Tải bản đầy đủ - 0 (trang)


Tải bản đầy đủ - 0trang



nett, 1970; Frankel and Hawkes, 1975a) to preserve the older more variable germplasm and wild relatives, which began to be referred to as genetic

resources. The term “genetic resources” per se excludes breeding lines

and recently released varieties (Frankel and Hawkes, 1975b), which are

composed of gene combinations rather than the genes themselves. The

two symposia also thoroughly reviewed the need for immediate and systematic exploration and collection on a worldwide scale of genetic resources of food and other commercial crops.

An International Board for Plant Genetic Resources (IBPGR) was

formed in 1972 by the Technical Advisory Committee of the Consultative

Group on International Agricultural Research to undertake the plant collection and conservation recommended by the symposia. With the establishment of the IBPGR, collection and conservation of representative

samples of genetic variability in landrace populations were accelerated and

a large number of samples began to accumulate in the cold stores of the

major genebanks.

Genetic resources merely stored safely are of little value to plant

breeders unless they are evaluated and the resulting data made widely

available. Evaluation is, therefore, an essential link between conservation

and use. In fact, Frankel (1987) categorically stated that genetic resources

have been utilized without elaborate characterization, but never without


The next step was to evaluate the collected samples to identify sources

of useful traits in order that the material be better utilized than in the past.

The evaluation process for large collections follows several distinct stages:

(a) seed multiplication and preliminary evaluation; ( 6 ) systematic evaluation of the entire collection; and (c) further evaluation of selected accessions (Chang, 1985). The utilization aspect of genetic resources was recently reviewed by Brown et al. (1989), wherein factors that are likely to

limit or facilitate this process are discussed.

Genetic resources workers discriminate between “characterization”

and “evaluation” (Erskine and Williams, 1980; Hawkes, 1985), although

this fact is not widely known. Characterization is defined as recording

information only once on those traits that are highly heritable, easily

visible, and expressed in all environments, for example, grain patterns and

isozyme profiles. Characterization provides a standardized record of

readily observable morphological characters that, together with passport

(origin) data, identify an accession in the genebank. Evaluation, on the

other hand, is the assessment of more variable traits for potential use in

breeding, such as plant height, time to maturity, disease resistance, and

protein content. This is done in several ways: growing the material in

different environments, exposing it to various abiotic and biotic stresses,

assessing grain quality, and selecting the best lines for the desirable attri-



butes. “Preliminary evaluation” refers to screening that is frequently

carried out during multiplication, at a single location and without the use of

replicates prior to the incorporation of the samples into the collection

(Damania et al., 1983; Erskine and Williams, 1980). The information generated by characterization and evaluation not only improves utilization of

the germplasm but also rationalizes storage space by identifying duplicates

and eliminating redundant germplasm.

In spite of the wide recognition given to the importance of conserving

and evaluating genetic resources (Frankel and Bennett, 1970; Hawkes,

1971, 1983; Frankel and Hawkes, 1975a; Harlan, 1975; Plucknett et al.,

1983; Holden and Williams, 1984; Rogalewicz, 1985), little was known

about the variability in the primitive forms, old landraces, and wild relatives of cereals and much work still remains to be done.

Detailed evaluation of stored populations of different origins allows an

understanding of the patterns of variability. The population structure of a

species is defined as the totality of ecological and genetical relationships

among individual members that may coevolve as a result of gene exchange, but may also diverge under localized forces of evolutionary

change (Jain, 1975). Landraces and primitive cultivars are products of

many years of crop evolution, and it is vital to preserve their genetic

composition during and after evaluation. Cases have been reported where

polymorphic cereal populations have undergone radical changes in their

genetic composition in one growing cycle (Shevchuk, 1973). However, in

the case of samples collected from village markets or those that are subjected to biased sampling methods, it is sufficient to safeguard and maintain their genes and not necessarily their gene frequencies within populations.

It is now widely recognized that extensive surveys of geographic areas

for genetic variability and computerized documentation of the evaluation

results are needed for the utilization of large collections of cereal genetic

resources. The problems of describing geographic variability data and

development of statistical methods for categorizing sets of population

samples from diverse localities have been discussed by Gabriel and Sokal

(1969), and the analysis of variability between and within genotypes and

environments has been discussed by Freeman and Dowker (1973). Computers with appropriate statistical packages have greatly facilitated this

task of documentation.

This chapter reviews the status of evaluation and documentation of

genetic resources of wheat and barley. The work of a multidisciplinary

team of evaluators that include germplasm scientists, taxonomists,

cytogeneticists, pathologists, biochemists, and physiologists is presented, followed by conclusions and suggestions for future avenues of research.




The evaluation of cereal genetic resources collections goes back to the

time when their value to crop improvement began to be appreciated by the

breeders. Cultivated wheat has been the most extensively evaluated crop

among cereals, which is in keeping with the prominent position it enjoys in

terms of production tonnage and importance as a food crop.

Qualset and Puri (1973) evaluated heading time in a world collection of

durum wheat (Triticumdurum Desf.) and presented results for each geographic area from which samples were analyzed. They found a wide range

of heading time in about 3700 samples studies and identified those that

were highly photosensitive. Puri and Qualset (1978) also researched effect

of seed and seeding rate on yield and other characteristics of durum wheat

and found a positive correlation between large seed size and yield. In

another study, geographic patterns of phenotypic diversity for qualitative

traits of more than 3000 samples of durum wheats were evaluated in the

United States Department of Agriculture (USDA) world collection at

Tulelake, California, by Jain et al. (1975). Observations were recorded on

leaf sheath glossiness, glume pubescence and color, awn color, kernel

color, and basal spike node fertility. Variability for each character was

found usually within each geographical region. Although Jain et al. (1975)

admit that the collection was small and not representative, they found

centers of diversity among material from Ethiopia, India, and the Mediterranean countries. Some of the areas known to be important sources of

genetic resources were poorly represented in the study, a fact emphasizing

the need for intensification of efforts in exploration and conservation.

Spagnoletti Zeuli and Qualset (1987) have reported an evaluation study on

the same durum wheat entries for spike characters. Five clusters were

delineated among 26 country origins and an east to west clinal pattern was

detected that represented a gradient in unimproved to improved types.

In a previous study, Porceddu (1976) evaluated 2400 wheat landraces

from the world collection of durum wheat and recorded information on

growth habit, beginning of shooting, heading time, culm length, and number of elongated internodes. Distinct differences were found to exist

among samples from different countries. Multivariate analysis showed

that samples from Mediterranean countries were very similar, probably

due to old trade links. Similarity in other samples from Cyprus, Egypt,

Jordan, and Palestine was also noted. Another discovery was the significant similarity between accessions from Turkey and the United States.

This was attributed to the history of transport of germplasm from Turkey

and subsequent inclusion in U.S. varieties. Konzak et al. (1973) also

evaluated wheaL germplasm from Turkey for reaction to mildew at



Pullman, Washington, U.S.A., and estimated yellow berry content of the

grain produced. Porceddu and Scarascia-Mugnozza (1983) carried out

similar studies for ascertaining variability in landraces of durum wheat

from Algeria, Ethiopia, and Italy and found that there was clear separation

between Ethiopian and Italian material, but Italian landraces were more

variable among themselves than Ethiopian ones and material from Algeria

was more variable than both the other two. Hence, differences among

landrace populations from the same country were highly significant for all

1 1 characters studied, except kernel weight and spike density. Using

multivariate analysis also, Kosina ( 1980) evaluated structure and caryopsis quality of some hybrids of spring wheat.

Ehdaie and Waines (1989) described genetic variability in T. aestiuum

from Iran. They concluded that local landraces, such as those found in

Iran, could be improved by selection for shorter genotypes with fewer

tillers per plant, but with larger and heavier grains. Morphological and

physiological variability in T. aestiuum collected from Afghanistan was

also reported comprehensively by Tani and Sakamoto (1987).

There are traits, such as resistance to diseases and tolerances to certain

types of soils, for which variability can only be observed at particular sites.

Such traits are economically important and every effort must be made to

record and document them by carrying out evaluation at sites where the

incidence of that particular stress is the greatest, such as the so-called

disease “hot spots.” For example, for screening against resistance to

Septoria tritici (leaf blotch), ICARDA uses a humid and high-rainfall site

located on the Mediterranean coast in Syria in addition to artificial inoculation. For experiments on tolerance to salinity, a drought-affected site on

the shores of salt lake Jabboul in northern Syria is used. Jana et al. (1983)

evaluated 3000 durum wheat accessions from various countries at this site,

and 10 lines were found to be highly tolerant to combined stresses of

salinity and drought. However, it is known that salinity is highly variable in

the field and if experiments do not comprise several replicates, laboratory

confirmation with tests such as chlorophyll influorescence (Smillie and

Nott, 1982) may be used to help identify salt-tolerant lines.

Selected bread wheat and barley plants from landraces grown in Nepal

and Pakistan were examined and variability for certain qualitative traits

described by Witcombe (1975). Barley from Nepal was found to be more

variable than barley from Pakistan, whereas in the case of wheat the

reverse was true. Murphy and Witcombe (198 I ) further analyzed landraces

of wheat from northern India by growing single plants under glasshouse

conditions in Wales, U.K. Multivariate analysis of data on quantitative

traits was used to distinguish between introduced material and indigenous

germplasm on the basis of means recorded on single plants. This data

Tài liệu bạn tìm kiếm đã sẵn sàng tải về


Tải bản đầy đủ ngay(0 tr)