PHYTOCHROME STATES IN ETIOLATED PEA SEEDLINGS: FLUORESCENCE AND PRIMARY PHOTOREACTIONS AT LOW TEMPERATURES

Emission spectra of the red phytochrome form (Pr) and fluence time-response curves of the Pr fluorescence intensity changes were measured in etiolated pea seedlings at low temperatures (80–150 K) in connection with its phototransformations into the initial photoproduct (Lr) and back upon actinic red (667 nm) and far-red (696 nm) illumination. The variable fluorescence reaches 45% at 85 K and decreases with the rise of temperature. Three kinetic components of the changes were found in the direct (Pr→Lr) and back (Lr→Pr) photoreactions belonging to three states of phytochrome: “slow”, “fast” and “very fast” (respective indices: s, f and vf). The amplitudes of the components and rate constants to reach photoequilibrium were determined in the direct and back photoreactions at different temperatures, and from this, their quantum yields, extent of the Pr?Lr phototransformation and activation energy of the reactions were evaluated for the three Pr and Lr states. The yields differ from each other by approximately a factor of 10 and those for the direct and back photoreactions are close to each other. The proportion of the amplitudes of the variable fluorescence of the three phytochrome states changes with temperature and upon the Pr→Lr photo-transformation and the Pr states differ in the position of their emission spectra by 3–5 nm. A close similarity between the Pr and Lr properties was observed, which implies a symmetrical scheme of their photoreactions. It is suggested that the three phytochrome species may originate in different conformational states of the chromophore and they independently transform in parallel photoreactions into the respective photoproducts: Prvf?Lrvf, Prf?Lrf and Prs?Lrs.