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Quantum Requirement for Photosynthesis in Sedum praealtum during Two Phases of Crassulacean Acid Metabolism ¹

Maurice S. B. Ku

Horticulture Department, University of Wisconsin, Madison, Wisconsin 53706, Biochemistry Department, University of Georgia, P. O. Box 5677, Athens, Georgia 30604, United States Department of Agriculture, Science and Education Administration, Richard B. Russell Agricultural Research Center. P. O. Box 5677, Athens, Georgia 30604

The quantum requirement (QR) for photosynthesis in Sedum praealtum, a Crassulacean acid metabolism plant, was compared with that of wheat, a C₃ plant, and maize, a C₄ plant, at 30 C. During the deacidification phase in S. praealtum, approximately 16 moles quanta were absorbed per mole malate consumed. This is equivalent to 16 moles quanta per mole CO₂ fixed, assuming 1 mole CO₂ is assimilated per mole malate decarboxylated. This QR for Crassulacean acid metabolism is similar to that of the C₃ or C₄ plant under atmospheric conditions, even though there are considerable differences in the biochemistry of photosynthesis. During late-afternoon C₃-like fixation of atmospheric CO₂ in S. praealtum, the QR was relatively high with values of 41 under 21% O₂ and 19 under 2% O₂. During the deacidification phase in S. praealtum, the relatively low QR can be accounted for by the repression of photorespiration and saturation of photosynthesis from the elevated CO₂ concentration in the leaves during malate decarboxylation.

¹ This research was supported in part by the College of Agricultural and Life Sciences, University of Wisconsin, Madison, and by National Science Foundation Grant PCM 77-09384 to G.E.E.

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