The role of ascorbate in the protection of thylakoids against photoinactivation

The inactivation of electron transport upon preillumination of isolated, stroma 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, whilst the coupling of ATP synthesis to electron transport was not affected. The inactivation concerned both the transfer of excitation energy to the reaction centres, and the reaction centres themselves. Ascorbate protected against photoinactivation of the electron transport from H^sub 2^O to NADP or to methylviologen, much less the electron transport depending only on PS 1. The protection by ascorbate required its well known action as a cofactor of de-epoxidation of violaxanthin and the consequent formation of qE: under conditions where de-epoxidation was inhibited (presence of DTT or uncouplers) qE was also suppressed and ascorbate protection was abolished. Ascorbate did not p rotect the thylakoids against inactivation caused by H^sub 2^O^sub 2^in the dark.The latter was shown to concern mostly PS 2 electron transport.[PUBLICATION ABSTRACT]