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Metabolism and Enzymology

Relationship between Photosynthetic Electron Transport and Stromal Enzyme Activity in Pea Leaves ¹

Toward an Understanding of the Nature of Photosynthetic Control

Department of Applied Genetics, Johns Innes Institute, Colney Lane, Norwich, NR4 7UH, United Kingdom, Department of Biology, University of Essex, Colchester, CO4 3SQ, United Kingdom, Laboratoire du Métabolisme, INRA, Route de St-Cyr, 78026 Versailles, France

The responses of the quantum efficiencies of photosystem (PS) II and PSI measured in vivo simultaneously with estimations of the activities and activation states of NADP-malate dehydrogenase, chloroplast fructose-1,6-bisphosphatase, and ribulose-1,5-bisphosphate carboxylase were used to study the relationship between electron transport and carbon metabolism. The effects of varying irradiance and CO₂ partial pressure on the relationship between the quantum efficiencies of PSI and II, and the activity of these enzymes shows that the interrelationships vary according to the limitations placed on the system. The relationship between the quantum efficiencies of PSII and PSI was linear in most situations. In response to increasing irradiance, the activity of all three enzymes increased. In the case of NADP-malate dehydrogenase this increase was well correlated with the estimated flux of electrons through PSI and PSII. The other two enzymes showed a more complex relationship with the estimated flux of electrons through both photosystems. These relationships are consistent with the known interactions between these stromal enzymes and the thylakoids. The response to varying CO₂ partial pressure is more complex. The efficiencies of PSI and II declined with decreasing CO₂ partial pressure and the activity of each enzyme varied uniquely. However, there are clear correlations between the activities of the enzymes and the flux of electrons through the photosystems. In contrast to the data obtained under conditions of varying irradiance, there is clear evidence of photosynthetic control of electron transport when the CO₂ concentration is varied.

¹ This work was supported by the United Kingdom Agricultural and Food Research Council via a grant-in-aid to the John Innes Institute and the Institut National de la Recherche Agronomique, France. Support was also given to J. H. from the Perry Foundation, Boreham, Chelmsford, CM3 3AX, Essex, U.K., and the Organization for Economic Cooperation and Development, Paris, France. B. G. received support from the Organization for Economic Cooperation and Development.

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