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Three years ago we established a long-term cryogenic storage project for apple germplasm and utilized grafting of buds obtained from stored dormant shoot sections as the major viability assay. Grafting, however, is time consuming and requires considerable skill. Electrolyte leakage and oxidative browning tests were used as alternative viability assays. Using leakage from individual buds in a multiwell analyzer, we examined modifications of the electrolyte leakage test and analyzed the kinetics of leakage in an attempt to determine whether the test can predict grafting success. The results suggest that more buds were viable than were estimated by the grafting test. In vitro culture is being examined to test this and to determine if practical recovery is feasible for diversity within the germplasm collection.
Clonally propagated crops, unlike seed-propagated crops, require intense and costly maintenance, generally in ex situ field gene banks. Consequently, large germplasm collections of tree species especially, are difficult to conserve in a well-replicated fashion and are vulnerable to damage from environmental stresses. Accordingly, long-term storage in liquid nitrogen presents a viable conservation alternative. To assess effectiveness of one approach to cryopreservation, dormant buds from 64 apple (Malus ×domestica Borkh. and other Malus spp.) accessions were collected and preserved in liquid nitrogen using a dormant-vegetative-bud method. Buds were retrieved from liquid nitrogen storage, rehydrated, and grafted onto rootstocks to determine survival. Mean recovery was 76% for 40 cold-hardy accessions, 66% for 20 moderately cold-hardy accessions, and 24% for four cold-tender accessions (range: 16% to 100%). Only four accessions had ≤25% recovery while 54 accessions had ≤50% recovery and 35 accessions had ≤75% recovery. No significant decline in recovery of these accessions by bud grafting occurred after 4 years of liquid nitrogen storage.