Cryopreservation of Excised Embryos and Embryonic Axes

Seeds are categorized into two main groups according to their response to desiccation and their storage physiology: orthodox (desiccation-tolerant) and recalcitrant (desiccation-sensitive) seeds (Roberts 1973). A third category of seeds are those that are relatively desiccation tolerant but do not withstand desiccation down to water contents as low as those tolerated by orthodox seeds. These seeds are freezing sensitive and are referred to as intermediate seeds (Ellis et al. 1990, 1991).

Seed is the most preferred plant propagule for ex situ germplasm conservation due to low storage cost, ease of seed handling and regeneration of whole plants from genetically diverse materials (Chin 1994; Pritchard 1995). While orthodox seeds are acquiescent to storage under conventional gene bank conditions for centuries, (i.e. 3–7% seed water content at –20°C ) (FAO/IPGRI 1994), cryopreservation is the only available option for longterm storage of non-orthodox seeds. However, in circumstances where storage of the whole seed of a non-orthodox species is constrained by desiccation and freezing sensitivity on one hand, and by its relatively large seeds on the other, excised embryos and embryonic axes are an alternative option. In other cases where viability of lipid-rich orthodox seeds under conventional storage conditions is drastically reduced due to the thin seed coat coupled with lipid peroxidation, as in the case of peanuts (Arachis hypogaea), germplasm curators can resort to cryopreservation of the excised embryonic axes (Gagliardi et al. 2002). Several studies on recalcitrant and intermediate species empirically determined that excised embryos and embryonic axes (in most cases) are more tolerant to desiccation and subsequent cryoexposure than whole seeds (e.g. Bajaj 1984; Radhamani and Chandel 1992; Normah et al. 1994; Makeen et al. 2005)