Superoxide radicals are not the main promoters of acceptor-side-induced photoinhibitory damage in spinach thylakoids

Superoxide anion radical formation was studied with isolated spinach thylakoid membranes and oxygen evolving Photosystem LI sub-thylakoid preparations using the reaction between superoxide and Tiron(1,2-dihydroxybenzene-3,5-disulphonate) which results in the formation of stable, EPR detectable Tiron radicals. We found that superoxide was produced by illuminated thylakoids but not by Photosystem II preparations. The amount of the radicals was about 70% greater under photoinhibitory conditions than under moderate light intensity. Superoxide production was inhibited by DCMU and enhanced 4-5 times by methyl viologen. These observations suggest that the superoxide in illuminated thylakoids is from the Mehler reaction occurring in Photosystem I, and its formation is not primarily due to electron transport modifications brought about by photoinhibition, Artificial generation of superoxide from riboflavin accelerated slightly the photoinduced degradation of the Photosystem II reaction centre protein D1 but did not accelerate the loss of oxygen evolution supported by a Photosystem II electron acceptor. However, analysis of the protein breakdown products demonstrated that this added superoxide did not increase the amount of fragments brought about by photoinhibition but introduced an additional pathway of damage. On the basis of the above observations we propose that superoxide radicals are not the main promoters of acceptor-side-induced photoinhibition of Photosystem II.