QUANTITATIVE STUDY OF THE SLOW DECLINE OF CHLOROPHYLL ALPHA-FLUORESCENCE IN ISOLATED-CHLOROPLASTS

A detailed study of the photo-induced decline in chlorophyll a f fluorescence intensity (Kautsky phenomenon) in coupled isolated chloroplasts from a high level (P) to a low stationary level (S) is presented. 1. 1. A linear relationship between P → S quenching and intrathylakoid H+ concentration was found. When the light-induced proton gradient was abolished by uncoupling, the fluorescence emission at room temperature was lowered proportionally to increased H+ concentration in the medium. 2. 2. Fluorescence spectra at -196°C of samples frozen at the P and S states showed no significant differences in the Photosystem I/Photosystem II ratio of fluorescence emission. Furthermore, freezing to -196°C reversed the P → S quenching. This indicates that the P → S quenching is not related to an increase of spillover of excitation energy from Photosystem II to Photosystem I. 3. 3. When Mg2+ was added to thylakoids suspended in a medium free of divalent cations, the inhibition of spillover required lower Mg2+ concentrations (half saturation at 0.6 mM) than the restoration of the P → S quenching (half saturation at 2 mM Mg2+). Increased proton concentration in the medium also inhibited spillover. 4. 4. The results are interpreted in terms of two sites of Mg2+ and H+ effects on excitation deactivation in Photosystem II. One site is located on the outer face of the thylakoid membrane; action of both Mg2+ and H+ at this side diminishes spillover. The second site is located on the inner face of the membrane; as Mg2+ is displaced there by protons, a non-photochemical quenching of Photosystem II fluorescence is induced, which is manifested by the P → S decline. © 1979.