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Plant and Cell Physiology, 1995, Vol. 36, No. 8 1629-1637
© 1995

Bioenergetics of Carbon Assimilation in Intact Chloroplasts: Coupling of Proton to Electron Transport at the Ratio H+/e=3 Is Incompatible with H+/ATP=3 in ATP Synthesis

Yoshichika Kobayashi1, Werner Kaiser2 and Ulrich Heber2

1 Department of Forestry, Faculty of Agriculture, Kyushu University Hakozaki, Higashi-ku, Fukuoka, 812 Japan
2 Julius-von-Sachs-Institut für Biowissenschaften, Universität Würzburg D-97082 Würzburg, Germany

During a transition from aerobic to largely anaerobic conditions light-saturated carbon assimilation of intact chloroplasts was not decreased although both the transthylakoid proton gradient and ATP levels declined. After a dark period under anaerobiosis, illumination failed to initiate carbon assimilation. ATP increased only transiently in the light and then returned to the dark level. Under such conditions, the addition of electron acceptors such as oxygen, oxalacetate or nitrite resulted in the increase of ATP levels and carbon assimilation was initiated. Assimilation continued under anaerobiosis in the presence of reduced proton gradients and reduced ATP levels after electron acceptors added in addition to bicarbonate were reduced.

Cyclic electron transport was inhibited when anaerobiosis did not permit linear electron transport. It was induced in this situation by micromolar concentrations of oxygen or when, under anaerobiosis, DCMU decreased PSII activity. Oxygen inhibited cyclic electron transport by draining electrons from the cyclic pathway only when electron donation from PSII was weak. The observations give evidence of the delicate redox balance required for cyclic electron transport.

Since H+/e=3 in linear electron transport, the observations of effective carbon reduction under a decreased transthylakoid proton gradient and decreased levels of ATP are incompatible with H+/ATP=2 or 3. They are compatible with H+/ATP=4.

(Received May 1, 1995; Accepted October 3, 1995)
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