THE ROLE OF ASCORBIC-ACID IN PHOTOSYNTHETIC ELECTRON-TRANSPORT

The role of the ascorbate-ascorbate peroxidase system in photosyntheti c electron transport was investigated in stroma-free thylakoids. It wa s found that high rates of light-dependent oxygen evolution are observ ed in the presence of ascorbate and H2O2, provided that membrane-bound ascorbate peroxidase is active, to produce monodehydroascorbate. It w as found that monodehydroascorbate photoreduction is inhibited by DCMU and DBMIB, and that it showed Emerson enhancement. Monodehydroascorba te was shown to compete for electrons with ferredoxin-NADP at the redu cing side of Photosystem I. The photoreduction of monodehydroascorbate generated Delta pH across the membranes, and was stimulated by uncoup lers. It is concluded that monodehydroascorbate is an electron accepto r at the reducing side of Photosystem I.