Relationships between quantum yield for CO2 assimilation, activity of key enzymes and CO2 leakiness in Amaranthus cruentus, a C-4 dicot, grown in high or low light

In C-4 photosynthesis, a part of CO2 fixed by phosphoenolpyruvate carboxylase (PEPC) leaks from the bundle-sheath cells. Because the CO2 leak wastes ATP consumed in the C-4 cycle, the leak may decrease the efficiency of CO2 assimilation. To examine this possibility, we studied the light dependence of CO2 leakiness (phi), estimated by the concurrent measurements of gas exchange and carbon isotope discrimination, initial activities of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) and pyruvate, orthophosphate dikinase (PPDK), the phosphorylation state of PEPC and the CO2 assimilation rate using leaves of Amaranthus cruentus (NAD-malic enzyme subtype, dicot) plants grown in high light (HL) and low light (LL). was constant at photon flux densities (PFDs) 200 mol m(2) s(1) and was around 0.3. At PFDs 150 mol m(2) s(1), increased markedly as PFD decreased. At 40 mol m(2) s(1), was 0.76 in HL and 0.55 in LL leaves, indicating that the efficiency of CO2 assimilation at low PFD was greater in LL leaves. The activities of Rubisco and PPDK, and the phosphorylated state of PEPC all decreased as PFD decreased. Theoretical calculations with a mathematical model clearly showed that the increase in with decreasing PFD contributed to the decrease in the CO2 assimilation rate. It was also shown that the conventional quantum yield of photosynthesis obtained by fitting the straight line to the light response curve of the CO2 assimilation rate at the low PFD region is seriously overestimated. Ecological implications of the increase in in LL are discussed.