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

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.

INDUCTION OF SEED GERMINATION IN Lactuca sativa L. BY NANOSECOND DYE LASER FLASHES*,†

“…From comparison of these double-flash kinetics with the kinetics of Pf: formation in seeds it had been concluded that the accumulation of intermediates in the phytochrome phototransformation pathway(s) and their subsequent reactions are responsible for this phenomenon (Haupt el al., 1981;Scheuerlein, 1983). Similar conclusions were drawn from other *A preliminary report of this work was presented at the IX International Congress on Photobiology phytochrome-mediated responses in vivo and from spectrophotometric measurements in situ (Kraml and Schafer, 1983), as well as in vitro (Scheuerlein, 1984). For a better understanding of the role that phytochrome intermediates could play in physiological responses, improved spectral purity of the flashes and better kinetic resolution are needed.…”

Action Spectrum for Interaction between Visible and Far-Red Light on Face Chloroplast Orientation in Mougeotia

“…The low energy, phytochrome-mediated chloroplast response and the response characterized here lead to the same kind of chloroplast movement (to face position), but it seems that they are mediated by different transduction chains. This conclusion is consistent with the results showing that this type of chloroplast response is strongly dependent upon irradiance in the FR (low Pfr/Pto, ratio) (1 1, 18) while in the low energy phytochrome-mediated response it occurs in the R (high Pfr/Ptow ratio) (9,10).…”

“…The data do not conform to the known properties of the low energy phytochrome effects that are induced by small total light fluences (low irradiance times short time) and that show full or nearly full reversibility during successive exposure to R and FR (9,10). In the case of low energy phytochrome-mediated chloroplast response in Mougeotia, flash durations of a few milliseconds are enough to induce chloroplast orientation (10 and GL on leaflet movement in Albizzia (15,16) and concerning the generation of a bioelectrical field potential of the soybean hypocotyl (17). Tanada (16,17) suggested that this kind of interaction may be active in the modification of the properties of membranes, leading to changes in their permeability.…”

Physiological Mechanism of the Auxin-Induced Increase in Light Sensitivity of Phytochrome-Mediated Growth Responses in Avena Coleoptile Sections

“…It should be noted that sections irradiated within 10 min after harvest into buffer containing no IAA show no response to VLF R, indicating that the bulk of the endogenous IAA has diffused out ofthe sections (26 (12). However, the fact that LAA added at 60 min after irradiation does not confer any sensitivity to VLF R, indicates that some early process subsequent to phytochrome photoconversion is probably affected by IAA.…”