1. Plant Genetic Resources: General Perspective - R.S. Paroda and R.K. Arora

Introduction
Change from nomadic life to settled agriculture
Dynamics of plant domestication
Regions of crop plant diversity
Spectrum of genetic resources
Value of crop plant diversity
Threat to genetic diversity
Conservation of genetic diversity
Summary
References
Appendix I. World centres of diversity of cultivated plants (Hawkes, 1983)
Appendix II. Cultivated plants and their regions of diversity (refer Fig. 3)

Introduction

Plant genetic resources are the most valuable and essential basic raw materials to meet the current and future needs of crop improvement programmes. A wider genetic base, thus, assumes priority in plant breeding research aimed at developing new varieties for increased crop production. This diversity comprises native landraces, local selections, elite cultivars and wild relatives of crop plants. The collection and conservation of this diversity in a systematic manner is the primary responsibility of all plant genetic resources institutes/centres: In fact, in the national context, in all those countries which hold native genetic diversity, the scientists, planners and policy makers have a challenge as to how best the existing genetic diversity can be preserved and utilised for the benefit of mankind. Plant genetic resources are thus our heritage which need conservation for posterity.

Man's interest in agriculture started about 10,000 years ago and, during this long period, transition from 'gathering' to 'growing' of plants occurred. In this process, a wide array of crop variability got generated by natural means and through both conscious or unconscious selection. Gradually, a new wealth of variability also got generated/adapted and diversified by crop introductions in the exotic environment or through migration of human population. Associated with this process was the keenness of human mind to explore the rich global diversity of plant wealth in general so as to judiciously tap the potential of useful flora.

Historically, mankind has used only about 5,000 plant species worldwide to meet food and other needs. This number is just a fraction of the total world flora. With population growth, we are increasingly dependent on most productive plants. Today, only about 150 plant species are important in meeting the food (calories) needs of humans worldwide. The short list of the most commonly used foods is what stands between us and the estimated worldwide carrying capacity in pre-agricultural times (Wilkes, 1984). This short list contains:

Cereals	: Wheat, rice, maize, barley, oats, sorghum, millets, rye.
Oilseeds	: Coconut, cotton seed, sunflower.
Legumes	: Soybean, peanut, common beans (Phaseolus spp.), pea, chickpea, cowpea.
Root crops	: Potato, sweet potato, cassava, yam and taro.
Sugar crops	: Sugarcane, sugarbeet.
Vegetables	: Tomato, cabbage, onion, squash.
Fruits	: Banana, orange, apple, pear, melon, mango.

The cereals represent twice the tonnage of all the other crops (if the potato with its high water content is discounted) and nearly three times the calories. Both tea and coffee are major world crops but these caffeine containing beverages are not included because they add essentially no calories to human nutrition. This list is primarily a calorie list and does not recognise the important role of low calorie vegetables and fruits in supplying vitamins, minerals and proteins to human nutrition. The list also does not include regional foods that locally may supply more than half of the calories consumed. In addition, pasture forages and fibre crops have been omitted (Wilkes, 1984).

Hence, greater dependence on fewer plant species, 20 to 30 in global context (Harlan, 1975), gradually, has resulted in the loss of native genetic resources which are otherwise essential as building blocks of genetic diversity. In order to safeguard and conserve this diversity, the large share of which is held by the developing world, different countries/national and international organisations have in recent past shown their concern for proper conservation of genetic resources. The cultivated plants truly form the very basis of modern civilization and their domestication in historical context constitutes the agricultural revolution, the signs of which are still visible in traditional cultivation sites in the developing world. In this chapter, emphasis is laid on some of these findings/concepts relating to the origin of agriculture, the process of plant domestication and the geographical regions where genetic richness in plant resources occurs. Further, the importance to safeguard and conserve such- genepools has been highlighted.

Change from nomadic life to settled agriculture

To quote the great scientific philosopher, Dr. Jacob Bronowski, 'the genesis of cultivated plants is a true fairy tale of genetics as if coming of civilization had been blessed in advance by the spirit of the abbot Gregor Mendel'. The change from nomadic life to settled agriculture has had a fascinating past. What were the forces that led to domestication of plants? Where were the sources/centres which provided the initiation of agriculture? When did the science of plant breeding begin and blossom to guide the crop evolutionary processes through man's intervention? Why does agricultural research give so much emphasis on the importance of crop plant diversity? These are some of the basic questions, answers to which can apprise the students of botany and agriculture of the value of plant genetic resources our 'Mother Earth' is so greatly endowed with. These points have been discussed in the context of plant genetic diversity, largely in the Indian perspective.

Conclusions drawn from circumstantial evidences show that agriculture started about 10,000 years ago. A well known hypothesis for the origin of agriculture is the Rubbish-Heap hypothesis. It says that early humans gathered nutritious roots and seeds for their food. Such plants actively colonized the bare areas around their dwellings, which were rich with the discarded rubbish. This natural process was obviously based on man's known food gathering activities and selection of only those useful plants which he found most suitable in tune with local habitats.

The plants selected by man must have been preadapted to agriculture and they must be of particular interest to man because of their large food reserves. Three stages seemed to have operated in this continuous process. In the first stage, the preadapted wild plants with weedy tendencies and large reserves of food began to colonize the open ground or kitchen middens around man's dwellings. Also, people probably accidentally dropped seeds from the natural habitats on the same ground and thus reinforced the process. In the second stage, perhaps the seeds were regularly harvested from the plots around the dwellings, and fenced to protect them from domesticated cattle and wild herbivores. At this stage, the early man was also selecting mutants for increased yields, palatibility and other desirable traits, and the plants might have evolved a series of better adapted types that were able to take advantage of the richer soil conditions. The third stage of sowing/placement of seeds in soil at the right time and carefully guarding them at all stages until harvest must have come very late at a time when man really began to know his plants. This stage marks the end of pre-agricultural phase and perhaps led to considerable increase of human population as a result of increased food supplies. From here, the human families would have begun to move away from their original areas, eventually reaching regions where the wild progenitors of crop plants no longer grew. These families could continue to harvest the food plants in the new region only if they had taken some seeds with them. As the population continued to spread, the new tradition of keeping and saving seeds, a tradition born out of necessity, may have been established. This explanation can obviously account for both seed and tuber/root crops (Hawkes, 1983; Paroda et al, 1986).

Dynamics of plant domestication

Domestication is an evolutionary process operating under the influence of human activities. Being evolutionary, obviously it is relatively a slow process and exhibits gradual progression from the wild state to a state of incipient domestication. Diverse forms that differ more and more from their progenitors develop. The possible changes in plant species (in different directions) due to domestication are listed below:

1. Adapting to a greater diversity of environments and a wider geographical range;

2. Different/specific ecological preference;

3. Flowering and fruiting simultaneously/uniformly;

4. Lack of shattering or scattering of seeds and sometimes may have lost the dispersal mechanism completely;

5. Increased size of fruits and seeds which often reduces the dispersal efficiency;

6. Change from a perennial to annual habit;

7. Loss of seed dormancy;

8. Loss of photoperiodic response;

9. Lack of normal pollinating organs;

10. Change in breeding system (may result from a change in flower morphology, or a change from self-incompatibility to self-compatibility);

11. Loss of defensive adaptations, such as hairs, spines, thorns, etc.;

12. Loss of protective coverings and sturdiness;

13. Improvement towards palatability, chemical composition rendering them more likely to be eaten by animals;

14. Increased susceptibility for diseases and pests;

15. Developing of seedless parthenocarpic fruits;

16. Multiplied vegetatively.

For prominent characters, the evolutionary changes that take place during plant domestication are listed in Table 1 (Hawkes, lecture delivered at the NBPGR, 1978).

Table 1. Evolutionary changes during plant domestication

Character		Wild forms	Domesticated forms
1. Viability in competition with other species		Good	Poor
2. Food reserves (size of fruit, etc.)		Medium, not very succulent	Large, succulent
3. Variability of storage organ used by man (size, shape, colour)		Small	Great
4. Physiological adaptation		Medium to narrow range	Wide range
5. Dispersal mechanisms such as
	(a) Rachis or rachilla of cereals	Brittle	Non-brittle
	(b) Stolons in potato	Long	Short
	(c) Explosive dehiscence (legume-pods etc.)	Present	Absent
	(d) Pores for seed dispersal (Papaver somniferum)	Present	Absent
	(e) Cereal awns	Present	Absent or reduced
6. Protective devices, such as
	(a) Thorns or spines	Present	Absent
	(b) Bitter or poisonous flavour	Present	Absent
	(c) Dense indumentum of hairs	Present	Absent
7. Sexual reproduction (as in potato, sweet potato, etc.)		Present	Absent or reduced
8. Habit		Perennial	Annual
9. Seed germination uniformity		Not synchronous	Synchronous and uniform
10. Breeding mechanism		Outbreeding	Inbreeding

Cultivation practices adopted by man also had a significant role in the domestication process, the cultivated field presenting a different environment from the wild habitat. The crop evolutionary process obviously includes changes as affected by changed environment of a cultivated field. Not only this, the selection pressure associated with cultivation practices also results in production of weedy races (plants which are competitive with cultivated races but retain some important characters of the wild races) and consequent occasional crossing between the two, leading to a setting up of differentiation-hybridization cycle and release of more potential variability. This is one remarkably elegant evoloutionary process wherein barriers to geneflow maintain identity of the two types and, at the same time, limited exchange of genes releases variability. Deliberate selection practices by man from the released variability provide a new order of selection pressure making the population an array of deliberately chosen components. The total potential range of variation is also fragmented into landrace populations or primitive cultivars. Different cultivars are grown for different purposes or to fit different ecological niches of the agricultural system. For example, the man selects glutinous and non-glutinous rice, long and short grained rice, aromatic rice, etc. In maize, the man's selection is for popping, boiling, eating tender cob, flour quality, etc.

Broadly, three factors that operate in the selection process of domesticated species are: (i) selection of desirable traits by the cultivator while sowing; (ii) changed micro-environment through cultivation practices; and (iii) differentiation - hybridization cycles between crop-weed pairs and man's selection from them. So the dynamics of domestication has resulted in great morphological changes without substantial change in the genetic background. However, speciation rarely occurs under domestication. Under domestication, modification(s) induced ultimately lead to the end products which are generally radically different in appearance from their wild progenitors.

Thus, most domesticated plants and all the food plants are, by and large, the product of a long selection process. As already elaborated, we call this evolutionary process as domestication. In the process of domestication, food plants have quite literally crossed a threshold. Their survival is keyed to preparation of the ground, decreased competition with other plants, sowing of the seed in the proper season, protection of plants during their growth and finally collecting seed. Thus, the process of domestication has 'tamed' these plants to make them dependent on humans.

On the global scene, the human population has enormously increased such that we are held captive by our domesticated food plants, that is we are totally dependent on the high yields of these few selective cultivated plants. By and large, a dependent, though viable relationship exists among plant domestication, genetic diversity and human population growth (Wilkes, 1984; Fig.1).

Fig. 1. Plant domestication, genetic diversity and human population growth (Wilkes, 1984)

Regions of crop plant diversity

Nuclear centres and regions of diversity

A study of the origin of agriculture/cradles of agriculture and the spread of agriculture provides clues to the geographical distribution of centres of plant domestication. These centres would obviously be located in areas where maximum diversity of crop(s) is found. Only later, the spread of agriculture could have taken place to more regions of crop plant diversification. de Candolle (1886) was perhaps the first who indicated about such regions where the initial plant domestication might have taken place. The Russian scientist, Nicolai Ivanovic Vavilov (1951), later indicated the cradles of agriculture in a more elaborate manner. He believed that time is the only factor that influenced the dispersal of a species and its increase of variation. Based on this hypothesis of presence of greatest diversity, he named eight centres where agriculture developed independently. He identified these as 'Centres of Origin' of crop plants (Fig. 2). Also, these centres were characterized by the accumulation of dominant genes in the centre and the recessive genes in the periphery. Appendix I lists crop plant species that originated in these centres.

Fig. 2. The eight centres of origin according to N.I. Vavilov (Harlan, 1971)

The criteria used by Vavilov in determining the centre of origin for a crop plant/species were (Hawkes, 1983):

1. Differentiating plants into specific and infraspecific taxa on morphological and genetic basis;

2. Determining the area of distribution of such species and groups of species;

3. Establishing the distribution of genetic diversity and determining the geographical centres where this is at its maximum, the centre where diversity of genetically allied species is concentrated, especially those centres with endemic forms/characters;

4. Correlating the above distribution/diversity with the areas of concentration of nearest wild relatives;

5. Comparing centres of origin of group of cultivated plants with certain specialized parasite(s), and;

6. Support the above, seek linkages/evidences from archaeology, linguistics and history.

Nuclear centres and regions of diversity

Megagene centres
Centres and non-centres

Definitely, sites of early farming which were discovered through efforts of archaeologists can substantiate the presence of a cradle of agriculture at a place. Such sites of early farming have been discovered in Thailand (11,000 B.C.), Near East (9,000 B.C.) and Mexico (6,000 B.C.). Hawkes (1983) suggested that generally agriculture began not once but several times, more or less simultaneously and in different regions of the world. His concept clearly envisaged centres of agricultural origin from which farming spread into one or more regions for which he proposed the name 'Nuclear centres and regions of diversity'. He linked the nuclear centres with the archaeological evidence to provide strong proofs of agricultural origins. According to him, the following nuclear centres and regions of diversity occur (Table 2).

Table 2. Nuclear centres and regions of diversity of domesticated plants (Hawkes, 1983)

Nuclear centres	Regions of diversity	Outlying minor centres
A. Northern China	I China	1 Japan
	II India	2 New Guinea
	III South-East Asia	3 Solomon Islands, Fiji and South Pacific
B. The Near East	IV Central Asia	4 Northwestern Europe
	V The Near East
	VI The Mediterranean
	VII Ethiopia
	VIII West Africa
C. Southern Mexico	IX Meso-America	5 United States, Canada
		6 The Caribbean
D. Central toSouthern Peru	X Northern Andes	7 Southern Chile
	(Venezuela to Bolivia)	8 Brazil

There are several regions where crops actually did not originate. In these regions, there is paucity or absence of wild progenitors and absence of archaeological remains and such other data, to suggest antiquity of a crop species. These are the regions into which the crops perhaps spread from the nuclear centres in the past and where further spatial isolation in time and intensive human selection played a prominent role in the increase of genetic diversity (Hawkes, 1983). Such regions are listed in the second column of Table 2.

Hawkes (1983) further identified small 'minor' centres for several crops e.g. (i) New Guinea - Sugarcane (Saccharum officinarum) and (ii) Solomon Isles to Fiji for Musa species. These are listed in the third column of Table 2.

In distinguishing the nuclear centres of agricultural origins from Vavilov's broader 'centres', Hawkes has tried to combine his views with other authors contributing to the knowledge of origin of agriculture and centres of diversity. The contributions of Zhukovsky and Harlan, in particular, are important to mention.

Megagene centres

Zhukovsky (1965), a close associate of Vavilov, proposed 12 megagene centres of crop-plant diversity. The new areas added to Vavilov's eight centres were Australia, whole Africa and Siberia followed by revision of the boundaries to make 12 centres. Microgene centres of wild growing species related to our crop plants, where the cultigens first originated, were also demarcated. Zeven and Zhukovsky (1975) have dealt this elaborately in the 'Dictionary of cultivated plants and their centres of diversity', listing species for different megagene centres, and the range and extent of the distribution of genetic/varietal/specific diversity, etc. In a further revised version of this book, Zeven and de Wet (1982) prefer the term region to centre. These twelve regions have by and large wider coverage and more acceptability (Fig. 3). Appendix II lists the prominent crop plants for which rich diversity occurs in these 12 regions.

Centres and non-centres

Harlan (1971) recognised only three main 'Centres', each with a more or less connected but large and diffuse non-centre. These are given in Table 3 and Fig. 4.

Table 3. Harlan's centres and non-centres (Harlan, 1971)

Centre	Non-centre
North Chinese Centre - B1	South-East Asian and South Pacific non-centre - B2
North East Centre - A1	African non-centre - A2
Meso American Centre - C1	South American non-centre - C2

Harlan also recognised smaller areas/pockets of varietal and/or racial diversity within a Vavilovian Centre, and he termed these as 'Micro centres'. Such small areas, as in Turkey and Africa (Harlan, 1975), contain varietal diversity of several crops in the plains and/or mountains.

Spectrum of genetic resources

Fig. 3. The twelve mega centres of cultivated plants (Zeven and Zhukovsky, 1975)

Fig. 4. Centres and non-centres of agricultural beginnings (Harlan, 1971)

Fig. 5. The spectrum of plant genetic resources (Chang, 1985)

Figure 5 depicts the full spectrum of genetic resources that can be found in a cultigen of great diversity and ancient agricultural origin, such as rice on wheat (IBP, 1966; Creech and Reitz, 1971; Chang, 1976, 1985). The major categories may be briefly described (Chang, 1976).

1. Related wild species and weed races in the same genus, and related genera found in the regions of primary or secondary diversity. This class may include undomesticated wild species which are consumed by man as food or for other uses, e.g. forestry species, medicinal plants and pasture species.

2. Unimproved landraces (folk varieties) and special purpose types from the areas of diversity which are adapted to specific ecological niches or provide special dietary/religious needs. Inherent diversity is a unique feature of the landraces. Many landraces are varietal mixtures.

3. Pure-line selections or open-pollinated commercial varieties from old agricultural areas where production levels remain largely unchanged in the last half century.

4. Obsolete varieties which can be found only in germplasm collections. Ecostrains of obsolete cultivars may persist in other areas.

5. Advanced cultivars, modern elite varieties (HYVs) and F₁ hybrids developed by scientific breeding and grown in areas of modern intensive agriculture. Composites and synthetics evolved through plant breeding also belong to this class.

6. Other products of plant breeding programme or genetical studies, which include breeding lines, breeding stocks, mutants, gene markers, genetic stocks, induced polyploids, aneuploids, intergeneric and interspecific hybrids, and cytoplasmic sources.

Value of crop plant diversity

For plant breeders, in their endeavour directed towards increased agricultural production, there is a pressing need for more genetic diversity to work upon, to cater to varied kinds of problems and needs. The wider the range of choice a breeder will have in selecting the appropriate kind of diversity, the better will be the chances for his success for any particular goal. Earlier the breeders were content to go not much further for their material than the old landraces and varieties that were then available in their own countries or from neighbouring ones, but in the last 50 years or so, breeders are requiring a much wider range of genetic diversity. The situation is particularly serious when developed varieties get immediate popularity. Here, the crops possess a 'narrow genetic base', and uniformity holds a hidden danger for it can offer an open doorway to attacks by pests and pathogens. With a uniform crop variety or a range of rather similar varieties, once a strain of a pathogen becomes adapted to attack it, the whole crop can be lost in a very short time. For example, in the United States, a single source of cytoplasm that had been used in developing the majority of the corn belt hybrids, also transmitted cornblight susceptibility to all other varieties that contained it. The vulnerability of these maize varieties became evident in 1970 when the advent of southern corn blight led to the widespread destruction of corn crop.

The wealth of landraces, which could not even be called varieties and which the farmers grew earlier, are gradually disappearing. A collection and study of these has revealed that these exhibit many useful characteristics, and can help in crop improvement programmes. Similarly, many wild relatives of crop plants can be useful in present day breeding programmes. One can thus understand the dire need for conservation of plant genetic diversity. Its value in the future will be much more than what can be imagined at present, considering the diversified crop improvement programmes, technologies and human needs. Table 4 highlights this, and the genetic composition and productivity level of different gene sources (Chang, 1985).

Table 4. Genetic composition, productivity level and potential value in breeding of different gene sources (Chang, 1985)

Group	Diversity within group	Homogeneity within a strain or population	Agronomic or commercial value	Genetic potential in breeding
Modern elite cultivars	Low to moderate	Very high	Very high	Moderately high
Principal commercial types	Moderately low to moderate	Moderate to high	Moderately high	Moderate
Minor varieties	Moderately high to high	Moderately low to moderate	Moderate	Moderately high
Speciality types	Moderately low to moderate	Moderately high	Moderately low	High
Obsolete types	Moderate to high	Moderately high to high	Moderately low to moderate	Moderately low
Breeding stocks	Moderately low to moderately high	Moderate for lines; low for bulks	Most variable	Moderate to high
Mutants	Moderately low to moderate	Moderately high to high	Mostly low; few moderately high to high	Mostly low
Primitive types	Moderately high to high	Low to moderate	Moderately low	Moderately high to high
Weed races	Moderately high to high	Low to moderately low	Low	Moderate to moderately high
Wild species	Moderately low to moderate	Low to moderately low	Very low	Moderate to high

Threat to genetic diversity

Three processes that affect the crops that are cultivated now, i.e., genetic erosion, genetic vulnerability and genetic wipeout are not mutually exclusive but are, in fact, interlocked by the demands of an increasing human population and rising expectations. It is necessary thus to focus here on these processess as elaborated by Wilkes (1984).

Genetic erosion: The technological bind of improved varieties is that they eliminate the resource upon which they are based. Over the past 10,000 years, crop plants have proliferated to an innumerable number of locally adapted genotypes. These landraces and folk varieties of indigenous and peasant agriculture have been the genetic reservoir for the plant breeder in crop improvement. Suddenly this genetic diversity is being replaced with a relatively small number of varieties bred for high yields and other adaptations necessary for high input agriculture. In addition, the scarcity of land is forcing changes in land use and agricultural practices resulting in the disappearance of habitat which harbour the wild progenitors and weedy forms of our basic food plants. As a result of these two trends, there is urgent need to collect and conserve the diverse genetic materials that remain. In a world where per capita resources are decreasing as the human population grows, the concept of a sustainable future is becoming increasingly more important. Biological diversity is one of the components of any sustainable future that includes human beings. Safeguarding this resource through scientific management should be our goal.

Genetic vulnerability: Genetic vulnerability is the risk of high input agriculture with commercial food crops varieties typical of developed nations. Genetic erosion, the gradual persistent loss of plant genetic diversity, is most typically but not exclusively, a phenomenon of landraces in developing nations. Genetic vulnerability is the 'thin ice' of a narrow genetic base. Never before have there been such widespread monocultures (dense, uniform stands of billions of plants) covering thousands of acres, all genetically similar. The narrowness of the genetic base is responsible for greater risk of crop failure as occurred in the wheat stem rust of 1954 or the southern corn blight of 1970 in the US. The Irish potato famine in the 1840s is a classic example of genetic vulnerability.

Genetic wipeout: The third threat to crop plant germplasm is the rapid and wholesale destruction of a wealth of potential species constituting genetic resources. Social disruptions/instability can eliminate such promising diversity.

Quite literally, as Wilkes (1984) points out, the genetic heritage of a millenium in a particular valley can disappear in a single bowl of porridge if the seeds are cooked and eaten instead of saved as seed stock. Equally dramatic is the discarding of a genetic collection because a curator retires or the collection is no longer of use to the institution. These delicate points need to be taken care of, should such eventualities arise and germplasm fully safeguarded. Institutional arrangements by which the perpetuation of genetic collections can be coordinated nationally, regionally and internationally need to be further streamlined.

Conservation of genetic diversity

Conservation systems/programmes

Genetic conservation programmes are directed towards the long-term preservation of genetic resources either in-situ or ex-situ so that the potential for continuing evolution or improvement would be sustained. Thus, conservation is more inclusive than preservation, the latter provides only for maintenance but not for evolutionary modifications under different environments (Chang, 1985).

It is impractical to conserve all the available gene resources and the primary goal should be to conserve as many representative samples of existing germplasm as human resources permit. Priority should be given to those being threatened by extinction or displacement and genetic erosion. The targets of conservation may be one or more of the following: (i) a nucleotide pair - a mutation, (ii) a desirable or favourable allele of a gene, (iii) a gene - complex controlling a desirable trait, (iv) a co-adapted gene complex, (v) a chromosomal segment, and (vi) a cytoplasmic component or components. Genetic conservation partly covers the known genes and partly aims to save plant materials of yet unknown genetic potential, such as the populations of threatened landraces/rare or endemic taxa of wild relatives. The latter category must necessarily be the primary responsibility of the conservationists because of the large number of species included. Germplasm is non-replaceable and its protection must be safeguarded.

Conservation systems/programmes

A comprehensive genetic conservation programme for a given crop using ex-situ preservation should include the following components (Chang, 1985): (i) survey, (ii) acquisition/exploration and collection, (iii) maintenance/multiplication/rejuvenation, (iv) evaluation, (v) documentation, (vi) distribution/exchange, (vii) preservation, (viii) training, and (ix) collaborative network. Plant quarantine facilities and measures should form part of a large genetic resources centre. Methods of conserving plant germplasm were classified by Frankel and Soulé (1981) as follows:

1. Conservation in-situ

(a) Protection of vast tracts to conserve plants and animals in entire biomes - extinction of species is deterred, but this has little impact on useful plants.

(b) Wild species in natural communities.

(c) Domesticates - landraces in their areas of cultivation.

2. Conservation ex-situ

(a) Domesticates in mass reservoirs (Simmonds, 1962) or genetic reserves (Dinoor, 1978) under the dynamic type.

(b) Forest reserves (provenances) in the wild.

(c) Conservation of seeds, plants, plant parts, cells, tissues and meristem cultures under a static environment (genebanks). Botanical gardens or parks also belong to this category as the last line of defence, but they also generate useful information and provide educational benefit.

Base collections

Seeds of 5 - 6 percent moisture content are sealed and stored between -10 to -20°C for long-term conservation; the scope is generally comprehensive; a centre holding such a collection assumes responsibility for viability tests records and other associated matters. The seeds are distributed not for use but for rejuvenation (regeneration). A duplicate site is needed for security.

Active collections

Seeds are dried to 8 ± 1 percent moisture content, sealed and stored in medium-term storage (slightly above 0°C), from which samples are drawn for regeneration, multiplication and distribution, evaluation and documentation. Active collections complement the base collection and are often associated with plant breeding or plant quarantine stations. Records on origin, handling and distribution of accessions should be maintained.

Working collections of plant breeders and other disciplines are not considered part of the genetic conservation system. Collections of parents and other breeding materials are generally held in short-term storage and their maintenance is subject to the whims and fancies of the breeder (Reitz, 1976; Wilkes, 1983). The evaluation data should be made available to other interested workers for use in breeding programmes. Therefore, promising breeding materials, such as F₁ hybrids, F₂ and F₃ bulks, composite crosses, etc. should also be carefully stored for a period of time in order to enable other breeders to be able to have access to the materials, if and when needed (Jensen, 1962).

Summary

The importance of plant genetic resources as basic materials for crop improvement has been highlighted. The circumstances leading to settled agriculture, and the dynamics of plant domestication resulting into changes in plants from wild to cultivated forms has been discussed. A broad picture has been presented on the centres of origin/diversity in crop-plants and recent views on this topic are expressed. The spectrum of genetic diversity covering different categories of genetic resources is indicated. The importance of crop plant diversity for increased food production is stressed, both in terms of its collection and conservation. The concern on genetic uniformity and genetic vulnerability vis-a-vis genetic erosion has been emphasized. Finally, the subject of genetic conservation has been introduced, both for in-situ and ex-situ systems.

References

Candolle, A. de. 1886. Origin of cultivated plants. Haffner, New York.

Chang, T.T. 1976. Manual of genetic conservation of rice germplasm for evaluation and utilisation. IRRI, Los Baños, Philippines.

Chang, T.T. 1985. Principles of genetic conservation. Iowa State Journal of Research 59: 325-348.

Creech, J.L. and L.P. Reitz. 1971. Plant germplasm now and for tomorrow. Adv. Agron. 23: 1-49.

Dinoor, A. 1978. Germplasm preservation. Plant Genetic Resources Newsletter. 34:18-19.

Frankel, O.H. (Ed.). 1973. Survey of crop plant resources in the centres of diversity. FAO/IBP, Rome.

Frankel, O.H. and M.E. Soulé. 1981. Conservation and evolution. Cambridge University Press, Cambridge.

Harlan, J.R. 1971. Agricultural origins: centres and non-centres. Science 174: 468-474.

Harlan, J.R. 1975. Crops and man. American Society of Agronomy, Madison, Wisconsin. 295 p.

Hawkes, J.G. 1983. The diversity of crop plants. Harvard University Press, Cambridge, Mass. 184 p.

IBP. 1966. Plant genepools. International Biological Programme Newsletter. 5: 48-51.

Jensen, N.F. 1962. A world germplasm bank for cereals. Crop Sci. 2: 361-363.

Paroda, R.S., S. Mauria and R.K. Arora. 1986. From wild plants to present day crop varieties - the continuum of plant genetic resources. J. IARI PG School. 9 (2): 9-18.

Reitz, L.P. 1976. Improving germplasm resources, pp. 85-97. In Agronomic research for food (Ed., F.L. Patterson). American Soc. of Agronomy. Spl. Publ. 26, Madison, Wisconsin.

Simmonds, N.W. 1962. Variability in crop plants: its use and conservation. Biol. Rev. 37: 442-465.

Vavilov, N.I. 1951. Phytogeographical basis of plant breeding. The origin, variation, immunity and breeding of cultivated plants (K. J. Choster, translator). Chronica Botanica. 13: 366 p.

Wilkes, G. 1983. Current status of crop plant germplasm: CAC critical review. Plant Sci. 127: 137-181.

Wilkes, G. 1984. Germplasm conservation towards the year 2000. Potential for new crops and enhancement of present crops. In Plant genetic resources: a conservation imperative (Eds., C. Yeatman, D. Kefton and G. Wilkes). American Assoc. for the Advancement of Sciences. Washington D.C., USA.

Zeven, A.C. and J.M.J. de Wet. 1982. Dictionary of cultivated plants and their regions of diversity. PUDOC, Wageningen. 259 p.

Zeven, A.C. and P.M. Zhukovsky. 1975. Dictionary of cultivated plants and their centres of diversity. PUDOC, Wageningen. 219 p.

Zhukovsky, P.M. 1965. Genetic and botanical irregularities in the evolution of cultivated plants. Genetika Mosc. 1. 41-49 (in Russian, English summary).

Appendix I. World centres of diversity of cultivated plants (Hawkes, 1983)

1. China

Avena nuda, naked oat (secondary centre of origin)
Glycine max, soybean
Vigna angularis, adzuki bean
Phaseolus vulgaris, common bean (recessive form; secondary centre)
Phyllostachys spp., small bamboos
Brassica juncea, leaf mustard (secondary centre of origin)
Prunus persica, peach
Citrus sinensis
Sesamum indicum, sesame (endemic group of dwarf varieties; secondary centre)
Camellia (Thea) sinensis, China tea

Oryza sativa, rice
Eleusine coracana, African millet
Cicer arietinum, chickpea
Vigna aconitifolia, moth bean
Vigna umbellata, rice bean
Macrotyloma uniflorum, horse gram
Vigna unguiculata, asparagus bean
Solanum melongena, eggplant
Raphanus caudatus, rat's tail radish
Colocasia antiquorum, taro
Cucumis sativus, cucumber
Gossypium arboreum, tree cotton, 2X
Corchorus capsularis, jute
Piper nigrum, pepper
Indigofera tinctoria, indigo

Dioscorea spp., yam
Citrus maxima, pomelo
Musa spp., banana
Cocos nucifera, coconut

Triticum aestivum, bread wheat
Triticum compactum, club wheat
Triticum sphaerococcum, shot wheat
Secale cereale, rye (secondary centre)
Pisum sativum, pea
Lens culinaris, lentil
Cicer arietinum, chickpea
Sesamum indicum, sesame (a centre of origin)
Linum usitatissimum, flax (a centre of origin)
Carthamus tinctorius, safflower (a centre of origin)
Daucus carota, carrot (basic centre of Asiatic varieties)
Raphanus sativus, radish (a centre of origin)
Pyrus communis, pear
Pyrus malus, apple
Juglans regia, walnut

Triticum monococcum, einkorn wheat
Triticum durum, durum wheat
Triticum turgidum, Poulard wheat
Triticum aestivum, bread wheat (endemic awnless group; a centre of origin)
Hordeum vulgare, cultivated two-rowed barley (endemic group)
Secale cereale, rye
Avena byzantina, red oat
Cicer arietinum, chickpea (secondary centre)
Lens culinaris, lentil (a large endemic group of varieties)
Pisum sativum, pea (a large endemic group; secondary centre)
Medicago sativa, blue alfalfa
Sesamum indicum, sesame (a separate geographic group)
Linum usitatissimum, flax (many endemic varieties)
Cucumis melo, melon
Amygdalus communis, almond
Ficus carica, fig
Punica granatum, pomegranate
Vitis vinifera, grape
Primus armeniaca, apricot (a centre of origin)
Pistacia vera, pistachio (a centre of origin)

Triticum durum, durum wheat
Avena strigosa, hulled oats
Vicia faba, broad bean
Brassica oleracea, cabbage
Olea europaea, olive
Lactuca sativa, lettuce

Triticum durum, durum wheat (an amazing wealth of forms)
Triticum turgidum, Poulard wheat (an exceptional wealth of forms)
Triticum dicoccum, Emmer wheat
Hordeum vulgare, barley (an exceptional diversity of forms)
Cicer arietinum, chickpea (a centre of origin)
Lens culinaris, lentil (a centre of origin)
Eragrostis abyssinica, tef
Eleusine coracana, finger millet
Pisum sativum, pea (a centre of origin)
Linum usitatissimum, flax (a centre of origin)
Sesamum indicum, sesame (basic centre)
Ricinus communis, castor bean (a centre of origin)
Coffea arabica, coffee

Zea mays, corn
Phaseolus vulgaris, common bean
Capsicum annuum, pepper
Gossypium hirsutum, upland cotton
Agave sisalana, sisal hemp
Cucurbita spp., squash, pumpkin

Ipomoea batatas, sweet potato
Solanum tuberosum, potato
Phaseolus lunatus, lima bean
Lycopersicon esculentum, tomato
Gossypium barbadense, sea island cotton (4X)
Carica papaya, papaya
Nicotiana tabacum, tobacco

Solanum tuberosum, potato

Manihot utilissima, manioc
Arachis hypogaea, peanut
Theobroma cacao, cacao (secondary centre)
Hevea brasiliensis, rubber tree
Ananas comosus, pineapple
Passiflora edulis, purple grandilla or passion fruit

Appendix II. Cultivated plants and their regions of diversity (refer Fig. 3)

1. Chinese-Japanese Region

· Prosomillet, Fox tail millet
· Soybean, Adzuki bean
· Leafy mustard
· Orange/Citrus, Peach, Apricot, Litchi
· Bamboo, Ramie, Tung oil tree, Tea

· Rice
· Rice bean, Winged bean
· Cucurbits/Ash gourd
· Mango, Banana, Rambutan, Durian, Bread fruit, Citrus/Lime, Grape fruit
· Bamboos, Nutmeg, Clove, Sago-palm, Ginger, Taros and Yams, Betel nut, Coconut

· Eucalyptus, Acacia, Macadamia nut

· Rice, Little millet

· Black gram, Green gram, Moth bean, Rice bean, Dolichos bean, Pigeonpea, Cowpea, Chickpea, Horse gram

· Eggplant, Okra, Cucumber, Leafy mustard, Rat's tail radish, Taros and Yams

· Citrus, Banana, Mango, Sunnhemp

· Sesame, Ginger, Turmeric, Cardamom, Arecanut, Sugarcane, Black pepper

· Wheat (Bread/Club/Shot), Rye
· Allium/Onion, Garlic, Spinach, Peas, Beet root, Faba bean
· Lentil, Chickpea
· Apricot, Plum, Pear, Apple, Walnut, Almond, Pistachio, Melon, Grape
· Hemp/Cannabis

· Wheat (Einkorn, Durum, Poulard, Bread), Barley, Rye/Secale

· Faba bean, Chickpea, French bean, Lentil

· Brassica oleracea, Allium, Cucumis, Melon, Grape, Plum, Pear, Apple, Apricot, Pistachio, Fig, Pomegranate

· Safflower

· Lupins, Medics

· Wheat (Durum, Turgidum), Oats
· Brassica oleracea, Lettuce, Beet root, Colza
· Faba bean, Radish
· Olive, Trifolium/Berseem, Lupins, Crocus, Grape, Fennel, Cumin, Celery, Linseed

· Wheat (Durum, Emmer, Poulard, Bread)
· African rice, Sorghum, Pearl millet, Finger millet, Tef
· Cowpea, Bottle gourd, Okra, Yams, Cucumber
· Castor bean, Sesame, Niger, Oil palm, Safflower
· Cotton, Kenaf
· Kola, Bambara groundnut, Date palm, Ensete, Melons

· Peach, Pear, Plum, Apricot, Apple, Almond, Walnut, Pistachio, Cherry
· Cannabis, Mustard (black), Chicory, Hops, Lettuce

· Potato, Sweet potato, Xanthosoma
· Lima Bean, Amaranth, Chenopodium, Cucurbita, Tomato
· Papaya, Pineapple
· Groundnut, Sea island cotton (Brazil-Paraguay: Cassava, Cacao, Rubber tree, Passion fruit)

· Maize, French bean, Potato, Cucurbita, Pepper/Chilli, Amaranth, Chenopodium, Tobacco, Sisal hemp, Upland cotton,

· Jerusalem artichoke, Sunflower, Plum, Raspberry, Strawberry