Energy‐Dependent Reverse Electron Flow in Chloroplasts

An ATP‐driven rise in fluorescence yield (Q reduction) and cytochrome f oxidation in light‐triggered chloroplasts are demonstrated. Both effects are shown to be dependent on the creation and maintenance of the light‐triggered state. ATP‐induced reduction of Q depends upon the presence of an electron donor in the dark with a redox potential up to, but not exceeding, 300 mV. The effects are abolished by uncouplers or energy transfer inhibitors. 3‐(3,4‐Dichlorophenyl)‐1,1‐dimethylurea inhibits the ATP‐driven reduction of Q but not that of cytochrome oxidation. A location for a coupling site and the site of action of electron‐transport inhibitors is suggested on the basis of these observations.