Romanian populations of Norway spruce are induced to set terminal buds by four inductive cycles of 8 h light/16 h darkness. To distinguish between circadian and hourglass timekeeping for the photoperiodic control of budset, seedlings were raised in continuous light at 300 µmol m-2s-1 at 20°C 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 µmol m-2s-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 night-break was applied close to the critical nightlength (CNL) of 6–7 h 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.
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Vince‐Prue et al. (2001) studied this question.