PHOTOCONVERSION OF PHYTOCHROME IN VIVO STUDIED BY DOUBLE FLASH IRRADIATION IN Mougeotia AND Avena

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Abstract
Abstract— The kinetics of phytochrome phototransformation from the red‐absorbing form (Pr) to the far‐red‐absorbing form (Pfr) in vivo at 22°C were studied using a double flash apparatus with 1‐ms flashes. Photoconversion by simultaneous flashes of red light saturates at a low Pfr level, indicating the possible attainment of a photoequilibrium between the excitation of Pr and the photoreversion of intermediates in the course of the I‐ms flashes. At saturation energy, simultaneous flashes resulted in about 50% as much Pfr as was produced by saturating irradiation with 5 s red light. Intermediates of the phototransformation pathway were analysed by separating two red or a red and a far‐red flash by variable dark intervals. In both plants phototransformation intermediates with half‐lives < 1 ms occur, but they are too short‐lived to characterize by our method. The subsequent intermediates have half‐lives of about 7 ms and 150 ms in A vena, 2 ms and 10 ms in Mougeotia. The conversion from Pr to Pfr seems to be completed 1 s after the red flash in Avena. In the alga Mougeotia, Pfr formation seems to be finished within only 50 ms after the inducing red flash. The kinetics obtained from physiological and spectrophotometric experiments with Avena mesocotyls are almost identical. These observations indicate that the physiological response corresponds directly to the amount of Pfr produced and not to phototransformation intermediates or “cycling” between Pr and Pfr.