The C 4 -pathway of photosynthesis. Evidence for an intermediate pool of carbon dioxide and the identity of the donor C 4 -dicarboxylic acid.

1. Leaves were exposed to (14)CO(2) under steady-state conditions for photosynthesis. The kinetics of entry or loss of label in pools of CO(2) and other compounds was examined during the period of the pulse and a ;chase' with (12)CO(2). 2. With maize the kinetics of labelling of the major CO(2) 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 CO(2). 4. The size and turnover time of the CO(2) and C(4) acid pools was calculated. These results provided the basis for estimating the concentration of CO(2) 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 C(4)-pathway and the operation of a CO(2) concentrating mechanism to serve ribulose diphosphate carboxylase.