Light Dependences Of The Co2 Leakiness, Quantum Yield Of Co2 Fixation And Activation State Of Key Enzymes In A C4 Plant, Amaranthus Cruentus, Grown In High- And Low-Light

Abstract

In C4 photosynthesis, some CO2 leak from the bundle-sheath cells to the mesophyll cells or intercellular air spaces. Because the CO2 leak wastes ATP consumed in the C4 cycle, it may decrease the CO2 assimilation rate, particularly at low incident photon flux density (PFD). In this study, the effects of CO2 leakiness (f) on the quantum yield for CO2 assimilation were investigated in detail using Amaranthus cruentus L. (NADmalic enzyme subtype, dicot) grown in high-light (HL) and low-light (LL). We measured the light responses of ϕ, CO2 assimilation rate, activities of key enzymes in the C4 photosynthesis [ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) and pyruvate, orthophosphate dikinase (PPDK)] and activation state of phosphoenolpyruvate carboxylase (PEPC). ϕ increased with a decrease in PFD below 150 µmol quanta m−2 s−1, and ϕ was 0.55 in LL and 0.76 in HL leaves at 40 µmol quanta m−2 s−1. It is possible that the increase in f results in the slightly decrease in the CO2 assimilation rate at low PFD, which leads to the overestimation of the quantum yield for CO2 assimilation.