Genetic variation, once considered unlimited, is fast eroding as modern varieties replace traditional cultivars over large areas and natural habitats are getting destroyed. Genetic variation must be conserved, both to combat new pests and diseases that emerge from time to time, and to produce better-adapted varieties in changing environments.

The International Crops Research Institute for Semi-Arid Tropics (ICRISAT) based in Hyderabad has made conservation of germplasm one of its major objectives. It has set up a genebank that now serves as a world repository for the collection and preservation of the seeds of five mandate crops of ICRISAT - sorghum, pearl millet, chickpea, pigeonpea, and groundnut. Through natural selection and countless generations of cultivation by farmers, these crops have become adapted to the semi-arid tropics of Asia, Africa and Latin America.

With over 1,30,500 varieties of seeds of these five crops collected from 130 countries, the ICRISAT genebank is the largest among the international genebanks. Incredible but true, there are almost 50,000 varieties of sorghum seeds alone and 5,000 varieties of ragi (small millet). Hundreds of varieties, now conserved in the ICRISAT genebank, have disappeared from their natural habitats in Africa and Asia.

According to N Kameswara Rao, Senior Scientist in-charge of the genebank, all newly acquired germplasm are first evaluated and screened by a team of ICRISAT and national programme scientists. Typically, these tests include stress tolerance, disease and pest resistance and quality characteristics. It is only after several years of detailed scientific evaluation and screening that new genetic stocks are identified for use in crop improvement as new sources of breeding material for desirable characteristics.

Seed storage under controlled environmental conditions - low temperature and low relative humidity - prolongs germinability and eliminates the need for frequent regeneration of germplasm, which is expensive and involves risk to genetic integrity.

Explaining the storage facilities, Kameswara Rao says a short-term storage maintained at 18-20 degree Celsius and 30-40 per cent relative humidity, is used to hold seeds while they are dried and prepared for medium and long-term storage. Six medium-term storage rooms maintained at 4 degree Celsius and 20-30 per cent relative humidity help conserve seed samples for 20-25 years. Three long-term storage units at minus 20 degree Celsius conserve the seeds dried to 5-6 per cent moisture content and hermetically sealed in aluminum foil packets for more than 50 years. Germplasm lines which do not produce adequate seeds for conservation are maintained as live plants in a botanical garden and screen houses.

Germplasm samples for conservation are multiplied during the post-rainy season. To minimize genetic drift, adequate number of plants are grown and sampled equally in constituting new seed stocks. Genetic integrity is maintained by pollination control while regenerating cross-pollinating crops like sorghum, pearl millet and pigeonpea. Pre-storage deterioration in seed quality is minimized by harvesting promptly when maturity is reached.

ICRISAT's genebank is designed to withstand natural disasters. For further safety, long-term conservation collection is duplicated in other genebanks. For example, the entire pigeonpea germplasm is now being prepared for long-term base conservation in the genebank of the National Bureau of Plant Genetic Resources, Indian Council of Agricultural Research, New Delhi. Similarly, duplicates of a large portion of chickpea germplasm are conserved at the International Center for Agricultural Research in the Areas (ICARDA), Aleppo, Syria, and about 56 per cent of the sorghum germplasm is duplicated at the National Seed Storage Laboratory, Fort Collins, Colorado, USA.

ICRISAT has been successful in assembling and conserving germplasm as part of the global effort for the conservation of biodiversity. The greatest impact is in conserving the vanishing germplasm and making diverse material readily available for use in crop improvement for the semi-arid tropics. ICRISAT's Genetic Resources Unit continues to assemble germplasm from unexplored areas of diversity and make it freely available for use in crop improvement for the benefit of humankind. It has distributed more than half-a million seed samples to crop improvement scientists, academic institutions and general users in 133 countries.