Freeze Preservation - In this approach, cells and tissues are stored at -196°C in liquid nitrogen. Theoretically, the cells and tissues can be stored in live state indefinitely, and cells/tissues recovered after thawing should be unchanged and live.
But in practice, viability generally declines with the duration of cryopreservation, and thawed cells/tissues show variable degrees of damage, which they may be able to repair under appropriate media and culture conditions.
It is desirable to preserve organised tissues like shoot tips, and somatic and zygotic embryos to minimise the risk of genetic instability associated with un organised tissues like cell suspensions and protoplasts. The techniques are being refined for many plant species, and are being used to conserve germplasm derived from wild relatives, and biotechnologically derived germplasm in designated international culture collections.