Mechanism of C4 photosynthesis in Chloris gayana: pool sizes and kinetics of 14CO2 incorporation into 4-carbon and 3-carbon intermediates.

In one group of C₄ species, including Chloris gayana, C₄ acids are decarboxylated via phosphoenolpyruvate carboxykinase to give phosphoenolpyruvate as the initial C₃ product. This paper presents an analysis of the kinetics of labeling of various photosynthetic intermediates in Chloris gayana leaves exposed to ¹⁴CO₂, and the pool sizes of these intermediates, primarily to provide information about the subsequent metabolism of phosphoenolpyruvate. Saturation labeling of the C-4 of aspartate and malate, and the C-1 of 3-phosphoglycerate, indicated photosynthetically active pools of 0.45, 0.22, and 0.95 μol/mg chlorophyll, respectively. For aspartate and 3-phosphoglycerate, the total leaf pools and the photosynthetic pools were of similar size, but the total pool of malate was about 100 times larger than the photosynthetically active pool. From the relative rates of labeling of phosphoenolpyruvate, pyruvate, alanine, and C-1, C-2 plus C-3 of aspartate, during steady-state ¹⁴CO₂ assimilation, relative pool sizes were calculated to be about 10:11:78:100, respectively. Pulse/chase labeling of leaves provided estimates of relative photosynthetic pool sizes in the ratio of about 6:15:90:100, respectively, where aspartate is arbitrarily assigned a value of 100 in both cases. Notably, labeling of alanine was consistent with its derivation from the C-1, C-2 plus C-3 carbons of aspartate, and the alanine pool was at least eight times larger than the phosphoenolpyruvate pool that showed similar labeling kinetics. Results were consistent with the view that at least most of the phosphoenolpyruvate produced by C₄ acid decarboxylation is metabolized via alanine.