Ca2+ requirement for photosynthetic oxygen evolution of spinach and mangrove photosystem II membrane preparations

CaCl2 significantly increased the photosynthetic oxygen‐evolution activity of photosystem II membrane preparations isolated from spinach and the mangrove Avicennia marina, but not that of spinach thylakoids. This increase is composed of two parts; one due to Cl−, the other to Ca2+. Under Cl−‐sufficient conditions photosystem II particle preparations required only 2 mM Ca2+ for maximum oxygen evolution, irrespective of the presence of the 23‐ and 18‐kDa polypeptides. It is suggested that there is no immediate interaction between Ca2+ and the 23‐ or 18‐kDa polypeptides and that instead Ca2+ acts directly on the oxygen‐evolving centre, perhaps via another, as yet unidentified protein.