Increases in the fluorescence F-0 level and reversible inhibition of Photosystem II reaction center by high-temperature treatments in higher plants

Effects of high temperatures on the fluorescence F-m (maximum fluorescence) and F-o (dark level fluorescence) levels were studied and compared with those of the photochemical reactions of PS II. These comparisons were performed during and after the high temperature treatments. The following results were obtained; (1)increases in the F-o level at high temperatures were partly reversible, (2) the F-m level in the presence of dithionite in spinach chloroplasts decreased at high temperatures and also showed a partial reversibility, (3) photoreductions of pheophytin a and QA were reversibly inhibited at high temperatures parallel to the decrease in the difference between the F-m and F-o levels, and (4) the decrease in the fluorescence F-m level seemed to be related to denaturation of chlorophyll-proteins. All the data suggested that, as well as the separation of light-harvesting chlorophyll a/b protein complexes of PS II from the PS II core complexes, partly reversible inactivation of the PS II reaction center at high temperatures is the cause of the increase in the F-o level.