Plant cryopreservation involves the storage of plant tissues (usually seed or shoot tips) in liquid nitrogen (LN) at-196 C or in the vapour phase of LN at-135 C in such a way that the viability of stored tissues is retained following re-warming (Day et al., 2008; Hamilton et al., 2009). Cryopreservation is usually applied to species with recalcitrant (ie dehydration sensitive) seeds that are not storable by any other means, or preservation of specific cultivars of vegetatively propagated crop plants like banana or potato, or for unique ornamental genotypes (Halmagyi et al., 2004; Kaczmarczyk et al., 2011a; Panis et al., 2005). Another reason to utilise cryostorage is to conserve endangered plant species, particularly where seeds may be extremely scarce or of doubtful quality and/or the species is threatened with imminent extinction (Decruse et al., 1999; Mallon et al., 2008; Mandal & Dixit-Sharma, 2007; Paunescu, 2009; Sen-Rong & Ming-Hua, 2009; Touchell et al., 2002).

The main advantage of cryopreservation is that once material has been successfully cooled to LN temperatures, it can be conserved in principle indefinitely, because at these ultra-low temperatures no metabolic processes occur. Replenishing a small volume of LN weekly in cryo-dewars is the only on-going maintenance operation usually required in cryostorage. There are further advantages to this approach: the low costs of storage, minimal space requirements and reduced labour maintenance compared to living collections and even when compared to maintenance of tissue cultures at room temperature. Once in storage, there is no risk of new contamination by fungus or bacteria, and cryogenically stored material has been reported to retain genetic stability (Harding, 2004). Depending on the species, small cryopreserved samples may take several weeks to re-establish shoot cultures, and several months to a year may be required to produce micropropagated plants capable of transfer to soil under greenhouse conditions and (following weaning) into the field.