPHORUS

LEACHED FROM VEGETATION

The transport of P from the terrestrial to aquatic environments may occur through the leaching and washoff of P from growing and decaying plant material (Gburek and Broyan, 1974; McDowell et al., 1980; Schreiber and McDowell, 1985; Schreiber, 1985; Sharpley, 1981). Numerous studies have suggested that the leaching of vegetation in different stages of growth and decay may account in part for the seasonal fluctuations in soluble P transported in runoff from various watersheds (Kleusner, 1972; Wells et al., 1972; Gosz et al., 1973; White and Williamson, 1973; Burwell et al., 1975; McDowell et al., 1980). Similarly, Muir et al. (1973), Burwell et al. (1974),

308

A.

N. SHARPLEY AND R.

G. MENZEL

and Gburek and Heald (1974) attributed differences in amounts of P transported to differences in the type of vegetation from watershed to watershed. Increased losses of soluble P in runoff from alfalfa plots (33 g P/ha) compared to forested (4 g Piha), oats (16 g P/ha), and corn plots (1 1 g P/ha), were attributed to the larger amounts of P leached from alfalfa (Wendt and Corey, 1980). In addition, White et al. (1977) reported that nutrient transport in surface runoff was significantly related to runoff amounts and that 50-75070 of the data variation was explained by this factor. The unexplained variation was partially attributed to plant cover, growth, and stage of decomposition. The extraction of cut vegetation by deionized water has shown that large differences in the amounts of P leached can occur with vegetation type and that they increased dramatically following freezing and thawing of the vegetation (Timmons et al., 1970; Bromfield and Jones, 1972; Cowen and Lee, 1973; White, 1973). Using a multiple-intensity rainfall simulator, Schreiber and McDowell (1985) measured 125 g P/ha (6.3% of wheat P) leached from wheat straw residue (4500 kg/ha loading rate) during a 25-mm rainfall of 25 mm/hr. Although the P losses generally increased as wheat residue loading rate increased, the percentage of P removed from the wheat residue decreased, due possibly to an easier pathway of nutrient movement from the residue surface to runoff at lower residue loadings (Schreiber, 1985). In one study, Sharpley (1981) found that growing cotton, sorghum, and soybean plants could maintain concentrations of soluble P in plant leachate (0.018-0.154 mg P/liter) similar to those released from unfertilized soil. For healthy mature plants, leached P accounted for approximately 20% of soluble P transported in surface runoff. Under conditions in which plants became P deficient and senescing plants, however, canopy leachate contributed the major proportion (90%) of the soluble P transported in surface runoff. Depending on the relative rates of infiltration and runoff, a portion of plant leachate may infiltrate the soil and be recycled. AND TRANSPORT OF PARTICULATE PHOSPHORUS D. DETACHMENT

Sources of sediment and particulate P in streams include eroding surface soil, subsoil, and stream sediments derived from streambanks and channel beds. The primary source of sediment in watersheds with a permanent vegetative cover, such as forest or pasture, is from streambank erosion. This sediment will have characteristics similar to the subsoils or parent material of the area, which are often P deficient. In cultivated watersheds, however, streambank erosion constitutes a smaller proportion of the sediment eroded, due to surface soil erosion. Although attempts to identify sediment sources have been made using sediment mineralogy (Klages and Hsieh, 1975; Wall and Wilding, 1976; Sawhney and Frink, 1978), erroneous

THE IMPACT OF SOIL AND FERTILIZER PHOSPHORUS

309

conclusions can be made if preferential erosion of clay minerals occurs during transport (Jones et al., 1977; Murad and Fischer, 1978; Rhoton et al., 1979). The degree of enrichment of P in runoff sediment due to the preferential transport of finer-sized particles and lighter organic matter discussed earlier is expressed as an enrichment ratio. For P, this is calculated as the ratio of the concentration of P in the runoff sediment to that in the source soil. Enrichment ratio values of 1.3 for total P and 3.3 for 0.001 M H,S04 extractable P for a silt loam situated on a 20-25% slope were observed by Rogers (1941), whereas Massey and Jackson (1952) observed values between 1.9 and 2.2 for “water soluble plus pH 3 extractable” P for silt loams in Wisconsin. Other ER values have ranged from 1.5 to 8.9 for total P (Knoblauch et al., 1942; Neal, 1944; Stoltenberg and White, 1953; Duffy et al., 1978). Sharpley (1985b) observed that the enrichments of Bray-1 (2.45) and labile P (2.89) were greater than those for other forms of P (total, inorganic and organic) (1.48) for six soils using simulated rainfall. The relatively greater enrichment of available P forms was attributed to less aggregation of runoff sediment compared to source soil reducing the physical protection of P. Phosphorus desorption-sorption characteristics, buffer capacity, sorption index, and equilibrium P concentration (EPC) were also enriched in runoff sediment compared to source soil. Massey and Jackson (1952), Menzel(1980), and Sharpley (1980) reported that the logarithm of particulate P ER was linearly related to the logarithm of soil loss (kg/ha). A similar relationship between soil loss and the ER of labile (”P) and bioavailable P (0.1 M NaOH) and of P sorption-desorption characteristics (P buffer capacity, P sorption index, and EPC) was also measured for several soils under simulated rainfall (Sharpley, 1985b). Although the texture of the soils studied ranged from Bernow fine sandy loam (8% clay) to Houston Black clay (50% clay), regression equations of the logarithmic soil loss-enrichment ratio relationship were similar. Different regression coefficients were obtained for different nutrient forms, however, with equation (1) holding for bioavailable, Bray 1, total, particulate, and organic P , P buffer capacity, and P sorption index: for labile P: and for EPC:

In ER = 1.21 - 0.15 In Soil loss (kg/ha) In ER = 2.48 - 0.35 In Soil loss In ER = 1.63 - 0.25 In Soil loss

(1) (2) (3)

The development of these general relationships between ER and soil loss provide a method of estimating the transport of these P forms with sediment (Sharpley, 1985b). Inclusion of these relationships in water quality models will improve the estimation of biological productivity of surface water in response to inputs of nutrients from agricultural runoff and allow a better description of P-runoff sediment interactions.

3 10

A. N. SHARPLEY AND R. G. MENZEL

E. CHANGES BETWEEN PHOSPHORUS FORMS DURING TRANSPORT Interchange between soluble and particulate P can occur during transport in stream flow. These transformations are accentuated by the selective transport of fine materials, which have a greater capacity to sorb or desorb P and will, thus, be important in determining the short-term potential of runoff to increase algal growth. In addition, soluble P may be removed by stream macrophytes (Stake, 1968; McColl, 1974; Vincent and Downes, 1980) and particulate P deposited or eroded from the stream bed with a change in stream velocity. Thus, the amounts of soluble and particulate P entering lakes and impoundments can be quite different from those entering stream flow. The direction of the exchange between soluble and particulate forms will depend on the concentrations of sediment and soluble P in stream flow and the equilibrium P concentration of the sediments contacted, which will include suspended, stream bank, and bottom material. The extent of these changes will depend on the labile or desorbable P content of the sediment material contacted and rate of stream flow. The equilibrium P concentration (EPC,) is defined as the soluble P concentration that is supported by a solid sample at which no net sorption or desorption takes place (White and Beckett, 1964; Taylor and Kunishi, 1971). If the soluble P concentration of runoff or stream flow falls below the EPC, of the suspended stream bank material contacted, P will be desorbed from the material. If, however, the soluble P concentration increases above the EPC,, P will be sorbed by the suspended or streambank material contacted. Changes in soluble P concentration during stream flow may occur with the entry of subsurface runoff having a low soluble P concentration or surface runoff having a high concentration, respectively. The above processes assume that sufficient desorbable P is present on the sediment for the EPC, to be reached and that the rate of desorption or contact time is sufficient for equilibrium to occur during runoff. If the sediment concentration of stream flow is high, then equilibrium may be attained due to rapid P desorption quickly reaching the soluble P concentration in equilibrium with sediment (Kunishi et al., 1972; Schuman et al., 1973; McColl et al., 1975). The input of sediment from heavily P-fertilized soils may increase the soluble P concentration of stream flow dramatically (Taylor and Kunishi, 1971). If, however, the sediment concentration of stream flow is low, the attainment of the EPC, will be limited by the capacity of the desorbable P pool of the sediment contacted. In this case, the reaction mainly occurs with streambank and bottom material that the stream contacts on its way to the watershed outlet. Streambank material is usually P deficient and has a high P sorption capacity. A decrease in the soluble P concentration during base stream flow, when the sediment concentration

THE IMPACT OF SOIL AND FERTILIZER PHOSPHORUS

31 1

was low, has been observed by Taylor and Kunishi (1971), Gburek and Heald (1974), Johnson et al. (1976), and Sharpley and Syers (1979). Soluble P concentrations of 0.10-0.13 mgfliter of runoff from fertilized fields were reduced to 0.009 mg/liter by sorption during movement downstream (Kunishi et al., 1972). A linear inverse relationship between soluble P concentration and logarithm of sediment concentration of runoff from cropped and grassed watersheds has been observed (Holt et al., 1973; Burwell et al., 1975; Nielson and Mackenzie, 1977; Sharpley et al., 1981~).Sharpley et al. (1981~)reported that the slope of the relationship for several watersheds was related to the P sorption capacities of suspended particulate material in runoff from the watersheds. It is apparent, therefore, that changes in bioavailability of P can occur between the point where it enters runoff flow and where it enters a lake or impoundment. Although some unavailable P forms may be convertedto available forms in transit downstream, data reported suggest that the predominant reaction causes available forms to be converted to unavailable forms. Consequently, the extent to which transformations between soluble and particulate P occur during stream flow must be considered in terms of modeling P movement and the potential bioavailability of P in surface waters.

IV.

IMPACT OF PHOSPHORUS ON THE AQUATIC ENVIRONMENT

A close relationship between the total P concentration of lake water and the average algal standing crop in a wide variety of lakes has been observed (Dillon and Rigler, 1974, 1975; Vollenweider, 1975; Schindler, 1977). The occurrence of algal blooms, dissolved oxygen depletions, and fish kills in Horseshoe Lake, Wisconsin, was partially attributed to high P inputs from agricultural and natural drainage by Peterson et al. (1973). In contrast, however, for the economic production of fish, ponds usually require the continuous addition of fertilizer (US Department of Agriculture, 1971). The high production of aquatic plants results in greater fish poundage, due to an increased worm, insect larvae, and other aquatic animal community feeding on the plants. In addition, the high fertility can reduce light penetration. The U.S.Department of Agriculture (1971) recommends that 112 kg of 8-8-2 fertilizer per surface hectare of water be used for the first 3-5 years, with a subsequent annual application of 45 kg/ha superphosphate. The forms and amounts of P in lake systems are a function of the input of P from external sources, its output from the lake, and the interchange of P among the various sediment and water components. The interaction between soluble and particulate P forms is controlled by chemical,

312

A. N. SHARPLEY AND R. G.MENZEL

biochemical, and physical processes. Although soluble P is immediately available for algal uptake, particulate P may provide a long-term source of available P to algae growth through desorption to the surrounding lake water (Bjork, 1972; Larsen et al., 1975; Cooke et al., 1977). Thus, the processes controlling the bioavailability of particulate P must be considered in designing programs to control accelerated eutrophication. Soluble and particulate P may be removed from the biotic zone by the natural processes of phytoplankton uptake and deposition. The process of accelerated eutrophication has been temporarily reversed in several eutrophic and hypereutrophic lakes by the inactivation of biologically available P with the addition of alum (Peterson et al., 1973; Cooke et al., 1978). A.

SOLUBLE PHOSPHORUS

The most available form of P to algae in the aquatic environment is soluble P (Vollenweider, 1968; Bartsch, 1969). Walton and Lee (1972) reported that soluble P was essentially 100% available, using algal assay procedures and a variety of waters. A number of investigators, however, have found that soluble P as measured by the molybdate method (Murphy and Riley, 1962) is not completely available to support algal growth (Rigler, 1968; Lean, 1973a,b: Dick and Tabatabai, 1977; Stainton, 1980). This results from a possible reduction in condensed phosphates, hydrolysis of organic P compounds, and reaction with arsenate during analysis, all of which will contribute to an overestimation of the true soluble P concentration. This discrepancy is relatively great for waters of low P concentration, such as are normally found in lakes, while the percentage error is much lower with concentrations found in streams, rivers, or wastewater discharges. Lee et al. (1979) suggested that from a lake management point of view, the discrepancy at low soluble P concentrations is of no major consequence as P control programs must be directed at sources of high concentrations. Boyd and Musig (1981) observed that planktonic communities in samples of water from fish ponds absorbed an average of 41070 of 0.30 mg/liter additions of soluble P within 24 hr. Over a longer period of time (2 weeks), these concentrations declined to 10% of that originally present due to the added removal of P by sediment. B. PARTICULATE PHOSPHORUS In oligotrophic and sometimes in eutrophic waters where soluble P concentrations are depleted by vigorous algal growth, concentrations may be as low as 0.001 mg/liter (McColl, 1972). Under these conditions, P may be desorbed from the suspended or deposited sediment material. In fact, Bannerman et al. (1975) calculated that approximately 10% of the external P

THE IMPACT OF SOIL AND FERTILIZER PHOSPHORUS

313

loading of Lake Erie (1.3 x lo7 kg/ha) resulted from the desorption of P from lake sediments. Furthermore, several studies have reported that particulate P can support biological growth, even though the soluble P concentration is low (Healy and McColl, 1974; Golterman, 1977; Allan and Williams, 1978). Consequently, the capacity of sediment entering lakes to supply or remove P is important in an evaluation of the importance of P in the aquatic environment. The processes involved in the sorption and desorption of P by sediment material in a lake are analogous to those occurring in stream flow discussed earlier. The direction of P movement will be governed by the soluble P concentration of the lake water and the desorbable P content or EPC, of the sediment material. The rate and extent of P interchangebetween sediment P and the surrounding lake water is controlled by the forms of P contained in the sediment and soluble P concentration of the interstitial water. The forms of P contained in the sediment have been reviewed extensively by Syers et al. (1973b). The potentially mobile forms are P sorbed on hydrous Fe and Al oxides and CaCO,. In addition to the chemical mobility of sediment material, its physicalmobility will also affect the interchangebetween particulate and soluble P, and subsequently its bioavailability. The physicalmobility of sedimententeringa lake will be a function of its texture and lake water temperature and turbulence. The relative densities and temperatures of the inflow and lake water will determine whether sediment enters the surface or bottom waters of the lake. As it enters the lake its turbidity may reduce the depth of the photic zone. Coarse-textured sedimentswill settlerapidly and be available to algae in the photic zone for short periods only. In contrast, fine-texturedsedimentswill remain in the photic zone for a longer period of time. The bioavailability of this sediment will be further increased by the fact that it will be enriched in P compared to coarser material. Removal of fine-textured sediments from the photic zone may be enhanced by bioflocculation in the presenceof certain algae (Avnimelechand Menzel, 1984). The mobilty of particulate P can increase if the sediment settles from an oxic photic zone into a deoxygenated hypolimnion. The desorbed P can then be redistributed during periods of lake turnover. In a study of the P dynamics of two shallow hypereutrophic lakes in Indiana, Theis and McCabe (1978) found that the soluble P concentration of lake water was reduced by sorption during oxic periods and increased by desorption during anaerobic periods. The increased mobility of particulate P under anaerobic compared to aerobic conditions is attributed to a reduction of Fe(II1) to Fe(I1) (Li et al., 1972; Syers et al., 1973b; Patrick and Khalid, 1974).

c.

BIOAVAILABILITY OF PHOSPHORUS

Due to the importance of sediment as the major source of P entering the aquatic environment from agricultural land and its ability to sustain algal

3 14

A. N. SHARPLEY AND R. G . MENZEL

growth, several methods to estimate the bioavailability of this P source have been proposed. The availability of P to algae can be determined by an algal culture test (EPA, 1971). However, more rapid chemical extraction procedures, which simulate removal of P by algae, have been proposed for the routine determination of particulate P bioavailability (Dorich et al., 1985). Chemical extractants that have been used to measure the bioavailability of particulate P are NaOH (Sagher et al., 1975, Golterman, 1976; Cowan and Lee, 1976; Armstrong et al., 1979; Logan et al., 1979); NH,F (Porcella et al., 1970; Dorich et al., 1980); anion exchange resins (Wildung and Schmidt, 1973; Cowan and Lee, 1976; Armstrong et al., 1979; Huettl et al., 1979); and citrate-dithionite-bicarbonate (CDB) (Logan et al., 1979). It is suggested that the weaker extractants and short-term resin extractions represent P that could be utilized by algae in the photic zone of lakes under aerobic conditions. In contrast, the more severe extractants (CDB) represent P that might become available under reducing conditions found in the anoxic hypolimnion of stratified lakes. Caution must be exercised, however, in relating P bioavailability of sediment material determined by these chemical extractions and the potential of the sediment to increase algal growth (Lee et al., 1979; Sonzogni et al., 1982). In turbid stratified lakes the surface photic zone may be relatively thin compared to the mixed layers above or below the thermocline. In addition, suspended sediment material often contains large amounts of silt-sized aggregates of clay, which will settle more rapidly from the photic zone than smaller particles, possibly reducing the actual availability of particulate P. Consequently, bioassays may produce erroneous estimates of available particulate P unless the physiochemical properties of the waterbody and sediment are considered in determining the appropriate bioassay to be used. Although these above chemical extraction procedures have identified which particulate P fractions can be utilized by algae, there is no evidence that all of the chemically extracted P is algal-available. Thus, Hegemann et al. (1983) suggested that a quantitative assessment of algal-available particulate P will depend upon the development of long-term (> 100 day) algal assay procedures. The bioavailability of P attached to suspended sediment transported in tributaries to lakes and of sediment deposited in lakes is summarized in Table IV for several studies. It is evident that a large variability in the bioavailability of sediment P exists, which reflects the dynamic nature of the physiochemical processes governing the transport, P mobility, and deposition of eroded soil material. Phosphorus associated with suspended sediment can be considered to be of short-term bioavailability due to sedimentation from the biotic zone. In contrast, P associated with deposited sediments is potentially bioavailable for a much longer period of time. Wildung et al. (1974) reported that the P content of the sediment in several lakes in Oregon was

Table N Percentage Bioavailability of Sediment P Transported in Several Lake Tributaries Draining Agricultural Watersheds and in Deposited Lake Sediments

Bioavailabilitf Reference

Location

Suspended sediment in tributaries Dorich et al. (1980) Indiana

Description Agricultural

De Pinto el al. (1981)

Great Lakes

Agricultural

Logan et al. (1979)

Lake Erie

Agricultural

Central Canada Lake Ontario

Prairie Lakes Postglacial Glacial Basin C. L. Memphremagog S. L. Memphremagog Riviere-du-sud Rijuland water Calcareous Noncalcareous Bottom sediments

Deposited sediment Allan and Williams (1978) Bannerman et al. (1975) Carigan and Kalff (1980)

Quebec

Klapwijk et al. (1982) Sagher et al. (1975)

Netherlands Wisconsin

Williams et 01. (1980)

Great Lakes

Procedure

(Qo)

Bioassay NH, F NaOH HCl Bioassay NaOH CDB* NaOH CDB

21 9 8 4 0-4 4-38 9-27 14-42 29-56

CDB NaOH NaOH NaOH Resin Resin Resin Bioassay NaOH NaOH NTA~ NaOH Resin

14-37 30-60 2-8 13-18 8 25 19 0-41 60-95 80-85 30 27 21

Total P Wkg)

0.2-0.7

0.5-1.2 0.5-1.2

0.5-1.3 1 .O-1.5 0.9-1.0 0.8-0.9 0.8-1.2 0.4-4.8 0.6-3.9 0.4-1.4

'Tercentage total particulate P bioavailable. bCDBand NTA representcitrate-dithionate-bicarbonateand 0.01 Mneutralized nitdoacetic acid extractableP, respectively.

316

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N. SHARPLEY AND R.

G. MENZEL

directly related to the biological productivity of surface waters and served as a significant source of P to these waters, supporting increased biological growth. Carignan and Kalff (1980) found that submerged macrophytes depended overwhelmingly on sediments for their P supply. Even under hypereutrophic lake conditions, sediments contributed the major proportion (72010) of P utilized during growth. It has been suggested, moreover, that these aquatic plants may supply P to overlying waters by excretion during growth and upon senescence (Carignan and Kalff, 1980). The water renewal time of a lake plays an important role in the dynamics and extent of P exchanges in a lake. With a short residence time, outflow of water from a lake can be a more important route for P removal than sedimentation. When the residence time of a lake exceeds a few months, most of the P inflow is retained in the lake sediments. Because of this process, impoundments and small lakes have been used as efficient traps (especially for particulate P) to improve downstream water quality (Rausch and Schreiber, 1977). It is apparent, however, that the amounts of P stored in lakes can build up to unacceptable levels, resulting in a permanent deterioration in water quality. In fact, a reduction in the external load of P upon the highly eutrophic Lake Trummen in Sweden did not bring about the desired improvement in water quality until the upper layers of the P-rich sediment were removed (Bjork, 1972).

D. ARTIFICIAL REMOVAL OF PHOSPHORUS FROM LAKES In order to control or reduce the increased biological productivity of lakes and impoundments, the inputs of P must first be reduced. The diversion of P inputs, however, does not always bring about a prompt and sufficient reduction in lake water concentration, due to internal recycling from P-rich sediments (Larsen et al., 1975; Cooke et al., 1977). A reduction in the P concentration of lake water and the inactivation of the recycling mechanisms may be brought about by chemical amendments (Jernelov, 1970; Peterson et al., 1973; Cooke et al., 1978; Kennedy, 1978). Several points need to be considered in the restoration of lake water quality by P precipitation and inactivation. These include the chemicals used, dosage, effect of the additives on benthic fauna, and method and time of application. The most commonly used chemicals are aluminum sulfate and sodium aluminate, due to the stability of flocculated Al hydroxides with redox changes. The removal of P is brought about by precipitation of AlPO,, by coagulation or entrapment of P-containing particulates, or by sorption of P on the surfaces of A1 hydroxide polymers (Recht and Ghassemi, 1970; Eisenreich et al., 1977). The maximum dosage of A12(S0,)3 for the long-term control of P cycling may be determined by Al,(S04)3addition to lake water samples until the

THE IMPACT OF SOIL AND FERTILIZER PHOSPHORUS

317

dissolved Al concentration reaches 0.050 mg AlAiter (Kennedy, 1978), a concentration Everhart and Freeman (1973) found to be nontoxic to fish. Very little direct laboratory or field evidence on the effect of Al on the aquatic biota exists, although several studies have shown no apparent effect on fish (Kennedy and Cooke, 1974; Bandow, 1974; Sanville et al., 1976) or benthic invertebrates (Narf, 1978) following full-scale lake treatments. A predetermined amount of A12(S04),is applied as a slurry from the lake surface if P removal from the epilimnion is required. If control of P release from sediments is required then application to the hypolimnion is necessary. As d2(so4)3removes dissolved organic P inefficiently (Browman et al., 1973; Eisenreich et al., 1977), applications should be made in early spring when the major proportion of P in lake water is inorganic (Browman et al., 1977; Eisenreich et af., 1977). The continued presence of organic P may be significant, as Heath and Cooke (1975) observed that certain nuisance bluegreen algae can produce a phosphatase enzyme under P-limiting conditions, that is capable of mineralizing organic to inorganic P at rates sufficient to support algal blooms. Application time will not be critical for treatment of P desorption from lake sediments. However, the relative importance of lake sediments as a P source should be assessed prior to Al,(SO4), application. For example, lakes receiving substantial inputs of clay in addition to P may contain sediments with high sorption capacities for P. The application of A12(S04),to just below the surface of Horseshoe Lake, Wisconsin, resulted in a significant decrease in the P content of both the epilimnion and hypolimnion (Peterson et al., 1973). Prior to application, the lake had experienced algal blooms and fish kills which were partially attributed to agricultural inputs of P. Born (1979) observed that although hypolimnion P increased slightly each year after application, it never reached pretreatment levels, thus giving approximately 8 years of control. The hypolimnetic application of Al,(S04), to the eutrophic West Twin Lake, Ohio, resulted in an 88% reduction in total P concentration of the lake water (Kennedy, 1978). Continued water quality monitoring by Kennedy (1978) indicated that the layer of Al(OH), deposited on the sediments reduced P release to overlying waters by 98%. Three years later the lake was mesotrophic.

V.

CONCLUSIONS

Fertilizer P use presents no direct problem to the terrestrial environment. The use of P fertilizer is essential to maintain adequate crop production for an ever-increasing population. Its application can reduce the nutrient enrichment of surface waters by establishing an increased vegetative cover

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A. N. SHARPLEY AND R. G . MENZEL

on eroding soils. These benefits to the environment must be considered along with a potentially detrimental indirect effect of fertilizer P use. Heavy metal and radionuclide contaminants not removed during fertilizer manufacture may xcumulate in the soil. No threat to human health has been reported at the present time, although the continued application of impure fertilizer materials to acid soils may lead to problems with crops susceptible to contaminant uptake, especially that of cadmium. Most of the public and scientific attention regarding environmental effects of P fertilizer has been focused on the aquatic environment due to the role of P in increasing the biological productivity of lakes and impoundments. Considerable research has been conducted to quantify the losses of soil and fertilizer P from various land management practices. However, we are still unable to relate P inputs to a lake or impoundment to a quantitative description of water quality. Furthermore, the effect of P concentration on algal growth receives continued attention, while little information is available on how lake macrophytes are affected, even though macrophytes present a more serious economic problem than algae in many lakes. Research should be directed towards improving the partitioning models for soluble and particulate P transport in runoff and in lakes and impoundments. This should focus on the mechanisms of exchange between desorbable or labile P and solution and methods to routinely quantify the amounts of desorbable or bioavailable P on various materials. With the accumulation of P at the soil surface under conservation tillage practices, existing soil P test procedures may need to be reevaluated. This may include changes in soil sampling frequency, timing, and depth, in addition to the use of chemical extractants capable of removing easily mineralizable organic P from the surface plant residue built up. As the crop canopy can contribute a major proportion of the soluble P transported in runoff, surface soil and plants must be considered as a continuum and the pool of desorbable P in both soil and plant material determined. The measured size of this pool will depend upon the experimental conditions of analysis, therefore extraction mediums, so1ution:soil ratios, and contact times relevant to either the terrestrial or aquatic environments must be used. In the case of the aquatic environment, the fact that the desorbed P can be continuously removed from the system by algal growth must be considered. In the light of research on the kinetics of P exchange between desorbable P and solution in the terrestrial and aquatic environments and during transport from the terrestrial environment, more accurate and widely applicable models simulating P transport from watersheds can be expected. This information should be used to improve the prediction of both the amounts and forms of P transported into lakes and impoundments. These models can then be used as tools to aid management decisions to reduce P

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loss in runoff and at the same time to increase crop yields to maintain adequate food production for an increasing population. In addition, these models may also identify areas of further research.

ACKNOWLEDGMENT The authors wish to acknowledge the pioneering phosphorus and water quality research of Dr. J. C. Ryden, who died suddenly May 3, 1986, in London, England.

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ADVANCES IN AGRONOMY, VOL. 41

BIOTECHNOLOGY APPLICATIONS IN WEED MANAGEMENT. NOW AND IN THE FUTURE Kriton K. Hatzios Department of Plant Pathology, Physiology and Weed Science Virginia Polytechnic Institute and State University Blacksburg, Virginia 24061

I.

INTRODUCTION

Genetic improvement of plants has challenged agronomists throughout the long history of agriculture. Extensive research in the last few decades has resulted in dramatic increases in agricultural crop production (Barton and Brill, 1983). However, recent advances in agricultural biotechnology have stimulated additional interest in research aimed at establishing genetic engineering as a practical facet of plant breeding and modern crop production. In a broad sense, biotechnology is commonly defined as “the application of biological organisms (microorganisms, plants, animals), systems, or processes to provide desirable goods and services” (Anonymous, 1984). Agricultural biotechnology then is based on the application of advanced concepts and techniques of biological sciences such as recombinant DNA, genetic engineering, some enzyme processes, cell fusion, plant cell and tissure culture, clonal propagation, monoclonal or polyclonal antibodies, embryo and other germ cell manipulations, and process or system engineering (i.e., fermentation) that are relevant to specific agricultural programs (Anonymous, 1984). The prospects and specific aspects of applying genetic engineering and other biotechnological procedures in plant agriculture have been reviewed by a number of scientists (Ammirato et al., 1984; Barton and Brill, 1983; Brown, 1984; Olson, 1986; Owens, 1984; Perani et al., 1986; Torrey, 1985; WilkeDouglas et al., 1986; Zaitlin et al., 1986). One of the areas of plant agriculture in which biotechnology is expected to have immediate practical applications is undoubtedly the field of weed management (Hardy, 1986; Jaworski, 1987). Weeds have long been recognized as major pests causing significant losses in agricultural productivity. Annual losses due to weeds are responsible for a 10% reduction in total 325 Copyright 0 1987 by Academic Press, Inc. All rights of reproduction in MY form r s a v e d .

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agricultural productivity of the United States, amounting to a loss of $12 billion (Shaw, 1982). In addition, farmers spend about $3.6 billion for chemical weed control and about $2.6 billion for cultural, ecological, and biological methods of weed control. Thus, the total losses caused by weeds and the cost for their control are presently estimated at more than $18 billion in the United States of America (Shaw, 1982). Synthetic organic herbicides represent the most effective tool available to farmers worldwide in their fight against weeds. A fundamental reason for the widespread use of herbicides in modern agriculture is their ability to control selectively a wide range of weeds in a variety of crops. The empirical synthesis-screening approach, however, by means of which most of the currently available herbicides were developed, has become increasingly ineffective (Hardy, 1986). In addition, the lack of broad crop tolerance and soil residues resulting in carryover problems limit the broad crop use of many herbicides. Finally, a few weeds have developed resistance to herbicides, such as the s-triazines, and it is desirable to understand, manage, circumvent, or exploit this resistance (LeBaron and Gressel, 1982). Recent advances in agricultural biotechnology provide us with new options and alternative approaches in dealing with these problems. Crops with specific herbicide resistance and herbicides designed by “biorational” synthesis are expected to be among the earliest products of plant biotechnology (Hardy, 1986). At present, four major areas within the field of weed management offer attractive opportunities for the application of biotechnology. These include (1) the development and use of bioherbicides, i.e., the use of plant pathogenic organisms such as fungi, bacteria, and viruses as biological control agents; (2) the discovery and use of naturally occuring herbicides; (3) the genetic improvement of crop tolerance to herbicides; and (4) the use of genetically engineered microorganisms for other areas of weed management such as the decontamination of herbicide spills or the protection of selected crops against herbicide injury. The principles and mechanisms involved in the potential application of biotechnology in these areas of weed management and herbicide technology as well as specific examples are described in the following sections of this review. Herbicides and related chemicals designated by common names approved by the Weed Science Society of America or the British Standards Institution are identified by chemical names in Table I.

II. PRODUCTION AND USE OF BIOLOGICAL WEED CONTROL AGENTS A.

STRATEGIES FOR

WEED CONTROL BY MICROBES

The concept of deliberately using organisms to control a pest constitutes the fundamental basis of all biological pest control systems. Extensive

Table I Chemical Names of Herbicides and Other Modifiers Mentioned by Common Name in the Text

Common name Acifluorfen Alachlor Amitrole Atrazine Benomyl Bentazon Benzadox Bialophos Butylate Chloramben Chlorofenprop-methyl Chlorpropham Chlorsulfuron Cinmethylin Dalapon Dietholate Diphenamid Diuron

EPTC

Fluazifop Fluorodifen Flurazole Glufosinate Glyphosate Imazaquin

MCPA

Mecoprop Methoxyphenone Metolachlor Metribuzin Molinate Monuron Paraquat Phenmedipham Picloram Propanil Pyrazon Quinclorac Sulfometuron Terbutryn Tridiphane Trifluralin 2,4-D 2,4,5-T

Chemical name 5-[2-Chloro~-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid 2-Chloro-N-(2,6-diethylphenyl)-N-(methox~ethyl)acetamide 1H-l,2,4-triazol-3-amine '-(l-methylethyl)-l,3,5-triazine-2,4-diamine 6-Chloro-N-ethyl-N Methyl l-(butylcarbamoyl)-2-benzimidazole-carbamate 3-(l-Methylethyl)-( lH)-2,1,3-benzothiadiazin4(3H)-one-2,2-dioxide (Benzamidooxy)acetic acid

~-2-Amino-4-[~ydroxy)-(methyl)phosphinoyl]-butyryl-~-alanyl-~-

alanine S-ethyl bis(2-methylpropyl)carbamothioate 3-Amino-2,5-dichlorobenzoicacid Methyl-2-chloro-3-(4-chlorophenyl)propionate 1-Methylethyl-3-chlorophenylcarbamate 2-Chloro-N-[ [(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl] benzenesulfonamide exo-1-Methyl-4-(l-methylethyl)-2-[(2-methylphenyl)methoxy]-7oxabicyclo[2.2.llheptane 2,2-Dichloropropanoic acid 0,O-diethyl-0-phenylphosphorothioate N,Ndimethyl-a-phenylbenzeneacetamide N '-(3,4-dichlorophenyl)-N,N-dimethylurea S-ethyl dipropyl carbamothioate ( f )-2-[4-[[5-(Trifluoromethyl)-2-pyridinyl]oxy]phenoxy] propanoic acid p-Nitrophenyl-a,a,c~,-trifluoro-2-nitro-p-tolyl ether Phenylmethyl 2-chloro4-(trifluoromethyl)-S-thiazole-carboxylate ~.~-2-Amino-4-~ydroxy)-(methyl)phosphinoyl-butyric acid N-(phosphonomethy1)glycine 2-[4,5-Dihydro-4-methyl-4-( 1-rnethylethyl)-5-oxo-lH-imidazol-2-yl-3quinolinecarboxylic acid (4-Chloro-2-methy1phenoxy)aceticacid ( + )-2-(4-Chloro-2-methylphenoxy)propanoicacid (4-Methoxy-3-methylphenyl)-(3-methylphenyl)methanone 2-Chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-l methylethyl) acetamide 4-Amino-6-(1,l -dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one S-ethyl hexahydro-1H-azepine-l-carbothioate N '-(-4-chlorophenyl)-N,N-dimethylurea 1,l '-Dimethyl4,4'-bipyridiniumion 3-[(Methoxycarbonyl)amino]phenyl(3-methylphenyl)carbamate 4-Amino-3,5,6-trichloro-2-pyridinecarboxylic acid N-(3,4-dichlorophenyl)propanamide 5-Amino-4chloro-2-phenyl-3(2H)-pyridazinone 3,7-Dichloro-8-quinolinecarboxylicacid 2[[[[(4,6-Dimethyl-2-pyrimidinyl)amino]carbonyl]amino]sulfonyl] benzoic acid N-(1,1-dimethylethyl)-N'-ethyl-6-(methylthio)-1,3,5-triazine-2,4diamine 2-(3,5-Dichlorophenyl)-2-(2,2,2-trichloroethyl)oxirane 2,6-Dinitro-N,N-dipropyl-4-(trifluoromethyl)benzenamine (2,4-Dichlorophenoxy)acetic acid (2,4,5-Trichlorophenoxy)acetic acid

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research over the past two decades has resulted in the establishment of two major strategies for the biological control of weeds: the classical and the bioherbicide approach (Charudattan and Walker, 1982; Templeton et al., 1986).

In the classical approach, a biocontrol agent is simply introduced or released into a weed population to establish itself and control the weed population, requiring no further manipulation (Templeton et al. 1979). Insects, fungi, nematodes, fish, and other biological systems have all been used with varying degrees of success as biocontrol agents in the classical approach of biological weed control (Charudattan and Walker, 1982). The bioherbicide approach employs the massive, usually annual, release of a biocontrol agent into specific weed-infested fields to infect and kill susceptible weeds (Templeton, et al., 1986). In modern literature, the term “bioherbicide” refers to microbial plant pathogens which are applied as sprays that uniformly kill or suppress the growth of weeds (Templeton and Smith, 1977). Thus, in the bioherbicide approach, microbial plant pathogens are applied to target weeds in a manner similar to chemical herbicides. This strategy has received considerable attention and the subject has been reviewed extensively (Charudattan and Walker, 1982; Quimby and Walker, 1982; Scheepens and van Zon, 1982; TeBeest and Templeton, 1985; Templeton and Smith, 1977; Templeton et al., 1979, 1986). Fungi, bacteria, and viruses offer great promise for use as bioherbicides in modern weed management. At present, however, only the potential use of fungal pathogens as mycoherbicides has been studied in depth (Scheepens and van Zon, 1982; Templeton et al., 1986). Phytopathological agents developed as bioherbicides may not be very effective in nature but are made effective for biological control by applying an abnormally high inoculum pressure at an appropriate time (Scheepens and van Zon, 1982). Thus, in the bioherbicide approach, the climax of an epidemic is reached artificially early in the season. Fungal pathogens that are virulent (capable of causing injury), host specific, and genetically stable but constrained naturally by low inoculum production and poor dissemination are good candidates for development as mycoherbicides (Templeton et al., 1986). The exploitation of plant pathogenic bacteria as bioherbicides seems feasible because they can be host specific and can easily be cultivated in vitro (Scheepens and van Zon, 1982). In contrast, plant pathogenic viruses are more problematic since they are often not host specific and a vector is usually necessary for their transmission from one plant to another. Other factors influencing the potential use of plant disease-causing organisms as bioherbicides include: low virulence, stringent temperature and moisture requirements, wounding requirements, and specific physiological interactions with the host plant (TeBeest and Templeton, 1985). The utilization of biological weed control agents in weed management

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329

offers many advantages including (1) a high degree of specificity for the target weed; (2) no effects on nontarget and beneficial plants or man; (3) absence of weed resistance development; (4) absence of residue buildup in the environment; and ( 5 ) potential impact from biotechnological research and development (Khachatourians, 1986; Templeton et al., 1986). Some drawbacks associated with the production and use of bioherbicides include 1. Bioherbicides have to be registered with the Environmental Protection Agency (E.P.A.) and the registration process may be lengthy. 2. Suppression or killing of weeds by bioherbicides may be a slow process. 3. Stability of bioherbicides under field conditions is highly dependent on environmental conditions. 4. Production of a bioherbicide for large-scale application may be an expensive process. 5 . Numerous fungi need to be discovered and developed as bioherbicides because of the high degree of specificity of these agents (Khachatourians, 1986; Templeton et al., 1986). As a consequence of these advantages and disadvantages, present work with bioherbicides is concentrated on their use for the control of species that escape standard chemical control. These hard-to-control weeds, spread across a wide area, present an economic opportunity that could fit into an established chemical control program. Thus, at present, bioherbicides are not seen as alternatives to chemical herbicides but as complementary adjuncts to current weed management systems (Templeton et d., 1986).

B.

COMMERCIAL PROSPECTS AND CONSIDERATIONS

The concept of using fungi, bacteria, and even viruses as bioherbicides is biologically feasible with several host-pathogen combinations. Today, however, only fungal pathogens have been exploited commercially as mycoherbicides, marketed for practical applications. Detailed overviews of several aspects pertaining to the commercialization of bioherbicides such as large-scale production, formulation, application, storage, and market potential have been reported by Quimby (1986) and Templeton et al. (1986). The potential commercialization of a microbial phytopathogenic agent is dependent greatly on whether this microbe possesses properties that allow it to be handled like a chemical herbicide or not. Ideally, a commercialized bioherbicide should be long-lived and insensitive to manipulations in the industrialization process (Kenney, 1986). Thus, determination of the life cycles of endemic pathogens of major weeds and development of methods for mass production of stable reproductive units (spores) of these pathogens

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are of paramount importance for the commercialization of microbial pathogens as bioherbicides. In addition, the small size of current markets of bioherbicides, their marketability in situations where a selective host range is required, and stabilization of the product under field conditions are factors or obstacles that need to be addressed during the commercialization process (Khachatourians, 1986; Templeton et ul., 1986). Utilization of innovative approaches for large-scale production and stabilization under field conditions has resulted in the commercialization and registration of two fungi as mycoherbicides. A formulation of the soilborne fungus Phytophthorupulmivora (butler) Butler was registered in 198 1 as the first selective mycoherbicide for the control of a strangler (milkweed) vine (Morreniu odorutu Lindl.) in Florida citrus groves under the trade name DeVineO (Abbott Laboratories, North Chicago, Illinois) (Kenney, 1986; Ridings, 1986). Phytophthoru pulmivoru is a facultative parasite that produces a lethal root rot of its host plant (strangler vine) and persists saprophytically in the soil for extended periods of time (Ridings, 1986). Host-specific strains of this fungus are formulated as liquid suspensions consisting largely of chlamydospores of the fungus (Kenney, 1986). These suspensions are normally applied with boom and nozzle sprayer systems to the soil surface under tree canopies. Rapid loss in spore viability required Abbott Laboratories to provide strict quality control checks on viability and to market DeVineO on a “made-to-order” basis (Kenney, 1986). Thus, DeVineO must be ordered prior to the season in which it will be applied, and it is distributed and stored under refrigeration. DeVineO proved to be an extremely effective mycoherbicide. Groves treated with DeVineO in 1978-1980 are still seeing 95-100% control of strangler vine from a single treatment, in spite of a continuous infestation of new seedlings originating from wind-blown seeds (Kenney, 1986). The second commercially developed mycoherbicide is marketed under the trade name CollegoO by the TUCO division of the Upjohn Company, Kalamazoo, Michigan (Bowers, 1986; Smith, 1986). CollegoO is a formulation of Colletotrichum gloesporoides (Penz.) Sacc. f. sp. aeschynomene, an endemic anthracose fungus, registered in 1982 for the selective control of northern jointvetch [Aeschynomene virginica (L.) B.S.P.] in rice (Oryza sutivu L.) and soybean [Glycine mux (L.) Merr.] fields of Arkansas, Louisiana, and Mississippi (Smith, 1986; Templeton et al., 1986). In 1983, a total of 619 acres were treated with CollegoO in rice and soybean fields of the aforementioned southern states (TeBeest, 1986). Colletotrichum gloesporoides is a facultative sporophyte that causes a lethal stem and foliage blight of its host weed when inoculated with spores (Smith, 1986). CollegoO is a dry powder containing 15% spores (conidia) of C . gloesporoides as active ingredient and 85 9’0 inert ingredients. The formulation is rehydrated and resuspended in a sugar medium before being mixed

BIOTECHNOLOGY APPLICATIONS IN WEED MANAGEMENT

33 1

with water in the spray tank of an application system (TeBeest and Templeton, 1985). CollegoO is normally applied aerially to rice and soybeans at dusk, but tractor-mounted spray equipment could be also used for applications in soybean fields (TeBeest and Templeton, 1985). Necessary precautions that must be taken when using DeVineO and College@ in the field include the prevention of exposure of these formulations to wetting agents, fertilizers, and chemical pesticides that are detrimental to the viability of their spores (TeBeest and Templeton, 1985). Recent field and laboratory tests demonstrated that Collego@ can be integrated with several chemical herbicides used in soybeans, such as acifluorfen and bentazon, in tank-mixed application (TeBeest and Templeton, 1985). However, tank-mixtures of CollegoO with the herbicide propanil, an important week killer in rice, were incompatible (TeBeest and Templeton, 1985). Although weeds can become resistant to mycoherbicides following repeated applications of these biocontrol organisms in an intensive weed management system, so far, this has not been the case with CollegoO and DeVine@. Development of biotypes of northern jointvetch with resistance to Colletotrichum gloesporoides has not been observed during the 14-year period of field tests conducted with this mycoherbicide (Templeton, 1986). Similarly, there is presently no indication of any changes in virulence or specificity of the fungus C. gloesporoides during this time. Apart from DeVineO and CollegoO , a number of other endemic fungi are in various stages of research and development as mycoherbicides, including the following: 1. Colletotrichum malvarum (A. Braun and Casp.) Southworth, an anthracnose-inducingfungus, for the control of prickly sida (Sidaspinosa L.) in cotton (Gossypium hirsutum L.) and soybean (Kirkpatrick et al., 1982). 2. Alternaria cassiae Jurair and Khan, a blight-inducing fungus, for the control of sicklepod (Cassia obtusifolia L.) in soybean and cotton (Walker and Riley, 1982). 3. Alternaria macrospora Zimm.for the control of spurred anoda [Anoda cristata (L.)Schlecht.] in cotton (Walker, 1981; Walker and Sciumbato, 1979). 4. Cercospora rodmanii Conway, a leaf spot-inducing fungus, for the growth suppression of water hyacinth [Eichhorniacrassipes (Mart .) Solms] in waterways (Conway et al., 1978) 5. Colletotrichum coccodes (Wallr.) Hughes for the control of velvetleaf (Abutilon theophrasti Medic.) (Gotlief et al., 1984).

c.

BIOTECHNOLOGY AND BIOHERBICIDE PRODUCTION

Many aspects of the biology of phytopathogenic agents that affect their bioherbicide potential could be improved greatly with the application of

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innovative technological procedures. Biotechnology, in particular, is expected to have a far-reaching impact on the production and use of phytopathogenic agents as bioherbicides in the future. The efficacy, industrial production, application, and cost-effectiveness of biological weed control agents could all be enhanced by applying biotechnology. Advances of fermentation and formulation technology of plant pathogens used as bioherbicides are needed to improve the costeffectiveness and industrial production of these biological weed control agents. Experience obtained with the two commercially developed mycoherbicides indicates that opportunities exist for improvements in the capacity of a fungus to yield spores in submerged fermentation or to tolerate adverse drying procedures (Templeton ei al., 1986). Techniques such as genetic engineering may allow us to bring about changes in the genome of fungal or bacterial plant pathogens that could result in an increase of their favorable properties when used as bioherbicides. Isolation of the genetic determinants of virulence, specificity, sporulation capacity, toxin production, and tolerance to climatic stresses as well as their transfer from pathogen to pathogen appear now quite promising. Virulence of a host-specific pathogen could be increased by strain selection or genetic improvement of this pathogen. Early work by Sands and Rovira (1972) demonstrated that genetic changes in plant pathogenic agents could be induced, making them less virulent to crop species and more virulent to weeds. Fungi that produce a sexual stage may be improved by crossing compatible strains. Advances in protoplast fusion technology with fungi make it possible now to use parasexual crossing with strains that do not produce a sexual stage (Templeton et al., 1986). In a way, this biotechnological approach will enable us to produce biological analogs of successful bioherbicides. Pathogen strain improvement may be also achieved by mutagenesis induced either by irradiation or by chemical treatment. This approach seems particularly promising for the selection of strains of bioherbicides that are tolerant to chemical pesticides. In a recent report. TeBeest (1984) used chemical mutagenesis to select several benomyl-tolerant strains of the mycoherbicide College@ . These strains contained all the desirable properties of the naturally occuring strains of C. gloesporoides. This successfulexample illustrates that development of strains of phytopathogenic agents resistant to selected chemicals will allow the wider use of bioherbicides in integrated pest management programs of selected crops (Templeton et al., 1986).

Ill. NATURALLY OCCURRING HERBICIDES As a consequence of worldwide growing concern about the environmental aspects of selected pesticides, the need for the use of easily degradable

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pesticides with good selectivity is greater than ever. Recent advances in microbial and plant biochemistry have stimulated scientific interest into the possible role of secondary plant products and microbial toxins as natural pesticides (Cutler, 1984; Glick et al., 1984; Duke, 1986a,b; Kurz and Constabel, 1985; McLaren, 1986; Misato and Yamaguchi, 1984; Rice, 1984; Sandermann, 1985; Seikizawa and Takematsu, 1983). Furthermore, advances in plant cell culture, fermentation technology, molecular genetics, and genetic engineering make it now possible to exploit biotechnologically plants and microorganisms as potential sources of naturally occurring chemical compounds that could be developed as herbicides. At present, two major areas of research appear attractive because of their potential commercial applications. They include (1) isolation and characterization of microbialtoxins or secondaryplant metabolitesthat could be used effectively as herbicides, and (2) evaluation of plant secondary metabolites and microbial toxins with novel chemistries which could be used as leads for the chemical synthesis of new herbicides. Selected examples illustrating the potential of these approaches are discussed briefly in the following sections. A. UTILIZATION OF PLANT AND MICROBIAL PHYTOTOXINS AS HERBICIDES The use of biologically derived chemicals as herbicides has already achieved respectable levels. In principle, this approach is based on fermenting bacteria and fungi, testing fermentation broths for activity, and isolating active compounds from these broths (Adler et al., 1985; Egorov and Landau, 1983). Plant cell cultures could be used in a similar fashion (McLaren, 1986; Rhodes and Kirshop, 1982). Although both of these procedures are tedius and long-term operations, they do possess practical and economical merit. Japan is presently considered to be the most advanced country in the development of microbial pesticides and pharmaceuticals (Dibner, 1985). Pesticides of microbial origin, commonly referred to as “agricultural antibiotics,” are highly specific for target organisms and supposed to be inherently biodegradable because they are synthesized biologically (Misato and Yamaguchi, 1984). In addition, according to Duke (1986a), purified natural compounds appear to have many practical advantages over bioherbicides as weed control agents such as longer self life, a wider range of storage conditions, a broader environmental window for application, lower storage space requirements, and greater ease of application. Extensive research during the last two decades has demonstrated that several plant secondary metabolites (allelochemicals) as well as fungal and microbial toxins possess good herbicidal activity. A brief list of selected naturally occurring compounds that exhibit promising herbicidal activity is

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presented in Table 11. Detailed reviews of this topic are available (Balandrin et al., 1985; Dodge and Knox, 1986; Duke, 1986a, b; Fellows, 1986; Fischer and Bellus, 1983; Misato and Yamaguchi, 1984; Putnam, 1985). Of the naturally occurring compounds listed in Table 11, only bialophos is currently marketed under the trade name HerbiaceaeO as a commercially Table 11 Examples of Microbial or Plant Phytotoxins with Promising Herbicidal Activity Natural phytotoxin

Plant or microbial source

Reference

Anisomycin Bialophos

Streptomyces sp.

Yamada et a/. (1972)

Streptomyces hygroscopicus Streptomyces viridochromogenes

Cytochalasins

Phomopsis sp.

Mase (1984) Tachibana (1987) Cole et al. (1981)

Cercosporin

Phloridzin Phosalacine Psoralen Rhizobitoxine

Cercospora sp. Pseudocercosporella capsella Coffee plants Scytonema hofmanni Sorghum plants Spurge plants Streptomyces saganonensis Black walnut trees Aspergillus terreus Fusarium moniliforme Penicillium sp. Xylaria sp. Macrophomina phaseolina Apple roots Kitasatosporia phosalacinea Psoralea plants Rhizobium japonicum

Stemphyloxin 1 Tabtoxin Tentoxin Toyocamycin Trimethylxanthine Viridiol Ziniol

Stemphylium botryosum Pseudomonas tabaci Alternaria alternata Strephtomyces toyocanensis Coffee plants Gliocladium virens Alternaria carthami

Caffeine Cyanobacterin Dhurrin Gallic acid Herbicidins Juglone Mevinolin Moniliformin Patulin Phaseolinone

Durbin (1981) Putnam (1985); Duke (1986a) Gleason er al. (1987) Putman (1985) Putnam (1985); Duke (1986a) Takiguchi et al. (1979) Reitveld (1983) Bach and Lichtenhaler (1983) Cole et al. (1973) Putnam (1985) Karl et a/. (1986) Dhar et a/. (1982) Putnam (1985) Omura et a/. (1984) Putnam (1985); Duke (1986a) Giovanelli et a/. (1971); Lieberman (1979) Barash et al. (1982) Langston-Unkefer et al. (1984) Duke (1986a) Yamada et al. (1972) Rizvi et a/. (1980) Howell and Stipanovich (1984) Robeson and Strobel (1984)

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developed herbicide in Japan (Mase, 1984;Tachibana, 1987). Bialophos is a microbial product isolated from the fermentation broth of Streptomyces hygroscopicus and S. viridochromogenes and exhibits strong herbicidal activity against a wide spectrum of grass and broadleaf weeds following application to their foliage (Mase, 1984; Misato and Yamaguchi, 1984). Bialophos exhibited very little phytotoxicity when applied to the soil, and this is believed to be due to its remarkable biodegradability by soil microorganisms (Misato and Yamaguchi, 1984). A detailed description of the biosynthetic pathway used by Streptomyces hygroscopicus to synthesize bialophos was recently reported by Imai et al. (1986).Utilization of blocked mutants and of biosynthetic intermediates was instrumental in the elucidation of this pathway. Several of the biosynthetic genes of bialophos were recently cloned (Imai et al., 1986). In sensitive plants, bialophos is metabolized to phosphinothricin { L-2-amino-4-[(hydroxy)-(methyl) phosphinoyll-butyric acid}, a phytotoxic metabolite which inhibits the enzyme glutamine synthetase in such plants (Lea et d., 1984). Glutamine synthetase is important in ammonia assimilation and the genes coding for this enzyme in alfalfa (Medicago sativa L.) plants have been recently cloned (Tischer et al., 1986). Treatment of plants with phosphinothricin causes ammonia to accumulate at levels exceeding those known to uncouple photophosphorylation and as a result CO, assimiliation is greately reduced (Kocher, 1983; Lea et al., 1984). Phosalacine is another microbial compound containing phosphinothricin and its herbicidal behavior is similar to that of dialophos (Omura et al., 1984). Of the other natural compounds listed in Table 11, tentoxin has been studied the most. Tentoxin is a cyclic tetrapeptide produced by Alternaria alternata and causes marked chlorosis to many grass and broadleaved weed species. Several crops such as corn (Zea mays L.) and soybeans are tolerant to this toxin (Duke et al., 1980; Durbin and Uchytil, 1977). However, in spite of its clear-cut crop selectivity and excellent activity, tentoxin has not been developed commercially as a herbicide. The role of several hostspecific phytotoxins produced by Alternaria sp. in host-parasite interactions has been reviewed by Ueno (1987). With respect to most allelochemicals from higher plants, a major problem that hinders their development as herbicides is their limited selectivity and lack of stability (Duke, 1986a). Recent advances in genetic engineering appear promising in overcoming these problems. Some of the plant enzymes mediating the biosynthesis of secondary plant products have already been isolated. The induction of such enzymes has been studied at the levels of transcription and translation, and cDNAs for some biosynthetic enzymes have been cloned as a prerequisite for the isolation of the respective genes (Hahlbrook et al., 1985). However, although attempts to introuce genes

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coding for the biosynthesis of natural pesticides into crop plants are underway in several laboratories, this would be a long-term process because several genes may have to be simultaneously transferred to achieve this goal (Sandermann, 1985).

B.

SYNTHETIC DERIVATIVES OF NATURALLY OCCURRING COMPOUNDS AS HERBICIDES

The problems of high phytotoxicity, limited crop selectivity, and instability under field conditions associated with several naturally occurring compounds hinder their commercial development as herbicides. These problems can be overcome by a biorational synthesis of more selective and stable analogs of these chemicals. Thus, microbial toxins and allelochemicals provide us with novel chemistries that could be manipulated in order to produce commercial herbicides. Selected examples of commercially developed herbicides which are chemically similar to naturally occurring phytotoxic compounds are listed in Table 111. In most of the presented cases, it is unclear whether these herbicides were developed by means of a biorational design based on natural phytotoxin chemistry or through random screening. The development of the herbicide methoxyphenone as a synthetic analog of the microbial toxin anisomycin represents one of the few successful efforts employing the biorational chemical synthesis of a herbicide based on a Table I11 Commercially Developed Herbicides Based on Natural Chemistry Natural product

Plant or microbial source

Herbicide

Manufacturer/ country

Anisomycin Cineole

Streptomyces sp. Widespread in plants

Methoxyphenone NihodJapan Cinmethylin She1MJ.S.A.

Benzoxazinones (hydroxamic acids) Iprexil

Graminae plants

Benzazin

BASF/Federal Republic of Germany

Benzadox

Fusaric acid Moniliformin

Iprex pachyon Fusarium sp. Fusarium moniliforme

Gulf/U S .A, Dow/U.S.A.

Quinolinic acid

Nicotiana tabacum

Phosphinothricin

Streptomyces viridochromogenes

Picloram 3,44ibutoxymoniliformin Quinclorac Glufosinate

.

CIBA-GEIGY/ Switzerland BASF/Federal Republic of Germany HoechstIFederal Republic of Germany

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natural chemistry (Munakata et al., 1973). Methoxyphenone is marketed in Japan as a selective herbicide for the control of barnyardgrass [Echinochloa crusgalli (L.) Beauv.] in rice and is easily degraded in soil (Duke, 1986a; Munakata et al., 1973). The ammonium salt of glufosinate, a synthetic racemic mixture based on the chemistry of phosphinothricin, has been recently introduced by Hoechst Aktiegesellschaft Co. (West Germany) as a nonselective herbicide marketed under the trade name Basta@ (Kocher, 1983). As discussed earlier, phosphinothricin is the active ingredient of the microbial herbicide bialophos (Herbiaceae@). Iprexil and its synthetic analog benzadox appear to act as proherbicides. Following their application to plants they are activated by being converted to amino-oxyacetic acid, a potent inhibitor of pyridoxyl phosphate-requiring enzymes (Duke, 1986a; Fischer and Bellus, 1983). Fusaric acid is a marasmin produced by many species of Fusarium fungi and has been detected in infected tomato (Lycopersicon esculentum Mill.) plants and wilted cotton (Owens, 1969). Picloram, a chlorinated analog of fusaric acid, has been marketed as a herbicide for many years. Peterson et al. (1974) reported that picloram caused desiccation and wilting in red maple (Acer rubrum L.) plants resembling the symptoms caused by fusaric acid. Similarities in physiological actions of picloram and several other synthetic or natural derivatives of picolinic acid have been also reported by Chang and Foy (1982). These examples demonstrate that the structures of naturally occurring phytotoxins can serve as leads for the synthesis of new successful herbicides. Undoubtedly, the use of biorational design for the discovery of new herbicide chemistries will become increasingly important in the future following the isolation and characterization of additional microbial toxins and allelochemicals from higher plants.

IV. GENETIC IMPROVEMENT OF CROP TOLERANCE TO HERBICIDES OF PLANT TOLERANCE TO HERBICIDES A. MECHANISMS

As mentioned earlier, one of the most important characteristics of chemical herbicides is their ability to control selectively a wide spectrum of weeds in a variety of crops. Such selectivity implies that some crop or weed species are able to survive and grow at agriculturally recommended rates of a herbicide but not at rates which are several times higher. When this happens we talk about crop or weed tolerance to this particular herbicide. The continuous application of herbicides for weed control in modern crop production systems, however, creates a new selective pressure that could lead to

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the development of resistant biotypes of a weed that was formerly susceptible to a given herbicide. The ability of these biotypes to grow in the presence of a herbicide is termed resistance. In contrast to plant tolerance, however, plant resistance is characterized by the ability of resistant biotypes to survive and grow at any feasible rate of application of a specific herbicide and not only at its agriculturally recommended rates (Duessing, 1984). In general, it is believed that herbicide tolerance and/or resistance is naturally present in plant systems before the introduction of a particular herbicide (Gressel, 1986; LeBaron, 1984). Moreover, physiological resistance and tolerance to any herbicide can only develop within the framework of metabolic processes that are present in the plant cell (Duessing, 1984; Gressel, 1986). Extensive biochemical investigations on the development of plant resistance and tolerance to several herbicides has identified the involvement of five major mechanisms (Duessing, 1984; Hatzios and Penner, 1982). These mechanisms include the following: 1. altered uptake and translocation or compartmentation of the herbicide in the resistant or tolerant plants 2. extensive metabolic detoxification of the herbicide in the resistant or tolerant plants 3. modification of the target site of a herbicide in the resistant or tolerant plants 4. increased production of a target enzyme in the resistant or tolerant plants 5 . increased synthesis of substrates able to reverse the herbicide-induced inhibition of growth in the resistant or tolerant plants A brief discussion of these mechanisms including the citation of selected examples will be given in the following paragraphs. Our present knowledge of the genetic basis of these mechanisms is also discussed. This information is vital for a better understanding of the discussion on the application of biotechnological techniques for the improvement of crop tolerance to herbicides that will be given later. I . Altered Uptake, Translocation, or Compartmentation

Resistant or tolerant plants in this class survive by preventing the herbicide from reaching its site of action (target site). Use of radiolabeled herbicides has demonstrated modified translocation and vascular compartmentation as the cause for the observed tolerance of cucumber (Cucumissativus L.) to the herbicide atrazine (Werner and Putnam, 1980), horseweed [Conyza linefolia (L.) Cronq.] to paraquat (Fuerst et al., 1985), and soybean to metribuzin (Falb and Smith, 1984). Autoradiograms for all three plant

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species clearly showed that herbicide translocation in atrazine-resistant cucumber, paraquat-resistant horseweed, and metribuzin-resistant soybean was limited to leaf veins and insufficient amounts of each herbicide reached the mesophyll cells containing the target site (chloroplast) of these photosynthesis-inhibiting herbicides. Alternative mechanisms conferring tolerance to other plants against these herbicides are also available (Gressel, 1986; Hatzios and Penner, 1982). 2. Metabolic Detoxification

The selective activity of most of the currently marketed herbicides is dependent on their differential metabolism by tolerant and susceptible plant species (Hatzios and Penner, 1982). Tolerant plants are able to render a phytotoxic molecule inactive by enzymatic or nonenzymatic conversion before the active herbicide reaches its site of action. Corn, for example, is tolerant to atrazine because of its ability to metabolize this herbicide by conjugating it to the tripeptide glutathione (GSH) (Frear and Swanson, 1970; Hatzios and Penner, 1982). The reaction is enzymatic, being catalyzed by an atrazine-specific GSH-sulfotransferase (Frear and Swanson 1970; Guddewar and Dauterman, 1979). Most (but not all) of the available corn hybrids used today possess a dominant gene for the biosynthesis of the atrazine-specific GSH-Stransferase. Additional mechanisms for the metabolic detoxification of atrazine in corn or other plants include hydrolytic and dealkylation reactions, and these may be the causes of the increased tolerance to atrazine in some of the weed species that do not possess plastid level tolerance to this herbicide (Gressel et al., 1983; Hatzios and Penner, 1982). Gressel et al., (1983) reported that biotypes of the weed Senecio vulgaris which developed resistance to atrazine at the chloroplast level had also coevolved much higher levels of degradative enzymes catalyzing N-dealkylation of atrazine. In addition to the atrazine-specific GSH-sulfotransferase, corn hybrids possess a number of other GSH-sulfotransferase isozymes which are specific for detoxifying the herbicides alachlor (Mozer et al., 1983) and EPTC (Lay and Niland, 1985). The important function of glutathione and GSH-S-transferase enzymes in herbicide metabolism by higher plants has been reviewed (Hatzios and Penner, 1982). GSH-S-transferase (GST) enzymes have been identified and partially characterized from leaves and roots of several plant species (Diesperger and Sandermann, 1979; Edwards and Owen, 1986; Frear and Swanson, 1970; Guddewar and Dauterman, 1979; Mozer et al., 1983). At least three isozymes, designated as GST I, GST 11, and GST 111, differing in their purification characteristics and substrate preference, have been identified in etiolated corn tissue (Mozer et al., 1983). GST I is expressed constitutively in corn roots and leaves and is

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capable of conjugating glutathione to chloroacetanilide herbicides such as alachlor. Treatment of corn seeds with chemical herbicide safeners used to protect crop plants from injury, such as flurazole, increased the activity of GST I and induced the appearance of a novel GST activity (GST 11) which has greater detoxification activity on these herbicides (Mozer et al., 1983). The structural analysis of a corn gene coding for GST I has been reported by Shah et al. (1986a). The inbred corn line Missouri 17 contained a single gene for GST I, whereas the hybrid line 3780A contained two genes (Shah et al., 1986a). The amino acid sequence of the corn GST I gene showed no apparent sequence homology with the published sequences of animal GSTs (Shah et al., 1986a). Table IV presents selected examples of GSTs and other plant enzymes that have been reported to play a key role in the metabolic detoxification of several important herbicides. The purification and properties of 0- or Table IV Plant Enzymes Known to Metabolize Selected Herbicides Herbicide

Enzyme

Plant source

Reference

Monuron, diuron 2,4-D Arylamines Atrazine

N-demethylase Hydroxylase Peroxidase GlutathioneS-transferase GlutathioneS-transferase GlutathioneS-transferase GlutathioneS-transferase

Cotton, soybean Peas, cucumber Peas Corn

Frear et al. (1969) Makeev et al. (1977) Corbett and Corbett (1983) Guddewar and Dauterman (1 979) Mozer et at. (1983); Edwards and Owen (1986) Frear et 01. (1983a)

GlutathioneS-transferase Deaminase N-glucosyltransferase Amidase

Soybean

Frear and Swanson (1973); Diesperger and Sandermann (1979) Frear et al. (1985)

Soybean Tomato

Fedtke (1983) Frear et al. (1983b)

Rice

Frear and Still (1968) Gaynor and Still (1983) Frear (1968)

Alachlor Acifluorfen Fluorodifen

Metribuzin Metribuzin Metribuzin Propanil Chloramben Pentachlorophenol

N-glucosyltransferase O-glucosyltransferase

Corn Soybean Peas

Soybean Soybean, wheat

Schmitt et at. (1985); Schmitt et al. (1986)

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N-glucosyl and 0- or N-malonyl transferases from wheat and soybeans which conjugate the 4-hydroxy derivative of 2,4-D and substituted chloroanilines, respectively, have been reported recently by Schmitt et al. (1986). The isolation and characterization of cDNA clones coding for plant enzymes which detoxify herbicides in higher plants (Table IV) will facilitate molecular genetic manipulations of these enzymes in the future. In addition, the potential transfer of bacterial genes which detoxify herbicides into plant genomes, following the modification of such genes for expression in plants, should be exploited for increasing the crop selectivity of herbicides. A recent example illustrating the potential of this approach is the isolation of a bacterial gene from Streptomyces sp. which codes for an enzyme capable of acetylating phosphinothricin, the active ingredient of the herbicides bialophos and glufosinate (Newmark, 1987). Acetylated phosphilothricin does not inhibit the activity of the enzyme glutamine synthetase. The bacterial gene coding for the enzyme which acetylates phosphinothricin has been engineered into several crop plants, such as tomato, tobacco (Nicotiana tabacum L.), and potato (Solanum tuberosum L.), making them tolerant to this herbicide (Newmark, 1987).

3. Modifcation of Herbicide Target Sites To exert their phytotoxic action most herbicides have to bind to a specific receptor site, which is usually a protein or an enzyme involved in plant metabolism. Resistance to such herbicides could develop if proteins or enzymes can be modified to discriminate functionally between the herbicide and the normal substrate for the binding site. Several examples have been reported in the literature that illustrate the importance of this mechanism in the development of resistance to selected herbicides. a. Plastid-Mediated Resistance to Triazine Herbicides. The most extensively studied example is the development of weed biotypes resistant to the herbicide atrazine. Following repeated applications of this herbicide in corn fields of the midwestern United States and of other countries, several weed biotypes have become tolerant to this herbicide through a plastid-mediated mechanism of resistance (LeBaron, 1984; LeBaron and Gressel, 1982). Atrazine is a photosynthesis-inhibiting herbicide known to act on a plastoquinone-binding membrane protein of 32 kilodalton (kd) termed “QBprotein” (Arntzen et al., 1983). The QBprotein is the site of herbicide binding and serves as the second stable electron acceptor of photosystem I1 of the photosynthetic apparatus located in the chloroplast of higher plants and green algae. Weed biotypes that have developed atrazine resistance at the chloroplast level share the following common traits: 1. Photosynthetic electron transport in resistant chloroplasts is

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KRITON K. HATZIOS

insensitive to inhibition by any triazine herbicide. 2. A high-affinity triazine binding site cannot be detected in thylakoid membranes from resistant chloroplasts. 3. The chloroplast-associated resistance is inherited maternally. The QB protein is coded for by the psbA gene, which has been sequenced from several atrazine-sensitive plants such as spinach (Spinacea oleracea L.) (Zurawski et al., 1982), tobacco (Nicotiana debeneyi) (Zurawski et al., 1982), and soybean (Spielman and Stutz, 1983) and from atrazine-resistant weeds such as Amaranthus hybridus (Hirschberg and McIntosh, 1983), Solanum nigrum (Golubinoff et al., 1984), and Sinapis alba (Link and Langridge, 1984). In addition, the psbA gene has been sequenced from atrazine-sensitive and atrazine-resistant green algae and cyanobacteria (Curtis and Haselkorn, 1984; Erickson et al., 1984; Golden and Sherman, 1984; Golden and Haselkorn, 1985; Karabin et al., 1984; Mets et al., 1986). The sequences of the psbA gene from several species have demonstrated that the 32-kd Q B protein is very highly conserved. The resistance of higher plants, green algae, and cyanobacteria to herbicides acting as inhibitors of electron transport mediated by photosystem I1 (e.g., triazines, ureas) has been correlated with a single mutation of the chloroplast psbA gene. Nucleotide changes resulting in the substitution of selected amino acids located at positions 219-275 of the psbA gene product are currently viewed as responsible for the observed plastid-mediated resistance to these herbicides (Fedtke and Trebst, 1987; Arntzen, 1986). In most of the studied cases, the mutated QB protein is rendered insensitive to triazine herbicides as a result of a single nucleotide change that replaces serine at position 264 of the QB protein of susceptible weed biotypes with glycine (Arntzen, 1986; Mets et al., 1986). In the cyanobacterium Anacystis nidulans, resistant mutants originated from a change of the serine at 264 to an alanine at the same position (Golden and Haselkorn, 1985). The molecular structure of the photosynthetic reaction center of the purple bacterium Rhodopseudomonas viridis was recently elucidated, using X-ray crystallography analysis, by Michel’s group at the Max Planck Institute in West Germany (Deisenhofer et al., 1985). Following this first description of the high-resolution structure of bacterial membrane protein D,, which corresponds to the QB protein of higher plants, a number of models describing the herbicide binding domain of the Q B protein and showing the location of amino acids which have been identified as being altered in herbicide-resistant membranes have been constructed by Arntzen (1986) and Trebst (1986). Such models facilitate predictions of mutations which could impart herbicide resistance not obtained from random mutagenesis. The psbA gene from resistant weed biotypes or green algae and cyanobacteria has been cloned and is considered very promising for the

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transfer of atrazine resistance to susceptible crop plants such as soybeans (Gressel, 1985, 1986; Sandermann, 1985). Mets et al. (1986) reported recently that in genetically mixed chloroplasts, the resistance alleles of the psbA gene appear to be recessive. They suggested that the exploitation of psbA in the engineering of plant resistance via gene transfer techniques will most likely require the replacement or inactivation of the endogenous sensitive allele before useful resistance will be expressed. An alternative approach could involve the increase of the product of the resistant gene relative to the sensitive protein by increasing the expression of the resistant gene with an altered promoter. b. Resistance to Glyphosate. Glyphosate, a nonselective herbicide, is known to inhibit an enzyme of the shikimic acid pathway, 5-enolpyruvylshikimate-3-phosphate synthase (Amrhein et al., 1983; Steinrucken and Amrhein, 1980). The enzyme, commonly abbreviated as EPSP synthase, has been highly purified from Escherichia coli and from pea (Pisum sativum L.) seedlings (Duncan et al., 1984; Mousdale and Coggins, 1984). The microbial enzyme is coded for by the aroA gene, which has been cloned from E. coli (Comai et al., 1983) and Salmonella typhimurium (Rogers et al., 1983). The mechanism of glyphosate-resistant strains of Salmonella and Aerobacter aerogenes (Schultz et al., 1984) has been shown to involve a modification of the target EPSP synthase resulting in a greatly reduced affinity of the enzyme for glyphosate. Molecular studies with the glyphosate-resistant aroA allele obtained from mutagenized Salmonella showed that a single base pair change resulting in a proline-to-serine amino acid substitution at the lOlst amino acid of the protein was responsible for the development of resistance to this herbicide (Stalker et al., 1985). Attempts to introduce the cloned aroA gene resistant to glyphosate to sensitive crop plants have been reported (Comai et al., 1985) and they will be discussed in the next section. c. Resistance to Sulfonylurea and Imidazolinone Herbicides. Although structurally unrelated, the sulfonyulurea and imidazolinone classes of herbicides have a similar mechanism of phytotoxic activity which involves the inhibition of the enzyme acetolactate synthase (Chaleff and Mauvais, 1984; Shaner et al., 1984). This is the first specific enzyme for the biosynthesis of the branched-chain amino acids, valine, leucine, and isoleucine, in higher plants and microorganisms (Chaleff and Mauvais, 1984; LaRossa and Schloss, 1984). Resistance to sulfonylureas and imidazolinones is accomplished by a modification of the target enzyme in resistant mutants which renders it insensitive to these herbicides (LaRossa and Smulski, 1985; Shaner and Anderson, 1986). Several isozymes of acetolactate synthase (ALS), designated as ALS I, ALS 11, and ALS 111, are known to exist in microorganisms (LaRossa and Schloss, 1984). The microbial enzyme from the enteric bacteria Salmonella typhimurium and E. coli which is mutated to yield resistance to these herbicides is coded by the ilvB gene (LaRossa and Smulki, 1985). Tobacco mutants resistant to the sulfonylurea herbicides

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chlorsulfuron and sulfometuron-methyl have been isolated recently by direct selection in tissue culture or by selection following mutagenesis (Chaleff and Ray, 1984; Chaleff, 1986). The resistance of tobacco to sulfonylurea herbicides is associated with two loci, and the obtained mutants are commonly designated as SURA and SURB (Chaleff, 1986). The resistance of yeast to sulfonylurea herbicides has been characterized by molecular biology precedures, revealing an altered structural gene for A L S encoding a resistant A L S protein in which a proline amino acid residue in the sensitive A L S is replaced by a serine (Hardy, 1986). d. Resktance to Dinitroaniline Herbicides. The existence of a biotype of goosegrass [Eleusine indica (L.) Gaertn.] which developed resistance to the dinitroaniline herbicide trifluralin following repeated treatments with this herbicide has been reported by Mudge et al. (1984). This biotype was also resistant to other dinitroaniline herbicides tested (Mudge et al., 1984). Vaughn (1986) reported recently that an alteration in the target protein involved in the action of the dinitroaniline herbicides is responsible for the development of resistance in weed biotypes of goosegrass. Biochemical analysis showed that the dinitroaniline-resistant biotypes of goosegrass had a tubulin subunit with an altered mobility as compared to the sensitive biotype (Vaughn, 1986). 4. Increased Production of a Target Enzyme

Increased activity of a target enzyme is another mechanism that may confer resistance to selected herbicides. Such a mechanism has been demonstrated empirically with glyphosate-resistant mutants of Aerobacter and cultured plant cells (Amrhein et al., 1983). In E. coli, overexpression of the enzyme EPSP synthase was achieved in a controlled way by placing the normal aroA gene on a multicopy plasmid and then transferring it back into the host bacteria (Rogers et al., 1983). These bacteria then make nearly 100-fold more EPSP synthase compared to wild-type strains which are susceptible to the herbicide glyphosate. Increased EPSP synthase levels have been also achieved by gene duplication following the use of sequential increases in glyphosate concentrations in plant tissue culture media. Glyphosate-resistant cultures of carrot (Daucus carota L.) and of Corydalissemprevirens have been obtained by this procedure (Amrhein et al., 1983; Widholm, 1984). As mentioned earlier (Section 111), L-phosphinothricin (the active ingredient of the herbicide’s bialophos and glufosinate) is a mixed competitive inhibitor of the enzyme glutamine synthetase (GS), which plays a central role in the nitrogen metabolism of higher plants. Donn et al. (1984) selected several suspension cell lines of alfalfa that were 20- to 100-fold more resistant to this herbicide. GS enzyme levels were three- to seven-fold elevated in the variant cell line, suggesting that resistance to L-phosphinothricin was due to an amplification of a glutamine synthetase gene and overproduction of the glutamine synthetase enzyme (Donn et al., 1984). The nucleotide

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sequence of the amplified alfalfa glutamine synthetase gene was characterized by Tischer et al. (1986). Several regions of homology were observed when the sequence of this gene was compared to glutamine synthetase sequences from Anabaena and Chinese hamster (Tischer et al., 1986). The minimum size of a glutamine synthetase gene amplification unit in the alfalfa tissue culture line which was resistant to L-phosphinothricin was determined to be 35 kb (Tischer et al., 1986). Overproduction of acetolactate synthase due to the presence of the IL V2 and ilvG genes, which encode ALS in yeast or E. coli, respectively, has been reported as an alternative cause of resistance development to the sulfonylurea herbicide sulfometuron-methyl (Falco and Dumas, 1985).

5. Increased Synthesis of Substrates Able to Reverse the Herbicide-Induced Inhibition Many of the currently used herbicides resemble structurally normal cellular substrates and they may function as antimetabolites by mimicking the structural features or chemical properties of these natural substrates. The inability of an enzyme or protein to discriminate between the normal substrate and the herbicide may be responsible for the action of this herbicide (Duessing, 1984). Potentially, a cell could tolerate this type of herbicide action by increasing the intracellular concentration of the natural substrate. Specific examples demonstrating the involvement of this mechanism in the development of crop or weed resistance to a particular herbicide have not yet been reported. The selection, however, of cultured mutant cell lines of corn (Hibberd et al., 1980), rice (Schaeffler and Sharpe, 1981), tobacco (Widholm, 1976), and carrot (Cella and Iadarola, 1983) that are resistant to amino acid analogs illustrates the feasibility of this mechanism. Plant tissue cultures of these resistant mutants have yielded plants with 10- to 50-fold higher levels of the competing amino acid compared to the susceptible cells (Hibberd et al., 1980; Cella and Iadarola, 1983). B.

GENETIC MANIPULATION OF CROP TOLERANCE TO HERBICIDES

Our current knowledge of the mechanisms conferring resistance or tolerance to all marketed herbicides is far from complete. Recent advances, however, in our understanding of the biochemical and physiological factors that render selected plant species resistant to specific herbicides and the genetic basis of these factors have stimulated considerable interest in the utilization of genetic manipulations for the improvement of crop tolerance to given herbicides. Extensive research during the last decade has resulted in

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the isolation and characterization of several plant and microbial genes which regulate the tolerance or resistance of plants to particular herbicides. Specific examples were presented and discussed in the previous section of this review. Uptake, translocation, and compartmentation of selected herbicides in higher plants are complex physiological processes often determined by multiple genes. The genetic exploitation of such systems to alter the uptake, translocation, and compartmentation of selected herbicides and confer crop tolerance to these herbicides is extremely difficult (Comai and Stalker, 1984). Genetic analyses of crop tolerance to herbicides arising from herbicide detoxification have shown that this type of resistance is dominant and is often expressed by a single gene (Comai and Stalker, 1984; Hatzios and Penner, 1982; Martin, 1985). Identification and characterization of genes encoding for enzymes involved in herbicide detoxification, as well as advances in our understanding of the processes involved in the regulation of such genes, offer attractive opportunities for the manipulation of crop tolerance to herbicides. Selected plant enzymes catalyzing the metabolic detoxification of specific herbicides are listed in Table IV. In most cases, however, the genes coding for these enzymes have not been characterized. A report by Shah et al. (1986a) represents the first successful attempt at cloning and sequencing a plant gene coding for the enzyme GST I involved in the detoxification of chloroacetanilide herbicides in corn. In a follow-up study, Wiegand et al. (1986) showed that the expression of this gene can be regulated chemically with the use of exogenously applied herbicide safeners. Safeners such as flurazole were shown to act at the transcriptional level, inducing a three- to four-fold increase in the steady state level of the mRNA encoding the GST I enzyme in corn tissues grown from flurazole-treated seeds (Wiegand et al., 1986). Isolation of bacterial genes coding for enzymes detoxifying herbicides should be also exploited for improvement of crop tolerance to herbicides. A number of bacterial genes have now been successfully introduced and expressed in plants by using plant promoters, signal sequences, and poly(A) + signals, etc. (Fraley et al., 1986a). The initial isolation and subsequent in vitro mutagenesis and selection of resistance may be easier in bacteria than in higher plants. The aforementioned successful transfer of a degradative gene from Streptomyces sp., which acetylates phosphinothricin into solanaceous crops, illustrates the potential of this approach (Newmark, 1987). In situations in which herbicide detoxification results from a chain of metabolic reactions rather than a single reaction, genetic manipulation of herbicide tolerance would be more complicated. Genes encoding for all metabolic enzymes degrading a herbicide and its metabolites would be

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needed, resulting in the development of oligogenic resistance systems (Comai and Stalker, 1984). Plant resistance to herbicides accomplished by a modification of the herbicide target site or increased production of target enzymes and/or substrates is often determined by single genes. Such systems are far more desirable for exploitation because their genetic manipulation would require the transformation and transfer of a single gene. In fact, the feasibility of engineering this mechanism of herbicide tolerance is viewed currently as being greater than that of any other agriculturally important trait (Comai and Stalker, 1984; Hardy, 1986; Netzer, 1984). Generation of herbicide tolerance or resistance is of immediate scientific interest because it provides us with a direct selection marker in studying plant gene expression and regulation. In addition, the introduction of genes confering herbicide resistance to economic crops could be utilized to improve the agricultural uses of currently registered herbicides. Crop plants resistant to herbicides can be engineered genetically by four major approaches including (1) classic plant breeding techniques and mutation breeding; (2) in vitro mutant selection at the cell or tissue level; (3) mutant selection by somatic hybridization; and (4) transfer of cloned genes into susceptible crop plants. 1. Classic Plant Breeding Techniques and Mutation Breeding

Numerous examples illustrating the genetic variability of the responses of crops to herbicides have been observed and have been reviewed by Martin (1985). Success in exploiting the intraspecific variation of plant responses to herbicides by means of classic breeding approaches has been limited (Martin, 1985; Beversdorf, 1985a). Standard breeding approaches work only for species that are sexually compatible, a condition that applies to very few crop plants and weeds (Beversdorf, 1985b; Beversdorf et al., 1980; Chaleff, 1981). The sources of genes utilized by plant breeders for desirable characteristics are usually limited by the normal reproductive barriers that distinguish species. Classic breeding techniques have been, however, successful for the development of triazine-resistant crop varieties. The occurrence of a triazine-resistant biotype of bird’s rape (Brassica campestris L.) has allowed the use of breeding for the development of commercially useful triazineresistant varieties of canola (Brassica napus L.) (Beversdorf et al., 1980; Souza-Machado and Bandeen, 1982). Beversdorf et al. (1980) succeeded in transferring the cytoplasmically inherited resistance to atrazine from bird’s rape to canola by means of backcrossings between these two cross-fertile species and selection for chromosome number. Atrazine-resistant canola (cv. OAC Triton) is presently cultivated in Canada and approximately

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200,000acres seeded with atrazine-resistant canola were expected to be produced in 1985 (Netzer, 1984). Under ideal conditions, the yield of both atrazine-resistant and atrazine-susceptiblecanola crops is equivalent. Under conditions of high temperature and drought, however, the yield of atrazineresistant canola is reduced by about 20% (Beversdorf, 1985a,b; Netzer, 1984). The poor agronomic performance of the triazine-resistant canola has been a limiting factor for the further utilization of breeding for atrazine resistance to other Cole crops and mustards (Beversdorf, 1985a,b). Mutation breeding, which includes mutagenesis of seeds followed by screening of seedlings for resistance, offers good promise for manipulating herbicide resistance at the seedling or whole plant level (Chaleff, 1981). This approach, however, is likely to require several backcrosses of the isolated, resistant mutant lines to eliminate undesirable mutations resulting from random mutagenesis. Early attempts by Pinthus (1972) in utilizing mutation breeding resulted in the selection of wheat (Triticum aestivum L.) mutants with increased seedling resistance to the herbicide terbutryne and of tomato mutants resistant to diphenamid. Populations of these plants grown from seeds treated with the mutagen ethyl methane sulfonate (EMS) were used in selecting the aforementioned mutants. More recently, Sommerville et al. (1985a) reported the successful selection of chlorsulfuron-resistant seedlings of the small crucifer Arabidopsis thaliana following mutagenesis with EMS. Similarly, Chaleff (1986) was successful in screening and selecting soybean mutants with increased tolerance to the sulfonylurea herbicides chlorsulfuron and sulfometuron-methyl by means of mutation breeding. Tolerance of these mutants to sulfonylureas was conferred by recessive nuclear mutations which, unlike the mutations recovered in tobacco (Chaleff and Ray, 1984), did not affect the sensitivity of the enzyme acetolactate synthase (ALS) to these herbicides. Although conventional selection techniques and mutation breeding are laborious and slow processes, these successful examples illustrate the value of these approaches to investigators attempting to genetically improve crop tolerance to herbicides. These approaches should not be overlooked. 2. In Vitro Mutant Selection at the Cell or Tissue Level

Attempts to achieve herbicide resistant crops by using cell culture techniques have been made for more than 10 years (Gressel, 1984). Herbicides that interfere with the metabolic activities of whole plants are expected to inhibit the growth of cultured cells or tissues as well. In such cases, herbicide-tolerant mutants can be selected by culturing cells or tissues in the presence of a herbicide concentration that is toxic to normal cells. Manipulation of large cell populations, studies on the direct interaction

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between cells and inhibitors, and regeneration of plants from selected cell lines are the main steps involved in this approach. The potential application of cell and tissue cultures in mutant selection for herbicide resistance has been reviewed (Chaleff, 1983; Gressel, 1984, 1985; Meredith and Carlson, 1982; Widholm, 1978). Successful examples demonstrating the value of this approach include the development of tobacco plants resistant to the herbicides amitrole (Barg and Umiel, 1977; Singer and McDaniel, 1984), bentazon (Radin and Carlson, 1978), chlorulfuron (Chaleff and Ray, 1948), glyphosate (Singer and McDaniel, 1985), paraquat (Miller and Hughes, 1980), phenmedipham (Radin and Carlson, 1978), and picloram (Chaleff and Parsons, 1978). Clover (Trifolium repens L.) plants resistant to phenoxyacetic acid herbicides have been also selected through this approach (Oswald et al., 1977). A novel selection scheme devised by Radin and Carlson (1978) and known as the “green island” technique has received considerable attention in studies associated with the selection of herbicide resistance since it combines mutant studies at both the whole plant and tissue culture levels. Radin and Carlson (1978) sprayed immature leaves of mutagenized haploid tobacco plants with the herbicides bentazon and phenmedipham. Following herbicide treatment and leaf expansion, the small green islands that appeared on the otherwise yellow leaves were excised and placed on a culture medium known to induce shoot regeneration. Plants regenerated from these cultures, however, retained only a small level (ranging from 13 to 21%) of the resistance to the herbicides bentazon and phenmedipham (Radin and Carlson, 1978). The in vitro selection and isolation of tobacco mutants resistant to chlorsulfuron and sulfometuron-methyl herbicides has attracted considerable attention and the potential commercial exploitation of such tobacco mutants is currently under way (Chaleff, 1986). Genetic analysis of plants regenerated from mutant cell lines showed that the resistance of these mutants is the result of a single semidominant mutation occurring at two unlinked loci (Chaleff and Ray, 1984). An altered form of the ALS enzyme is produced in the tobacco mutants, rendering them insensitive to sulfonylurea herbicides. Corn cell lines (e.g., XA17) exhibiting greater than 100-fold enhanced tolerance to imidazolinone herbicides such as imazaquin have been selected recently by Shaner and Anderson (1986). Plants were regenerated from the resistant lines in the presence of toxic levels of imidazolinones and herbicide resistance was expressed at the whole plant level. Genetic analysis showed that the inheritance of imidazolinone resistance in corn is controlled by a single dominant gene (Shaner and Anderson, 1986). In contrast to the situation of the atrazine-resistant canola, the respective resistance of tobacco and corn plants to sulfonylurea and imidazolinone herbicides is not associated with any yield penalty (Hardy, 1986).

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Glyphosate-tolerant plant cell cultures containing elevated levels of EPSP synthase activity have been reported by Nafziger et al. (1984), and Amrhein et al. (1983). A glyphosate-tolerant cell line of petunia (Petunia hybrida) established after stepwise selection on increasing amounts of glyphosate has been reported by Steinrucken et al. (1986). This cell line of petunia (MP4-G) overproduces EPSP synthase 15- to 20-fold, suggesting that the molecular basis for the glyphosate tolerance of this line is the amplification of the gene coding for EPSP synthase. Another example demonstrating the involvement of gene amplification in the expression of herbicide tolerance is the selection of alfalfa cell lines with increased tolerance to L-phosphinothricin (the active ingredient of the herbicides bialophos and glufosinate) (Donn et al., 1984). In generating crop corn resistance to herbicides by means of gene amplification, however, one should not overlook the potential loss of the extra copies of the expressed gene after the selective pressure is removed. This is a common problem associated with the amplification of mammalian genes, and studies with amplified plant genes have also identified this problem (Comai et al., 1985). In spite of its potential and the achieved successes, however, the use of in vitro selection techniques at the cell or tissue levels is limited by several factors (Meredith and Carlson, 1982). First, the absence of many differentiated functions in cultured cells must be considered when selecting for herbicide resistance. Not all metabolic processes are “turned on” in all cells at all times in the life cycle of a plant (Gressel, 1985; Meredith and Carlson, 1982). Consequently, photosynthesis-inhibiting herbicides are often inactive in nongreen cells, and root-metabolized herbicides may not be damaging to green cells. Second, it should be recognized that inhibition of the growth of a cell culture by a given herbicide may result in spite of the absence of its primary function. The observed effect may then be due to secondary modes of action and therefore the selection of mutants resistant to these secondary actions may not be enough for the development of resistance under field conditions. In addition, the potential interference of the components of the culture medium with the action of a given herbicide should be examined and a cell culture appropriate for this particular herbicide must be obtained. An additional obstacle is the difficulty involved in regenerating plants from cell cultures. Although establishing a cell culture from virtually any plant seems to be a feasible process, regeneration is presently possible for only a limited number of plant species (Chaleff, 1981, 1983; Meredith and Carlson, 1982). Corn and soybeans, for example, two of the most important agricultural crops, cannot be easily regenerated from tissue or cell cultures at the present time. To avoid some of these problems, the selection and use of algal mutants has been suggested as a complementary system when the most sensitive site of a herbicide is homologous between algae and crop plants (Mets, 1985).

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Studies on algal mutants that confer resistance by altering the sensitive site can provide critical information needed for the genetic engineering of herbicide resistance in higher plants. In particular, this approach has shown considerable promise for the engineering and transfer of the atrazineresistant psbA gene, which is highly conserved in higher plants, algae, and cyanobacteria (Mets, 1985; Mets et al., 1986). The small crucifer, Arubidopsis thaliana, has several attractive features including a short life cycle and small-sized genome, and its use as a model system for plant genetics and molecular biology has become quite popular in recent years (Sommerville et al., 1985b).

3. Mutant Selection by Somatic Hybridization The lack of sexual compatibility between many crop plants and weeds was cited earlier as an important factor limiting the application of classic breeding techniques in genetic manipulations for the development of crops tolerant to selected herbicides. Sexual incompatibility among plants, however, can be bypassed by the use of somatic cell genetic procedures such as protoplast fusion. Protoplasts are plant cells devoid of their cell walls. Under appropriate conditions, protoplasts from different plants can be induced to fuse in culture, combining their genetic information to create a new hybrid, one that could never have happened in nature (Skelsey, 1984). The recombinant protoplasts can then be induced to reform their cell walls, proliferate, form calli, and regenerate, as shown schematically in Fig. 1. Sexually incompatible species can be hybridized and useful hybrids can be selected for several characteristics including herbicide resistance. The goal is to introduce a nuclear or cytoplasmic trait, such as those controlled by the DNA of chloroplasts and mitochondria, into the crop without reducing the yield and quality of the product. To date, only hybrid protoplasts from closely related species have been induced to regenerate from culture (Skelsey, 1984). Application of protoplast fusion to engineer herbicide resistance has attracted considerable attention because of the close botanical relations of several atrazine-resistant weeds to selected agronomic crops (Table V). Attempts however, to utilize this technique in practice have met only with partial success. Binding et al. (1982) were the first to demonstrate the feasibility of protoplast fusion for transferring atrazine resistance from the resistant biotypes of the black nightshade (Solanurn nigrum L.) weed to the botanically related potato crop. However, although the goal was to get a crop with a single desirable trait of the weed (Le., atrazine resistance), the hybrid formed resembled more closely the weed than the potato crop (Gressel et ul., 1984).

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352 SUSCEPTIBLE PLANT

1 1

T I S S U E CULTURE

SELECTION & REGENERATION

1 1

SUSCEPTIBLE PLANT

RESISTANT PLANT

1

PROTOPLASTS

PROTOPLASTS

,lo,

SOMATIC HYBRID

1

NEW V A R I E T I E S

Schematic representation of the production of herbici, -resistant plants through FIG. somatic hybridization.

Use of irradiation to destroy the nuclei of donor protoplasts containing the desirable trait has been suggested as a way to improve the results of protoplast fusions (Netzer, 1984). Such fusions have been accomplished by Shahin et al. (1984) by disabling the nuclei of the protoplast of the triazineresistant nightshade prior to fusion with protoplasts of triazine-susceptible tomato plants. Pelletier et al. (1983) used protoplast fusion to incorporate triazine tolerance into cytoplasmic male-sterile lines of canola. The wide application of this approach, however, is limited by the aforementioned limitations in regenerating specific plants from cell cultures or fused protoplasts. The limitations in the use of protoplasts for somatic hybridization and transformation of cereal plants as well as alternative approaches for such manipulations were reviewed by Ozias-Akins and Lijrz (1984). 4.

Transfer of Cloned Genes into Susceptible Plants

The direct transfer of cloned genes into cells of sensitive plants is presently viewed as the most powerful approach for genetically manipulating crop tolerance to herbicides. An ideal gene transfer method would involve

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Table V Atrazine-Resistant Weeds That Are Botanically Related to Economic Crop# Atrazine-resistant weed

Crop plant

Chenopodiaceae Chenopodium album (common lambsquarters) A triplex patula

Beta vulgaris (sugarbeet) Beta vulgaris (red beet)

Compositae Senecio vulgaris (common groundsel) Ambrosia artemisifolio (ragweed)

Helianthus annus (sunflower) Carthamus tinctorius (safflower)

Cruciferae Brassica campestris (wild turnip)

Brassica campestris (turnip rape) Brassica napus (canola) Brassica oleracea (cabbage)

Solanaceae Solanum nigrum (black nightshade)

Solanum tuberosum (potato) Lycopersicon esculentum (tomato) Nicotiana tabacum (tobacco)

uAdopted from Valiulis (1984).

genetic replacement, in which the transforming gene exchanges with the endogenous gene by homologous recombination or gene conversion (Mets et al., 1986). The development of successful systems for the genetic transformation of higher plants is dependent on the availability of engineered vectors and the development of cell culture techniques that permit efficient DNA delivery, selection of transformants, and regeneration of intact plants (Skelsey, 1984). Several aspects of the genetic transformation of higher plants have been reviewed by Fraley et al. (1986a) and by Horsch et al. (1985). In principle, the procedure is simple and is illustrated graphically in Fig. 2. However, in practice, there are several obstacles that must be overcome before this technique is fully utilized. At present, various vectors can be used to transfer DNA from one plant to another. The most promising and widely used is the Ti plasmid of Agrobacterium tumefaciens (Caplan et al., 1983; DeCleene and De Ley, 1976). This bacterium has the unique ability to enter a plant cell and insert a small piece of its plasmid DNA into the chromosomes of the host plant, causing a plant disease known as crown gall (Caplan et al., 1983). By means of recombinant DNA technology, the gene to be transferred is spliced into the Ti plasmid of Agrobacterium tumefaciens. The engineered bacterium, carrying the recombinant DNA in its plasmid, is then introduced into excised leaf disks or plant cells growing in culture. The bacterium enters the cells and the plasmid inserts a part of its DNA into the chromosomes of the plant

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RESISTANT PLANTS

1

RESISTANT GENES

SUSCEPTIBLE PLANTS Vector #

1

PROTOPLASTS I

REGENErloN SELECTION

&

EXPRESSION OF

RESISTANT GENE

3

NEW RESISTANT VARIETIES FIG.2. Schematic representation of the production of herbicide-resistant plants through gene manipulation and transfer.

cells, introducing thus the cloned gene into the plant genome. The introduction and stable integration of foreign genes into plant cells is now routine in certain plant species with engineered versions of the Ti plasmid vectors from Agrobacterium tumefaciens (Fraley et al., 1986a, b; Horsch et al., 1985). The commercial potential of the Ti plasmids as vectors for the transfer of cloned genes into eukaryotic cells is limited by several factors including (1) the possible pathogenicity of Ti plasmids, since Agrobacterium tumefaciens is a tumor-inducing phytopathogenic bacterium causing the crown gall disease of many economic crops; (2) the limited host range of Ti plasmids, which includes only dicotyledonous plants; and (3) the inability of researchers to target the insertion site of Ti plasmids (Thomas and Hall, 1985). Recent advances, however, have resulted in the construction of Ti plasmids lacking the disease-causing capability of the normal plasmids (Fraley et al., 1986a). Such plasmids, known as disarmed Ti plasmids, are used routinely to introduce the genes for antibiotic resistance into plants. Cells from host plants have been regenerated into whole plants with the gene for antibiotic resistance still being expressed. In addition, transformation of monocotyledonous plants such as asparagus (Asparagus officinalis L.) and several members of the Liliales by Ti plasmid vectors has been demonstrated (Hernalsteens et al., 1984; Hooykaas-van Slogteren et al., 1984). However, attempts to transform corn and several other monocotyledonous crops have been unsuccessful so far. The inability of vector systems to transfer foreign DNA to organelles (e.g., chloroplasts) of plant cells is presently an obstacle that needs to be considered. The need for vectors whose insertion site can be targeted is particularly important for the

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transfer of mutated psbA genes into susceptible chloroplasts of crops such as soybean to make them resistant to atrazine or other herbicides interfering with the function of photosystem 11. Successful attempts in transferring foreign DNA to chloroplasts by means of an Agrobacteriurn-mediated vector system have been reported recently (De Block et af., 1985). Viruses and viroids have been considered also as potential vectors for the transfer of foreign DNA into plants. Among viral-based vectors, the cauliflower mosaic virus (CaMV), a DNA plant virus, has received more attention than others because it provides a single genetic system for the reproduction and transfer of foreign DNA in plant cells (Hull and Covey, 1985). However, insertion into CaMV DNA is constrained by the small number of regions that can be modified without destruction of the viral functions that are essential for infectivity (Thomas and Hall, 1985). An alternative route for the use of viruses as vectors of foreign DNA into cells of higher plants is the method of the so-called agroinfection (Grimsley et a f . , 1986). This technique is based on the observation that bacterial plasmids containing viral genomes are infectious (Lebeurier et a f . , 1980; Walden and Howel, 1982). The potential application of this approach was demonstrated recently by Grimsley et a f . (1986), who showed systemic infection of turnips (Brassica rapa L., Rapifera group) through wounding and inoculation with strains of Agrobacterium tumefaciens in which more than one genome of CaMV had been placed in the T-DNA of the tumor-inducing plasmid. Once inside the plant, the viral genome escaped from the T-DNA, causing symptoms typical of CaMV infection. Thus, the combined use of the tumor-inducing plasmid of A. tumefaciens and viral DNA opens new avenues to molecular biology applications that are not possible with each system alone (Grimsley et af., 1986). The use of transposable elements (TEs) for plant transformation has also been viewed as an attractive approach (Doring, 1985). The best studied TE in plants is the Ac-Ds system, which operates in corn and which could be subject to regulation (Thomas and Hall, 1985). Upon insertion, plant transposons can give rise to a variety of chromosomal changes in the vicinity of the insertion site (Doring, 1985). The recent reports of successful transfer of the maize (corn) Ac element into tobacco and tomato and its subsequent transposition within these plants suggest that these elements may be applicable to a wide variety of both monocot and dicot systems for mutagenesis and subsequent isolation of TE-tagged genes involved in herbicide resistance (Doring, 1985). Other alternative systems for the transfer of cloned genes into plant cells include microinjection, direct DNA uptake by protoplasts (electroporation), or using pollen as a vector system. In microinjection, DNA is injected directly into the nucleus of a specific plant cell, using a fine glass needle which is less than a micron wide at its tip.

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It is a delicate process and sophisticated instrumentation is needed for the application of this technique. The history, present status, future potential, and limitations of the application of micromanipulation techniques such as microinjection in the transformation of higher plants were reviewed by Crossway et al. (1986). Direct gene transfer (electroporation) describes the phenomenon of uptake of genes into plant cells (protoplasts) through the naked plasma membrane and the functional integration of these genes into the host genome. The method is independent of biological vectors and it makes use of the fact that DNA can pass through plasma membrane and that protoplasts can be totipotent. Theoretically there is no host range problem. The method, however, requires protoplasts, and totipotent protoplasts are not readily available for every desired plant species, especially the cereal crops. The present status of electroporation and its future potential for the transformation of plant cells has been reviewed by Potrykus et al. (1985). An electroporator is used to apply short high-voltage pulses to facilitate the entrance of the foreign DNA into the shocked protoplasts. At present, the use of recombinant DNA technology for engineering herbicide resistance in crop plants has been successful only in a few cases. This technology is limited to certain plant species for which transformation vectors and regeneration procedures are available. Comai et al. (1985, 1986) tested whether the expression of a bacterial gene encoding a glyphosateresistant EPSP synthase could confer tolerance to tobacco or Brassica plants against the herbicide glyphosate. They used the Ti plasmid system of A. tumefaciens as a vector to express a mutant allele of the aroA gene of Salmonella typhimurium coding for an EPSP synthase in which a single amino acid substitution of a proline to serine causes a decreased affinity for glyphosate. The octopine synthase gene (ocs) promoter was used to facilitate the expression of this chimeric gene in tobacco and Brassica plants. Analysis of gall tissue from tobacco and Brassica infected with A . tumefaciens pK12 containing this chimeric gene showed that 50% of the total EPSP synthase activity present had kinetic properties of the mutant bacterial enzyme. Tobacco plants were also transformed and regenerated from cocultivation with Agrobacterium rhizogenes containing the same chimeric gene (Comai et al., 1986). These results demonstrated that the expression of a bacterial mutant aroA allele is expressed in transferred plants and confers to them tolerance against the herbicide glyphosate. The construction of a chimeric gene containing a mutated EPSP synthase gene isolated from glyphosate-resistant Petunia hybrida and the cauliflower mosaic virus 355 promoter has been reported recently by Shah et al. (1985b). A binary vector, pMON530, was used for Agrobacteriummediated transformation of this chimeric EPSP synthase gene into petunia cells (Shah et al., 1986b). Transformed petunia as well as regenerated

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transgenic plants were tolerant to glyphosate (Shah et al., 1986b). The resistance, however, in the transformed petunia resulted from an overproduction of the EPSP synthase induced by the amplified mutated gene, rather than a decreased affinity of the EPSP synthase for glyphosate as in the case of tobacco and Brassica discussed earlier. The successful transfer of mutated bacterial genes conferring resistance to tobacco plants against the sulfonylurea herbicides has been also reported (Chaleff, 1986; Falco et al., 1984). A chimeric gene was constructed by inducing the fusion of the coding sequence of a chlorsulfuron-resistantALS gene from yeast to the promoter and leading peptide of a tobacco RuBPsmall subunit gene (Falco et al., 1985).Following its transfer into tobacco cells via a Ti plasmid vector system,this chimericgenewas expressedand increasedslightly (three-fold)the tolerance of tobacco calli to the herbicide chlorsulfuron (Fraley et al., 1986a). These examplesillustratethe potential for using gene transfer systemsto transform crop plants and improve their tolerance to selected herbicides. The routine use of tobacco and petunia plants in such studies is encouraged by the relatively modest effort and short time frame (3 to 4 weeks) required for the production of transgenic plants from these species (Fraley et al., 1986a). Severalobstacles must be overcome before these gene transfer methods could be extended to other crops such as cereals and legumes, which are agronomically more important.

V.

ADDITIONAL USES OF GENETICALLY ENGINEERED MICROORGANISMS IN WEED MANAGEMENT

Apart from the previously discussed uses of microorganisms in selected areas of weed management (e.g., bioherbicides, production of natural phytotoxins, and genetic transformation of plants), genetically engineered microorganisms may be useful for solving a number of other difficulties involved in modern weed management and herbicide technology. Specific areas of weed management which have been recently recognized as quite promising for innovative applications of genetically engineered microorganisms include (1) microbial degradation of soil-applied herbicides; (2) development and use of microbial herbicide safeners; and (3) use of microorganisms as biocatalysts in the production of synthetic herbicides. The principles and selected examples illustrating the potential applications of genetically engineered microorganisms in these areas are discussed in the following paragraphs.

THAT A. PLASMID-BORNEGENESYSTEMS DEGRADE SOIL-APPLIED HERBICIDES Biodegradation is a very important factor affecting the environmental persistence of most organic herbicides used in modern agriculture.

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Biodegradation, or mineralization, of herbicides in water and soils is almost always the result of microbial activity. Soil microbes utilize either adaptive or constitutive enzyme systems which can alter biochemically a pesticide into a utilizable nutrient and energy source (Alexander, 1981; Ghosal et al., 1985; Kaufman and Kearney, 1976; Torstensson, 1980). Apart from the physicochemical characteristics of a given soil, the microbial degradation of a herbicide can be influenced also by the rate and frequency of its application, the cropping system, and the presence of other pesticides applied simultaneously or sequentially to the same soil (Kaufman et al., 1985). Frequent, repeated applications of a given pesticide to the same soil may lead to an increase of microbial populations to the point that succeeding application of this or other pesticides are ineffective on target pests because of the accelerated rates of their biodegradation. Such soils are currently referred to as “conditioned,” “problem,” or “history” soils (Kaufman et al., 1985; Roeth, 1986). Although this phenomenon has been known for some time (Audus, 1951), the recent documentation of the accelerated degradation of thiocarbamate herbicides in soils with previous thiocarbamate herbicide exposure has attracted considerable attention (Fox, 1983; Kaufman et al., 1985; Roeth, 1986). Thiocarbamate herbicides such as EPTC are used extensively in corn fields for the control of selected annual and perennial grass weeds. Soils conditioned to the rapid degradation of thiocarbamate herbicides are found primarily in the midwestern states where the bulk of the United States’ corn production is centered. However, the existence of thiocarbamateconditioned soils has been also documented in southern states and in other countries (Murdock et al., 1984; Rahman et al., 1979; Roeth, 1986). Accelerated microbial degradation has been also demonstrated with the phenylacetamide herbicide diphenamid and the chloracetanilide herbicides alachlor and metolachlor (Kaufman et al., 1985; White et al., 1987). The ability of soil and water microorganisms to degrade a wide variety of pesticides and other industrial chemicals has been known for some time (Alexander, 1981; Ghosal et al., 1985; Kaufman and Kearney, 1976; Torstensson, 1980). A number of herbicides are completely mineralized by selected soil microganisms and specific examples are presented in Table VI. For simplicity, in Table VI only the names of the genera of these soil mocroorganisms are given. It should be emphasized, however, that in most cases only defined strains of these microorganisms are capable of degrading the herbicides listed in Table VI. At present, the mechanism@)of the accelerated or natural biodegradation of herbicides in soils are poorly understood. In analogy to mechanisms involved in the development of bacterial resistance to antibiotics, it has been postulated that degradative genes for soil-applied herbicides may be carried on bacterial plasmids which can be traded freely among various

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Table VI Microbial Strains Known to Metabolize Selected Herbicides in Soils Herbicide Phenoxyacetates (2,4-D, MPCA, 2,4,5-T)

Chlorpropham Pen tachlorophenol

Pyrazon Dalapon Glyphosate Alachlor

Metolachlor

Propanil Phenylureas EPTC EPTC Molinate

Microbe

Alcaligenes sp. Achromobacter sp. Arthrobacter sp. Aspergillus sp. Brevibacterium sp. Corynebacterium sp. Nocardia sp. Pseudomonas sp. Strepromyces sp. Aspergillus sp. Pseudomonas sp. Arthrobacter sp. Flavobacterium sp. Pseudomonas sp. Phenylobacterium sp. Pseudomonas sp. Pseudomonas sp. Chaetomium globosum Rhizoctonia solani Fusarium oxysporum Unspecified actinomycete Chaetomium globosum Bacillus circulans Bacillus megaterium Fusarium sp. Mucor racemosus Fusarium solani Baccilus sp. Pseudomonas sp. Arthrobacter sp. Fluorescent Pseudomonads Mycobacterium sp. Flavobacterium sp. Streptomyces sp. Fusarium sp.

Reference Don and Pemberton (1981) McCormick (1985) McCormick (1985) McCormick (1985) McCormick (1985) McCormick (1985) McCormick (1985) Kilbane et al. (1982) McCormick (1985) McCormick (1985) Cripps and Roberts (1978) Staniake and Fink (1982) Ghosal et al. (1985) Karns et al. (1983) Eberspacher and Lingens (1981) Cripps and Roberts (1978) Moore et al. (1983) McCormick (1985) McCormick (1985) Kaufman and Blake (1973) Krause et al. (1985) McGahen and Tiedje (1978) Saxena et al. (1987) Saxena et al. (1987) Saxena et al. (1987) Saxena et al. (1987) McCormick (1985) McCormick (1985) McCormick (1985) Tam et al. (1986) Meredith et al. (1987) Mueller et al. (1987) Imai and Kuwatsuka, (1986a,b)

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microorganisms to speed adaptation to these herbicides (Don and Pemberton, 1981; Fox, 1983; Pemberton, 1981; Roeth, 1986). However, only in relatively few cases have the enzymes that attack herbicide molecules and cause a loss in their phytotoxicity been documented. Studies on the isolation and cloning of microbial genes encoding for herbicide degradation are also limited. The role of plasmids in microbial evolution and their potential involvement in pesticide degradation have been reviewed by Chakrabarty (1982), Clarke (1984), Reanney (1976), and Reineke (1984). Pemberton and Fisher (1977) presented evidence that the microbial degradaton of the phenoxyacetic acid herbicides 2,4-D and MCPA is specified by plasmid-borne genes. The presence of a conjugal plasmid encoding for the ability of a strain of Alcaligenes paradoxus to degrade 2,4-D and MCPA has been documented (Don and Pemberton, 1981; Fisher et al., 1978). Chakrabarty and his co-workers have studied extensively the role of transmissible and nontransmissible plasmids in the microbial degradation of 2,4,5-T (Chatterjee and Chakrabarty, 1981; Chatterjee et al., 1982; Kellog et al., 1981; Kilbane et al., 1982, 1983). Tam et al., (1986) have reported the isolation of an Arthrobacter spp. (strain A56) from EPTChistory soils which was capable of degrading this herbicide. Meredith et al. (1987) and Mueller el al. (1987) reported the isolation of several strains of fluorescent pseudomonads from butylate-history soils which were also capable of growing on culture media containing EPTC as a carbon source. In both cases, the enhanced degradation of EPTC by these soil bacteria was thought to plasmid-mediated, and a 50.5-kb plasmid has been partially characterized from the A56 strain of Arthrobacter spp. (Tam et al., 1986). Another indication of plasmid transfer in connection with herbicide degradation comes from the work of Senior et al. (1976), who showed that a strain of Pseudomonasputida acquired the ability to grow on the herbicide dalapon through the evolution of an extant dehalogenase. Dehalogenases (also known as halidohydrolases) catalyze the hydrolytic removal of halides from halogenated alkanoic and, particularly, chlorinated acetic and propionic acids such as the herbicide dalapon (Goldman et al., 1968; Little and Williams, 1971; Slater et al., 1979). Beeching et al. (1983) suggested that plasmid-mediated transfer of dehalogenase genes is the most likely reason for explaining the increased occurrence of different bacteria possessing dehalogenases after challenging the mixed microbial flora with herbicidal halogenated alkanoic acids. However, in a study utilizing the PP3 strain of Pseudomonas putida, which possesses no detectable plasmids, Slater et al. (1985) suggested that the dehalogenase genes were present on chromosomally located transposable elements and that spontaneous mutations involved excision of these elements. The frequency of the excision events was strongly dependent on environmental conditions. Thus, both mechanisms

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(plasmid-mediated transfer and excision of chromosomally located transposable elements) appear to be important in the evolution of bacterial mutants that can degrade and grow on specific herbicides following the repeated exposure of soil microflora to these herbicides. Plasmid-borne genes coding for degradative enzymes can be transferred to selected bacteria, and such genetically engineered mirobes could find a number of practical applications. Ongoing studies on the biochemistry and genetics of herbicide metabolism by soil microflora could definitely maximize the effective utilization of the biological degradation process for the feasible waste disposal of persistent pesticides and other industrial chemicals (Chakrabarty, 1985; Ghosal eta/., 1985; Kearneyet a/., 1987; Steiertand Crawford, 1985; Summers, 1985; Trevors, 1985). In addition, a better understanding of the biochemistry and genetics of the microbial degradation of thiocarbamate and chloroacetanilide herbicides in history soils could result in the development of chemical inhibitors of soil microorganisms to eliminate this problem. In fact, chemicals marketed as extenders are already available for use as short-term aids for coping with the enhanced microbial degradation of thiocarbamate herbicides (Roeth, 1986). The chemical dietholate is currently used as an additive in the formulation of the thiocarbamate herbicides EPTC and butylate (e.g., ERADICANE! EXTRA@)to act as a microbial inhibitor and extend the persistenceof these herbicidesin history soils (Roeth, 1986). Applied with EPTC in history soils, dietholate doubled the half-life of EPTC, slowed ’‘C02 evolution from I4C-labeled EPTC, and increased grass control by this herbicide (Obrigawitch et al., 1982). However, dietholate did not extend the persistence of EPTC in nonhistory soils nor restored the lag period to EPTC degradation in history soils (Roeth, 1986). Contradictory results and questions on the efficacy of dietholate as an extender of thiocarbamate herbicide in history soils have been reported by a number of investigators (see review by Roeth, 1986).

B.

MICROBIAL HERBICIDE SAFENERS

The term “herbicide safener” is commonly used to describe a chemical compound which protects crop plants against injury from herbicides (Hatzios, 1983). Other terms such as antidote, protectant, and antagonist are also used for the same purpose. At present, all herbicide safeners marketed commercially are synthetic chemicals developed by means of random or empirical screening (Hatzios, 1983). Recent advances in biotechnology, however, appear quite promising for the potential development and use of microbial herbicide safeners in the near future. Microbes carrying plasmids with herbicide degradative genes could be used as crop protectants to increase the selectivity of soil-applied herbicides. In principle, the idea is simple. Genes encoding for degradative enzymes which break down specific herbicides, isolated from plants or microbes,

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could be cloned into appropriate plasmids and transferred to bacteria (e.g., Pseudornonas fluorescens) that colonize the roots or seeds of susceptible crop plants. Such a system could then protect these susceptible plants from soil-applied and root-absorbed herbicides. Although the practical utilization of such a system may seem remote at present, recent success in utilizing a similar system for the microbial control of a root-feeding insect of corn demonstrated the possible use of this approach (Kaufman, 1986). This model system utilized the cloning of the genes coding for the active endotoxin of Bacillus thuringiensis kurstani into isolates of Pseudornonas fluorescens, which colonize the roots of corn. Data from monitoring studies of the risks associated with the release of these genetically engineered bacteria into the environment showed that this approach is safe. Release and use of this bioinsecticide is associated with low pathogenicity, low capacity for genetic exchange, and limited environmental persistence (Kaufman, 1986). Additional support for the potential development of microbial herbicide safeners comes from earlier research conducted in Poland showing that herbicide injury to sugar beets could be reduced with the use of selected bacterial fertilizers such as azotobacterin (Geller and Nikolaenko, 1972; Wegrzyn, 1975). An alternative approach for the use of microorganisms as safeners for the protection of crop plants against herbicide injury could include the direct insertion of microbial genes encoding for herbicide degradative enzymes into the chromosomes of susceptible plants. The goal of this approach is to manipulate the plant genetically and transform it to a producer of the desirable enzyme which provides protection against injury from a given herbicide. Partial success with this approach has been reported by Meusen and Zabeau (1986), who attempted the insertion of the gene coding for the endotoxin of Bacillus thurigiensis into the chromosomes of tobacco plants in order to make them resistant to insects. Coating corn seeds with the altered P . fluorescens via a propriatary process developed by Monsanto Chemical Company produced a plant whose roots contained the endotoxin of Bacillus thurigiensis that inhibits the attack of the root worm (Kaufman, 1986).

This approach of microbial herbicide safening could be extended further to include the use of endophytic bacteria which live internally in the xylem of higher plants. Genetically engineered endophytic bacteria could be modified to produce large amounts of specific amino acids whose biosynthesis is known to be inhibited by selected classes of herbicides such as the sulfonylureas, imidazolinones, and glyphosate (see Section IV for a detailed discussion). Thus, it is feasible that genetically engineered endophytic microorganisms could be used as herbicide safeners in the future.

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c.

363

U S E OF MICROORGANISMS AS BIOCATALYSTS IN THE PRODUCTION OF SYNTHETIC HERBICIDES

Apart from their potential use as bioherbicides or as sources of novel phytotoxins and leads for new herbicide chemistries, microorganisms could be further exploited as biological catalysts for the synthesis and commercial production of sophisticated plant protection chemicals (Alder et al., 1985; Bewick et al. 1986). The most obvious attraction of biocatalysts is that they make it possible to utilize the higher stereo- and regiospecificity of enzyme reactions and to perform transformations which may be difficult or impossible to perform chemically. Isolation and characterization of microorganisms which can invert the optical configuration of the S-enantiomer present in racemates of herbicides containing an asymmetric carbon atom in their molecules is of utmost importance for the commercial production of the optically resolved and herbicidally active R-enantiomer (Bewick et al., 1986; Fedtke, 1982). Activity, specificity, and physical properties represent some of the key factors which need to be considered in the screening of microorganisms as biocatalysts. Although research in this area has been limited, the potential of this biotechnological approach is enormous. Table VII illustrates the successful application of this approach in the production of the herbicidally active enantiomers of selected herbicides including members of the phenoxy carboxylic acid, phenoxy-phenoxy carboxylic acid, and pyridinyl-phenoxy carboxylic acid groups. Such herbicides exhibit excellent grass activity and good crop selectivity and they are very important in modern weed management programs of major crops such as soybeans and winter cereals. Genetically engineered microorganisms could be also exploited for their potential to produce biosurfactants, which can enhance the activity of chemical herbicides under field conditions. Banerjee et al. (1983). reported recently that a culture specifically developed for the degradation of the

Table VII

Use of Microorganisms as Biocatalysts in the Production of the Active Enantiomers of Selected Herbicides Herbicide

Active enantiomer

Biocatalysts

Reference

Chlor fenprop-methyl

R R

Clostridium klujveri Rhodococcus sp.

Fedtke (1982)

Fluazifop-butyl Mecoprop (Duplosan@)

R

Not given

Koning (1986)

Bewick et al. (1986)

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KRITON K. HATZIOS

herbicide 2,4,5-T produces a potent biosurfactant (emulsifier) which forms stable emulsions with 2,4,5-T. Such emulsions may be very useful for the herbicidal application of 2,4,5-T in the field as a uniform film.

VI.

CONCLUSIONS AND FUTURE PROSPECTS

The potential applications of biotechnology in addressing selected issues related to weed management and herbicide technology are quite promising. Bioherbicides, naturally produced herbicides, and constructed microbes with novel degradative ability have been already developed and used commercially. In addition, the concept of using genetic engineering techniques for the generation of herbicide resistance or tolerance in sensitive crops has created great excitement. The commercialization and marketing of the mycoherbicides DeVine@ and College@ illustrates the great potential of the use of phytopathogenic agents as bioherbicides to control selected weeds. At present, bioherbicides are viewed as complementary adjuncts to current weed management practices rather than as alternatives to chemical herbicides. In particular, the future potential of mycoherbicides is seen in areas that are currently served inadequately by chemical herbicides. According to Templeton et al. (1986) such areas include (1) control of parasitic weeds; (2) control of weeds closely related to crops (crop mimics), in which case a high degree of selectivity is necessary; (3) control of weeds resistant to chemical herbicides; and (4) control of weeds infecting small, specialized areas where development of chemical herbicides would be too costly. Advances in our knowledge of the interactions of plants and phytopathogenic organisms at the molecular level will increase our understanding of the role of toxins in pathogenesis, toxin production, and its genetic control by the pathogen. Such knowledge could lead to the commercialization of new bioherbicides or the synthesis of new chemical herbicides developed by means of a biorational design based on the chemistry of natural toxins. It is expected that in the near future, chemical herbicides would be developed by new approaches, based on the principles of biotechnology and chemistry, rather than by the currently dominant process of empirical synthesis coupled with biological screening. The commercial exploitation of the microbial toxin, L-phosphinothricin, which is the active ingredient of the microbial herbicide bialophos and of the chemically synthesized herbicide glufosinate illustrates this point. Plant cell cultures and fermentation broths of microorganisms are expected to have a major impact in agriculture through the novel production of agrochemicals such as herbicides used in weed management. Additional

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advances are also,expected in the area of biocatalysis benefiting from the use of plant or microbial cell enzymology, which is presently superior to the skills of organic chemists in performing complex biotransformations. Learning more about the plasmid-determined resistance of soil microbes to relevant herbicides will allow us to employ effectively such resistance in several microbially related processes such as the decontamination of soil residues of persistent herbicides or the development of microbial herbicide safeners. Advances in genetic technology will also facilitate our efforts to understand, manage, circumvent, and exploit the resistance of plants to selected herbicides. Isolation and characterization of plant or microbial genes coding for mutations altering the sensitivity of specific target proteins to herbicides or for herbicide-detoxifying enzymes will improve our understanding of the biochemical and genetic basis of plant resistance or tolerance to herbicides. Such information coupled with advances in recombinant DNA technology will enable us to engineer herbicide-resistant determinants and develop herbicide-resistant crop plants. At a theoretical level, progress in this area has been enormous. However, at the practical level, success has been only partial. Presently, only one herbicide-resistant crop, atrazine-resistant canola, developed from classic breeding techniques rather than sophisticated genetic engineering technology, is marketed in Canada. The development of tobacco and petunia plants resistant to the herbicides glyphosate and chlorsulfuron by means of gene transfer and transformation techniques indicates that genetic engineering of herbicide resistance is feasible. The full utilization of biotechnological procedures as tools for incorporating herbicide resistance to major agronomic crops (e.g., cereals and legumes) is presently limited by a number of unresolved problems such as a limited pool of genes of interest, lack of appropriate vector systems for gene transfer, and inefficient methods for the regeneration of selected crops from cell or tissue cultures. Future research, hopefully, will address and solve these problems. For a more fruitful application of biotechnology in engineering herbicide resistance several considerations should be examined or reevaluated in future investigations. According to Gressel(l985) and Widholm (1978) such considerations include the following: 1. A given herbicide should be considered only when it more cost effectively controls specific problem weeds in a given crop better than other presently available herbicides. 2. All biochemical mechanisms conferring plant tolerance or resistance to herbicides should be considered and exploited for practical application. 3. If we are looking for resistance at the mode of herbicide action level, we need more information on the biochemical mechanisms involved and their genetic basis.

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4. There is a need for the development of better culture systems from which economic crops (e.g., corn and soybean) can be regenerated. 5. The in vitro selection systems already available should be tested to determine whether plant improvement can be accomplished. 6 . In isolating plant mutants, learn from the plant breeders: isolate many mutants and evaluate their cost and effectiveness. 7. There is a need for advances in technology for transferring resistant genes into plants: in addition to Ti plasmid vectors, new vectors for gene transfer to monocotyledonous plants or cell organelles (e.g., chloroplast) are currently needed. 8. Herbicide resistance incorporated into a sensitive crop should not cause potential yield reduction or alteration of the quality of the marketed product. 9. In achieving the overall goal of engineeringherbicide-resistantcrops, a close cooperation between genetic engineers and plant breeders is imperative. Once the required techniques have been developed, it may be easier and less costly to transfer herbicide resistance into crop plants than to develop new herbicides.

Biochemical and genetic advances may facilitate also the development of chemical additives that could regulate the activity of currently marketed herbicides. Such bioregulators could either potentiate the activity of a herbicide on target weeds (synergism) or reduce the activity of a herbicide on nontarget crops (antagonism or safening). The commercialization of tridiphane, a potent inhibitor of plant glutathione sulfotransferase enzymes, as a synergist of triazine herbicides on grass weeds (Ezra et al., 1985) and of safeners such as flurazole, which acts as a chemical regulator of the gene coding for the GST I enzyme and confers tolerance on corn against chloroacetanilide herbicides (Shah et al., 1986a) illustrates the practical ramifications of this approach. Apart from the four main areas of weed management that were discussed in this review as promising fields for applying biotechnology, other areas of weed management or herbicide technology may be considered for such applications. Among them, the utilization of monoclonal antibodies and immunoassays in trace analysis of herbicide soil residues is becoming widely appreciated. Immunological assays offer considerable potential for rapid, inexpensive, and sensitive methods of analysis of agrochemicals or their metabolites and they could be used either as supplements or as alternatives to instrumental methods of analysis. For more information on this subject the reader is referred to the reviews by Hammock e t a / . (1987), Mumma and Brady (1987), and Vanderlaan et a / . (1987). ACKNOWLEDGMENTS

I thank the many scientists around the world who provided reprints of their published research during the preparation of this chapter. I express my appreciation to my colleague Dr.

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C. L. Cramer for her critical review of the manuscript of this review. The author’s own work on the molecular biology of the enhanced herbicide biodegradation is supported by the USDA Competitive Research Grant 85-CRCR-1-1905.

REFERENCES Alder, E.F., Wright, W.L., and Brown, I.F. 1985. In “Agricultural Chemicals for the Future“ (J.L. Hilton, ed.), pp. 147-155. Rowman & Allanheld, Totowa, New Jersey. Alexander, M. 1981. Science 211, 132-138. Ammirato, P.V., Evans, D.A., Flick, C.E., Whitaker, R.J., and Sharp, W.R. 1984. Trends Biotechnol. 2, 53-58. Amrhein, N., Johanning, D., Schab, J., and Schultz, A. 1983. FEBS Lett. 157, 191-196. Anonymous. 1984. “High Technology Industries: Profiles and Outlooks-Biotechnology.” U. S. Department of Commerce, Washington, D.C. Amtzen, C.J. 1986. Abstr. Int. Congr. Pestic. Chem., 6th IUPAC, Ottawa 3S-04. Amtzen, C.J., Steinback, K.E., Vermaas, W., and Ohad, I. 1983. In “Pesticide Chemistry: Human Welfare and the Environment” (J. Miyamoto, P.C. Kearney, S. Matsunaka, D.H. Hutson, and S.D. Murphy, eds.), Vol. 3, pp. 51-58. Pergamon, Oxford. Audus, L.J. 1951. Plant Soil 3, 170-192. Bach, T.J., and Lichtenhaler, H.K. 1983. Physiol. Plant. 59, 50-60. Balandrin, M.F., Klocke, J.A., Wurtele, E.S., and Bollinger, W.H. 1985. Science 228, 1154-1 160.

Banerjee, S., Duttagupta, S., and Chakrabarty, A.M. 1983. Arch. Microbiol. 135, 110-1 14. Barash, I., Pupkin, G., Netzer, D., and Kaufman, Y. 1982. Plant Physiol. 69, 23-27.

Barg, R., and Umiel, N. 1977. Z. Pflanzenphysiol. 83, 437-447. Barton, K.A., and Brill, W.J. 1983. Science 219, 671-676. Beeching, J.R., Weightman, A.J., and Slater, J.H. 1983. J . Gen. Microbiol. 129, 2071-2078. Bellus, D., and Fisher, H.P. 1979. Adv. Pestic. Sci. 2, 373-382. Beversdorf, W.D. 1985a. Abstr. Natil. Meet. ACS, 19Oth, Chicago No. AGRO-26. Beversdorf, W.D. 1985b. Proc. Soybean Res. Conf.., 14th pp. 64-67. Beversdorf, W.D., Weiss-Lerman, J., Erickson, L.R., and Souza-Machado, V. 1980. Can.J. Genet. Cytol. 22, 167-172. Bewick, D.W., Weissler, M. S., and Castle, D.L. 1986. Abstr. Int. Congr. Pestic. Chem., 6th IUPAC, Ottawa 619, No. 1A-05, Binding, H., Jain, S.M., Finger, J., Mordhorst, G., Nehls, R., and Gressel, J. 1982. Theor. Appl. Genet. 63, 273-277. Bowers, R.C. 1986. Weed Sci. 34 (Suppl. I), 24-25. Brown, W. L. 1984. “Genetic Engineering of Plants: Agricultural Research Opportunities and Policy Concerns.” National Academy Press, Washington, D.C. Caplan, A., Herrara-Estrella, L., Inze, D., Van Haute, E., Van Montagu, M., Schell, J., and Zambryski, P. 1983. Science 222, 815-821. Cella, R., and Iadarola, P. 1983. Plant Sci. Lett. 29 327-337. Chakrabarty, A.M. 1978. ASM News 44, 687-690. Chakrabarty, A.M. 1982. In “Biodegradation and Detoxification of Environmental Pollutants” (A.M. Chakrabarty, ed.), pp. 127-139. CRC Press, Boca Raton, Florida. Chakrabarty, A.M. 1985. Trends Biotechnol. 3, 32-38. Chaleff, R.S. 1981. “Genetics of Higher Plants: Application of Cell Culture.” Cambridge Univ. Press, New York. Chaleff, R.S. 1983. Science 219, 676-682.

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Sommerville, C. R., McCourt, P., Caspar, T., Estelle, M., and Keith, K. 1985b. “Plant Genetics,” pp. 651-660. Liss, New York. Souza-Machado, V., and Bandeen, J. D. 1982. Weed Sci. 30, 281-285. Spielman, A. and Stutz, E. 1983. Nucleic Acids Res. 11, 7157-7167. Stalker, D. M., Hiatt, W. R., and Comai, L. 1985. J. Biol. Chem. 260, 4724-4728. Stanlake, G. J., and Fink, R. K. 1982. Appl. Environ. Microbiol. 44, 1421-1427 Steiert, J. G., and Crawford, R. L. 1985. Trends Biofechnol. 3, 300-305. Steinrucken, H. C., and Amrhein, N. 1980. Biochem. Biophys. Res. Commun. 94, 1278-1212. Steinrucken, H. C., Schulz, A., Amrhein, N., Porter, C. A., and Fraley, T. A. 1986. Arch. Biochem. Biophys. 244, 169-178. Summers, A. 0. 1985. Trends Biotechnol. 3, 122-124. Tachibana, K. 1987. In “Pesticide Science and Biotechnology, 6th IUPAC Congress of Pesticide Chemistry” (R. Greenhalgh and T. R. Roberts, eds.), pp. 145-148. Blackwell, Oxford. Takiguchi, Y., Yoshikawa, H., Terahara, A., Torikata, A., and Terao, M. 1979. J. Anfibiof. 32, 857-867.

Tam, A. C., Behki, R. M., and Khan, S. U. 1986. Abstr. Inf. Congr. Pesfic. Chem. 6th, IUPAC, Ottawa, NO. 6C-08. TeBeest, D. 0 . 1984. Phyfopathology 74, 864 (Abstr.). TeBeest, D. 0. 1986. Proc. BASF Symp. Basic Aspect PIanf Protect. Res. Raleigh, Norfh Carolina (in press). TeBeest, D. O., and Templeton, G. E. 1985. Plant Dis. 69, 6-10. Templeton, G. E. 1982. Weed Sci. 30, 430-433. Templeton, G. E . 1986. Weed Sci. 34, (Suppl. I), 35-37. Templeton, G. E., and Smith, R. J., Jr. 1977. In “Plant Disease: An Advanced Treatise” (J. G. Horsfall and E. B. Cowling, eds.), Vol. 1. pp. 167-176. Academic Press, New York. Templeton, G. E., TeBeest, D. O., and Smith, R. J., Jr. 1979. Annu. Rev. Phyfopafhol. 17, 301-310.

Templeton, G. E., Smith, R. J., Jr., and TeBeest, D. 0. 1986. Rev. Weed Sci. 2, 1-14. Thomas, T. L., and Hall, T. C. 1985. BioEssays 3, 149-153. Tischer, E., DasSarma, S., and Goodman, H. M. 1986. Mol. Cen. Genet. 203, 221-229. Torrey, J. G. 1985. A m . Sci. 73, 354-363. Torstensson, L. 1980. In “Interactions Between Herbicides and the Soil” (R. J. Hance, ed.), pp. 15-178. Academic Press, London. Trebst, A. 1986. Z. Naturforsch. 41c, 240-245. Trevors, J. T. 1985. Trends Biotechnol. 3, 291-293. Ueno, T. 1987. In “Pesticide Science and Biotechnology, Sixth IUPAC Congress of Pesticide Chemistry” (R. Greenhalgh and T. R. Roberts, eds.), pp. 89-96. Blackwell, Oxford. Valiulis, D. 1984. Agrichem. Age 28, 53-60. Vanderlaan, M., Van Emon, J., Watkins, B., and Stanker, L. 1987. I n “Pesticide Science and Biotechnology, Sixth IUPAC Congress of Pesticide Chemistry” (R. Greenhalgh and T. R. Roberts, eds.), pp. 597-602. Blackwell, Oxford. Vaughn, K. C. 1986. Absfr. Weed Sci. Soc. A m . 26, 77. Walden, R. M., and Howell, S. H. 1982. Planf Mol. Biol. 2, 27-31. Walker, H. L. 1981. Weed Sci. 29, 505-507. Walker, H. L., and Riley, J. A. 1982. Weed Sci. 30, 651-654. Walker, H. L., and Sciumbato, G. L. 1979. Weed Sci. 27, 612-614. Wegrzyn, T. 1975. Rocz. Glebozn. 26. 79-89. Werner, G.M., and Putnam, A. R. 1980. WeedSci. 28, 142-146. White, D. R., Hatzios, K. K., Meredith, S., and Wilson, H. P. 1987. Virginia J . Sci. 38 (in press). Widholm, J. M. 1976. Can. J. Bof. 54, 1523-1529.

BIOTECHNOLOGY APPLICATIONS IN WEED MANAGEMENT

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Widholm, J. M. 1978. I n “Propagation of Higher Plants Through Tissue Culture: A Bridge Between Research and Application” (K. W. Hughs, R. Henke, and M. Constantin, eds.), pp. 189-199. Technical Info. Center, U.S. Dept. Energy, Washington, D‘. C. Widholm, J. M. 1984. Proc. Katzir Katchalsky Conf., Ilth, Jerusalem. Wiegand, R. C., Shah, D. M., Mozer, T. J., Harding, E. I., Diaz-Collier, J., Saunders, C., Jaworski, E. G., and Tiemeier, D. C. 1986. Plant Mol. Biol. 7 , 235-243. Wilke-Douglas, M., Perani, L., Radke, S., and Bossert, M. 1986. Physiol. Plant. 68, 560-565 Yamada, 0.. Kaise, Y., Futatsunga, F., Ishida, S., Ito, K., Yamamoto, H., and Munakata, K. 1972. Agric. Biol. Chem. 36, 2013-2015. Zaitlin, M., Day, P., and Hollaender, A., eds. 1986. “Biotechnology in Plant Science: Relevance to Agriculture in the Eighties.” Academic Press, Orlando, Florida. Zurawski, G. H., Bohnet, H. J., Whitfield, P. R., and Bottomley, W. 1982. Proc. Natl. Acad. Sci. U.S.A. 19, 7699-7703.

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ADVANCES IN AGRONOMY, VOL. 41

RECLAMATION OF ACIDIC MINED LANDS IN HUMID AREAS P. Sutton and W. A. Dick Department of Agronomy, The Ohio State University Ohio Agricultural Research and Development Center Wooster, Ohio 44691

I. INTRODUCTION Surface mining for minerals in the United States has affected over 1.5 million hectares (ha) of land (Paone et al., 1978). A major part of the disturbance is a result of surface mining (stripmining) for coal in the eastern portion of the United States. Although many of the states enacted reclamation laws relatively early, stripmined areas were often not adequately reclaimed (Bowling, 1978). It has been estimated that 0.63 million ha of land stripmined for coal in the humid region of United States require additional reclamation (Table I). Unreclaimed or poorly reclaimed lands often are barren because soil materials left on the surface after mining and reclamation were completed would not support plant growth. Such areas for which there were no further legal requirements for additional reclamation have been referred to as orphan or abandoned mined lands. Major environmental concerns related to these abandoned mined lands are acid mine drainage and excessive erosion that occurs due to inadequate vegetative cover. Approximately 100 times more erosion has been reported for the abandoned mined lands compared to similarly located forest lands (U.S. Environmental Protection Agency, 1976). Erosion not only reduces the productivity of the mined lands themselves, but also causes offsite damage by depositing sediments in road ditches, stream channels, water reservoirs, and some of the most productive agricultural lands in the area. A primary objective in plans for achieving satisfactory reclamation is to establish a permanent vegetative cover. Establishment of vegetation on abandoned mined lands is often hindered by low availability of plant nutrients and soil moisture and by unstable slopes due to hilly terrain. The main factor, however, in preventing vegetation from becoming established is acidity. During stripmining, strata in the overburden are exposed that may contain iron disulfide minerals (Hill, 1978). These minerals, when exposed 311 Copyright 0 1987 by Academic Press. Inc.

AU rights of reproduction in any form resewed.

378

P. SUTTON AND W. A. DICK Table I Land Surfnee Mined in the Humid Region of the United States as of July 1, 1977, Needing Reclamationu

States Alabama Arkansas Connecticut Delaware Florida Georgia Illinois Indiana Iowa Kansas Kentucky Louisiana Maine Maryland Massachusetts Michigan Minnesota Mississippi Missouri Nebraska New Hampshire New Jersey New York North Carolina Ohio Oklahoma Pennsylvania Rhode Island South Carolina Tennessee Texas Vermont Virginia West Virginia Wisconsin Total

Coal mines @a)

Sand and gravel (ha)

Other mined areas @a)

43,375 3,435

10,603 5,294 319 26 103,93 1 15,301 7,594 3,408 6,504 5,726 3,034 1,032 1,214 1,180 4,184 11,135 21,340 3,168 13,868 1,632 169 2,256 9,837 3,524 11,077 7,378 18,427

-

8,954 8,709 6,780 1,179 5,884 3,230 11,710 6,500 7,535 5,988 1,328 15,116 12,606 6,954 12,977 22,310 17,209 18,616 2,236 7,271 5,154 9,967 18,992 7,697 15,909 3,817 10,530 1,050 5,451 2,333 64,292 1,723 3,125 1,844 21,664

2,156 1,394 17,048 866 1,318 403 4,220

62,932 17,438 7,099 1,205 109,815 19,520 83,946 50,595 19,846 28,753 107,983 16,148 13,820 13,041 17,161 33,503 38,549 21,784 48,286 8,909 5,323 12,223 28,829 11,221 137,858 28,374 150,457 1,050 7,607 16,974 84,190 2,589 17,381 39,720 25,884

632,801

356,646

300,566

1,290,013

-

989 64,642 40,687 5,807 17,039 103,621

-

4,907

58

-

32,182

-

110,872 17,179 121,500

-

13,247 2,850

-

12,938 37,473

"From Bastian el al. (1982).

-

Total

RECLAMATION OF ACIDIC MINED LANDS

379

to air and moisture, oxidize to produce acid and soluble salts. The end result is a spoil material or mine soil that is far too acid for plant growth and drainage that is highly acidic and contains high concentrations of soluble salts. Although the acid and soluble salts will decrease with time because of weathering and leaching, the spoil will generally remain too acid for plant growth (Struthers, 1964). As a result, many of these areas, if left to amelioration by weathering, will remain without vegetative cover for many years. The spoilbanks may be ameliorated by addition of soil amendments such as liming materials, sewage sludge, papermill sludge, power plant fly ash, or by covering them with soil material so that plant growth can be established (Apel, 1983). Once the abandoned mined lands have vegetation growing on the surface, the regeneration of these areas for productive use has begun and offsite damages are minimized. In addition, establishment of vegetation on the abandoned mine lands also improves the aesthetics of the area. Ultimate use of these lands will be determined by land use capabilities, location, and objectives of the owners and will include agriculture, forestry, wildlife, and recreation. The purpose of this review is to summerize work describing soil (spoil) properties on abandoned mine lands in the eastern United States which affect plant growth and how various amendments, applied to these lands, ameliorate the soil properties. Information relevant to vegetation establishment will also be included in this review.

II. PROPERTIES OF SPOIL ON ACIDIC MINE LANDS A.

PHYSICAL PROPERTIES

Spoilbanks consist of a heterogeneous mixture of the overburden strata. The composition of the overburden material and the type of mining and reclamation will thus have a large influence on the physical properties found at the surface of a given spoilbank. Smith et al. (1976) have pointed out that surface erosion is less severe if the surface spoil layer contains a significant percentage of coarse fragments. The fraction of spoil material with a diameter of 2 mm or greater has been reported to range from 3.0 to 93% (Grub& al., 1971; Pedersen et al., 1980; Fehrenbacher et al., 1982). Unless intensive cultivation is planned, spoils may contain as much as 75% coarse fragments (by weight) without seriously reducing productivity (Smith et al., 1976). The effect of stony soils on stripmine productivity was summarized by Ashby et al. (1984).

380

P. SUTTON AND W. A. DICK

Compaction of spoils adversely affects plant establishment and growth by increasing resistance to penetration by roots and to the emergence of seedlings and by decreasing water infiltration by the reduction of pore sizes (Chapman, 1967;Larson and Vimmerstedt, 1983;Smith et al., 1971;Byrnes et al., 1982). Compacted coal shale below the scarified layer may act analogously to a hardpan, preventing the penetration of roots and resulting in the plant cover being susceptible to drought (Ayerst, 1980). Compaction of spoils is commonly revealed through bulk density measurements. Changes in bulk density are primarily determined by degree of overburden breakup during mining, mixture of overburden materials, texture of the material, and amount of traffic over an area by heavy equipment. Smith et al. (1971)reported bulk density for six mined spoils averaged 1.47 mg/m3 compared to 1.03 mg/m3 for undisturbed soils. Grading of spoil compacted the surface layer to a depth of 45-60 cm (Tyner et al., 1948). Fehrenbecher et al. (1982)found the bulk density of graded spoil was 1.7 mg/m3 at 54 cm depth and increased to 1.9 mg/m3 at 102 cm. In Germany, heavy, rubbertired earthmoving equipment resulted in compaction as measured by changes in porosity (Knabe, 1964). Porosities of 45-50'7'0 were observed for undisturbed loess soils but only 33-35070where the earthmoving equipment had traversed. Plant growth on spoils may be limited due to lack of available moisture. Spoil materials often have reduced water infiltration and water holding capacity compared to unmined soil. Growth stress is evident where spoil is extremely stony or course textured or severely compacted (Smith et al., 1971). Mine spoil in Pennsylvania retained about one-fourth as much plantavailable water as did natural soils (Pedersen et al., 1980). The percentage of available moisture in spoils has been reported to lie between 2.3 and 14.7% (Jones et al., 1975;Pedersen et al., 1980). Infiltration rates are also decreased as a result of mining. On undisturbed soil, infiltration rates ranged from 0.56to 2.34cm/hr but were only 0.08-0.74cm/hr on adjacent spoil (Jones et al., 1975). Type of vegetation and grading procedures influence infiltration, as reported by Grandt and Lang (1958): average infiltration rates for four spoils were 2.2 cm/hr for level grading, 2.3 cm/hr for strike-off grading, and 13.2 cm/hr where the spoil was left in ridges without any grading. It is difficult to improve infiltration once it has been adversely affected, as was noted by Smith et aL(1971): the measured infiltration rates for three iron ore spoils with ages of 85-103 years and undisturbed soil were 16.5 and 29.6 cm/hr, respectively. Slope aspect, soil moisture, and shading by plants influence temperature of spoil material with high temperature injury to plant seedlings being especially serious on black bituminous coal and black organic shales, but far less severe on light-colored mineral shales and sandstones. Even with midday air temperatures within a normal range, temperatures of the spoil surface can become so high that young seedlings can be killed. When air

RECLAMATION OF ACIDIC MINED LANDS

381

temperatures ranged between 30-35 "C, maximum temperatures on level spoil surfaces consistently reached 50-55 "C on the lightest-colored materials, and 65-70 "C on the darkest-colored materials (Deely and Borden, 1973). B. CHEMICAL PROPERTIES Coal-bearing rocks were formed originally as sediments in a reducing environment. When exposed in a stripmining operation, many of these rocks are placed in an oxidizing environment for the first time since their formation (Brant, 1964). Iron disulfides, occurring either as mascasite or pyrite, in the unmined coal and in the overburden above the coal are readily oxidized when exposed to the atmosphere (Caruccio, 1968). The general equations for the reactions are (Hill, 1978):

---

2FeS2 + 2H20 + 7 0 2 2FeS0, + 2H2S04 4FeS04 + O2 + 1 0 H 2 0 4Fe(H0)3 + 4H2S04 4FeSO. + O2 + 2H2S04 2Fe2(S04)3+ 2H104 FeS2 + 14Fe3++ 8H20 15Fe2++ 2SO:' + 16H+ Fe2(S04)3 + 6H20 2Fe(OH), + 3H2SO4

-

The fate of the ferrous iron formed from pyrite (Eq. 1) depends upon the pH of the system at the time it is formed. When hydrogen ions are not readily available and the pH is approximately 5 or above, the ferrous iron is oxidized to ferric hydroxide (Eq. 2A), which is insoluble. Sulfuric acid is also produced, which will cause a decrease in pH unless it is neutralized. If the pH is low, the ferrous iron is oxidized to form soluble ferric iron (Eq. 2B). The reactions described in Eqs. 2A and 2B are biological in nature, being catalyzed by Thiobacillusferrooxidans. The ferric iron formed (Eqs. 2A and 2B) may undergo several strictly chemical reactions. For example, the ferric iron may react with pyrite mineral to generate additional ferrous iron and large quantities of acid (Eq. 3A), with the ferrous iron acting as new substrate for the biological reactions (Eqs. 2A and 2B). Because ferric iron is soluble primarily under acid conditions, the reaction described in Eq. 3A will not occur until after the pH has been lowered to 4 or below by the formation of acid via the other mechanisms. However, once sufficient acidity is present for optimum Thiobacillusferrooxidans activity to occur and for ferric iron to remain in solution, the coupling of the biological reaction (Eq. 2B) with the chemical reaction (Eq. 3A) results in very rapid oxidation of pyrite to form sulfuric acid. Equation 3B describes a reaction in which the dissolution and hydrolysis of the ferric iron generates additional acid. If the spoil contains pyrite, the addition of alkaline materials such as limestone will neutralize much of the acidity produced. However, the drainage water that is discharged usually contains high levels of sulfate.

382

P. SUTTON AND W.A. DICK

Low pH often is generally the chief factor determining whether vegetation may become established on spoil material. Below pH 4.0 spoils are usually toxic for plant growth (Croxton, 1928; Tyner and Smith, 1945). However, the minimum pH necessary for plant growth varies depending on individual plant species. Stucky et al. (1980) reported that roots of reed canarygrass (Phalarisarundinacea L,), switchgrass (Panicum virgatum L.), and orchardgrass (Dacytlisglomerata L.) can tolerate pH values in spoil of 3.5, 3.7, and 4.2, respectively. Besides causing acid production and a low pH, chemical weathering of toxic spoil produces high concentrations of soluble salts. Soluble Al, Fe, and Mn and generally limited amounts of Ca and Mg are also produced. Five toxic spoils placed in an outdoor lysimeter yielded an average of 134.5 tons of salt per hectare to a depth of 1.2 m during the first year they were subjected to natural weathering (Struthers, 1964). Later results from this same study showed chemical yield decreased with duration of weathering but increased with increased precipitation (Vimmerstedt and Struthers, 1968). The lysimeter results support field observations that most toxic spoilbanks improve with time in regard to establishing vegetation. However, erosion on a toxic spoilbank removes the surface material that has been improved by weathering and exposes fresh, toxic material to plants. An important component of an effective rooting medium for plants is organic matter, as it has a desirable effect on soil tilth, water infiltration and retention, and plant nutrient supply. Overburden materials are generally low in organic matter and unless topsoil is saved and returned to the surface of the spoilbank, mining usually results in low levels of organic matter. In many of the earlier mining operations there was little attention given to saving topsoil because the damage resulting from barren spoils had not yet been fully recognized. However, organic matter levels on such lands can be restored through the return of plant residues to the spoil or by the addition of organic amendments. It is very difficult to restore, under natural conditions, organic matter levels to their initial concentrations, as work by Smith et al. (1971) has shown that natural soils contained higher organic matter contents than iron ore spoils even after 70-130 years. Caution must be taken when studying organic matter changes if coal fragments and dark shales are present in the spoil as positive erroneous results will be obtained (Geyer and Rogers, 1972). Due to low organic content of most acidic spoils, N is the primary limiting nutrient for plant growth (Mays and Bengston, 1978). However, in most cases a complete fertilizer (N, P, and K)is needed for plant establishment and growth on acid spoils in the eastern United States (Bennett et d., 1976; Berg, 1973, 1978; Mays and Bengston, 1974; Barnhisel, 1977; Plass and Vogel, 1973). A legume-grass association will eliminate the need for N

RECLAMATION OF ACIDIC MINED LANDS

383

fertilization to maintain vegetation (Woodruff and Blaser, 1970), but the successful growth of legumes requires a favorable soil pH and a favorable mineral balance. Application of dolomitic limestonewill counteract soil acidity and also has a long-lasting residual effect of supplying Ca and Mg. Initial applications of other elements during reclamation, when their requirements are determined by a soil test, generally last for decades because of the recycling of the nutrients (Wright etal., 1978). With plant removal, however, the need for additional soil testing and application of plant nutrients must be recognized and carried out. C. BIOLOGICAL PROPERTIES Acidic mined lands exhibit reduced biological activity compared to undisturbed soils. Biological activity, however, is important for the breakdown of litter so that plant nutrients can be released and recycled within the ecosystem and for improving soil physical properties (Neumann, 1973). Vimmerstedt (1983) reported earthworms were absent in newly topsoiled spoils. Dunger (1969a) found earthworms did not colonize an acid spoil after 15 years. Microbial growth and activity are also decreased as a result of mining and remain depressed in acid spoil that cannot support vegetation. Exceptions are the acidophillic bacteria such as Thiobacilli. Acidic mine spoils supported lower numbers of bacteria, actinomycetes, and fungi than undisturbed soil or revegatated spoil (Wilson, 1965; Muller, 1973; Lawry, 1977). The fungal taxain barren mine spoil were also less diverse in acid spoil than in the undisturbed soil (Lawry, 1977). Respiration (Hedrick and Wilson, 1956; Lawry, 1977; Stroo and Jencks, 1982) and enzyme activities (Stroo and Jencks, 1982) were lower in acid mine spoil compared to undisturbed soil. Lawry (1977) attributed the lower respiration activity to high levels of soluble trace metals which occur at low pH while Hedrick and Wilson (1956) thought insufficient nitrogen was the cause. Decomposition of organic matter was decreased in acid spoil but rapidly returned to the levels similar to that of unmined land (Lawry, 1977). Even though decomposition of litter may be slow in acid spoils, the production of vegetative matter is also low, such that the accumulation of organic matter occurs only slowly with time (Wilson, 1965).

111.

APPLICATION OF VARIOUS AMENDMENTS TO AMELIORATE ACIDIC MINE SPOIL

In early reclamation efforts, one treatment was to let extremely acid spoil

(PH< 4.0) weather, thus permitting the oxidation of acid-producing pyritic

384

P. SUTTON AND W. A. DICK

materials and leaching of the acid (Kohnke, 1950). This may require many years for some areas (Knabe, 1964). In areas with lower quantities of pyritic materials, the leaching treatment has proven successful (Riley, 1973). Proper amendment applications, however, achieve rapid vegetative cover. Prior to application of amendments to spoil, extensive grading often is conducted to shape the area to be reclaimed so that it will be more suitable for a specified use and to allow application of the amendments to be accomplished via conventional types of equipment. Grading and shaping a spoil bank should reduce water runoff and erosion (Struthers and Vimmerstedt, 1965) while increasing infiltration and the removal of salts and acids by leaching (Riley, 1973). Grading, however, will also result in compaction and may expose new unweathered pyrite-containing materials which will temporarily increase acid mine drainage. If establishment of vegetation to control erosion and improve aesthetics are the primary objectives of a particular reclamation project, consideration should be given to plans that will require limited or no grading. This would eliminate exposure of unweathered pyrite while simultaneously reducing one of the costly items (i.e., grading) in reclamation of abandoned mine lands. A. TOPSOIL Covering spoil with topsoil can be used to establish vegetation. Sutton (1979) covered a toxic spoil (PH 2.4) with 5, 10,15,20, and 25 cm of pH 6.5 soil material. Vegetative cover was established and maintained over 11 growing seasons when the soil depth was 15 cm or greater. Plant roots,

however, had only begun to penetrate the spoil below the soil cover to a depth of approximately 0.65 cm, indicating changes in the untreated spoil are rather slow. Limestone mixed into the surface of the spoil before a topsoiling operation should speed up root penetration (Barnhisel et al., 197,5; Vogel, 1975). This would be an important consideration if shallow layers of soil cover are applied or if steep slopes were to be covered. Root penetration between the soil cover and spoil would reduce the possibility of the cover being removed by sliding. When topsoil is used to cover toxic spoil the nutrient status can be determined by a soil test. Based on this information the approximate quantity of nutrients required for good plant growth can be calculated. However, plant root growth is limited to the soil cover material (Sutton, 1979), which is a much smaller volume than that generally exploited by plants. As a result, the recommended rates of application of plant nutrients based on traditional soil tests may underestimate the needs of plants growing on the reclaimed spoil. If good topsoil or A-horizon material is available, the growth medium for plants will also be improved in organic matter content

RECLAMATION OF ACIDIC MINED LANDS

385

and structure. In many areas, however, most of the cover material available will be subsoil or a mixture of B- and C-horizon materials.

Studies where power plant fly ash has been used to establish vegetation on toxic spoils have been summerized by Capp (1978). Six stripmine sites with pH ranging from 2.6 to 4.7 were treated with fly ash at rates of 830 to 4430 tonslha. After 1-10 years the pH at these sites ranged from 5.0 to 7.5 and the condition of vegetation was rated to good except in one case in which it was rated fair. The treatments had a surface effect only and the layers below the fly ash-treated material remained extremely acid. The principal effect of adding large quantities of fly ash is the neutralization of acid, although not all fly ash materials are alkaline in reaction (Plank and Martens, 1973). With the exception of N, fly ash also contains many of the elements for plant growth (Table 11). Boron, Mo, Zn, P, and K in fly ash are available to plants and can be used to correct deficiencies of these elements (Plank and Martens, 1973; Martens and Beahm, 1976). Toxic amounts of BoyMo, Se, and soluble salts, however, may pose a serious constraint to the application of fly ash on spoil for the purpose of establishing vegetation (Adriano et al., 1980). Severe toxicity symptoms in plants were observed when large amounts of fly ash (224 to 448 tonslha) were applied to stripmine areas (Bennett, 1971). The toxicity was attributed primarily to excess B in the fly ash. Before a fly ash is selected for use in reclamation of a toxic spoil the plant nutrients, soluble salts, and acidity should be determined and evaluated in relation to plant growth and the chemistry of the area to which it will be applied. C.

SEWAGE SLUDGE

Large quantities of sewage sludge have also been used to establish vegetation on toxic spoils (Hill et al., 1979; McCormick and Borden, 1973; Montague and Hill, 1976; Peterson and Gschwind, 1973; Lejcher, 1972; Sutton, 1979; Haghiri and Sutton, 1982). The plant nutrient content of sewage sludges is highly variable but is generally sufficient in N and P and limiting in K (Halderson and Zenz, 1978; Jewell, 1982). Application rates required to ameliorate toxic spoils (50-500 tonslha) will also supply more plant nutrients than are required for plant growth (Table 11). A result of the combination of high application rates and high N content is the formation of nitrates at levels which may exceed accepted pollution standards. The possibility of combining lower rates of sludge treatment with lime and other

386

P. SUTTON AND W. A. DICK

organic residues to create a medium suitable for revegetation, thus reducing the nitrate leaching hazard, has been suggested by Urie et al. (1982). D. PAPERMILL SLUDGE A sludge by-product of the papermill industry has been tested as an amendment for establishing vegetation on toxic spoils and has provided excellent results (Hoitink and Watson, 1982; Hoitink et al., 1982; Haghiri and Sutton, 1982). Papermill sludges contain large amounts of basic cations and plant nutrients (Tables I1 and 111). However, they also contain high levels of readily decomposable organic matter which can be beneficial in altering the physical and chemical properties of the toxic spoil but have an inhibitory effect on the amount of plant-available N. Dolar et al. (1972) reported that sludges with a C/N ratio greater than 20:l will likely result in N immobilization. A mixture of primary and secondary sludges applied at rates of 56-336 tons/ha (dry weight basis) to a spoil containing 0.82% sulfur and a pH of 2.6 was found to inhibit seedling emergence initially (Hoitink et al., 1982). However, the erosion control provided by the papermill sludge was excellent even without the use of a mulch cover. Although seedling emergence was delayed, eventually a complete cover of vegatation was established. With composted papermill sludge, delay in seedling emergence was not observed (Haghiri and Sutton, 1982).

E. SODIUM LAURYLSULFATE The bacterium Thiobacillus ferrooxidans accelerates acidification of pyritic material and significantly increases acid production (Kleinman, 1980). The use of the anionic detergent sodium lauryl sulfate (SLS) as a bactericide can reduce this acid production by 60-90% (Kleinmann and Erickson, 1982). The rate of SLS required to inhibit acid formation in a pyrite-containing spoil will vary among spoils depending on the amount of SLS that is sorbed. Sufficient SLS must be added to the spoil to overcome the sorption capacity of the spoil for SLS so that it will interact with the T. ferrooxidans in the soil solution or surrounding the pyrite-containing particles. Sodium lauryl sulfate is water soluable and will be rapidly lost from the spoil under field conditions. To overcome this problem, formulations of SLS in a slow-release matrix have been developed. By using these materials, acid production in toxic spoils can be controlled for a time period long enough for establishment of a vegetative cover.

387

RECLAMATION OF ACIDIC MINED LANDS

Table I1 Major Plant Nutrient Content of Fly Ash, Sewage Sludge, Papermill Sludge, and Soil Fly ash of bituminous coal'

Sewage sludgeb (Yo)

Papermill sludge' (q0)

-

0.03-17.6 0.04-6.1 0.008-1.9 0.1-25 0.03-2.0

0.008-2.33 0.0125-0.50

Element N P

K Ca Mi3

0.1-0.2 1.7-3.2 0.2-4.1 0.1-0.8

0.004-0.85

0.10-14.4 0.09-0.57

Soil" (VO)

0.02-0.4 0.0005-0.2 0.04-3.0 0.7-50.0 0.06-0.6

'From Capp (1978). bFrom Halderson and Zenz (1978). %om Dolar et al. (1972)and Haghiri and Sutton (1982).

Table Ill Metal Content of Fly Ash, Sewage Sludge, Primary Papermill Sludge, and Soil'

Element

Fly ash of bituminous coalb

Sewage sludgeC

Papermill sludged

1.22%

Al

127,000

-

As B Ba Cd co Cr cu Fe Ga Hg La Mn Mo Ni Pb sc Sr Zn

82

6-230 4-757 21-8,980 1-18 17-99,000 84-10,400 400-90,000 -

36 974 0.3 35 172 132 86,600 100 0.1 99 145 33 11 15 22 794 20

-

33 23 -

36 0.67%

-

18-7,100 5-39 10-3,515 13-19,700 13-27,EOO

'In mg/kg except where noted. bFrom Adriano et al. (1980). Trom Halderson and Zenz (1978). dFrom Haghiri and Sutton (1982).

537

-

22 47 319

Soilb

4-30070 0.1-40 2-100 100-3,000 0.01-7 1-40 5-3 ,000 2-100 O.7-55% 15-70 100-4,000 0.2-5 10-1,000 2-100 10-25 500-4,000 10-300

388

P. SUTTON AND W. A. DICK

F.

COMBINATION OF AMENDMENTS

When materials such as sewage sludge, power plant fly ash, and papermill sludge have been used to ameliorate acidic spoils, relatively high rates of these materials have been applied. Frequently the high rates are required to raise the pH of the spoil to a level suitable for plant growth. It may be more economical to apply a combination of limestone and other amendments, thereby reducing the quantity of the material to be transported. However, if sufficient supplemental amendments such as limestone are not added to neutralize the acid produced by additional oxidation of the pyrite, the spoil will again become toxic to plants (Sutton, 1970, 1983). Further research to evaluate the use of combinations of amendments for vegetation establishment is needed.

G . METALCONTENT OF AMENDMENTS A major concern when applying high rates of amendments, especially industrial sewage sludges, is the addition of heavy metals that may accumulate in the food chain or in surface and subsurface waters. The metal content of fly ash, sewage sludge, papermill sludge, and soils (Table 111) depends primarily on the source from which the material was derived. The amounts of potentially toxic metals added to a spoil can be controlled by limiting the rates applied at any one time. If the land use after reclamation is to be for agricultural production or animal grazing, agricultural sludge utilization practices and restrictions should be followed (Table IV). If toxic spoil is to be vegetated primarily for erosion control, a single application of

Table IV Recommended Cumulative Limits for Metals of Major Concern Applied to Agricultural Croplane

Soil cation exchange capacity (cmol/kg)b Metal (kglha)

< 5 kg/ha

Cadmium Copper Nickel Zinc Lead

6

11

140 140 280 560

280 280 560 1,120

5-15 kg/ha

aFrom USEPA (1983). %oil must be maintained at pH 6.5 or more.

>I5 kg/ha 22 560

560 1,120 2,240

RECLAMATION OF ACIDIC MINED LANDS

389

sludge at a rate sufficient for rapid establishment of a vegetative cover is desirable because of economic reasons. Although little information is available, some concern has been expressed about the fate of certain organic compounds such as trihalomethanes and dioxin that may be present in sludges. Additional research is clearly needed to assess the organic compounds that are contained in sludges and their behavior in the environment.

IV.

SEEDING AND MANAGEMENT OF AMENDMENT-TREATED SPOIL A.

LIMESTONE AND FERTILIZER

Application of limestone and plant nutrients, in addition to those contained in the primary amendment, are often required before seeding can take place. This is especially relevant when fly ash and sodium lauryl sulfate are used as primary amendment. The rate of limestone required to adjust the spoil pH to a proper level for vegetative establishment may be difficult to determine because (1) methods to determine the neutralization potential of the amendment cannot be rapidly conducted or are inaccurate and (2) both the active and total potential acidity of the spoil material must be known (Barnhisel, 1977). A laboratory procedure which estimates the lime requirement based on the total potential acidity of spoil material has been developed by Smith et al. (1974). The acid estimates are obtained from total pyritic sulfur contents without differentiation of pyritic types, which may vary in reactivity. If the amendment material produces a cover that essentially seals the spoil from oxygen or inhibits acid production via other means, it is not necessary that sufficient neutralizing power be added to the spoil to counteract all the potential acidity in the vegetative rooting zone. The type of limestone added to spoil is of importance as spoils with a high concentration of sulfate ions, limed with dolomitic limestone, may limit plant establishment and growth due to the high soluble salt levels that result from the solubility of MgSO, (Evangelou and Thom, 1984). The amount of fertlizer require is usually determined on the basis of standard soil tests and the potential of the amendment to supply essential plant nutrients. Availability of N in an organic amendment, for example, may require that the amendment be mineralized. The rate of mineralization is thus as important consideration in determining the total amount of plantavailable nutrients to be applied at time of seeding.

390

P. SUTTON AND W. A. DICK

B. MULCHES

Mulches serve as a soil protectant and enhance plant establishment (Kay, 1978). The soil is protected by reducing raindrop impact, by reducing water

flow and soil movement by trapping sediment on the sites, and by increasing water infiltration. Mulches enhance plant establishment by holding seed and fertilizer in place, retaining moisture, preventing crusting, and modifying temperatures of the soil. The type and rate of amendment, the depth of mixing, and the time of seeding all influence the need for a mulch. Hoitink et al. (1982), for example, found that when 221 tons/ha of papermill sludge were used as a soil amendment, a mulch was not necessary for controlling erosion before vegetation became established. Frequently, however, sites are seeded immediately after site preparation has been completed, which may not necessarily coincide with the recommended seeding dates for the area. A mulch will provide protection for germinating seedlings and increase the chances for survival under less than favorable conditions. Types of mulches available for use in land reclamation are organic materials, rock, and chemicals. The most commonly used materials are organic, such as residues from agricultural and wood-processing industries (Vogel, 1981). The effectiveness of an organic mulch is roughly related to the size and shape of the particles. Long, narrow particles are superior to finely ground products. Rates of application for straw and hay mulches should be 3.0-4.5 tons/ha (Vogel, 1981). If erosion control is a primary concern, between the time of seeding and establishment of vegetation rates as high as 8.0-10 tons/ha may be required. A crimper is commonly used to anchor straw mulches so that they will not be removed by wind. Recommended rates of application for bark and wood chips for most seedlings is 85-115 m3/ha (0.95-1.3 cm deep). Where soils are droughty or the effective rooting zone is shallow, rates of 115-190 m3/ha (1.3-1.9 cm deep) are recommended for greater conservation of soil moisture (Vogel, 1981).

Myer et al. (1972) reported crushed stone, gravel, and wood chip mulches showed great potential for erosion control on steep barren slopes, especially where conventional methods might not be satisfactory. For slopes of 20%, 224-448 tons/ha of stone and gravel or 34-56 tons/ha of wood chips provided excellent soil stabilization. Erosion could be controlled much better with 2.5 cm of stone and gravel and a slightly deeper wood chip mulch than 5.2 tons/ha of straw. Soil stabilizers are organic and inorganic chemical products that are applied in water solutions to soil surfaces to stabilize the soil against wind and water erosion (Vogel, 1981). These materials are designed to coat and

RECLAMATION OF ACIDIC MINED LANDS

391

penetrate the soil surface and bind the soil particles together. They are used mainly to control only temporarily until vegetative cover is sufficiently established to protect the site. In the humid East, the effectiveness of chemical stabilizers is relatively short-lived compared to most mulches (Vogel, 1981).

C. PLANT SPECIES Each plant species has its own growth characteristics that determine its value in stabilizing soil. Grasses and legumes are the most effective plant materials for controlling erosion in the early stages of reclamation. Trees and shrubs, however, have an advantage of providing a permanent or semipermanent cover on disturbed areas with little or no additional care and maintenance. The selection of the proper species for planting should be based on chemical and physical properties of the spoil, topographic influences, climate, use, and management planned. Many studies have been conducted to determine adaptation of various plant species for revegetation and stabilization of stripmined lands in the eastern United States (Bennett, 1971; Brown, 1971; Vogel and Berg, 1968; Ruffner, 1978; Tyner and Smith, 1945; Grandt and Lang, 1958). Listed in Table V are grasses, legumes, trees, and shrubs that have shown some success in providing cover on abandoned mined lands. D. MANAGEMENT OF SEEDED AREAS During and after vegetation has been established on abandoned mined lands, these areas must be managed so the vegetation will continue to provide protection against soil erosion. This may require the addition of plant nutrients in cases in which part of the vegetative production is removed as a crop. The need for application of limestone and fertilizer for agricultural purposes can best be determined by soil testing. Once trees and shrubs are established, they will survive on relatively infertile areas without the need for additional fertlizers. In reclaimed areas which have a low soil organic matter content and which are seeded to a grass-legume mixture, the legumes will virtually disappear from the stand by the third or fourth year. If this occurs, application of additional N may be required so that the grasses will maintain sufficient vigor and growth to control soil erosion, especially on sloping areas. Also, areas to be used as pasture for livestock should not be overgrazed as this will cause a reduction in plant stands to the point where erosion will no longer be controlled.

392

P. SUTTON AND W. A. DICK

Table V Seeding Rates and Lower pH Limit at Which Plant Growth Can Be Expected for Various Plant Species Used in Establishing Vegetation on Abandoned Mined Landsf ~~~

Plant species Common name Grasses Redtop Big bluestem Smooth bromegrass Bermudagrass Orchardgrass Japanese millet Weeping lovegrass Ky.-31 tall fescue Annual ryegrass Perennial ryegrass Deert on gu e Switchgrass Pearl millet Reed canarygrass Timothy Rye Foxtail millet Indiangrass Sorghum Sudangrass Winter wheat Legumes Crownvetch Flatpea Sericea lespedeza Korean lespedeza Common lespedeza Kobe lespedeza Birdsfoot trefoil Alfalfa Yellow sweetclover White sweetclover Red clover White clover Ladino clover

~

Seeding rate (Wha)

Scientific name

Mixtures

Alone

Lower pH limit

Agrosiis gigantea Andropogon gerardi Bromis inermis Cynodon dactylon Dactylsis glomeraia Echinochloa crusgalli var. frumeniacea Eragrostis curvala Festuca arundinacea Lolium muliiflorum Lolium perenne Panicum clandesiinum Panicum virgaium Pennesetum americanum Phalaris arundinacea Phleum praiense Secale cereale Setaria iialica Sorghastrum mufans Sorghum bicolor Sorghum sudanense Triiicum aestivum

2.2-4.5 4.5-9.0 11.2-16.8 3.4-5.6 5.6-8.9 8.9-13.4

3.4-6.8 4.5-16.8 16.8-22.4 7.8-13.4 11.2-16.8 22.4-28 .O

4.0-4.5 4.5 5.O 4.0 4.5 4.5

Coronilla varea Laihyrus sylvestris Lespedeza cuneaia Lespedeza siipulacea Lespedeza siriaia Lespedeza siriaia var Kobe Lotus corniculatus Medicago saiiva Meliloius officinalis Melilotus arfa Trifolium praiense Trifolium repens Trifolium repens

.

-

28-44.8 28-44.8 89.7-134.5

4.0 4.5 4.5 4.5 4.0 4.0-4.5 4.0-4.5 4.5 4.5-5.0 4.5 4.5 4.5 4.5-5.0 4.5-5.0 4.5

5.6-11.2 22.4 11.2-22.4 6.7-13.4 8.9-1 6.8 8.9-16.8

16.8-22.4 33.6 11.2-16.8 22.4-28.0 28.0-33.6 28.0-33.6

5.0 4.0-4.5 4.5 5.0 4.5 4.5

5.6-9.0 4.5-13.4 4.5-7.8 4.5-7.9 4.5-9.0 2.2-4.5 2.2-4.5

11.2-13.5 13.4-20.2 11.2-16.8 11.2-16.8 9.0-13.4

4.5 5.5 5.5

2.2-3.4 11.2-16.8 4.5-7.8 5.6-1 1.2 6.7-9.0 2.2-5.6 9.0-13.4 5.6-9.0 4.5-7.8 33.6-67.3 11.2-16.8 5.6-13.4 16.8-22.4 16.8-22.4 33.6-67.3

22.4-39.2 22.4-28 .O 22.4-28.0 13.5-16.8 5.6-13.4 22.4-28.0 9.0-13.4 89.7-134.5 22.4-33.6

-

-

5.5 5 .O

5.5 5.5

(continued)

393

RECLAMATION OF ACIDIC MINED LANDS

Table V (Continued) ~~

~

Seeding rate (kg/ha)

Plant species

i=ommon name

Scientific name

Mixtures

Shrubs Indigobushb Silky dogwoodb Autumn oliveb Shrub lespedeza* Japan lespedezab Thunberg lespedezab Amur honeysuckle Morrow honeysuckle Tatarian honeysuckle Fragrant sumac Shining sumac Bristly locust

Amorpha fruticosa Cornus arnomum Elaeagnus umbellata Lespedeza bicolor Lespedeza japonica Lespedeza thunbergii Lonicera maackii Lonicera morrowii Lonicera rarorica Rhus aromatica Rhus capallina Robinia fertilis

.56 Seedlings Seedlings 1.21-3.36 1.21-3.36 1.21-3.36 Seedlings Seedlings Seedlings Seedlings Seedlings 2.24-5.61

Trees Conifer Eastern redcedar Japanese larch White spruce Norway spruce Jack pine Shortleaf pine Austrian pine Red pine Pitch pine Eastern white pine Scotch pine Loblolly pine Virgina pine

Juniperus virginiana Larix leptolepis Picea glauca Picea abies Pinrus banksiana Pinus echinata Pinus nigra Pinus resinosa Pinus rigida Pinus strobus Pinus sylvestris Pinus taeda Pinus virginiana

-

Acer rubrum Acer saccharinum Acer saccharum Alnus glutinosa Betula nigra Betula pendula Castanea mollissima Fraxinus americana Fraxinus pennsylvanica Juglans nigra Liquidambar styraciflua Liriodendron tufipifera

-

Hardwoods Red maple Silver maple Sugar maple European black alder* River birch European white birch Chinese chestnut White ash Green ash Black walnut Sweetgum Yellow poplar

-

-

-

-

-

-

-

Alone

Lower pH limit

4.0 4.0 4.0 4.5 4.5 4.5 5.0 5 .O 5.0 4.5 4.0 3.5

5.0 4.0-4.5 4.5-5.0 4.5-5.0 4.0 4.5 4.0 4.0-4.5 4.0 4.0 4.0 4.0 3.5-4.0

4.5 4.0 4.5 3.5-4.0 4.0 3.5-4.0 4.5 4.0 4.0 5.5 4.0 4.5

(continued)

394

P. SUTTON AND W. A. DICK Table V (Continued) Seeding rate Wha)

Plant species Common name Hardwoods (cont.) Osage orange Crab apple American sycamore Hybrid poplars Eastern cottonwood Black cherry White oak Bur oak Northern red oak Black locustb

Scientific name

Mixtures

Alone

-

-

Maclura pomifera Malus sp. Platanus occidentalis Populus sp. Populus detoides Prunus serotina Quercus alba Quercus macrocarpa Quercus rubra Robinia pseudoacacia

-

-

-

-

-

-

Lower pH limit 4.5 4.5 4.0-4.5 4.0-4.5 4.5 4.5 5.0 4.0 4.0 4.0

Trom Vogel(l981). hitrogen fixers.

V.

CHANGES IN SOIL PROPERTIES AS AFFECTED BY ADDITION OF AMENDMENTS

A. PHYSICAL Covering toxic spoil with topsoil results in a medium for plant growth that is very similar to that of the applied topsoil. However, because of the discontinuitybetween the topsoil and spoil, the behavior of water in the soil will be changed compared to the soil’s location prior to its application on the spoil. In addition, removal and placement of the topsoil can result in compaction, which can adversely affect porosity, pore-size distribution, and bulk density. The most important factor in predicting the amount of soil compaction produced by soil-moving equipment is soil moisture content at time of operation (Bymes et al., 1982). The texture of fly ash is similar to that of a silt loam (Chang et al., 1977). When fly ash is added to a spoil the particle size distribution of the resulting soil is modified. In the case of a fine or a course-textured spoil,the modification results in the formation of a medium-textured soil (Capp, 1978). The silty nature and lack of stable aggregates of fly ash amendment caused the surface of amended spoil to be susceptible to wind and water erosion (Townsend and Hodgson, 1973). Until vegetation is established, the fly ash is highly susceptibleto being eroded and moved off the reclamation site. Water holding capacity, as measured by available water, increased 4% for spoils and 6% for coal mine refuse when amended with fly ash (Capp,

RECLAMATION OF ACIDIC MINED LANDS

395

1978). Fly ash applied at a rate of 336 tons/ha to acidic spoils improved water infiltration to a depth of 122 cm (Plass and Capp, 1974). Chang et al. (1977) noted hydraulic conductivity of soils increased with small amounts of fly ash added but declined rapidly as fly ash volume increased. Fly ash applied to spoil in a 1:1 ratio decreased bulk density from 1.38 to 1.15 mg/m3 (Capp and Gillmore, 1973). A decrease in bulk density of a heavy-textured spoil is considered beneficial to plant growth. Fly ash may also have a negative impact by being involved in a pozzolanic reaction, the formation of a stable cement when the fly ash is wetted in the presence of liming material. This will result in the formation of a crust and may increase bulk density, reducing water movement and root penetration (Chang et al., 1977; Hodgson and Townsend, 1973). If soil aggregates are formed due to a pozzolanic reaction, then percolation of water through soil would be increased (Plass and Capp, 1974). The pozzolanic reaction was not a problem mentioned by Capp (1978) when summerizing research conducted to investigate the use of fly ash on acidic coal mine spoils. Hodgson and Townsend (1973) reported cementation can be inhibited by adding relatively small quantities of sewage sludge, peat, or ground straw. The pozzolanic reaction is more likely to be a problem with unweathered fly ash. Sewage sludge, derived from organic and inorganic matter removed from waste water at sewage treatment plants, ameliorates physical properties of heavy-textured spoils by improving aeration, friability, and water infiltration. Columns of spoil treated with sludge were found to absorb water faster, and 10% more water passed through the treated columns compared to the controls (Peterson and Gschwind, 1972). Application of 235-435 tons/ha of sewage sludge decreased the bulk density in the upper 7.5 cm of spoil from 1.61 to 1.10-1.19 mg/m” (Peterson et al., 1979). Hinesly et al. (1982) reported water-stable aggregates greater than 0.25 mm increased from 12.2% in samples from untreated plots to 42.1% in spoil amended with 896 tons/ha of sewage sludge. The available water holding capacity was increased from 14.8 to 21.1% with this treatment.

B.

CHEMICAL

Topsoil is generally used in reclamation to provide a cover over acidic spoil.

As such the chemical properties of the reclaimed area will be identical to that of

the topsoil applied. Often it is necessary, however, to add lime or plant nutrients to the topsoil prior to seeding to provide a more suitable medium for plant growth. The changes in chemical properties of the topsoil which result, however, are small compared to changes in spoil when amendments such as fly ash and sewage sludge are mixed directly into the spoil. Also, the influence of the topsoil cover on the underlying spoil is small and occurs only slowly with time.

396

P. SUTTON AND W.A. DICK

Amendment of toxic spoil with fly ash affects pH, content of soluble salts, cation exchange capacity and base saturation, and nutrient and metal availability to plants. The effect of fly ash on spoil acidity will depend on the pH and buffering capacity of the ash. Plank and Martens (1973) tested eight fly ashes and found some were alkaline while others were acidic. The use of alkaline fly ash has been shown to raise the pH of acidic spoil to the point where plants can be grown. One to 10 years after applying 336-1792 tons/ha of fly ash to acidic spoil (PH 2.3-4.7), Capp (1978) found that the pH of the treated sites ranged from 5.1 to 7.4. The fly ash treatment had a surface effect only and layers below the fly ash-treated zone remained extremely acid. The soluble salt content of fly ash depends largely on the coal source, lagooning process, and age of ash. Unweathered fly ash may result in salt injury when applied in high rates to spoils (Adriano et al., 1980). Electrical conductivities of saturation extracts exceeding 4.0 dS/m will result in reduced plant growth (USDA, 1954). Electrical conductivities ranging from 0.18 to 14 dS/m have been reported for most fly ashes (Hodgson and Holliday, 1966). A fly ash with an electrical conductivity of 1.5-2.3 dS/m applied to spoil of 0.21,2.3, and 3.2 dS/m resulted in electrical conductivities of 0.66, 2.3, and 2.5 dS/m, respectively (Capp, 1978). Due to leaching, older fly ash-amended sites have lower soluble salt contents than when the fly ash is first applied. Chang et al. (1977) reported that cation exchange capacity of a fly ash they used as an amendment to be 1.4 cmol/kg. Fly ash applied to an acidic spoil increased exchange capacity from 12 to 19 cmollkg after the first growing season (Capp and Engle, 1967). The increase was attributed to fine roots produced by the thick turf rather than the addition of fly ash. Adriano et al. (1980) concluded that in most cases the direct effect of fly ash application would be to lower the cation exchange capacity of soils. Most of the research concerning the plant nutrient status of fly ash has focused on the change in the growth and chemical composition of plants rather than chemical changes in the soil as measured by soil test (Adriano et al., 1980; Capp, 1978). Abnormal plant growth has been reported when plants were grown on fly ash alone due to B toxicity (Townsend and Hodgson, 1973), Al and Mn toxicities (Rees and Sidrak, 1956), or to P deficiency (Hodgson and Townsend, 1973). Capp (1978) summarized reclamation experiments conducted on acidic spoils with applications of 336-1792 mg/ha of fly ash with a range of 0.05-0.45 g/kg B. Vegetation was established but in some cases B was as high as 300 mg/kg in the plant tissue. Generally a B content above 75 mg/kg is considered toxic (Allway, 1968). Adams et al. (1972) also noted toxicity symptoms on plants grown on fly ash-amended spoil. These symptoms were generally found the first year after seeding and disappeared after continued growth. Leaching was thought to account for the decreased B uptake after several years.

RECLAMATION OF ACIDIC MINED LANDS

397

Amendment of acid spoil with sewage sludge also has an ameliorating effect on the chemical properties of the spoil with changes in pH probably being most significant. Many sewage sludges have a pH that is near neutral and alkalinity that will neutralize the acid conditions formed in many mine wastes and spoils (Hill et al., 1979). Many studies have reported an increase in pH by sewage sludge amendment (Mathias et al., 1979); Stucky and Zoeller, 1980; Haghiri and Sutton, 1982; Sutton and Vimmerstedt, 1974; Lejcher, 1972). The ameliorating effect may be evident for several years, as Sutton (1979) reported that the pH of the 1-10-cm amended spoil layer was 5.4 when the spoil was treated with 659 tons/ha of sludge compared to 2.4 without sludge. McCormick and Borden (1973) have postulated that sewage sludge may increase pH by increasing ammonia and ammonium concentrations as a result of anaerobic decomposition of the sludge. Sewage sludges often contain large amounts of heavy metals. The level of metals in plants or leachate of a sludge-amended spoil is influenced by the level in the sludge, rate of sludge application, spoil pH and texture, time after application, and type of crop grown. The most important variable, however, seems to be pH. A sludge-amended spoil at pH 3.5 resulted in higher concentrations of metals being extracted by 0.1 A4 HCl (Fig. 1) or taken up by plants (Stuckey and Bauer, 1979) than a similarly treated spoil at pH 4.7. An increase in pH, which can be brought about by sludge application, decreases the concentration of most metals via precipitation or absorption mechanisms (Table VI). Plants established where the pH was greater than 5.5 accumulated lower quanities of metals compared to plants grown on spoil with a mean pH less than 5.5 (Stuckey et al., 1980.) The organic content of sludges can range from 65 to 480 g/kg with a median ranging from 270 to 330 g/kg (Sommers, 1977). Addition of sewage sludge to spoils increases the organic carbon content of the amended spoil. Spoil containing 6.1 g/kg organic carbon was treated with 211-383 tondha, and after 7 years the treated spoil had an organic carbon content of 36 g/kg (Peterson et al., 1982). Sewage sludge applied to spoil at rates of 0,224,448, and 896 tons/ha resulted in organic carbon concentrationsof 24,28,47, and 69g/kg, respectively(Hinesly et al., 1982). Stucky and Zoeller (1980) applied 0, 224, 336, and 448 tons/ha of sewage sludge to an acidic spoil and measured organic carbon approximately 5 months later. The amended spoil organ carbon levels, from lowest to highest amendment rate, were 17,43,56, and 63 g/kg. The cation exchange capacity of organic matter increases from near zero at a pH of 4 or below to 200 cmol/kg at pH 7 (Coleman and Mehlich, 1957). An increase of 1 g/kg of organic matter in a spoil will represent an approximate increase in cation exchange capacity of 0.2 cmol/kg. An application of 448 tons/kg of sewage sludge with a cation exchange capacity of 40 cmol/kg increased the cation exchange capacity of an acidic spoil from 13 to 24 cmol/kg (Stucky and Zoeller, 1980).

398

P. SUTTON AND W. A. DICK

200 100 -

99

50 20 -

ga

l05-

x CI, \

F

2I-

0.5 -

0.1 -

0.2

:u

Zn

Ni

Gd

Cr

Mn

Pb

FIG.1. Effect of pH on concentrations of metals in sludge-amended spoil extracted with

0.1 M HCI. (Adapted from Stucky and Bauer, 1979.)

Table VI Removal Mechanism of Heavy Metals In Soil' ~~

Element

Removal mechanismb

Element

Removal mechanismb

Aluminum Arsenic Beryllium Boron Cadmium Chromium Cobalt Copper Fluoride Iron

PR,S AD,S PR AD,W AD,CE,S AD,CE,S AD,CE,S AD,CE,S AD,S PR,CE,S

Lead Lithium Manganese Mercury Molybdenum Nickel Selenium Silver Zinc

AD,CE,S CE, W PR,CE,S AD,CE,S AD,S AD.CE,S =,w AD,CE,S AD,CE,S

%om USEPA (1976). bAD, Adsorption with iron or aluminum hydroxide, pH-dependent; AE, anion exchange; CE. cation exchange; PR, precipitation, pH-dependent (iron and manganese are also subject to changes by oxidation-reduction reaction); S, strong strength of removal; W, weak strength of removal.

RECLAMATION OF ACIDIC MINED LANDS

399

Application of sewage sludges may also increase electrical conductivity to levels that are inhibitory to germination of salt-sensitive crops. When 336 and 448 tons/ha of sewage were applied to an acid spoil the electrical conductivity values increased from 2.0 to 3.0 and 3.5 dS/m, respectively (Stucky and Zoeller, 1980). During the growing season these values decreased due to leaching and plant growth. Hinesly et al. (1982) reported that the electrical conductivity value of spoil treated with 896 tons/ha of sludge increased from 2.2 to 6.6 dS/m resulting in a 50% reduction in corn grain yields. Crops grown on sewage sludge-amended spoil have shown increased uptake of plant nutrients and heavy metals. Mathias et al. (1979) found that sludge application to an acid spoil generally increased plant concentrations of N, P,Ca, Mg, Cd, Cu, Zn and decreased concentrations of K and S. In a field lysimeter study with two sludges, there was and increase in plant concentration of N which correlated with increased application rate (Haghiri and Sutton, 1982). However, the N concentrations decreased as much as 54% after three growing seasons. The metal concentrations of Al, Cu, Fe, Ni, Mn, Zn, and Cd in the plants also decreased with time. Papermill sludges also increase pH; soil test levels of P, K, and Ca; and organic matter levels (Hoitink and Watson, 1982; Hoitink et al., 1982; Haghiri and Sutton, 1982; Watson and Hoitink, 1985). However, papermill sludges with C:N ratios of 140-15O:l will depress plant growth due to the immobilization of available N by microorganisms degrading the sludge. This can be corrected by delaying the planting of grasses or applying N to offset the temporary shortage due to immobilization. Sludges from secondary activated biological treatment processes contain sufficient N in relation to organic C so as not to limit N for plant growth (Hoitink and Watson, 1982; Watson and Hoitink, 1985). C.

BIOLOGICAL

The effect of topsoil, sewage sludge, papermill sludge or other amendments on the biological activity of the amended spoil has not received the same amount of research effort as has the study of changes in the physical and chemical properties. Regardless of the amendment applied, once vegetative cover is established, biological activity rapidly increases. Microarthropods become established in the early stages after reclamation, and they are the initial dominant invertebrate decomposers (Curry and Cotton, 1983). Earthworms were found to colonize brown coal-lignite mines planted with alder (Alder glutinosa) after 5 years but after 15 years the movement of earthworms from the edge of the unmined areas into the more acid areas was minimal (Dunger, 1969a). Site age, shade, and moisture content were found to be the most important parameters in establishing earthworm populations in mined sites restored for agriculture (Standen et al.,

400

P. SUTTON AND W. A. DICK

1982). Earthworm numbers and biomass in pasture lands reclaimed from opencast coal mines (Rushton, 1986) were greater in the oldest (15 years) reclaimed sites than on the more recently (2 and 7 years) reclaimed or unmined sites. The species distribution was also affected by mining and reclamation, with control sites being dominated by Aporrectodea longa and Lumbricus terrestris whereas Ap. longa and Allolobophora chlorotica were dominant on reclaimed sites. In Ohio Vimmerstedt (1983) observed that surface mining reduced earthworm populations to low levels but that L. terrestris could be reintroduced to vegetated minesoils that ranged in pH from approximately 4 to above neutral. Vegetation on the reclaimed calcareous sites influenced earthworm growth with larger earthworms found under Alnus glutinosa than under black locust (Robinia pseudacacia). An important activity of earthworms in reclaimed mine spoils is their ability to incorporate and decompose litter, thus altering soil-forming processes (Vimmerstedt, 1983). Dunger (1969b) noted that in the initial stages after reclamation, when microarthropods were dominant, less than 5% of the litter was decomposed. After 10 years earthworms were removing 70% of the litter fall and accounted for 94% of invertebrate respiration. Increasingly being recognized as an important component of a successful reclamation program is an active microbial community (Tate and Klein, 1985). Microbial processes are highly sensitive to the same environmental factors that affect plant growth. Therefore, treatments applied to barren spoils to improve plant growth will also stimulate microbiol activity. Wilson (1965) observed that the numbers of bacteria, fungi, and actinomycetes increased in spoils as vegetation became established. The type of vegetation also has an effect, however, as the numbers of microorganisms were higher in areas vegetated by forage than in areas with trees. Herbaceous vegetation also supported higher populations of microorganisms than other types of vegetation on kaolin spoils (Ostrosina et al., 1984). A more complete review of the literature than can be provided here on the role the microbial community plays in the reclamation of spoil material can be found in the book edited by Tate and Klein (1985). Topics covered included nutrient cycling, the impact of microorganisms on the recovery of soil and waste ecosystems, algae ecology in disturbed lands, soil enzyme activities as predictors of reclamation progress, and the impact of mycorrhizal relationships on mineland reclamation. Most of the concepts and research findings discussed in that review are not derived from the studies of reclamation of abandoned toxic mine spoil, although they may be applied to such cases. Greater rersearch effort is needed to assess adequately the changes in biological properties of spoil when various types of amendments are applied and to determine the role of various components of the biological sector in successful reclamation.

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401

SUMMARY OF LONG-TERM RESULTS

Several studies of long-term results obtained from surface-mined land reclamation have been reported. Jencks et al. (1982) found that after 16-18 years, acid mine spoils seeded with black locust (Robunia pseudoacacia L.) had soil N levels slightly higher than in an adjacent native soil with a mixed hardwood stand. An evaluation of a 30-year-old plantation on stripmined land in east central Ohio indicated only a 23% survival of planted trees on noncalcareous spoils. Soil compaction during grading reduced tree survival and growth (Larson and Vimmerstedt, 1983). Smith et al. (1971) evaluated characteristics of 70-130-year-old iron ore spoils in West Virginia. The natural soil was superior to the spoils in bulk densities, porosity, soil structure, infiltration, organic matter levels, and texture in the surface horizon. The mine spoils were found to be superior in total available water holding capacity and plant rooting depth. Because various types of amendments and reclamation practices may have long-term effects, additional information is clearly needed to evaluate the relative long-term success of the amendment alternatives for reclaiming abandoned mined lands in relation to future land use demands and capabilities.

ACKNOWLEDGMENT Support provided by State and Federal funds appropriated to the Agricultural Research and Development Center, The Ohio State University. Journal Article No. 172-86.

REFERENCES Adams, L. M.,Capp, J. P., and Gillmore, D. W. 1972. Compost Sci. 13, 20-26. Adriano, D. C., Page, A. L., Elseewi, A. A., Chang, A. C., and Straughan, I. 1980. J. Environ. Quai. 9,333-344. Allway, W. H. 1968. Adv. Agron. 20, 235-274. Apel, L. M. 1983. “Leachability and Revegetation of Soild Waste from Mining,” EPA-600/S2-82493. U.S. Environmental Protection Agency, Cincinnati, Ohio. Ashby, W. C., Vogel, W. G., Kolar, C. A., and Philo, G. R. 1984. In “Erosion and Productivity of Soils Containing Rock Fragments” (J. D. Nichols, P. L. Brown, and W. J. Grant, eds.), pp. 31-44. Am. SOC.Agron., Madison, Wisconsin. Ayerst, J . M. 1980. Reclamation Rev. 1, 27-33. Barnhisel, R. I. 1977. Reclamation of surface mined coal spoils. Kentucky Agric. Exp. Sta. Lexington. CRSIAG No. D6-E762. Barnhisel, R. I., Powell, J. L., and Akin, G. W. 1975. Natl. Coal Assoc. Symp. Surf. Mining Reclamation 3rd. 2, 140-1 5 1. Bastian, R. K., Montague, A., and Numbers, T. 1982. In “Land Reclamation and Biomass

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Production with Municipal Wastewater and Sludge” (W. E. Sopper, E. M. Speaker, and R. K. Bastian, eds.), pp. 13-54. Pennsylvania State Univ. Press, University Park, PA. Bennett, 0. L. 1971. In “Revegetation and Economic Use of Surface-Mined Land and Mine Refuse Symposium” (D. M. Bondurant, ed.), pp. 23-25. School of Mines, West Virginia Univ., Morgantown. Bennett, 0. L., Armiger, W. H., and Jones, J. N., Jr. 1976. In “Land Application of Waste Materials,” pp. 195-215. Soil Conserv. SOC.of Am., Ankeny, Iowa. Berg, W. A. 1973. In “Ecology and Reclamation of Devastated Land” (R. J. Hutnik and G. Davis, eds.), Vol. 1, pp. 93-104. Gordon & Breach, New York. Berg, W. A. 1978. In “Reclamation of Drastically Disturbed Lands” (F. W. Schaller and P. Sutton, eds.), pp. 653-661. Am. SOC.of Agron., Madison, Wisconsin. Bowling, K. C. 1978. In “Reclamation of Drastically Disturbed Lands” (F. W. Schaller and P. Sutton, eds.), pp. 95-116. Am. SOC.of Agron., Madison, Wisconsin. Brant, R. A. 1964. Ohio J. Sci. 64, 68-75. Brown, J. H. 1971. In “Revegetation and Economic Use of Surface-Mined Land and Mine Refuse Symposium” (D. M. Boundurant, ed.), pp. 26-28. West Virginia University, Morgantown. Bymes, W. R., McFee, W. W., and Steinhardt, G. C. 1982. “Soil Compaction Related to Agricultural and Construction Operations.” Purdue Univ. Agric. Exp. Stn. Bull. 397, West Lafayette, Indiana. Capp, J. P. 1978. In “Reclamation of Drastically Disturbed Lands” (F.W. Schaller and P. Sutton, eds.), pp. 339-353. Am. SOC.of Agron., Madison, Wisconsin. Capp, J. P., and Engle, C. F. 1967. In “Fly Ash Utilization Symposium,” pp. 269-279. U.S. Bureau of Mines Information Circ. 8640. U.S. Government Printing Office, Washington, D.C. Capp, J. P., and Gillmore, D. W. 1973. In “Third Symposium of Ash Utilization,” pp. 258-279. IC-8640. U.S. Bureau of Mines, Washington, D.C. Caruccio, F. T. 1%8. In “Second Symposium on Coal-Mine Drainage Research,” pp. 107-151. Bituminous Coal Research, Monroeville, PA. Chang, A. C., Lund, L. J., Page, A. L., and Warneke, J. E. 1977. J. Environ. Qual. 6, 267-270.

Chapman, A. G. 1967. “How Strip-land Grading Affects Tree Survival and Growth.” Southern Illinois School of Agric. Publ. 29, Carbondale. Coleman, N. T., and Mehlich, A. 1957. In “Soil, the 1957 Yearbook of Agriculture,” pp. 72-85. U S . Department of Agriculture, Washington, D.C. Croxton, W. C. 1928. Ecology 9, 155-175. Curry, J. P., and Cotton, D. C. F. 1983. In “Earthworm Ecology: From Darwin to Vermiculture” (J. E. Satchel], ed.), pp. 215-228. Chapman & Hall, New York. Deely, D. J., and Borden, F. Y. 1973. In “Ecology and Reclamation of Devastated Land” (R. J. Hutnik and G. Davis, eds.), Vol. I, pp. 69-79. Gordon & Breach, New York. Dolar, S. G., Boyle, J. R., and Keeney, D. R. 1972. J. Environ. Qual. 1,405-409. Dunger, W. 1969a. Pedobiologia 9, 146-151. Dunger, W. 1969b. Pedobiologia 9, 366-371. Evangelou, V. P., and Thom, W. 0. 1984. In “Conference on Reclamation of Abandoned Acid Spoils,” Dep. of Agronomy, University of Kentucky, Lexington, Fehrenbacher, D. J., Jansen, 1. J., and Fehrenbacher, J. B. 1982. SoilSci. SOC. Am. J. 46, 353-359.

Geyer, W. A., and Rogers, N. F. 1972. J. Soil Water Conserv. 21, 114-116. Grandt, A. F., and Lang, A. L. 1958. “Reclaiming Illinois Strip Coal Land with Grasses and Legumes.” Univ. of Illinois Agric. Exp. Stn. Bull. 628, Urbana. Grube, W. E., Jencks, E. M., Singh, R. N., Smith, R. M., and Wilson, H. A. 1971. “Mine Spoil Potentials for Water Quality and Controlled Erosion.” Water Pollution Control

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Research Ser. 14010 EJE 12/71. U S . Environmental Protection Agency, Washington, D.C. Haghiri, F., and Sutton, P. 1982. In “Land Reclamation and Biomass Production with Municipal Wastewater and Sludge” (W. E. Sopper, E. M. Speaker, and R. K. Bastian, eds.), pp. 433446. Pennsylania State Univ. Press, University Park, Pa. Halderson, J. L., and Zenz, D. R. 1978. In “Reclamation of Drastically Disturbed Lands” (F. W. Schaller and P. Sutton, eds.), pp. 355-337. Am. SOC.of Agron., Madison, Wisconsin. Hedrick, H. G., and Wilson, H. A. 1956. Proc. West Virginia Acad. Sci. 28, 11-15. Hill, R. D. 1978. In “Reclamation of Drastically Disturbed Lands” (F. W. Schaller and P. Sutton, eds.), pp. 687-704. Am. SOC.of Agron., Madison, Wisconsin. Hill, R. D., Hinkle, K. R., and Klingensmith, R. S. 1979. In “Utilization of Municipal Sewage Effluent and Sludge on Forest and Disturbed Land” (W. E. Sopper and S. N. Kerr, eds.), pp. 425-443. Pennsylvania State Univ. Press, University Park, PA. Hinesly, T. D., Redborg, D. E., Ziegler, E. L., and Rose-Innes, I. H. 1982. In “Land Reclamation and Biomass Production with Municipal Wasterwater and Sludge” (W. E. Sopper, E. M. Speaker, and R. K. Bastian, eds.), pp. 339-352. Pennsylvania State Univ. Press, University Park, Pa. Hodgson, D. R., and Holliday, R. 1966. Chem. Ind. 785-790. Hodgson, D. R., and Townsend, W. N. 1973. In “Ecology and Reclamation of Devastated Land” (R. J. Hutnik and G. Davis, eds.), Vol. 2, pp. 247-271. Gordon & Breach, New York. Hoitink, H. A. J., and Watson, M. E. 1982. In “Land Reclamation and Biomass Production with Municipal Wasterwater and Sludge” (W. E. Sopper, E. M. Speaker, and R. K. Bastian, eds.), pp. 301-306. Pennsylvania State Univ. Press, University Park, Pa. Hoitink, H. A. J . , Watson, M. E., and Sutton, P. 1982. In “Abandoned Mine Reclamation Symposium” (N. Smeck and P. Sutton, eds.), Chap. 5. Dep. of Agronomy, Ohio State Univ., Columbus. Jencks, E. M., Tyron, E. H., and Contri, M. 1982. Soil Sci. Soc. Am. J. 46, 1290-1293. Jewell, W. J . 1982. In “Land Reclamation and Biomass Production with Municipal Wastewater and Sludge” (W. E. Sopper, E. M. Speaker, and R. K. Bastian, eds.), pp. 448-480. Pennsylvania State Univ. Press, University Park, PA. Jones, J. N., Jr., Armiger, W. H., and Bennett, 0. L. 1975. Nail. Coal Assoc. Symp. Surf. Mining Reclamation, 3rd 2, 185-194. Kay, B. L. 1978. In “Reclamation of Drastically Disturbed Lands” (F. W. Schaller and P. Sutton, eds.), pp. 467-483. Am. SOC.of Agron., Madison, Wisconsin. Kleinmann, R. L. P. 1980. Univ. Kentucky Symp. Surf. Mining Hydrol. Sedimentol. Reclamation, pp. 333-337. Kleinmann, R. L. P., and Erickson, R. M. 1982. Univ. Keniucky Symp. Surf. Mining Hydrol., Sedimentol. Reclamation, pp. 617-622. Knabe, W. 1964. Ohio J. Sci, 64, 75-105. Kohnke, H. 1950. Adv. Agron. 2, 317-349. Larson, M. M., and Vimmerstedt, J. P. 1983. “Evaluation of 30-year-old Plantations on Stripmined Land in East Central Ohio.” Ohio State Univ.-Ohio Agric. Res. and Dev. Center Res. Bull. 1149, Wooster. Lawry, J. D. 1977. Environ. Pollui. 14, 195-205. Lejcher, F. R. 1972. In “National Symposium of Watersheds in Transition,” pp. 371-376. American Water Resources Assoc. and Colorado State Univ., Fort Collins. McCormick, L. H., and Borden, F. Y. 1973. In “Ecology and Reclamations of Devastated Land” (R. J. Hutnik and G. Davis, eds.), Vol. I , pp. 239-250. Gordon & Breach, New York.

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Martens, D. C., and Beahm, B. R. 1976. In “International Ash Utilization Symposium.” pp. 657-664. Morgantown Energy Research Center, Morgantown, West Virginia. Mathias, E. L., Bennett, 0. L., and Lundberg, P. E. 1979. In “Utilization of Municipal Sewage Effluent and Sludge on Forest and Disturbed Land” (W. E. Sopper and S. N. Kerr, eds.), pp. 307-314. Pennsylvania State Univ. Press, University Park, PA. Mays, D. A., and Bengtson, 0. W. 1974. “Fertilizer Effects on Forage Crops on Strip-Mined Land in Northeast Alabama,” p. 23. National Fertilizer Development Center, Muscle Shoals, Alabama. Mays, D. A., and Bengston, G. W. 1978. In “Reclamation of Drastically Disturbed Lands” (F, W. Schaller and P. Sutton, eds.), pp. 307-327. Am. Soc. of Agron., Madison, Wisconsin. Montague, A., and Hill, R. D. 1976. “The Potential for Using Sewage Sludge and Compost in Mine Reclamation.” Mining Pollution Control Report. Ind. Environ. Res. Lab, Cincin4 nati, Ohio. Muller, K. 1973. In “Ecology and Reclamation of Devastated Land” (R. J. Hutnik and G. Davis, eds.), Vol. 1, pp. 325-334. Gordon & Breach, New York. Myer, L. D., Johnson, C. B., and Foster, G. R. 1972. J. Soil Wuter Conserv. 21, 264-269. Neumann, U. 1973. In “Ecology and Reclamation of Devastated Land” (R. J. Hutnik and G. Davis, eds.), Vol. 1, pp. 335-348. Gordon & Breach, New York. Ostrosina, W. J., Marx, D. H., and May, J. T. 1984. ReclamafionReveg. Res. 3, 1-15. Paone, J., Struthers, P., and Johnson, W. 1978. In “Reclamation of Drastically Disturbed Lands” (F. W. Schaller and P. Sutton, eds.), pp. 11-22. Am. SOC.of Agron., Madison Wisconsin. Pedersen, T. A., Rogowski, A. S., and Pennock, R., Jr. 1980. Soil Sci. SOC. Am. J. 44, 321-328. Peterson, J. R., and Gshwind, J. 1972. J. Environ. Qual. 1, 410-412. Peterson, J. R., and Gshwind, J. 1973. In “Research and Applied Technology Symposium on Mined-Land Reclamation,” pp. 187-1%. Bituminous Coal Research, Monroeville, PA. Peterson, J. R., Pietz, R. I., and Lue-Hing, C. 1979. In “Utilization of Sewage Effluent and Sludge on Forest and Disturbed Land” (W. E. Sopper and S. N. Kerr, eds.), pp. 359-368. Pennsylvania State Univ. Press, University Park, PA. Peterson, J. R., Lue-Hing, C., Gshwind, J., Pietz, R. I., and Zenz, D. R. 1982. In “Land Reclamation and Biomass Production with Municipal Wastewater and Sludge” (W. E. Sopper, E. M. Speaker, and R. K. Bastian, eds.), pp. 322-338. Pennsylvania State Univ. Press, University Park, Pa. Plank, C. O., and Martens, D. C. 1973. J. Soil Water Conserv. 28, 177-179. Plass, W. T., and Capp, J. P. 1974. J. Soil Wuter Conserv. 29, 119-121. Plass, W. T., and Vogel, W. G. 1973. “Chemical Properties and Particle Size Distribution of 39 Surface-Mine-Spoilsin Southern West Virginia.” USDA Forest ServiceResearch Paper NE-276. Forest Exp. Sta., Upper Darby, PA. Rees, W. J., and Sidrak, G. H. 1956. Planf Soil 8, 141-157. Riley, C. V. 1973. In “Research and Applied Technology Symposium on Mined-Land Reclamation,” pp. 159-177. Bituminous Coal Research, Monroeville, PA. Ruffner, J. D. 1978. “Plant Performance on Surface Coal Mine Spoil in Eastern United States.” USDA Soil Conservation Service SCS-TP-155. Morgantown, West Virginia. Rushton, S. P. 1986. Pedobiologia 29, 27-32. Smith, R. M., Tyron, E. H., and Tyner, E. H. 1971. “Soil Development on Mine Spoil.” West Virginia Univ. Agric. Exp. Stn. Bull. 604T, Morgantown. Smith, R. M., Grube, W., Arkle, T., and Sobeck, A. 1974. “Mine Spoil Potentials for Soil and Water Quality,” EPA 67012-74470. U.S. Environmental Protection Agency, Cincinnati, Ohio. Smith, R. M., Sobek, A. A., Arkle, T., Jr., Sencindiver, J. C., and Freeman, J. R. 1976. “EXtensive Overburden Potentials for Soil and Water Quality,” EPA-600/2-76-184. U.S. Environmental Protection Agency, Cincinnati, OH.

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Sommers, L. E. 1977. J. Ennviron. Qual. 6 , 225-239. Standen, V., Stead, G. B., and Dunning, A. 1982. Pedobiologio 24, 57-64. Stroo, H. F., and Jencks, E , M. 1982. Soil Sci. Soc. Am. J. 46, 548-553. Struthers, P. H. 1964. Ohio J. Sci. 64, 125-131. Struthers, P. H., and Vimmerstedt, J. P. 1965. Ohio Rep. 50, 99-101. Stucky, D. J., and Bauer, J. 1979. In “Utilization of Municipal Sewage Effluent and Sludge on Forest and Disturbed Land” (W. E. Sopper and S. N. Kerr, eds.), pp. 379-387. Pennsylvania State Univ. Press, University Park, PA. Stucky, D. J., and Zoeller, A. L. 1980. Reclamation Rev. 3, 141-147. Stucky, D. J., Bauer, J. H., and Lindsey, T. C. 1980. Reclamation Rev. 3, 129-139. Sutton, P. 1970. Ohio Rep. 55, 99-101. Sutton, P. 1979. Ohio Rep. 64, 72-75. Sutton, P. 1983. Ohio Rep. 68, 13-14. Sutton, P., and Vimmerstedt, J. P. 1974. Compost Sci. 15, 22-23. Tate, R. L., and D. A. Klein (eds). 1985. “Soil Reclamation Processes, Microbiological Analyses and Applications.” Dekker, New York. Townsend, W. N., and Hodgson, D. K. 1973. In “Ecology and Reclamation of Devastated Land” (R.J. Hutnik and G. Davis, eds.), Vol. 1, pp. 45-56. Gordon & Breach, New York. Tyner, E. H., and Smith, R. M. 1945. Soil Sci. Soc. Am. Proc. 10. 429436. Tyner, E. H., Smith, R. M., and Galpin, S. L. 1948. J. Am. Soc. Agron. 40, 313-323. Urie, D. H., Losche, C. K., and McBride, F. D. 1982. In “Land Reclamation and Biomass Production with Municipal Wastewater and Sludge” (W. E. Sopper, E. M. Speaker, and R. K. Bastian, eds.), pp. 386-398. Pennsylvania State Univ. Press, University Park, PA. U.S. Department of Agriculture. 1954. “Agriculture Handbook No. 60.” USDA, Washington, D.C. U.S. Environmental Protection Agency. 1976. “Erosion and Sediment Control, Surface Mining in the Eastern U.S., Vol. 1,” EPA-625/3-76-006. U.S. Government Printing Office, Washington, D.C. U.S. Environmental Protection Agency. 1983. “Process Design Manual for Land Application of Municipal Sludge,” EPA-625/1-83-016. U.S. Environmental Protection Agency, Cincinnati, OH. Vimmerstedt, J. P. 1983. In “Earthworm Ecology: From Darwin to Vermiculture” (J. E. Satchell, ed.), pp. 229-240. Chapman & Hall, New York. Vimmerstedt, J. P., and Struthers, P. H. 1968. In “Second Symposium on Coal Mine Drainage Research,” pp. 152-163. Bituminous Coal Research, Monroeville, PA. Vogel, W. G. 1975. Natl. Coal Assoc. Symp. Surf. Mining Reclamation 3rd 2, 152-170. Vogel, W. G. 1981. “A Guide for Revegetating Coal Minesoils in the Eastern United States.” Gen. Tech. Rep. NE-68. Northeastern Forest Experiment Station, Broomall, PA. Vogel, W. G., and Berg, W. A. 1968. J. Soil Water Conserv. 23, 89-91. Watson, M. E., and Hoitink, H. A. J. 1985. Ohio Rep. 70, 19-21. Wilson, H. A. 1965. “The Microbiology of Strip-Mine Spoil.” W. Va. Univ. Agric. Exp. Stn. Bull. 506T, Morgantown. Woodruff, J. M.. and Blaser, R. E. 1970. “Establishing and Maintaining Turf on Steep Slopes along Virginia Highways.” Annual Report Virginia Dep. of Highway and U.S. Bureau of Public Roads, Dep. of Agronomy, Virginia Polytechnic and State Univ., Blacksburg. Wright, D. L., Perry, H. D., and Blaser, R. E. 1978. I n “Reclamation of Drastically Disturbed Lands” (F.W. Schaller and P. Sutton, eds.), pp. 553-582. Am. SOC.of Agron., Madison, Wisconsin.

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ADVANCES IN AGRONOMY, VOL. 41

WATER AND QUALITY LOSS DURING FIELD DRYING OF HAY Andy D. Macdonald and E. Ann Clark Department of Crop Science, University of Guelph Guelph, Ontario, Canada N1G 2W1

I.

INTRODUCTION

The challenge faced by producers of dry hay can be simply stated. A hay producer must (1) take herbaceous vegetation containing roughly four units of water for each unit of dry matter (DM), which would amount to 10 tons/ha' of water in a hay crop yielding 3 tons/ha (18% FW basis); (2) manage it so that 95% of the water evaporates as quickly as possible; then (3) collect, package, store, and use the dry hay with minimal losses of DM and nutritional content. Why, then, is haymaking considered such a risky proposition, given that a well-watered stand of forage can transpire twice this amount of water on a single, bright summer day (Thompson, 1981)? How can it be that some 75% of the hay produced in England and Wales is unable to provide ruminant maintenance requirements (Charlick et al., 1980)? What accounts for the lessening of interest in dry hay, and the concomitant increase in silage in Great Britain, as documented by Wilkinson (1981), and elsewhere in recent years? What are some of the conflicts between water loss and nutrient conservation? How will recent innovations in haymaking equipment affect both biological and economic efficiency? The status of haymaking technology has been thoroughly reviewed in recent decades (Carter, 1960; Klinner and Shepperson, 1975; Wilkinson, 1981), with additional excellent contributions in the physiology of water loss (Harris and Tullberg, 1980) and modeling of field drying (Thompson, 1981). The present study surveys current knowledge of how haymaking practices influence water loss and accompanying dry matter and quality losses, with an emphasis on recent work.

II. PATTERNS OF WATER LOSS A.

SAFE STORAGE MOISTURE FOR DRYHAY

Moisture content may be expressed on a fresh weight (FW) basis, such as that hay weighing 6 tons/ha at 50% moisture (FW basis) would contain 'Tons are metric

tons. 407 Copyright 0 1987 by Academic Press, Inc. All rights of reproduction in any form reserved.

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50% of 6 tons or 3 tons/ha of water. Alternatively, water content can also be expressed relative to dry matter (DM) content, such that the same hay would have equal parts water and DM, or a moisture content of 1.0 (DM basis). In this chapter, moisture content will be expressed on a FW basis, except where specifically noted. The objective of haymaking is to conserve the yield and nutritional value of fresh-cut forage by drying it as quickly as possible to a level at which the activity of microbial decomposers is halted. The moisture content theoretically required to prevent microbial activity is 10-12% (Nash, 1978), but in practice, dry hay is baled and stored at from 15 to 20% moisture (Feldman and Lievers, 1978; Friesen, 1978; Jones and Harris, 1980), which incurs low but acceptable levels of microbial damage. Safe moisture content for dry hay storage varies with time of year, and with type of bale package. In Great Britain, Nash (1978) indicated that dry hay could be safely packaged at a somewhat higher moisture content for winter than for summer storage (18 versus 16%, respectively), because cold winter temperatures would reduce the activity of fungal decomposers. Required moisture content for safe storage is inversely related to the size of the bale package, such that small rectangular bales may be safely harvested a few percentage points below that required for large round bales (Scales et al., 1978) (see Section IV,B,S).

B. PHASES OF DRYING Rate of water loss during field drying of cut hay typically declines exponentially, such that each additional percentage drop in moisture content requires progressively more time (Firth and Lesham, 1976; Robertson, 1983; Hale, 1986). Although the pattern of loss is a smooth curve under constant environmental conditions, the drying cycle may be conveniently divided into two or three different phases, which differ in duration, in rate of water loss, and in type of resistances to water loss. The rapid initial drying phase occurs when the stomata are open, plant resistance is minimal, and the vapor pressure deficit (VPD) between plant tissues and ambient air is maximal. As this phase is often brief and can be difficult to detect, it is typically considered within the next phase, when the stomata have closed and cuticular, stomatal, and boundary layer resistance become significant barriers to water loss. Water loss can still be rapid in this second and longer phase, if efforts are made to reduce forage packing with periodic disturbance. Heavy, highmoisture hay, particularly if composed of juvenile, leafy tissues rather than stiff, reproductive stems, tends to compact or settle under its own weight, reducing air circulation within the windrow and increasing boundary layer

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resistance (see Section III,C,l). The importance of opening up or disturbing the windrow at this time is shown by Nash (1978), who cited rates of water loss in this phase ranging from 0.5 to l%/hr (FW basis) in undisturbed swaths, to 2%/hr in tedded swaths, to 3%/hr for swaths subjected to both conditioning and tedding (see Sections IV,B,2 and 4). The final drying phase, which commences when moisture content reaches about 45% (Nash, 1978), is less responsive to drying management and more sensitive to the weather than are previous phases. In the often protracted, final drying phase, water bound by osmotic and matric forces is increasingly difficult to dislodge, although progressive water loss can also make the windrow itself lighter and more open, reducing boundary layer resistance. Protracted drying also retards subsequent regrowth, and hence, timing of harvest and N regimes, which can encourage windrow damage and weed invasion.

111.

FACTORS INFLUENCING WATER LOSS DURING FIELD DRYING OF HAY A.

CLIMATIC FACTORS

Predictive models of forage drying have identified irradiance as having the greatest influence on both alfalfa (Medicago sativa L.) and grass drying rate, with other significant contributions from the drying potential of the environment, expressed as potential evapotranspiration (PET) or vapor pressure deficit (VPD), soil moisture, and windrow density (Dyer and Brown, 1977; Thompson, 1981; Rotz and Chen, 1985; Savoie and Mailhot, 1986). As noted by Thompson (1981), “The old saying urges the initiation of haymaking when the flux density of direct short-wave radiation is large, and experience confirms that this is usually good advice.” The problem is that good haymaking weather is typically infrequent and unpredictable. In southern England, Murdock (1980) reported that there are an average of 2-2.5 periods of good drying weather lasting 3 days or more in the interval between May and August. The average length of the favorable drying intervals is 6-8 days. Dyer and Brown (1977) simulated the probability of successful hay drying in four different regions of Ontario, based on rainfall, temperature, and irradiance data collected between 1921 and 1970. For the time interval corresponding to recommended first hay cut, the probability of making dry (23%) hay in 4 days or less declined from a high of 50% at the southernmost site to 35% at the northernmost site (42-49” latitude). The uncertainty of both the timing and duration of good haymaking weather and the strong weather dependence of dry hay production contribute to the risks associated with haymaking.

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Hay crops are typically cut at various times during the season, and required drying time to achieve safe baling moisture varies during the season. Dyer and Brown (1977)calculated that the probability of making dry hay (23%) varied among the three harvest intervals during the season at each of four locations in Ontario. At Guelph (43"Nlatitude), for example, the probability of making dry hay in 4 days or less varied from 50% in early June, to 60% in late July, down to 5% in early September. They concluded that to have at least a 50% probability of reaching safe baling moisture in the Guelph area would require 4 drying days for May, June, and July, 5 days for August, and 7 days for the first half of September, values which correspond fairly well to observed practice. Relative humidity (RH) is one environmental factor which warrants special emphasis because its influence of hay drying is often underestimated. The RH which creates the gradient which influences water loss from drying hay is that within the windrow adjacent to the plant tissue (Clark and McDonald, 1977; Thompson, 1981). Under controlled drying conditions, alfalfa arrayed in a thin layer reached 20% moisture in 25 hr at 45% RH, but required 47 hr, or almost twice as long, at 70% RH (Crump, 1985). When treated with K2C03,the influence of RH was comparable, such that attaining 20% moisture required 6.2versus 11.2 hr at 45 and 70% RH, respectively. Because hay is hygroscopic or water absorbing, RH also influences the equilibrium moisture content of hay approaching safe moisture storage levels. Nash (1978)calculated that air of 80% RH will be in equilibrium with hay at 22% moisture. Similarly, 60% RH air sets the lower limit of hay moisture at 13YO.Diurnal trends in RH partition the day into periods which are favorable and unfavorable for water loss (Clark and McDonald, 1977). Dew condensation onto the surface of drying hay increases as the hay dries (Tullberg and Minson, 1978), such that, particularly under cool, fall conditions, daily drying rate becomes a balance between nighttime rewetting and daytime evaporation (Crump, 1985). Treatments which cause fastest drying also allow fastest rewetting, thus eliminating any advantage gained during the daytime (Feldman and Lievers, 1978). Wilman and Owen (1982)noted that rewetting is greater in an immature than in a mature grass hay.

B. PLANTFACTORS Detection and quantification of the various components of plant resistance to water loss typically require drying under single-stem or thinlayer conditions, to avoid boundary layer effects. In the field, however, the relevance of plant factors in retarding overall drying varies with the magnitude of other factors, especially windrow resistance.

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I.

41 1

Stomatal Resistance

When fully open and intact, a plant leaf acts as a freely evaporating surface, allowing water to move readily through the system (Williams and h e r , 1957). Estimates of stomatal density for the upper and lower surfaces of alfalfa leaves have varied from 1090 and 880 stomata/mm2, respectively (cited in Pedersen and Buchele, 1960) to 169 and 188 stomata/mm2 (cited in Harris and Tullberg, 1980). Stomatal density on grass leaves appears to be an order of magnitude less than that on legume leaves (various, cited in Harris and Tullberg, 1980). The resistance of open stomata varies from 50 to 500 sec/m for mesophytic plants (Nobel, 1983). When hay is placed in a windrow, progressive water loss and shading within the windrow block encourages stomatal closure, which increases resistance to water loss by one to two orders of magnitude (Harris and Tullberg, 1980). Although stomata have been reported to close as early as 1 hr after cutting in grasses (Murdock, 1980), or at a moisture content of 2.3 kg water/kg DM (70% FW basis) (Savoie et a/., 1984), approximately 20-30070 of water loss occurs prior to stomatal closure (Harris and Tullberg, 1980). Because stomatal closure applies a significant brake to transpirational water loss, efforts have been made to keep stomata open artificially, using applications of fusicoccin, a toxin produced by the fungus Fusicoccum amygdali Del., which wilts plants by preventing stomatal closure (Morris, 1972; Turner, 1970), and other chemicals (Harris and Tullberg, 1980). Under controlled drying conditions, alfalfa to which fusicoccin had been applied 3 hr prior to cutting reached 40% moisture in half the time required by unsprayed control shoots (Turner, 1970). In field-dried trays, fusicoccin reduced drying time to reach 22% moisture from 54 to 46 hr, or in effect, to 3 days instead of 4. Thus, stomatal closure, while significant, was less apparent under field than under controlled drying conditions. 2.

Cuticular Resistance

The cuticle consists of cutin, an extracellular, insoluble polymer, together with associated soluble waxes. The cuticle coats the outer surface of epidermal cells, forming a continuous layer which inhibits moisture loss, as well as protecting against pests. Cuticular resistance is high, ranging from 2500 to 10,OOO sec/m in crop plants (Thompson, 1981: Nobel, 1983), and as such, the intact cuticle presents a formidable barrier to water loss. The hydrophobic nature of the cuticle comes from fatty acid groups, while that of the waxes is due to long-chain fatty acids esterified with long-chain monohydric alcohols

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(Kolattukudy, 1981). Because of the hydrophobic nature of the cuticle, most biocide, growth regulator, or desiccant formulations contain a detergent or wetting agent to facilitate uptake. The location and chemistry of cuticular waxes have been examined for white clover (Trifolium repens L.) and red clover (T. pratense L.) (Hall, 1967), and for perennial ryegrass (Lolium perenne L.) and orchardgrass (Dactylis glomerata L.) (Harris et al., 1974). Species-specific differences in wax composition arise from variations in the alcohol, fatty acid, and straight-chain hydrocarbon constituents (Chibnall et al., 1933; Hamilton and Power, 1969). Removing or modifying the cuticle to reduce its effectiveness as a moisture barrier increases drying rate (Dernedde, 1980; Harris and Shanmugalingam, 1982). Altering the surface waxes without modifying the wax in the internal layers of the cuticle increased the drying rate of red clover leaves, suggesting that external waxes may exert an overriding influence on drying rate (Hall and Jones, 1961). Physical removal of the epidermis and adhering cuticle increased drying rate of red clover leaflets, petioles, and thick stems by as much as 13-, 11-, and 4-fold, respectively, compared to that of intact tissues (Harris and Shanmugalingam, 1982). In red clover, Harris and Shanmugalingam (1982) found that younger, thinner stems (2-mm diameter) dried more slowly than older, thicker stems (4-mm diameter), but when the epidermis was removed, drying rate of thin stems was twice that of thick stems. They concluded that the permeability of the cuticle to water increases with age in spite of the fact that the thickness of the cuticle also increased with age. Harris and Tullberg (1980) and Meidner (1986) cited evidence that the thickness of the cuticle per se does not appear to relate to its effectiveness in water retention. Wieghart et al. (1983) inferred that weathering would have depleted the protective cuticle to a greater extent in older than in younger alfalfa leaves, thus accounting for faster drying in more mature plants. Meidner (1986) demonstrated great variations in cuticular conductance between and within species, depending on age and growing environment. Significant genetic and environmental influences on epicuticular wax content of alfalfa cultivars and clones were reported by Galeano et al. (1986). As a methodological note, the influence of the growing environment on cuticular formation is of interest. Meidner (1986) reviewed literature indicating that cuticular conductance varies with age, with temperature and irradiance during growth, and with degree of hydration of the leaf. Hull (1958) reported that greenhouse-grown plants can have a much thinner cuticle than do field-grown plants, although, as noted, thickness is not necessarily predictive of resistance. Potassium carbonate, which reportedly enhances drying by reducing cuticular resistance, reduced drying time required to reach 20% moisture by 77% on field-grown alfalfa (Crump,

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1985). However, the same chemical, applied to the same greenhouse-grown cultivar, which was then dried under an identical temperature, irradiance, and RH regime, reduces drying time by only 47%. Quantitative and/or qualitative changes in the cuticle might be expected to influence responsiveness to conditioning (see Section. IV,B,3) and hence, applicability of results derived from greenhouse-grown plants to the field (Crump, 1985).

3. Intra- and Extracellular Resistance Water retention via osmotic and matric forces increases during the drying cycle (Jarvis and Slatyer, 1970; Firth and Leshem, 1976; Murdock, 1980). Increasing the osmotic potential of leaves of several nonforage species either by direct introduction of glucose or by high irradiance reduced cell permeability to water and decreased transpiration rates (Boon-Long, 1941). While quantitative evidence is limited, it appears that intra- and extracellular factors, apart from stomata1 and cuticular resistance, retard drying primarily in the final drying phase (Jones and Harris, 1980). C. MANAGEMENT FACTORS I.

Windrow Resistance

Freshly cut hay is often piled directly into a central pile, or windrow, rather than being left in a swath where it was cut. Windrowing serves to limit surface area exposed to weathering damage and reduces losses when hay is picked up by the baler. However, concentrating forage into windrows also reduces surface area available for interception of incident irradiance and increases the density of the cut crop to about five times that of the standing crop (Larsen and Rider, 1985). For a 3-ton/ha (18%) crop with an initial moisture content of 8O%, initial windrow density would be 1.25 kg DM and 5 kg water/m2 of windrow surface. In addition, the physical structure of the windrow or swath imposes a constraint on air circulation adjacent to the drying tissues. The frictional drag or aerodynamic constraint is termed boundary layer resistance, as detailed in Clark and McDonald (1977) and Jones and Harris (1980). Thus, practices designed to reduce weathering and enhance quality of conserved feed may, in fact, prolong drying. Jones and Harris (1980) stated that windrow resistances are most limiting early in the drying cycle, while plant resistances become paramount later on. Clark and McDonald (1977) demonstrated differential drying of vertically stratified layers within a grass windrow. The rate of water loss from the surface layer was initially greater than that of the internal layers, due to both greater incident irradiance and a shorter diffusive pathlength. When

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the surface layer dried to the point where plant resistances became overriding, moisture began to move out of the underlying layers, but at a much slower rate. Slower interior drying is caused by the limited penetration of incident energy, the smaller VPD, and the longer diffusive path length within the windrow. Jones and Harris (1980) reported that incident irradiance 2 cm deep in a windrow was only half that at the surface, while only 10% of available energy penetrated to the base. Heterogeneity in drying within a windrow can result in nonuniformity of moisture content within a windrow and, ultimately, within a bale. Using a mixture of perennial ryegrass, timothy (Phleum pratense L.), and white clover, Wilman and Owen (1982) observed faster drying in a wide, thin windrow than in a narrow, thick windrow (220 versus 728 g DM/m2, respectively), but only down to 70% moisture. In later stages of drying, the thinner windrow was disadvantageous, because the broader exposed surface incurred a greater uptake of moisture from the soil, stubble, rain, and dew. They concluded that forage should be swathed and then windrowed at 40% moisture rather that windrowing just prior to baling. 2. Agronomic Factors

Practical methods of facilitating water loss from cut hay include growing grasses and legumes in mixture to lessen the impact of the slower drying legumes. In monoculture, Ciotti and Cavallero (1980) found that drying rate of orchardgrass was faster and haymaking losses were lower than in alfalfa. In mixture, drying rate was faster and haymaking losses were significantly lower than in monocrop alfalfa. In addition to conditioning (see Sections IV,B,2 and 3), other agronomic approaches include (1) leaving a tall stubble to lessen soil contact and promote air movement under and within the windrow; and (2) cutting small fields on each of several days, rather than all at once, to lessen the risk of rainfall damage, as well as prevent overdrying losses. Other alternatives include baling at 3040% moisture for subsequent barn drying (beyond the scope of this paper), or baling at 20-30% moisture in conjunction with chemical preservatives (see Section V).

D. FORAGESPECIES I.

Grasses

Rate and pattern of water loss from plant tissues vary with tissue type, starting moisture content, maturity, and nitrogen (N) fertilization. Due partly to greater evaporative surface per unit tissue volume, leaf blades dry

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faster than stems (Firth and Leshem, 1976). In annual ryegrass (Lolium multiflorum Lam.), Jones (1979) reported that rate of water loss was three time greater in leaves than in stems during the rapid, initial drying phase and seven times greater during the slower, protracted drying phase. By the time the leaves had dried to safe storage moisture, stem water content remained four to eight times greater than that of the leaves, depending on the stage of maturity when harvested. Murdock (1980) concluded that grass leaves dry 10-15 times faster than stems, and further, that as much as 30% of stem water is actually lost through the leaves. Jones (1975) used the analogy of grass leaves as wicks drawing moisture out of the stems until some limiting level of leaf dryness was reached. Using thin-layer drying under controlled environmental conditions, Jones (1979) found that vegetative tillers, in which leaves constituted 80% of total dry weight, dried in less than one-third the time required by tillers at ear emergence, when leaves represented only 40% of dry weight. After ear emergence, however, drying time again decreased due to lower plant water content and increased exposure of the stem to the drying environment (Harris et a/., 1974). Thus, Jones and Harris (1980) concluded that drying rate in vegetative tillers was comparable to that in post-ear emergence tillers. Starting moisture content, or that prevailing when forage is first cut, directly affects both initial drying rate and required drying duration to reach safe storage moisture (Hart and Burton, 1976; Savoie and Mailhot, 1986). As grasses mature, moisture content gradually declines from a high of 85% or more at the leafy, vegetative stage to less than 65% moisture at the seed-set stage (Nash, 1978; various cited in Harris and Tullberg, 1980). A comparable range for alfalfa was reported to be 85-74%, with some variations among tissue types (Harris and Tullberg, 1980). Moisture content varies among species and with time during the season. Starting moisture content in 6-week regrowths varied significantly among five grass species harvested at intervals throughout the growing season (Morris, 1972). Starting moisture content and initial rate of water loss were generally lowest in the fine-leaved sheep fescue (Festuca ovina L.), while starting moisture content was generally highest in perennial ryegrass, particularly in midseason. While young, leafy forage dries quickly when arrayed in a thin layer, it dries more slowly than mature forage when windrowed (Dexter, 1947). Wilman and Owen (1982) concluded that difficulties in drying a young and heavy grass crop have discouraged British farmers from cutting grass early, when it is most nutritious, and, further, have resulted in recommended N levels for grass hay being half that for grass silage. Increasing N from 112 to 637 kg/ha increased starting moisture content of Coastal bermudagrass (Cynodon ductylon (L.) Pers.) by 8% (Hart and Burton, 1976), while 150

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kg/ha N applied to perennial ryegrass increased starting moisture by only 3% over that of the control (Wilman and Owen, 1982). Genetic variation in drying rate both within (Klinner and Shepperson, 1975; Murdock, 1980; Owen and Wilman, 1983) and between species (Harris et al., 1974; Jones and Prickett, 1981; Owen and Wilman, 1983) suggest that rate of grass drying may be enhanced by breeding. Using individual cultivars of each species, Jones and Prickett (1981) found that tall fescue (Festuca arundinacea Schreb.) dried faster than timothy, perennial ryegrass, and annual ryegrass, under lab conditions. The deeper root system of tall fescue was interpreted as lessening the need for a heavy cuticle to act as a barrier to water loss, thus indirectly enhancing drying rate. In comparisons involving 140 grass cultivars from seven grass species, harvested at hayable maturity for up to 3 years, Owen and Wilman (1983) also found that tall fescue dried fastest, followed in order by annual ryegrass = meadow fescue (FestucapratensisL.) > timothy = orchardgrass > perennial ryegrass = hybrid ryegrass (L. perenne x L. multiforum). Perennial and hybrid ryegrass required twice as long as tall fescue to reach hayable moisture. In comparisons involving a wide range of leaf types on vegetative tillers, Morris (1972) noted that drying rate increased with increase in the surface area to dry weight ratio, or for redtop (Agrostis giganteu Roth.), with the presence of stolons. Grass species with broad leaves were considered easier to dry than perennial ryegrass or sheep fescue. 2. Legumes

Morphological differences between grasses and legumes, including a higher initial moisture content and a larger proportion of stem to leaf dry weight, typically cause legumes to dry more slowly than grasses (Jones and Harris, 1980). Because of their greater perceived nutritional value and the high cost of chemical N, legumes are often managed more intensively than grasses in North America, although the converse is true in Europe. Legumes may be sown and fertilized with greater care, resulting in greater stand density and yield, and further, are typically harvested at a younger stage, resulting in hay of greater nutritional potential. However, managing for higher yields and lesser maturity at harvest may also cause higher initial moisture content and slower drying. Thomas et al. (1981) reported that grasses dried faster than alfalfa and mature alfalfa dried faster than alfalfa cut at early bloom stage, as confirmed by Wieghart et al. (1983). Differences in initial moisture content among legume species influenced treatment comparisons in a study by Crump (1985). Correcting for such differences using initial DM content as a covariate revealed treatment effects not apparent in the original data.

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Species-specific differences in drying rate affect the feasibility of successful haymaking. For example, alfalfa dries relatively quickly, and is thus the species of choice for haymaking, while the much slower drying of red clover encourages its conservation as silage rather than hay. In a comparison involving alfalfa, sweetclover (Melilotus alba Desr.), sainfoin (Onobrychis viciaefolia Scop.), alsike clover (Trifolium hybridum L.), and red clover, Clark et al. (1985) reported that drying time to 20% moisture varied from 44 to 86 hr among species. Drying was fastest in alfalfa and slowest in red clover. Stem diameter accounted for 47% of the variation in drying rate. Qualitative differences between the cuticles of grasses and legumes were inferred by Chung and Verma (1986), who reported that chemicals which accelerated drying in leguminous crops had no effect on grass crops. Harris and Tullberg (1980) reviewed the literature on stem-to-leaf transfer of water and concluded that leaf transpiration substantially affects stem moisture loss. Tullberg (1975) demonstrated that at least 35% of stem water was lost through the leaves in alfalfa, and that the stem-to-leaf pathway remained viable until plant moisture content approached 40%. Stem moisture loss through leaves was also inferred by Clark et al. (1985), who contrasted water loss in the initial and final phases of drying (see Section I1,B) of internode sections of red clover. In intact sections, rate of water loss was 153% faster in the initial than in the final phase of drying. When blades were removed, however, rate of drying did not differ between the two phases, an effect which was independent of internode age.

IV.

LOSSES DURING FORAGE CONSERVATION, STORAGE, AND HANDLING

Carter (1960) summarized much of the earlier work detailing DM and nutrient losses in haymaking. Based on nine published trials comparing field- with barn-cured hay, total losses between cutting and feeding averaged 25 and 15%, respectively. In eight of these trials for which data was available, average loss of crude protein (CP) in field-dried hay was 34%, or 25% greater than DM loss. Wilkinson (1981) summarized results of nine trials showing that in vivo DM digestibility of field-dried hay was 8.1 percentage units lower than that at cutting (64.4 versus 72.5'7'0, respectively). In addition, based on 26 comparisons, intake (of digestible DM) of field-cured hays was 18% lower than that of the same grass at cutting. Dulphy (1980) also reported an 18% reduction in intake of field-dried hay, relative to that of fresh material. Reduced intake was associated with decreased soluble constituents and increased cell wall contents, which were attributed to respiration, leaf loss, and weathering damage occurring in the field.

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The magnitude of these losses in yield and quality of forage during drying and haymaking underscores the inherent conflict between managing for loss of water while striving to retain yield and quality. Losses of dry matter and nutrients in the conservation process may be considered in three phases: respiratory and weathering losses during drying; harvest losses associated with cutting, conditioning, raking, tedding, and baling; and storage and handling losses. A.

RESPIRATORY AND WEATHERING

LOSSES

Plant and microbial respiration throughout field drying can reduce harvested yield by 2-8% under good drying conditions and by 16% under poor conditions (Klinner and Shepperson, 1975). When drying is delayed by extremely wet and humid conditions, as much as 30% of initial DM can be lost due to respiration (Rees, 1982). Tullberg (1975) suggested that respiration losses in bulk samples of alfalfa can reach a maximum of 4% DM loss per day, independent of plant maturity. However, photosynthesis may also continue after cutting, partially offsetting respiratory losses (Greenhill, 1959).

Following cutting, plant respiration continues, but at a declining rate (Wood and Parker, 1971) -until plant moisture content reaches about 3 0 4 % (May-Brown and Harris, 1974; Martin, 1980; and others cited in Klinner and Shepperson, 1975). Even when whole plant moisture content has reached 30-40%, the slower-drying parts of the plant will continue to respire until they too reach that point (Rees, 1982). Rewetting due to dew or rainfall prolongs respiration and increases overall loss. Using thin-layer drying, Simpson (1961) found that crushing forage stems stimulated respiration, but as crushing also accelerated drying and caused respiration to cease earlier, overall respiration losses were reduced. Protracted field drying exposes hay to potential leaching and weathering losses, which can significantly reduce not simply DM but also digestibility of the conserved forage. Leaching of soluble nutrients from the cut plant material is the principal component of weathering damage, followed by leaf and bloom loss and molding (Hill, 1976). Timing of rainfall is perhaps more critical than amount, as tolerance to rainfall declines with drying time. The relative integrity of the cuticle and cellular membranes in fresh-cut forage prevents loss of soluble nutrients such as nonstructural carbohydrates and potassium (Murdock and Bare, 1963). Loss of digestibility, amounting to 5 percentage units or more of digestible organic matter (Nash, 1978), and decreased voluntary intake can accompany even modest rainfall. In 21 comparisons between grass hays which were field-cured with and without rainfall damage, Wilkinson (1981)

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reported a decrease in digestible DM intake of 8 percentage units. Following rainfall ranging from 25 to 38 mm on grass and grass-legume hay, DM intake was reduced by 5 and 20 g/kg0.75,respectively, and DM digestibility was reduced by 4 and 8.8 percentage units, respectively (Milligan, L. P., Mathison, G. W., Weisenberger, R. D., and Kennedy, P. M. 1981. Forage damage-effects on feeding value. Univ. of Alberta. Unpublished mimeo). Incorporating both effects, digestible DM intake was reduced, on average, by 12% and 30% in the grass and grass-legume hays, respectively. Collins (1983) reported that susceptibility to weathering-induced reductions in DM and quality varied between alfalfa and red clover, and also with the timing and duration of wetting. Simulated rainfall of 2.5 cm applied after 24 hr of field drying increased mean DM loss from 8.1% for the unwetted control to 17% and as much as 25.8% for red clover cut at first flower. Rainfall of 4.1 cm over a 4-day period increased mean DM loss from 10.5% for the control to 43.4%, with concomitant reductions in in vitro DM digestibility (IVDMD) from 66.4 to 48.1%. Reductions in total nonstructural carbohydrates were greater than those of N or IVDMD. Collins (1983) reviewed literature suggesting that legumes are more sensitive to weathering damage than are grasses.

B. HARVESTING LOSSES Losses of dry matter and nutritional value during drying and baling depend on the type of mower used for cutting, on the postcutting or conditioning treatment, on timing and method of raking and tedding, and on the type of baler and hay moisture content at harvest (Klinner and Shepperson, 1975). 1. Mowing

a. Equipment. The process of cutting or mowing a standing crop of forage may be accomplished with a variety of implements, some of which are self-propelled, while others are pulled behind or beside a tractor. Cutting width of these units is fixed, typically ranging from 2.0 to 4.2 m, a characteristic which influences not simply the number of passes needed to cut a field, but also the density of the resulting windrow. Concentrating forage from a 4.2-m cutting width into a l-m windrow results in a heavier, more compactible windrow than does that from a 2.0-m cutting width. Unlike cutting width, cutting height is adjustable on all units and typically ranges from 5 to 10 cm aboveground. Cutting height influences stubble height, which in turn, influences soil contact and boundary layer resistance (see Section IlI,C,l). In addition, height of cutting affects both yield and

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quality of harvested forage. Taylor and Rudman (1965) reported an 11-15% reduction in growth rate of steers when offered grass hay cut at 6.5 versus 10 cm aboveground, presumably due to a greater contribution of older stem tissue in the lower cut forage. The term swather is often used to mean a self-propelled device which simply cuts and lays down forage either in a swath or in a windrow, and as such, may serve a dual purpose for cutting small grains or rapeseed (Brassicasp.). A subsequent improvement was the mower-conditioner (m/c), which is either self-propelled or pulled behind a tractor. As the name implies, the m/c conditions (see Section IV,B,2), as well as cuts the forage. The cutting action for both the swather and the m/c is typically provided by a reciprocating, linear cutter bar, which relies on the rapid lateral motion of a toothed knife against a stationary guard to cut standing forage. Newer m/cs can also be fitted with disk-type rotary mowers instead of the traditional cutter bar. The cutting action in a flail-type mower relies on a series of freeswinging, hardened steel flails, which are attached to a rapidly spinning central cylinder oriented parallel to the ground. Flail-type mowers lacerate and chop the cut forage into 10- to 20-cm lengths, which has the useful effect of thoroughly mixing old with young plant parts, reducing selective feeding by steers (Taylor and Rudman, 1965). A more recent innovation is the rotary mower, which appears to be replacing the cutter-bar mower, especially in Europe. Rotary mowers use a system of rotating disks or drums with pivoting knife sections to replace the reciprocating knife of the conventional cutter-bar mower. b. Losses. Klinner (1975) showed that DM losses were slightly less when hay was cut with a reciprocating or cutter-bar mower than with a rotary mower, possibly due to the greater physical damage inflicted on plant tissues by repeated, high-speed contact during rotary mowing. Results cited in Svensson (1978) were consistent with those of Klinner (1975), and it was further noted that a flail-type mower caused considerably more DM loss than did a rotary mower, again due to repeated laceration of the crop. Svensson (1978) reported that DM losses were 5-20% for a conventional cutter-bar mower, 5-25% for a rotary mower, and 1540% for a flail-type mower when all machines were operated at a typical operating speed of 6.1 km/hr. Von Bargen (1978) measured slightly higher DM losses with a rotary mower than with a conventional cutter-bar mower when both were operated at 6.4 km/hr. When travel speed was increased by 50%, however, DM loss increased with the cutter-bar mower but was unaffected with the rotary mower. Reducing the flail speed or increasing the forward speed of flailtype mowers may decrease yield in heavy crops due to an increase in stubble height. Effective stubble height is increased because the flails push the material over, making contact higher on the stem. At the same time,

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42 1

however, DM losses are reduced with a lessening of flail speed or an increase in travel speed because of concomitant reductions in laceration of plant material. The primary advantage of rotary mowers lies in their ability to handle heavily fertilized or lodged forage and to avoid stone damage, thus requiring less frequent repairs. Because of these advantages, Klinner and Shepperson (1975) found that the newer horizontial rotary mowers of the disk and drum type were replacing cutter-bar mowers in Great Britain, despite their higher cost and slightly higher tractor power requirement. Clothier and Taylor (1980) compared drying time of a perennial ryegrass and white clover mixture cut by disk- versus drum-type rotary mowers, both with and without conditioning treatments. When unconditioned, drying was faster in swaths cut with the disk- than with the drum-type mower. When conditioned, drying time to safe baling moisture was unaffected by mower type. Differences in swath architecture, with that produced by the disktype mower having a lower bulk density, could encourage faster drying in windrows cut by a disk- compared to a drum-type rotary mower (Clothier and Taylor, 1980). 2. Mechanical Conditioning

Klinner and Shepperson (1975) define crop conditioning as a mechanical or chemical action designed to increase the rate of evaporation of crop moisture. In practice, mechanical conditioning reduces cuticular resistance to water loss by crushing or crimping (Greenhill, 1959; Clothier and Taylor, 1980) or by surface abrasion (Klinner and Hale, 1980). In addition, the process of propelling the conditioned forage backwards from the rollers, such that it strikes the apron and is shaped into a windrow by the adjustable baffles, produces an open and airy structure, which minimizes boundary layer resistance. Conditioning is typically accomplished at the same time as mowing, often using the same machine. The equipment which is used to condition by crushing is termed a mower-conditioner or haybine. Crushing is a process by which plant material is pulled through a pair of solid, high-speed, metalor rubber-coated rollers, often of an intermeshing herringbone design, which split or damage the stem longitudinally. Conditioning by crimping uses a device called a crimper, which is similar to a m/c, except that the open rather than solid rollers cause the cuticle to be broken laterally. Surface abrasion differs from crushing in that the waxy cuticle of the leaves and stems is modified or damaged, typically with tufted brushes, but the stem itself is not split open. Abrasion is caused by unequal brush speed as the plant material passes between opposing rollers.

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Under good drying weather, conditioning may increase drying rate by as much as 75% during the initial stages of drying (Dernedde, 1980), with a lesser effect in later stages. First-cut crops, which tend to be thicker stemmed, often respond well to mechanical conditioning, while the finer stems characteristic of later cuts may pass through the conditioner more or less untreated. The benefit of conditioning may not be observed if the drying environment is poor or the windrow is heavy (Crump, 1985) and is usually more apparent on legume than on grass crops, as shown by Ciotti and Cavallero (1980). They found that rate of moisture loss in orchardgrass (0.34% moisture per saturation deficit hour) (SDH) was unaffected by conditioning, while that of alfalfa increased from 0.19 to 0.27% per SDH. The main advantage of surface abrasion over crushing or crimping is that the structural integrity of the windrow remains intact, leaving the windrow more resistant to compaction. A weakly structured windrow will constitute greater boundary layer resistance, thus retarding outward passage of water evaporating from the inner plant tissues. Dernedde (1980) compared drying patterns in forage cut with a two-drum mower and then conditioned by either crushing with a m/c or by surface abrasion with a tedder (see Section IV,B,4). Conditioning by crushing weakened stem strength, which promoted windrow collapse and slower drying. Conversely, while tedding tended to leave a light and fluffy windrow, it was not possible to vary the degree of abrasion applied, resulting in excessive leaf shattering. Equipment described by Klinner and Hale (1980) used tufted or solid plastic rotary brushes instead of metal or rubber rollers to condition the crop, which reduced total power requirement by 25% compared to a metal spoke rotor, and achieved similar drying rates (Klinner and Hale, 1980). Mechanical conditioning has been reported to increase leaching losses if the cut hay is subjected to rainfall during the drying cycle (Von Bargen, 1978; cited in Harris and Tullberg, 1980). Both increased microbial degradation and shattering losses during subsequent raking, baling, or handling have also been associated with conditioning (Martin, 1980). However, overall DM losses in conditioned hay are typically less than in unconditioned hay because the risk of additional leaching and shattering losses is more than offset by decreased drying time, which reduces respiration and weathering losses (Murdock and Bare, 1963). 3.

Chemical Conditioning

Chemical conditioning refers to the use of chemical agents which reportedly alter cuticular resistance and thus enhance drying rate. Faster drying improves both yield and quality of hay by reducing duration of respiratory losses, lessening the risk of weathering damage, and hastening hay removal, which encourages faster regrowth. Paraquat, a bipyridyl

WATER AND QUALITY LOSS DURING DRYING OF HAY

423

herbicide, (Arnold and Barrett, 1978), potassium carbonate (K,CO,) (Tullberg and Angus, 1972; Clark et al., 1985; Rotz and Davis, 1986a,b; Crump, 1985), methyl esters (Wieghart et al., 1983), and various other chemicals reviewed in Harris and Tullberg (1980) have been tested as potential drying agents. Concerns about chemical conditioners typically involve (1) possible side effects on animal acceptability (2) the excessive amounts of water required to carry and disperse the product, and (3) nonuniformity of application. The effect of chemical conditioners on animal intake is product specific and needs further testing. Early work with K,CO, involved application rates as high as 400-800 literslha (Tullberg and Minson, 1978; Wieghart et al., 1983), a volume of water which constitutes either significant additional weight or frequent tank refilling. Reducing application rate to more feasible volumes of 150-300 literdha, as recommended for some of the newer products, reduced product effectiveness (Rotz and Davis, 1986a). When higher concentrations of the active ingredients, modifications in pump pressure, and changes in nozzle type failed to compensate for lesser application rates, it was concluded that high carrier volumes may be necessary for effective performance. Uneven coverage of fresh-cut forage by either liquid or powder formulations, due to both variation in windrow density and applicator malfunction, creates pockets of overly moist hay in an otherwise dry bale. Windrow density varied by a factor of 3.4:l in a trial at a British research station, although 99% of the variation was within f 15% of the mean (Charlick et al., 1980). In the same trial, 95% of the variation in moisture content was within *2Vo of the mean, a factor of critical importance when application rate varies with moisture content of the forage. The resulting heterogeneity in bale moisture, which would probably be larger under farm conditions, creates pockets of mold which can spread outward, causing unacceptable variation in hay quality within and among bales. Solution of this problem will await improvements in applicator design and methodology (Klinner and Holden, 1978; Charlick et al., 1980; Rotz and Davis, 1986a). For example, K2C03solutions were found to work most consistently when applied in conjunction with a roll-type conditioner, in part because the rollers distributed the chemical more uniformly as the forage passed through (Rotz and Davis, 1986a). However, variations in nozzle type, including flat-fan, hollow-cone, and solid-cone nozzles, at both high and low pump pressure did not influence drying rate. Solutions of K,CO, have been found to accelerate legume drying under both single-layer conditions (Tullberg and Angus, 1972; Crump, 1985) and in the field in Australia (Tullberg and Minson, 1978), in Michigan (Rotz and Davis, 1986b), and in Alberta (Crump, 1985). Because drying rate is enhanced more in stems than in leaves, K,CO,-treated stems can reach safe baling moisture before leaves overdry and shatter, thus improving the

424

ANDY D. MACDONALD AND E. ANN CLARK

leafistem ratio and overall quality, as well as yield (Tullberg and Minson, 1978). The quantities of potassium added to hay by applying K2C03as a chemical conditioner are so low that it is unlikely to influence nutritive value (Tullberg and Minson, 1978). K2C03reportedly acts by modifying the wax platelet structure of the cuticle, thus permitting water loss via a continuous liquid film from inner to outer surfaces, as discussed in Harris and Tullberg (1980). KzC03reduced drying time by 42-57070 in a study with alfalfa, sweetclover, sainfoin, alsike clover, and red clover (Clark et al., 1985). All legumes responded to treatment, but species differed in degree of response, with sweetclover drying rate most affected by the product. Chung and Verma (1986) demonstrated that K2C03was ineffective with grasses. Weighart et al. (1983) reported an additional enhancement in drying rate when KzC03 was combined with methyl esters and a surfactant, although subsequent tests have focused primarily on KzC03(Rotz and Davis, 1986a,b). Harris et al. (1974) indicated that a combination of both mechanical damage and thermal or chemical surface treatment could form the basis for more effective methods of increasing the drying rate of field crops cut for conservation. In a study conducted in Alberta, Crump (1985) reported that combining mechanical and chemical (K2C03)conditioning significantly increased drying rate over KzC03 alone in three of five trials, and over mechanical conditioning alone in two of five trials. She concluded that the effects of mechanical and chemical conditioning may be partly additive or that the distribution of the product was improved by passing through the rollers. Rotz and Davis (1986b) studied the effect of mechanical (intermeshing rubber rollers) and chemical (K2C03)conditioning treatments alone and in combination on an alfalfa hay crop. While mechanical conditioning increased the drying rate of first-cut alfalfa only, chemical conditioning improved the rate in all cuts, provided that drying conditions were favorable. When combined, the two treatments slightly increased drying rate of firstcut alfalfa over that which was mechanically conditioned, and in subsequent cuts, over that which was chemically conditioned. Field losses were not significantly changed by treatment. 4. Raking and Tedding

Tedders and rakes are used to open up the windrow, which promotes drying by increasing airflow and enhancing penetration of radiant energy into the windrow. As observed by Fitzherbert (1523, quoted by Nash, 1978), ". . . good teddying is the chief poynte to make good hay. . . ." Rakes are also used to invert windrows following rainfall, to expose the moist underside to better drying conditions, or to collect a loose swath and place it in a windrow prior to baling.

WATER AND QUALITY LOSS DURING DRYING OF HAY

425

A properly made windrow has the small-stemmed, quick-drying, leafy portions of the plant surrounded by the coarse-stemmed, slow-drying part of the plant. Because leaves dry faster than stems, arraying tissues in this order exposes the slow-drying stems to the most favorable drying environment, thus encouraging a more uniform drying of the hay crop (but see also Pedersen and Buchele, 1960). Proper and careful raking can contribute to optimal windrow architecture. The windrow produced by most mowers is compact and lumpy, and as windrow resistance is initially the primary limiting factor, drying rate increases following tedding early in the drying cycle down to a moisture content of 50%, with subsequent raking and turning (Nash, 1978). Because of damage to stem strength caused by mechanical conditioning, the windrow often compresses under its own weight, thus requiring periodic tedding to keep it open. Frequent disturbance ensures maximal ventilation, particularly to the innermost and underlying parts of the windrow profile (Clark and McDonald, 1977). Jones and Harris (1980) also noted that tedding serves to mix layers within the windrow resulting in more uniform drying. Tedding is seen with lesser frequency in North America than in Europe, perhaps because top quality hays are predominantly composed of grasses in Europe, while the focus is on legumes in North America. Grasses are less vulnerable to leaf loss and can thus tolerate greater agitation during the drying cycle (Nash, 1978). Native grass hay lost 10-15% DM, while grass-alfalfa hay lost 15-25 %, when raked at similar moisture contents (Friesen, 1978). Wilkinson (1981) reported average losses between cutting and baling in hay subjected to four postcutting treatments as 38.9% for alfalfa but only 19.1% for grass. Frequency and timing of tedding relative to moisture content strongly influence potential DM losses. Murdock and Bare (1963) found that tedding the same day as cutting resulted in a greater yield and faster drying than did tedding 1-2 days after cutting. Jones and Harris (1980) reported that tedding was most beneficial to drying in the range of 67-50010 moisture, after which plant resistances dominate. Tedding may also be more suited to immature than to' mature tissues. Dexter (1947) and Wilman and Owen (1982) observed that immature, leafy material tends to pack down into the swath to a greater extent than does more mature, stemmy material, such that tedding is more appropriate on juvenile than on mature forage. The more vigorous action of tedders is designed for use early in the drying cycle, when the forage is moist and relatively insensitive to agitation. When it is necessary to disturb drier material, the gentler rake is preferred. The two most common types of rakes are the side-delivery rake and the finger-wheel rake. The conventional side-delivery rake uses an angled, tine-spring tooth beater, oriented parallel to the ground, to gather or turn a windrow. The finger-wheel rake consists of several large, ground-driven spring-toothed

426

ANDY D. MACDONALD AND E. ANN CLARK

wheels, oriented perpendicular to the ground, which roll the hay to one side to form a windrow. Dry matter losses may occur as a result of raking or turning a hay crop, with the magnitude of loss depending on the type of rake, the moisture content at raking, and the yield and species composition of the hay. DM losses ranged from 5 to 11% for a side-delivery rake and 0-12070 for a fingerwheel rake, with lowest losses occurring when raked above 40% moisture, and highest losses occurring when raked at 28% moisture (cited in Friesen, 1978). Dobie et al. (1963) reported that raking hay at 10-15'70 moisture reduced yield and nutritional value by 25 and 30%, respectively, when compared to raking at 40-50% moisture. A short, light crop raked at low moisture content is most vulnerable to raking losses.

5. Baling

a. Equipment. Balers harvest and package dry hay into managable units for transport, storage, and feeding. The shape, size, and weight of bale packages varies among balers, from the conventional square or rectangular bales to large round bales (LRBs) (Table I). Hay may also be packaged in cubes, in large rectangular bales weighing up to 0.9 tons, and in hay stacks or loaves weighing up to 7 tons (Larsen and Rider, 1985). The various large bale packages have become increasingly popular because they substitute mechanical energy for human labor, a direction which many have perceived as cost effective. However, the size and weight of the various large package systems imply complementary investment in specialized equipment to retrieve the bales from the field and feed them to Table I Descriptive Statistics on Hay Packaging Systems

Package Bales Conventional Largeround Large rectangular

Dimensions (cm)

Density (Wm')

36 x 46 x variable

160-300

100-170 x 100-180 122 x 122 x 200-280

110-250 240

Stacks

260-750 x 210-360 x 240-520

Cubes Pellets

4 X 4 X 5

400

0.5 x 2.5 x 2.5-4.5

550

55-95

Source Friesen (1978) Larsen and Rider (1985) Larsen and Rider (1985) Larsen and Rider (1985) Friesen (1978) Nash (1978) Larsen and Rider (1985)

WATER AND QUALITY LOSS DURING DRYING OF HAY

427

livestock. Lechtenberg (1978) noted that large hay packages require specialized feeding management to lessen feeding waste. As such, he emphasized that the labor and mechanization advantages of the large bale packages must be reconciled with greater potential feeding and storage losses. Labor efficiencies are considered to be the basis for the popularity of LRBs, with average handling rates of about 2.5 to 5.6 tons/manhour (Friesen, 1978; Kjelgaard et al., 1981; Heslop and Bilanski, 1986). Particularly with restricted labor availability, the possibility of baling without hired help is considered a bonus on many contemporary farms. However, as noted by Von Bargen (1978), careful management is required to ensure that the quality of LRB hay equals that of conventional square bales. Although conventional bales are vulnerable to weathering, and thus, require cover for storage, they do allow for the greater freedom in utilization, because they are light enough to be moved manually and can be fed either mechanically or manually (Larsen and Rider, 1985). Self-propelled bale wagons have been designed to recover bales from the field mechanically and to transport them to a storage area. Bale throwers or ejectors are an increasingly popular addition to many conventional balers because they reduce conventional baling to a one-person, one-pass operation, analogous to LRB baling. Average handling rates for conventional bale packages are on the order of 0.8-2.1 tons/manhour (Friesen, 1978; Kjelgaard et al., 1981). 6. Losses. Losses of DM at baling may occur at pickup or in the bale chamber itself and are affected by moisture content, by species composition, and by type of baler. Excessive moisture at baling permits microbial activity, which engenders potential bale heating, respiratory losses of carbon dioxide, changes in chemical composition, and decreases in digestibility of protein and energy. At the other extreme, hay which has overdried prior to baling is subject to higher DM and nutrient losses at baling (Dobie et al., 1963). Friesen (1978) reported the practice of baling dry hay at night, after dewfall, to reduce baling losses. Conventional rectangular baling incurs losses ranging from 3 to 8070, while losses associated with baling as LRBs range from 1 to 15% (cited in Martin, 1980). The Prairie Agricultural Machinery Institute (P.A.M.I., 1977) indicated that bale chamber losses from large round balers could be reduced by baling conditioned hay and traveling at the fastest possible ground speed while baling. In addition, increasing the cutting width and reducing power takeoff speed when harvesting light crops can reduce bale chamber losses (P.A.M.I., 1977). Kjelgaard et al. (1981) found that baling losses decreased when windrows were combined before baling, although the process of raking windrows together increased losses before baling to the point at which there was little if any benefit. Baling losses vary with species and with plant maturity. Shepherd (1959)

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ANDY D. MACDONALD AND E. ANN CLARK

found that susceptibility of alfalfa, white clover, and subterranean clover (T. subterranean L.) leaves to shattering loss was least when plants were harvested at a juvenile stage and increased with both plant maturity and time in the season. Nash (1978)concluded that leaf shattering in alfalfa, even under favorable conditions, accounted for losses of 15% of total crop weight, and up to 3045% of total weight under poor conditions.

c.

STORAGE AND HANDLING LOSSES

Losses occurring during transport from the field to storage area, from microbial activity in storage, and from processing to feeding can reduce the quantity and quality of conserved feed. Friesen (1978)reported that DM losses during transport alone can vary from 1 to lo%, depending on the type of crop, the length of time the bales had been left in the field, and the type of pickup used. 1. Storage

The source and magnitude of storage losses will depend on moisture content at baling and whether the dry hay is stored with or without cover. Hay stored at 20% moisture should lose no more than 5% DM due to respiration, although DM losses for hay stored at 25010 moisture can reach 10'70, and hay baled at 35-40% moisture will lose 15-20'70 DM due to respiration (Nash, 1978). Storage losses of CP in alfalfa varied from 17.9 to 29.2070, respectively, for LRBs stored in the barn versus outdoors without cover (Heslop and Bilanski, 1986). Equivalent losses of digestible DM were 4.2 and 23.4'70,respectively. Losses of DM in storage ranging from 8 to 32% have been observed when large hay packages were stored outside (Lechtenberg, 1978). Losses specific to outside storage are due to weathering around the bale exterior, particularly where bales touch each other or the soil. a. Losses Due to Excessive Moisture at Baling. Hay that is cut at proper maturity, dried quickly in the field, baled at safe baling moisture content, and stored indoors is green, leafy, and shows no mold growth (Miller et al., 1967; Martin, 1980). However, if hay is baled in excess of safe baling moisture, often under pressure of unfavorable weather, molding begins and hay color deteriorates to gray or brown. Because the criteria most associated with top quality hay include bright green color, a high 1eaf:stem ratio, and freedom from the odor and dustiness indicative of mold growth, these organoleptic or sensory changes in hay quality can markedly affect its real and perceived nutritional value. Hay baled at a moisture content above about 20% deteriorates during

WATER AND QUALITY LOSS DURING DRYING OF HAY

429

storage, due both to direct microbial degradation and to associated heating damage (Klinner and Shepperson, 1975). Heating to a temperature of about 70°C indicates significant microbial activity within stored material, while temperatures above 70°C indicate exothermic chemical oxidation (Currie and Festenstein, 1971). The typical pattern of temperature rise in wet-baled hay is an initial peak at 2-3 days, followed by a second, higher peak, often within the first 7-10 days (Nelson, 1966). The first peak represents cellular respiration, while the second is indicative of fungal respiration (Nash, 1978). Fungi such as Aspergillus glaucus are primarily responsible for the heating and subsequent breakdown of complex carbohydrates and proteins in stored hay (Martin, 1980;Lacey et al., 1981). Nelson (1966)noted that bale density and moisture content at baling affected bale heating in an additive fashion. As bale density increased at a given moisture content, bale heating increased and resultant quality decreased. Alfalfa hay baled at 27% moisture heated more when packaged as LRBs than as conventional square bales (Scales et al., 1978). Moldy hay represents not simply poor quality livestock feed, but can also constitute an actual health hazard for both livestock and livestock operators, particularly in humid climates. Thermophilic actinomycetes, in particular Micropolyspora faeni and Thermoactinomyces vulgaris , are favored by the high temperature reached in wet-baled hay (Nash, 1978). A crucial health facet of hay conservation which has long been underestimated is that human and livestock illnesses such as “farmer’s lung” and bovine lung disease are caused by these actinomycetes (Klinner and Shepperson, 1975). b. Losses Due to Weathering. Although LRBs were originally designed for outdoor storage in dry environments (Helsop and Bilanski, 1986), the rounded shape of LRBs, which reportedly acts to shed rather than absorb water, encourages outdoor storage even in humid environments. Other factors promoting outdoor storage include their size, awkward shape for stacking, and associated difficulties with transport. As a result, recent studies on weathering losses in storage have focused on LRBs, although the results would be generally applicable to other hay packages as well. Helsop and Bilanski (1986)cited data contrasting reported LRB weathering losses of 4-8070 in drier parts of Western Canada versus 5-100% for the more humid region of Ontario. They suggested that even in Ontario, however, bale storage on well-drained soil with at least 60 cm between bale rows can keep weathering losses below 20Vo. Many hay producers are prepared to accept the associated spoilage and weathering losses rather than undertake the expense of building indoor storage space. In two different experiments, weathering and respiration losses during 1 year of outdoor storage accounted for a 27-30% loss of DM in grass LRBs (Lechtenberg, 1978). In the same studies, outdoor storage on crushed rock reduced losses to 15%, compared with indoor storage losses of

430

ANDY D. MACDONALD AND E. ANN CLARK

8%. The weathered portions of grass LRBs stored outdoors for 2 years were found to be inadequate for supplying even the maintenance energy requirements of nonlactating beef cows, although the quality of the inner, unweathered portion did not change during storage. Lechtenberg et al. (1978) also reported that LRB deterioration was similar with both grass and grass-legume hay. Heslop and Bilanski (1986) attributed storage losses in LRBs left outdoor to the following: (1) bale settling allows about one-third of the bale circumference to touch the ground, from which moisture can travel up as far as 30 cm; and (2) because the outer 15 cm represents fully 20% of bale volume, 42% of the bale volume is affected if only the outer 15 cm around the top plus 30 cm from the bottom edge are weathered. They further concluded that it is profitable to store LRBs indoors in a humid climate if outdoor weathering losses exceed 14%. At each of five levels of annual hay protection, indoor storage of LRBs was cheaper than that of square bales, using a bale thrower (Heslop and Bilanski, 1986). Atwal et al. (1984) reported losses of 9 and 40% for alfalfa LRBs baled at 20% moisture and then stored either indoors or outdoors, respectively. They concluded that the increases in acid detergent fiber (ADF) and cellulose and the decreases in digestibility of DM, energy, and crude protein in alfalfa LRBs stored outdoors justified the cost of indoor storage in high precipitation areas.

2. Handling Losses Studies summarized by Lechtenberg (1978) estimated feeding losses ranging from 3.7% when large bales were fed from a feed rack, to 35-46% when cattle were given access to large haystacks. Hay wastage when fed without a rack to pastured cattle varied from 11 to 34%, as the amount fed varied from a 1- to an %day supply. He calculated that feeding without a rack at &day intervals would require 52% more feed than feeding daily from a rack.

V.

PRESERVATION OF WET HAY

Harvesting and packaging at greater than safe baling moisture content, in concert with chemical preservatives, is one method of reducing field losses and improving conserved forage quality. Preservatives such as propionic acid (Knapp et al., 1976; Kjelgaard et al., 1981) and ammonia (Knapp et al., 1975; Lechtenberg et al., 1978) applied at baling or shortly thereafter act to inhibit the activity of microorganisms.

WATER AND QUALITY LOSS DURING DRYING OF HAY

43 1

Although various organic acids have been tested, only propionic acidbased products have consistently retarded microbial activity and heating (Charlick et al., 1980, Kjelgaard et al., 1981). Propionic acid applied at 1% of fresh weight at time of baling prevented heating and dry matter loss in hay baled at 32% moisture (Knapp et al., 1976). Objections to the use of propionic acid stem from its corrosiveness and high volatility, which lead to high application losses (Klinner and Holden, 1978). When air temperatures were 18, 20, and 22”C, average field application losses of propionic acid were 41, 52, and 58070, respectively. Ammonium propionate was found to be both less volatile and less corrosive but also less effective per volume of applied product than propionic acid in inhibiting molds, although both were economically feasible and could be ingested without harm to livestock (Charlick et al., 1980). At 23”C, retention of ammonium propionate was 78% versus only 34% for propionic acid. Ammoniation has been used primarily to preserve and enhance quality of lesser-quality hay and straw baled at high moisture content, by increasing total N, IVDMD, and DM intake (Knapp, et al., 1975). Anhydrous ammonia applied a rate of 1.0% (fresh weight basis) successfully prevented heating in alfalfa hay bales at 32% and sealed in plastic bags, whereas the temperature of the untreated control baled at the same moisture exceeded 50°C and the hay became extremely moldy (Lechtenberg et al., 1978). Grotheer et al. (1980) reported that anhydrous ammonia applied at 1-3070 (DM basis) was effective in preserving and enhancing nutritive value in dry (80.2% DM) and high-moisture (67.5% DM) coastal bermudagrass hay packaged as LRBs in plastic bags. Ammoniation increased crude protein equivalent and IVDMD by 65.8 and 21.5070, respectively. Caution must be taken when handling and applying anhydrous ammonia since breathing ammonia vapor is dangerous. Concentrated ammonia may also form a flammable mixture with the air. Another concern with highly ammoniated feeds was voiced by Henning et al. (1986), who outlined the risk of “crazy cattle syndrome” in sheep and cattle fed ammoniated, highmoisture, high-quality hay. Symptoms of ammonia toxicity include nervous behavior, circling, convulsions, and potentially, death. Weiss et al. (1986) also reported that the crazy cattle syndrome in sheep was associated with metabolic acidosis and high blood pyruvic and lactic acid levels resulting from ammoniated orchardgrass hay. Hay treated with 4% ammonia caused hyperexcitability in four of five calves nursing cows fed ammoniated hay, although hay treated with 1-3% ammonia (DM basis) appeared to cause no ill effects on the cattle, as noted earlier by Saenger et al. (1982). Henning et al. (1986) preferred urea instead of anhydrous ammonia for preserving high-moisture hay, because of its effectiveness in controlling bale temperature, its ease of handling and reduced health hazards, and the lack

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ANDY D. MACDONALD AND E. ANN CLARK

of necessity for postbaling treatment. They found that 80% of the initial urea N was converted to ammonia and other N forms within 13 days after baling, inhibiting yeast and mold but not bacterial growth.

VI.

CONCLUSIONS

Resolving the inherent conflict between enhancing water loss while causing minimal losses of DM and nutritional value of hay is the central issue in haymaking technology. Recent trends toward using rotary rather than cutter-bar mowers, conditioning by abrasion rather than crushing and crimping, employing tedding to promote early drying (in North America), packaging in large bales rather than conventional square bales, and storing LRBs indoors rather than outdoors represent innovative approaches to this central dilemma. Biological efficiency is not necessarily synonymous with profitability, however, as illustrated by the trade-off between mechanization, labor costs, and forage quality in baling, storing, and using LRBs. Losses of DM during haymaking and use, as estimated by various authors, are presented in Table I1 and summarized into three haymaking scenarios in Table 111. Percentage losses occurring at each level are applied to the product of the preceeding level, a linear approach which avoids and probably oversimplifies the interactive nature of various haymaking practices. Given these provisos, however, conserved forage DM potentially consumed by livestock represents, at best, only three-quarters of what was available in the standing crop. Quality losses in terms of concentrations of protein and nonstructural carbohydrates, digestibility, and intake may be conservatively estimated to reduce potential nutritional value of conserved feed to perhaps 50% of that of the standing crop, even in the optimistic scenario. Clearly, the scope for improvement at every level of haymaking technology is large. Climatic limitations impose risk and unpredictability on haymaking and use, particularly in humid regions. However, haymaking problems and solutions differ somewhat between Europe and North America, because of the greater reliance on grasses versus legumes, respectively, for top quality hay. Legumes are more sensitive to handling, and as such, are vulnerable to greater losses, particularly with tedding, raking, and baling. Legumes also appear more vulnerable to weathering losses, both in drying and in storage. Understanding the limits to drying will contribute to refining hay management, to designing novel mechanical or chemical approaches to drying enhancement, and to breeding forages for faster drying.

433

WATER AND QUALITY LOSS DURING DRYING OF HAY Table I1 Estimated Dry Matter Losses, Partitioned by Source, during Haymaking, from Cutting to Feeding Dry matter loss (070)

Descriptive comments

Cutting and conditioning 2-5 1 Cutter-bar mower 1-4 Mechanical conditioning 14 5-20 5-25 15-40

Source

Cutter-bar mower Rotary mower Flail-type mower

Martin (1980) Cited in Von Bargen (1978) Cited in Von Bargen (1978) Wilkinson (1981) Svenson (1978) Svenson (1978) Svenson (1978)

Alfalfa Alfalfa Good haymaking weather Controlled drying Poor haymaking weather

Klinner and Shepperson (1975) Cited in Turner (1970) Pedersen and Buchele (1960) Nash (1978) Wilkinson (1981) Rees (1982)

Respiration 2-8,

up to 16 10 5-15 5-10 8 30

Raking and tedding 5-1 1 Side-delivery rake 0-12 Finger-wheel rake 5-25 Raking 0.Vtedding At 70-80'7'0 moisture 2.5%/tedding At 20'370 moisture Alfalfa, tedded at 50% moisture 5% of leaf

Friesen (1978) Friesen (1978) Martin (1980) Wilkinson (1981) Wilkinson (1981) Cited in Von Bargen (1978)

Rainfall damage 9.6 17.0 43.4

Coastal bermudagrass, 5 cm Legumes, with 2.5 cm Legumes, with 4.1 cm

Hart and Burton (1970) Collins (1983) Collins (1983)

Pickup losses Alfalfa Alfalfa Good conditions Large round bales (LRB) LRB bale chamber losses, good conditions As above but poor conditions Conventional bales Conventional bales LRB LRB and stacker Poor conditions

Friesen (1978) Cited in Turner (1970) Pedersen and Buchele (1960) Nash (1978) Von Bargen (1978) P.A.M.I. (1977)

Baling 1-5 20 20 10-15 0.5-15 <5 17 3-8 3 1-15 10 30-45

P.A.M.I. (1977) Martin (1980) Kjelgaard e t a / . (1981) Martin (1980) Kjelgaard el a/. (1981) Nash (1978) (continued)

Table 11 (Continued) Dry matter loss (To)

Source

Descriptive comments

Storage

8 5 3 9 2 4-8 5-100 27-32 15 20

Lechtenberg (1978) Nash (1978) Wilkinson (1981) Atwal el a/. (1984) Scales et a/. (1978) Heslop and Bilanski (1986) Heslop and Bilanski (1986) Lechtenberg (1978) Lechtenberg (1978)

Grass LRB indoors Indoor respiration Indoors LRB indoors, 20% moisture LRB indoors LRB outdoors, dry climate LRB outdoors, humid climate LRB outdoors 1 year LRB outdoors 1 year on crushed rock LRB outdoors, good management, humid climate LRB outdoors, 6 months

Heslop and Bilanski (1986) Scales el a/. (1978)

40 Handling and feeding Per handling in feedlot 1-2 Max, with tubgrinder 10 Transport 1-10 Good management, controlled 5 access LRB in rack 3.7 Free access, I-day supply 11 Free access, 8-day supply 34 Free access, large package 40-50 Free access, large package 35 -46

Von Bargen (1978) Cited in Von Bargen (1978) Martin (1980) Von Bargen (1978) Lechtenberg (1978) Lechtenberg (1978) Lechtenberg (1978) Von Bargen (1978) Lechtenberg (1978)

Table 111 Estimates of Total Dry Matter Losses in Haymaking and Use, Assuming Optimistic, “Realistic,” and Pessimistic Scenarios, Synthesized from Table I1 Optimistic Source of loss Relative to fresh cut material Cutting/conditioning Respiration Rakinghedding RainfaWweathering Baling Storage Handling/transport/ feeding Forage dry matter potentially consumed

Loss (Yo)

Realistic

Left (Yo)

Loss (To)

100

5 5 5 0 5 5 5

95 90 86 86 81 77 74

Left (To)

Pessimistic Loss (Vo)

100 10 10 10 10 10 10-2ff 10

74

OAssuming either indoor or good outdoor storage.

90 81 73 66 59 53-47 48-43 48 or 43

Left (070)

100 20 15 20 15 20 30 30

80 68 54 46 37 26 18 18

WATER AND QUALITY LOSS DURING DRYING OF HAY

43 5

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Index Actinomycetes acidic mined lands and, 400 hay drying, loss during, 429 Alder, acidic mined lands and, 399 Alfalfa hay drying, loss during, 409,410, 412, 415-417 agronomic factors, 414 harvesting losses, 422, 424, 425, 428 preservation, 431 respiratory losses, 418, 419 storage losses, 430 mineral nutrient uptake and, 207 phosphorus and, 308 root systems and, 107, 111, 118, 138 utilization, 121, 122 weed management, biotechnology and, 335 genetic manipulation, 350 tolerance, 344, 345 Algae acidic mined lands, and, 400 phosphorus and, 297, 303, 318 artificial removal from lakes, 317 bioavailability, 313, 314 impact on aquatic environment, 311-313 weed management, biotechnology and, 342, 350 Alkalinity, acidic mined lands and, 397 Alternuria alternata, weed management, biotechnology and, 335 Alternuria cassiae, weed management, biotechnology and, 331 Alternaria macrospora, weed management, biotechnology and, 331 Aluminum acidic mined lands and, 382, 396, 399 linseed and fiber flax and, 226, 281, 285, 286 mineral nutrient uptake and, 209, 210, 215 phosphorus and, 307, 313 artificial removal from lakes, 316, 317 sorghum, culture techniques and, 163 Amino acids linseed and fiber flax and, 267 weed management, biotechnology and, 343

A Absorption acidic mined lands and, 397 linseed and fiber flax and, 238, 270 mineral nutrient uptake and, 195, 196, 198, 199, 214 Acetolactate synthase, weed management, biotechnology and, 343-345, 348, 357 Acetylation, weed management, biotechnology and, 341, 346 Acetylene reduction, mineral nutrient uptake and, 209 Acetylene reduction method, intercropping systems and, 74, 85 Acid detergent fiber, hay drying, loss during, 430 Acidic mined lands, 377-379, 401 amendments, 383, 384, 387 combination, 388 fly ash, 385 metal contents, 388, 389 papermill sludge, 386 sewage sludge, 385, 386 sodium lauryl sulfate, 386 topsoil, 384, 385 seeding, 391-394 biological, 399, 400 chemical, 395-399 limestone, 389 mulches, 390, 391 physical, 394, 395 spoil properties biological, 383 chemical, 381-383 physical, 379-381 Acidity linseed and fiber flax and, 226, 232, 280-282, 284-286 nutrient disorders, 258, 264-266, 270, 27 1 mineral nutrient uptake and, 208, 21 1, 215 phosphorus and,299 Acifluorfen, weed management, biotechnofogy and, 331 439

440

INDEX

Amitrole, weed management, biotechnology and, 349 Ammonia acidic mined lands and, 397 hay drying, loss during, 430-432 linseed and fiber flax and, 229 mineral nutrient uptake and, 175 weed management, biotechnology and, 335 Ammonium, intercropping systems and, 80, 83 Ammonium propionate, hay drying, loss during, 431 Aneuploidy, Arachis and, 9 Anion linseed and fiber flax and, 254 mineral nutrient uptake and, 207 phosphorus and, 314 Anisomycin, weed management, biotechnology and, 336 Anther culture, sorghum, culture techniques and, 154, 155 Anthesis intercropping systems and, 59 root systems and, 139 Antibiotics, weed management, biotechnology and, 358 Antibodies, weed management, biotechnology and, 325 Arachis, 1-3, 34, 35 cytogenics, 8-10 germplasm, 14, 22 disease resistance, 14, 15, 20, 21 insect resistance, 16-19, 21, 22 hybridization, 10-13 origin, 7, 8 plant collection, 4-7 taxonomy, 3-5 utilization, 22 direct hybridization, 24-28 gene transfer restrictions, 23, 24 intersectional hybridization, 28-30 interspecific hybridization, 30-32 potential, 32-34 Arachis glabrata, 1, 3, 9, 10 germplasm, 14, 15, 19 utilization, 24 Arachis quaranitica, 3 Arachis hypogaea, 1, 2, 34, 35 cytogenetics, 8, 9 germplasm. 15, 20-22

hybridization, 10- 13 origin, 7, 8 plant collection, 6 potential, 32-34 taxonomy, 3 , 4 utilization, 23-31 Arachis repens, 1, 14, 15 Arachis tuberosa, 3 Arachis villosulicarpa, 1, 18 Area Time Equivalent Ratio, intercropping systems and, 47-50, 52 Arginine, linseed and fiber flax and, 267-269 Argobacterium, weed management, biotechnology and, 355, 356 Argobacterium tumefaciens, weed management, biotechnology and, 353-356 Arsenic, phosphorus and, 299 Asparagus, weed management, biotechnology and, 354 Atrazine, weed management, biotechnology and, 365 genetic manipulation, 347, 351, 353, 355 tolerance, 338, 339, 341-343

B Bacteria acidic mined lands and, 400 hay drying and, 431 weed management, biotechnology and, 326, 332 biodegradation, 358, 360, 361 commercial prospects, 329 genetic manipulation, 346, 354-357 herbicides, naturally occurring, 333 microbes, 328 safeners, 362 tolerance, 341 Baling, hay drying, loss during, 426-428 Barley intercropping systems and, 42, 43, 46,78 linseed and fiber flax and, 231 acidity, 281 nutrient disorders, 261 uptake, 234 mineral nutrient uptake and, 175, 182, 189, 215 concentration, 202 photosynthate partitioning, 212

441

INDEX solution, 205 temperature, 198 root systems and, 95, 99, 136, 138 Bean intercropping systems and, 50, 51, 62, 73 combinations, 43, 44 productivity, 63, 66-68 linseed and fiber flax and, 269 mineral nutrient uptake and, 205 root systems and, 92, 111, 112, 119 Bentazon, weed management, biotechnology and, 331, 349 Benzadox, weed management, biotechnology and, 337 Bialophos, weed management, biotechnology and, 334, 335, 337, 341, 364 Biocatalysts, weed management, biotechnolOgy and, 363-365 Biodegradation, weed management, biotechnology and, 357-361 Bioflocculation, phosphorus and, 313 Bioherbicides, weed management, biotechnology and, 328, 329, 364 bioherbicide production, 331, 332 commercial prospects, 329, 330 Biosurfactants, weed management, biotechnology and, 363, 364 Biotechnology, weed management and, see Weed management, biotechnology and Boron acidic mined lands and, 385, 3% linseed and fiber flax and, 226 diagnostic criteria, 274 nutrient disorders, 255-258, 260, 269 yield quality, 255 yield responses, 234 Brassica, weed management, biotechnology and, 356, 357 Broad sense heritability, root systems and, 117-1 19

Buffering, root systems and, 134 C

linseed and fiber flax and, 226 acidity, 281 diagnostic criteria, 274 genotype, 284 nutrient disorders, 256-261, 268 salinity, 278, 280 yield quality, 254, 255 yield responses, 233, 238, 239, 241 mineral nutrient uptake and, 175, 184, 203

Callus Arachb and, 30, 31 weed management, biotechnology and, 351, 357

Callus culture, sorghum, culture techniques and, 153-155, 157-160, 162-164 Cambium, root systems and, 118, 119 Canola, weed management, biotechnology and, 347-349, 352, 365 Carbohydrates hay drying, loss during, 418, 419, 429, 432

linseed and fiber flax and, 241 mineral nutrient uptake and, 211, 212 root systems and, 111 sorghum, culture techniques and, 151, 162

Carbon acidic mined lands and, 386, 397, 399 linseed and fiber flax and, 267 mineral nutrient uptake and, 180 phosphorus and, 297 sorghum, culture techniques and, 163 weed management, biotechnology and, 360

Carrot, weed management, biotechnology and, 345 Cassava intercropping systems and, 83 combinations, 42 light, 55 productivity, 65 mineral nutrient uptake and, 189, 205, 206,212

Cadmium acidic mined lands and, 399 mineral nutrient uptake and, 202, 206, 207, 215

phosphorus and, 297, 299, 300 Calcium acidic mined lands and, 382, 383, 399

Cation acidic mined lands and, 386, 397 linseed and fiber flax and, 255, 281 mineral nutrient uptake and, 207 phosphorus and, 306 Cation exchange capacities, intercropping systems and, 60

442

INDEX

Cattle, hay drying and, 430,43 1 Cauliflower mosaic virus, weed management, biotechnology and, 355, 356 Caulorhizae, 14, 15, 17, 20 Cell culture, sorghum and, see Sorghum, culture techniques and Cellulose hay drying, loss during, 430 linseed and fiber flax and, 251, 252, 254 Cercospora rodmonii, weed management, biotechnology and, 331 Cereal linseed and fiber flax and, 224, 225 nutrient disorders, 256 uptake, 235 yield responses, 229 root systems and, 92 weed management, biotechnology and, 352, 357, 363

Cereal-legume intercropping systems, see Intercropping systems Chickpea intercropping systems and, 43 root systems and, 115 Chlamydospores, weed management, biotechnology and, 330 Chlorine, linseed and fiber flax and, 226, 239, 280

Chloroacetanilide, weed management, biotechnology and, 361, 366 Chloroanilines, weed management, biotechnology and, 341 Chloroplasts, weed management, biotechnology and, 342, 343, 351 Chlorosis linseed and fiber flax and, 262-264, 266, 270, 286

weed management, biotechnology and, 335

Chlorsulfuron, weed management, biotechnology and, 348, 349, 357, 365 Chromium, phosphorus and, 299 Chromosomes Arachis and, 8-13 germplasm, 15, 22 potential, 32, 33 utilization, 23, 25-27, 29-31 root systems and, 118 sorghum, culture techniques and, 148, 165

weed management, biotechnology and, 347, 353, 355

Citrate-dithionite-bicarbonate,phosphorus and, 314 Climate acidic mined lands and, 391 hay drying, loss during, 409, 410, 432 intercropping systems and, 41 root systems and, 123, 129, 130 Cloned genes, weed management, biotechnology and, 352-357 Clones hay drying, loss during, 412 weed management, biotechnology and, 335 genetic manipulation, 346 tolerance, 341-343 Clover hay drying, loss during, 412, 417 harvesting losses, 421, 424, 428 respiratory losses, 419 window resistance, 414 linseed and fiber flax and, 224, 270, 281 mineral nutrient uptake and, 198, 204, 206, 207, 209, 211, 212, 215

root systems and, 116, 117 Colchicine Arachis and, 23, 25 root systems and, 104 Colletotrichum coccodes, weed management, biotechnology and, 331 Colletotrichum gloesporoides, weed management, biotechnology and, 330, 331

Colletotrichum malvarum, weed management, biotechnology and, 331 Compartmentation sorghum, culture techniques and, 163 weed management, biotechnology and, 338, 346

Copper acidic mined lands and, 399 linseed and fiber flax and, 226 diagnostic criteria, 274 nutrient disorders, 261 yield quality, 255 yield responses, 234 mineral nutrient uptake and, 202, 203, 215

Corn acidic mined lands and, 399 linseed and fiber flax and, 231, 232, 269 phosphorus and, 308

443

INDEX root systems and, 99, 100, 106, 118 weed management, biotechnology and, 366 biodegradation, 358 genetic manipulation, 349, 350, 355 herbicides, naturally occurring, 335 safeners, 362 tolerance, 339-341, 345 Cotton phosphorus and, 308 root systems and, 112, 119 weed management, biotechnology and, 337 Cowpea intercropping systems and, 51, 53, 60,84 combinations, 42-44 nitrogen, 70, 71, 73, 14, 76-79, 82, 83 productivity, 62, 63, 67 water, 57, 58 root systems and, 112, I13 Crazy cattle syndrome, hay drying and, 43 1 Crude protein, hay drying, loss during, 417, 430 Cucumber, weed management, biotechnology and, 338, 339 Cuticular resistance, hay drying and, 411-413 Cutin, hay drying, loss during, 41 1 Cyanobacteria, weed management, biotechnology and, 342, 350 Cylindrocladium black rot, Arachis and, 21, 33 Cytology, Arachis and, 9, 1 1 , 12

D Dehalogenase, weed management, biotechnology and, 360 Deionization, phosphorus and, 308 Dentrification, intercropping systems and, 79-81, 83 Desorption, phosphorus and, 318 impact on aquatic environment, 313 transport to aquatic environment, 303, 306, 307, 309, 310 Dessication, sorghum, culture techniques and, 165 Detoxification, weed management, biotechnology and, 339-341, 346 Dicotyledons, root systems and, 94, 107-1 10

Dieback, linseed and fiber flax and, 269, 270 Dietholate, weed management, biotechnology and, 361 Diffusion, mineral nutrient uptake and, 193-196 Dinitroaniline, weed management, biotechnology and, 344 Diphenamid, weed management, biotechnology and, 348 Diploids Arachis and, 1 , 8 , 9, 12, 13, 35 hybridization, 25-30 potential, 32 root systems and, 104 sorghum, culture techniques and, 148 Disease, linseed and fiber flax and, 224, 235, 240 Disease resistance Arachis and, 2, 14-21, 34. 35 root systems and, 128 sorghum, culture techniques and, 152 DNA linseed and fiber flax and, 283 weed management, biotechnology and, 325, 365 genetic manipulation, 351, 353-356 Drought resistance Arachis and, 2 root systems and, 103, 105, 106, 112 genetics, 119, 120 phenotypic plasticity, 136 utilization, 122, 125, 127, 129, 132 sorghum, culture techniques and, 164, 165 Drought resistance index, root systems and, 129, 130 Dry matter, hay drying, loss during, 407, 417, 418, 432-435 harvesting losses, 420-422, 425-427 preservation, 431 respiratory losses, 418, 419 storage losses, 428, 430 water Loss, 408

E Earthworms, acidic mined lands and, 399, 400 Edaphic niches, root systems and, 92-94, 123, 130, 139

444

INDEX

Endotoxin, weed management, biotechnology and, 362 Enzymes mineral nutrient uptake and, 196 phosphorus and, 298 weed management, biotechnology and, 325, 366 genetic manipulation, 346 herbicides, naturally occurring, 335 microorganisms, 360-362, 365 tolerance, 338-341, 343-345 EPSP synthase, weed management, biotechnology and, 344, 350, 356, 357 Equilibrium P concentration, phosphorus and, 309, 310, 313 Erectoides 4, 7, 9-12 germplasm, 17, 20 utilization, 30 Erosion acidic mined lands and, 377 amendments, 384, 388 seeding, 390, 391 soil properties, 394 spoil properties, 382 phosphorus and, 297, 298, 318 transport to aquatic environment, 301-303, 308 Escherichia coli, weed management, biotechnology and, 343-345 Ethyl methane sulfonate, weed management, biotechnology and, 348 Eukaryotes, weed management, biotechnology and, 354 Eutrophication, phosphorus and, 297 artificial removal from lakes, 317 bioavailability, 316 impact on aquatic environment, 312 transport to aquatic environment, 303 Extracellular resistance, hay drying, loss during, 413 Ewtranervosae, 7, 9 germplasm, 1 8 hybridization, 1 1 , 12 F

Fatty acids, hay drying, loss during, 41 1 Fermentation, weed management, biotechnology and, 332, 364 herbicides, naturally occurring, 333, 335

Ferric hydroxide, acidic mined lands and, 381 Fertilizer, acidic mined lands and, 389, 391 Fertilizer phosphorus, impact on environment, see Phosphorus, impact on environment Fiber flax, mineral nutrition of, see Linseed and fiber flax, mineral nutrition of Fish phosphorus and, 298, 311 weed management, biotechnology and, 328 Flax rust, linseed and fiber flax and, 256, 284 Flowing nutrient solutions, mineral nutrient uptake and, see Mineral nutrient uptake Flufosinate, weed management, biotechnology and, 341 Flurazole, weed management, biotechnology and, 340, 346 Fly ash, acidic mined lands and, 385, 387 388 seeding, 389 soil properties, 394-396 Fresh weight, hay drying, loss during, 407-409 Fungi acidic mined lands and, 400 Arachis and, 15 hay drying, loss during, 408, 41 1, 429 mineral nutrient uptake and, 189 weed management, biotechnology and, 326, 332 commercial prospects, 329-33 1 herbicides, naturally occurring, 333, 337 microbes, 328, 329 Fusaric acid, weed management, biotechnology and, 337 Fusarium, weed management, biotechnology and, 337 Fusarium wilt, linseed and fiber flax and, 232, 239, 256 Fusicoccin, hay drying, loss during, 41 1 Fusicoccum arnyqdali, hay drying, loss during, 41 1 Fusion, weed management, biotechnology and, 325

INDEX

445

weed management, biotechnology and, 335

Genetics, root systems and, 117 control system, 118-120 heritability, 117, 118 Genotrophs, nutrient induced, linseed and fiber flax and, 282, 283 Genotype Arachis and, 15, 20, 22, 28, 31, 32 intercropping systems and, 74 linseed and fiber flax and, 280 differences, 283, 284 nutrient induced genotrophs, 282, 283 mineral nutrient uptake and, 171, 184, 215

sorghum, culture techniques and, 153, 154, 157, 159, 160, 163, 164

Genotypic variation, root systems and, see Root systems, genotypic variation in Germplasm Arachis and, 1-3, 14, 22, 34, 35 disease resistance, 14, 15, 20, 21 hybridization, 13 insect resistance, 16-19, 21, 22 plant collection, 6, 7 potential, 33, 34 utilization, 22, 30 root systems and, 111, 116, 125, 129, 132 sorghum, culture techniques and, 152, 154, 158

Gibberellic acid, Arachis and, 23, 24, 31, 32 Glucose, hay drying, loss during, 413 Glutamine synthetase, weed management, biotechnology and, 335, 344, 345 Glutathione, weed management, biotechnology and, 339, 340 Glyphosate, weed management, biotechnology and, 343, 344, 365 genetic manipulation, 349, 350, 356, 357 safeners, 362 Grass acidic mined lands and seeding, 391, 392 soil properties, 399 spoil properties, 382 hay drying, loss during, 409,41 1, 414, 416

harvesting losses, 420, 422, 425 respiratory losses, 419 phosphorusand,298 sorghum, culture techniques and, 153

Groundnut, intercropping systems and, 56, 57, 63

combinations, 43, 44 nitrogen, 68-71, 73, 74, 78, 79 Groundnut rosette virus, Arachis and, 15 GSH-S-transferase (GST), weed management, biotechnology and, 339, 340, 346,366

H Haploids Arachis and, 32 sorghum, culture techniques and, 154, 166

weed management, biotechnology and, 349

Harvesting losses, hay drying and, 419 baling, 426-428 chemical conditions, 422-424 mechanical conditions, 421, 422 mowing, 419-421 raking, 424-426 Hay drying, 407,432-435 handling losses, 430 harvesting losses, 419 baling, 426-428 chemical conditioning, 422-424 mechanical conditioning, 421, 422 mowing, 419-421 raking, 424426 preservation of wet hay, 430-432 respiratory losses, 418, 419 storage losses, 417, 418, 428-430 water loss agronomic factors, 414 climate, 409,410 drying, 408, 409 grasses, 414-416 legumes, 416, 417 plant factors, 410-413 storage moisture, 407, 408 windrow resistance, 413, 414 Herbicide, weed management, biotechnology and, see Weed management, biotechnology and Heterosis, root systems and, 105, 120

446

INDEX

Heterozygosity root systems and, 134 sorghum, culture techniques and, 157 Hexaploids, Arachis and, 12, 25, 26

Homology Arachis and, 12, 13

weed management, biotechnology and, 340, 344, 350, 353

Homozygosity, sorghum, culture techniques and, 151, 152, 154, 166 Hormones Arachis and, 7 mineral nutrient uptake and, 193 Horseweed, weed management, biotechnology and, 338, 339 Humidity hay drying, loss during, 410, 429 intercropping systems and, 58, 81 Humidity, acidic mined lands and, see Acidic mined lands Hurley system, mineral nutrient uptake and, 173-176, 198, 204

Hybridization Arachis and, 1, 34, 35 cytogenetics, 8, 9 diploids, 25-28 germplasm, 15, 22 intersectional, 28-30 interspecific, 10-13 potential, 32, 34 utilization, 23, 24 in vitro techniques, 30-32 hay drying, loss during, 416 root systems and, 99, 100, 105 utilization, 121, 128, 132 sorghum, culture techniques and, 148 applications, 166 callus, 154 protoplast culture, 162 somaclonal variation, 165 somatic embryogenesis, 157, 158 weed management, biotechnology and, 339, 340

Hydrolysis, phosphorus and, 298, 312 Hydrophobicity, hay drying, loss during, 411, 412

Hydroponics, root systems and, 105, 136 Hypolimnion, phosphorus and, 313, 317

I

Imazaquin, weed management, biotechnology and, 349 Imidazolinone, weed management, biotechnology and, 343, 349 I n vitro dry matter digestibility, hay drying, loss during, 419, 431 Inflorescence linseed and fiber flax and, 248 sorghum, culture techniques and, 150, 153, 158, 159

Insect resistance Arachis and, 16-19, 21, 22, 33, 34 sorghum, culture techniques and, 152 Insects, weed management, biotechnology and, 328 Intercropping systems, 41, 42, 85 competitive relationships, 52-54 light, 53, 55-57 nitrogen, 58, 59 phosphorus, 59, 60 potassium, 60, 61 water, 57, 58 crop combinations, 42-46 density, 61-63 LER, 50-52 nitrogen, 68, 69, 72, 84 budgeting, 82, 83 fuation, 72-76 losses, 79-82 maize, 68-72 transfer, 76-79 plant arrangement, 63-65 productivity evaluation, 43, 47-50 sowing, 65-68 International Crops Research Institute for the Semi-Arid Tropics, 55, 72, 83 Interpational Rice Research Institute, root systems and, 103, 104, 125, 127 Intracellular resistance, hay drying, loss during, 413 Iodine, linseed and fiber flax and, 222. 246 yield quality, 251, 253, 254 Iron acidic mined lands and, 381, 382, 399 linseed and fiber flax and, 226, 286 acidity, 281 diagnostic criteria, 274 genotype, 284

447

INDEX nutrient disorders, 260, 262-266 uptake, 241, 242 mineral nutrient uptake and, 203, 206 phosphorus and, 307, 313 Iron disulfide, acidic mined lands and, 377 Irradiance, hay drying, loss during, 409, 412-414

Irradiation, weed management, biotechnology and, 352 Irrigation linseed and fiber flax and, 225 nutrient disorders, 256, 257 salinity, 277 yield components, 242, 245, 251 yield responses, 227, 228. 231 root systems and, 93, 128, 132, 136, 138

intercropping systems and, see Intercropping systems weed management, biotechnology and, 357

Lignin linseed and fiber flax and, 251, 260 sorghum, culture techniques and, 151 Lime acidic mined lands and, 379, 385, 389, 395

linseed and fiber flax and, 260, 280, 282 root systems and, 107 Limestone, acidic mined lands and, 383, 389, 391

Linoleic acid, linseed and fiber flax and, 222, 253

Linolenic acid, linseed and fiber flax and,

K

222, 253

Linseed and fiber flax, mineral nutrition of, Karyotype, Arachis and, 9, 10

L Land Equivalent Ratio, intercropping systems and, 47-51, 83, 84 advantages, 51, 52 disadvantages, 52 nitrogen, 68-72 productivity, 62, 66-68 Large round bales, hay drying, loss during, 426-432

Leaching acidic mined lands and, 379 amendments, 384, 385 soil properties, 3% hay drying, loss during, 418, 422 intercropping systems and, 81-83 phosphorus and, 307, 308 Lead mineral nutrient uptake and, 202, 215 phosphorus and, 299 Leaf area index, intercropping systems and, 55

Leafspots, Arachis and, 20, 33, 34 Legume acidic mined lands and seeding, 391, 392 spoil properties, 382, 383 hay drying, loss during, 414, 416, 417 harvesting losses, 422-424 respiratory losses, 419

221-223, 284-286

acidity, 280-282 agronomy, 224. 225 crop growth, 223, 224 genotype differences, 283, 284 nutrient induced genotrophs, 282, 283 nutrient disorders boron, 255-258 calcium, 258-261 copper, 261 diagnostic criteria, 271-276 iron, 262-264 magnesium, 264 manganese, 264, 265 molybdenum, 265, 266 nitrogen, 266, 267 phosphorus, 267 potassium, 267, 268 sulfur, 268, 269 zinc, 269-271 production, 223 salinity, 277-280 uptake major nutrients, 234-241 micronutrients, 241, 242 yield components, 242 major nutrients, 242-251 micronutrients, 251 yield quality, 255 nitrogen, 25 1-253 phosphorus, 253, 254 potassium, 254

448

INDEX

yield responses, 225, 226 major nutrients, 226-234 micronutrients, 234 Loam, acidic mined lands and, 394 Lodging linseed and fiber flax and, 227, 229 root systems and, 116 Lower leaf scorch, linseed and fiber flax and, 265, 266

M Macrophytes, phosphorus and, 310, 318 Magnesium acidic mined lands and, 382, 383, 399 linseed and fiber flax and, 226 acidity, 281 diagnostic criteria, 275 nutrient disorders, 260, 264 salinity, 278, 280 yield quality, 255 yield responses, 233, 239, 241 mineral nutrient uptake and, 203 Maize intercropping systems and, 50, 51, 83, 84 combinations, 42-44 light, 55-57 nitrogen, 68-74, 76-78, 81-83 phosphorus, 60 potassium, 61 productivity, 48-50, 62-68 water, 58 linseed and fiber flax and, 273 mineral nutrient uptake and, 189, 205, 212

root systems and, 138 sorghum, culture techniques and applications, 164, 166 callus, 153 protoplast culture, 160 somatic embryogenesis, 158, 160 Manganese acidic mined lands and, 382, 396, 399 linseed and fiber flax and, 226, 285, 286 acidity, 281 diagnostic criteria, 275 genotype, 284 nutrient disorders, 260-265 uptake, 241, 242 yield components, 251

yield quality, 255 yield responses, 234 Mascasite. acidic mined lands and, 381 Meiosis, Aruchb and, 8, 9, 11, 13 utilization, 23, 26 Meristematic activity, sorghum, culture techniques and, 160, 166 Meristems, linseed and fiber flax and, 256 Methoxyphenone, weed management, biotechnology and, 336, 337 Metribuzin, weed management, biotechnology and, 338, 339 Microarthropods, acidic mined lands and, 399,400

Microbes, weed management, biotechnology and, 326, 328, 329 Microbial herbicide safeners, weed management, biotechnology and, 361, 362, 365, 366

Millet, intercropping systems and combinations, 42, 43, 46 light, 56 productivity, 68 water, 57 Mineral nutrient uptake, 171-173, 214, 215 external concentration, 184-186 composition, effects of, 204-21 1 diffusive pathway into roots, 193-196 nonsteady state, 189-193 plant composition, 202-204 steady state, 185, 187-189 Hurley system. 173-176 photosynthate partitioning, 211, 212 plant growth, 177-180 plant requirements, 180-184 temperature, 1%-202, 213 Mineral nutrition of linseed and fiber flax, see Linseed and fiber flax, mineral nutrition of Mineralization acidic mined lands and, 389 intercropping systems and, 77 phosphorus and, 298 weed management, biotechnology and, 358

Minirhizotron, root systems and, 105, 114 Mitochondria, weed management, biotechnology and, 351 Moisture acidic mined lands and, 377, 379

449

INDEX

genetic manipulation, 346, 347 herbicides, naturally occurring, 335 somatic hybridization, 351, 352 tolerance, 342-345 in vitro selection, 348-351 Mycoherbicides, weed management, biotechnology and, 332, 364 commercial prospects, 329-33 1 microbes, 328 Mycorrhiza mineral nutrient uptake and, 171, 184,

seeding, 390 spoil properties, 380 hay drying, loss during climate, 410 drying, 409 grass, 415, 416 harvesting losses, 422, 425, 427 legume, 416, 417 respiratory losses, 418 storage, 407, 408, 415 storage losses, 428, 430 windrow resistance, 414 linseed and fiber flax and, 225, 226 nutrient disorders, 255, 260 yield responses, 229, 234 root systems and, 107, 118, 122 weed management, biotechnology and,

189, 215

root systems and, 91

N Narrow sense heritability, root systems and, 117-120

328

Necrosis, linseed and fiber flax and, 259,

Molybdenum, linseed and fiber flax and,

265, 266, 269

226

acidity, 281 diagnostic criteria, 275 nutrient disorders, 261, 264-266 Monoclonal antibodies, weed management, biotechnology and, 366 Monocotyledons, root systems and, 94-99 Morphology Arachis and, 4, 9, 27 hay drying, loss during, 416 intercropping systems and, 41, 42, SO, 72 linseed and fiber flax and, 282 mineral nutrient uptake and, 199 root systems and, 91, 93, 94 dicotyledons, 107, 111-115 genetics, 118 monocotyledons, 95, 99, 100, 105 phenotypic plasticity, 137, 138 utilization, 127, 128, 132 sorghum, culture techniques and, 148, 164, 165

Mowing, hay drying, loss during, 419-421 Mulches, acidic mined lands and, 390, 391 Mutation root systems and, 115, 120, 130 sorghum, culture techniques and, 164, 165

weed management, biotechnology and, 332. 366

biodegradation, 360, 361 breeding, 347, 348 cloned genes. 355-357

*

Nematodes, weed management, biotechnology and, 328 Nitrate linseed and fiber flax and, 238, 265, 281 root systems and, 100 Nitrogen acidic mined lands and, 401 amendments, 385, 386 soil properties, 399 spoil properties, 382 hay drying, loss during grass, 414-416 legume, 416 preservation, 43 1, 432 respiratory losses, 419 intercropping systems and, 41, 42, 58, 59, 83-85

budgeting, 82, 83 competitive relationships, 58, 59 fixation, 72-76 losses, 79-82 productivity, 68-72 transfer, 76-79 linseed and fiber flax and, 284 diagnostic criteria, 273, 275, 276 genotype, 282. 283 nutrient disorders, 264-270 salinity, 278 uptake, 235-240 yield components, 242-251 yield quality, 251-254 yield responses, 226-234

450

INDEX

mineral nutrient uptake and, 180, 196, 202-204, 206-209, 211, 215 phosphorus and, 297 root systems and, 91

weed management, biotechnology and, 344

Nitrogenase, mineral nutrient uptake and, 209, 210, 215

Norghum, sorghum, culture techniques and,

acidic mined lands and, 395 root systems and, 123 Pest resistance, sorghum, culture techniques and, 151 Petunia, weed management, biotechnology and, 350, 356, 357, 365 Phenmediphan, weed management, biotechnology and, 349 Phenol, sorghum, culture techniques and, 151, 153, 158, 164

154, 157

Nutrient solutions, mineral nutrient uptake and, see Mineral nutrient uptake

0 Oats intercropping systems and, 42, 43 linseed and fiber flax and acidity, 281 diagnostic criteria, 273 nutrient disorders, 261 uptake, 235 root systems and, 101, 102, 129, 130 Oilseed rape, mineral nutrient uptake and, 196, 199, 204, 205, 207, 212, 214, 215

Oxygen, phosphorus and, 298, 31 1

P

Phenotype root systems and, 92, 140 dicotyledons, 115 genetics, 117 monocotyledons, 100 plasticity, 133-1 39 utilization, 121, 132 sorghum, culture techniques and, 149, 151

Phenotypic plasticity root systems, genotypic variation in, 133 characteristics, 134-136 description, 133, 134 experimental evidence, 136-139 sorghum, culture techniques and, 157 Phenoxyacetic acid, weed management, biotechnology and, 349, 360 Phloem, linseed and fiber flax and, 241, 251, 278

Papermill sludge, acidic mined lands and, 385, 387, 388

seeding, 390 soil properties, 399 Paraquat hay drying, loss during, 422 weed management, biotechnology and, 338, 339, 349

Pea linseed and fiber flax and, 256 root systems and, 113, 117, 120, 137 weed management, biotechnology and, 343

Peanut, see also Arachis root systems and, 113, 114 Peanut mottle virus, Arachis and, 20 Peanut rust, Arachis and, 33 Peanut stunt virus, Arachis and, 20 Peg, Arachis and, 3, 7, 23, 24, 31 Pentaploids, Arachis and, 23 Pepper, root systems and, 116 Percola tion

Phosphate linseed and fiber flax and, 236 mineral nutrient uptake and, 171, 175, 182, 187-190, 195

photosynthate partitioning, 212 solution, 206, 208, 211 temperature, 199 phosphorus and, 297, 312 Phosphinothricin, 335, 337, 341, 346 L-Phosphinothricin, 344, 345, 350, 364 Phosphoric acid, phosphorus and, 299 Phosphorus acidic mined lands and, 385, 396, 399 intercropping systems and, 58-60, 74 linseed and fiber flax and, 284, 285 diagnostic criteria, 273, 275, 276 genotype, 282-284 nutrient disorders, 261, 266, 267, 270, 273

salinity, 278 uptake, 235-241 yield components, 242, 243, 246-251

INDEX yield quality, 252-254 yield responses, 226-234 mineral nutrient uptake and, 202, 203, 206, 211 root systems and, 99, 100, 103, 105, 106 Phosphorus, impact on environment, 297, 298, 317-319 artificial removal from lakes, 316, 317 beneficial effects, 298 detrimental effects, 299-301 impact on aquatic environment, 311, 312 bioavailability, 313-316 particulate, 312, 313 soluble, 312 transport to aquatic environment, 301, 302 amounts, 302-305 desorption, 303, 306, 307 leaching, 307, 308 particulate, 308, 309 transformation, 310, 31 1 Photic zone, phosphorus and, 313, 314 Photophosphorylation, weed management, biotechnology and, 335 Photosynthate partitioning, mineral nutrient uptake and, 211, 212, 214, 215 Photosynthesis, mineral nutrient uptake and, 178 Phytopathogenic agents, weed management, biotechnology and, 33 1, 332, 364 Phytophthroa palrnivora, weed management, biotechnology and, 330 Phytotoxins, weed management, biotechnology and, 333-336, 339 biocatalysts, 363 biodegradation, 360 tolerance, 343 Picloram, weed management, biotechnology and, 337, 349 Pigeonpea intercropping systems and combinations, 42, 43 competitive relationships, 53 LER, 51 light, 55-57 nitrogen, 72 phosphorus, 60 potassium, 61 productivity, 48-50, 63, 65 water, 81

45 1

Plasmid, weed management, biotechnology and, 353-355, 366 biodegradation, 357-361 Plastid, weed management, biotechnology and, 339, 341-343 Plastoquinone, weed management, biotechnology and, 341 Pollen mother cells, Arachis and, 9, 28 Pollen stainability, Arachis and, 11, 28, 29 Pollen tubes, Arachis and, 24 Pollination Arachis and, 3, 12, 23, 26, 27, 32 linseed and fiber flax and. 224 sorghum, culture techniques and, 150, 152, 157, 166 Polyploids, Arachis and, 9, 23, 31 Porosity acidic mined lands and, 380, 401 root systems and, 138 Potassium acidic mined lands and, 385, 399 hay drying, loss during, 418, 424 intercropping systems and, 60, 61 linseed and fiber flax and acidity, 281 diagnostic criteria, 273, 276, 278 genotype, 282, 283 nutrient disorders, 267, 268 salinity, 280 uptake, 235, 238-241 yield components, 246, 248-251 yield quality, 253, 254 yield responses, 226, 227, 231, 233, 234 mineral nutrient uptake and, 175, 178, 182, 187-192, 194, 195, 215 concentration, 202, 203 photosynthate partitioning, 21 1 temperature, 198 root systems and, 99, 131 Potassium carbonate, hay drying, loss during, 412, 423, 424 Potato, weed management, biotechnology and, 341, 351 Potential evapotranspiration, hay drying, loss during, 409 Pozzolanic acid, acidic mined lands and, 395 Prexul, weed management, biotechnology and, 337 Propionic acid, hay drying, loss during, 430, 43 1

452

INDEX

Protein, see also Crude protein Arachis and, 22 hay drying, loss during, 427, 429, 432 intercropping systems and, 43, 84 linseed and fiber flax and, 222, 223, 285 nutrient disorders, 261, 266, 267 yield quality, 253, 254 weed management, biotechnology and. 341, 343-345, 365

Protoplast culture, sorghum, culture techniques and, 160, 162 p b A gene, weed management, biotechnology and, 342, 343, 350, 354 Pyrite, acidic mined lands and amendments, 384, 386, 388 seeding, 389 spoil properties, 381

Q Qe protein, weed management, biotechnology and, 341, 342

R

yield components, 248 yield responses, 229. 232 Regeneration, sorghum, culture techniques and, 155-162, 165, 166 Relative growth rate, mineral nutrient uptake and, 182, 183, 185, 186, 189, 194, 195, 202, 214

Relative humidity, hay drying, loss during, 410.413

Relative Yield Total, intercropping systems and, 43, 47-50, 52 Respiratory losses, hay drying and, 418, 419 Rhizobium, mineral nutrient uptake and, 184

Rhizomatosae, 7, 9 germplasm, 20 hybridization, 11, 12 utilization, 30 Rhizosphere linseed and fiber flax and, 263, 270, 280 mineral nutrient uptake and, 171, 172, 184, 208

Rice intercropping systems and, combinations, 43,46

Radiation, mineral nutrient uptake and, 177, 178

Radioactivity, phosphorus and, 299 Radionuclides, phosphorus and, 301, 318 Radish, mineral nutrient uptake and, 182, 187, 202, 212

Radium, phosphorus and, 297 Rain, root systems and, 93, 94, 105, 125, 130

Rainfall hay drying, loss during, 409 agronomic factors, 414 harvesting losses, 422,424 respiratory losses, 418, 419 windrow resistance, 414 intercropping systems and, 57, 81 linseed and fiber flax and, 225, 228, 233 phosphorus and, 301 transport to aquatic environment, 302, 307-309

Raking, hay drying, loss during, 424-426 Rapeseed hay drying, loss during, 420 linseed and fiber flax and, 222, 224 salinity, 277 uptake, 235, 236

mineral nutrient uptake and, 189 root systems and, 93, 102, 103 genetics, 118, 120 utilization, 124-127 weed management, biotechnology and, 330, 331, 337

RNA, linseed and fiber flax and. 283 Root systems, genotypic variation in, 91, 92, 139, 140

dicotyledons, 94, 95, 107-110, 115-117 alfalfa, 107, 111 bean, 111, 112 cotton, 112 cowpea, 112, 113 pea, 113 peanut, 113, 114 soybean, 114, 115 tomato, 115 edaphic niches, 92-94 genetics, 117-120 monocotyledons, 94-97 barley, 95, 99 corn, 99. 100 forage grasses, 100, 101 oat, 101. 102 rice, 102-104

453

INDEX rye, 104 sorghum, 104-106 wheat, 106, 107 phenotypic plasticity, 133 characteristics, 134-136 description, 133, 134 experimental evidence, 136-139 utilization, 120, 121 alfalfa, 121 crop improvement, 130-132 oats, 129, 130 rice, 124-127 soybean, 122-124 wheat, 121, 122, 127, 128 water flow, 95 Rootworm, root systems and, 100 Runoff acidic mined lands and, 384 phosphorus and, 298, 318, 319 amounts, 303-305 desorption, 306, 307 leaching, 307, 308 particulate, 309 transformation, 310, 311 transport to aquatic environment, 301, 302

Rye, root systems and, 104 Ryegrass hay drying, loss during, 412, 415, 416 harvesting losses, 421 windrow resistance, 414 mineral nutrient uptake and, 175, 177, 178, 180, 182, 187-192, 194-196, 214, 215 concentration, 202 photosynthate partitioning, 21 1, 212 solution, 205-207 temperature, 198 root systems and, 117

S Safflower, root systems and, 116, 137 Salinity linseed and fiber flax and, 226, 257, 277-280, 284

mineral nutrient uptake and, 215 sorghum, culture techniques and, 162, 163

Salmonella, weed management, biotechnology and, 343

Salt, acidic mined lands and, 379, 399 Saturation deficit hour, hay drying, loss during, 422 Sedimentation, phosphorus and, 298 artificial removal from lakes, 317 bioavailability, 314, 315 transport to aquatic environment, 308-310

Sewage sludge, acidic mined lands and, 385-388, 395, 397, 399

Sheep, hay drying and, 431 Sodium linseed and fiber flax and, 226 diagnostic criteria, 273 salinity, 278, 280 yield quality, 254, 255 mineral nutrient uptake and, 175 Sodium chloride linseed and fiber flax and, 277, 278, 280 sorghum, culture techniques and, 163 Sodium lauryl sulfate, acidic mined lands and, 386 Soil phosphorus, impact on environment, see Phosphorus, impact on environment Somaclonal variation, sorghum, culture techniques and, 164, 165 Somatic embryogenesis, sorghum, culture techniques and, 155-161, 164 Sorghum intercropping systems and combinations, 43, 45, 46 competitive relationships, 53

LER, 51

light, 55 nitrogen, 72, 73, 78 potassium, 61 productivity, 62, 65 water, 57 phosphorus and, 308 root systems and, 104-106, 131, 136 Sorghum, culture techniques and, 147-150, 166, 167

anther, 154, 155 breeding goals, 150-152 callus, 153, 154 potential applications. 162 adjunct, 165, 166 model system, 162. 163 screening, 163, 164 somaclonal variation, 164. 165

454

INDEX

protoplast culture, 160, 162 somatic embryogenesis, 155-161 Sorghum bicolor 6.) Moench, 148, 149 Sorption acidic mined lands and, 386 phosphorus and artificial removal from lakes, 317 impact on aquatic environment, 313 transport to aquatic environment, 301, 309-31 1

Sowing linseed and fiber flax and, 225 diagnostic criteria, 239 yield responses, 230, 231 mineral nutrient uptake and, 180 root systems and, 100 Soybean intercropping systems and, 53. 83 combinations, 42-45 LER, 51 light, 55, 56 nitrogen, 68-70, 73, 75, 76, 78, 79 productivity, 48, 49, 63, 65 linseed and fiber flax and, 232, 236, 286 mineral nutrient uptake and, 199, 205 phosphorus and, 308 root systems and, 114, 115, 122-124 weed management, biotechnology and,

acidic mined lands and, 389, 399 linseed and fiber flax and, 226, 285 nutrient disorders, 267-269 yield components, 244 yield responses, 233, 234 mineral nutrient uptake and, 203 Sulfuric acid, acidic mined lands and. 381 Sunflower linseed and fiber flax and, 223 acidity, 281 nutrient disorders, 269 yield components, 248 yield quality, 252 mineral nutrient uptake and, 206, 212 root systems and, 116 Sunflower oil, linseed and fiber flax and, 222

Superphosphate linseed and fiber flax and, 233, 261, 282, 285

phosphorus and, 301, 311 Symbiosis intercropping systems and, 74 mineral nutrient uptake and, 171, 196, 206, 207, 209, 215

root systems and, 91 Symplasm, mineral nutrient uptake and, 194- 196

330, 331

biocatalysts, 363 genetic manipulation, 350 herbicides, naturally occurring, 335 tolerance, 338, 339, 341-343 Spinach, weed management, biotechnology and, 342 Staple Land Equivalent Ratio, intercropping systems and, 48-50 Stomata1 resistance, hay drying, loss during, 41 1

Streptomyces, weed management, biotechnology and, 341, 346 Streptomyces hygroscopicus, weed management, biotechnology and, 335 Sucrose, linseed and fiber flax and, 261, 267-269

Sugar cane intercropping systems and, 42 sorghum, culture techniques and, 162 Sulfonylurea, weed management, biotechnology and, 343-345, 349, 362 Sulfur

T Tedding, hay drying, loss during, 424-426 Temperature acidic mined lands and, 381 hay drying, loss during, 409,413, 429, 43 1

intercropping systems and, 79 linseed and fiber flax and, 225 nutrient disorders, 263, 270 uptake, 239 yield responses, 229 mineral nutrient uptake and, 171, 172, 175, 196-202, 204, 212, 213, 215

phosphorus and, 313 root systems and, 100, 101, 104, 138, 139 weed management, biotechnology and, 328

Tentoxin, weed management, biotechnology and, 335 Terbutryne, weed management, biotechnology and, 348

455

INDEX Tetraploids Arachis and, 1, 8, 13, 24-26, 32 root systems and, 104 Thiobacillusferrooxidans, acidic mined lands and, 381, 386 Thiocarbamate, weed management, biotechnology and, 358, 361 Tillage, phosphorus and, 303 Timothy, hay drying, loss during, 414, 416 Tissue culture, sorghum and, see Sorghum, culture techniques and Tobacco, weed management, biotechnology and, 365 genetic manipulation, 348, 349, 355-357

tolerance, 341-345 Tomato mineral nutrient uptake and, 205 root systems and, 92, 115, 120, 137 weed management, biotechnology and,

hay drying and, 431 linseed and fiber flax and, 229-231 uptake, 237 yield quality, 252 Ureide, intercropping systems and, 74

V Vanadium, phosphorus and, 299 Vapor pressure deficit, hay drying, loss during, 408, 409,414 Vigna subterranea, 3 Virus Arachis and, 15, 21 weed management, biotechnology and, 326

commercial prospects, 329 genetic manipulation, 355 microbes, 328 Volatilization, intercropping systems and, 79-8 1, 83

337, 341

genetic manipulation, 348, 352, 355 Tomato spotted wilt virus, Arachis and, 20 Topsoil, acidic mined lands and, 383-385 394, 395, 399

Toxins, weed management, biotechnology and, 332, 333, 335, 356, 364 Translocation, weed management, biotechnology and, 337, 338, 346 Transposable elements, weed management, biotechnology and, 355 Triazine, weed management, biotechnology and.341-343,

366

genetic manipulation, 347, 352 Tridiphane, weed management, biotechnology and, 366 Trifluralin, weed management, biotechnology and, 344 Trifofiumsubterraneum, 3 Trihalomethane, acidic mined lands and, 388

Triploids, Aruchis and, 12, 15, 25, 26, 33 Tubulin, weed management, biotechnology and, 344 Tumor, weed management, biotechnology and, 355

U

W Washoff, phosphorus and, 307 Water acidic mined lands and, 394 intercropping systems and, 57, 58, 79 linseed and fiber flax and, 251 mineral nutrient uptake and, 199 phosphorus and, 297 bioavailability, 316 impact on aquatic environment, 311-313

transport to aquatic environment, 302, 306, 308, 309

root systems and, 91, 95 dicotyledons, 111, 112, 114-116 monocotyledons, 101-103, 105-107 phenotypic plasticity, 138 utilization, 123, 127, 128 weed management, biotechnology and, 33 1

Water loss, hay drying and, see Hay drying Water use efficiency, intercropping systems and, 58 Waterlogging, linseed and fiber flax and, 258-260, 262

Watersheds, phosphorus and, 307, 308, Uranium, phosphorus and, 297 Urea

311, 315

Weathering losses, hay drying and, 418, 419

456

INDEX

Weed management. biotechnology and, 325-327, 364-366 biocatalysts, 363, 364 biodegradation, 357-36 1

bioherbicide production, 331, 332 commercial prospects, 329-331 genetic manipulation, 345-347 cloned genes, 352-357 mutant selection, in vitro, 348-351 mutation breeding, 347, 348 somatic hybridization, 351, 352 herbicides, naturally occurring, 323, 333 phytotoxins, 333-336 synthetic derivatives, 336, 337 microbes. 326, 328, 329 microbial herbicide safeners, 361-363 tolerance, 337. 338 detoxification, 339-341 substrates, 345 target enzyme, 344, 345 target sites, 341-344 translocation, 338, 339 Weeds, linseed and fiber flax and, 223-225 uptake, 235 yield responses, 232 Wheat intercropping systems and combinations, 43, 46 nitrogen, 77, 78 nutrients, 58 linseed and fiber flax and, 223, 224, 286 acidity, 281 nutrient disorders, 256, 261 uptake, 235-237 mineral nutrient uptake and, 173, 205 phosphorus and, 308

root systems and, 106, 107 genetics, 117. 118 phenotypic plasticity, 138 utilization, 121, 127, 128, 131. 132 weed management, biotechnology and, 341, 347

Windrow, hay drying and climate, 409 grass, 415 harvesting losses, 419,420, 422, 425. 427

Windrow block, hay drying and, 411 Windrow resistance, hay drying and, 410, 413, 414

Withertop, linseed and fiber flax and, 258 259. 280, 284

X Xylem intercropping systems and, 57 linseed and fiber flax and, 251 mineral nutrient uptake and, 178 root systems and, 95, 107, 111, 115 utilization, 125, 127-129 weed management, biotechnology and, 362

z Zinc acidic mined lands and, 385, 399 linseed and fiber flax and, 226, 286 nutrient disorders, 262-264, 269-273 uptake, 235, 239 yield quality, 255