Cation effects on the fluorescence of isolated chloroplasts.

At room temperature, the fluorescence of chloroplasts in vivo and in vitro originates nearly exclusively from the chlorophyll of the oxygen evolving system II of the photosynthetic apparatus. The yield and the kinetics of this light emission are closely coupled to the primary photochemical events of photosystem II. Fluorescence studies, therefore, have contributed much to our knowledge about the water oxidizing photoact. According to Duysens and Sweers i(3) the fluorescence yield is determined by the oxidation state of the primary electron acceptor Q: when Q is oxidized, the fluorescence is quenched and, consequently, a low yield is observed; since reduced Q1(Q-) does not quench, a maximal fluorescence yield indicates a complete reduction of the electron acceptor pools containing Q. Several anomalies in the fluorescence kinetics of intact algae have led to the assumption of an non-homogeneous pool Q containing, even after the primary activation step (8), active and inactive oxidants Qa and Qi' (1,2, 4,12). Both forms of Q are thought to quench the fluorescence in their oxidized states, albeit with slightly different effectiveness. A photoreduction of the total pool Q requires an activation of any Qi' to Qa, which according to some authors (1,4,12) is sensitized by photosystem I. In this communication, we wish to report on an overriding influence of the ionic environment on the fluorescence yield of isolated chloroplasts. It will be shown that the fluorescence yield can be changed at will simply by varying the composition of the suspension medium. Accordingly, the fluorescence yield may possibly be governed not only by the oxidation state olf the Q-pool, but also by ion induced structural changes.