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

Abstract

Seedlings of P. nil, strain Violet, were subjected to a long dark period and exposed to 5 or 30 minutes of red light at different times. When the dark period was preceded by continuous illumination, red light given about 8 hours after the onset of darkness produced maximum inhibition, and a 2nd interval of inhibition was observed when the red light was given 30-40 hours after the onset of darkness, regardless of the length of the dark period. Thirty minutes of red light given 16-24 hours after the beginning of the dark period slightly promoted flowering. These phenomena suggest that one of the timing mechanisms which is initiated at the beginning of the dark period is an endogenous circadian rhythm. When the plants were subjected to an 8-hour dark period followed by an 8-hour light period before the main dark period, the time of effectiveness of red light interruption was somewhat extended and delayed at both the 8- and 32-hour points. This effect is considered to depend on another endogenous circadian rhythm which is initiated at the beginning of the light period. It is supposed that at least 2 kinds of endogenous circadian rhythms are participating in the photoperiodic reaction of Pharbitis.