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The whole set of genetic material of a species of plant is known as germplasm of the organism .It is based on the knowledge of germplasm that various breeding techniques of plants are developed. Hence the storage or preservation of germplasm is important. Conventionally seeds were used to store the germplasm. But in case where seeds cannot be used for regeneration of plants or in cases where shoot and root tissue is not stable, it is important to preserve them this can be achieved by mechanisms like:

(i) Cryopreservation:
The freeze preservation of culture of cells or tissues in liquid nitrogen at -196 degree C is known as cryopreservation. The technique involves four steps:

The procedure of freezing may be conducted slowly, rapidly or initial freezing by dropping temperature slowly and followed by a rapid decrease in temperature. In order that the plants are not affected by the sudden decrease in temperature, treatment of cells with plant vitrification solution helps cells and tissue to overcome the harsh temperature. The medium was added with cryoprotectant like DMSO, glycerol, and proline to the culture medium to protect cells from injury. The addition of cryoprotectant protects the cell by prevention of large crystals inside cell, protect from water loss from cell. The frozen cells are stored in a refrigerator containing liquid nitrogen. The temperature of such refrigerator is maintained at or below -130 degree C. Organised tissues like shoot tips, somatic and zygotic embryos are usually chosen for storage. Alternatively cells can be immobilised in sodium alginate and then cryopreserved.

Thawing of cultures is done in a rapid process. The freeze preserved culture is dipped in a water bath containing water at about 37-40 degree C for about 90 seconds. This process in done rapidly so that no ice crystals are formed. The thawed culture is washed several times to remove cryoprotectant. In the recent times, the cryprotectant is removed by diluting. This is done by fixing the culture along with a cryoprotectant onto a disk and is kept on a suitable medium. This disk is frequently transferred into a fresh medium. This frequent transfer dilutes out the cryoprotectant.

The culture which is freeze preserved need to be thawed and cultured to bring it back to normal life. The optimum conditions of freeze preserved plants have to be determined for developing a successful reculture. After cryopreservation, some plants tend to show special requirement for growth which was not necessary under normal propagation of the corresponding plants. For eg: tomato shoot tips when cryopreserved, thawed and recultured, the culture required some levels of abscisic acid in their medium in order to initiate and develop shoot tip from callus formed.

It is found that mostly meristematic cells survive cryopreservation than other cells. In plants where the germplasm cannot be stored in seeds or other parts the cryopreservtaion provides a good option of storage and future usage.

(ii)Slow growth cultures:
Slow growth of cultures involves limiting the conditions of growth so that the culture does not grow and propagate in ordinary pace. This can be achieved by limiting the factors affecting the growth. This provides an attractive alternative to cryopreservation as the procedure is cost effective and simple comparatively. The various factors affecting the growth of cultures are:

The lowering of temperature beyond optimum level was found to affect the cultures by lowering the growth pace.
Nutrient restriction: The limiting of certain nutrient which is vital for growth and differentiation helped in achieving the slow growth culture.
Growth regulators: In some case where temperature and nutrient control was not seen to be effective the culture is added with some growth regulators which regulate the growth of culture. The various growth regulators added include tri-idobenzoic acid (TIBA), chlormequat(CCC), abscisic acid(ABA).

Osmotic concentration: the level of osmotic concentration is another important method by which the slow growth of cultures can be achieved. The high levels of sucrose, mannitol or sorbitol were shown to reduce the growth of cultures.

Other factors: certain other factors such as oxygen concentration, culture vessel used for culturing, restricting the illumination received by cultures all affect the growth of cultures.

DNA clones:
The germplasm can also be conserved in DNA segments cloned into appropriate vectors but the process demands high expertise and is costly.

Artificial seeds:
Another mechanism of germplasm conservation is by desiccating embryos and storing it as artificial seeds. This has proved to be an effective mechanism, but was possible only with somatic embryos and in certain cases by shoot tips. The process of germplasm conservations offers several advantages like cost effective, availability of germplasm of specific plants to propagate, small storage space, and longer terms of storage. It also includes risks such as cell damage by cryopreservation, high technology involved etc.
Today’s preservation techniques has been developed on the base of natural occurring phenomenon’s like sporulation ,germination, hibernation, adaptation etc, that is found in different animals and in microorganisms. All above and many other sophisticated preservation techniques are today being used in giant projects that are going on. One of such project is Millennium Seed Bank Project. This project is an international conservation project which aim to preserve rare and endangered species of plant from around the globe. This project is coordinated by Royal Botanical Garden, Kew which is launched in year 2000.
As per the plan they will be collecting and preserving around 50,000 species of plants which fall under such categories. They also collect seeds from the entire native UK’s flora. Once such seeds are collected, they do research and develop new methods for their preservation as well as propagation in future. Each plant’s seed has its own protocol for germination and propagation in this bank. This way they will preserve all valuable flora form around the world.
The most important thing is that the rare species are being developed and propagated with these preserved seed using various techniques like tissue culture and other micro-propagation etc. This project is the largest and successful preservation of valuable seeds which can be used in future as and when required. These seeds are preserved in liquid nitrogen, or below temperature like -20 to -80 °C.
The collect seeds are billion in number and variety. By today, these stocks represents around 12-15 percent of total of globe. It means still there are many species which will be preserved and can be used in future depending upon their number in existence.
Such projects are not only a hope for plants under threat of extinction but also saves medicinal and other valuable plants on which we are dependent.
Plant genetic source efficiency can be considered from two factors. In situ conservation includes the organization and/ or servicing of natural supplies where species are permitted to stay in maximum environments with the lowest of management. On the other hand, ex situ conservation includes the use of botanic landscapes, field farms, seeds shops and gene financial banks. Within each type of conservation there are numerous techniques and associated problems. The studies performed in the division have particular importance to certain factors of ex situ conservation of plant germplasm, viz. seeds stores and gene-banks.