Contrasting types of photoperiodic response in the control of dormancy

The effect of night‐break or day‐extension treatments with red (R) and/or far‐red (FR) light were examined in the control of dormancy and extension growth in Weigela florida and Picea abies. Dormancy in Weigela (as assessed by the continued production of new leaves) was completely prevented by a 30 min night‐break treatment with R: the effect of R was prevented by a subsequent exposure to 30 min FR. A day‐extension treatment for 8.25 h with R or with R+FR also completely prevented dormancy, irrespective of whether it was given before or after a short day (SD) in sunlight. There was no significant difference between any dayextension treatment, nor between day‐extension treatments and a night‐break with R. Dormancy in Picea (as assessed by the maintenance of shoot growth) was also delayed by a night‐break with R but less effectively than the most effective day‐extension. A day‐extension treatment for 9 h with R light was more effective when it preceded than when it followed a SD in sunlight. The addition of FR in the second but not in the first 9 h of a 17 h photoperiod increased its effectiveness. It is concluded that the mechanisms of the control of dormancy must differ in the two species. A day‐extension for 16 h with light from tungstenfilament lamps increased stem elongation in Weigela when compared with an R night‐break. This resulted from an increase in internode length and there was no significant difference between the two treatments in their effect on leaf production.