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

1. Leaves were exposed to ¹⁴CO₂ under steady-state conditions for photosynthesis. The kinetics of entry or loss of label in pools of CO₂ and other compounds was examined during the period of the pulse and a ‘chase’ with ¹²CO₂. 2. With maize the kinetics of labelling of the major CO₂ 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 Amaranthus leaves except that the C-4 of aspartate rather than malate was apparently the primary source of CO₂. 4. The size and turnover time of the CO₂ and C₄ acid pools was calculated. These results provided the basis for estimating the concentration of CO₂ 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 C₄-pathway and the operation of a CO₂ concentrating mechanism to serve ribulose diphosphate carboxylase.