The response of mesophyll conductance to short-term variation in CO2 in the C4 plants Setaria viridis and Zea mays

In two C-4 species, mesophyll conductance increases with short-term exposure to decreasing pCO(2) and limits photosynthetic capacity below ambient levels, whilst carbonic anhydrase imposes a further limitation only at very low pCO(2).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 CO2 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 A degrees C in Zea mays. Additionally, we quantified the effect of finite g(m) calculations of leakiness (I center dot) 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 I center dot compared with values calculated with infinite g(m) depending on whether the observed C-13 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 I center dot. At pCO(2) below ambient, the photosynthetic rate was limited by CO2 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 CO2 assimilation at low pCO(2) and improve photosynthetic efficiency under situations when CO2 is limited, such as drought.