THE ROLE OF ASCORBIC-ACID IN PHOTOSYNTHETIC ELECTRON-TRANSPORT

The role of the ascorbate-ascorbate peroxidase system in photosynthetic electron transport was investigated in stroma-free thylakoids. It was found that high rates of light-dependent oxygen evolution are observed in the presence of ascorbate and H2O2, provided that membrane-bound ascorbate peroxidase is active, to produce monodehydroascorbate. It was found that monodehydroascorbate photoreduction is inhibited by DCMU and DBMIB, and that it showed Emerson enhancement. Monodehydroascorbate was shown to compete for electrons with ferredoxin-NADP at the reducing side of Photosystem I. The photoreduction of monodehydroascorbate generated Delta pH across the membranes, and was stimulated by uncouplers. It is concluded that monodehydroascorbate is an electron acceptor at the reducing side of Photosystem I.