Kinetic Studies on Pigment Systems Concerned with the Photoperiodic Response in Pharbitis nil

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Abstract
In Pharbitis nil, strain Violet, far-red light given during the 1st half of a 48-hour dark period inhibited flowering. The amount of far-red light required for maximum inhibition at a given time (saturating energy) remained constant (360 kergs/cm2) at least during the first 8 hours of the dark period. Even if the far-red irradiation was preceded by red irradiation, 360 kergs/cm2 of far-red light was enough to cause maximum inhibition. Far-red light given at the beginning of the dark period inhibited flowering and this inhibitory effect was completely reversed by subsequent irradiation with red. The amount of red energy required to nullify the far-red effect completely was about 82.5 kergs/cm2. Even if more than a saturating amount of far-red anergy was given the same amount of red energy was enough to cause complete nullification of the far-red effect. These results clearly show that the amount of far-red absorbing pigment (Pfr) remained relatively constant during the dark period. To produce maximum conversion of Pfr to Pr 360 kergs/cm2 of far-red were required and 82.5 kergs/cm2 of red energy were enough to reconvert Pr to Pfr. Further results suggest that red radiant energy absorbed by some pigment other than phytochrome was also effective for flower inhibition, and this effect could not be reversed by subsequent irradiation with far-red. The inhibitory effect of far-red was controlled by phytochrome and the far-red effect was reversed by red light at any time during the dark period.