Mechanism of C4 Photosynthesis

Abstract

We sought to characterize the inorganic carbon pool (CO2 plus HCO3-) formed in the leaves of C4 plants when C4 acids derived from CO2 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 C4 plants. The kinetics of labeling of the inorganic carbon pool of leaves photosynthesizing under steady state conditions in 14CO2 closely paralleled those for the C-4 carboxyl of C4 acids for all species tested. The inorganic carbon pool size, determined for its 14C content at radioactivity saturation, ranged between 15 and 97 nanomoles per milligram of leaf chlorophyll, giving estimated concentrations in bundle sheath cells 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 CO2 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 C4 plants during steady state photosynthesis will comprise largely of CO2; that is, CO2 will only partially equilibrate with bicarbonate. This predominance of CO2 is believed to be vital for the proper functioning of the C4 pathway.