Variation in freezing tolerance, water content and carbohydrate metabolism of floral buds during deacclimation of contrasting blackcurrant cultivars

As a result of climate change, temperature patterns are expected to become increasingly irregular with longer and more frequent episodes of unseasonable warm spells during the winter season. Warm spells may promote premature loss of freezing tolerance and bud burst in woody perennials, thereby increasing the risk of tissue damage by subsequent frosts. This study investigated the variation in kinetics of deacclimation and bud break and associated changes in carbohydrate metabolism and water status in floral buds of six blackcurrant (Ribes nigrum) cultivars in response to a simulated warm spell (16/11 °C day/night). In three of the cultivars, the rate of deacclimation showed an almost logarithmic course, whereas the other three cultivars exhibited greater deacclimation resistance and a sigmoid deacclimation pattern. The timing and rate of bud development, and their relationship with deacclimation varied greatly amongst cultivars, indicating genotypic variation in time-dependent responses of freezing tolerance and bud break to warm temperatures. In all six cultivars, deacclimation and growth resumption were strongly associated with rehydration. In contrast, changes in carbohydrate metabolism were mostly associated with deacclimation. Evaluation of phenological responses of the same cultivars under field conditions showed that cultivars which were fast flushing in response to an experimental warm spell also exhibited early bud break under natural conditions, indicating that cultivar differences in phenological responses are consistent under different temperature conditions.