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

Abstract

In many short-day and long-day plants the action spectrum for inhibition or promotion of flowering by a light interruption of the dark period shows a peak in the red (660 m,u). The effect of this red light interruption is reversed by succeeding exposure to far-red light (735 m,u) (2, 4). The pigment involved in these changes has been obtained in a partially purified state and is called phytochrome. Hendricks (5) proposed that the Pfr (far-red absorbing form of phytochrome) converts to Pr (red absorbing form of phytochrome) slowly in darkness, and that the dark conversion time of Pfr to Pr provides a control mechanism of timing in the photoperiodic response. The reduction of Pfr to a certain level is presumed to be required for photoperiodic induction. Salisbury and Bonner (7) gave Xanthiuml various radiant energy treatments of red light during the inductive dark period, and showed that nearly equal amounts of the red light were required for the maximum response when applied any time after the first 3 hours of the dark period. From this experiment they supposed that the phytochrome conversion was nearly completed within the first 2 or 3 hours of the dark period. More recent work with the phytochrome system involved in cauliflower heads showed a similar dark conversion (3). Konitz (6), on the other hand, reported that in Cheto poditum the amount of red light required for the maximum response varied during the dark period. In Pharbitis (10) the flower-inhibiting effect of a red light interruption is not reversed by subsequent irradiation with far-red, but far-red light given after a red light interruption intensifies the inhibitory effect. On the other hand, the flower-inhibitory effect of far-red light is reversed by subsequent irradiation with red light (10). Both red and far-red irradiations are inhibitory to flowering at the 8-hour point of the dark period, and these effects are almost additive when both were given in that order (red followed by far-red) or simultaneously (10). These results can not be understood with the present general concept of the physiological action of the phytochrome system. In the present experiments the pigment systems involved in the photoperiodic response of Pharbitis were investigated in more detail.