Time-resolved monitoring of flash-induced changes of fluorescence quantum yield and decay of delayed light emission in oxygen-evolving photosynthetic organisms

The present contribution describes a new experimental setup that permits time-resolved monitoring of the rise kinetics of the relative fluorescence yield, Phi (rel)(t), and simultaneously of the decay of delayed light emission, L(t), induced by strong actinic laser flashes. The results obtained by excitation of dark-adapted samples with a train of eight flashes reveal (a) in suspensions of spinach thylakoids, Phi (rel)(t) exhibits a typical period four oscillation that is characteristic for a dependence on the redox states S-i of the water oxidizing complex (WOC), (b) the relative extent of the unresolved "instantaneous" rise to the level Phi (rel)(rise)(100 ns) at 100 ns and the maximum values of Phi (rel)(t) attained at about 45 mus after each actinic flash, Phi (rel)(m)(45 mus) synchronously oscillate and exhibit the largest values at flash nos. 1 and 5 and minima after flash nos, 2 and 3, (c) opposite effects are observed for the normalized extent of the rise kinetics in the 100 ns to 5 mus time domain of relative fluorescence yield, Phi (rel)(5 mus) - Phi (rel)(100 ns), i.e., both parameters attain minimum and maximum values after the first/fifth and second/third flash, respectively, and (d) analogous features for the "fast" and "slow" ns-kinetics of the fluorescence rise were observed in suspensions of Chlamydomas reinhardtii cells. A slight phase shift by one flash is ascribed to physiological differences. The applicability of this noninvasive technique to study reactions of photosystem TI, especially the reduction kinetics of P680(.+) and their dependence on the redox state S-i of the WOC, is discussed.