Phaseshifting of the photoperiodic flowering response rhythm in Pharbitis nil by red‐light pulses

The control by light of the flowering response rhythm in the short‐day plant Pharbitis nil Choisy cv. Violet was examined by giving a single pulse of light at various times between 1 and 6 h after a 24‐h light period. When the first circadian cycle of the rhythm was monitored, it was found that a pulse of red light given at 1, 2 or 3 h into a 72‐dark period caused a 1‐h delay of the phase of the response rhythm, while a pulse at 6 h caused a 2‐h delay. These results support the hypothesis that, when red‐light pulses are given at hourly intervals, they are as effective as continuous light in preventing the onset of dark timing because they repeatedly return the rhythm to the circadian time at which it is apparently suspended in continuous light. The perception of and response to continuous light and red‐light pulses are also briefly discussed.