Energetic Factors Affecting Carbon Dioxide Fixation in Isolated Chloroplasts

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Abstract
Light- and HCO3--saturated (10 mM) rates of O2 evolution (120-220 .mu.mol O2/mg chlorophyll per h), obtained with intact spinach chloroplasts, are decreased up to 3-fold by changes in assay conditions such as omission of catalase from the medium, the use of high (.gtoreq. 1 mM) Pi, inclusion of NO2- as an electron acceptor, or bright illumination at low partial pressures of O2. These inhibitions may be reversed by addition of uncoupling levels of NH4Cl or of antimycin concentrations that partially block cyclic electron transfer between cytochrome b6 and cytochrome f. Measurements of the pH gradient across the thylakoid membrane with the fluorescent probe, 9-aminoacridine, indicate that changes in .DELTA.pH are sufficient to account for both the inhibited and restored rates of electron transport. The rate of HCO3--saturated photosynthesis may be restricted by a proton gradient back pressure under these conditions. The rate of O2 evolution is also decreased 3-fold when ambient CO2 (0.063 .mu.mol HCO3- at pH 8.1) is used in place of saturating HCO3- and chloroplasts are illuminated aerobically with catalase and a low level (0.25 .mu.mol) of K2HPO4. Only inhibitory effects are observed with additions of antimycin or NH4Cl. Under these conditions, excessive photophosphorylation or a large pH gradient does not limit the rate of photosynthesis.