Fractional control of photosynthesis by the QB protein, the cytochrome f/b 6 complex and other components of the photosynthesic apparatus.

In order to obtain information on fractional control of photosynthesis by individual catalysts, catalytic activities in photosynthetic electron transport and carbon metabolism were modified by the addition of inhibitors, and the effect on photosynthetic flux was measured using chloroplasts of Spinacia oleracea L. In thylakoids with coupled electron transport, light-limited electron flow to ferricyanide was largely controlled by the QB protein of the electron-transport chain. Fractional control by the cytochrome f/b 6 complex was insignificant under these conditions. Control by the cytochrome f/b 6 complex dominated at high energy fluence rates where the contribution to control of the QB protein was very small. Uncoupling shifted control from the cytochrome f/b 6 complex to the QB protein. Control of electron flow was more complex in assimilating chloroplasts than in thylakoids. The contributions of the cytochrome f/b 6 complex and of the QB protein to control were smaller in intact chloroplasts than in thylakoids. Thus, even though the transit time for an electron through the electron-transport chain may be below 5 ms in leaves, oxidation of plastohydroquinone was only partially responsible for limiting photosynthesis under conditions of light and CO2 saturation. The energy fluence rate influenced control coefficients. Fractional control of photosynthesis by the ATP synthetase, the cytochrome f/b 6 complex and by ribulose-1,5-bisphosphate carboxylase increased with increasing fluence rates, whereas the contributions of the QB protein and of enzymes sensitive to SH-blocking agents decreased. The results show that the burdens of control are borne by several components of the photosynthetic apparatus, and that burdens are shifted as conditions for photosynthesis change.