| Abstract: | The protective action of co-solutes, such as sucrose and glycinebetaine, against the thermal inactivation of photosystem II function was studied in untreated and Mn-depleted photosystem II preparations. It was shown that, in addition to the reactions that depend on the oxygen evolving activity of the photosystem, those that implicate more intimately the reaction center itself are protected by high concentrations of osmolytes. However, the temperature required to inhibit oxygen evolution totally in the presence of osmolytes is lower than that required to eliminate reactions, such as P680 (primary electron donor in photosystem II) photo-oxidation and pheophytin photo reduetion, which only involve charge separation and primary electron transport processes. The energy storage measured from the thermal dissipation yield during photoacoustic experiments and the yield of variable fluorescence are also protected to a significant degree (up to 30%) at temperatures at which oxygen evolution is totally inhibited. It is suggested that a cyclic electron transport reaction around photosystem II may be preserved under these conditions and may be responsible for the energy storage measured at relatively high temperatures. This interpretation is also supported by thermoluminescence data involving the recombination between reduced electron acceptors and oxidized electron donors at - 30 and - 55 °C. The data also imply that a high concentration of osmolyte allows the stabilization of the photosystem core complex together with the oxygen-evolving complex. The stabilization effect is understood in terms of the minimization of protein-water interactions as proposed by the theory of Arakawa and Timasheff (Biophys. J., 47 (1985) 411--414). |
