Deacclimation and reacclimation of apple (Malus × domestica Borkh.), blackcurrant (Ribes nigrum L.) and raspberry (Rubus idaeus L.) shoots and buds under controlled conditions

Abstract In the future climate scenario the transient warm spells during winter are predicted to become more frequent and potentially increase the risk of frost damage in perennial horticultural crops. The aim of our study was to examine, whether apple (Malus × domestica Borkh.), blackcurrant (Ribes nigrum L.), and raspberry (Rubus idaeus L.) cultivars differing in winter hardiness have a different susceptibility to deacclimation during a warm spell and the ability to reacclimate in response to a subsequent cold period. Shoot samples were collected from commercial orchards in Finland on Feb. 6, 2019. Samples were subjected to a deacclimation treatment (+5°C, 4 d) followed by a reacclimation treatment (-7°C, 7 d). Controlled freezing tests were conducted immediately after sampling, and at the end of deacclimation and reacclimation treatments to determine frost hardiness (FH). FH was most stable in those cultivars that are known to be hardy in the Finnish climate conditions, i.e. ‘Pirja’ apple, ‘Mikael’ and ‘Ojebyn’ blackcurrants, and ‘Maurin Makea’ raspberry. Buds were consistently less hardy than shoots, the difference being largest in blackcurrant, 25°C. The buds of all cultivars, except ‘Pirja’ apple, deacclimated during four days at +5°C, but only ‘Ben Tron’ blackcurrant buds were able to reacclimate. The depth of dormancy was determined by forcing the shoot samples in a greenhouse to examine the relationship of the FH stability to the dormancy status in the cultivars. Sixty to 95 % of apple, 98 to 100 % of blackcurrant, and 55 to 67 % of raspberry buds broke during five weeks in forcing. Only the apple cultivars exhibited cultivar differences in the dormancy status in the beginning of February. ‘Pirja’ despite having the most stable FH of the apple cultivars, showed the most complete and fastest bud break in forcing. Consequently, the susceptibility to deacclimation during a warm spell and the ability to reacclimate at subsequent low temperatures were not related to the status of dormancy in these cultivars.