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

Red light given in the inductive dark period inhibits flowering in short day plants, and this effect may be reversed by far-red light given shortly after the red irradiation. The pigment system involved in these reactions is called phytochrome. In Pharbitis, however, the flower-inhibitory effect of red light is not reversed by succeeding irradiation with far-red (6, 7, 8, 13). The flowering response of Pharbitis is inhibited by far-red light given at the beginning of the dark period, and this inhibitory effect is completelv reversed by red light applied shortly after the far-red irradiation (6, 7). On the other hand, in Xanthiulm far-red light given at the beginning of the dark period promotes flowering anid shortens the critical dark period by some 2 hours (1). Takimoto and Ikeda (14), working with Pharbitis, found that far-red light given at the beginning of the dark period slightly promoted flowering when the dark period was shorter than 13 hours but inhibited flowering when thedark period was longer than 13 hours. It was also reported that farred interruptions applied in a long dark period inhiibited flowering when given in the first 16 hours of the dark period (13). Maximum inhibition was obtained when the far-red light was given 8 hours after the beginning of the dark period. Nakayama et al. (8) suggested that the inhibitory effect of far-red light given at the 8-hour point is a result of absorptioln by Pr (red absorbing form of phytochrome) rather than Pfr (far-red absorbing form of phytochrome). On the other hand, from the fact that 1)oth red and far-red light inhibit flowering at the 8-hour point, Salisbury (9) pointed out the possibility that optimal flowering in Pharbitis may require a mixture or balance of Pfr and Pr at the 8hour point. In most of these early works the plants were subjecte(l to short days consisting of 24-hour cycles. However, when plants are subjected to very long cycles (48-72 hours), they show a rhythmic response to red light interruptions. Recently Carpenter and