Sites Along the Electron‐Transport Chain Controlled by the Energy‐Conversion System in Chloroplasts

The light‐induced absorbance changes of electron transport carriers were measured in intact (class I) chloroplasts which catalyzed oxygen evolution with phosphoglyceric acid or CO₂ as electron acceptors. In these chloroplasts the reduction of cytochrome b₅₆₃, induced by light at 720 nm, increased in the presence of ADP or in the presence of uncouplers such as gramicidin and NH₄Cl. In the presence of ADP there was a decrease in the light‐induced reduction of C550 and in the oxidation of cytochrome f. The effect of ADP was augmented when phosphoglyceric acid was used as an electron acceptor in class I chloroplasts. A decrease in the photoreduction of C550 was also observed in the presence of the uncoupler gramicidin in class I and class II, with benzylviologen as an electron acceptor, and in class II chloroplasts with ferricyanide. The uncoupler gramicidin caused an increase in the light‐induced reduction of cytochrome b₅₆₉. Based on these effects of ADP or uncoupler on the steady state of the light‐induced redox level of the carriers, we proposed sites along the electron transport chain in which the rate of electron flow is controlled by the energy conversion system.