The role of ascorbate in the protection of thylakoids against photoinactivation

The inactivation of electron transport upon preillumination of isolated, st roma free thylakoids has been studied. Inactivation is defined here as the loss of activity which is not reversed upon relaxation of qE. It was found that both PS 2 and PS 1 dependent electron transport were inactivated, whil st the coupling of ATP synthesis to electron transport was not affected. Th e inactivation concerned both the transfer of excitation energy to the reac tion centres, and the reaction centres themselves. Ascorbate protected agai nst photoinactivation of the electron transport from H2O to NADP or to meth ylviologen, much less the electron transport depending only on PS 1. The pr otection by ascorbate required its well known action as a cofactor of de-ep oxidation of violaxanthin and the consequent formation of qE: under conditi ons where de-epoxidation was inhibited (presence of DTT or uncouplers) qE w as also suppressed and ascorbate protection was abolished. Ascorbate did no t protect the thylakoids against inactivation caused by H2O2 in the dark. T he latter was shown to concern mostly PS 2 electron transport.