The response of mesophyll conductance to short-term variation in CO(2) in the C(4) plants Setaria viridis and Zea mays

Mesophyll conductance (g(m)) limits rates of C(3) photosynthesis but little is known about its role in C(4) photosynthesis. If g(m) were to limit C(4) photosynthesis, it would likely be at low CO(2) concentrations (pCO(2)). However, data on C(4)-g(m) across ranges of pCO(2) are scarce. We describe the response of C(4)-g(m) to short-term variation in pCO(2), at three temperatures in Setaria viridis, and at 25 °C in Zea mays. Additionally, we quantified the effect of finite g(m) calculations of leakiness (ϕ) and the potential limitations to photosynthesis imposed by stomata, mesophyll, and carbonic anhydrase (CA) across pCO(2). In both species, g(m) increased with decreasing pCO(2). Including a finite g(m) resulted in either no change or increased ϕ compared with values calculated with infinite g(m) depending on whether the observed (13)C discrimination was high (Setaria) or low (Zea). Post-transitional regulation of the maximal PEP carboxylation rate and PEP regeneration limitation could influence estimates of g(m) and ϕ. At pCO(2) below ambient, the photosynthetic rate was limited by CO(2) availability. In this case, the limitation imposed by the mesophyll was similar or slightly lower than stomata limitation. At very low pCO(2), CA further constrained photosynthesis. High g(m) could increase CO(2) assimilation at low pCO(2) and improve photosynthetic efficiency under situations when CO(2) is limited, such as drought.