Studies on Red Light Interruption in Relation to Timing Mechanisms Involved in the Photoperiodic Response of Pharbitis nil

In a previous paper (2) it has been suggested that at least 3 kinds of timing mechanisms are involved in the photoperiodic response of Pharbitis nil. The first timing component is similar to an hourglass in that a linear increase in the flowering response results with increasing duration of the dark period. This component is very sensitive to temperature. The second component is an endogenous circadian rhythm which starts at the beginning of the light period, and for which the temperature sensitivity has not been determined. The third component, which is temperature insensitive, starts at the beginning of the dark period and has a very light sensitive phase with the maximum sensitivity about 8 hours after the onset of darkness. The first and the second components have been detected by giving different lengths of dark periods, and the third component by giving a red light interruption during 24- or 48-hour dark periods. In the previous paper (2) an effect of red light interruptions given during the last half of a 48-hour dark period was noted but was not discussed. The present experiments were designed to obtain more detailed information on these timing mechanisms with emphasis upon the effects of red light interruptions given at different times during a long dark period.