The perception of dusk signals in photoperiodic time‐measurement

A critical duration of darkness must be exceeded for the photoperiodic induction of flowering in short‐day plants. This requires detection of the light/dark transition at dusk and the coupling of this information to a time‐measuring system. Lowering the Pfr/Ptot, ratio photochemically at the end of the day did not accelerate the onset of dark timing in Pharbitis nil Choisy cv. Violet. Time‐measurement was initiated when, with no change in spectral quality, the irradiance fell below a threshold value. Thus, if the light/dark transition at dusk is sensed by a reduction in Pfr, this reduction can be achieved as rapidly through thermal reactions as through photochemical ones. When given at hourly intervals during a 6‐h extension of a 24‐h main light period in white light, pulses of red light were as effective as continuous red light in delaying the onset of timing; pulses every 2 or 3 h were less effective. The effectiveness of intermittent red light indicates that phytochrome is the photoreceptor and the requirement for frequent exposures suggests that Pfr is lost rapidly in the dark. However, the red light pulses could not be reversed by far‐red light, which argues against this hypothesis. An alternative explanation is that the perception of light as being continuous occurs only when “new” Pfr is regenerated sufficiently frequently. The nature of the coupling of the dusk signal to the time‐measuring system is discussed and it is suggested that the effect of each red light pulse is to delay the phase of the photoperiodic rhythm by 1–3 h.