(13)CO(2) labeling kinetics in maize reveal impaired efficiency of C(4) photosynthesis under low irradiance

C(4) photosynthesis allows faster photosynthetic rates and higher water and nitrogen use efficiency than C(3) photosynthesis, but at the cost of lower quantum yield due to the energy requirement of its biochemical carbon concentration mechanism. It has also been suspected that its operation may be impaired in low irradiance. To investigate fluxes under moderate and low irradiance, maize (Zea mays) was grown at 550 µmol photons m(−2) s(−l) and (13)CO(2) pulse-labeling was performed at growth irradiance or several hours after transfer to 160 µmol photons m(−2) s(−1). Analysis by liquid chromatography/tandem mass spectrometry or gas chromatography/mass spectrometry provided information about pool size and labeling kinetics for 32 metabolites and allowed estimation of flux at many steps in C(4) photosynthesis. The results highlighted several sources of inefficiency in low light. These included excess flux at phosphoenolpyruvate carboxylase, restriction of decarboxylation by NADP-malic enzyme, and a shift to increased CO(2) incorporation into aspartate, less effective use of metabolite pools to drive intercellular shuttles, and higher relative and absolute rates of photorespiration. The latter provides evidence for a lower bundle sheath CO(2) concentration in low irradiance, implying that operation of the CO(2) concentration mechanism is impaired in this condition. The analyses also revealed rapid exchange of carbon between the Calvin–Benson cycle and the CO(2)-concentration shuttle, which allows rapid adjustment of the balance between CO(2) concentration and assimilation, and accumulation of large amounts of photorespiratory intermediates in low light that provides a major carbon reservoir to build up C(4) metabolite pools when irradiance increases.