Towards a cryopreserved germplasm collection of apple - results of dormant bud cryopreservation in the mild maritime winter climate of Hawkes Bay, New Zealand

Apple (Malus spp.), the third largest fruit crop in the world, is adaptable to both cold and subtropical climates. The wide genetic variability within Malus provides the basis for many international breeding programmes. Much of this genetic diversity is held in field repositories represented by either single or multiple plants. To overcome the risks associated with maintaining field germplasm, dormant bud cryopreservation has been successfully adopted for apple collections from cold winter climates, however, this method has not been well-established for warmer climates. In this study we demonstrate the use of winter dormant buds for establishing a cryopreserved collection of Malus germplasm with buds sourced from a field genebank collection growing in the maritime winter climate of Hawkes Bay, New Zealand. Winter harvested single-nodal sections of 18 apple cultivars were subjected to a two-step freezing protocol and cryopreserved in vapour phase of liquid nitrogen. Following two years of trials, most genotypes recorded >40% viability after the buds were recovered from cryopreservation and subsequently T-bud grafted onto rootstocks. The results indicate that the relatively warmer winter climate of Hawkes Bay is comparable to much colder climates for successfully cryopreserving winter dormant buds for the establishment of an apple genebank. The two-step initial freezing procedure using a modified domestic freezer and the use of T-bud grafting for assessing viability under field conditions enables our method to be economical with resources. There were differences in survival between genotypes and also between growing seasons that require further investigation. Our results will form the basis for the establishment of a cryopreserved genebank of Malus spp. despite the source material being grown in the moderate climate of Hawkes Bay, New Zealand where winter freezing temperatures are rare.