Inhibition of photosystem II electron transport in chloroplasts by fatty acids and restoration of its activity by Mn 2+

Fatty acids, especially C1a-unsaturated acids, represent a substantial portion of the thylakoid lipids [l] and play an important role in the maintenance of the structural and functional integrity of the chloroplast membrane [2,3] . Interesting enough, free fatty acids, which accumulate in the membrane due to lipid hydrolysis under special conditions such as aging in vitro [4-71, seem to influence the structure and function of chloroplasts in a specific way. For instance, exogenous C18-unsaturated fatty acids enhanced chloroplast swelling and inhibited light-induced shrinkage [7]. They also caused a sequential inhibition of photosystern II and photosystem I electron transports and the respective photosynthetic phosphorylations [8] . However, in spite of much evidence that fatty acids at appropriate concentrations are specific inhibitors of the photosystem II electron transport [8,9] , their precise inhibition site is still unknown. In order to localize this site, several specific inhibitors of photosystem II (DCMU and Tris-washing) and artificial electron donors (1 ,5-diphenylcarbazide and MnCl,) were tested in various combinations, with or without fatty acids, for their action on the photoreduction of 2,6-dichlorophenol indophenol. The results indicate that the inhibition site of unsaturated fatty acids lies on the oxidizing side of photosystem II, probably between the hypothetical carriers Y2 and Y3 (see scheme, fig. 3). An additional result of this investigation was the finding that Mn2+ interacts with some unknown electron carriers of the electron transport chain located between H20 and the pigments complex II, allowing a shunt between Y, and Y,, bypassing thereby the fatty