Light Perception and Time Measurement in Photoperiodism
The perception of light in photoperiodism is essentially attributed to the pigment phytochrome and, insofar as flowering is concerned, this is possibly the only photoreceptor. There are various possible interactions of phytochrome with the photoperiodic clock. Irrespective of the nature of the timer, transitions from light to dark must be perceived, and phytochrome is a candidate for such a function. Two different models have been proposed to approach the problem of photoperiodic time measurement. The first one views the clock as an hourglass which measures time by the accumulation or destruction of some unknown product; this model of the clock implicates a nonoscillatory timer. The second involves an endogenous oscillation, and the circadian clock has been implicated. In Chenopodium rubrum, the suggestion has been made that both light-on and light-off signals may control the phase of the rhythm, but as the duration of the photoperiod increases, there can be a change in their effectiveness for rephasing the rhythm.