Mechanism of c(4) photosynthesis: the size and composition of the inorganic carbon pool in bundle sheath cells.

We sought to characterize the inorganic carbon pool (CO(2) plus HCO(3) (-)) formed in the leaves of C(4) plants when C(4) acids derived from CO(2) assimilation in mesophyll cells are decarboxylated in bundle sheath cells. The size and kinetics of labeling of this pool was determined in six species representative of the three metabolic subgroups of C(4) plants. The kinetics of labeling of the inorganic carbon pool of leaves photosynthesizing under steady state conditions in (14)CO(2) closely paralleled those for the C-4 carboxyl of C(4) acids for all species tested. The inorganic carbon pool size, determined from its (14)C content at radioactivity saturation, ranged between 15 and 97 nanomoles per milligram of leaf chlorophyll, giving estimated concentrations in bundle sheath cells of between 160 and 990 micromolar. The size of the pool decreased, together with photosynthesis, as light was reduced from 900 to 95 microeinsteins per square meter per second or as external CO(2) was reduced from 400 to 98 microliters per liter. A model is developed which suggests that the inorganic carbon pool existing in the bundle sheath cells of C(4) plants during steady state photosynthesis will comprise largely of CO(2); that is, CO(2) will only partially equlibrate with bicarbonate. This predominance of CO(2) is believed to be vital for the proper functioning of the C(4) pathway.