The Role of Secondary Quinone Acceptor Q B of Photosystem II in Photoinhibition of Isolated Wheat Chloroplasts

Abstract

Summary The role of semiquinone anion radicals in photoinhibition of isolated wheat ( Triticum aestivum ) chloroplasts was investigated by subjecting the chloroplasts to high light stress in the presence or absence of DCMU and DBMIB. The decrease in the efficiency of PS II photochemistry measured as F v /F m ratio and oxygen evolution after photoinhibition of isolated wheat chloroplasts was less in the presence of DCMU. We suggest that the protective effect of DCMU is due to its binding to the 32 kDa Q B -binding protein and reducing the probability of formation of semiquinone anion and other free radical species that have been suggested to be involved in photoinhibition damage. The hypothesis was also tested by using DBMIB during photoinhibition. DBMIB is known to reduce the plastoquinone pool, resulting in an increase in the semiquinone ion population. A greater extent of reduction of F v /F m and oxygen evolution was observed when chloroplasts were photoinhibited in the presence of DBMIB. The results suggest an involvement of reduced semiquinones in the photoinhibition of wheat chloroplasts. A partial recovery of variable chlorophyll fluorescence in the presence of 20 mM hydroxylamine was also observed in chloroplasts subjected to light stress.