Circadian timekeeping for the photoperiodic control of budset in Picea abies (Norway spruce) seedlings

Romanian populations of Norway spruce are induced to set terminal buds by four inductive cycles of 8 h light/16h darkness. To distinguish between circadian and hourglass timekeeping for the photoperiodic control of budset, seedlings were raised in continuous light at 300 mu molm(-2)s(-1) at 20 degreesC for 10 weeks. They were then exposed to an extended night regime consisting of three cycles of 8 h light/40 h dark with 4-h or 1-h nightbreaks (120 mu molm(-2)s(-1)) applied to groups of plants at intervals during the extended night. Following a final cycle of 8 h light/16 h dark to maximize budset, the plants were transferred to continuous light. Budset was delayed when the nightbreak was applied close to the critical nightlength (CNL) of 6-7h or about 22-23 h later in the extended night, consistent with circadian rather than hourglass timekeeping. Confidence intervals were calculated for the times to maximum effect of the night-breaks.