The efficiency of C4 photosynthesis under low light conditions: assumptions and calculations with CO2 isotope discrimination

Leakiness (phi), the proportion of carbon fixed by phosphoenolpyruvate carboxylation that leaks out of the bundle-sheath cells, determines C-4 photosynthetic efficiency. Large increases in phi have been described at low irradiance. The underlying mechanisms for this increase remain uncertain, but changes in photorespiration or the energy partitioning between the C-4 and C-3 cycles have been suggested. Additionally, values of phi at low light could be magnified from assumptions made when comparing measured photosynthetic discrimination against C-13 (Delta) with the theoretical formulation for Delta. For example, several simplifications are often made when modelling Delta to predict phi including: (i) negligible fractionation during photorespiration and dark respiration; (ii) infinite mesophyll conductance; and (iii) CO2 inside bundle-sheath cells (C-s) is much larger than values in mesophyll cells (C-m). Theoretical models for C-4 photosynthesis and C-4 Delta were combined to evaluate how these simplifications affect calculations of Delta and phi at different light intensities. It was demonstrated that the effects of photorespiratory fractionations and mesophyll conductance were negligible at low light. Respiratory fractionation was relevant only when the magnitude of the fractionation factor was artificially increased during measurements. The largest error in estimating phi occurred when assuming C-s was much larger than C-m at low light levels, when bundle-sheath conductance was large (g(s)), or at low O-2 concentrations. Under these conditions, the simplified equation for Delta overestimated phi, and compromised comparisons between species with different g(s), and comparisons across O-2 concentrations.