The Effect on Flower Formation in the Long-day Plant Silene armeria L. of Interrupting Long Photoperiods by Short Periods of Darkness

Abstract

Interrupting long photoperiods by short periods of darkness — indicated as «Day Interruption» - resulted in an accelerated flower formation when applied early during the photophase as compared with late application, the total amount of light being the same in both cases. Day Interruption primarily influenced floral induction, whereas realization was influenced to a much smaller extent or not at all. It is not darkness as such but absence of light that is the determining factor. Different effects of Day Interruption according to the time of its application can be understood as different effects of the daily photoperiodic action, i.e. weak at the onset, but increasing sharply near the critical daylength. Suppression of the light action by darkness at a time when the effect of light is weak, results in earlier flower formation than suppression at a time when this effect is strong. The combined results of Day Interruption and Night Interruption lead to the conclusion that the photoperiodic action of light during a 24 h-cycle follows a circadian rhythm.