Cryopreservation (liquid nitrogen-196 °C) represents the only safe and cost-effective option for long-term conservation of plant germplasm. Classical cryopreservation techniques, which are based on freeze-induced dehydration, are suitable for freezing undifferentiated cultures and apices of cold-tolerant species. New cryopreservation techniques, which are based on vitrification, are successfully employed with all explants, including cell suspensions and calluses, apices, and somatic and zygotic embryos of temperate and tropical species of medicinal plants. The development of cryopreservation protocols is significantly more advanced for vegetatively-propagated species than for recalcitrant seed species. Technology has been developed for long-term cryopreservation of nuclear genetic diversity in the form of pollen. Besides the already existing role of pollen cryobanks in breeding, there are many promising applications which have come into focus with the recent advances in allied bioscientific areas. There are a growing number of examples where cryopreservation is employed on a large-scale for different types of materials, including seeds with orthodox and intermediate storage behavior, dormant buds, pollen, biotechnology products, and apices sampled from in vitro plantlets of vegetatively-propagated species. In addition to germplasm conservation, cryopreservation also ensures genetic stability and retention of biosynthetic potential. Various explants have been used for cryopreservation of medicinally important plants. The present review takes stock of the developments in this area with a special reference to medicinal plants.

The most promising approach to secure long-term conservation lies in cryopreservation, that is, storage at ultra-low temperature in liquid nitrogen. Cryopreservation effectively suspends metabolism and therefore should eliminate time-related biochemical phenomena. Thus, the challenge in cryopreservation relates to the transition to and from the exposure to that temperature for a particular period of time. There is a growing awareness of the need to conserve plant genetic resources, not just to maintain biodiversity, but also to support plant breeding and biotechnology programs. Cryopreservation, the storage of viable biological material at ultra-low temperatures, provides a means for the long-term stable storage of plant germplasm.

Cryopreservation is a promising option for safe long-term storage of germplasm of those species which can easily be regenerated into whole plants of vegetatively-propagated species and species with recalcitrant seed. Cryopreservation may also use a technique for the long-term storage of many orthodox species (Stanwood, 1980). Because of the quality of cryopreservation and its high safety, the cost-intensive conservation activities, i.e., the preparation for freezing, etc. which are responsible for over 90% of total costs, can be reduced to once in every 50 years or even longer (Schafur-Menuhr, 1996). Hence, these preparation costs may be distributed over the whole conservation period of 50 years as annuity. Consequently, cryopreservation shows the lowest annual unit costs for ex situ conservation, with $22 per accession and year (Virchow, 1999).

The different types of cultures that have been investigated with a view to their storage by cryopreservation range from protoplasts and cell suspension to meristems, shoot tips and embryos (Withers, 1990). In general, the degree of difficulty in cryopreservation appears to be relating to the level of attention devoted to the development of techniques. The progress over recent years gives some encouragement to the view that it will be possible to carry out cryopreservation of a range of species with some degree of reproducibility and with adequately high success rates.

Genetics & Evolution Journal, Cryopreservation, Medicinal Plant Systems, Plant Germplasm, Cryopreservation Protocols, Biotechnology Programs, Plant Breeding Programs, Cryogenic Techniques, Cryopreservation Technology, Chromosomal Aberration.