How the perception of light by phytochrome is linked to the production of gibberellin and anthesin in longday and short-day plants is not clear. One idea is that plants measure the amount of Pfr present. Flowering in short-day plants would be inhibited by Pfr, and these plants would not flower until very little or no Pfr remained after a long night. Po flower, long-day plants would require some minimum level of Pfr, which would not be available if the nights were too long but this explanation is not viable because Pfr vanishes within a few hours after the dark period begins.
Alternately, levels of phytochrome may influence an internal biological clock that keeps track of time. Phe clock establishes a free-running circadian rhythm of about twenty-four hours, this clock needs to be constantly reset to parallel the natural changes in photoperiod as the seasons change. Phytochrome interacts with the clock to synchronize the rhythm with the environment, a prospect that is strengthened by night-break experiments, where the time of the light flash during the night is critical. In the case of the Japanese morning glory, there are times during the night that a red light flash completely inhibits flowering and other times when it has no effect. In these experiments, the phase of the rhythm of the clock defines the nature of the interaction with phytochrome.
Studies on the relationship between flowering and day length have focused on the production of gibberellin and anthesin. Phere is evidence that the production of inhibitors by leaves is also under photoperiodic control. This has been demonstrated for photoperiodic tobacco plants and for some peas. In the case of the pea, the inhibitory effect is most obvious for short days, but lower levels of inhibitors continue to be produced as the days grow longer.
See also: Photoperiod, Temperature, Genetic Control of Flowering
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