Chilling requirements of contrasting black currant (Ribes nigrum L.) cultivars and the induction of secondary bud dormancy

Chilling requirements for bud dormancy release and subsequent flower development in contrasting black currant (Ribes nigrum L.) cultivars were assayed at temperatures ranging from +10 to 10 degrees C using single node cuttings, entire detached shoots, and intact plants. While single node cuttings underestimated the chilling requirements of intact plants, severed shoots and intact plants produced similar results. Chilling at -5 degrees C for 14 weeks or more were optimal for breaking of bud dormancy and promotion of flower development in most cultivars, flower development usually having greater chilling needs than bud break itself. Within certain limits, extension of the chilling period compensated for non-optimal chilling temperature. However, while exposure to -10 degrees C for 8 weeks caused bud dormancy release, continued chilling for another 8 weeks inhibited bud break completely. We propose that excessive chilling induces secondary bud dormancy, a principle that is well established in seeds, but has to our knowledge, not been recognized for bud dormancy before. Marked genotypic differences were found and discussed. The observed severe chilling requirements of black currants concur with the reported vulnerability of this crop to declining winter chill in the wake of the ongoing global warming. Furthermore, elevated autumn temperature was found to induce a particularly deep dormancy state that further increases the chilling need. (C) 2014 Elsevier B.V. All rights reserved.