Photoinhibition of Electron Transport Activity of Photosystem II in Isolated Thylakoids Studied by Thermoluminescence and Delayed Luminescence

The effect of photoinhib i t ion on the pr imary ( O a ) and secondary ( O b ) quinone acceptors of photosystem I I was investigated in isolated spinach thylakoids by the methods of thermoluminescence and delayed luminescence. The amplitudes of the Q (at about 2 °C) and B (at about 30 °C) thermoluminescence bands which are associated w i th the recombination of the S ;OA and S2QB charge pairs, respectively, exhibited parallel decay courses dur ing photoinhibi tory treatment. Similarly, the amplitudes of the flash-induced delayed luminescence components ascribed to the recombination of S 2 0 A and S2OB charge pairs and having half life-times of about 3 s and 30 s, respectively, declined in parallel wi th the amplitudes of the corresponding Q and B thermoluminescence bands. The course of inhib i t ion of thermoluminescence and delayed luminescence intensity was parallel w i th that of the rate of oxygen evolut ion. The peak positions of the B and Q thermoluminescence bands as well as the half l ife-times of the corresponding delayed luminescence components were not affected by photoinhib i t ion. These results indicate that in isolated thylakoids neither the amount nor the stabil ity of the reduced OB acceptor is preferential ly decreased by photoinhibi t ion. We conclude that either the primary target of photodamage is located before the O b binding site in the reaction center of photosystem I I or Q A and OB undergo simultaneous damage.