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Articles

Energetic Factors Affecting Carbon Dioxide Fixation in Isolated Chloroplasts ¹

Biology Department, Upton, New York 11973, Brookhaven National Laboratory, Upton, New York 11973

Light- and HCO₃^–-saturated (10 millimolar) rates of O₂ evolution (120 to 220 micromoles O₂ per milligram chlorophyll per hour), 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 (1 millimolar) inorganic phosphate, 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 pH are sufficient to account for both the inhibited and restored rates of electron transport. It follows that 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 (0.63 millimolar 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 millimolar) 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.