Simultaneous dormancy induction interferes with short day floral induction in black currant (Ribes nigrum L.)

Blackcurrant is a short day plant that ceases growing and initiates flowers at photoperiods shorter than a critical length. Surprisingly, however, the flowering response was found to be several-fold stronger in the near-critical photoperiod of 15 h than in shorter photoperiods down to 10 h ( Heide and Sonsteby, 2011 ). This unusual response of a short day plant was confirmed by the present experiments where plants were exposed to 15 and 10 h photoperiods for 5 weeks at 18 °C. Also, plants exposed to naturally decreasing day-length from July, produced significantly more flowers than plants exposed to the shorter day-lengths in August. Furthermore, plants exposed to controlled photoperiods decreasing from 15 h to 10 h responded in much the same way as those in constant 15 h, while the reverse changes from 10 h to 15 h mimicked the response to constant 10 h photoperiod. Superior flower induction was always associated with a gradual reduction in growth rate followed by a delayed cessation of growth, while the sparse-flowering plants underwent an early and abrupt cessation of growth. We therefore conclude that the prompt growth cessation induced by an abrupt change to SD well below the critical length also interferes with flowering by arresting floral initiation at an early stage. Accordingly, the optimum photoperiod for flowering in black currant must be short enough to allow flower initiation and still long enough to prevent premature dormancy.