Cold tolerance and chilling requirements for breaking of bud dormancy in plants and severed shoots of raspberry (Rubus idaeusL.)

SummaryCold hardiness and the chilling requirements for breaking bud dormancy in red raspberry (Rubus idaeus L.) were studied by exposing dormant plants and severed shoots of the cultivars ‘Glen Ample’ and ‘Asker’ to temperatures ranging from +10ºC to –20ºC for periods of 7 – 21 weeks. Temperatures in the +5ºC to –5ºC range were optimal for breaking bud dormancy and the promotion of flower development across all chilling periods, while +10ºC and –10ºC were outside the optimum chilling temperature range. The effect of the various temperatures varied with the duration of the chilling period. However, even at optimal chilling temperatures, both bud burst and flowering were promoted and advanced in both cultivars by sustained chilling for up to 21 weeks. Experiments with intact potted plants and severed canes of ‘Glen Ample’ produced almost identical dormancy-breaking results. Neither plants nor severed canes of ‘Glen Ample’ survived storage at –10ºC or –20ºC, whereas canes of ‘Asker’ survived at –10ºC.We therefore conclude that 20 or more weeks of chilling at near-freezing temperatures are required for full dormancy release and the promotion of flowering along the entire length of the raspberry cane, which is a pre-requisite for large fruit yields. Therefore, erratic bud break and less consistent fruit yields may be expected in raspberry crops in the wake of ongoing climatic warming with declining winter chill.