Effects of High Temperatures on Photosynthetic Systems: Fluorescence Fo Increases in Cyanobacteria

Oxygen evolution is one of the most heat-sensitive sites in photosynthetic organisms. To investigate effects of heat stress on photosynthesis, high-temperature (HT) -induced fluorescence Fo (minimum fluorescence) increases and Fm (maximum fluorescence) decreases had been studied by many workers (1). In higher plants, the Fo increase was attributed to irreversible detachment of light-harvesting chlorophyll a/b protein complexes from the reaction center (RC) complexes of Photosystem (PS) II, to partly reversible inactivation of PS 11 (2, 3), and to dark reduction of QA (4). The Fm decrease is related to inhibition of oxygen evolution (5, 6). However, Fm levels in the presence of 3- (3,4-dichlorophenyl) -1,1-dimethylurea and NH2OH, which cancelled the inhibitory effect of heat stress on oxygen evolution, were also decreased by HT treatments, showing that RCs of PS II were inactivated by high temperature-stress and that the inactivated PS II RC complexes become less fluorescent (6). In cyanobacteria, however, little is known about the effects of HTs on photosynthetic systems and on these fluorescence parameters. In this report, we show that the Fo increase in cyanobacteria is due to partly reversible release of phycobilisomes from the RC complexes of PS II and to partly reversible inactivation of PS II RC.