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VI. Documentation of Genetic Resources

VI. Documentation of Genetic Resources

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and effort would be saved if a lengthy description of the method and

intervals used for recording each character is avoided. The IBPGR has

been convening small working groups of scientists for the purpose of

arriving at an internationally agreed upon list of descriptors for describing

information for wheat (IBPGR, 1985) and barley (IBPGR, 1982). However,

experience has shown that it is too time-consuming to record observations

on all descriptors mentioned in the descriptor lists, as the method of

recording and the selection of characters are very much dictated by the

region and the needs of local breeders at each institution concerned. It is

also necessary to set out data in a standard format using a generally agreed

upon series of descriptors and descriptor states for the crop. In this way

the data can be entered into computers, retrieved, and exchanged among

institutions with the least possible confusion and optimum efficiency

(Hawkes 1985).

For the purpose of utilization, systematic analysis and description of

samples is useful in both distinguishing between populations and identifying duplicates, as well as in providing information on the extent of

variation within a given germplasm collection. It is axiomatic that the more

documentation on a collection, the greater the chance of its rational utilization. Information from the site where a particular sample was collected

may be extremely important. For instance, at ICARDA, germplasm that is

described as having a short maturity period receives immediate attention

of the breeders as this trait is very useful to escape drought and high

temperatures during grain filling in the dry areas. Therefore, information

recorded by germplasm collectors at site would be very valuable later

when the samples are evaluated.

Peeters (1988) studied statistically a large barley germplasm collection

at Cambridge and reported that despite extensive collecting activity in

recent years and subsequent exchange between countries, combinations

of characters have remained substantially different in germplasm by country gene pool. Material from the United States now contains more variability in toto than material from any other country. Subsequently, Peeters

and Martinelli (1989) used hierarchical cluster analysis to classify entries

from this collection according to their degree of similarity and concluded

that this statistical analysis procedure could be used as a tool to classify

entries to their respective gene pools even when no passport data are

available.

Often those responsible for entering data recorded at a collecting site

into a computer data base believe that lengthy descriptive notes made at

the collection site, for example, notes on disease observations or peculiarities of the habitat, are not relevant and hence should be omitted. Nothing

could be more erroneous. Although it is recommended that passport data



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must not be encumbered with vast amounts of morphologicaldescriptions,

it should certainly contain disease and habitat data and, more importantly,

comments from the farmers as to the useful features that distinguish their

material from the rest.

Inadequate passport data very often inhibit effective utilization of collected germplasm. It has been repeatedly pointed out to collectors and

genebank managers that passport data divulge extremely valuable and in

many cases the only available information on the ecological adaptation of

an accession and hence no effort should be spared to fill this important gap

in documentation of germplasm (Frankel, 1987).

Systematic description of samples for discrete traits has been limited to

cataloging the phenotypic variation because of constraints in relating genotype with phenotype. Quantitative morphoagronomic traits are also

currently used in characterization. These traits are controlled by several

genes, each contributing a small effect that is quite often blurred by the

environment. Consequently, the correlation between genotype and phenotype is obscured.

Certain evaluation studies have used ranking as a method of describing

results of economically important traits such as yield. This ranking may

change from one site to another for some quantitative characters such as

plant height and days to heading (Damania, 1983). Such unstable characters cannot be adequately described when studied at a single location.

Thus, the concept of multilocation testing becomes imperative.

We cannot commit ourselves to hard and fast rules regarding the selection of a representative sample, but it must be stressed that an evaluation

that partially covers the total variability can only be of limited value at

best. That is, if raw data are misinterpreted or incorrectly fed into the main

data base, self-consistency is lost and the entire task becomes futile.

Unfortunately, not all the samples assembled in our genebanks were collected with the aim of preserving genetic variability of populations in

danger of extinction. On the contrary, several genetic resources collecting

expeditions were targeted to filling certain gaps, such as finding resistant

lines to specific stresses or studying relationships between wild and cultivated species. Therefore, genetic material from such expeditions represents only a fraction of the existing variability present in a particular area.

This being so, it can provide useful genes for current breeding goals, but

may be inadequate for tomorrow’s needs (Porceddu, 1976).

The major collections of wheat and Barley now contain several thousand accessions. Such numbers may be too large for detailed evaluation.

In response to this, there is a recent trend toward developing “core

collections,” which are subsets upon which detailed evaluation work may



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be concentrated (Brown, 1989). The remainder of the large collection

constitutes a reserve still maintained in storage and available when a

desired trait cannot be found in the core (Chapman, 1989).

The ability of genetic resources managers to respond to requests from

breeders for material depends very much on the adequate description of

the accessions and the ability to query the information in a computerized

data base. Hitherto, insufficient emphasis has been placed on recording

passport data, and their absence is a major constraint to curators in assessing the range of variability in their collections and in identifying gaps

(Williams, 1989).



VII. SUMMARY AND CONCLUSIONS

Genetically uniform cultivars are employed by cereal farmers in the

major cereal-producing countries of the world. Because plant breeding is

essentially a process for exploitation of genetic variability, breeders could

also examine means of conserving already existing genetically variable

germplasm as well as creating new varieties. Mak and Harvey (1982) have

described the composite cross technique that creates, as well as exploits,

genetic variability, using the USDA barley world collection as a model.

This may be one of the ways to proceed for other cereal crops.

When precise objectives of evaluation are known at the outset, the task

becomes relatively simple. In the case of most wild and primitive forms,

evaluation aims to reveal potentially useful variability for direct use in the

breeding programs. This may necessitate initial characterization in

nurseries and cataloging of passport information, followed by a more

detailed field study in collaboration with the end-users of the germplasm.

Inferences regarding geographical variability, even on the basis of evaluation of a world collection, cannot be considered truly representative as

they represent findings based on the composition of a collection that may

be comprehensive for some regions and deficient for others. Furthermore,

variability studies based on collections made several years ago may not

accurately reflect the variability to be found at present in the same area. It

is presumed that, after observation of dramatic degradation of the environment and genetic erosion, there would be considerable decline in variability if not extinction of indigenous germplasm in several previously generich regions in the world (Hawkes, 1981). The utilization of germplasm

collections in crop improvement for the major cereals has revealed the

following:



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1. The use of exotic germplasm. The successful use of landraces and

wild species in cereal improvement has been more extensive in the developed countries, which lack indigenous germplasm, than in the less developed countries. In recent years the International Agriculture Research

Centers (IARC) have contributed substantially to generation and distribution of improved adapted germplasm with genes from landraces and wild

forms to the national programs and other institutions, as the germplasm

developed for more favorable environments has not succeeded in the dry

areas.

2. Constraints to the use of exotic germplasm. Many plant breeders

were reluctant to devote a greater part of their resources for the exploitation of landraces and wild species in the past. This was because the

potential value of these germplasms for the stressed environments was not

fully appreciated. However, in recent years, varieties targeted for lowinput, rain-fed agricultural systems possess genes from adapted landraces

and even direct usage of selections of the best lines isolated from landraces

have been recommended for release.

3. Support for plant genetic resources programs. Extensive use of landraces, primitive forms, and wild species will be more tenable for harsh

environments when the process of conservation, evaluation, documentation, and exchange of germplasm is strengthened and adequately funded.

Donor countries and international agencies could increase support for

utilization of indigenous landraces and primitive forms in the recently

established breeding programs of the developing world.

4. Use of computers and statistical program packages. Computer programs designed for analyzing a large quantity of evaluation data have

greatly reduced the time and effort needed for arriving at tangible results.

This in turn has led to the publication ofgermplasm catalogs, which have

facilitated dissemination of information on genetic resources collections to

actual users, allowing for greater utilization of the services rendered by

genebanks. However, breeders prefer to receive a short list of accessions

with specific traits to choose from rather than large genetic resources

catalogs.



Electrophoretic techniques that permit rapid mass screening of samples

are increasingly recognized as powerful research tools for the study of

genetic variations in populations. A wider application of gel electrophoresis in the evaluation of plant genetic resources is expected. The use

of restriction fragment length polymorphisms (RFLP) for evaluating genetic diversity has been described (Bernatsky and Tanksley, 1989), and is

the best available means for detecting differences at the DNA level on

samples of reasonable size. It would be useful if such techniques were

utilized to a greater extent than at present on genetic resources collections.



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The breeding objectives of the developed countries, mostly located in

favorable environments, are different from those of the West Asia and

North Africa region. The advanced breeding programs in the former

countries began utilizing exotic landraces about 100 years ago and have

fully exploited them, so they no longer seek variability but only single

genes from the wild relatives and grasses of the tertiary gene pool. The

breeding objectives for West Asia and North Africa, on the other hand, are

to develop varieties adapted to withstand harsh environments and low

inputs. Hence, selections from landraces and crosses with wild and pnmitive forms are undertaken to produce well-adapted germplasm for targeted

agroecological zones.

In recent years, world collections of cereals have been evaluated by

many scientists working in different countries who were searching for

economically useful genes or gene combinations. Confidence has been

expressed that such materials are a usable source of breeding stocks,

although they still require thorough assessment. Large-scale evaluation, if

carried out thoroughly, is an expensive, arduous, and time-consuming

process. Therefore, it is imperative to carefully select the traits that one

wishes to evaluate in consultation with the breeders. Further, not all

material in a collection may be of immediate interest. Priorities need to be

discussed and selection of the traits made on the basis of their importance

to the actual user. Such a procedure will assure optimal utilization of

physical facilities, manpower, and financial resources. Finally, breeding

objectives change, sometimes rapidly, and hence evaluation needs to be

adaptive to a certain extent to succeed.

ACKNOWLEDGMENTS

The author thanks Drs. W. Erskine, S. Ceccarelli, and J . Valkoun for their comments and

suggestions on a draft of this review and the Government of ltaly for financial support.



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ADVANCES IN AGRONOMY, VOL. 44



MODELING CROP ROOT GROWTH

AND FUNCTION

Betty Klepper and R. W. Rickman

United States Department of Agriculture

Agricultural Research Service

Columbia Plateau Conservation Research Center

Pendleton, Oregon 97801



I . Introduction

11. Early Models

111. Desirable Model Features

1v. Model Components

A. Root Classification

B. Root Growth Parameters for Modeling

C. Root-Soil Relationships in Growth

D. Root-Soil Relationships in Uptake Functions

V. Some Existing Root Growth and Function Models

VI. Limitations to Development of Root Growth Models

References



I. INTRODUCTION

Many crop growth models that quantify plant uptake of water and

solutes from soils require a quantitative description of the root system and

its location in the soil profile. They require information like root length

density distribution with depth and changes in that distribution over time.

To sample the soil-root system and measure its properties over time in the

growing season is labor-intensive. Therefore it is useful to have models

that relate the generation of new root material and the decay of old roots to

plant properties and to soil conditions at various profile depths. Furthermore, root growth and death models are needed for use in calculating

fluxes of carbohydrates into below-ground organs for the detailed, physiologically based crop growth models presently being built. Such information as numbers and locations of root meristems, elongating zones, and

aging zones, with their associated rates of water and nutrient uptake and

their suites of exudates, are needed for interfacing to models that will

eventually be written to describe rhizosphere microbial dynamics and



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