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Cryopreservation as a tool for the conservation of Eucalyptus genetic variability: concepts and challenges

An essential criterion of Eucalyptus tree breeding programmes employed by the commercial forestry sector is the maintenance of potentially important species and genotypes for periods commensurate with field testing. Current methods for this purpose include conservation stands, clonal hedges and maintenance of cuttings in greenhouses and hydroponic systems. However, these methods, especially the maintenance of conservation stands and clonal hedges, require large tracts of land, which are now unavailable. The latter methods also have other disadvantages in that they are labour intensive and may be expensive to maintain. The conservation method of cryopreservation (the maintenance of plant cells, tissues and organs at subzero temperatures) may offer a cost-effective alternative with significant benefits, including the maintenance of plant material for theoretically unlimited periods. This field of science has experienced significant breakthroughs in the last two decades, especially in terms of the understanding of the physical, chemical and biological processes that occur during the freeze–thaw cycles. Consequently, several effective mechanisms of reducing the risk of ice crystal damage have been developed, resulting in the successful cryopreservation of numerous wild and domesticated plant species. Apart from germplasm conservation, cryopreservation may also be employed as a means of maintaining genetically engineered transformants while field testing is conducted. This is particularly advantageous considering the large numbers of transformants that can be generated during genetic engineering trials. Cryopreservation should, therefore, be considered a potentially important tool in commercial breeding strategies. This paper details the benefits and advantages of the technology, and also identifies and explains some of the key challenges that still exist, especially those associated with the cryopreservation of highly hydrated and/or desiccation-sensitive material such as Eucalyptus axillary buds.