Advances in Agronomy, Volume 23

ADVANCES IN

AGRONOMY VOLUME 23

CONTRIBUTORS TO THIS VOLUME

MARTINALEXANDER

J . G . CADY JOHN L. CREECH R. B. DANIELS C. L. FOY

E. E. GAMBLE M. G . HALE W. W. HECK

H. E. HECCESTAD CHARLES S. HELLINC YOSHIAKI ISHIZUKA PHILIP C. KEARNEY B. A. LAKHDIVE

ROBERT B. MUSCRAVE DALEN . Moss RAJENDRAPRASAD G . B. RAJALE LOUISP. REITZ F. J. SHAY

ADVANCES IN

AGRONOMY Prepared under the Auspices of the AMERICAN SOCIETY OF AGRONOMY

VOLUME 23

Edited by N. C . BRADY

Roberts Hall, Cornell University, Ithaca, New York ADVISORY BOARD

W. L. COLVILLE W. A. RANEY 1. J . JOHNSON J . R. RUNKLES R. B. MUSGRAVE G . W. THOMAS 1971

ACADEMIC PRESS

New York and London

COPYRIGHT 6 1971, BY ACADEMIC PRESS,h C . ALL RIGHTS RESERVED NO PART OF THIS BOOK MAY BE REPRODUCED IN ANY FORM, BY PHOTOSTAT, MICROFILM, RETRIEVAL SYSTEM, OR ANY OTHER MEANS, WITHOUT WRITTEN PERMISSION PROM THE PUBLISHERS.

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LIBRARY OF CONQRESS CATALOG CARD

NUMBER: 50-5598

PRINTED IN THE UNITED STATES OF AMERICA

CONTENTS CONTRIBUTORS TO

VOLUME 23 ....................................................................

PREFACE.................................................................................................... ERRATA for Volume 22 .................................................................................

ix xi

xiii

PLANT GERM PLASM NOW A N D FOR TOMORROW JOHN L. CREECH A N D LOUISP. REITZ

......................................................... I. Introduction ...... II. Ill. IV. Where Germ Plasm Comes from ... V. Collecting and Assembling Germ P ........................... VI. Evaluation, Increase, and Maintena VII. How Germ Plasm Is Used VIII. Germ Plasm Collections ..._.............. IX.

i

4 6 9 12 17 24 37 45 47

THE RELATION BETWEEN GEOMORPHOLOGY A N D SOIL MORPHOLOGY A N D GENESIS R. B. DANIELS,E. E. GAMBLE, A N D J. G. CADY

I. II.

51

111. IV.

V. Summary ................................, ........................................................ References .......................................................................................

53 55 62 84 86

FACTORS AFFECTING ROOT EXUDATION M. G. HALE, C. L. FOY,A N D F. J. SHAY 1. 11.

Introduction ..................................................................................... History ...................................., ., . ., , ., ., ............................................ V

89 92

vi

CONTENTS

111.

IV V. VI. VII. VIII.

Sources of Exudate and Pattern of Root Exudation .......... Interactions of Organic Nitrogen Sources and Exudation ......................... Effects on Carbon and Nitrogen Balance in Plants ... Foliarly Applied Compounds and Root Exudation .................................. Root Exudation-Mineral Nutrient-Mi Effects of Environment on Root Exudation ...............................

.............................................................

93

95 95 97 101

I03 107

I07

NATURE, EXTENT, AND VARIATION OF PLANT RESPONSE TO AIR POLLUTANTS H. E. HEGGESTAD A N D W. W HECK 1. Introduction. ........................................................................ Nature of Plant Response ................................... I I I. Extent of Response ......... 1V. Variation in Rtsponse as R V. Discussion ............................................................................. V1. Summary .............................. References ............................................................................

Ill

11.

140

BEHAVIOR OF PESTICIDES IN SOILS CHARLES

s. HELLING,PHILIP c .

KEARNEY. AND

MARTIN

ALEXANDER

I. Introduction ................................... 11. Processes Affecting Pesticides in Soils

I l l . Effect of Pesticides on Soil Community ................................................ IV. Implications ................................... V. Summary ............ References .....................................

208

PHYSIOLOGY OF THE RICE PLANT YOSHlAKl I. II. 111. IV.

ISHIZUKA

Introduction ..................................................................................... Static Approach: Analytical Studies of the Growth of the Rice Plant ......... Dynamic Approach: Fundamental Research for Higher Yield .................. Conclusion .......... ........... References .......................................................................................

241

243 275 309 3 I0

CONTENTS

vii

PHOTOSYNTHESIS A N D CROP PRODUCTION D A L EN . Moss I.

II. IV. V. VI. V11. V I 11.

AND

ROBERTB. MUSCRAVE

itro iction ..................................................................................... Yield and Net Photosynthesis Breeding for Photosynthetic Efficiency ................................................ Selecting for Low I' Traits in High I' Species ...................... Selecting for Photosynthetic Rates within Species ................................ Breeding for Plant Type ........................................................ Managing Photosynthesis ...................................................................

17 3 I7 32 0 327 327 328 33 1 334 334

NITRIFICATION RETARDERS A N D SLOW-RELEASE NITROGEN FERTILIZERS R A J E N D RPRASAD. A G. B. RAJALE,A N D B. A. L A K H D I V E

............................................. ultural Chemicals ..... Nitrification Retarders .............................. Slow-Release Nitrogen ........................................ Coated Fertilizers ............................................................. Concluding Remarks ............. .................... ...............................................................................

337 342 345 358 37 I 375 376

Author Index .............................................................................................. Subject Index .............................................................................................

385 405

I. 111. IV. V. VI.

Introduction

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CONTRIBUTORS TO VOLUME 23 Numbers in parentheses indicate the pages on which the authors’ contributions begin.

MARTIN ALEXANDER ( 147), Department of Agronomy, Cornell University, Ithaca, New York J. G . CADY(5 l ) , Department of Geography and Environmental Engineering, The Johns Hopkins University, Baltimore, Maryland JOHN L. CREECH ( I ) , Agricultural Research Service, United States Department of Agriculture, Beltsville, Maryland R. B. DANIELS (5 I ) , North Carolina Agricultural Experiment Station, Raleigh, North Carolina C. L. FOV(89), Department of Plant Pathology and Physiology, Virginia Polytechnic Institute and State University, Blacksburg, Virginia E. E. GAMBLE (5 I ) , North Carolina Agricultural Experiment Station, Raleigh, North Carolina M . G . HALE(89), Department of Plant Pathology and Physiology, Virginia Polytechnic Institute and State University, Blacksburg, Virginia W. W. HECK( I I I), Environmental Protection Agency, North Carolina State University, Raleigh, North Carolina H. E. HEGGESTAD ( 1 I I), Agricultural Research Service, United States Department of Agriculture, Beltsville, Maryland CHARLESS . HELLING( 147), Agricultural Research Service, United States Department of Agriculture, Beltsville, Maryland YOSHIAKI ~ S H I Z U K A(24 I), Emeritus Professor, Hokkaido University, Japan; Asian and Pacific Council, Food and Fertilizer Technology Center, Taipei, Taiwan PHILIP C. KEARNEY ( 147), Agricultural Research Service, United States Department of Agriculture, Beltsville, Maryland B. A. LAKHDIVE (337), Division of Agronomy, Indian Agricultural Research Institute, New Delhi, India DALE N . MOSS (317), University of Minnesota, St. Paul, Minnesota ROBERT B. MUSGRAVE(3 17), Cornell University, lthaca, New York RAJENDRAPRASAD (337), Division ofAgronomy, Indian Agricultural Research Institute, New Delhi, India G . B. RAJALE(337), Division ofAgronomy, Indian Agricultural Research Institute, New Delhi, India LOUISP. REITZ( 11, Agricultural Research Service, United States Department of Agriculture, Beltsville, Maryland. F . J. SHAV(891, Department of Plant Pathology and Physiology, Virginia Polytechnic Institute and State University, Blacksburg, Virginia ix

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PREFACE

I n recent years the value of science to society and in turn the obligation of society to support science have become widely questioned. Some national leaders have expressed the feeling, likely shared by many laymen, that science and technology are inherently evil. In their view, pressing human problems such as pollution, crime, and even poverty are caused directly or indirectly by technological developments. Students, joined by irate taxpayers, are concerned about the seeming irrelevance of most scientific endeavors. They see no pay-off to society from the public inputs supporting scientific research, and they are concerned with the apparent independence of scientists who follow their own disciplineoriented curiosities rather than addressing themselves to the solution of human problems. Fortunately, one has little difficulty justifying, even to the pragmatist, the benefits to society of agronomic research. The eight articles appearing in this volume exemplify this fact. Each of them reviews the state of knowledge on subjects of considerable importance to man. Concern with factors affecting the quality of the environment is demonstrated through reviews on pesticides in soils, plant response to air pollutants, and slowrelease nitrogen fertilizers. Man’s ability to feed himself is the direct or indirect concern of papers dealing with rice physiology, the world’s plant germ plasm base, and factors affecting photosynthesis and crop production. Two other papers concern the state of our knowledge of important soil and plant processes. I t is quite appropriate for the scientist to justify his existence to society as a whole and especially to those who support him and his work. Likewise, the scientist has an obligation to inform not only his immediate colleagues but the larger scientific community as well of his accomplishments and failures. Reviews such as those presented in this volume are important components of the world-wide scientific communication system. They represent the first step in fulfilling an important obligation of scientists to society. N . C . BRADY Ithaca, N e w York August, 1971

xi

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ERRATA Volume 22 Page 163, Eq. (3), the fraction 4/3 should he enclosed in parentheses Page 168, Eq. (7). the left-hand side should read: k + H ) ( a c / a r ) + c ( a H / a t ) ;helow Eq. (7). c ' should read c ' . ~ Page 168, line 12, the ratio y/H should be enclosed in parentheses Page 173, Eq. (15), the second term should read ( W u / L ) Page 187, Eq. (22). the second equals sign should he a minus sign Page 342, line 4 should read: This would equal 5.0 mg of Mg per 100 g of soil

...

Xlll

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ADVANCES IN

AGRONOMY VOLUME 23

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PLANT GERM PLASM NOW AND FOR TOMORROW John 1. Creech and Louis P. Reitz Agricultural Research Service, U.S. Department of Agriculture, Beltsville, Maryland

I. Introduction ..................................................................................... 11. Germ Plasm Definitions ..................................... .................................................... 111. Our Needs for Germ Plasm. ............................... IV. V A. Seed Germ P ......................... B. Vegetatively VI. Evaluation, lncre A. Small Grain

.................................................... VII.

VIII.

A.

Cereal Grains

C.

Horticultural

.................................................... .................................................... ....................

................................................... A. Number of Items .............................................................

IX. Programs for the Future ......................................

I.

I 4 6 9 12 14

IS 17 18 22 24 25 29 31

35 37 37 39 44 45 47

Introduction

Modern crop husbandry is based on an exceedingly small segment of the plant kingdom. These efficient crops are the cumulative results of man’s research. But in this generation and for the future, plant breeders will be dependent for raw materials on the genetic resources that occur in the “gene centers” of the world. Such centers may be the geographic origins of the parental species or cultivation centers where primitive varieties have been handed down and improved by farmers over generations. Many of these progenitor types and weedy relatives of crops have been the principal sources of pest resistance and unique traits essential to successful recent crop improvement. If they are allowed to be displaced 1

2

JOHN L. CREECH A N D LOUIS P. RElTZ

and destroyed, these genetic resources cannot exert their particular influence on modern plant breeding. These irreplaceable genetic resources are seriously threatened with depletion by overgrazing and related malpractices, or through the replacement of “land races” by improved varieties. This fact has not attracted the attention of enough scientists. The impact of such losses is difficult to relate to our immediate circumstances of high crop yields, and this has produced complacency. However, it does not require much imagination to appreciate the need for genetic diversity in the face of the current threat to production by serious diseases, such as corn leaf blight (Helminthosporium maydis), or the first discovery of leaf rust (Hemileia vastatrix) of coffee in the Western Hemisphere in January 1970. The latter disease eliminated coffee production in Ceylon prior to 1900 and caused a shift to tea production. Recognizing that these resources are essential to agriculture in developed and developing countries alike, international organizations such as the Food and Agriculture Organization of the United Nations (FAO), the International Biological Program ( I BP), and The Rockefeller Foundation have encouraged the development of national and international programs to attack the problem of dwindling germ plasm reserves. To emphasize this goal and lay the scientific foundations for action programs, an international Technical Conference was held in Rome, September 18-26, 1967, under the auspices of FAOlIBP ( I 967). The conferees proposed that there be greater coordination among nations concerned with the exploration, conservation, and utilization of plant resources for agriculture and forestry. This would include existing institutions, such as botanic gardens and arboretums, agricultural and forestry research stations, agencies dealing with seed distribution, and international organizations which undertake programs of agricultural development anywhere in the world. The recommendations urged surveys to determine plants that are threatened by imminent destruction in their natural habitats and by decline in existing collections. Such surveys would permit priorities to be established so that the species and areas of greatest danger of genetic erosion would receive first consideration. I t would be anticipated that the scientists with special knowledge of local and regional conditions with respect to special plant groups could be encouraged to provide the needed information. It was recommended that institutions engaged in the study of variation of plants serve as centers to train young scientists in the field of phytogeography, tactics of plant exploration and collection, and related matters as a means of developing facilities and staff for national and international

P L A N T GERM P L A S M N O W A N D FOR T O M O R R O W

3

centers for regional exploration activities in developing countries or groups of countries with common needs. Concern was also expressed for the development of improved systems of evaluation and exchange of primitive stocks and other cultivars so as to make these essential materials equitably available and to avoid excessive duplication of effort. This would require international accord for the treatment, storage, and retrieval of genetic information which is becoming of increasing significance as the available germ plasm expands and the evaluations become more complex. This will require specialists concerned with methods to standardize climatic, ecological, phytogeographic, and taxonomic data, as well as the basic plant characteristics which are to be considered for standardization. The conference encouraged national programs to protect genetic resources habitats and to develop seed banks by drawing up plans which would create programs for national germ plasm maintenance. The members recognized that sophisticated storages in every country are unrealistic and urged that those storages now in operation, or under construction, accept international responsibilities as an interim means of protecting germ plasm. The Society for Economic Botany, in cooperation with the International Biological Program (US1BP/U M), organized a symposium at Columbus, Ohio, on September 5 , 1968, to focus attention on the urgent need to protect, conserve, and properly utilize the world’s plant resources [see Econ. Bot. 23(4) ( 1 969)]. The Symposium was designed to support the recommendations of the 1967 F A 0 Technical Conference on Exploration, Utilization, and Conservation of Plant Gene Resources. It provided a forum for alerting a segment of the U.S. biological community to our dependence on foreign sources of crop germ plasm and the needs for national and international cooperation in rescuing and conserving these irreplaceable resources. Although much is known of the origins and centers of diversity of our crop plants, American biologists were asked to consider how best to sample this diversity, ways and means to utilize germ plasm, conserve it for the future, and be alert to opportunities for international cooperation in these efforts. The Symposium was prompted by the realization that the problem of dwindling germ plasm resources is not being attacked in the United States in a coordinated fashion. There are Federal and other agency programs involved with the introduction, evaluation, and conservation of crop germ plasm. Some of these have been in effect for many years and provide a basic service to American plant science, but they have not pooled their efforts for maximum impact on this broad problem. There has been some encouraging progress. Recently, a Crop Evolution Labora-

4

JOHN L. CREECH A N D LOUIS P. RElTZ

tory was established at the University of Illinois. The mission of this Laboratory is largely concerned with the origins and evolution of cultivated plants and their relationships to wild relatives. The work of the Laboratory will nicely complement action programs for the collection and objective evaluation of crop breeding stock. There are also provisions in the Plant Gene Pools objectives of US1 BPlUM to give leadership to this overall problem area and to develop national working groups to implement the broad recommendations of the International Biological Program. Thus, I BP is promoting international recognition of the problem and is providing a framework for more meaningful cooperation on a worldwide basis. The objectives that are realistic for specific action to be undertaken by scientists in the United States are as follows: 1. To conduct national surveys of primitive and wild genetic resources of plants held by institutions and scientists in the United States; and to identify the scope, quality, and usefulness of these gene resources; 2. To assemble, analyze, and retrieve this information through use of automatic data processing; and to disseminate results of the survey to I BP, FAO, Agency for International Development (AID), institutions and scientists here and in other nations; 3. To cooperate in national programs to use and perpetuate this base of genetic variability utilizing existing facilities and proposing additional ones; and to develop information on these resources from a genetic, agricultural, and other scientific standpoint and to fully exploit their diversity; 4. To participate with IBP, AID, and other institutions in developing priorities for urgent germ plasm collecting in threatened biological and geographical areas. In keeping with the concepts of these various recommendations, the present paper is an effort to describe the current activities that have an impact on the future of germ plasm in the United States. As a nation so dependent on the world’s plant resources for raw genetic materials, we are obligated to assume a leadership role in the use and conservation of these essential inputs to plant breeding. As scientists, we would indeed be less than responsible to future generations if we do not give high priority to measures that will protect these dwindling resources from deterioration. The paper serves also to identify the various agencies and institutions that have expressed a commitment to germ plasm. II.

Germ Plasm Definitions

Germ plasm has become a standard term in agricultural research. It has been employed most extensively in dealing with crop improvement.

PLANT GERM PLASM N O W A N D FOR TOMORROW

5

Crop germ plasm has been applied to individual donors of genetic traits at one extreme to large assemblages of breeding stocks at the other extreme. A number of related terms are beginning to appear in the literature, and it seems appropriate to attempt in this paper to define some of these terms as they relate to the use and conservation of the world’s plant resources. Crop germ plasm can be defined as the array of plant materials, assembled or not, that serves as a basis for crop improvement, or related research. In this sense, germ plasm is a generic term, but its chief characterization is that of a reservoir of genes to meet the needs of plant breeders. It allows for a range of plant materials that may be as narrow and specific or as broad and widely represented as is necessary to meet the objectives of the user. I t also implies documentation of the components and continuity of this material. Collections of germ plasm fall into two major categories. Working collections are held in adequate storage, documented, and available for immediate use. Working collections are usually specific crops and may acquire the title of a world collection when the number and geographical origin of accessions has reached such a magnitude as to result in worldwide requests for seed. Sometimes world collection implies a broadly represented collection even though there is no attempt to distribute samples. Conserved stocks are broad segments of germ plasm held in national and international seed storages for long-term conservation (conservation centers). These stocks duplicate working collections but are released only when the latter have been depleted. The flow of germ plasm is from holders of working collections to long-term storages. National seed storages accept all categories of germ plasm, have precise policies governing acceptance of seed, and attempt to provide the optimum temperature and humidity conditions to maintain seed viability for extended periods of time. In the United States the USDA’s National Seed Storage Laboratory, Fort Collins, Colorado, is a center for conserved stocks. A similar storage exists at the National Institute of Agricultural Sciences, Hiratsuka, Japan (Ito and Kumagai, 1969). FAO, in cooperation with the Turkish government has constructed a cold storage at Izmir, Turkey, to preserve the germ plasm collected throughout Turkey. Unlike the rather precise facilities which can be provided to protect reserves of seed-reproduced plants, working stocks of vegetatively propagated crops constitute our base of conserved materials. Because of the tremendous cost of holding vegetative stocks, duplication appears impractical. It is for this very reason that the loss or abandonment of these types of collections is so detrimental.

6

JOHN L. CREECH A N D LOUIS P. REITZ

I n contrast to the broad aspect of germ plasm, the term genetic stocks of crop plants, as defined by the Crop Science Society of America, is restricted to stocks of specific genes and gene combinations that have direct usefulness in genetic analysis. A characteristic of genetic stocks is that such materials frequently are maintained by individual workers. However, the Preservation of Genetic Stocks Committee, CSSA, is attempting to identify holders of genetic stocks and to encourage deposition of these stocks in the National Seed Storage Laboratory, Fort Collins, Colorado. A new encompassing germ plasm term was coined by the F A 0 Panel of Experts on Plant Exploration and Introduction (1970a). This is the genetic resources center. The purpose of delineating such centers is to provide for an umbrella-type organization to combine the objectives of long-term conservation (conservation center) and the propagation and distribution of plant materials held in one or more working collections. The essential requirements recommended by the F A 0 panel for the establishment and functioning of a genetic resources center include the following: A genetic resources center must provide for long-term conservation of seed and maintain vegetative collections. It should be operated by a professional staff with background and facilities for the propagation and distribution of germ plasm. These facilities do not need to be a physical part of the main center and one or more associated institutions with established collections could serve as subcenters. Because of the stringent quarantine safeguards necessary for international movement of plant materials, close collaboration with plant quarantine officials is essential. Some centers in countries out of the crop cultural range can serve as intermediate quarantine propagation stations in the manner that the Plant Introduction Station, Glenn Dale, Maryland, and Kew Gardens, England, do for coffee and cocoa, respectively. The center is responsible for organizing the increase of collections and arranging for the transfer of germ plasm from working collections to longterm storage. This will require methods for documenting and accumulating additional information. The center would be rzsponsible for providing data to a central information system at FAO, Rome. Ill.

O u r Needs for G e r m Plasm

Plants cover the land areas of the earth; some are found in the sea. In its totality, this embraces our heritage of germ plasm. Each organism owes its existence to the genes it contains which set in motion a train of events leading to its attained uniqueness. The value of germ plasm can only be equated with the magnitude of the benefits from plants to man’s total ex-

PLANT GERM PLASM N O W A N D FOR TOMORROW

7

istence and to the preservation of the environment in which he lives. I n recent years, man has realized that the latter engulfs the entire planet including our atmosphere. Most people do not regard germ plasm in such a broad setting, but think only of special needs for single, or a few, genes to utilize in reaching a momentary objective in plant breeding, or to find a new variant useful in achieving some particular goal. Yet, when the question is raised about what and how much we should be concerned, the list invariably grows and encompasses our entire flora. Even deleterious factors are sought as genetic markers or as a means to some positive goal. The world’s plants are the survivors of gene pools created by cross breeding, mutations, natural selection, or by other means. The germ plasm exists as recombinants, mostly in farmer’s seed in the case of cultivated crops; some exists in nature and some in living herbaria or collections. Under the dynamics of world change, the products of evolution and of plant breeding are quickly exploited over wide areas. One result seen repeatedly in cultivated crops is a dramatic change in the germ plasm comprising the cultivated species grown at the beginning and at t h e end of a single decade. This is now becoming applicable to every major agricultural country and in a more serious way, to developing nations. The recent highly successful transplantation of Mexico-derived wheat varieties to the Mideast and elsewhere totaling 20 million acres over a 5-year period is a good example of the extent of change. The United States, Mexico, Canada, and Australia may be cited where repeated replacement of contemporary wheat varieties by newer ones is an accepted pattern of agriculture. One thus has the option to discard or conserve the superseded varieties. Concern is often expressed that irreplaceable germ plasm is being lost. Of course, it is! The recent trend over large tracts of land whereby ancient varieties of local evolutionary development are being replaced by rather limited germ plasm, gives strong support to that concern. Land denuded by stripping off the timber or by overgrazing has erased irretrievable germ plasm. Further man-made improvement in crops through breeding will largely be traceable to genes extant in living herbaria or collections. Only at times of dire need do breeders supplement these by crash programs to collect breeding stocks among existing gene pools in the wild. More often they wait for, or hope to induce, the desired mutations. New crops are being sought to better serve man’s needs. Unless suitable germ plasm can be found which is amenable to the purpose and capable of manipulation to meet agronomic standards, these goals cannot be met. The range of worthwhile goals covers crops for medicinal purposes, industrial uses, pest repellents, and above all, nutritious foods. Research

8

JOHN L. CREECH A N D LOUIS P. REITZ

is being conducted on plants to find constituents for treatment and control of cancer; to discover new annual sources of fiber; to develop less hazardous insecticides; to extend the principle of biological resistance to more diseases and insect pests; and to provide new energy-trapping food plants to supplement those we have pushed almost to the limit. Established crops, such as sorghum, rice, wheat, maize, soybeans, and alfalfa, have been improved over the years by adding genes from alien sources to local or regional varieties. The objective has been to increase productivity, enhance stability, improve quality, and reconstruct plants to increase their physiological efficiency. Much work has gone into breeding for disease and insect resistance, and the need for new genes for this purpose is a continuing one. There is no better way than through genetic mechanisms to reduce losses from biological competition. Human population density is so great in certain areas and man’s dependence upon annual crop abundance so absolute that famine is axiomatic if we are faced with a major epiphytotic, if a new insect ravages the crops, or if planted crops are destroyed by other natural catastrophes. So, man’s dependence now on stable crops and his growing need for every greater volume of food production leaves little place for complacency about germ plasm both in the short and long views. Our need for germ plasm preservation has immediate and long-term aspects, and it varies considerably among the species. Some species are utilized in the wild form, vis., native grassland, timber, tundra, and some tree nut crops. For these to become extinct is conceivable since it is not feasible to preserve these materials with the same ease as for beans, oats, or clover. However, as we encroach on the wild plantings of forest species and grazing plants, and are faced with the reestablishment of cover on denuded areas, the same need for germ plasm exists as in the case of a food plant. Furthermore, a diversity of forms from which improved types for reestablishment may be developed approaches that heretofore thought about only in the case of cultivated crops. Wilderness areas and parks help to preserve such species. There is a constant relationship between the number of diverse genes available and the number of true-breeding recombinations which may theoretically be obtained. This is expressed by 2” in which n is the number of alleles. Ten alleles could be arranged in 2 ’ O or 1024 different truebreeding combinations or genotypes. Hence, one might have over 1000 items in a collection and be concerned with only 10 diverse alleles. By adding one new gene, the recombinations jump to 2048. Therefore, it is well to consider what the gene pool contains in contrast to the possible number of recombinants or genotypes. For various reasons, all recombinations are not obtained; some may not even be viable. Nearly all

PLANT GERM PLASM N O W A N D FOR TOMORROW

9

populations sampled will reveal the duplication over and over of most of the genes. One way to relate genes to populations is given in Fig. 1.

Species diversity

FIG. I . Relationship of gene base to species diversity. A small gene base can provide for considerable variability among populations that are derived from it.

IV.

W h e r e Germ Plasm Comes from

Germ plasm of crops is derived from three main sources. These are ( I ) the wild species and primitive forms of crops in primary centers of diversity, (2) plant migrants to secondary centers of culture where their diversity may be augmented, and (3) the unique products of plant breeding. The present-day theories of the origins of germ plasm are mostly refinements of Vavilov’s concept of the phytogeographic basis for plant exploration (Vavilov, 1950). While these modern interpretations, based on the ever-increasing knowledge of plant resources are significant, it is still Vavilov’s earlier concept that is employed in determining the gaps in our genetic resources, establishing the priorities for procuring wild and primitive crop progenitors, and conducting the necessary explorations to secure the material. In terms of cultivated plants we tend to think of the origins as those areas which contain the broadest array of genetic diversity. However, the concept is not always valid, cannot be irrevocably proved, and disallows multiple origins. Briefly summarized, Vavilov’s main centers of origin (now considered as centers of diversity) are listed below with the number of species of some degree of use to man and the approximate number of crops of considerable world importance contributed by each center. 1 . China- 136 species, 1 I crops 2. India- I17 species, 15 crops a. Indo-Malaysia-55 species, 4 crops 3. Middle Asia-42 species, 15 crops 4. Near East - 83 species, 20 crops 5. Mediterranean 84 species, 6 crops 6. Ethiopia-38 species, 15 crops 7. Mexico-Central America- 49 species, 9 crops

10

JOHN L. CREECH A N D LOUIS P. REITZ

8. South America (Peru, Ecuador, Bolivia)-45 species, 7 crops a. Chile-4 species, 1 crop b. Brazil, Paraguay - I3 species, 6 crops These are the primary centers from which the crop species were first moved for domestication. In view of the remarkable shifts of many crop species to distant lands for cultivation purposes, there has emerged the concept of secondary centers of diversity. In many respects, these secondary centers have become significant sources of germ plasm, especially in respect to different biological ecosystems. In primary centers of diversity the species is in competition with other natural elements of its environment, whereas in secondary centers new biological and physical stresses become selective factors. Primitive cultivars in these secondary centers are important segments of germ plasm because of man’s involvement in assuring their success although the age element is extremely brief in comparison to the natural evolvement of crop species in their primary centers of origin. While we regard the materials from primary and secondary centers as the main components of germ plasm, to these must be added the materials developed through highly advanced plant breeding techniques (Hanson, 1969). This source of germ plasm includes such significant contributions as induced polyploidy, mutations, and the combining of multiple traits into useful breeding lines not likely to be discovered either in progenitor species or primitive varieties. A simplified flow diagram of the origins of our germ plasm might be as depicted in Fig. 2. The most diverse component of our germ plasm is the contribution of the primary centers. In potato germ plasm held in the working collection Primary centerswild soecies of croDs and close relatives, immediate primitive cultivars I

I

,

Secondary centersprimative cultivors, land races with selective traits for new ecosystem stress

^

I

berm

Advanced breedina linescombined troits, controlled heterotic responses, induced mutations, apomictic lines, etc.

FIG.2. Sources of germ plasm relative to amount and diversity of contributions.

PLANT GERM PLASM N O W A N D FOR TOMORROW

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at Sturgeon Bay, Wisconsin, 97% of the introductions added since 1958 are wild collections or primitive types (Ross and Rowe, 1965). These are important sources of such traits as frost resistance (Ross and Rowe, 1969) and immunity to virus X (Solanum acaule Bitter), immunity to viruses A and Y ( S . stoloniferum Schlecth., S . chacoense Bitter), field resistance to late blight ( S . demissum Lindl.), and nematode resistance ( S . vernei Bitter & Wittm. ex Engl.). Recently, Radcliffe and Lauer (1966) screened 395 potato introductions involving 65 species, and found resistance to green peach and potato aphids in 6 species of limited distribution in Mexico. Thus, within the broad concepts of primary centers of diversity, microcenters yielding specific genetic traits are being identified. Contributions of secondary centers are apt to be less identifiable except when the germ plasm is evaluated in an organized manner. In the collection of pepper (Capsicum frutescens L.) screened at the Southern Regional Introduction Station, Experiment, Georgia (Sowell and Langford, I963), resistance to bacterial spot (Xanthomonas vesicatoria) was not found in accessions from South America, where C . frutescens is native. I t was located in introductions from India, where selection pressure was exerted on the host by the pathogen during the period of their association after both had been introduced into India. It is interesting to note that the varieties and selections of Cofea arabica L. which show the greatest degree of resistance to coffee rust (Hemileia vastatrix) races are descendants of a plant of variety K E N T which was discovered in a plantation at Mysore, India, in I9 I 1 . The rust had invaded India around 1870 and devastated most of the plantations. Although both Cofea arabica and the rust are native to northeastern Africa (Ethiopia), no particularly resistant types have been found among the collections from there. The contributions made by plant breeders and geneticists through the manipulation of germ plasm is often overlooked in relation to the quality of our germ plasm resources. Many of these research products would not likely survive in primary centers of germ plasm since they exert a neutral or negative influence on genetic continuity and are dependent on man’s intervention to realize their benefit. The new short wheats are a perfect example of the production of a germ plasm complex that includes photoperiodic insensitivity, shortened stature through the use of the introduction NORIN 10, broadly based disease resistance, and the ability to produce high yields with appropriate irrigation and fertilizer. In the wild, the character of short stature, so important to the new strains, might well be regarded by the plant collector as a deficiency and something not to be collected, or not even observed in a population of taller plants.

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JOHN L. CREECH A N D LOUIS P. REITZ

The H I G G I N S buffelgrass Pennisetum ciliare (L.) Link, may be cited as an example of an artificially produced crop variety developed by control and manipulation of apomixis (Anonymous, 1968). A single sexual plant (discovered in 1958) was used as a female parent in crosses with different biotypes of apomictic buffelgrass. Selected apomictic progeny from these crosses possessed combinations of characters (improved vegetative vigor, seed yield, and drought resistance) that had not been observed previously in collections of this species. Subsequent research has revealed that apomictic reproduction in buffelgrass is inherited like other genetically controlled characteristics, and can be manipulated. V.

Collecting and Assembling Germ Plasm

Collecting and assembling germ plasm has become a formidable task and of such a magnitude that no one individual can assume the responsibility for all of its aspects. Rather, private, state, regional, national, and international organizations need to act on a coordinated basis, if we are to collect and utilize effectively more than a fraction of the world’s genetic resources while they exist. This requires an organization and personal commitment to the principle of good management of germ plasm, a need that was recognized as early as 1898 by David Fairchild. He saw that plant introduction had special needs involving people and facilities for plant exploration, quarantine procedures, special propagation techniques, as well as distributing seed or plants and documenting the results achieved. It is interesting to note that the need to conserve germ plasm had not been judged to be essential. But this was in an era when there was rather free access to most of the world by the developed nations, and germ plasm did not appear to be threatened. The general philosophy expressed by Fairchild (1938) and the interest and enthusiasm developed by the team of scientists he assembled was to have a profound effect on American agriculture. Their efforts resulted in a long succession of plant explorations to centers of diversity. A system of documentation was devised and persists today in the form of the Plant Inventory. This is the published documentation of seeds and plants introduced by the U.S. Department of Agriculture. The first of the Federal Plant Introduction Stations was established, now represented by those at Chico, California; Miami, Florida; Savannah, Georgia; and Glenn Dale, Maryland (Hodge et al., 1956). It was in this era that individual plant explorers, N. E. Hansen, F. N. Meyer, P. H. Dorsett, H. L. Westover, C. R. Enlow, W. Popenoe, and scores of other scientists made their personal contributions to plant introduction (Ryerson, 1967). After World War I I, a committee of scientistladministrators headed by Director R. E. Buchanan, Iowa AES, recognized the need for organized

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Federal/State cooperation in the introduction, evaluation, and maintenance of “new crops.” No longer did the system of reliance on individual scientists to receive and evaluate germ plasm, often discarding what appeared nonessential, serve our national interests. The deliberations of this committee resulted in the system of regional plant introduction stations to serve the needs of scientists in regional associations of states. These stations are located at Agricultural Experiment Stations at Geneva, New York (Regional Project NE-9); Ames, Iowa (Regional Project NC7); Experiment, Georgia, (Regional Project S-9); and Pullman, Washington (Regional Project W-6) (Hodge and Erlanson, 1955). They also envisioned and caused the Potato Introduction Station, Sturgeon Bay, Wisconsin (IR-l), to be established in 1950 and the National Seed Storage Laboratory to be established at Fort Collins, Colorado, in 1958. There are two additional developments worthy of note here. In order to prevent the extinction of native strains of maize, The National Academy of Sciences - National Research Council formed a Committee on Preservation of Indigenous Strains of Maize (Clark, 1956). This Committee sponsored a project to collect, preserve, and study for future use as many varieties and pioneer corns as possible. The Committee, in cooperation with The Rockefeller Foundation, the Government of Brazil, and the USDA established seed centers in Mexico, Colombia, Brazil, and Maryland, USA. Over a 3-year period (195 1-54) some 12,000 variants of Indian corn were collected through cooperative efforts of collaborator scientists in the Caribbean area, Mexico, Central America, ColombianAndean area, Brazil-Eastern South America, the United States, and Canada. These materials have been in demand by plant breeders ever since their assemblage. The need to collect and evaluate ornamental plants on a sustained basis was recognized by The Longwood Foundation (Longwood Gardens), Kennett Square, Pennsylvania, in 1956. U p to this time exploration for ornamental plants had been supported by private institutions, but, with more restrictive quarantines and less private access to many parts of the world, there was need for a Federal program. The Longwood Foundation proposed ajoint project to the Agricultural Research Service to encourage the advancement of ornamental horticulture through the discovery and introduction of new or little-known plants of the world which would have potential value for research and ultimately, the people of the United States. Since 1956, funds from Longwood Gardens have supported 12 ornamental explorations to Europe, parts of the U.S.S.R., the Himalayas, most of the Far East, New Guinea, Brazil, and Australia. For the first time, the private sector and government had joined forces to support a sustained effort in ornamental plant exploration.

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JOHN L . CREECH A N D LOUIS P. REITZ

The principal means of assembling germ plasm are direct exploration, exchanges of plant material between scientists, and the transfer of increased stocks from existing working collections into more comprehensive ones. We are involved in assembling seed and vegetative germ plasm of hundreds of species and each has its own particular requirements for increase, evaluation, and conservation. A. SEEDGERMPLASM The exploration for, and introduction of, seed-reproduced crops involves direct collecting in the primary and secondary centers of diversity. Explorations are no longer conducted with the broad sweep of the earlier collectors, who considered the collecting of all crops encountered as their responsibility. Instead, we concentrate on collecting to improve the diversity of germ plasm for specific crops. The explorations are conducted in a systematic fashion by crop specialists. Most plant collecting explorations are of short duration, rarely exceeding four or five months, coinciding with the time of seed ripening for the species of primary interest as governed by altitudinal or latitudinal range. Working with efficiency, a well-equipped team of two collectors, familiar with the terrain, is unlikely to exceed an average of 10 collections daily or about 1000 samples for a trip. A striking conclusion by Bennett (1970) is the awareness that such a level of activity cannot possibly meet the needs to salvage genetic resources being depleted at an alarming rate. In order to improve, the situation requires the establishment of exploration priorities, international cooperation, trained personnel, and adequate funds to operate as the National Academy provided for maize. As a means of mounting a sustained effort in plant collecting, the F A 0 Panel of Experts on Plant Exploration and Introduction recommended that short-term practical training courses in plant collecting be given as well as support to university courses in the use and conservation of genetic materials; that personnel for training be drawn from developing as well as developed countries: and that individual governments consider ways to collect its indigenous germ plasm. In addition, the F A 0 offered to act as a clearinghouse for informing interested scientists of forthcoming explorations through the Plant Introduction Newsletter ( 1970b). The art of field collecting has also changed, and considerable attention is now paid to sampling techniques as well as population distribution. Field sampling procedures are now designed to review the fullest possible variability of the species. Collecting teams should therefore include broadly trained agronomists, geneticists, entomologists, and pathologists. It is essential that they document the greatest amount of useful data for the scientist making subsequent evaluations. Both random and directed

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sampling methods of collection, aimed at filling the gaps, are employed (see Fig. 2 ) and accurate documentation as to the character of sampling has become of increasing significance as a means of communication between the collector and the evaluator. In view of the complexity of modern field exploration, certain organizations, particularly FAO, are especially helpful in providing information about agricultural and geographical conditions in remote areas and often provide access to competent local collectors (FAO, 1969b). I n the United States, the New Crops Research Branch, Plant Science Research Division, Plant Industry Station, Beltsville, Maryland 20705, is capable of providing considerable basic information of this nature and provides collectors with its facilities for introduction, quarantine inspection, and accessioning. In return for such service, collectors are expected to share their collections with the USDA as a means of providing for the needs of other scientists. A most recent concept in collecting is the development of exploration centers in regions of crop diversity from which to conduct field operations. The Crops Research and Introduction Center, Izmir, Turkey, supported jointly by F A 0 and Turkey is the first active center of this nature (Schulz-Schaeffer, 1970). Here, the foreign collector can receive technical help and supplies as well as facilities for seed cleaning and preparation for shipment, and other facilities essential to success. Considerations are being given to a somewhat similar center for potato collecting in South America, supported cooperatively by several U.S. institutions. Such centers offer continuity of purpose, a means of recollecting by local staff members, and could become training centers as well as conservation centers.

B. VEGETATIVELYPROPAGATED GERMPLASM Collecting vegetatively propagated plants presents most of the problems of seed collecting explorations. I n addition, other difficulties are associated with transit survival, danger of insect and disease importation, and the high cost of propagating and developing vegetative collections. Because of these factors, many more vegetative collections have been lost as compared to seed introductions. Among the crops most frequently collected by vegetative means are bananas (Musa spp.), manioc (Manihot esculenta Grantz), pineapples (Ananas cornosus (L.) Merr.), potatoes (Solanurn spp.), sugarcane (Saccharurn oBcinarum L.), sweet potatoes (Ipornoea batatas (L.) Lam.), taro (Colocasia esculenta (L.) Schott), and yam (Dioscorea spp.); a vast array of deciduous and tropical fruit and nut crops, certain forage species (Cynodon, Digitaria, Andropogon, Brachiariu), and the perennial Arachis

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JOHN L . CREECH A N D LOUIS P. REITZ

species are maintained vegetatively. I n addition, many ornamentals are collected as bulbs, tubers, corms, cuttings, and grafting material. The international movement of vegetative stocks is complicated by quarantine regulations. The long periods of time required to grow populations, and the excessive cost of developing tree crop collections has discouraged intensive efforts with these crops. Meanwhile, the loss of wild stands of fruit tree species, often cut for fuel or razed to provide land for intensive cropping, and abandonment of orchards of old varieties accelerates. Recently, through the use of U.S. PL 480 funds in India, Poland, and Yugoslavia, it has been possible to collect and establish in-country collections of germ plasm. Yugoslavia has an excellent base of fruit and nut crop diversity with potentials for frost and drought resistance. The Agricultural College at Skopje is surveying the native fruit species of Macedonia and assembling these as a source of budwood and seeds for distribution. PL 480 funds permitted the collecting of seed stocks of small grains in Yugoslavia to search for resistance to the cereal leaf beetle. Plant exploration by local scientists with its opportunities for continuity has been one of the most enterprising results of the use of PL 480 currency. The USDA coffee and cocoa germ plasm collections illustrate an important aspect of a vegative collection in lieu of seed reserves. Coffee and cocoa seed is rather short lived and yet the time required to raise new seedlings to fruiting is several years. In addition, there are disease and insect problems that restrict the movement of these crops. In cooperation with USAID, varietal collections were assembled at the U.S. Plant Introduction Station, Miami, Florida, where, despite occasional frosts, both coffee and cocoa can be grown outdoors successfully. Neither crop is cultivated commercially and possible infection by coffee or cocoa pests is unlikely. These facts have made this an ideal extra-tropical isolation germ plasm collection center. Over the 15 years that the coffee collection has been developed, there has been a rather limited demand for material since the most serious disease, coffee leaf rust, had never been reported in the Western Hemisphere. I n 1970, coffee leaf rust was first reported from several locations in Brazil. Although the outbreak was restricted to the poorer producing regions where less than 10% of the production occurs, the disease is now a threat to production throughout the hemisphere and has caused producing countries to initiate programs to develop resistance combined with high yield and quality in Coffea arabica. The Miami collection with its safeguards plus the quarantine facilities at the U.S. Plant Introduction Station, Glenn Dale, Maryland, offers breeders a reliable source of germ plasm without the danger of introducing

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additional rust races or other diseases as might be the case if importations were made directly from Africa where important coffee types and the rust occur. In the case of sweet potato (Ipomoea bataras) (Yen, 1970), the building of a working collection at the Otara Vegetable Research Station, D.S.I.R., New Zealand, was developed over a period of seven years of collecting clonal material from seven American areas, Polynesia, and Melanesia. At the end of this period, 175 varieties were assembled, evaluated comparatively, and are available to sweet potato breeders. However, this collection may be abandoned unless efforts are successful to transfer the collection elsewhere. A similar effort is being conducted at the Federal Experiment Station, Mayaguez, Puerto Rico, where the true yam (Dioscorea spp.) is being assembled with USAID support. Yam varieties have been collected on farms, in markets, and from other stations in the Caribbean. The introductions undergo varietal identification and determination of tuber characteristics, disease screening, and quality observations. The ultimate objective is to establish a documented, disease-free working collection for use in the tropics as a source of breeding lines or to upgrade the quality of varieties now being grown. VI.

Evaluation, Increase, and Maintenance of Germ Plasm

Evaluation of germ plasm really begins with sample collection. Either some selectivity is exercised in choosing among plants from which collections are taken or a bulking of many source plants is utilized according to the plan, type of material available, and objective. When a random sample is taken, this implies that an appraisal of the mission’s objectives has been made whereby an effective use can be made of the material for later studies. Complete records about the collected items are essential and are a vital part of the evaluation. Generally, deficient plants, e.g., diseased plants, are rejected at the collecting site, but some valuable germ plasm may be unwittingly overlooked. Samples of adequate size often can be evaluated without an intermediate multiplication. Single fruits, seeds, panicles, etc., except for surface descriptive purposes, must be increased for further study. Quarantine restrictions must be considered and met when applicable. This may require a seed generation in detention, hence, the size of the original sample should be compatible with plans to meet quarantine needs and an increase for evaluation. Scions, cuttings, or other vegetative parts are subject to quarantine detention and propagation. Because this entails virus indexing procedures, it is essential that the collector be judicious in choice of the material. This

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JOHN L. CREECH A N D LOUIS P. REITZ

should be collected from individual specimens and not be a mixture from several trees of a variety. Otherwise, each propagating piece must be indexed because of the uneven and masked distribution of viruses. This is particularly applicable when samples are obtained from collections. Again, the documentation becomes extremely important in these followup operations of propagation, identification, and virus eradication. In the United States, quarantine and the initial increase of seed stocks frequently are combined in one operation. Small grains are grown in a detention nursery at Mesa, Arizona, where the growing crop is observed several times for evidence of seedborne diseases. The seed is treated before sowing with a disinfectant and all weed seeds, smut balls, etc., are screened out or removed by hand. Other crops, some requiring vegetative propagation, are handled in a comparable manner at the U.S. Plant Introduction Station, Glenn Dale, Maryland, and the Regional Plant Introduction Stations. These regional stations may increase seed introductions themselves or arrange for increase at other experiment stations in the region. The introductions are evaluated by teams of Federal/State agronomists, horticulturists, pathologists, and entomologists. Thus, when the seed is catalogued for general distribution, documentation relates to the collector’s notes and the observations made at the initial increase. As added information is accumulated through subsequent evaluation and use by crop specialists, this is provided to scientists requesting material. A portion of all collections is held at the regional station as working stock, and samples are also sent to the National Seed Storage Laboratory. To avoid unnecessary duplication of effort, the regional stations have developed crop priorities among themselves for multiplication and evaluation, and responsibility for working collections. A. SMALLGRAINCEREALS WORKING COLLECTIONS Wheat, oats, rice, barley, and other self-pollinated annual seed crops are relatively easy to handle in large numbers in a small space. Single plants, hills sown with perhaps 20 seeds, single or multiple rows spaced 30 to 60 cm apart and of any desired length (usually 1 or 2 meters) may be used to increase the seed of these crops. Simultaneously, many observations can be made for purposes of description and evaluation of the items. Evaluation at this stage must remain subordinate to multiplication of the seed, the primary purpose. Winter hardiness, specific disease reaction, yield, etc., are evaluated under optimum conditions to detect varietal differences. Obviously, extreme tests might eliminate many stocks which should be preserved for later tests of a different nature where they might prove to be excellent germ plasm.

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PLANT GERM PLASM NOW A N D FOR TOMORROW

Our method of handling accessions of wheat, oats, and barley, both new and old, is illustrated in Fig. 3 (Reitz and Craddock, 1969).

__ -- ------- c Decontamination - ------ - - - . -Accessioning - -Propagotion

in detention Mesa, Arizona

Maintenance

11 I Evaluation

Breeding

Other uses

I

FIG. 3. Flow chart outlining the steps in acquiring and maintaining germ plasm in the USDA collection of small grains.

The working collections are kept at Beltsville, Maryland, in a room held at about 50°F and 50% RH. The reserve location is the National Seed Storage Laboratory, Fort Collins, Colorado. One or two additional safety storage locations are maintained for each crop, but not all lines are at one geographical address. Since the call on our supplies by breeders and others who use the seed depletes most of the seed in four to six years, it is necessary to regrow accessions on a schedule of about 10,000 items annually. Mesa, Arizona, and Aberdeen, Idaho, have been satisfactory locations for this work because of favorable climates and irrigation facilities. Mesa accommodates nearly all types of cereals without regard to the peculiar growth requirements of some accessions. Mesa is sufficiently isolated from other grain areas to be safely used as a quarantine or detention center to intercept latent plant diseases that may have been

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JOHN L. CREECH A N D LOUIS P. REITZ

brought into the country when the seed was imported. Seedborne diseases such as viruses and smuts are examples. Seed in our collections is available to research workers of all kindspublic, private, domestic, and foreign. Evaluation studies include resistance to insects, disease-causing organisms, adverse environments, and production hazards. Nutritional and other unique genes of many kinds are sought. Occasionally, varieties are utilized for direct cultivation. The collections are replenished either when seed requests have diminished the supplies, or when viability begins to decline. Since only about 200 g of each item are held initially and some are depleted at a greater rate than others, the inventory of seed on hand becomes uneven. This tedious task has now been built into the data-processing machine program. The computer is programmed to print a list of the depleted items when inventories run low, and these items are returned to the field for multiplication the next year. All small grain collection items with descriptive data are entered on punch cards, then put on tape from which printouts are made. Coded information gives the genus, species, name or designation, accession number-Cereal Investigations (C.I.) and Plant Introduction (P.1.) - source of seed, origin of variety, kernel color, growth habit, spike or panicle characteristics such as density, awn, chaff color, pubescenes, etc., secondary type description if mixed when received, and general plant height. Such information is obtained routinely as the items are added to the collection and serves three functions: (1) it provides a history and basic description of each item, (2) it is a guide to authenticate items when being regrown to replenish the seed inventory, (3) it provides a basis for breeders to request certain types of major interest. There is need for germ plasm users to cooperate with the curator of a collection at this point in time since adequate evaluation for particular uses will involve special equipment, testing facilities, genetic analyses, etc. Data so derived, however, should be shared with the curator and part of them added to his total record. The latter is especially true when the entire collection is evaluated uniformly for some unique trait. Resistance to bunt, stem rust, sawfly, and cereal leaf beetle, as well as protein and lysine content have been determined for our entire wheat collection. Large segments have been evaluated for other resistances, agronomic value, and other characteristics. The chart (Reitz and Ward, 1958) (Fig. 4) illustrates how the curator and collection are intimately related to initial evaluations (central circle), less involved with programs where major responsibility rests with others (such as the International Rust Nursery), and remotely related to sustained breeding programs, and commercial production.

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Bulks comprised of compatible accessions are appealing provided equal competitive opportunity is maintained and evaluation procedures take this fully into account. However, if possible, when accessions are bulked,

Evaluation

FIG.4. Degrees of involvement of a collection, and its curator, are illustrated for wheat. The inner circles suggest that kinds of data the curator desires on all items. Once promising items are identified, these go into remote testing programs or into completely separated activities (peripheral arcs).

there needs to be a conserved collection of the original lines, developed prior to bulking; viz, stocks of the 12,000 primitive maize collections held as well as the 2000 bulked lines. Evaluation for one response is no guide to the usefulness of an item for some other purpose. Each purpose and case must be assessed independently although several evaluations may be obtained from one seeding. Another related problem is the difficulty of controlling extraneous influences. For example, a plant already defoliated by insects or a leaf blight cannot be evaluated for leaf rust reaction. This seems so obvious; yet it is a common weakness in evaluation programs.

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Procedures for handling the increase, maintenance, and evaluation outlined here are applicable to other self-pollinated crops such as beans, soybeans, self-pollinating grasses, and millets. Seed size, plant growth, the evaluation objective, and other factors alter specific procedures. Obstacles include specific requirements for photoperiod, vernalization, optimum soil and air temperature conditions, and protection during development. Much germ plasm is lost because these factors are inadequately met during the maintenance and replenishment phases. In some cases the evaluation is meaningless because soil-plant interactions or parasiteplant interactions were disregarded or not adequately assessed.

B. CROSS-POLLINATED SPECIES Rye, corn, sorghum, cotton, alfalfa, red clover, many turf and pasture grasses, fruit and forest trees, sugar beets, and other crops cross-pollinate to such a degree that the procedure outlined for small grains must be modified. Spatial isolation is recommended but becomes difficult or impossible to achieve with large numbers. Manual self pollination is effective in sorghum merely by placing a bag over the panicle during pollinating time. Bagged ears of corn may be pollinated with a bulk of that line’s own pollen but some inbreeding depression will occur. The latter would not directly affect the germ plasm resource unless it caused population drift by selectively causing some gene carriers to be depressed and lost. This problem is partially overcome by developing race groups, as has been done in corn and alfalfa, and maintaining the races in isolation. The use of border rows (which are discarded) to provide a pollen dilution gradient mitigates the contamination. Sorghum breedefs heretofore limited to sorghum varieties that flower under comparatively long daylengths such as those in the milo, kafir, and feterita groups, will soon share in a genetic bonanza. Under a Rockefellersponsored program in India and, more recently, an ARS-sponsored Public Law 480 grant, an extensive reservoir of sorghum stock, called the IS Collection, has been established by Indian scientists. The scientists assembled, characterized, and classified nearly 12,000 stocks from 44 countries. Because sorghum is a tropical crop and flowering is influenced by day length, many potentially valuable varieties do not flower under U.S. conditions. To offset this, an elite group of sorghums from the IS Collection is being used in the U.S. sorghum conversion program currently underway at the Federal Experiment Station, Mayaguez, Puerto Rico, and at Texas Agricultural Experiment Station, Lubbock. There, ARS and Texas geneticists are crossing exotic types with early dwarf varieties to recover desirable exotic characteristics and combine them with heights and

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maturities needed in the United States. From this conversion program, 63 new genetic lines were released to U S . breeders during 1970, and approximately 1000 elite items will eventually become available. Evaluation and maintenance of long-lived species is more difficult and time-consuming. Treatment to cause early flowering in perennial forms has made much progress in recent years thereby enabling breeders to obtain a seed generation at tolerable intervals. The perennial habit is an advantage, however, because one sowing or planting might persist forever, or until it is willfully destroyed. Opinons differ about the number of items comprising an adequate size of collection. These range from prime concern for the basic species, whether wild or under domestication, to thousands, even millions of individuals to represent the crop. The latter extreme imposes a heavy burden which virtually no institution can bear. The present collection of about 22,000 accessions of wheat in the USDA Collection and a like number of oats and barley combined requires the attention of one scientist and four assistants. This is a dynamic program of research evaluation as well as acquisition and maintenance. One difficulty regarding size is the basic concept about germ plasm. It is more than a bank of genes, but it need not embrace all the possible cross-combinations among genetic entities. Some germ plasm has merit because of the unique gene interactions represented; much of it is single-gene based (Ward, 1962). Every collection of any size quickly shows duplication of genes and phenotypes. For example, a half dozen awn types in wheat about covers the range we see; the same is true of most other morphological characteristics. Therefore, additions to our collection duplicate much of what we already have. I n our case, we are concerned with resistances, efficiency factors, physiologic capacity, and nutritional values. Selective additions are more significant than more items at random, especially if new geographically located gene pools can be sampled. The general situation is illustrated in Fig. 5 regarding successive samplings in the same gene pool. The assumptions are purely for illustrative purposes. It is evident that resampling leads to fewer and fewer new unique forms and a correspondingly larger number of duplicates. It has been conjectured that our barley collection of 12,000 items probably embraces a majority of the world’s barley genes. The conclusion is not that we have enough barley germ plasm, but that a concentrated effort should be redirected toward the missing portion, if definable. Those who propose that the base species are enough to retain would depend upon commercially available stocks, natural and induced mutations, and introgressive evolutionary forces to provide variability that might be needed later. The question was discussed in the 1940’s when

JOHN L. CREECH A N D LOUIS P. REITZ

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hybrid corn came into prominence whether any effort should be made to preserve the open-pollinated varieties of central U.S. Most of the genes resided in inbred lines, it was argued. Fortunately, a stubborn few corn

;'I

pq 000

I

15.000

I

Second

First

Third

Fourth

FIG. 5 . Four successive samplings of a population containing 10,000 unique forms. Ten thousand samples were taken each time. Assuming a duplication rate of 1/2, the four samplings would yield a total of 9375 unique forms on a descending frequency of 5000 from the first sample to 625 from the fourth sample.

breeders maintained O.P. corn, and on numerous occasions this has been the source of new and useful variation. For example, it is doubtful that any of our corn borer resistant lines would be available because essentially all of them were developed after 1940 from open-pollinated stocks. New varieties of limited pedigree sometimes sweep all others out of an extensive region, as indeed the new semidwarf wheats are doing in parts of Mexico, India, etc. This contributes to germ plasm losses. VII.

How G e r m Plasm is Used

Direct use has been the historical means of incorporating introduced germ plasm into agriculture throughout the world. Most of the crops grown in the United States originated elsewhere. Native Western Hemispheric crops - maize, common beans, peanuts, pumpkins, sweet potatoes, sunflowers, manioc, potatoes, rubber, and tobacco -found many homes in other continents. The geographical origin of plants has been

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discussed by Vavilov (1957), J. R. Harlan (196 I , 1966), Frankel and Bennett ( 1970), and many others. The dissemination of these crop species by man is an equally discussed subject. I n a symposium volume on Germ Plasm Resources (J. R. Harlan, 1961a; Cullinan, 1961; Weiss, 1961) it is clearly shown that origin, greatest variability, and greatest use of germ plasm may be in separate geographical areas, and that the use of germ plasm extends into all facets of molding the species subunits to fit a prevailing environment or to suit man’s subsistence and economic needs. The permutations are endless, it seems; simple generalizations become inadequate, and a full treatment of how germ plasm is used is both too tedious to set down and too detailed to gather. The range of plant variation is astounding, and yet, when one examines the pedigrees of varieties being grown commercially it is evident that only a small fraction of the available germ plasm has been incorporated. This means that ( I ) only a limited amount of the available germ plasm has been used successfully, ( 2 ) some of the germ plasm may be worthless, (3) pedigrees may not reveal the entire ancestry and may not be correct, (4)individual breeders tend to work within a relatively narrow range of material and inbreed rather extensively because local adaptation restricts merit of extreme genotypes, (5) all needs to be supplied by available germ plasm are not identified at one time and even the needs are not all recognized at once, and (6) biological systems are not static. We did not realize, for example, that resistance to the cereal leaf beetle was needed until the insect was introduced accidentally about 1962. How could anyone evaluate small grains for resistance? Higher protein and better balance for humans among the essential amino acids have recently come into prominence, and it is no suprise that variability has likewise been discovered in the germ plasm of several of these crops. Many evaluations and uses of germ plasm given in the paragraphs which follow are from progress reports by various Branches of the Plant Science Research Division, ARS, and from the annual and other reports from the four Federal/State Cooperative Regional Introduction Stations to their Technical Committees. A.

CEREALGRAINS 1 . Maize

Indian corn, or maize, quickly achieved world significance soon after the discovery of America and its early colonization and exploration. In fact, maize was so rapidly disseminated that arguments arose as to whether maize, in fact, did originate in Central America or whether it may have had an origin involving other continents. Certainly some of the maize forms outside America are strikingly unique, but they are logically

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explained by the merging of segments of germ plasm brought together under new stress environments. Much of the same thing happened to maize in North America in the evolution of varieties such as Reid Yellow dent, so highly productive in the Ohio and Mississippi Valleys (Jenkins, 1936, pp. 464-466). Valuable maize germ plasm has been found among introduced materials such as Zapalote Chico, P.I. 2 I74 13, a white dent corn recognized and used as a source of resistance to the corn earworm. This cultivar originated in southern Mexico but is easy to grow as far north as Minnesota because it is comparatively independent, or unaffected, by length of day. It is 8-10 rowed, short eared (about 5 inches) and has very dented kernels. P.I. 217407, or ‘Ladyfinger’ popcorn, has been widely used as a source of northern corn blight resistance, Helminthosporium turcicum. This introduction is a late maturing, prolific, high quality popcorn, very similar to the ancient popcorns of Peruvian graves. The high lysine germ plasm includes Opaque-2, a mutant form, and floury endosperm. A number of problems remain to be solved in corn improvement for which new germ plasm is sought. A higher level of resistance to stalk rots, resistance to other diseases, especially leaf blights, better male sterile cytoplasmic systems, and resistance to insects attacking the roots, stalks, and ears, to name only a few.

2 . Sorghum Sorghum is an ancient crop, but a highly successful immigrant in the United States. The crop in multiple forms has contributed immensely to agriculture in the southern, Central Plains, and southwestern States of the United States. Grain, silage, pasture, sirup, broom, and other types are grown. Much of the basic germ plasm in the United States was introduced 75- 100 years ago. Unknown processes, including natural crossing and mutations, resulted in an enriched germ plasm even as the crop was being grown by farmers. At least five distinct forms of milo appeared on farms during a 30-year period (Martin, 1936, p. 536). This evolution and cross breeding with kafir was the basis for developing types suited to combine harvesting. Numerous introductions have contributed to the improvement of sorghum for the production of grain, forage, and sirup. Within the sorghum genus perhaps the most valuable germ plasm yet introduced was contained in four accessions (P.I.’s 221569, 221570, 221572, and 221688) collected in Nigeria in 195 1. They produce grain with yellow endosperm and have higher levels of vitamin A than other sorghums. P.I. 221688 is the ‘Short Kaura’ variety grown in Nigeria, and the other three represent different seed lots of the variety ‘Kaura.’ All are too late for commercial

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plantings in this country but many sorghum hybrids released by both private and public breeders during the last 10 years contain germ plasm from these yellow endosperm sorghum introductions. In 1967, a U S D A sorghum exploration to Ethiopia was undertaken in cooperation with The Rockefeller Foundation. Materials collected from this highly successful venture plus other sorghum introductions received from India and elsewhere are increased and evaluated through the sorghum germ plasm conversion program at the Federal Experiment Station, Puerto Rico, and the Texas AES. New germ plasm is sought to provide better resistance to several diseases, especially stalk rots, and resistance to insects, notably a new strain of aphid in the Central and Southern Plains. Germ plasm to make sorghum adapted and productive in short season environments and where low temperatures preclude its production at present seem likely developments and would extend the range where the crop could be grown. Germ plasm sources and problems to be solved have recently been presented (Wall and Ross, 1970) in which sorghum of Asia, Africa, and the United States, are featured. Programs are being conceived whereby sorghum will play a greater role in agriculture of tropical America.

3 . Wheat Wheat has long been associated with man’s migrations and, because of its direct use as human food and wide adaptation, has been distributed worldwide. The story of wheat germ plasm in North America was recently reviewed (Reitz and Craddock, 1969). Many writings deal with the development, early history, germ plasm resources, and use of the crop (Clark, 1936). The success of plant introductions in the phenomenal development of modern wheat varieties is a singular achievement. In a crop where the varieties change completely in a span of a decade, a broad base of diversity is essential. Norin 10 (P.I. 156641), introduced from Japan in 1946, was the key to semidwarf wheat breeding in the Pacific Northwest and in Mexico (Reitz, 1970). Another plant introduction, P.I. 178383, illustrates the significance of maintaining accessions irrespective of the immediate evaluation. This is an accession of wheat collected from Turkey by J . R. Harlan in 1948. It has the appearance of a hard red type but its milling, starch, and baking properties are that of a soft red wheat. It has poor milling quality. P. I . 178383 is susceptible to leaf rust, has weak straw, and lacks winter hardiness. P.I. 178383 was early found to be highly resistant to stinking smut and was little regarded for any other use until 1960 when stripe rust suddenly

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JOHN L. CREECH A N D LOUIS P. REITZ

became epiphytotic and was devastating to wheat production in some areas of the Pacific Northwest. It is now known that P.I. 178383 has good resistance to four races of stripe rust (Puccinia striiformis), 35 races of common bunt (Tilletia caries, and T .foetida), and 10 races of dwarf bunt (Tilletia controversa), all of which occur in the Pacific Northwest. The accession also has usable tolerance to flag smut (Urocystis tritici) and snow mold (Fusarium and Typhula spp.). Thus P.I. 178383 (except for its bunt resistance) was neglected for a number of years following its introduction and later became one of the prominent breeding lines used in Montana, Idaho, Washington, and Oregon. I t is an excellent example of a single P.I. accession contributing multiple disease resistance. In addition to its disease resistance, P.I. 178383 has excellent seedling emergence. It is now used as the new “standard” for the comparison of emergence data in the Pacific Northwest. The final utilization of the remarkable characteristics of P.I. 178383 demonstrates the value of maintaining germ plasm even though tests for some lesser traits show it to be of little or no commercial value. This introduction is one parent in several varieties now in use on U.S. farms. Germ plasm.needs remain unfilled for a broad spectrum of resistance to all root and foot rots including snowmold, augmented resistance to streak, yellow dwarf, and soilborne mosaic viruses; there is a need for additional genes for rust resistance, mildew, Septoria, and numerous other diseases. The Hessian Fly has been partially controlled by use of resistance from many sources. In some cases control has been temporary because of the evolution of new virulent cultures of fungal and insect pests. 4 . Other Cereal Grains

The germ plasm requirements for barley, oats, rice, and rye have likewise been met to some extent by assessment and use of stocks from many countries. The variations used for each crop follow a pattern similar to that discussed for maize, sorghum, and wheat. The story is never complete because the beginnings of the species, the accumulated diversity from chromosome rearrangements, introgression from wild distantly related species, mutations from natural or man-induced causes must become known and be recounted. The germ plasm resources for these crops and how they have been used were recently reviewed by Reitz and Craddock (1969; see also H. V. Harlan and Martini, 1936; Stanton, 1936; J . W. Jones, 1936; Adair et al., 1966; Frankel and Bennett, 1970; Reid et al., 1968; Vavilov, 1957; Khush and Stebbins, 1961; B. D. Webb et al., 1968; Chandler, 1970). Higher nutritional quality as well as pro-

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cessing quality is being emphasized in recent work. The discovery of Hiproly (P.I. 60693) barley from Ethiopia in 1924, having high protein and lysine content, is exciting news (Munck et af., 1970).

B. GRASSA N D FORAGE Many introductions have contributed new cultivated species to the agriculture of the United States. Much of the early history of activity bringing species into the United States and of the variability conducive to breeding improved forms was discussed in the 1937 Yearbook of Agriculture (see especially McKee and Pieters, 1937; Vinall and Hein, 1937; Tysdal and Westover, 1937; Pieters and Hollowell, 1937). Direct introduction continues to be fruitful for U.S. agriculture but most improvement has come from the use of exotic forms in breeding programs. Commencing in the mid- 19303, an expanded program of forage crop improvement was initiated in the United States by public and more recently by private breeders with the result that the numbers of varieties have been greatly increased, regional and local adaptation more sharply defined, higher levels of productivity achieved, and some disease, insect, and nematode problems controlled. Recent interest has centered around expanded use of warm-season pasture grasses in the American tropics and southern warm-temperature regions of the United States. Many of the native grasses of South Africa (Digitaria, Hemarthria, Chloris, and Pennisetum) appear to have desired characteristics for this purpose. Oakes (1965; Oakes and Langford, 1967) has discussed some of the potentials of Digitaria for cold tolerance. This becomes a paramount limiting factor in attempting to extend the production range and increased yields to higher tropical elevations. Consequently, explorations for native grasses of South Africa are receiving high priority in the USDA plant collecting program. There are many worthy examples of forage legume and grass introductions which have contributed either as direct varieties or as parents in the development of other varieties. Annual ryegrasses, Lolium multijlorum Lam., collected in Uruguay in the 1950’s, have contributed resistance to crown rust in ryegrasses now grown in the south. Most important of these introductions are P.I. 193145, T.O. 1882, and P.I. 20 1980, separate introductions of a Uruguayan variety La Estanzuela 284, and two late maturing accessions, P.I. 194394 and 194395. A bromegrass, Bromus biebersteinii Roem. & Schult., P.I. 172390, collected in Turkey in 1949 was used by the USDA Soil Conservation Service and the Idaho AES to develop REGAR bromegrass, released in 1966. REGAR has excellent seedling vigor, develops numerous leaves and has a

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strong regrowth habit with early spring development and heavy forage production. Agropyron elongaturn (Host) Beauv., P.1. 98526, was collected in the Soviet Union in 1932. It illustrates an accession that was increased and distributed in its introduced state and by 1953 was grown on 114,000 acres in the United States and certified in Nebraska as ‘Nebraska 98526.’ Alfalfa introductions (Medicago spp.) are constantly under evaluation. Diseases, insect pests, and poor adaptation in certain regions are compelling reasons for breeders to continue the search for alfalfa germ plasm. Thus the recent outbreak of the eastern biotype of the alfalfa weevil threatened alfalfa’s position as a major forage plant in several States. Owing to the difficulty of maintaining alfalfa germ plasm as distinct entities, considerable research has been devoted to the development of broad gene pools as a means of concentrating resources in composite populations hoping to obtain new gene combinations and at the same time producing enough seed for use by breeders and continued conservation of the pool. Some of the most recent releases of alfalfa germ plasm find their origin among the earliest introduction of the USDA. TRAVOIS, released by the South Dakota AES in 1962, contains genes of three introductions, P.I.’s 24455, 28070, and 28071 collected in Russia by N. E. Hansen in 1908 and 19 10. These yellow-flowered alfalfas were perpetuated in nurseries in South Dakota and ultimately utilized in the natural hybrids out of which TRAVOIS was developed. The USDA released a broadly based alfalfa population, AWPX3, in 1968, as a source of resistance to various stages of the life cycle of the alfalfa weevil. The parentage includes 13 plant introductions collected in nine different countries between 1952 and 1960. This is a fine illustration of the variability which can be traced to a multitude of ecological regions. Stem nematodes are a serious alfalfa pest in areas of the southwestern United States where the successive culture of alfalfa crops creates a high nematode population and ultimate loss of the crop. Resistance to the nematode was found in an introduction, P.I. 141462 collected in Iran in 1940. When this resistance was transferred to commercial varieties, the savings represented millions of dollars to alfalfa growers. Bermudagrass, long frowned upon as a serious pest in cultivated fields, is the leading turf grass in the South. It is a highly variable species and several lawn types have been developed in recent years. Several of these improved varieties resulted in part or entirely from introductions of Bermudagrass.

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C. HORTICULTURAL CROPS The first 900 pages of the U.S. Department of Agriculture Yearbook for 1937 was devoted largely to variability and breeding opportunities with vegetables, fruit, nut, garden, and ornamental crops. Plant introductions have been and are used effectively in horticultural crop breeding. Scientists have turned to these germ plasm resources increasingly as specialization, mechanization, and decline of resistance in commercial varieties to diseases and insects become limiting production factors. The few examples used here are typical of the many benefits accrued from incorporating characters from seemingly undesirable species into cultivated ones. Virtually none of these is native to the continental United States although some are native to Latin America. Thus, U.S. industry is largely based on alien germ plasm. 1. Vegetable Crops

Common bean germ plasm of immense value has come from Mexico (Gentry, 1968). Resistance to Fusarium root rot, halo and common blight, and to two insects was found in some 5400 lines representing many local Indian varieties, but many of those have been only partially screened. Recently, demand for these accessions by scientists in foreign countries has created an additional drain on the working stocks held at the Regional Introduction Station, Pullman, Washington. Many of the beans are difficult to increase because of lack of adaptation. By growing the collection in the winter at the Federal Experiment Station, Mayaguez, Puerto Rico, we are able to produce two generations a year and overcome daylength problems. In cooperation with The Rockefeller Foundation, the Agricultural Research Service is now engaged in a germ plasm increase program of beans (Phaseolus) plus other edible legumes to provide breeding material for the general improvement of these important protein crops. In addition to the seasonal advantage, winter increase at the Federal Experiment Station permits staff pathologists to evaluate the collection for diseases and viruses not generally encountered in continental United States. One of the most important and useful cucumber introductions is P.1 220860, SHOGOIN, collected in Korea in 1948. This introduction is the source of the gynoecious character, which, when utilized in breeding lines and varieties, makes possible the large-scale production of hybrid cucumber seed. N o less than 18 other cucumber introductions show significant degrees of resistance to diseases. P.I. 2008 15, a cucumber collected in Burma in 1952, has been widely used as a source of bacterial wilt resistance in the United States and Canada. A single cucumber intro-

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duction collected in India, P.1. 197087, has contributed the anthracnose resistance contained in several varieties. In 1956, an attractive butterhead lettuce from Israel, P.1 21 1 1 18, RINAT HAKFAR, was evaluated at the Regional Introduction Station, Geneva, New York. Its main features were thick leaves, large heads, and exceeding slowness to bolt. From seed distributed by the Geneva, New York, Introduction Station, the Canada Department of Agriculture released a new variety BUTTERKING which extended the butterhead season by 6-10 days. In 1964, BUTTERKING won a bronze medal in the All-American trials. The continuing success in finding melon introductions with disease resistance is a remarkable credit to plant introduction. In 1937, a wild melon, P.I. 1241 1 1 , was collected in the hills of India. Subsequently, the winter melon industry was struck by a serious new race of powdery mildew. Published statements claim that the resistance found in P.1. 1241 1 I saved the $5 million melon crop for California. Over the succeeding years, cantaloupe varieties with powdery or downy mildew resistance from various P.I.’s have appeared in a number of states. Peas collected in their Mediterranean homeland and elsewhere in the Middle East have been essential in breeding for disease resistance. Three pea varieties released by the New York AES, Geneva, carry a dominant gene for resistance to pea enation mosaic virus found in a pea introduction from Iran, P.I. 140295. Scientists at the Washington AES have located 10 introductions of peas with extremely high resistance to a new race of wilt. As a wild plant, Pisum sativum L., appears on the decline and this is equally true of local varieties. Replacement by modern European varieties, eradication of “weed” species, and grazing threaten this essential germ plasm. Dr. Howard S. Gentry, USDA plant explorer, has conducted two explorations in 1969 and 1970 to the Mediterranean centers of Pisum diversity from Sicily to Ethiopia. Because there are many areas of Ethiopia where primitive agriculture is still practiced, there is a high degree of variability there. Over 170 seed samples have been collected and sent to the Regional Introduction Station, Geneva, New York, for increase. Both explorations received the support of FAO, Rome, through their offices in Turkey and Ethiopia. It is estimated that more than 100 tomato varieties carry wilt resistance tracing back to P.I. 79532, Lycopersicon pimpinellifolium (Jusl.) Mill., collected in Peru in 1929. This has saved growers millions of dollars every year. Among the more recent interests is that of locating tomato accessions capable of germinating in cold soil and setting fruit under low temperatures. Introductions from the U.S.S.R., P.l.’s 262929 and

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262934, have been reported by the Geneva, New York, Introduction Station as carrying these traits. Dwarf, determinate-flowering plant types are being introduced rapidly wherever mechanical harvesting of tomatoes is possible. The pioneering work of G. C. Hanna in California (Hanna, 1966; R. E. Webb, 1966; R. E. Webb and Bruce, 1968) led to the development of varieties adapted for machine harvesting combined with disease resistance. The variety SAN MARZANO introduced from Italy in 1930, was found to bear fruit tolerant to abuses of machine harvesters. It was crossed with earlymaturing varieties of determinate growth habit. Later, disease resistance was added. This development and subsequent progress has revolutionized the canning industry. Perhaps the most singular report on tomato germ plasm available appears in the North Central Regional Research Publication 172 (Skrdla ef al., 1968). Here, 2658 accessions are described as to horticultural characters and reaction to two major diseases, the leaf blight phase of early blight and tobacco mosaic virus. Plant introductions, genetic stocks, and breeding materials are included among the entries. 2. Fruit and Nut Crops

Introductions of fruit and nut tree crops are handled by the Federal Introduction Stations because of quarantine propagation and the time factors involved in evaluation and distribution. The apple, pear, peach, and many other fruit crops were introduced almost at once by early colonists who came to America (McCrory, 1958). Migrants, Indians, birds, and animals were involved in the widespread distribution of highly variable germ plasm. Depending upon species, this germ plasm provided superior chance seedlings or sports, later propagated by cuttings or grafts, from which whole orchards were established. Forty of the 55 American apple varieties listed by Magness in the 1937 Yearbook of Agriculture were developed from chance seedlings, and most of the remainder, while of unknown origin, presumably were from seedlings, also. Many of these and other newer varieties are triploid. New fruit varieties are bred by the usual scientific methods, often through consortiums of breeders in several State AES’s. New germ plasm in great diversity to help solve the limitations of production and marketing will continue to be sought. But the most disturbing fact is the decline of existing germ plasm in old collections. By 1842, there were about 1000 pear varieties known in the United States (Howlett, 1957). Many of the varieties that occupied an important place are no longer obtainable. In an effort to describe, evaluate, and conserve the early fruit introductions, several projects have been supported by the regional research programs. In the North Central States two regional

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bulletins on evaluation of fruit introductions (Howlett, 1957; McCrory, 1958) were published. In the Western region, support has been provided for pear collections in Oregon. An inter-regional project, 1R-2, has been established for the holding and distribution of virus-free stocks of desirable fruits. The repository program was begun in 1955 and includes over 600 Prunus clones and 90 Malus clones. New accessions are indexed at Prosser, Washington, before being placed in the isolation collection at Moxie, Washington. Clones are indexed annually and must remain virus-free for five years before they can be distributed. The development of distribution centers as IR-2, provides a source of authenticated, disease-free plant material and, in the long run, may replace the many collections of unknown health status from which grafting material is now distributed.

3. Ornamental Crops The arboretums and public gardens situated in every major city of the United States play a significant role in the introduction, evaluation, and maintenance of ornamental plants. These objectives have been pursued with vigor since Colonial days. But because of the increasing stringency of plant quarantines, the Federal Government has played an increasing role in the area of introduction. The public gardens are thus assuming a greater and perhaps more urgent role of adaptive evaluation as a result of increasing environmental stress on previously standard shade trees and garden plants. Numerous ornamentals have been selected, developed, and released by arboretums and botanic gardens, through the nursery trade and ultimately to the homeowner. The BRADFORD ornamental pear is an example of an introduction that has achieved notable success in recent years (Whitehouse et al., 1963). This variety was developed from seed introduced in 1918 from China as a potential source of disease resistance. The 100 pounds of seed of Pyrus calleryana Dcne., was one of the last important pieces of exploration work of Frank N. Meyer, who died in China shortly thereafter. In the early 1950’s one of the trees growing at the Glenn Dale Introduction Station was selected for street tree trial because of its handsome flower and foliage qualities. An experimental planting was developed in a newly constructed subdivision and observed over several years. The BRADFORD became an immediate success and has been called “the tree for all seasons” because of its beauty throughout the year, including bright red autumnal leaf coloring. In addition, it appears resistant to prevalent insects and is resistant to fire-blight. A second new ornamental of significance because it demonstrates the importance of evaluation and maintenance following introduction is the

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privet (Ligustrum vulgare L.) (Dodge et d., 1965). The Arnold Arboretum sent Dr. Edgar Anderson to the Balkans in 1934 to collect ornamentals among other plants. There he collected seed of a privet in the dry, barren hills near Sarajevo, Yugoslavia. The seed was sent to the USDA and assigned P.I. No. 107630. Plants were widely distributed in 1937. After only 2 years, observers in several rigorous climates including Cheyenne and Sheridan, Wyoming, and Sioux Falls, South Dakota, reported on the superior hardiness performance of this privet. Because of its sustained excellent hardiness rating during regional trials in the 1950's and its superiority over Amur River North privet, this introduction was named C HEYENNE by the USDA in 1965. I n this instance the collector deliberately selected his material from an area of severe cold, and subsequent trials bore out the validity of his selection. CHEYENNE

D. NEW CROPS New approaches and objectives in plant screeing and exploration are to procure germ plasm of wild species not previously considered for crop purposes (Q. Jones and Wolff, 1960; Creech, 1970b). The search is for species which are promising sources of industrial end products, such as fiber and pulp from annual species, unique industrial oils, gumlike products, waxes, pharmaceuticals, insecticides, and other products with specific chemical properties. This complex effort requires the cooperation of the chemist, economic botanist, agronomist, and economist. First, the literature about a potentially useful plant group is surveyed jointly by a botanist and a chemist. Collections are then obtained within the genera selected for consideration. For example, Vernonia anthefmintica (L.) Willd., a weed from India, is attractive on account of the unique epoxyacid in its seed oil, which may be useful for protective coatings, synthetic rubber and other industrial purposes (Berry et al., 1970). Unfortunately, the agronomic characteristics of this species need considerable improvement. To obtain a wider range of variation, and to determine the center of origin of the Vernonia species, explorations were conducted by the USDA in 1966 to Africa, where Vernonia anthefmintica appears to have originated. This species and its relatives were collected to clarify their taxonomic affinities as well as to obtain species with more useful characters, such as greater seed size and oil content, and improved plant habit. After this preliminary evaluation, a pilot plant stage to study the commercial utilization of the new crop was made. Thus the pattern of development for this entirely new crop follows that for established crops. The search for plant precursors of cortisone (Correll el al., 1953, represents the most successful venture into this field. It did not necessarily establish Dioscorea as a successful domesticated crop. However,

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this program, in which centers of species in Africa, Central and North America were explored, and the chemical analyses completed, established Dioscorea as the most promising source of steroids, displacing Strophanthus, Agave, Veratrum, Trillium, and Yucca. Further intense exploration for Dioscorea species decreased the field to Dioscorea composita Hemsl., D . floribunda Mart. & Gal., and D . spiculiflora Hemsl., out of scores of species sampled. Harvesting in the wilds of Mexico still provides the crude material for steroid intermediates. The effort to develop productive commercial plantings continues as the wild supply diminishes. Several potential new crops, unlike Dioscorea, will depend on successful establishment as cultivated crops for their utilization. Of these, the most promising are crambe (Crambe abyssinica Hochst. ex R. E. Fries) and kenaf (Hibiscus cannabinus L.). Crambe is the first new oilseed to be grown commercially as a direct result of the new crops program (White and Higgins, 1966). Crambe has been widely tested for adaptability and yield and was grown commercially for several years. The Indiana AES has an active program underway and has released the variety PROPHET, which was derived from the Swedish introduction, P.I. 247310, and the variety I N D Y , an introduction of Crambe hispanica L., P.I. 279346. Several thousand acres of crambe are now grown in Canada. Crambe seed is high in oil, and the oil contains about 60% of erucic acid, a fatty acid not available in other domestically produced oil seeds. The oil has superior properties in the lubrication of molds for continuous casting of steel. The oil may also be converted to a variety of promising products for industrial outlets, such as, coatings, molded plastics for gears, adhesives, extended objects, and films. The utility of kenaf as a pulp and papermaking raw material has been demonstrated by preliminary agronomic and utilization research. Extensive field testing needs to be conducted with emphasis in the Southeast (White et al., 1970). Excellent yields have been obtained in plot studies but several factors must be resolved. Susceptibility to root-knot nematodes is the major production problem; optimum fertilizer, varietal standardization and harvesting require additional research. The varieties EVERGLADES 71 and EVERGLADES 41, which evolved from P.1. 207883, are extensively used in experimental plantings. A Kenyan accession, P.I. 292207, which appears to be segregating for resistance to two rootknot nematode species, is included in the breeding program. Screening of kenaf germ plasm for nematode resistance is being done at Savannah and Tifton, Georgia, and breeding efforts are carried out at Savannah and Mayaguez, Puerto Rico. The Florida AES has embarked on a rather corn-

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prehensive breeding program, and from their experimental plantings, yarn and fabric have been made as well as paper. In addition, kenaf leaves, young stems, and growth tips contain rather high contents of crude protein. These by-products may be useful as commercial feed. The search for additional new crop leads continues. As of January 1969, some 8300 samples have been analyzed by the Northern Marketing and Nutrition Research Division, Peoria, Illinois, for oil and protein. A total of 1278 samples have been assessed for fiber potentials. From the broad botanical-utilization screening program, about 50 new seed lipid components have been discovered, are present in realistic percentages, and have unique characteristics to suggest practical utility. From these evaluations come the plant leads for preliminary agronomic investigations. These “new crops” represent a broad array of plant families and species, most of which are scarcely known except in taxonomic and related systematic research. VIII.

A.

Germ Plasm Collections

NUMBER OF ITEMS

In order to assess the strength of our existing genetic resources and to identify the needs for additional germ plasm, partial surveys of the extent of current collections have been made by FAO, The Rockefeller Foundation, and the Agricultural Research Service. In 1969 the Crop Ecology and Genetic Resources Branch, FAO, initiated a preliminary survey of world plant genetic resources. Holders of germ plasm provided rather simple information on species held, approximate number of samples, and conditions of storage. The inquiries, sent to about 500 individuals, revealed that there were some 2 million items held by the respondents. A view of the early returns of the survey (FAO, 1970b) is alarming in that only a small portion of this material (28.5%) is in special storage facilities. Furthermore, there is an obvious degree of duplication, although this in itself is not detrimental. It serves as a safeguard against losses which may result from storage conditions. The United States is a major contributor to this wealth of conserved germ plasm. The working stocks of the Agricultural Research Service, USDA, are estimated to be about 120,000 i.tems. Similarly, collections are held by State Agricultural Experiment Stations, the Soil Conservation Service, private institutions, and industry. In many of these collections, information is documented and retrieved by automatic data processing. Some collections, held by individual scientists, are not adequately documented nor are they brought to the attention of the general scientific community.

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The American Horticultural Society, through a grant from Longwood Foundation, has established and operates a Plant Records Center for the purpose of locating and documenting collections of woody ornamental plants in the United States. An excellent team approach to collecting the data has been devised whereby the Plant Records Center specialists arrange visits to each participating institution and retrieve the raw data in cooperation with the local staff. This principle could well apply to efforts on a large scale, dealing with the economic crops. Estimates of the extent of vegetative collections, except for those maintained in botanic gardens and arboretums, are more difficult to ascertain than seed collections. Consequently, many vegetative collections are abandoned without notice. Collections of some major crops are well documented; for example, an excellent sugarcane working collection of 1,420 lines of five species is maintained by the USDA at Canal Point, Florida. In the lesser crops, however, thousands of varieties are held in a casual manner, mostly by individual farmers. The true yams (Dioscorea), a most important crop of West Africa, could well benefit by efforts to assemble and document the variation in this crop. At present there is little attempt to identify holders of yams or assemble composite collections. Strict quarantine regulations control the movement of vegetative stocks and constant vegetative regeneration intensifies and spreads viruses. We scarcely know what increments of increase in yields might be achieved or improved varieties developed if a total examination of the yam could be undertaken. We are aware of the high cost of maintaining vegetative collections and little research has been directed toward itriproved techniques. In potatoes, however, conversion to true seed has been especially rewarding. Where applicable, meristem culture may be an answer to maintaining vegetative collections under the controlled conditions of growth chambers. The USDA has made surveys of the collections of fruits and nuts maintained by Federal and State Experiment Stations (Fisher, 1963a,b, 1964; Larson, 1961). The preliminary information indicated that of some 8600 clones reported, 44.6% were apples; 30% were stone fruits; 15.6% were pears; 4.5% were nuts; 5.3% were miscellaneous items. This survey did not include citrus, but a Florida survey of citrus collections around the world lists 70 collections in 40 countries, including 13 in the United States. Again, duplication reflects the complications of developing acceptable standards for vegetative collections. F A 0 held a conference on the propagation of tropical and subtropical fruits in 1969 (FAO, 1969a). The recommendations of this conference expressed the need to establish germ plasm collections of tropical fruit trees and to strengthen the interchange of information on the status of living collections. It also proposed

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collecting missions to areas where fruit tree germ plasm is being destroyed by monoclonal culture in the case of varieties, and as a result of elimination of forests in relation to wild species.

B.

LONG-TERM STORAGE CENTERS

1 . The National Seed Storage Laboratory The establishment of the National Seed Storage Laboratory by the U .S. Department of Agriculture in 1958 was a milestone in the conservation of germ plasm. This Laboratory is situated at Colorado State University, Fort Collins, Colorado. I t has the responsibility for long-term storage of our conserved stocks of seeds of crop and forestry plants. The basic Laboratory mission is to conserve important types of seed: document each accession; maintain them perpetually by the most effective control of viability and by rejuvenating the seed stocks; and publish periodic inventories of the seeds in storage. In addition to these activities, the Laboratory staff conducts investigations into the factors governing the long-term survival of seeds in storage and methods of storing the seeds. Unlike working collections, the NSSL does not distribute the stored seeds as long as they are available elsewhere in the United States. As specific seed samples in general working collections are no longer in demand, the NSSL will maintain them until there is a need for such collections again. Among the 78,000 seed lines in the NSSL, “world collections” outnumber all others. These are wheat, oats, barley, sorghum, soybeans, flax, tobacco, cotton, safflower, and sesame. Attempts are made to expand collections of obsolete commercial varieties. When Federal, State, and private breeders release new varieties or breeding lines, they are encouraged to deposit them promptly. The NSSL collection also includes genetic stocks, old open-pollinated lines, inbreds, plant introductions, differential hosts for pathogens, and virus indicators. Only seeds of high viability are placed in storage. When received, germination tests are run to determine the degree of viability. If the germination is satisfactory, the seeds are placed in one of the cold storage rooms where they remain for five years except where some lots may be borderline in viability. The latter are retested in two years, but five years is the standard interval. When viability of seed declines during storage it is necessary that the seeds be reincreased in such a manner that the genetic characteristics of the original seed will be retained. Reincrease work will be done under contract with a scientist with the special knowledge or in an area where

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JOHN L. CREECH A N D LOUIS P. RElTZ

the crop is adapted. All reincrease work will be done in isolation, or by sibbing or selfing according to the crop requirements. The value of the Laboratory will be realized more in the future than in the present. Requests for seeds of obsolete varieties have been filled, and as time progresses these kinds of requests will be increasingly evident. Collections which might be valueless when held under poor storage conditions in other places will be available at the Laboratory. In this respect, the Laboratory deserves much of the credit for rescuing the classical Blakeslee Duturu collection and other valuable genetic stocks. In the past, the working collections were reincreased every five years except where demand depletes stocks rapidly. By depositing small samples of the collections in the Laboratory, the increase interval can now be extended to 10 years with assurance that materials can be recovered from the NSSL. Under the provisions of the Plant Variety Protection Act, the NSSL could well serve in a new capacity. The Act requires that a viable sample of basic seed necessary for propagation of the variety will be deposited and replenished periodically in a public repository. This seed will serve as the reservoir for public use after the termination of the period of protection provided under the Act. The original policies under which the Laboratory operates were drawn up in 1959 by representatives of the Seed Trade, Commercial Plant Breeders, State Experiment Stations, and the Agricultural Research Service. Minor changes in policies were made in 196 I . The most important of these was a provision permitting a donor to deposit seed with a 5-year restriction on distribution as stated under Item f. The statement under Item j absolves the Laboratory of any responsibility for nomenclature, and Item n emphasizes the absence of Federal endorsement of any varietal material deposited in the Laboratory. The policies governing the NSSL are included below for the guidance of concerned scientists. a. The Laboratory is a Federal facility and all seed accepted for storage becomes Federal property. b. Only seed will be accepted for storage. c. Valuable seed stocks will be accepted by the Laboratory from Federal and State institutions, commercial seed interests, and private individuals. The basic criterion for acceptance is its potential use in plant breeding and genetic studies or fundamental biology. Information as to history and genetic composition and complexity is required for the retrieval of certain genotypes. d. Any bona fide research worker of the United States, its territories and possessions, may receive seed from collections stored at the Labora-

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tory subject to the restrictions in Item f. However, seed will not be provided by the Laboratory if available commercially or in working stocks of research agencies. The Laboratory will suggest sources of supply. e. The Laboratory will have no responsibility in relation to commitments with foreign countries. A11 requests from foreign sources will be sent to headquarters of the New Crops Research Branch, Beltsville, Maryland, where decisions in relation to foreign countries will be made. f. Both public and private donors of specific lots of valuable seed stocks or seed of new varieties, who wish to do so, may retain for a period not to exceed five years the exclusive right to withdraw or permit withdrawal of portions of such seed provided the optional restriction is clearly indicated at the time the seed lot or sample is deposited. No seed collection may be withdrawn in its entirety. After such time limit has expired, and on seed lots or samples deposited without this restriction, all seed deposited in the Laboratory shall be available to any bona fide research worker, whether public or private, of the United States, its territories, or possessions. g. The Laboratory will not hold bulk supplies or seasonal stocks: it is not a warehouse or seed distributing center. Rather, it is a germ plasm bank for valuable stocks to be held over the years for the use of research workers when needed. h. The Laboratory will issue periodic inventories of the stocks held in storage to inform research workers of material available. i. Only clean seed of reasonably high germination is acceptable for storage. If seed of low viability (below 60-65% germination) is received, it will be held on a tentative basis until the donor is able to provide replacement seed of higher viability (75% germination or better). j. N o charge will be made by the Laboratory for the service of furnishing seed. The Laboratory will use every care in keeping good records, but it is not responsible for errors which may occur in the original documentation. The varietal name supplied by the donor will be accepted by the Laboratory. k. When seed has been accepted officially, the Laboratory will be responsible for the increase of stocks if, during storage viability drops to a point where there is danger of loss of the accession or stocks have become depleted as a result of seed distribution. 1. The Laboratory will not assume responsibility for replenishment of stocks if the accessions received are sub-minimal in quantity or viability. However, if obsolete varieties are received not meeting the preceding acceptable standards, the Head of the Laboratory in consultation with the appropriate specialists in the Plant Science Research Division may make arrangements for increase.

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JOHN L. CREECH A N D LOUIS P. REITZ

m.The principal objective of the Laboratory is long-time holding of valuable seed, Research projects will be carried on at the Laboratory related to the Laboratory’s objective, i.e., physiological problems in seed viability and longevity. n. The acceptance of seed of a commercial variety by the Laboratory shall not be considered in any way a Federal endorsement as to the value of the variety. In addition to the above-revised policy, recommendations have been made as to what constitutes “valuable seed.” It is recognized that such a definition will vary greatly depending upon the significance attached to the present commercial value of the crop involved and the individual research worker’s evaluation whether he be a geneticist, horticulturist, agronomist, or pathologist. However, the following categories of crop seed will be accepted by the Laboratory: N e w Varieties: All newly released varieties, whether of private, public, or commercial origin, including reselections from varieties continuing in current use. Current Varieties: Varieties currently in use and under registration by respective crop group organizations, or otherwise documented as to specific origin and distinguishing characteristics. In this group would be included those varieties approaching obsolescence which have been superseded by new or currently popular varieties. Open-Pollinated Varieties: Stocks representing earlier varieties or types of specific crops which have been or will be replaced in the commercial field by hybrids. Inbred Lines: Parental lines of known genetic composition widely used in combination for hybrid production. Obsolescent Germ Plasm: Samples representing hold-over material from earlier research programs and of no immediate interest. Genetic Stocks: Includes materials of academic and genetic interests such as marker genes, mutants, translocations, monosomics, trisomics, and other chromosome aberrations. Replenishment of such stocks, if in a heterozygous state, will remain the obligation of the donor. With the latter type of stocks the Laboratory serves only as an insurance against loss. Plant Introductions: From Regional and Federal Introduction Stations or other agencies as seed is increased beyond “working stocks.” Differential Host Varieties: Used or being used as differential hosts for differentiating pathogenic races. Virus Indicator Plants: Used in indexing plant viruses. Physiologically Useful Species: Used in physiological studies or physiological assays. All inquires as to minimum quantities of seed required for specific

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crops, documentation, condition or quality, and other routine information should be addressed to the National Seed Storage Laboratory, USDA, Fort Collins, Colorado 8052 1. 2 . Other Storages National seed storages exist in a few other countries. A national center for conservation of germ plasm has been operating in Japan since 1966 at the National Institute of Agricultural Sciences, Hiratsuka, Kanagawa (Ito and Kumagai, 1969). Although the Hiratsuka storage has a similar responsibility as our Fort Collins storage for long-term preservation of seeds, it also serves as a distribution center for working stocks. This has resulted in some excellent research on containers for dual storageshipment purposes without having to disturb the seed during packaging. F A 0 and the Turkish Government established a Crop Research and Introduction Center at Izmir, Turkey, in 1964. A storage has been constructed to conserve the germ plasm native to Turkey and adjacent countries under optimum conditions of humidity and temperature (SchulzSchaeffer, 1970). Like the facility in Japan, this storage will serve for both long-term preservation of seed and distribution of working stocks. It also provides for exploration activities and other related aspects of the broad concept of a genetic resources center. Because of the support by FAO, the lzmir Center has taken on an international responsibility which might not endure should this support be discontinued. The International Rice Research Institute serves as an excellent example of a genetic resources center for a single crop. Founded in I962 at Los Banos, Laguna, Philippines (Chang, 1970; Chandler, 1970), lRRl is the effort of The Rockefeller Foundation, in cooperation with the Government of the Philippines, Ford Foundation, NSF, NIH, AID, and International Business Machines, Inc. This Institute has made spectacular progress in rice improvement. The germ plasm activities include preservation of primitive germ plasm and the development of an understanding of the complex system where there is a high frequency of outcrossing, low seed fertility, and other genetic factors combined with operating a storage where high temperature and high humidity prevail. The Institute holds over 900 wild forms of 38 entities as well as thousands of varieties of Oryza sativa L. As seed is increased these are transferred to the USNSSL as a backstop since the storage conditions are not considered truly adequate. Although the needs for long-term storage facilities for seed are recognized as essential, we are only on the threshold of this development. No one nation can hope to preserve all the species adequately. On the other hand, it is unrealistic to believe that every nation requires an expensive,

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JOHN L. CREECH A N D LOUIS P. REITZ

sophisticated seed storage. It falls on those nations with the greatest resources and stability to cooperate in such ventures. It is generally agreed that there is an urgent need to establish regional centers which will be concerned with long-term conservation of germ plasm in the Mediterranean, Western and Central Europe, and Scandinavia. There is also an awareness in the U.S.S.R. of the inadequacies of the current facilities for preserving the enormous number of collections (1 75,000) now held at the Vavilov All-Union Institute of Plant Industry, Leningrad (Brezhnev, 1970). It is an alarming fact that deficiencies exist all along the line in our efforts to establish stable centers to preserve germ plasm, and these will be compounded as efforts to collect germ plasm are emphasized. C. GAPS I N GERMPLASM One of the results of assessing the extent of our germ plasm collections is to identify gaps which can be closed by planned explorations that give priority to threatened materials. The sorghum germ plasm committee established by The Rockefeller Foundation in cooperation with the Crop Evolution Laboratory, University of Illinois, conducted a survey of collections and concluded that conspicuous gaps exist in wild and weedy forms of the genus and that many systematic collections have been lost due to lack of equipment and staff. There are incomplete collections of Chinese kaoliangs, Ethiopian and Sudanese varieties, primitive types from West Pakistan and other areas of Southeast Asia. Despite a major collection of sorghums in India, an absence of genetic barriers between the cultivated and wild forms, relatively few sorghum relatives have been used in breeding or are present in adequate variation in collections. As a consequence it is not known whether these species can be used successfully although they may be important sources of insect and disease resistance. But the threat to these resources by grazing and other agricultural pursuits continues unabated. A similar situation has been described for rice germ plasm. Although the present world collection consists of about 12,000 (Chandler, 1970) accessions representative of considerable varietal diversity, local races from such remote areas of northeast India across to northeast Thailand are deficient. Fortunately, through the use of PL 480 funds in India, an attempt is being made to systematically collect primitive and less important varieties in northeast India. Perhaps the greatest barrier to securing reserves of rice from some of these areas is the political and military conditions affecting these regions.

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Concern has also been expressed for the glaberrima (Oryza glaberrima subsp. stapfi (Roshev.) Chev.) cultivated in Africa as well as for the species which may be the clue to the evolution of this crop. Since this species appears as a weed as well, there is a deliberate erosion of it. With few sources of funds, such as PL 480, in the affected areas, collections from important areas of West Africa are unlikely. Realizing that some priorities for “rescue” operations to secure threatened germ plasm are necessary, the F A 0 Panel of Experts on Plant Exploration and Introduction ( I970a) recommended that the following are urgent projects. 1. The exploration and collection of primitive and wild wheats in Turkey, Iraq, Iran, Afghanistan, and West Pakistan because these resources are rapidly being displaced by introduced wheats. 2. The exploration and collection of glaberrima rices in West Africa as an important resource for future rice improvement. 3. The exploration and collection of native West African Dioscorea species which are being displaced by other root crops as well as subject to loss when propagating stocks have been used for food in times of stress. In reaching these priorities, the major concern has been for the rapidity with which agricultural development has proceeded and primitive crop varieties have declined. Germ plasm in those areas where subsistence agriculture has not changed drastically appears to be safe for the present as compared to regions where there has been a concerted effort by national and international groups to upgrade technology. Also some areas are much more accessible from the viewpoint of successive explorations. The series of explorations for primitive forms of beans (Phaseolus vulgaris L.) in Mexico, conducted by the USDA in 1966- 1967, resulted in a rich assortment of races which the local Indians selected and increased according to local environmental and ethnic demands. Fortunately, from the germ plasm aspect modern varieties have not made serious inroads in this traditional system of agriculture. In some parts of the world, decisions to require the growing of new varieties without a means of preserving the discarded ones has been detrimental to our germ plasm base.

IX.

Programs for the Future

We have now reached a time when serious consideration needs to be given to the direction our overall activities in germ plasm will take. The question is not so much what we can do, but whether the priorities that will be assigned by scientists and administrators to the conservation of germ plasm will meet the goals.

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In 1966, the National Academy of Sciences attempted to foresee the general trends of the plant sciences over the next decade. A section of the report (Anonymous, 1966) dealt with the future needs for basic research on the 15 food plants on which the world depends for its major sources of food energy. These are rice, wheat, corn, sorghum, and barley; sugarcane and sugar beet; common bean, peanut, and soybean; potato, sweet potato, and cassava; and banana and coconut. The report stressed the need to concentrate on basic biological characteristics of these principal food plants and to inventory and assess world resources of these and other plants which form the basis to today’s agriculture. It is interesting to note that much of the research accounting for our present agricultural abundance was conducted 25 or more years ago. We are in the era of research to meet the needs of the year 2000 and beyond. With new and menacing problems of the environment facing us, the need to accumulate as diverse a base of variability as possible becomes increasingly important. It has been made clear that these natural resources are not inexhaustible and the decline of primitive genetic resources continues unabated. Since U.S. agriculture is and will be dependent on the majority of these crops and others, it is to our national interests to be certain that the germ plasm we hold will fulfill the needs of future plant breeders. Among the specific programs for the future which are essential and which can be translated into programs for action are : (a) the development of plant resources information centers to cope with the expanding demand for expert information on the composition, use, and sources of plants beneficial to man; (b) the development of national and international clonal repositories to protect the dwindling resources of vegetatively propogated plants; (c) the development of more effective national and international surveys of holders of collections to make these genetic resources widely available; (d) the increased exchange of plant germ plasm among nations, and the initiation of such exchanges where they do not exist. A center, such as that recommended in recent conferences, to provide basic information on all kinds of economic plants is fundamental. In order to find out everything possible about crops as unique biological systems, a rich source of germ plasm is absolutely essential. While the National Academy of Sciences emphasized the 15 major crops, it is obvious that the need for germ plasm applies to basic inquiries into all plant species useful to man. Beyond the basic food crops, man’s needs for plants to provide adequate shelter, improved health, and aesthetic surroundings must be served.

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ACKNOWLEDGMENT The evaluations and use of germ plasm given as examples are largely from progress reports of the Plant Science Research Division and from annual and other reports o f the Coordinators of Regional Research Projects NE-9, Geneva, New York; NC-7, Ames, Iowa: S-9, Experiment, Georgia; W-6, Pullman, Washington; and IR- I , Sturgeon Bay, Wisconsin. REFERENCES Adair. C. R.. Atkins, J. G . . Beachell, H. M., Evatt, N. S., Everett, T. R., Green, V. E., Jr., Jodon, N. E., Johnston, T. H., Mikkelsen, D. S., Miller, M. D., Shaw, W. C., Smith, R. J., Jr., Thysell, J. R., and Webb, B. D. 1966. U . S . , Dep. Agr., Agr. Handb. 289, 1-124. Anonymous. 1966. “The Plant Sciences-Now and in the Coming Decade,” pp. 100-1 14. Nat. Acad. Sci., Washington, D.C. Anonymous. 1967. Annual report, 1966. Int. Rice Res. Inst., Los Banos, Philippines. Anonymous. 1968. Texas, Agr. Exp. Sta., Circ. L-731. Bennett, E. 1970. I n “Genetic Resources in Plants” (0.H. Frankel and E. Bennett, eds.), pp. 157- 178. Blackwell, Oxford. Berry, C. D., Lessman, K. J., White, G. A., and Earle, F. R. 1970. Crop. Sci. 10, 178-180. Brezhev, D. D. 1970. I n “Genetic Resources in Plants” (0.H. Frankel and E. Bennett, eds.), pp. 157- 178. Blackwell, Oxford. Chandler, R. F. 1970. “Catalog of Rice Cultivars and Breeding Lines (Oryza sativa L.) in the World Collection of the International Rice Research Institute.” Int. Rice Res. Inst., Los Banos, Laguna, Philippines. Chang, T . T. 1970. Sabrao Newsletter 2,59-64. Clark, J. A. 1936. Yearb. Agr. (US.Dep. Agr.) pp. 207-302. Clark, J. A. 1956. Econ. Bot. 10, 194-200. Correll, D. S., Schubert, B. G., Gentry, H. S., and Hawley, W. 0. 1955. Econ. But. 9,307375. Creech. J. L. 1963. Chemurgic Dig. 21,7-9. Creech, J. L. 1970a. Span. 13, I I 2- I I 4. Creech, J. L. 1970b. I n “Genetic Resources in Plants” (0.H. Frankel and E. Bennett. eds.). pp. 22 1-229. Blackwell. Oxford. Cullinan, F. P. 1961. I n “Germ Plasm Resources,” Publ. No. 66,pp. 91-102. Amer. Ass. Advance. Sci., Washington, D.C. Dodge, A. F., Ackerman, W. L., and Winters, H. F. 1965. Amer. Horf. Mag. 44,92-98. Fairchild, D. 1938. “The World Was My Garden.” Scribner’s, New York. FAO. 1967. “Report of the FAOllBP Technical Conference on the Exploration, Utilization, and Conservation of Plant Genetic Resources.” FAO, Rome, Italy. FAO. 1969a. “Report of the F A 0 Conference on Propagation of Tropical and Subtropical Fruits.” FAO, Rome, Italy. FAO. 1969b. “Report pf the Third Session of the F A 0 Panel of Experts on Plant Exploration and Introduction.” FAO, Rome, Italy. FAO. 1970a. “Report of the Fourth Session of the F A 0 Panel of Experts on Plant Exploration and Introduction.” FAO, Rome, Italy. FAO. I970b. “Plant Introduction Newsletter,” No. 23. FAO, Rome, Italy. Fisher. H. H. 1963a. U S . , Dep. Agr., A R S ARS 34-37-1, 1-324.

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Fisher, H. H. 1963b. US.,Dep. Agr., ARS ARS 34-37-2, 1-303. Fisher, H. H. 1964. U.S., Dep. Agr., ARS ARS 34-37-3, 1-292. Frankel, 0. H., and Bennett, E., eds. 1970. “Genetic Resources in Plants.” Blackwell, Oxford. Gentry, H. S. 1968. Econ. Bot. 23, 55-69. Hanna, G . C. 1966. In “Proceedings of the National Conference on Mechanization of Tomato Production,” pp. 9- 16. Nat. Canners Ass., Washington, D.C. Hanson, A. A. 1969. Proc. Int. Seed Test. Ass. 34,369-382. Harlan, H. V., and Martini, M. L. 1936. Yearb. Agr. (US.Dep. Agr.) pp. 303-346. Harlan, J. R. 1961a. I n “Germ Plasm Resources,” Publ. No. 66, pp. 3-19. Arner. Ass. Advance. Sci., Washington, D.C. Harland, J. R. 1966. In “Plant Breeding” (K. J. Frey, ed.), pp. 55-83. Iowa State Univ. Press, Ames. Hodge, W. H., and Erlanson, C. 0. 1955. Advan. Agron. 7,189-2 1 I . Hodge, W. H., Loomis, H. F., Joley, L. E., and Creech, J. L. 1956. Nut. Hort. Mag. pp. 86106. Howlett, F. S. 1957. Agr. Exp. Sta., Ohio, Res. Bull. 790, 1-131. Jenkins, M. T . 1936. Yearb. Agr. (US.Dep. Agr.) pp. 455-522. Ito, H., and Kumagai, K. 1969. Japan Agr. Res. Quart. 4,32-38. Jones, J. W. 1936. Yearb. Agr. (US.Dep. Agr.) pp. 415-454. Jones. Q., and Wolff. 1. A. 1960. Econ. Bot. 14,37-55. Khush, G. S., and Stebbins, G. L. 1961. Amer. J . Bor. 48,723-730. Lamon, R. E. 1961. In “Germ Plasm Resources,” Publ. No. 66, pp. 327-336. Amer. Ass. Advance. Sci., Washington, D.C. McCrory, S. A. 1958. South Dakota Agr. Exp. Sta., Bull. 471, 1-39. McKee, R., and Pieters, A. J. 1937. Yearb. Agr. (U.S. Dep. Agr.) pp. 999-1031. Magness, J. R. 1937. Yearb. Agr. ( U . S . Dep. Agr.) p. 604. Martin, J. H. 1936. Yearb. Agr. (US.Dep. Agr.) pp. 523-560. Munck, L., Karlson, K. E., Hagberg, W., and Eggum, B. 0. 1970. Science 168,985-987. Oakes, A. J. 1965. Trop. Agr. 42,323-331. Oakes, A. J.. and Langford, W. R. 1967. Agron. J . 59,387-388. Pieters, A. J., and Hollowell. E. A. 1937. Yearb. Agr. (US.Dep. Agr.) pp. 1190-1214. Radcliffe, E. B., and Lauer, F. I. 1966. Minnesota, Agr. Exp. Sra., Sta. Bull. 253, 1-23. Reid, D. A., Wiebe, G . A., Dahms, R. G., Dickson, A. D., Harlan, J. R.,Moseman, J. G., Olien, C. R., Price, P. B., Shands, R. G., Shaw, W. C., and Suneson, C. A. 1968. US., Dep. Agr., Agr. Handb. 338, I - 127. Reitz, L. P. 1970. Science 169, 952-955. Reitz, L. P., and Craddock, J. C. 1969. Econ. Bot. 23,315-323. Reitz, L. P., and Ward, D. J. 1958. In “First International Wheat Genetics Symposium,” pp. 143-159. Manitoba, Winnipeg, Canada. Ross, R. W., and Rowe, P. R. 1965. Wisconsin, Agr. Exp. Sta., Bull. 533, 1-73. Ross, R. W., and Rowe, P. R. 1969. Amer. Potato J . 46,5-13. Ryerson, K. A. 1967. In “Proceedings of the International Symposium on Plant Introduction,“ pp. 1- 19. Tegucigalpa, Honduras. Schulz-Schaeffer. J. 1970. Now 6, 8-9. Skrdla. W., Alexander, L. J., Oakes, G., and Dodge, A. F. 1968. Norfh Central Reg. Res. Bull. 172,l-1 10. Sowell, G., Jr., and Langford, W. R. 1963. Proc. Amer. Soc. Hort. Sci. 83,609-612. Stanton, T. R. 1936. Yearb. Agr. ( U . S . Dep. Agr.) pp. 347-414. Tysdal, H. M., and Westover, H. L. 1937. Yearb. Agr. (US.Dep. Agr.) pp. 1122-1 153.

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Vavilov, N. I. 1950. Chron. Bot. 13, 1-366. Vavilov, N. I. 1957. “World Resources in Cereals, Leguminous Seed Crops, and Flax and Their Utilization in Plant Breeding.” Acad. Sci. USSR, Moscow (transl. OTS 602 1127). Vinall, H. N., and Hein, M. A. 1937. Yearb. Agr. (U.S. Dep. Agr.) pp. 1032-1 102. Wall, J. S., and Ross, W. M. 1970. “Sorghum Production and Utilization.” Avi Publ., Westport, Connecticut. Ward, D. J. 1962. US.,Dep. Agr., Tech. Bull. 1276, 1 - 1 12. Webb, B. D., Bollich, C. N., Adair, C. R., and Johnston, T. H. 1968. Crop Sci. 8,361-365. Webb, R. E. 1966. In “Proceedings of the National Conference on the Mechanization of Tomato Production,” pp. 17-23. Nat. Canners Ass., Washington, D.C. Webb, R. E., and Bruce, W. M. 1968. Yearb. Agr. ( U . S . Dep.Agr.) pp. 103-107. Weiss, M. G. 1961. In “Germ Plasm Resources,” Publ. No. 66, pp. 103-1 16. Amer. Ass. Advance. Sci., Washington, D.C. White, G. A., and Higgins, J. J. 1966. US.,Dep. Agr., Prod. Res. Rep. 95, 1-20. While. G . A,. Cummins, D. G.. Whiteley. E. L., Fike, W . T., Greig, J. K., Martin, J. H , Killinger, G. B., Higgins, J. J., and Clark, T. F. 1970. US.,Dep. Agr., Prod. Res. Rep. 113, 1-38. Whitehouse, W. E., Creech, J. L., and Seaton, G. A. 1963.Amer. Horr. Mag. 42, 150-157. Yen, D. E. 1970. In “Genetic Resources in Plants” (0.H. Frankel and E. Bennett, eds.), pp. 341-350. Blackwell, Oxford.

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THE RELATION BETWEEN GEOMORPHOLOGY AND SOIL MORPHOLOGY AND GENESIS’ R. B. Daniels,

E. E. Gamble, and J. G. Cady

North Carolina Agricultural Experiment Station, Raleigh, North Carolina, a nd the Johns Hopkins University, Baltimore, Maryland

I. II. 111.

IV.

V.

Introduction .............. ......................................... Definition of Terms ........................................................... Evolution of Soil-Ge Criteria Used in Geo A. Surface Sequence, Depositional ........................ B. Surface Sequence, Erosional ................................................. C New Terminology .......................................................... Examples of Soil-Geomorphic Relations ............................. A. A Hillslope Study in Uniform Material .............. B. Australian Laterite and Solonetz ...................................................... C. New Mexico Desert-Argillic and Carbonate Horizons in Desert Soils .............. ............ ................ D. North Carolina Coastal ........................... ................ Summary ........... ...................................................... ..... References ...............................................................

I.

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s3 55 56 58 61 62 62 65 70 76 84 86

Introduction

Geomorphology is defined in the Glossary of Geology (Howell, 1957) as the “systematic examination of land forms and their interpretation as records of geologic history . . . .” Pedology is the science that studies the soils whose upper boundaries are the surface of the earth. A strong link should exist between these two sciences because they deal with parts of the same thing. Before World War 11 there was little conscious interchange of ideas between t h e two disciplines and even less application of the methods of one science to the solving of problems in the other. During World War 1 I , there was close collaboration among soil scientists and geologists in the Military Geology Unit of the U.S. Geological Survey. This started an interchange of ideas that has been increasing. During the ‘Paper No. 3367 of the Journal Series. A joint contribution from the Soil Conservation Service, USDA and the Department of Soil Science, North Carolina Agricultural Experiment Station, Raleigh, North Carolina.

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R. B. DANIELS, E. E. GAMBLE, A N D J . G . CADY

last twenty years there has been widespread effort to apply the concepts and knowledge of the stratigrapher and geomorphologist to problems of soil morphology and genesis. This report summarizes some of the progress made in recent years. Practically all work dealing with the distribution of soils on the earth’s surface employs some geomorphic concepts. In developing a short history of the interrelation between the two sciences, we will limit our review mainly to studies where geomorphic and allied approaches were used primarily to provide background information for soil genesis studies. This eliminates most studies of parent materials, of relations between distribution of glacial drift and soils, and of soils studied in relation to topography, slope, drainage, or erosion. It also eliminates most pure geomorphic studies, and we will not attempt to review the large volume of literature on buried soils. For those interested in the literature on buried soils, the papers by Thorp (1965), Morrison ( 1 968), and Ruhe (1968) are good places to begin. A paper by Hunt (1954) shows how stratigraphic principles can be used to determine the age of soils. We will define and illustrate the geomorphic criteria used in the various studies and define the terminology used. A limited number of studies that combine geomorphology and soil genesis work will be summarized. DEFINITION

OF

TERMS

Certain terms with precise meanings are used throughout the text. These and a few more general terms are defined as follows: Geomorphology. The science that studies the evolution of the earth’s surface. Geomorphologists are interested in how a surface evolved, what factor or factors were involved, and when these processes started and stopped. Stratigraphy. The science that studies the formation, composition, sequence, and correlation of rocks as parts of the earth’s crust (Howell, 1957). Land forms. “Features of the earth that together make up the land surface” (Ruhe, 1969). Weathering zones. Horizons or layers of rock or sediment altered from its original state, usually by subaerial weathering. Alteration may involve the loss of carbonate, weathering of minerals such as feldspar, and the removal or rearrangement of iron, and many other elements. The term “weathering zone” is used extensively by workers in the United States Midwest Pleistocene (Ruhe, 1954) and by Australian (Mulcahy, 1967; Northcote and Tucker, 1948) workers when describing laterite.

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Peneplain. A broadly undulating erosion surface that is almost a plain (“pene” meaning “almost”). Surface. A two-dimensional form that has length and width but no thickness. It may be a plane or it may be composed of compound curves. Geomorphic surface. A mappable feature that is “a portion of the landscape specifically defined in space and time” (Ruhe, 1969). The surface of the earth is made up of a large number of geomorphic surfaces. But there is the implication used throughout this paper that a geomorphic surface is something more than a part of the surface of the earth. It is a part that has been studied and mapped. A geomorphic surface may be depositional (formed by deposition of sediment) or erosional, or both. It may be a level plain, a straight slope, or may have a multicurvate or undulating configuration. It may be confined to one rock-type or sedimentary formation, or it may cut across several. A geomorphic surface may have formed in a short time throughout its extent, as in a lava flow or ash fall, or if erosional it may have taken a long time to develop. A geomorphic surface may be uplifed, lowered, faulted, or warped by tectonic movements, and it may be buried without being destroyed. But once a surface is eroded, it is destroyed because by definition it has no thickness. The erosion creates a new surface. To summarize, a geomorphic surface is a part of the surface of the land that has definite geographic boundaries and is formed by one or more agencies during a given time span. The time span may be relative or absolute. II.

Evolution of Soil-Geomorphic Concepts

Early workers in .soil genesis and classification used the classic ideas of geology and physiography to explain differences in soils and grouped soils on a geographic basis. But for a long time little attention was given to landscape age or to the origin of particular land surfaces. The distribution and genesis of laterite has intrigued geologists and soil scientists for years, and it is in the discussions of laterite that some individuals more or less pointed toward the relations between geomorphology and soils. Newbold ( 1 844), in discussing the laterite of southern India, said: “ I t is evident not only that it must have covered it (the peninsula) formerly to much greater extent than at present, but that it has since been much broken u p by the-subsequent denudation . . . . I t is impossible to compare these scattered and detached portions without imagining that the whole intervening country has once been covered with a great body of laterite, enormous masses of which have been removed by denudation” (quoted from Prescott and Pendleton, 1952). These and other comments of Newbold ( 1844, 1846) definitely pointed to the prob-

54

R. B. DANIELS, E. E. GAMBLE, A N D J . G. CADY

ability that laterite in this area was the result of previous weathering and that it was much older than the soils on the later erosional and depositional surfaces. This is one of the first reports where specific parts of the landscape and allied soils were at least relatively dated. Woolnough ( I9 I8), in discussing the physiographic significance of laterite, stated, “If this view is correct (laterite formed under peneplain not plateau conditions), then the importance of laterite from a historic point of view is greatly enhanced. It owes its present position on the summit of the plateau to the uplift of the peneplain.” Davis ( 1920) believed that laterite formed in the old age of the peneplanation cycle. These statements point to the geologists’ belief that remnants of old landscapes are preserved. Soil scientists frequently have a physiographic bias when they discuss the soils of a region. This is illustrated by the publications of Marbut et al. ( 19 13) and Coffey ( I9 12) in which soils were discussed by physiographic regions such as “soils of the Piedmont Plateau Province, etc.” Yet Coffey and Marbut understood and used soil-landform relations. Marbut (1928), probably citing Fowler’s work (1927), talks about the Tifton series being on a very old land surface. Coffey apparently recognized that soils on stream terraces in Alabama were younger than those on the adjacent uplands and that they should be considered as another group. Milne ( I 935) in his original article on the Catena noted that the profiles changed along the traverse from the ridgecrest to the stream in accordance with topography and its influence on drainage. The present topography was in part influenced by that of times past. He discussed a complex of soils in Tanganyika and hinted that the red earths may be relict soils, especially when associated with calcareous black clays (Milne et af., 1935). Milne et al. recognized how landscape evolution can affect soils when they wrote, “The great plateaux lying behind the eastward-facing ranges carry material which was graded down during a long process of peneplanation, and which therefore may (a) retain characters impressed during its first weathering, (b) have acquired new ones as its profile developed under conditions of sluggish drainage, and (c) by undergoing a fresh adjustment to conditions of rejuvenated drainage following the elevation of the peneplain into plateaux.” Milne et af. were well aware that the history of the land surface had considerable influenc‘e on soil properties. Probably more important was the understanding that different parts of the landscape may have had different histories. The influence of various factors on soil development has been discussed by early writers (Glinka, 1963), but Milne et al. are among the first to clearly point out some of the multiple factors involved in soil-landscape relations. Since 1940 emphasis has been on the history of the landscape in rela-

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tion to soils. The Australian and African workers have been concerned with soils that could not have developed under the present climate. Numerous publications from Africa have dealt with the influence of previous weathering on soils (Nye, 1954, 1955; Spurr, 1954; Ollier, 1959; Radwanski and Ollier, 1959; Watson, 1964; Jongen, I96 1 : Maigninen, 1961; Maud, 1965). Similar work in Australia emphasized the fossil nature of laterite (Crocker, 194 I , 1946; Prescott, 1944; Stephens, 1946, 1949; Northcote, 1946; Playford, 1954; Blackburn and Leslie, 1958). After the initial work on the fossil character of laterite, many Australian workers paid increasing attention to minor modification of the landscape by erosion and deposition (Butler and Hutton, 1956; Hutton and Stephens, 1956; Van Dijk, 1958, 1959; Butler, 1958). This later work challenged several established soil-geomorphic concepts. Butler ( 1959) tested the geomorphic theory that large areas of t h e landscape had been stable against his evidence of episodic erosion and sedimentation. He developed the K-cycle concept from this work. Butler’s work was used and amplified by Van Dijk (1959), Walker, (1962, 1963), and Churchward (1963). Soil-geomorphic work in the United States in the 1950’s and early 1960’s was in regions where Quaternary deposits and complex erosionaldepositional surfaces make up a large part of the landscape (Daniels and Jordan, 1966; Gile, 1970; Hawley, 1965; Parsons and Balster, 1966; Parsons et al., 1970; Balster and Parsons, 1969; Ruhe, 1967, 1969). These studies emphasized the relations between geomorphic surface and soils, with the weight being placed on time relations based on radiocarbon chronology. A somewhat similar course was followed in Australia during the 1960’s where soil studies established a history of the profile based on its known geologic history (Mulcahy, 1960; McArthur and Bettenay, 1960; Wright, 1962, 1963; Bettenay and Hingston, 1964; Ward, 1966; Turton et al., 1962; Blackburn et al., 1967; Bettenay et al., 1964). Present work uses geomorphic data to develop ideas about how soils form. Soil scientists are trying to find out when a process starts and how fast it goes. With this greater interest in soil process, there has been an increase in the need for stratigraphic studies, pollen analysis, radiocarbon dating and many other kinds of information. Ill.

Criteria Used in Geomorphic Work

Most geomorphic work involves several other fields of geology, but one of the most used is stratigraphy, or the study of the sequence and correlation of rocks. The principle of superposition is used to determine

56

R. B. DANIELS, E. E. GAMBLE, A N D J. G. C A D Y

sedimentary and geomorphic surface sequence. This principle simply says that younger beds occur on top of older beds, providing they have not been overturned. In Fig. lA, bed 2 is younger than bed I because the A

B

Bed

2

Bed

I

-~

C

wu/ Bed

I

D

I

FIG. I . Generalized illustrations of typical sedimentary sequences. Relative ages of the beds can be determined by the principle of superposition, i.e., bed 2 is younger than bed 1.

younger bed always overlies the older. In Fig. 1B the topographically higher bed 2 is younger than bed 1 because it still overlies bed 1. Although the landscape form is the same in Fig. IB and IC, the topographically higher bed 2 in Fig. IC is the youngest because it overlies bed 1. The age of bed 3 in Fig. ID is questionable. Beds 3 and 4 are younger than bed 2, but bed 3 may be older than, equal to, or younger than bed 4. No greater refinement of dating beds 3 and 4 in relation to each other can be made with the available information. There are two types of geomorphic surfaces, depositional and erosional (Fig. 2). As their names imply, one is formed by deposition of sediments, and the other by erosion. On many landscapes, a surface can have depositional and erosional elements (Fig. 2B), and the two elements are considered as one surface. A. SURFACE S E Q U E N C E , DEPOSITIONAL Depositional surfaces are the same age as the immediately underlying sedimentary laminae. These surfaces do not necessarily have the same

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57

age throughout because sediment can be deposited on one part of a plain and not another. A depositional surface, however, should be restricted to one depositional episode and it must be shown that there have been no A Deposltional /Surface A

7 sed\JEros!on Bed

Surface

\

I

B

B Depositional Surface

A

Erosional Element,

Depositional Surface B

Element, Erosion

C

I

Bed

I

\t

FIG. 2. Generalized relations between geomorphic surfaces and sedimentary deposits. The principle of superposition can be used to determine the relative ages of sediments and surfaces. A geomorphic surface is younger than the youngest bed it truncates.

subsequent additions or removal. Depositional surfaces usually are dated by the age of the underlying beds that were formed somewhat earlier, but in the same depositional episode. For example, a depositional surface is late Miocene if it is at the top of a late Miocene formation, or it is amidPleistocene surface if the underlying beds are mid-Pleistocene. Stratigraphic superposition determines the relative ages of depositional surfaces, but in many instances the younger surfaces are topographically lower (see Fig. 3A). In eolian landscapes, the depositional surface may have a wide range in altitude (Fig. 3B). The relations shown in Fig. 3A are common in river valleys and in belted coastal plains where the highest depositional surfaces are the oldest and the lowest ones are the youngest. The relations in Fig. 3B are common in eolian landscapes whether the material is sand or parna (sand-sized clay aggregates). Figure 3B also illustrates that a surface may have a simple or a complicated two-dimensional shape.

58

R. B. DANIELS, E. E. GAMBLE, A N D J . G. CADY

A.

OR M A R I N E

FLUVIAL

LANDSCAPES

Depositional Surface A , Bed

2\

Bed

I

J

b

B.

EOLIAN

e

Depositional Surface B

d 3

LANDSCAPES

-

Depositional

k Bed

d

3

I

FIG.3. Depositional surfaces; their age is determined by the age of the beds on which they occur. (A) Fluvial or marine landscapes. (B) Eolian landscapes.

B.

SURFACE S E Q U E N C E , EROSIONAL

Erosional surfaces are more complex than depositional surfaces in their distribution, relations to other surfaces, and dating. The following principles are very useful in determining the ages of erosional surfaces (Trowbridge, I92 I ; also see Ruhe, 1969). An erosional surface is younger than the youngest material it cuts, any structure it bevels, or any neighboring surface that stands at a higher level (Fig. 28). (There are some exceptions to the latter.) An erosional surface is older than valleys cut below it or deposits in that valley related to later alluviation, and it is older than any lower adjacent erosion surface. An erosional surface is the same age as the depositional surface it is graded to. The age of an erosional surface usually is determined in the field by observing what it slopes downward to and merges with in a smooth concave profile. In Fig. 2A, erosional surface B is younger than bed 2 and depositional surface A because it cuts, bevels, or truncates them. If the lower part of bed 2 were deposited 10,000 years ago, then depositional surface A is less than 10,000 years old. If depositional surface A is 8000 years old, then erosional surface B is less than 8000 years old. Figure 2B illustrates why an erosional surface is younger than a higher adjacent surface. Sur-

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59

face C truncates bed 3 and depositional surface B, therefore it is younger than either. Most landscapes consist of interfluves and valley slopes, and the relative ages assigned to erosional surfaces are based on relations of the slopes to alluvial fills. The alluvial fills are also good sources of material for radiocarbon dating. Figure 4 illustrates why radiocarbon dating is so Let us assume that bed 1 is Cretaceous and bed 2 is late Miocene. Deposit i o n o l Surface A

1

Surface B ,Erosional Element

010.000

60.000.000 yr

.

FiII,Bed

yr

3

14,000 yr

9,000 yr

FIG. 4. The age of a surface may be determined by its relation to other surfaces, sediments, and radiocarbon dates.

Because the erosional element of surface B’truncates bed 2, it is postMiocene, or younger than about 10 million years. The problem is that it could date from any time within this I0 million year period. Bed 3 is the alluvial fill associated with erosional surface B. The radiocarbon age of the base of the fill is 14,000 years. The subaerial element of erosional surface B, the sediment-air interface, is less than 14,000 years old because it slopes downward and grades to the top of bed 3, not to the base of bed 3. Another radiocarbon date near the top of bed 3 is 10,000 years. Thus, we now know by the relations shown in Fig. 4 that the erosional and depositional elements of surface B are less than 10,000 years old. But again we do not know how much less. If somewhere we can find datable material in beds younger than 3, then we can date surface B. If, for example, bed 4 is cut into bed 3 and has logs 9000 years old near its base, then we are justified in dating the erosional and depositional elements of surface B as being between 10,000 and 9000 years. We consider time zero of a geomorphic surface as the time when it was first exposed to subaerial weathering. On an erosional surface, this would be when erosion at that point stopped; on a marine depositional surface, it would be when the ocean withdrew and exposed the sediments to subaerial weathering. By this definition, the age of the geornorphic surface and the associated soil is the same. This definition may help overcome some of the confusion about soil-surface relations discussed by Moss ( I 968).

60

R. B. DANIELS, E. E. GAMBLE, A N D J . G. CADY

Throughout the geomorphic literature there is considerable emphasis on a “stepped sequence of surfaces,” such as shown in Fig. 4. In most landscapes, profiles constructed along interfluves from the drainage divide to the stream will show a “stepped sequence” of levels. These levels are easy to see in the field. But stepped interfluve summits do not always represent remnants of erosional or depositional surfaces, especially if they are narrow and convex across the top when viewed in cross section. The interffuve summits may be related to the youngest erosion cycle, but their form and altitude may be related to resistance of the rock or position between streams. Each case must be proved; an accordance of altitudes of interfluve summits may suggest, but does not prove by itself, the existence of a remnental geomorphic surface. Erosion frequently produces a large discontinuity in ages of soils from one surface to the next. In Fig. 4, soils on depositional surface A are approximately 10,000,000 years old, but a short distance away on surface B the soils are 9,000 to 10,000 years old. While this is a true statement, it does not give all the history of a site on surface B. Soils formed under old depositional surfaces such as A in Fig. 3 may have solums 10-15 feet thick and an associated weathering zone, or zones, that may be tens of feet thick, especially under some of the Australian laterities (Wright, 1963). I n Fig, 5 , erosional surface B

7

Depositional Surface A B Hor

/////////////////y

U

01

m

-

a

m

‘Y

- --- -

I-------”

Unweathered Material

-8,

Bed I

FIG. 5 . Truncation of weathering zones by erosional surfaces. The soil properties are determined in part by the nature of the outcropping weathering zone.

truncates bed 2, its weathering zones, and the solum of the soil under surface A. It should be obvious that a soil under surface B at point u will have a solum that cannot be distinguished from one under surface A. In this case the solum was formed under surface A, but it was truncated during the formation of surface B. Progressively downslope from point u are soils formed in weathering zones I , 11, and 111 until at point c the

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61

solum apparently was formed in fresh materials. A soil formed at point b has developed in the last 9000 to 10,000 years from materials weathered during an earlier period. The chronological age of the solum is 9000 to 10,000 years, but its characteristics have been determined in part by earlier weathering. This outcropping of weathering zones occurs in Iowa (Ruhe et al., 1953, Australia (Northcote and Tucker, 1948), and Africa (Ollier, 1959). The outcrop pattern of weathering zones is very important when the distribution of soils is considered, and it is one of the major reasons why geomorphic surfaces and their associated soils do not have the same aerial distribution (Gamble et al., 1970). One may consider it more or less academic that point a of Fig. 5 is on a chronologically young surface - Why not consider it as part of surface A because the soils are the same? The reason is that the age of a geomorphic surface must be proved by geomorphic criteria, not soil criteria. That part of surface A above point a has been destroyed by erosion, and it no longer exists at this point, although its underlying weathering zones are intact. Truncation of soil horizons can be used as geomorphic evidence, but not the properties of these horizons. If soil criteria are used to prove the geomorphic surface then the geomorphic work cannot be used to prove the soil. The recognition, delineation, and dating of geomorphic surfaces in the detail required for soil-geomorphic relations involves considerable field work. The relation of surfaces to the underlying sediments, weathering zones, and to adjacent surfaces must be determined in a three-dimensional examination. Erosional surfaces require tracing in the field because most topographic maps have contour intervals too large to show the detail needed. The age of an erosional surface on a valley side is determined by what it grades to (or slopes down to and merges with in a smooth concave profile): its relative elevation or form has little meaning. An example of the detail needed is shown by some monadnocks rising above the Piedmont in North Carolina. From a distance the side of a monadnock appears to slope down to and merge with the Piedmont Peneplain. But when looked at in detail, most of the monadnock’s surface grades to the ephemeral streams on its sides and to the perennial streams at its base. The surface, therefore, is the same age as the streams, not the Piedmont Peneplain. This example illustrates why direct tracing in the field is necessary to determine the age of a surface and why younger erosion surfaces can locally rise above older surfaces. C. NEW TERMINOLOGY As more geomorphic-soil studies were made, a number of soil scientists became dissatisfied with some of the older concepts of geomorphic sur-

62

R. B. DANIELS, E. E. GAMBLE, A N D J . G. CADY

faces. As a result, there have been several changes in nomenclature in a attempt to closely tie the geomorphic work to the soils (Butler, 1959; Don and Yaalon, 1968). Butler wrote ( 1959), “Their (geomorphologists) land surfaces, surfaces, or erosion surfaces are defined in terms of relative elevation, form and shape, or agency of formation, or in terms not involving soil criteria. Hence, it cannot be assumed that the definition of surfaces by either geologist or geomorphologist will bear any constant relationship to those defined by the pedologist.” He then proposed the term “groundsurface,” which was defined as “all those erosional and depositional surfaces and layers which have developed in a landscape during one interval of time and upon which a unit mantle of soils has developed.” Butler proposed the term “K cycle” to include the time interval from formation of the surface to its destruction by erosion or by deposition of other material. Detailed soil-geomorphic work has been done elsewhere using standard stratigraphic and geomorphic nomenclature (see Ruhe et al., 1966; Daniels and Jordan, 1966). If the geomorphic work is detailed and accurate, then the surfaces of interest to soil scientists and geomorphologists are the same. The authors believe that soil scientists should use the standard nomenclature of geologists so that all concerned with the subject can understand each other. IV.

Examples of Soil-Geomorphic Relations

There are numerous published soil-geomorphic studies. In this section we shall discuss some of these studies to illustrate the range and scope of the work. A. A HILLSLOPE STUDYI N UNIFORM MATERIAL Loess normally is a uniform material, and it should be ideally suited for studying the influence of various factors on soil formation. A cross section of an interfluve summit and adjacent valley slopes in Harrison County, Iowa (Daniels and Jordan, 1966) is shown in Fig. 6A. The material is Tazewell loess, and the sample sites of soils with slopes from I to 20% are shown. The traverse is from WNW to ENE. As would be expected from the study of Norton and Smith ( 1 930), some soil properties appear to be related to slope gradient. In the Iowa example, solum thickness and depth to carbonate decrease as slope gradient increases (Fig. 6B). There was no apparent difference in soil properties by aspect. With this much information available, a possible conclusion would be that the changes in soil properties shown in Fig. 6B are a true reflection of the effect of slope. The most probable explanation of this conclusion would

GEOMORPHOLOGY A N D SOIL MORPHOLOGY A N D GENESIS

63

be that steeper slopes result in less infiltration and more runoff, thus giving a drier site and less soil development per unit of time. A WNW

Lo

ENF

MI

M 5M \ 2 A

-60

\,

,

I 400

0

M4

I

800 Ft

B Solum Thickness

p e p f h t o Carbonate

y.88 6 - 3 4 6 x

n

20 0 20 Slope Gradient, P e r c e n t

FIG. 6. Hillslope study in Iowa. (A) Location of sampled profiles. ( B ) Relation between slope gradient and selected soil properties. (Idealized from Daniels and Jordan, 1966.)

How did geomorphic work improve the interpretation? First, through regional (Ruhe, 1969) and local stratigraphic studies it was found that the loess under broad flats or gently rounded summits has an upper noncalcareous or leached zone overlying calcareous loess. The distribution of these zones is shown in Fig. 7A. Local work, not illustrated, showed that this upper leached zone has about this same thickness under gently convex summits (as in Fig. 7A) or under level interfluves. This suggests that the centers of these gently rounded summits may be a depositional loess surface, and, if this is true, soil formation could date back approximately 15,000 years. The base of the leached loess is nearly level from the ridge crest to the steeper valley slope where it thins and curves downward, nearly paralleling the valley slope. This feature could be interpreted as a draping of the leached loess over calcareous loess. If this interpretation is made, it then follows that the landscape is the same age throughout. The curving downward of the thin leached loess under the valley slopes can also be interpreted as evidence that the valley slopes truncate the weathering zones of the loess. The small amount of leached loess on these slopes would then

64

R. B. DANIELS, E. E. GAMBLE, A N D J . G. CADY

postdate the erosion of the slopes. Based upon this last interpretation, we now have probable differences in age between the sample sites on the ridge crest and valley sides, and also differences in slope gradient. But we need additional evidence to test these interpretations. A

,

Lo

0

I

400

I 800 Ft

6. Loess

1800 y r Mullenix

FIG. 7. Hillslope study in Iowa. (A) Location of sampled profiles in relation to loess weathering zones. (B) Relation of profiles to Hatcher Alluvium. (Idealized from Daniels and Jordan, 1966.)

Aerial work gives additional evidence. The slopes from M5 to M8 and M2 to M4 grade downward to the top of the Hatcher alluvium (Fig. 7B). The top of this alluvium is about 1800 years old. There is no evidence of a later deposit on the Hatcher where it stands above the Mullenix bed. The slopes that grade to the top of the Hatcher must therefore be about I800 years old. We now have a slightly different history for the soils sampled across this loess landscape than that provided by Fig. 6 alone. We have reason to suspect that profiles M3, M4, M6, M7, and M8 have formed from calcareous loess during the last 1800 years. Sites M5 and M2 were eroded 1800 years ago, but they formed partly in noncalcareous loess. Site MI may have been uneroded during the last 15,000 years; but if eroded 1800 years ago, very little material was removed because the noncalcareous loess is as thick here as under broader flats. Therefore, many

GEOMORPHOLOGY A N D SOIL MORPHOLOGY A N D GENESIS

65

of the characteristics of the soil at MI can date back to the end of loess deposition. The relations shown in Fig. 6B are the result of a combination of slope gradient, character of parent material, and length of weathering. Because climatic and vegetative changes are known to have taken place in Iowa during the last 15,000 years (Ruhe and Scholtes, 1956), weathering intensities also may have changed. If we want to evaluate the influence of slope on soil properties, only profiles M3, M4, M6, M7, and M8 could be used. To have much statistical significance, many more samples would be needed. The above study illustrates the generally complex history of even a relatively young landscape. Without knowledge of the erosional modification of the landscape, the worker would conclude that all, or many, of the differences in the soils are the effects of differences in moisture regime. But other interpretations are made when the stratigraphic and geomorphic work are considered. Maximum ages of the erosional and depositional elements of the landscape are established. Radiocarbon dating gives precise age differences, and pollen studies suggest that the climate has changed within the time span of soil formation. Thus the factors of time, slope, aspect, and parent material become controlled variables to be used in explaining the differences in soils. But there is still the further complication of climatic change that cannot be fully evaluated. A N D SOLONETZ B. AUSTRALIAN LATERITE

The Merredin area of western Australia is an excellent example of an old, deeply weathered landscape being modified by later erosion and deposition. The soils on the modern land surface and the processes currently operating are influenced by the total history of the area, not by just the most recent episodes. For example, the Merredin landscape has large areas of lateritic soils but receives only about 12 inches of rainfall annually. The detailed work discussed here was done by Bettenay ( I962), Bettenay and Hingston (1961, 1964), and Bettenay et al. ( 1 964). Part of this work was generalized later by Mulcahy ( I 967). The geomorphic evolution of the Merredin area is fairly well understood. It has not been affected by Pleistocene sea level changes because it stands above the nickpoints of erosion cycles initiated by uplift along the Darling Fault Scarp. From its position above the nickpoints, one would think that the area would be stable because the streams had not cut deep valleys. But detailed work has shown that minor erosion and deposition have modified the formerly extensive mid-to-late-Tertiary surface and weathering zones. These modifications are believed to be associated with climatic changes during the Pleistocene and Recent times.

66

R. B . DANIELS, E. E. GAMBLE, A N D J . G. CADY

During late Tertiary time, the entire area from the old plateau to the center of the shallow valleys probably had lateritic soils with thick weathering zones (Fig. 8A). Riverine activity in the valley stripped the

A.

NEW

I OLD

PLATEAU Breakaway

\

PLATEAU

Duricrust Yellow I

6.

Fic. 8. Soils and sediments in the Merredin area, Australia. (A) Relation of soils to topography and weathering zones. (Idealized from Bettenay and Hingston, 1964.) (B) Sediments to the lee of salt lakes, Horizontal hatchure is riverine deposit; vertical hatchure is colluvium. (From Bettenay, 1962.)

lateritic profile down to the pallid zone, resulting in an initial lowering of the valley floor. Contemporaneous erosion of the old plateau did not uniformly remove all of the preexisting profile, but in places left materials that later hardened to duricrust. Nearly level areas of the old plateau were deeply eroded in places which later were filled with sandy materials weathered from the laterite. These deposits are the sand plains, and they are considered to be relatively young because locally they are mixed with lake parna. Extensive colluvial deposits 4-5 feet thick overlie the pallid zone on the floors and sides of tributary valleys. The colluvium was derived from up-

GEOMORPHOLOGY A N D SOIL MORPHOLOGY A N D G E N E S I S

67

slope erosion of the duricrust, the pallid zone, or bedrock, depending on the amount of truncation. The colluvium merges or interfingers with relatively thin riverine deposits overlying the pallid zone. Extensive salt lakes or salinas occur on the valley floor. These are believed to be part of a river system that was active in Tertiary time. Uplift dismembered the rivers, and wind further modified them during postTertiary arid phases (Bettenay, 1962). Eastward, to the lee side, of the salt lakes are gypsum dunes, sand, silt or clay lunettes, and sheets of lake parna overlying riverine and colluvial deposits (Fig. 8B). Dunes of gypsum are being added to today by material crystallizing on the salt lake surface. Although the old plateau is still covered by several feet of material weathered duringTertiary time, there probably is none of the original Tertiary surface remaining (Mulcahy, 1967). The surfaces and the associated sediments are Pleistocene or later, but the effects of Tertiary weathering are still expressed in the soils and in the processes operating on that landscape. Some of the soils of the old plateau have formed in materials weathered during Tertiary, but exposed or reworked by post-Tertiary erosion. The duricrust has formed by drying of materials exposed by truncation of the laterite. Sandy deposits 15 or more feet thick formed by erosion of the laterite smooth out the irregularly eroded surface of the old plateau. These sandy soils are Yellow Earths (Fig. 8A) and have only trace amounts of Na, K, Ca, and Mg. The clay fraction is mostly kaolinite. Soils on colluvial materials and on parts of the riverine sediments are Brown Solonetz. In the B horizons, they have a clay maximum; and many have concentrations of carbonate. They have reasonable amounts of potassium. Near the old plateau, soils in preweathered materials have thin A horizons over acid mottled kaolinitic clays. Some soils in riverine and eolian sediments toward the centers of the valleys are affected by saline ground waters. On the lee sides of salt lakes are modern gypsum dunes, sandy lunettes with minimal soil development, and clayey lunettes, calcareous and saline, with no or very little soil development (Fig. 8B). In the sheets of lake parna, the soils are rich in carbonate, soluble salt, and acid-soluble potassium. Some of the potassium is from eolian alunite, but the greater part is from illite (Bettenay, 1962). Soils in the Merredin area are closely related to the weathering, erosional, and depositional history of their sites. Yet not all soil properties seem to be consistent with the stratigraphy or geomorphology. For example, the high calcium carbonate and potassium content of the soils in colluvium is not consistent with their being derived from weathered

68

R. B. DANIELS, E. E. GAMBLE, A N D J. G. CADY

granite or lateritic materials. Their topographic position makes it seem unlikely that ground water ever approached the surface and, if it did, the ground water in the pallid zone should be low in calcium and potassium. Most likely, small additions of parna to the colluvium are responsible for the high carbonate and potassium content (Bettenay and Hingston, 1964). Although these additions may explain the potassium and calcium content of soils in colluvium, it does not explain the original source of these elements because the plateau and the valley are underlain by the Tertiary pallid zone that is very low in alkali and alkaline earth metals. More detailed work by Bettenay et al. (1964) gave considerable additional information on the source, distribution, and relation of alkali and alkaline earth metals to the past history of the area. A cross section in the Belka Valley, part of the Merredin area, is shown in Fig. 9. The

FIG.9. Water movement in the Belka Valley, Western Australia. (From Bettenay et al., 1964.)

aquifer is the pallid zone resulting from Tertiary weathering of granite and gneiss. Overlying the aquifer in most places is a dense clayey aquiclude, which has above it sandy bodies and fine textured soils. Saline patches developed at seep spots in sandy materials after the area was cleared of native vegetation. The saline areas near the salt pans have been growing in size. These saline areas are the result of decreased transpiration and some increase in water entering the aquifer. Runoff has increased since clearing, but the runoff waters do not become saline until they flow across saline soils. The stream systems flow periodically, and the salinity values of the flow drops to less than 100 me/l if flow persists for some weeks (Hingston, cited by Mulcahy, 1967). The salt deposits in

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69

the lake areas therefore probably represent only a few years’ accumulation, and the net result of runoff is ultimately to remove salts from the system by way of the major drainage (Bettenay et al., 1964). Water analyses and water-table data from wells spaced across the landscape were used to interpret the origin of the salt (Fig. 9). From the intake area to the outlet, the salinity of the ground water increased in a regular manner from about 100 to about 800 me Cl-/l (Fig. 1 OA). The salts could not be concentrated by downward movement from soils because the aquiclude is relatively impermeable and the underlying aquifer is under pressure. I t is reasonable to assitme t h a t most of this concentration of salts is not from the aquifer because it is highly weathered material, low in salts. Bettenay et al. believe that the increase in salts is due to loss of water, ultimately as vapor, from the aquifer upward through the aquiclude by the combination of positive aquifer pressure and capillary action (Fig. 9). Toward the center of the valley the aquifer comes closer to the surface, and the whole soil is within the saturation zone of the now highly saline waters. Although upward movement is slow, it is by this process that salts accumulate. Eolian activity can then redistribute these salts across affected parts of the landscape. Bettenay et al. found that although the total salt content of the ground water increased from the intake to the outlet, the proportion of Na, K, Ca, Mg, and SO, decreased relative to chloride (Fig. IOA). This decrease in alkali and alkaline earths is associated with a decrease in pH from neutrality to less than 4 and an increase in aluminum and iron in solution (Fig. 10B). Apparently there is an exchange between the acid (aluminum) clays of the aquifer and the highly saline ground waters. Local rocks are either highly weathered or they are fresh granite. In either case they are not a good source for the salts in the area. Rainwater analyses indicate that the major source of salts in the Belka Valley is atmospheric accession of ocean salts and local or regional terrestrial sources. These salts enter the system in the rainwater, and they are concentrated locally by the mechanisms outlined above. Interpretations about the distribution of soils on the landscape and their physical and chemical properties are based largely on what is known about the stratigraphic and geomorphic history. Other factors such as salt content, its source, and how it gets to a specific part of the landscape are geochemical problems, but there is no clean-cut separation from soil chemistry. Interpretations of chemical data are modified by and partly depend upon stratigraphic and geomorphic data. Yet the stratigraphic and geomorphic data by themselves tell only part of the history of the area that is of interest to the soil scientist because they do not explain the source of salts nor their mode of accumulation.

70

E; K&

R. B. DANIELS,

E.

E. GAMBLE, A N D J . G. CADY

soor

A

/

-8

E400 \ E400 _I a, \ _I

E

Mg

Ca

400 Salinity ( M e CI-/Ltler)

0

Inlet

800

Outlet

160

r

B.

Aprox

I

E

Q

LW L

0

400

800

Salinity ( M e CI-/Liler)

Inlet

f

Outlet

FIG. 10. Changes in chemical composition of ground water, Belka Valley, Western Australia. (A) Relation between me of Na, Mg, SO,, Ca, and K per liter and salinity. (B) Relation between pH and salinity and Fe and Al concentration and salinity. (From Bettenay el al., 1964.)

c.

NEW MEXICODESERT-ARGILLIC A N D CALCIUM CARBONATE HORIZONSI N DESERTSOILS 1 . Geologic Setting

Ruhe, Gile, Hawley, and others have studied the land forms and soils near Las Cruces, New Mexico, for the purpose of “determining their origin within the physiographic history of the region” (Ruhe, 1967). The vegetation of the area is desert grassland (Humphrey, 1958). The annual precipitation at Las Cruces is 8-9 inches, but it increases to about 15 inches in the mountains (Ruhe, 1967; Gile et al., 1970). Most of the precipitation falls during the hot summer months: July has a mean temperature of 80°F (E. L. Hardy, 1941). The climate of the Las Cruces area is roughly similar to the Merredin area studied by Bettenay et al. (1964), but the geologic histories and soils are different. The Australian area is modified, highly weathered Tertiary peneplain, whereas the Las Cruces area is a tectonically controlled river

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71

valley and an internally drained basin area that dates its oldest surfaces and weathering from mid-Pleistocene (Ruhe, 1967). The stratigraphic-geomorphic history has been discussed by Ruhe ( 1962, 1964, 1967), Hawley ( 1963, Hawley and Kottlowski ( 1969), Hawley et al. (1 969), and Gile et al. ( 1970). The area in mid-Pleistocene time was composed of the internally drained Jornada del Mureto and the Mesilla Basins. Each basin had two major components: piedmont slopes and basin floors. The piedmont slopes include erosional surfaces and constructional alluvial-fan and coalescent-fan surfaces, and the basin floors include alluvial flats and playa-lake plains. Cutting of the present Rio Grande Valley system began in late mid-Pleistocene time. Development of graded valley-border surfaces related to episodes of valley-floor stability started with the emplacement of the Rio Grande. There was erosion and sedimentation in the internally drained basins and fan piedmont areas (Table I) at the same time the valley-border surfaces developed. This produced landscape areas that have been relatively stable for periods ranging from several thousands of years to several hundreds of thousands of years. But in contrast to the often highly weathered materials of the Australian area, the soils in southwestern New Mexico formed in relatively fresh unconsolidated debris derived from rhyolite, tuff, andesite, monzonite, limestone, shales, and sandstones (Ruhe, 1967; Gile and Grossman, 1968). The surficial basin-fill deposits in many areas consist of sediments derived from one rock type, and this provides excellent parent material control for soil studies. The climate apparently has had alternating episodes of moist and dry conditions from mid-Pleistocene to the present, although the record is sketchy. Ruhe (1967) accepted local and regional pollen studies and hydrologic reconstruction of pluvial lakes north of the area as evidence of a wetter environment more than 9500 years ago. Gile et al. (1970) cite faunal evidence for a significant lowering of life zones believed to be associated with past glaciation. R u h e ( 1962) believed that in the Las Cruces area the past pluvial environments should have resulted in greater vegetative cover, landscape stability, and soil formation. The interpluvial environments, as the present one, resulted in weaker vegetative cover and, in many areas, unstable landscapes and severe erosion. Reworking by wind and at least small additions of dust to old surfaces is a possibility, if past interpluvials were as dry as or drier than the present climate (see Gile, 1966a). The evidence cited by Ruhe ( 1967) and Gile et al. ( 1970) indicates that climate has not been stable from mid-Pleistocene to present. Neither the magnitude of these fluctuations nor their duration and distribution in time can be given detailed treatment. This part of the history of the area

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R. B. DANIELS, E. E. GAMBLE, A N D J . G. CADY

TABLE I Geomorphic Surfaces and Soils in the Las Cruces Area" Major soils

Surface

Age and elevation ancestral Rio Grande above present flood-plain levels

Alluvial fan, piedmont, and apron Organ 1 100-4600 years

Issaks' Ranch Jornada I1

6400 years-latest Pleistocene Late Pleistocene

Jornada I

Mid-Pleistocene

Dona Ana

Mid-Pleistocene

Basins and scarplet surfaces Holocene and late Pleistocene Lake Tank Petts Tank La Mesa

Late Pleistocene M id-Pleistocene

Valley-border erosion surfaces Fort Seldon Group 1000-2600 years (43 ft) Fillmore Leasburg

7300 years-latest Pleistocene (65 ft)

Picacho

Late Pleistocene (70-90 ft)

Tortugas

Mid-late Pleisotcene (115-15Oft)

"From Ruhe (1967) and Gile er al. (1970).

Low carbonate parent materials

High carbonate parent materials

Haplustolls, Entisols, Camborthids, weak Haplargids Haplargids

Haplustolls, Entisols, weak Calciorthids

Haplargids, Paleargids, Calciorthids, Paleorthids Calciorthids, Paleorthids, Haplargids, Paleargids Paleargids, Calciustolls, Paleort hids

Calciorthids, Paleorthids

Torrerts, Haplargids Paleargids, Paleorthids, Haplargids

Entisols, Cam borthids Entisols. Camborthids, Haplargids, Calciorthids Haplargids, Paleargids, Calciorthids, Paleorthids -

Calciorthids -

Entisols Calciorthids

Calciorthids, Paleorthids

Calciorthids, Paleorthids

GEOMORPHOLOGY A N D SOIL MORPHOLOGY A N D GENESIS

73

would be valuable in soil genesis studies where one is concerned with rates of soil formation. Regardless of the magnitude of climatic fluctuation, apparently at no time since t h e Rio Grande Valley established its present course has the water table been close enough to the surface to influence the soils (Gile et al., 1970). This may be one of the major reasons why few saline soils occur in the area even though an abundance of sodium is in the parent rocks. 2 . Argillic Horizons Argillic horizons are of great interest to soil scientists because they are used as indicators of profile development. Horizons in the B position in the Las Cruces area range from raw alluvium to well expressed argillic horizons on the older surfaces (Gile, 1966b, 1970; Gile and Grossman, 1968; Gile ef al., 1970). Soils on historical (formed after the area was settled) surfaces show little or no evidence of soil formation. Soils on post Pleistocene-prehistoric surfaces have cambic horizons, and those on Pleistocene surfaces have argillic horizons. If the clay content is the same, argillic horizons are redder in soils on the older surfaces. Exceptions to these generalizations are found where soil fauna have destroyed these horizons and where they have been engulfed by carbonate (Gile et al., 1969). The morphology of the argillic horizon varies somewhat with texture. Many of the buried Pleistocene soils have prominent clay skins. But soils under the present surface with medium to fine textures do not have clay skins on ped faces although they are found in those parts of the argillic horizons that extend down pipes in the carbonate horizon. Evidence for clay illuviation in soils under the present surface is found in remnants of clay skins inside peds and in preferred orientation of clay around sand grains and pebbles. Apparently processes operating since late Pleistocene favored disruption of, or have not furnished stable sites for, clay skins. Gile and Grossman ( 1968) believe that the processes leading to clay skin disruption are carbonate accumulation, disruption by roots and fauna where the argillic horizon is close to the surface, and high-energy moisture changes involving montmorillonitic clay. Gile ( 1970) believes that silicate clay is currently accumulating in some soils. His belief is based on the fact that noncalcareous B horizons within the zone of wetting show evidence of clay illuviation. Gile et al. (1970) also show that 1339% of the dust presently falling in the Las Cruces area (0.1 kg/m2/yr) is clay-sized material. Thus there is considerable potential for adding clay-sized material to the surface of these soils. Given sufficient time, argillic horizons apparently could form even under present conditions. Prominent argillic horizons in soils in the Las Cruces area are thought

74

R. B . DANIELS, E. E. GAMBLE, A N D J . G . CADY

to have formed primarily during Pleistocene pluvials when more water was available for downward translocation of materials (Ruhe, 1967; Gile 1966b, 1970; Gile and Hawley, 1966; Gile and Grossman, 1968). The lower parts of these prominent argillic horizons are thought to be largely relict, but silicate clay presently is accumulating in their upper parts. The present apparently represents a period of low-intensity clay illuviation rather than a complete cessation of the process.

3. Calcium Carbonate Horizons Some of the more interesting work in the Las Cruces project has been done on calcium carbonate (lime)- its morphology, accumulation, and genesis. Gile et al. (1966) developed the concept of “stage of carbonate (lime) accumulation” that is applicable over wide areas (Fig. 11). The I

II

FIG. 1 I . Stages of calcium carbonate accumulation in gravelly soils near Las Cruces, New Mexico. The solid black areas are calcium carbonate. (From Gile et al., 1966.)

morphology resulting from lime accumulation in nongravelly soils is different from that in gravelly soils during the early stages, but later on they resemble each other. The various stages in both types of soils reflect an increase in the amount of lime accumulation and concurrent changes in morphology. In gravelly soils, the skeletal grains are coated with lime in the latter part of stage 111, and most of the pebble interstices are filled or plugged. At this stage, infiltration is markedly reduced and, with continued additions of lime, the horizon developes into stage IV. In stage IV, horizontal nearly pure calcium carbonate laminae overlie a lime plugged horizon. The laminar horizons, the tops of which look like poorly trowled concrete, vary considerably in thickness, and some show multiple layering. This laminar horizon has very low infiltration rates, high unconfined compressive strength, and high bulk density (Gile, 1961). It also is a barrier to roots (Gile e f al., 1966), and it physically forces the overlying material upward as the calcite crystallizes. Gile et al. believe that a plugged horizon (late stage 111) must develop

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75

before the laminar horizon can form. The numerous laminae suggest that the accumulation of the laminar horizon is a long-term process, and radiocarbon dates show that the laminae become younger upward (Gile et al., 1966). On the other hand, radiocarbon dates from a thick laminar horizon under the La Mesa surface suggest that some of these horizons can form quickly (Gile et al., 1970). Soils on all geomorphic surfaces except part of Lake Tank (Table I ) have some lime accumulation regardless of the rock type involved (Ruhe, 1967). Stage I is found in soils on the post-Pleistocene Fillmore surface and stage I 1 in soils on the Leasburg surface (> 7300 years to latest Pleistocene). Stages 111 and IV are found in soils on the late-Pleistocene Picacho and older surfaces (Gile et al., 1966). All sediments under old surfaces have thick horizons of lime accumulation. Ruhe ( I 967) believes that the lime in monzonitic and limestone sediments could be derived from the sediments themselves. But in rhyolitic gravels, these lime accumulations would require the decomposition of thick layers of the gravel. There is no evidence of decomposition of rhyolitic gravel. Therefore, other sources for the calcium must be sought where the sediments are dominated by rhyolite. Analyses of clay on a carbonate-free basis indicate that many calcic horizons also are zones of maximum clay (Gile, 1970; Gile et al., 1966; Ruhe, 1967). I t is suggested that the clay had to accumulate before being sealed and plugged by calcium carbonate because the permeability of these laminar or plugged horizons is almost nil. The above data is evidence that calcium carbonate engulfed part of former Bt horizons. Organic carbon sealed in a laminar calcium carbonate horizon from under the Picacho surface is 9550 years old. The underlying argillic horizon can be no younger than 9550 years because the overlying laminar calcium carbonate prevented further additions of clay. The last Wisconsin glacial period ended about 10,000 years ago. Ruhe suggests that the argillic horizon sealed under the laminar horizon is related to a wetter (pluvial) episode; and the lime deposition to a later drier (post pluvial) episode. Gile (1970) generally agrees with the time of argillic horizon engulfment, but he thinks that most of the lime of Recent age would have been emplaced in the lower part of the argillic horizon now developing above the laminar horizon rather than in the laminar horizon. Later radiocarbon data on organic carbon from laminar calcium carbonate horizons (stage IV, Fig. I 1) also suggest that the occluded organic matter is from pluvial periods. These pluvial periods were about 12,000 and 20,000 years ago. Dust traps in the study area collected about 0.1 kg/m2 per year (Gile et al., 1970), or about 1 cm of dust every 160 years. The dust added

76

R. B. DANIELS, E. E. GAMBLE, A N D J . G . CADY

about 0.2-0.4 g of CaCOa/m2/yr.Calcium is also brought in by precipitation, and Gile et al. estimated that about 2 g of CaCOJmz/yr is contributed to the soils from all atmospheric sources. They conclude that the major source of authigenic calcium carbonate is not from dry dust but from precipitation. Gile (1970) and Gile et al. ( I 966, 1970) generally accept the eolian origin for much of the calcium carbonate found in these soils. On the other hand, Ruhe ( 1 967) believes that pedogenic processes, eolian sources, groundwater and surface-water deposition may all have had a part in formation of a lime horizon in the Las Cruces area. Ruhe felt that it is not possible to separate the amount contributed by various processes. Later work shows that ground water probably has not been involved in lime deposition because the water table has always been 100 feet or more below the major surfaces (Gile et al., 1970). Some of the very intriguing questions raised by the Las Cruces work are: ( I ) What was the variation in calcium addition during pluvial periods? (2) What was the source area if dust was a transport vehicle during these pluvial periods? and (3) What would be the effect of pluvial periods on calcium carbonate morphology and placement in the profile? This work in the Las Cruces area again emphasizes that there are strong relations between geomorphic surfaces and soils. But it also shows that soils weathering for any length of time may have complex developmental histories. Evidence has been presented that pedogenic processes have either been discontinuous throughout time, or there has been considerable variation in intensity of process. This work shows that soil scientists should be cautious about conjectures regarding soil genesis based largely upon soil-profile characteristics. D. NORTHCAROLINA COASTALPLAIN 1 . Surficial Geology

The upper Coastal Plain near Raleigh, North Carolina, lies above an altitude of 275 feet. The upper Coastal Plain is a deeply dissected area with gently convex interfluves and a few narrow flats. These flats are remnants of pre-middle Coastal Plain depositional and erosional surfaces (Daniels et al., 1966),but they make up less than 10% of the area (Gamble et al., 1970). Average slope toward the ocean is 3-4 feet per mile across the remnants, but local slope gradients are near 1 foot per 100 feet (Daniels et al., 1970a). The Coats scarp,with a toe altitude of about 275 feet, marks the eastward limit of the upper Coastal Plain. The middle Coastal Plain.occurs as three stepped surfaces (Fig. 12) between the Coats and Surry scarps and has large areas of nearly level surface remaining. Regional slope to the east is 2-4 feet per mile and local slopes

GEOMORPHOLOGY A N D SOIL MORPHOLOGY A N D GENESIS

77

average less than 0.5 foot per 100 feet. These middle Coastal Plain surfaces are not absolutely flat but have a gentle swell and swale relief. UPPER COASTAL PLAIN

I

MIDDLE COASTAL P L A I N

FIG. 12. Idealized section of the Coastal Plain from west of the Coats Scarp to the Surry Scarp in North Carolina.

In the surficial sediments, quartz usually dominates the sand fraction, with minor amounts (< 10%) of feldspar and resistant heavy minerals. The clay fraction is largely kaolinite with less than 2 % mica. Textures range from clay to sand. The usual textural sequence is loamy sand to sandy loam in the lower beds grading upward into heavy sandy loam to sandy clay sediments at the surface. These sediments are 20-40 feet thick, and they usually overlie less permeable, more clayey, older formations. The surficial sediments are relatively thin compared to divide width in the middle Coastal Plain. This characteristic allows the lower beds to be saturated by water perched above the less permeable layer. The stream system usually trenches the surficial sediments and, in the upper Coastal Plain, cuts deeply into the underlying formations. This trenching allows the dissected edges of the surface to have deep water tables, but it may have little effect on water tables in the centers of broad divides such as those found in some areas of the middle Coastal Plain. The upper Coastal Plain surficial sediments are fluvial late Miocene to Pliocene deposits (Daniels et al., 1966; Daniels and Gamble, 1971). The middle Coastal Plain surficial sediments are fluvial Pliocene to possibly early Pleistocene deposits that may have some marine beds. The depositional and erosional surfaces in the upper Coastal Plain are late Miocene to Pliocene and have been weathering for about 10 million years. Surfaces in the middle Coastal Plain are Pliocene to possibly early Pleistocene age. The surfaces and the soils under them have been weathering

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R. B. DANIELS, E. E. GAMBLE, A N D J . G. CADY

from 2 or 3 to about 8 million years. This is a long time for surfaces to exist without modification (Mulcahy, 1961), but a recent study (Daniels et al., I97 I ) indicates that they are stable. Part of the surface stability is related to the vegetational and climatic history. There seems to be no evidence that this area has had an arid or semiarid climate since the Miocene. Most evidence suggests a Pliocene climate similar to that present (Isphording, 1970). The climatic stability suggests that the vegetative cover has been nearly the same throughout the history of these geomorphic surfaces. Vegetation shifted to more northern types during the Pleistocene (Whitehead, 1963, 1964, 1969; Frey, 1953), but it was a matter of degree, because many pollen diagrams show mixtures of northern and southern species. The present climate in the Coastal Plain has warm summers and mild winters. The average July temperature is about 80°F and the January temperature about 45°F (A. V. Hardy et al., 1967). Precipitation is about 48 inches per year. 2 . Water Regimes The surficial deposits in the middle Coastal Plain are thin and the underlying formations are clayey and have low permeability. On wide divides this means that the water must move laterally above the underlying formation for long distances before the soils can drain after wet seasons. The upper Coastal Plain, on the other hand, is deeply dissected and the remnants of the original flats are narrow. The water table stands in the solum for only short periods because of this remnantal characteristic. Although the streams cut through the middle Coastal Plain surificial sediments, only the edges of the large flats are well drained. Water-table data from the Coharie surface near Newton Grove, North Carolina, allows us to partially reconstruct past water regimes. When the surfaces were formed, and before the stream system incised, the water table probably stood at or near the surface much of the year. The extent of the flats at that time is unknown, but it must have been miles between drainage systems. After the streams incised, the depth to the water table would be partially controlled by depth of incision and by the distance from the edge of the surface. Figure 13 shows the yearly water table regime of the Coharie surface at Newton Grove, North Carolina. All sites used are on the swells of the surface, and the stream system has cut through the surficial sediments so that additional stream incision without narrowing the divides will have little effect on water table levels. Sites farther than 0.37 mile from the surface edge are extrapolations of actual data obtained from sites closer to the edge (Daniels et al., 1 9 7 0 ~ ) . If the divide is 2 miles wide, the water table in the center of the divide

79

GEOMORPHOLOGY A N D SOIL MORPHOLOGY A N D GENESIS

is within 20 inches of the surface about half the time, and within 5 inches about 25% of the time. If the divide is as narrow as 0.2 mile, the water table drops so that the isopleth for 50% saturation (half the time) is about A

---__

-----___ ----------_

_----

0

I

l

01

02

l

,

1

1

06

04

1

,

I

08 Divide

B

/’

0

I

10

I

I

12

I

,

14

I

,

16

I

,

18

I

2.0

Width.Miles

C

,.-.,

01 03 Divide Width, Miies

05 Divide W i d t h , M i l e s

Fic. 13. Water table levels at Newton Grove, North Carolina: (A) 2-mile wide divide; (B) 0.5-mile wide divide; (C) 0.2-mile wide divide. The 25%. SO%, and 75% lines are saturation isopleths, or the level at which the soil is saturated 25, SO, or 75% of the time.

55 inches and the isopleth for 25% saturation is about 36 inches below the surface. The greatest change in water-table depth occurs when the divide is less than about 0.4 mile wide (Fig. 13). The change in water table per unit of divide width is small if the divides are from 2 to 0.4 mile wide. There is a close relation between soil morphology and water table levels. At Newton Grove, North Carolina, the B horizon at the dissected edge of the surface is a yellowish red (SYR 5/6) sandy clay (Daniels and Gamble, 1967). Soils with this color and texture form a discontinuous band about 0.05 mile wide around the edge of the surface and the water table seldom rises above 60 inches. Away from the yellowish red edge is a zone of soils with strong brown B horizons about 0.02 mile wide that,

80

R. B. DANIELS, E. E. GAMBLE, A N D J . G. CADY

farther from the edge, is replaced by soils with yellowish brown B horizons. Several other soil properties have a progressive variation as the depth to the water table decreases across this sequence of soils. A2 horizons thin and their clay content increases (Fig. 14), and the clay and gibbsite

,A 2

Thickness

8 2 Clay C o n t e n t

‘A2

Clay Content

Lo

Gibbsite ,Content

--

Be H o r i z o n Thickness

20 60 I n c r e a s i n g Depth t o W a t e r Toble

FIG. 14. Generalized relations between selected soil properties and water table depth.

content of the B horizons decrease. The thickness of horizons with Be bodies (Daniels et al., 1968) increases to a maximum and then decreases. Only generalizations are given here, but these sequential changes have been discussed elsewhere (Daniels and Gamble, 1967; Daniels et al., 1967, 1968, 1970b, 1970~).It must be emphasized that these ordered patterns of change are on one geomorphic surface and in one nearly uniform parent material. The only differences that can be found are in the distance of the soil from the dissected edge of the surface, and the accompanying changes in the water table regime (Fig. 13). These morphological changes associated with the dry dissected edge of the surface are repeated on older and younger surfaces, and they are the rule rather than the exception. The B horizons on the dry edge are not always yellowish red, but they are brighter and may have redder hues than horizons away from the edge. This close connection between soil morphology, physical, and chemical properties, and water table levels seems to be a cause and effect relation because it is repeated at similar topographic positions over a wide area of the North Carolina Coastal Plain. In the middle Coastal Plain, dissection of the landscape determines the water table level. These soil-

GEOMORPHOLOGY A N D SOIL MORPHOLOGY A N D GENESIS

81

landscape associations are an example of how an external factor such as landscape dissection can be a control in soil morphology and genesis. There are several inferences about soil development that can be made from the relations between soil morphology and water-table levels. The first is that it would be possible for a soil on a remnant of a once broad divide to have had a water regime sometime in its history that was similar to that shown for each point in Fig. 13A. We do not imply that the bands of soils like those associated with the present dry edge will march across the landscape to the divide center. The water table regime associated with each soil will move across the landscape as dissection continues, but there may not be time or materials available to produce the soil patterns that we see now. If a broad divide could be narrowed at a uniform rate, the greatest change in water table, and possibly soil morphology, per unit of time would be where the divide has become less than 0.4 mile. This is where the water table starts to drop sharply. This suggests that when soils have been exposed to soil formation for long periods (2-6 million years in our example) their age is much less important than the internal environments. I n other words, soil formation is not necessarily a linear function of time (Daniels et al., 1970b). These relations also illustrate how soils on old stable surfaces may change when their environment changes. Processes, such as A2 horizon formation, may occur late in the history of the site in the center of the divide, yet the same process may have been active much earlier at other locations on the same surface. Thus it is necessary to know much more than how long a soil has been forming, because this information sets the time limits only. What we need to know is what has happened within these time limits.

3 . Soils with Plinthite Certain soils in the middle Coastal Plain and most of the soils in the upper Coastal Plain have a subsurface horizon that is strikingly mottled red, yellow, and gray. The red parts are iron concentrations. The pattern is reticulate with a horizontal orientation. Part or all of the red material will become hard if exposed to several cycles of wetting and drying. Other red iron concentrations with the same general appearance will not harden. The material that will harden is called plinthite, and the term will be used here to distinguish the material that will harden from the similar-appearing soft mottles that may be an incipient or earlier stage of the same process (Soil Survey Staff, 1960). Near Newton Grove, North Carolina, soils with less than 5 % plinthite make u p about 30% of the Coharie surface. The plinthite occurs 45 to 60 inches below the surface. I n the centers of the divide, these horizons are saturated with water 50-75% of the time (Fig. 13). The red mottles in

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R. B. DANIELS, E. E. GAMBLE, A N D J . G . CADY

these soils occupy 15-25% of the horizon, but only 3-5% of the horizon hardens upon exposure. This is incipient plinthite. Horizons with plinthite underlie about 5 5 % of the somewhat older Brandywine surface near Benson, North Carolina. Red mottles occupy about 25-30% of the horizon, and those that will harden make up about 15% of the total horizon. On the Brandywine surface, plinthite occurs in well and moderately well drained soils over large areas but is absent in some well drained soils near the dissected edge of the surface. Water perches for considerable periods above the horizontal platy plinthitic horizon. The true water table is below the plinthite horizon most of the year, as would be expected for a divide position less than 0.4 mile wide (Fig. 13). The divides in the Plain View surface of the upper Coastal Plain are about 0.2 mile wide, and plinthite makes up 25-30% of the plinthic horizons. Almost all red mottles harden upon exposure, and plinthite is under all of the Plain View surface in our study area. The true water table is always below 10 feet, yet water is held up in the plinthic horizon between 5 and 10 feet for several months each year. This perching is more a slowing down of water in the iron-rich zones than absolute impermeability as shown by the fact that all wells in plinthite will dry out during low rainfall periods. The distribution of soils with plinthite and the water table regimes associated with them at Newton Grove suggest that moderately high but fluctuating water tables are necessary to start plinthite formation. We believe that plinthite starts to form when a horizon is saturated 50-75 % of the time. During the initial stages of formation, this horizon does not perch water. When plinthite occupies about 10% of a horizon it perches water and keeps the horizon saturated for considerable periods. The plinthite horizon on the Brandywine surface has a water regime similar to that in the incipient horizon at Newton Grove, although the true water table is below 5 feet most of the year. Water regime of the soils on the Plain View surface is similar to that on the Brandywine except the saturation period of the plinthitic horizon is less. Plinthite occurs only on the local swells or high parts of the landscape on the Coharie and Brandywine surfaces. It fades and disappears as the soils become somewhat poorly drained. Thus, initial plinthite formation is a process in soils with somewhat restricted drainage. Its occurrence on the high parts of the landscape on slopes of 2-3 feet per mile almost precludes much lateral transfer of iron. If lateral transfer was the mechanism involved, then plinthite should occur in the low parts, not the high parts, of the landscape. Also if horizontal movement were active one should find a concentration gradient from weak to strong in the flow direction. None is found. Apparently, the concentration of iron is from within the

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83

pedon and not from outside sources. But more work needs to be done to find the source of iron. Is it brought from below as well as above, or is it segregated within the horizon? Plinthite occurs under a large part of the Brandywine surface except in the poorly drained soils. But it underlies all the higher older Plain View surface. The distribution of plinthite raises the question of whether or not it formed in former poorly drained soils on the Plain View surface after dissection lowered the water table and their drainage improved. For a poorly drained soil to develop into a well drained soil with plinthite would require considerable redistribution and possibly additions of iron. But our studies suggest that this does not happen because the configuration of the landscape and the mode of dissection make the preservation of poorly drained soils the exception rather than the rule. The gentle swell and swale landscape in the middle Coastal Plain is in reality a miniature drainage system with the swells forming the drainage divide and the swales forming the valleys and internally drained basins. After heavy rains, such as those accompanying hurricanes, runoff collects in the lows or swales. If the water is deep enough, it slowly moves from outlet to outlet until it connects with a drainage head. These drainage heads always end at or near a swale area of the undissected surface, suggesting that stream trenching and headward cutting is along the swales, or where the water collects. Thus, the poorly drained soils would be removed during landscape dissection, and the local swells or microdrainage divides would be the last to erode. Therefore, it is logical to assume that the soils with plinthite on the Plain View surface probably have been on the better drained parts of the landscape ever since plinthite formation started. They may never have been poorly drained. The increasing amount of plinthite from the Coharie to the Plain View surface is almost a textbook example of a soil geomorphic surface relation, i.e., the more strongly expressed soil properties occur in older surfaces. One interpretation of this type of data is that it is an age relation, or with increasing time there is increasing profile differentiation. We believe this interpretation is only partly true for several reasons. In the relation between plinthite and surfaces, one cannot assume a linear development of plinthite with time. I t does not necessarily follow that 10 times more plinthite will occur in a profile that has been weathering 10 times longer than another. Profiles on the Brandywine and Plain View surfaces have about the same amount of red mottling in the plinthitic horizon (20-30% of the mass), but they differ largely in the amount or percentage of mottles that will harden. On the Brandywine, about half the red mottles harden whereas all of them harden on the Plain View surface. The question is: when, in the development of plinthite, did all the

84

R. B. DANIELS, E. E. GAMBLE, A N D J . G . CADY

mottles on the Plain View surface become capable of hardening? Was it 10,000 years ago or 1,000,000 years ago? Profiles on the Plain View surface probably have reached the maximum degree of plinthite development possible under parent material and other controlling conditions. Additional time of weathering therefore will not change the degree of plinthite expression in these soils unless internal environments change so that plinthite can be destroyed. The increase in percentage of plinthitic soils from the younger to the older surfaces should not be interpreted as evidence of a constantly increasing area of soils with plinthite. There may be some increase in these soils during the initial stages of plinthite formation, from the Coharie to the Brandywine surface, for example. Plinthite is present under nearly all remnants of the Plain View surface, but its total area may be less because erosion may have removed plinthic as well as nonplinthic soils. Even though there is a relation between plinthite and surface (and it is partly a time function), one may not assume an age relation between a soil process and the geomorphic surface (Daniels et al., 1970b). The infrequent occurrence of plinthite on the younger Sunderland surface (Fig. 12) and its sporadic distribution on the Coharie are interpreted as evidence that plinthite formation did not start when the surface was first exposed to subaerial weathering. The Sunderland and Coharie surfaces may be 2 to 6 million years old, yet in that time they have developed essentially no plinthite, whereas the older Brandywine and Plain View surfaces have large amounts. Thus environmental conditions, some created by geomorphic processes, such as stream dissection, are more important in determining soil properties than a direct age sequence. V.

Summary

The studies reviewed represent a variety of relations between age and type of geomorphic surface and soils. The studies all show that conclusions about soil genesis based on conventional analysis and conventional consideration of present soil-forming factors usually are improved when geomorphic and stratigraphic information is added. It is a common practice to interpret a property as always indicating the same combination of factors. But in reality, a soil or other natural object can develop its features by a number of different possible routes. Knowledge about the history of a site, its relation to the surrounding landscape and especially the duration of its existence and exposure to various environments help clarify our understanding of the object we now see. Geomorphologists and pedologists have an overlapping interest in the same object. One studies the origin of the shape of the surface of the

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85

earth, the other studies the collection of natural bodies, soils, just beneath that surface. The geomorphic history of a site is also the history of the soil in that place. In addition to the shape of the land and its history, the origin of the material, past climates, water table regimes, outside additions such as dust and cyclic salt are also part of the geomorphic record. The pedologist studies soils in the field and laboratory to understand how their characteristics came to be and also to determine the importance of the characteristics - whether they are durable or transient. If he knows the history of the landscape in which his objects occur, in the way shown by the studies discussed here, he is in a position to interpret and decipher many things that might seem either too easily obvious or strange and anamalous. Since pedology is primarily a historical science, not an experimental one, solutions to its problems depend on having as much knowledge as possible about the natural variables that contribute to the resulting soil. The experiment has been run; we must interpret the results. The geomorphic data on process of development of the surface, its age and past climate and vegetation regimes provide far more control than is available in the properties of the soil itself and eliminates assumptions about the meaning of the direct observations. The studies discussed here are geomorphic in the main, but other fields of the earth and biological sciences are used. The contribution of geomorphology can and should be much more than a listing of the ages of surfaces and sediments (Wooldridge, 1949) because this is only the first step. If the concept that soil is the product of the interaction of the classical factors, these factors could well combine in different degrees to produce the same result on surfaces of different ages. One must avoid the trap involved in the easily made assumption that soils on old surfaces have passed through all the stages illustrated by what now exists on adjacent younger surfaces. This may be true, but it implies a continuity of factors in space and time-a very difficult hypothesis to prove. Frequently the assumption is made that rates of soil formation can be estimated by plotting some parameter as a function of time. This assumption may work in some areas where climate and other factors have been somewhat uniform across a sequence of surfaces. But most areas have had episodic fluctuations of soil-forming factors. The New Mexico and the Merredin, Australia, areas are examples. Thus the intensity of soil formation may vary greatly with time on one surface. The factor producing variations in intensity may be climatic as suggested for the New Mexico area, or it may be related to internal changes of soil environment, such as a drop in the water table, as suggested by work in North Carolina (Daniels et a/., 1970~). Geomorphic work is of maximum utility to the pedologist when it

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R. B. DANIELS, E. E. GAMBLE, A N D J . G. CADY

is used to develop a site history. Probably of greater importance, however, is that the pedologist may start asking new questions about morphology and genesis when such additional background material is available. REFERENCES

Balster, C. A,, and Parsons, R. B. 1969. Northwest Sci. 43, 1 16- I 19. Bettenay, E, 1962. J . Soil Sci. 13, lo- 17. Bettenay, E., and Hingston, F. J. 1961. CSIRO Austruliun Soils Lund Use Ser. No. 41. Bettenay, E., and Hingston, F. J. 1964. Australian J . Soil Res. 2, 173-186. Bettenay, E., Blackmore, A. V., and Hingston, F. J. 1964. Australian J . Soil Res. 2, 187210. Blackburn, G., and Leslie, T. I. 1958. CSIRO Australian Soil Publ. No. 12. Blackburn, G., Bond, R. D., and Clarke, A. R. P. 1967. CSIRO Australian Soil Publ. No. 24. Butler, B. E., 1958. CSIRO Australian Soil Publ. No. 10. Butler, B. E. 1959. CSIRO Australian Soil Publ. No. 14. Butler, B. E., and Hutton, J. T. 1956.AustralianJ. Agr. Res. 7,536-553. Churchward, H. M. 1963. Austrelian J . Soil Res. 1, 117-128. Coffey, G. N. 1912. US.Dep. Agr., Bur. Soil Bull. NO. 85. Crocker, R. L. 1941. Trans. Roy. Soc. South Australia 65, 103-107. Crocker, R. L. 1946. CSIRO Australian Bull. No. 193. Daniels, R, B., and Gamble, E. E. 1967. Geoderma 1, 117-124. Daniels, R. B., and Gamble, E. E. 1971. Univ. of Kentucky Press (in press). Daniels, R. B., and Jordan, R. H. 1966. U.S.Dep. Agr., Tech. Bull. 1348. Daniels, R. B., Gamble, E. E., Wheeler, W. H., and Nettelton, W. D. 1966. Southeast Geol. 7, 159-182. Daniels, R. B., Gamble, E. E., and Nelson, L. A. 1967. Soil Sci. 104,364-369. Daniels, R. B., Gamble, E. E., and Bartelli, L. J. 1968. Soil Sci. 106, 200-206. Daniels, R. B., Nelson, L. A., and Gamble, E. E. 1970a. Z. Geomorphol. [N.S.] 14, 175185. Daniels, R. B., Gamble, E. E., and Cady, J. G. 1970b. Soil Sci. SOC.Amer. Proc. 34,648653. Daniels, R. B., Gamble, E. E., and Nelson, L. A. 1 9 7 0 ~Agron. . Abstr. (abstr.). Daniels, R. B., Gamble, E. E., and Wheeler, W. H. 1971. Southeast Geol. (in press). Davis, W. M. 1920. Geol. Mag. 57,429-43 1 . Don. J., and Yaalon, D. H. 1968. Truns. Int. Congr. Soil Sci., 9th, Vol. 4, pp. 577-584. Fowler, E. D. 1927. Proc. Int. Congr. Soil Sci., 1st. Vol. 4, pp. 435-441. Frey, D. G. 1953. Ecul. Monogr. 23,289-3 13. Gamble, E. E., Daniels, R. B., and Nettleton, W. D. 1970. Soil Sci. Soc. Amer., Proc. 34, 276-28 1. Gile, L. H 1961. Soil Sci. SOC.Amer., Proc. 25,52-61. Gile, L. H. 1966a. Soil Sci. SOC.Amer., Proc. 30, 657-660. Gile, L. H. 1966b. Soil Sci. SOC.Amer., Proc. 30,773-781. Gile, L. H. 1970. Soil Sci. SOC.Amer., Proc. 34,465-472. Gile, L. H ,and Grossman, R. B. 1968. Soil Sci. 106,6-15. Gile, L. H., and Hawley, J. W. 1966. SoilSci. SOC.Amer., Pruc..30, 261-268. Gile, L. H., Peterson, F. F., and Grossman, R. B. 1966. Soil Sci. 101,347-360.

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Gile. L. H., Grossman. R. B., and Hawley, J. W. 1969. SoilSci. 108,273-282. Gile, L. H., Hawley, J. W., and Grossman. R. B. 1970. Guideb. Soil-Geomorphol. Field Con/:, 1970. Glinka, K. D. 1963. 4th ed. Israel Program Sci. Transl., Jerusalem. Hardy, A. V., Carney, C. B., and Marshall, H. V. 1967. North Carolina, Agr. Exp. Sta., Bull. 433. Hardy, E. L. 1941. US.Dep. Agr. Yearb. Agr.pp. 101 1-1024. Hawley, J. W. 1965. New Mexico Geol. Soc., Field Cont. Guideb. 16, Part 11, 188-198. Hawley, J. W., and Kottlowski, F. E. 1969. New Mexico, Bur. Mines Miner. Resour., Circ. 104, 89- I 15. Hawley, J. W., Kottlowski, F. E., Strain, W. S., Seager, W. R., King, W. E., and LeMone, D. V 1969. New Mexico, Bur. Mines Miner. Resour., Circ. 104,52-76. Howell, J. V. 1957. “Glossary of Geology and Related Sciences.” Amer. Geogr. Inst. Hunt, C . B. 1954. US.,Geol. Surv. Bull. 996C, 91-140. Hutton, J. T., and Stephens, C. G . 1956. J . Soil Sci. 7,255-267. Humphrey, R. R. 1958. Arizona, Agr. Exp. Sta., Bull. 299. Isphording, W. C . 1970. Amer. Ass. Petrol. Geol., Bull. 54,334-343. Jongen, P. 1961. Trans. Int. Congr. SoilSci., 7th 1960 Vol. 4, pp. 335-346. McArthur, W. M., and Bettenay, E. 1960. CSIRO Australian Soil Publ. No. 16. Maigninen, R. I96 I . Trans. I n t . Congr. Soil Sci.,7th, 1960 Vol. 4. pp. 17 1- 176. Marbut. C . F. 1928. “Soils, their Genesis, Classification and Development.” Mimeo. copy of lectures given at Graduate School, February to May, 1928. U S . Dep. Agr. Marbut, C . F., Bennett, H . H , Lapham, J. E., and Lapham, M. H. 1913. US.Dep. Agr., Bur. Soil Bull. No. 96. Maud, R. R. 1965. J . Soil Sci. 16,60-72. Milne, G. 1935. Soil Res. 4, 183-198. Milne, G., Beckley, V. A., Jones, G . H G., Martin, W. S., Griffith, G., and Raymond, L. W. 1935. Trans. I n t . Congr. Soil Sci.,3rd, 1935 Vol. 5, pp. 270-274. Morrison, R. B. 1968. I n “Means of Correlation of Quaternary Successions” (R. B. Morrison and H. E. Wright, Jr., eds.), pp. 1 - 1 13. Univ. of Utah Press, Salt Lake City. Moss, R. P. 1968. In “The Soil Resources of Tropical Africa” (R. P. Moss, ed.). pp. 29-60. Cambridge Univ. Press, London and N e w York. Mulcahy, M. J. 1960. J . Soil Sci. 11,206-225. Mulcahy, M. J. I96 I . Z . Ceomorphol. 5 , 2 I 1-225. Mulcahy, M. J. 1967. I n “Landform Studies from Australia and New Guinea” (J. M. Jennings and J. A. Mabbutt, eds.), pp. 21 1-230. Cambridge Univ. Press, London and N e w York. Newbold, T. J. 1844. J . Asiatic Soc. Bengal 13,984- 1004. Newbold, T. J . 1846. Roy. Asiatic Soc. 8, 277-240. Northcote, K. H. 1946. Trans. Roy. SOC.South Australia 70,294-296. Northcote, K. H., and Tucker, B. M. 1948. CSIRO Australian Bull. 233. Norton, E. A., and Smith, R. S. 1930. J . Amer. SOC. Agron. 22, 25 1-262. Nye, P. H. 1954. J . Soil Sci. 5,7-21. Nye, P. H . 1955.5. Soil Sci. 6 , 5 1 4 3 . Ollier, C . D. 1959.J. Soil Sci. 10, 137-140. Parsons, R. B., and Balster, C . A. 1966. Soil Sci. SOC.Amer., Proc. 34,485-49 I . Parsons, R. B., Balster, C. A,, and Ness, A. 0. 1970. Soil Sci. Soc. Amer., Proc. 34,485491. Playford, P. E. 1954. Australian J . Sci. 17, 1 I- 14. Prescott, J . A. 1944. CSIRO Australian Bull. 117.

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Prescott. J. A.. and Pendleton, R. L. 1952. Commonw. Bur. Soil Sci. Tech. Commun. 47. Radwanski, S. A., and Ollier, C. D. 1959. J. Soil Sci. 10, 149-168. Ruhe, R. V. 1954. Amer. Railway Eng. Ass., Bull. 514,639-645. Ruhe, R. V. 1962. J . Geol. 70, 151-167. Ruhe, R. V . 1964. Ann. Ass. Amer. Geogr. 54, 147-159. Ruhe, R. V. 1967. New Mexico, Bur. Mines Miner. Resour., Mem. 18. Ruhe, R. V. 1968. In “Loess and Related Eolian Deposits of the World” (C. B. Schultz and J. C. Frye, eds.), pp. 49-65. Univ. of Nebraska Press, Lincoln. Ruhe, R. V. 1969. “Quarternary Landscape in Iowa.” Iowa State Univ. Press, Ames. Ruhe, R. V., and Scholtes, W. H. 1956. Soil Sci. Soc. Amer., Proc. 20,264-273. Ruhe, R. V., Prill, R. C., and Riecken, F. F. 1955. Soil Sci. Soc. Amer., Proc. 19,345-347. Ruhe, R. V., Daniels, R. B.. and Cady, J. G. 1966. U . S . Dep. Agr., Tech. Bull. 1349. Soil Survey Staff. 1960. “Soil Classification, A Comprehensive System 7th Approximation.” US. Dep. Agr., Washington, D.C. Spurr, A. M. M 1954. Proc. Inter-Ayrican Soils Conf., 2nd, 1954 Vol. I , pp. 175-190. Stephens, C. 0 . 1946. CSlRO Australian Bull. 206. Stephens, C . G. 1949. J . Soil Sci. 1, 123-149. Thorpe, J. 1965. Soil Sci. 99, 1-8. Trowbridge, A. C. 1921. Stud. Natur. Hist. Iowa Univ. 9, 1-127. Turton, A. G., March, N. L , McKenzie, R. M., and Mulcahy, M. J. 1962. CSIRO Australian Soil Publ. No. 2. Van Dijk, D. C. 1958. CSIRO Australian Soil Publ. No. 1 I . Van Dijk, D. C. 1959. CSIRO Australian Soil Publ. No. 13. Walker, P. H . 1962. J . Soil Sci. 12, 167-177. Walker, P. H 1963. Australian J . Soil Res. 1, 223-230. Ward, W. T. 1966. CSIRO Australian Soil Publ. No. 23. Watson, J. P. 1964. J . Soil Sci. 15, 238-257. Whitehead, D. R. 1963. Ecology 44,403-406. Whitehead, D. R. 1964. Ecology 45,767. Whitehead, D. R. 1969. Southeast Geol. 10, 1-16. Wooldridge, S. W. 1949. J. Soil Sci. 1 , 3 1-34. Woolnough, W. G . I9 18. Geol. Mag. 55,385-393. Wright, R. L. 1962. J . Roy. SOC. Western Australia 45, 5 1-64. Wright, R. L. 1963.J. Geol. Soc. Australia 10, 151-163.

FACTORS AFFECTING ROOT EXUDATION M. G . Hale, C. 1. Foy, and F. J. Shay Department of Plant Pathology and Physiology, Virginia Polytechnic Institute and State University, Blacksburg, Virginia

I. Introduction ..................................................................................... 11. History ............................................ ....... ................. 111. Sources of Exudate and Pattern of Root ................................

IV. V. VI. VII. VIII.

1x.

Interactions of Organic Nitrogen Sources and Exudation ......................... Effects o n Carbon and Nitrogen Balance in Plants ................................. Foliarly Applied Compounds and Root Exudation .................................. Root Exudation-Mineral Nutrient-Microbial Interactions . Effects of Environment on Root Exudation ........................................... Summary ........................................................ References .....................................................................................

89 92 93 95

95 97 101

I03 I07 I07

I. Introduction

Absorption and desorption of substances extremely diverse in molecular size and in physicochemical and biological properties occur at the root surface. Substances (other than water) which are released from plant surfaces have been termed exudates by many investigators regardless of whether their appearance on the surface is the result of passive or active transport. In regions of the plant other than the root, exudation may occur from special structures called glands, but in roots no glands have ever been demonstrated. Possible sources of metabolites in exudates include intact living cells, moribund cells, or injured cells and tissues still attached to the root and the apoplast. At least one investigator has included sloughed cells and tissue in the term exudate (Rovira, 1969b). A large body of published work on exudation has been done by microbiologists and plant pathologists who were mainly concerned with exudates as a source of nutrients or growth factors for microorganisms and the attraction of microorganisms to roots. The role that root exudation plays in the physiology and life cycle of the plant itself and the mechanisms of exudation have been neglected. Our purpose is to bring together the body of information pertaining to the quantitative and physiological aspects of root exudation and to point out how the development of techniques for axenic culture of whole plants 89

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may aid this kind of research. In these two aspects this report differs significantly from that of Rovira (1969a). Some of the terminology pertaining to the absorption of ions and root exudation is given in Table I. The scientific names of plants referred to in the text by common name are listed in Table 11. TABLE I Concepts or Definitions of Terms Used in Reference to Root Exudation and Interactions of Roots with the Surrounding Medium Term Accumulation

Active transport

Absorption

Adsorption Apoplast Axenic Carriers

Excretion

Exudation

Exudates, root Free space (or “outer space”)

Concept or definition and literature reference Increase in concentration of a substance, usually both ions of a salt, in the vacuoles of cells as a result of an expenditure of energy by the cells. [Sutcliffe (1962) includes movement of ions against a concentration gradient and the passive movement resulting from establishment of Donnan equalibrium.] The process whereby an ion is moved against an electrochemcal potential gradient and is therefore dependent on a decrease in free energy in some metabolic process (Briggs et al., 1961, p. 131). A general term used for the entry of a substance into any plant cell or plant part by any mechanism. Desorption is the reverse process. Ions or molecules held by polar linkages to charged groups of cellular structures, and thus available for exchange. The continuum of nonliving cell wall material that surrounds and contains the symplast (Crafts, 1961, p. 142). Germfree, absence of other organisms. Compounds with which ions combine chemically in a labile form, and are transported across a membrane, after which the ions are released (Sutcliffe, 1962, pp. 40-46). Elimination of useless superfluous or harmful substances which may reach the exterior of the plant, or which may be stored in nonfunctional tissue. Exudation of applied pesticides may be excretion but the term is more applicable to animals than to plants. The appearance on a plant surface of substances from inside the plant whether as a result of passive movement or secretion. Frequently the pathway to the surface is established through wounds. Those substances which are released into the surrounding medium by healthy and intact plant roots (Rovira, 1969a). Defined as that part of a tissue or organ into and through which the solute and solvent from the external solution move readily. The volume of free space may vary slightly with solute and solvent but usually is around 10% (Briggs et al., 1961, p. 74; Devlin, 1969, pp. 238-239).

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FACTORS AFFECTING ROOT EXUDATION

TABLE I (continued) Concept or definition and literature reference

Term Leaching

Leakage as a result of the flow of solvent or dilute solution surrounding the organ (Tukey, 1966). Efflux by purely physical means (diffusion and exchange). May occur from symplast to the apoplast or from plant organ into the surrounding medium, or from vacuoles into cytoplasm (Sutcliffe. 1962, p. 27). Absorption of substances previously released or desorbed. That part of the rhizophere adjacent to the root surface. The external surfaces of the plant roots together with any closely adhering particles of soil or debris (Clark, 1949). That sone of soil in which the microflora is hfluenced by plant roots. I t is a poorly defined zone of soil in which one finds microbiological gradients from the root surface into the soil (Rovira, I965a). A component of the exosphere (Scott, 1969).

Leakage

Resorption Khizoplane

Rhizosphere

Secretion

Efflux by mechanisms involving metabolic energy (active transport). May result in loss to plant surface or into cell vacuoles (Levitt, 1969, p. 85). The continuum of interconnected protoplasts of the plant (Crafts, 1961, p. 142).

Symplast

TABLE I 1 Common Name, Scientific Name, and Reference for Plants Mentioned by Common Name in the Text

~

Alfalfa Apple Barley

Bean

Beet Broad bean Chrysanthemum Corn Cotton Cucumber Flax Oats

Reference

Scientific name

Common name _

_

_

_

~

Medicago sativum L. Pyrus malus L. Hordeum distichon L Hordeum vulgare L. C.V.C A M P A N A C.V. MARIS BADGER Phaseolus vulgaris L

Beta vulgaris L. Vicia faha L. Chrysanthemum sp. Zea mays L. Gossypium sp. Cucumis sativus L. Linum usitatissimum L. C.V. BISON & NOVELTY Avena sativa L.

Richter et al. (1968) Head (1964) Vancura (1964) Hiatt and Lowe (1967) Barber and Loughman ( 1967) Miller and Schmidt (1965a). Foy and Hurtt ( 1967), Schroth et al. (1966) MacDonald ( I 967) Pearson and Parkinson ( 196 I ) Woltz (1963) Morre et al. ( I 967) Schroth et al. ( I 966) Vancura ( I 967) West (1939) Schroth and Hildebrand ( 1964)

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M. G. HALE, C. L. FOY, A N D F. J . SHAY

TABLE I I (continued) ~~

Common name

Scientific name

Orchardgrass Pea

Dactylis glomerata L. Pisum sativum L.

Peanut P halarisgrass, Canarygrass Pine

Arachis hypogaea L. Phalaris caroliniensis L.

Potato Ragi, finger millet

Solanum tuberosum L. Eleusine coracana Gaertn.

Rattlebox Red clover

Crotalaria medicaginea Trifolium pratense L.

Rice Sicklepod Sorghum

Oryzae sativa L. Cassia tora L. Sorghum vulgare Pers.

Strawberry Subterranean clover Sunnhemp

Fragarin virginiana L. Trifolium subterraneum L. Crotalaria juncea L.

Timothy Tobacco

Phleum pratense L. Nicotiana tabacum L. Nicotiana rustica L. Lycopersicon esculentum

Tomato

Wheat

Pinus radiata, Pinus strobus L.

Triticum aestivum L. C.V.FEDERATION 41 Triticum vulgare C.V.CABO

Reference Couch and Bloom (1960) Ayers and Thornton ( I 968), Kerr ( I964), Schroth e f al. (1966) Hale (1968) Rovira (1959) Bowen (1968, 1969a,b) Slankis et al. (1964), Street ( I 966) O’Brien and Prentice ( I 930) Balasubramanian and Rangaswami (1969) Sullia ( 1 968) Bowen and Rovira ( 1 966), Couch and Bloom (1960) Ramchandra-Reddy (1968a) Sullia ( I 968) Balasubramanian and Rangaswami ( 1969) Hussain and McKeen (1963) Rovira ( I 959) Balasubramanian and Rangaswami (1969) Couch and Bloom ( 1960) Steinberg (1950) Steinberg (1952) Bowen and Rovira ( I 966), Subba Rao et a / . (1961). Rovira (1959) Ayers and Thornton ( I 968) Bowen and Rovira ( I 966)

II. History

As early as the nineteenth century the French botanist, DeCandolle (1832) ascribed an important role in the “soil sickness” problem to excretions from roots. The work that has ensued since then has been amply reviewed (Bonner, 1950; Borner, 1960; Evanari, 1961 ; Garb, 196 1 ; Grodzinsky, 1962; Loehwing, 1937; Lyon and Wilson, 1921; Rovira, I969a; Santovich, 1965; Schroth and Hildebrand, 1964; Street, 1966; Woods, 1960). Comprehensive treatment of the literature is beyond the

FACTORS AFFECTING ROOT EXUDATION

93

scope of the present discussion but some papers of historic interest are worthy of mention. Four such papers have been reviewed by Rovira (1965b): Knudson ( I 920) concluded that sucrose in the nutrient solution was absorbed by the roots of axenic plants, hydrolyzed and exuded by the roots as reducing sugars; Lyon and Wilson ( 1 92 1 ) found that nitrogencontaining organic compounds were released into sterile rooting medium; Cranner (1 922) showed that roots of seedlings and of mature plants released phosphatides; and O’Brien and Prentice ( 1 930) presented the first conclusive evidence of stimulation of specific organisms by root exudates when they showed that the cysts of potato eelworm (Heterodera Rostochiensis) were caused to hatch in the presence of root washings of potato, but not of some other plants. The evidence that some organisms in the rhizosphere are dependent upon roots for growth factors was demonstrated by West ( 1 939). Thiamine and biotin were found to be released from flax roots in amounts of 0.33 pg of thiamine and 0.06-0.08 pg of biotin in 1 week from five plants. It has been established subsequently that most of the metabolites found within plants may also be found outside of plants under various experimental conditions. Axenic culture techniques and continuous harvesting of exudate samples have revealed that there is an endogenous rhythm of exudation in cereal plants (Dubrov and Bulygina, 1967) and alfalfa (Richter et al., 1968). The rhythm was exhibited for amino acids and a yellow pigment. Compounds applied to foliage, or their metabolites, also may appear or reappear in the rooting medium (Blackman, 1956, 1958; Blackman et al., 1958; Crafts, 1961; Foy and Hurtt, 1967; Foy and Yamaguchi, 1965; Hurtt and Foy, 1965a; Klein, 1968; Levi, 1968; Linder et al., 1958, 1964; Mitchell et al., 1959; Preston et al., 1954; RamchandraReddy, 1968a,b; Sullia, 1968). These include such things as mineral nutrients, growth regulators, and pesticides. Ill. Sources of Exudate and Pattern of Root Exudation

There are several well documented examples of exudation from special structures. These include guttation from hydathodes as a result of root pressure; secretion from glands, such as those in nectaries of flowers and in salt glands found in some halophytes. N o glands have been demonstrated in roots of plants although formation of droplets containing a highly hydrated hexose polysaccharide have been observed at the tips of corn roots (Morre et al., 1967) and apple roots (Head, 1964). Mollenhauer ( 1967) has concluded that all terrestrial or epiphytic roots have some secretory activity from root cap cells.

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M. G . HALE, C. L. FOY, A N D F. J. SHAY

Many of the substances identified as root exudates are probably released as a result of injury by lateral or adventitious roots as they grow from their internal site of origin to the outside, by injury from cultural practices, or by injury from attacking organisms. Ayers and Thornton ( 1 968) discussed the possibility of injury causing exudation because, when they moved a root across filter paper, greater quantities of amino acids were released than if the root was undisturbed on the paper. Agitation with sand in the nutrient solution bathing the roots also caused an increase in amino acids exuded. McDougall ( 1968) was able to demonstrate that when wheat roots were deliberately cut or crushed considerable exudation occurred from the injured site. Some substances may originate from sloughed root cap, epidermal, and cortical cells. Visual examination of peanut roots growing in nutrient solution in a sterile environment revealed quantities of sloughed material clinging to the root axis and to the root cap (G. J. Griffin, M. G. Hale, and F. J. Shay, unpublished observations, 1969). Slight agitation caused quantities of this material to floc to the bottom of the container. Microscopic examination revealed that masses of root cap cells, recognizable because of the conical shape of the mass, and sheets of cortical cells make up the bulk of the material with few single cells present. Other masses of cells originate at the site of eruption of lateral roots. In 1919 Knudson made similar observations on corn and pea seedlings and found that such cells remained alive as long as 40 days after sloughing. He estimated the total weight would be about 20 mg per 42-day-old plant. Over the growth period of a root the quantity of organic matter released into the rhizosphere as sloughed cells and tissue probably plays an important role in microbial succession in root colonization. The qualitative and quantitative amounts of soluble nutrients released into the rooting medium from sloughed material are still not known. It is not clear what proportion of exudates may be the result of leaching from the apoplast of the root and what proportion may result from exchange reactions with inorganic nutrient ions (Hiatt and Lowe, 1967). Burstrom ( 1965) has suggested that modes of exudation from various sources may yield different groups of compounds. Loss from the apoplast may yield compounds of low molecular weight, whereas secretion from the vacuoles and cytoplasm may result in compounds representing the entire cell contents. The source of pressures or gradients which might cause exudation are largely unknown although their existence and involvement in absorption is well established. The polysaccharide exuded by root cap cells is probably the result of the activity of Golgi bodies and a positive turgor pressure (Morre ef al., 1967). Gradients of efflux of ions occur along roots (Bowen, 1968). Efflux of

FACTORS AFFECTING ROOT EXUDATION

95

chloride ions in the apical 2-3 cm in 24 hours was approximately 20% of that originally present in the apex of the root but was only 8.2% in a region 6 to 8 cm from the apex. A distribution pattern for efflux of I4C along roots was different. McDougall ( 1968) found that I4C was released at a fairly uniform level along the root of 5- to 6-day-old wheat plants except at the base of the root (about 8 cm from the tip) where large amounts were exuded. The compounds contained no ninhydrin-reacting substances, On the other hand, broad bean roots were found to exude amino acids from a region centered about 28 mm from the root apex (Pearson and Parkinson, 1961). Exocellular enzymes also are exuded. Chang and Bandurski ( 1964) used two methods to study root exudation from 4- to 5-day-old corn seedlings. In one method the seedlings, substrate, and cofactors were placed in the same test tubes, and enzyme activity was measured for a time. Total activity of free enzymes and those on the surface of the root yielded invertase, cellobiase, adenosine triphosphatase, pyrophosphatase, ribonuclease, and deoxyribonuclease. In method two, an aliquot of the solution bathing the roots was removed from the presence of the roots and substrate plus cofactors added. Only free, exuded enzymes were measured by this method and yielded invertase, ribonuclease, and deoxyribonuclease. N o attempt was made to ascertain the distribution pattern along the root and the number of enzymes found was limited by the assays used. IV. Interactions of Organic Nitrogen Sources and Exudation

Resorption of exudates or absorption of metabolites from organisms living on exudates from roots may affect the growth of plants. The effects of amino acids have been studied in this respect. Steinberg (1950) was able to show that amino acids in the rooting medium could produce the symptoms of frenching of tobacco and associated release of these amino acids by microorganisms in the soil with appearance of disease symptoms (Steinberg, 1952). A similar relationship has been shown for chrysanthemums (Woltz, 1963). Axenic culture of plants has been used to prove that amino acids can be absorbed from soil (Miller and Schmidt, I965b). The sources of nitrogen that can be used by higher plants and the interactions between microorganisms and plant nutrition have been reviewed by Barber ( 1968). V. Effects on Carbon and Nitrogen Balance in Plants

Attempts to measure productivity based on increases in dry weight need to account for the photosynthate lost through the roots just as ex-

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M. G. HALE, C. L. FOY, A N D F. J . SHAY

cretion of photoassimilated organic compounds must be given attention in studies of aquatic angiosperms and algae (Wetzel, 1969). Applications of 14C02to the aerial portion of the plant and subsequent detection of 14C have shown that significant quantities of carbon compounds leave the roots in forms other than COr (McDougall and Rovira, 1965; McDougall, 1968; Slankis et al., 1964). 14C appeared in the rooting medium in 4-5 hours after application to seedling wheat plants. It has also been demonstrated that labeled carbon moves out of the roots of one plant and into the roots of an adjacent plant labeling all parts of the plant within 21 hours (Ivanov et al., 1963). Rovira ( 1969b) has chosen to differentiate between diffusible exudates and nondiffusible exudates, the latter being sloughed cells and mucilaginous material. The nondiffusible material is not collected in most root exudate studies, and the contribution of the plant root to soil is underestimated to a considerable degree. Amounts of material lost through the roots under natural conditions are unknown largely because of the difficulty of measuring loss. Under experimental conditions it has been demonstrated that even slight agitation of the root system increased the amount of amino acids released into the nutrient solution. The maximum amount of ninhydrin-reactive compounds measured was 24 pg per 15 wheat seedlings after 14 days in the culture solution (Ayers and Thornton, 1968). This yield was obtained with agitation of the roots by swirling the nutrient solution, containing a small amount of sand, against the roots. Although undisturbed roots of wheat yielded no amino acids, those of peas did. Slightly more than 33 pg of amino-N per 5 undisturbed pea seedlings was released after 14 days in the culture solution. Also those plants of both species which grew in sand released more amino acids than did those which grew in nutrient solutions. The greater release was attributed to injury of the roots by the sand. Richter and co-workers ( 1 968) were able to establish a system whereby they could continuously harvest the nutrient solution without disturbance of the root system. Average daily yields of 0.24 pmole of amino-N per 7 alfalfa seedlings over a period of 57 days were obtained. They were able to show that the loss fluctuated in a rhythmic pattern for each of several amino acids and that the respiratory poison, dinitrophenol, decreased the loss of some amino acids but not others. This is one of the few attempts to correlate exudation with root metabolism. Vancura (1964) obtained 0.4-0.5 mg dry weight of exudate from roots of barley seedlings. This amounted to 7-10% of the dry weight of the tops and was the result of exposing roots to water stress and then relieving the stress.

97

FACTORS AFFECTING ROOT EXUDATION

Manorik and Belima (1969) believe that the best way to express the amount of exudate for comparisons between species and between experiments is on the basis of amount of exudate per unit area of absorbing root surface (Table 111). Such an expression would eliminate errors caused by differences in growth rate and nutritional status of the plant. TABLE 111 Micrograms of Amino Nitrogen in Root Exudates of Peas and Corn" Per gram of roots Variant

Per plant

Fresh

Dry

Above ground part, fresh

Peas Corn Peas :corn ratio

6.8 I 3.86 1.76

9.90 3.40 2.9 1

7 1.84 24.78 2.89

5.76 2.48 2.32

Per square decimeter of absorbing surface of roots 2 I .07 2.24 9.40

OData from Manorik and Belima 1969, Plenum, N e w York, by permission.

Measuring the absorbing root surface area, however, might be difficult. The expression micrograms lost per square decimeter is useful only for the diffusible or soluble exudate, and not necessarily for the nondiffusible exudate and sloughed cells. The amount lost per plant is a more readily obtainable figure. Any expression must also include a time unit. Some comparisons are made in Table IV by recalculating the data of several investigators into the amount lost per plant per week. Since the examples used are for seedlings of about the same age, such comparisons seem valid. Quantitative data of this kind are quite meager although there are data relating to semiquantitative analyses such as size of spots on chromatograms and relative effects on microbial populations in the rhizosphere. There is evidence which indicates that amounts of exudates occurring into soil may be greater than those occurring into nutrient solution (Table V), but the difficulties of removing roots without injury with the increased loss from wounds make direct comparisons questionable. It is evident that measurable losses of soluble organic material occur from roots and more quantitative data are needed from a variety of experimental conditions before the principles of root exudation can be fully elucidated. VI. Foliarly Applied Compounds and Root Exudation

Herbicides and other growth regulators have been observed to differ markedly with respect to rate of uptake and subsequent translocation,

W

00

TABLE I V Amounts of Amino Nitrogen, Sugars, and Total Substances Lost from Seedling Roots of Various Crop Species"

Species

Compound or group

Amount

Amount recalculated to pg/plant/week

24 pg/15 plants/l4 days 33 pg/15 plants/l4 days 0.29 pmole/72 plants/day 2.54 @/plant/ 15 days 3.16 pg/plant/ 15 days 1.72 pg/plant/15 days 1.20 pg/plant/30 days 0.104 pnole/plant/l4 days 1.7 to 3. I mg/plant/24 days

0.8 1 3.30 0.37 0.22 0.28 0.15 0.05 0.73 60- 104b

-

Reference

Amino N Wheat Peas Alfalfa Sorghum Sunnhemp Ragi Tomato Pinus radiata Phaseolus vulgaris

Ayers and Thornton (1968) Ayers and Thornton ( 1 968) Richter et al. ( 1968) Balasubramanian and Rangaswami Balasubramanian and Rangaswami Balasubramanian and Rangaswami Balasubramanian and Rangaswami Bowen (1968) Miller and Schmidt (1965b)

(1969) ( I 969) (1969) (1969)

Sugars Sorghum Sunnhemp Ragi Tomato Peanut

82 pg/plant/15 days 68 pglplantll5 days 42 pg/plant/l5 days 32 pg/plant/30 days 3.8 pglplantlweek

38.3 31.7 19.6 1.46 3.80

Balasubramanian Balasubramanian Balasubramanian Balasubramanian Hale ( I 968)

and and and and

Rangaswami Rangaswami Rangaswami Rangaswami

Total organic substances Barley Peanut

0.5 mg/plant/ I0 days 0.54 mg/plant/l88 days

350' 21

Vancura ( 1 964) Hale (1968)

"Grown in nutrient solution except as noted. "Plant grown in sterile soil and soil extracted for analysis; fertilized with KNOI. Some root injury on removal from soil. =Plants grown in sand, allowed to dry down and then watered.

( 1969)

(1969) ( 1969) ( 1969)

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FACTORS AFFECTING ROOT EXUDATION

TABLE V Comparison of Amino Acids Found in Autoclaved Soil before and after Growing Beans in I t for 24 days"," Concentration in soil after growth of Phaseolus vulgaris C.V. BLACK VALENTINE

(pg per 250 g dry soil)

Amino acid

Concentration in autoclaved soil

1"

2

3 479.3 464.4 76.2

Aspartic Threonine Serine Glutamic acid Glycine Alanine Valine Leucine Tyrosine Phenylalanine

232.9 405.5 105.5 -

377.4 249.5 137.2 I 14.6 70.8 468.2 136.2 185.8

379.8 225. I 156.8 69. I 169.4 182.9 933.9 273.3 131.1 597.7

139.1 135.0 186.0 704.0 427.6 409.0

Summation

743.9

1739.7

31 19.1

3020.6

"Data from Miller and Schmidt 1965a, Williams & Wilkins, Baltimore, Maryland, with permission. *Anoka soil with 36 mg K N O , added to each container. All roots were removed before extracting the soil with ammonium acetate. Since sterility of the root system and the rooting medium was maintained, the additional amino acids could come only from the bean plants. 'Plant number.

both into and out of roots, as revealed by autoradioaugraphy, isotope counting, and bioassay (Crafts, 1964; Foy and Yamaguchi, 1965). Similar differences in mobility patterns among compounds were observed after foliar treatments. Certain substances, e.g., substituted urea and s-triazine derivatives, apparently move in the apoplast, whereas others, e.g., 2,4-dichlorophenoxyacetic acid (2,4-D), have a stronger affinity for the symplasm and are phloem-mobile. Some compounds are known to be translocated not only from the shoots into roots, but out of roots into the culture medium. Accumulation and perhaps active secretion of certain of these substances are suspected. The loss from roots of trans-cinnamic acid (Bonner and Galston, 1944) is a classical example of the release of a naturally occurring growth regulator by plant roots. a-Methoxyphenylacetic acid (MOPA) was apparently the first exogenous compound reported as being excreted from roots (Preston et al., 1954). Subsequently, five other exogenous growth

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M. G . HALE, C. L. FOY, A N D F. J. SHAY

regulators were reported to be exuded from roots (Linder et al., 1958; Mitchell et al., 1959). More recently, the excretion of dicamba ( 2 methoxy-3,6-dichlorobenzoic acid) was reported by Linder et al. ( 1964) and the excretion of the important herbicidal derivative of picolinic acid, picloram (4-amino-3,6-trichloropicolinicacid), was reported by Hurtt and Foy ( 1 965a,b). Thirty-one compounds were examined by Foy and Hurtt ( 1 967) with respect to their ability to be excreted from the roots of bean plants following foliar application. From this series of compounds picloram, dicamba, and 2,3,6-trichlorobenzoic acid emerged as three of the most potent from the standpoint of injuring untreated plants growing in close proximity to the treated plants. The transfer of herbicide from treated to adjoining untreated plants via the respective root systems was demonstrated in soil, sand, and nutrient solution. Symptoms of herbicide injury were consistently observed on untreated plants when about 2 pg of either picloram or dicamba were applied to the corresponding adjacent plants. Picloram and dicamba were absorbed, translocated, and excreted, regardless of whether they were applied by leaf dipping, spraying, drop application, or application through a severed petiole. No detectable dicamba was released into nutrient solution from the stems of plants whose roots had been excised before application of the herbicide. In comparable studies, small amounts of picloram leaked out of the stems of treated plants but the quantities were small compared with those released by plants with intact root systems. Neither herbicide was released from plants whose roots had been steamed before herbicide application. The total amount of dicamba or picloram excreted into nutrient solution by the roots of intact plants over a 4-day interval was 10-15% of the amount applied as a foliar application (Hurtt and Foy, 1965a). Autoradiographs (dicamba-14C) and bioassay data indicated that neither herbicide was excreted when the phloem was girdled by steam before herbicide treatment. Both herbicides were excreted under aerobic conditions, but dicamba was not excreted by plants whose roots were grown under anaerobic conditions obtained by aerating the nutrient solution with nitrogen. The substitution of oxygen for air as a source of aeration did not increase the amount of dicamba excreted. Picloram, in contrast, was excreted under nitrogen aeration, and the substitution of air or oxygen for nitrogen did increase the amount of herbicide excreted (Hurtt and Foy, 1965b). Ramchandra-Reddy ( 1968a,b) applied antibiotics, trace elements, and metal chelates to foliage and concluded that changes in the rhizosphere populations depended upon the nature and concentration of substances used and the season of growth. Sullia (1968) applied 100 ppm each of gibberellic acid and indole-3-acetic acid to foliage of sickle pod and rattle-

FACTORS AFFECTING ROOT EXUDATION

101

box. There was an increase in total number of fungi observed but no change in the species present in the rhizosphere. Balasubramanian and Rangaswami ( 1969) observed that generally foliar applications of a 0. I % solution of nitrogen in the form of N a N 0 3 increased the concentrations of amino acids exuded by sorghum, sunnhemp, ragi, and tomato but applications of P as Na2HP04reduced the concentration. The results on sugars exuded were the reverse. N applications reduced total sugars exuded while P applications increased the sugar concentrations. There were also some changes in the kinds of amino acids exuded as a result of treatment. Considerable significance can be attached to the alteration of root exudates by foliar applications as a possible means of influencing rhizosphere populations of microorganisms and thus, perhaps, controlling root diseases of plants (Rovira, 1969a). Study of root exudation or excretion of organic compounds, particularly endogenous and exogenous plant growth-regulating substances, and their subsequent fate has been neglected. More consideration needs to be given to the role of root-colonizing microorganisms. VII. Root Exudation -Mineral Nutrient- Microbial Interactions

The effects of microorganisms on mineral nutrient availability are varied and depend upon plant species, age, microbial population, and the particular conditions of the experiment being conducted. Care must be exercised in interpreting experimental results from studies that do not use soil for the rooting medium and that deal with unnatural microbial populations. The incidence of microorganisms capable of dissolving organic phosphates is higher in rhizosphere soil than in nonrhizosphere soil (Greaves and Webley, 1965; Sperber, 1957). Bowen and Rovira (1966) demonstrated that in short-term uptake studies nonsterile clover and tomato plants took up 45% and 85% more phosphate than did sterile plants. Similar results for barley have been reported (Barber and Loughman, 1967) and for pine (Pinus radiata) (Bowen, I969b). The increased availability of phosphate caused by microorganisms is reflected by higher levels of nucleic acids, phospholipids, and phosphoproteins in the nonsterile plants. A greater loss of phosphate from nonsterile plants was noted when the plants were transferred to a phosphate-free solution (Bowen and Rovira, 1966; Barber and Loughman, 1967). A possible explanation is that the phosphate was released from the microorganisms at the root surface or that the microorganisms migrated into the ambient solution. Plant-microbial interactions affecting nutrient availability and/or trans-

102

M. G . HALE, C. L. FOY, A N D F. J . SHAY

location have been reported for several elements other than phosphorus. Sulfur translocation to the shoot of tomato plants infected with Fusarium and Trichoderma spp. was reduced 16-46% in the infected plants (Subba Rao et al., 1961). This may have been caused by either reduced translocation of sulfur as a result of infection, a reduction in the availability of the element, or a reduction in its absorption. Calcium and rubidium uptake and translocation in the roots and shoots of clover seedlings were reduced when previously sterile plants were inoculated with suspensions of rhizosphere microorganisms (Trolldenier and Marckwordt, 1962). Microorganisms also influenced the absorption of sodium and chloride ions by beet discs and the effects were shown to be comparable to those caused by low concentrations of growth depressants. It was suggested that the microorganisms released compounds which influenced the metabolism of the beet root cells (MacDonald, 1967). The interaction of plant roots and microorganisms may also be complicated by the presence of root exudates. Exudates may serve as a microbial substrate and/or they may play a direct role in liberating nutrients from the soil. For example, quinic and glutamic acid exuded from the roots of lucerne and clover have been shown to release phosphate from calcium, iron, and aluminum phosphates (Barber, 1968). The availability of mineral nutrients has been shown to affect root exudation quantitatively and qualitatively. Rovira ( 1959) found that varying the concentration of calcium from 5 X IW4 to 5 x lo-* M i n the rooting medium had little effect on amino acid loss from the roots of tomato, subterranean clover, and phalarisgrass. However, the lowest concentration of calcium used by Rovira was within the range of adequacy established for most plants (Bowen, 1969a). Therefore, the effect of calcium deficiency on exudation remains unknown. Quantitative and qualitative differences in the loss of amides and amino acids from sterile pine seedlings when grown under conditions of nitrogen deficiency, phosphate deficiency, and nutrient sufficiency were reported by Bowen ( I 969b) (Table VI). In the exudate collected from 2- to 4-week-old seedlings, the amido-amino nitrogen was 104.5 X mde/plant when grown in complete nutrient solution, and the plants grown in phosphate-deficient molelplant (Bowen, 1969b). The solution yielded a total of 248.5 x quantitative effect of nutrient deficiency on the exudation of individual amino acids and amides was variable, as may be seen in Table VI. The complexity of the plant-microbial exudate interactions and their effects on nutrient availability must be understood when one investigates the mechanisms of ion uptake; however, these interactions are usually overlooked. The review by Laties (1 969) emphasized a dual mechanism of ion uptake. One mechanism is prevalent at low ion concentrations and

FACTORS AFFECTING ROOT EXUDATION

103

TABLE VI Exudation of Amides and Amino Acids (Moles X IO-!'/Plant) from Roots of Pinus radiata Seedlings, 2 to 4 Weeks"." Nutrient solution

Amino acid or amide

Complete

Asparagine Glutamine" A-Aminobutyric acid u-Alanine Aspartic acid Glutamic acid Glycine Leucine Serine Threonine Valine Total amino nitrogen

10.9 23.6 5.2 I .6 4.4 6.0 7.3 3 .O 4.8 I .4 I .8 104.5

Phosphate deficient 32.5 52.0 13.8 2.8 9.6 19.7 14.0 5.6 8.0 2.0 4.0 248.5

Nitrogen deficient 3 .O 2.8 I .o 1.2 2.0 2.0 3.4 1.8 2.0 0. I 25. I

"Data from Bowen ( I969a). "Roots were of similar length in all treatments. 'Some arginine was also present, but only in small amount.

affects the accumulation of ions in the cytoplasm, and the other mechanism is operative at high ion concentrations and affects the movement of ions across the tonoplast. However, most of the experiments leading to these conclusions were conducted under nonaxenic conditions, and it has been shown that microorganisms affect both the uptake and translocation of inorganic ions. Barber (1968) suggested that the effects of microorganisms on nutrient availability would be greatest at low ion concentrations. Microorganisms have been found to inhabit a mucilaginous layer which covers some roots (Jenny and Grossenbacker, 1963; Brams, 1969) and as a result may play a role in absorption and root exudation. Most studies of the mechanisms of ion uptake employ solutionculture grown roots. A mucigel layer, if present under these conditions, may support a microbial population different from that of roots grown in soil and, hence, an erroneous understanding of absorption may result. VIII. Effects of Environment on Root Exudation

Definitive investigations of effects of environmental factors on root exudation are relatively few. There is some qualitative value in examining situations in which indirect evidence involving the rhizosphere effect

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M. G. HALE, C. L. FOY, A N D F. J . SHAY

has been reported. These studies, however, have not resulted in precise quantitative data of the kinds and amounts of exudates released under differing conditions. They show only that changes in environment of the plant affect microorganisms in the vicinity of the root. It would be helpful to know and understand how temperature, light intensity and soil moisture affect the exudation pattern of various crop plants. It is to be expected that light and temperature, as they affect photosynthesis and translocation, would have a profound effect on the amounts and kinds of exudates released. From the limited information available it is difficult or impossible to develop concepts about effects of the variables on exudation because of lack of uniformity of conditions, age, species of plant and methods of analysis from investigator to investigator. Schroth and Hildebrand (1 964) reported that each of four investigators who studied the exudation of amino acids from oats reported different results. Ayers and Thornton (1968) stated that there are “widely differing techniques that have been used to grow plants for studies of root exudation, varying from simple culture of seedlings on filter paper or in culture tubes, to elaborate systems using sand or soil as the rooting medium. In some cases, no distinction is made between materials emanating from roots and seeds or other portions of the plant. Others exclude seed exudates by special absorbents. Likewise, collection techniques have varied in the use of a mineral solution or distilled water rinses, with the roots intact in the rooting medium or after they are mechanically removed from the soil, sometimes with the aid of a sieve and bristled brush.” It is not strange that a systematic study of environmental effects on exudation has not yet been undertaken. There are many difficulties to overcome before plants can be cultured under controlled environmental conditions and under the axenic conditions necessary to eliminate complications created by the presence of other organisms. Procedures must be available for harvesting exudates in large numbers of samples and large amounts, keeping them sterile until analysis and analyzing them quantitatively. Few investigators have access to all necessary facilities and techniques. Then too, the justifica