EFFECT OF INTERMITTENT IRRADIATION ON PHOTOPERIODIC RESPONSES

Publisher Website
Google Scholar
Abstract
Several series of expts. are described in which intermittent irradiation was used to determine the interrelationships between the photochemical and non-photochemical reactions associated with photoperiodism. The results show that (1) the region of saturation of the photochemical reaction for most of the plants used is in the neighborhood of 1 footcandle, since further increases in irradiance failed to bring about corresponding increases in the photoperiodic responses; and (2) the greater length of the dark period between irradiation cycles, the less the efficiency of the radiant energy in promoting the long-photoperiod responses, with the efficiency falling off rapidly when the dark periods were around 90 min. or longer for spinach and Biloxi soybean. In all cases, continuous irradiation resulted in the highest efficiency. A theory is proposed concerning the kinetics of the photoperiodic reaction based on 2 relationships which appear to limit the photochemical reaction: the slow rate of the non-photochemical reaction which forms the substance to be photoactivated; and the low equilibrium conc. which this substance attains during long periods of darkness.