The C4-pathway of photosynthesis. Evidence for an intermediate pool of carbon dioxide and the identity of the donor C4-dicarboxylic acid

摘要

p1. Leaves were exposed to sup14/supCOsub2/sub under steady-state conditions for photosynthesis. The kinetics of entry or loss of label in pools of COsub2/sub and other compounds was examined during the period of the pulse and a ‘chase’ with sup12/supCOsub2/sub. 2. With maize the kinetics of labelling of the major COsub2/sub pool and of depletion of label during a ‘chase’ was consistent with this pool being derived from the C-4 of malate and being the precursor of the C-1 of 3-phosphoglycerate. 3. Similar results were obtained for iAmaranthus/i leaves except that the C-4 of aspartate rather than malate was apparently the primary source of COsub2/sub. 4. The size and turnover time of the COsub2/sub and Csub4/sub acid pools was calculated. These results provided the basis for estimating the concentration of COsub2/sub in the bundle-sheath cells or chloroplasts assuming the pool was largely restricted to one or other of these compartments. 5. These findings are considered in relation to current schemes for the Csub4/sub-pathway and the operation of a COsub2/sub concentrating mechanism to serve ribulose diphosphate carboxylase./p