Mesophyll conductance in Zea mays responds transiently to CO2 availability: implications for transpiration efficiency in C4 crops

Mesophyll conductance (g(m)) describes the movement of CO2 from the intercellular air spaces below the stomata to the site of initial carboxylation in the mesophyll. In contrast with C-3-g(m), little is currently known about the intraspecific variation in C-4-g(m) or its responsiveness to environmental stimuli. To address these questions, g(m) was measured on five maize (Zea mays) lines in response to CO2, employing three different estimates of g(m). Each of the methods indicated a significant response of g(m) to CO2. Estimates of g(m) were similar between methods at ambient and higher CO2, but diverged significantly at low partial pressures of CO2. These differences are probably driven by incomplete chemical and isotopic equilibrium between CO2 and bicarbonate under these conditions. Carbonic anhydrase and phosphoenolpyruvate carboxylase invitro activity varied significantly despite similar values of g(m) and leaf anatomical traits. These results provide strong support for a CO2 response of g(m) in Z. mays, and indicate that g(m) in maize is probably driven by anatomical constraints rather than by biochemical limitations. The CO2 response of g(m) indicates a potential role for facilitated diffusion in C-4-g(m). These results also suggest that water-use efficiency could be enhanced in C-4 species by targeting g(m).