Improved models of the effects of winter chilling on blackcurrant (<i>Ribes nigrum</i> L.) show cultivar specific sensitivity to warm winters

Sufficient chilling in winter is essential for many perennial crops to start growing in spring and to produce good yields. Using blackcurrants as an example we have developed improved models which can help identify varieties resilient to the variable winters expected as the climate warms. Controlled temperature experiments were used to calibrate 3 proposed models of chilling accumulation requirements for a number of commercial blackcurrant cultivars. The first model assumed a linear relationship between bud break and chilling accumulation, the second a quadratic relationship which allows for the possibility of over-chilling and the third, an asymmetric quadratic relationship in which the maximum achievable effectiveness is temperature dependent. The models were then applied to data on selected cultivars gathered from blackcurrant growers across the United Kingdom and the third model was found to provide the best fit for the data, suggesting that long warm winters do not have the same effect as short cold winters in terms of the satisfaction of chilling requirement. Further, the degree to which temperature affects maximum bud break varies by cultivar. We discuss the potential effects of differing timing of chill on the applicability of the models presented.