Rhythms as photoperiodic timers in the control of flowring in Chenopodium rubrum L

In C. rubrum, the amount of flowering that is induced by a single dark period interrupting continuous light depends upon the duration of darkness. A rhythmic oscillation in sensitivity to the time that light terminates darkness regulates the level of flowering. The period length of this oscillation is close to 30 hours, peaks of the rhythm occurring at about 13, 43 and 73 h of darkness.Phasing of the rhythm by 6-, 12- and 18-h photoperiods was studied by exposing plants to a given photoperiod at different phases of the free-running oscillation in darkness. The shift in phase of the rhythm was then determined by varying the length of the dark period following the photoperiod; this dark period was terminated by continuous light.With a 6-h photoperiod the timing of both the light-on and light-off signals is shown to control rhythm phasing. However, when the photoperiod is increased to 12 or 18 h, only the light-off signal determines phasing of the rhythm. In prolonged periods of irradiation-12 to 62 h light-a "durational" response to light overrides any interaction between the timing of the light period and the position of the oscillation at which light is administered. Such prolonged periods of irradiation apparently suspend or otherwise interact with the rhythm so that, in a following dark period, it is reinitiated at a fixed phase relative to the time of the light-off signal to give a peak of the rhythm 13 h after the dusk signal.In daily photoperiodic cycles rhythm phasing by a 6-h photocycle was also estimated by progressively increasing the number of cycles given prior to a single dark period of varied duration.In confirmation of Bünning's (1936) hypothesis, calculated and observed phasing of the rhythm controlling flowering in c. rubrum accounts for the photoperiodic response of this species. Evidence is also discussed which indicates that the timing of disappearance of phytochrome Pfr may limit flowering over the early hours of darkness.