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

The quantum requirement (QR) for photosynthesis in Sedum praealtum, a Crassulacean acid metabolism plant, was compared with that of wheat, a C(3) plant, and maize, a C(4) 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 CO(2) fixed, assuming 1 mole CO(2) is assimilated per mole malate decarboxylated. This QR for Crassulacean acid metabolism is similar to that of the C(3) or C(4) plant under atmospheric conditions, even though there are considerable differences in the biochemistry of photosynthesis. During late-afternoon C(3)-like fixation of atmospheric CO(2) in S. praealtum, the QR was relatively high with values of 41 under 21% O(2) and 19 under 2% O(2). 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 CO(2) concentration in the leaves during malate decarboxylation.