Relationships between quantum yield for CO₂ assimilation, activity of key enzymes and CO₂ leakiness in Amaranthus cruentus, a C₄ dicot, grown in high or low light

In C₄ 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 C₄ cycle, the leak may decrease the efficiency of CO₂ assimilation. To examine this possibility, we studied the light dependence of CO₂ leakiness (φ), 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). φ was constant at photon flux densities (PFDs) >200 μmol m^-2 s^-1 and was around 0.3. At PFDs <150 μmol m^-2 s^-1, φ increased markedly as PFD decreased. At 40 μmol m^-2 s^-1, φ 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 φ with decreasing PFD contributed to the decrease in the CO ₂ 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 φ in LL are discussed.