- Conservation of germplasm away from its natural habitat is called ex situ germplasm conservation. It can be achieved in the following five ways
- Seed gene banks
- Plant or field gene banks
- Shoot-tip gene banks
- DNA gene banks.
- In seed gene banks, germplasm is stored as seeds of various accessions.
- Virtually all gene banks are essentially seed gene banks. Seed conservation is quite easy, relatively safe and ordinarily needs minimum space. Under suitable conditions, seeds of many species can be stored for up to 50-100 years.
- Containers of glass, tin, plastic or a combination of these may be used for seed storage. Seeds are classified, mainly on the basis of their storability, into two major groups:
- Orthodox and
- This grouping of seeds was proposed by Roberts in 1973.
- Seeds of this type can be dried to a moisture content of 5% or lower without lowering their viability.
- Most crop seeds belong to this category. Such seeds can be easily stored for long periods; their longevity increase in response to lower humidity and storage temperature.
- The viability of this group of seeds drops drastically if their moisture content is reduced below 12-30%. Seeds of many forest and fruit trees, and of several tropical crops like citrus, cocoa, coffee, rubber, oil palm, mango, jackfruit, etc. belong to this group.
- Such seeds present considerable difficulties in storage. Therefore, germplasm of such plants are conserved by alternative approaches.
- The conditions for seed storage depend mainly on the duration of storage. Generally, seed bank collections are classified into three groups:
(1) base collections,
(2) active collections and
(3) working collections.
- This grouping increases the efficiency of use and the level of management of the collections.
- These consist of all the accessions present in the germplasm of a crop, which are stored at about - 20°C with 5% moisture content; they are disturbed only for regeneration.
- Germination tests are done every 5-10 years. When the germination of an accession falls below, usually, 95% of its germination at the start of storage, the accession is regenerated. For reasons of safety, duplicates of base collections should be conserved in other Germplasm banks as well.
- High quality orthodox seeds can maintain good viability upto 100 years.
- The accessions in an active collection are stored at temperatures below 15°C (often near 0°C) and the seed moisture is kept at 5%.
- The storage is for medium duration, i.e., 10-15 years.
- These collections are used for evaluation, multiplication and distribution of the accessions.
- Active collection are usually maintained by multiplying the seeds of their own accessions. But from time to time, base collection material should be used for regeneration of these collections.
- This is essential to prevent any appreciable shift in the genetic make up of the collections.
Field Gene Banks: Essentially, a field or plant gene bank is an orchard or a field, in which accessions of fruit trees or vegetatively propagated crops are grown and maintained. Field banks suffer from the following serious limitations:
- The accessions being actively used in crop improvement programmes constitute working collection.
- Their seeds are stored for 3-5 years at less than 15°C and they usually contain about 10% moisture.
- These collections are maintained by the breeders using them.
- They require large areas,
- They are expensive to establish and maintain, and are prone to damage from
- disease and insect attacks,
- They are man-made
- There are natural disasters and
- human errors in handling.
- Few good plant banks exist in India.
Shoot Tip Gene Banks
- In such gene banks, germplasm is conserved as slow growth cultures of shoot-tips and nodal segments. Their regeneration consists of sub culturing the cultures, which may be done every 6 months to 3 years. This approach offers the following advantages for the conservation of germplasm of vegetatively propagated crops and tree species.
- Genotypes of the accessions can be conserved indefinitely free from diseases and pests.
- They can be used for such crops, which either do not produce seeds or produce recalcitrant seeds.
- Subculture becomes necessary only after relatively long periods (every 6-36 months).
- Regeneration, i.e., subculturing, requires a comparatively very short time.
- In addition, cuttings, bulbs and tubers can be maintained under controlled humidity and temperature conditions; however, this approach is practical for the short and medium term storage, and it should be used in conjunction with a field gene bank.