Relative oxygen-evolution activity of resuspended PSII samples previously exposed to 47°C for various time periods. Oxygen evolution was measured in the presence of 5 mM CaCl2 (solid circles), 5 mM CaCl2 plus 50 mM MgCl2 (open triangles) and 55 mM CaCl2 (open circles). Additionally all media contained 15 mM NaCl, 1 M betaine, 20 mM MES-NaOH, pH 6, and 1 mM K3[Fe(CN)6] plus 0.25 mM DCBQ as electron acceptors. (Solid line), Single exponential decay with a rate constant of 1 min−1. (Dotted line), The small offset value discussed in the body of the text.