Evidence for two actions of light in the photoperiodic induction of flowering in Pharbitis nil

Using one-day-old light-grown seedlings of Pharbitis nil we have shown that there are two distinct responses to light during the inductive dark period. The first is the classic night-break, which inhibits flowering at a specific stage of the circadian rhythm (assumed to be the basis of dark time measurement). The second action is to control the phase of this rhythm. The two responses were compared at the 6th and 8th hour of darkness. They differed in their dose responses, and by using very short exposures it was possible to achieve one response without the other. The response of the rhythm to light displayed characteristics of other circadian rhythms; the direction and sensitivity of the phase shift changed between the 6th and 8th h, the rhythm was reset by longer exposures to light, and with one critical light treatment at the appropriate phase, the rhythm was apparently abolished. These results offer direct support for an external coincidence model in the photoperiodic control of floral induction.