Effect of Far-Red Light and its Interaction with Red Light in the Photoperiodic Response of Pharbitis nil

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Abstract
In young seedlings of P. nil, strain Violet, 5 minutes of far-red light interruptions given throughout a 48-hour dark period were most inhibitory to flowering at the beginning of the dark period. The inhibitory effect of far-red light decreased almost linearly with delay of far-red irradiation. When far-red light was given at the beginning or 8 hours after the beginning of the dark period followed by various lengths of dark periods, the flowering response did not increase with increased duration of the dark period beyond certain lengths. Far-red light given shortly after a red interruption was very inhibitory to flowering and this inhibitory effect decreased with increasing intervals of time between the red and far-red irradiations. However, red light applied after far-red repromoted flowering provided that the time of the red irradiation did not fall in an inhibitory phase of the light-off rhythm which was initiated by the beginning of the dark period. If the inductive dark period was interrupted with 5 minutes of red light followed by 5 minutes of far-red light at the 8-hour point, the flower-inhibiting effects of red and far-red light were almost additive. However, if the red and far-red light were given in reversed order they showed the same effect as a single red light interruption. Possible roles of far-red light in the photoperiodic response were discussed.