Molecular mechanism of high-temperature-induced inhibition of acceptor side of Photosystem II

High-temperature-induced inhibition of the acceptor side of Photosystem II (PS II) was studied in tobacco thylakoids using oxygen evolution, chlorophyll a (Chl a) fluorescence and redox potential measurements. When thylakoids were heated at 2 degrees C/min from 25 to 50 degrees C, the oxygen evolving complex became inhibited between 32 and 45 degrees C, whereas the acceptor side of PS II tolerated higher temperatures. Variable Chl a fluorescence decreased more slowly than oxygen evolution, suggesting that transitions between some S-states occurred even after heat-induced inhibition of the oxygen evolving activity. 77 K emission spectroscopy reveals that heating does not cause detachment of the light-harvesting complex II from PS II, and thus the heat-induced increase in the initial F-0 fluorescence is due to loss of exciton trapping in the heated PS II centers. Redox titrations showed a heat-induced increase in the midpoint potential of the Q(A)/Q(A)(-) couple from the control value of -80 mV to +40 mV at 50 degrees C, indicating a loss of the reducing power of Q(A)(-). When its driving force thus decreased, electron transfer from Q(A)(-) to Q(B) in the PS II centers that still could reduce Q(A) became gradually inhibited, as shown by measurements of the decay of Chl a fluorescence yield after a single turnover flash. Interestingly, the heat-induced loss of variable fluorescence and inhibition of electron transfer from Q(A)(-) to Q(B) could be partially prevented by the presence of 5 mM bicarbonate during heating, suggesting that high temperatures cause release of the bicarbonate bound to PS II. We speculate that both the upshift in the redox potential of the Q(A)/Q(A)(-) couple and the release of bicarbonate may be caused by a heat-induced structural change in the transmembrane D1 or D2 proteins. This structural change may, in turn, be caused by the inhibition of the oxygen evolving complex during heating.