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

Abstract

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 Phar-bitis, 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 the-dark period was longer than 13 hours. It was also reported that far-red interruptions applied in a long dark period in-hiibited 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 absorp-tioln by Pr (red absorbing form of phytochrome) rather than Pfr (far-red absorbing form of phyto-chrome). 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 8-hour point. In most of these early works the plants were sub-jecte(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 1 Received February 17, 1965. Hamner (3) reported that Biloxi soybean plants did not show a rhythmic response to far-red interruptions , but did show a clear rhythmic response to red light interruptions. Konitz (5), on the other hand, reported opposite results, i.e., Chenopodiuim plants showed a rhythmic response to far-red interruptions in that the far-red light exerted an effect quite opposite to red light at any point in the photoperiodic cycle. Thus, many of the results are conflicting and more extensive studies on the effect of far-red light may be required. In the present experiments Phar-bitis plants were subjected to very long dark periods at suboptimal temperatures and the effect of far-red light and its interaction with red light was studied in detail. The temperature was carefully controlled so that in each experiment the flowering responses would be at the most sensitive level.