Effect of Double Red Light Interruptions on the Photoperiodic Response of Pharbitis nil

Abstract

Seedlings of P. nil, strain Violet, grown under continuous illumination were subjected to a single dark period of 36-72 hours and exposed to 2 red light interruptions of 5 minutes, each given at different times. Irrespective of when the 1st red light was given the 2nd red light given at the 6- to 12-hour points was inhibitory to flowering and that given at the 16- to 24-hour points stimulated flowering. The effect of the 2nd red light was closely related to the time from the beginning of the dark period and not to the time from the 1st red light interruption. This suggests that the timing mechanism which is initiated at the beginning of the dark period still persisted after the 1st red light interruption. Whenever the 1st red light was given and also irrespective of the length of the dark period, the 2nd red light applied in the last 20 hours of the dark period inhibited flowering with a maximum about 16 hours before the end of the dark period. There was some interaction between the red light interruption and the following light period.