Differences in stomatal sensitivity to CO2 and light influence variation in water use efficiency and leaf carbon isotope composition in two genotypes of the C4 plant Zea mays

The ratio of net CO2 uptake (A(net)) and stomatal conductance (g(s)) is an intrinsic measurement of leaf water use efficiency (WUEi) however its measurement can be challenging for large phenotypic screens. Measurements of leaf carbon isotope composition (delta C-13(leaf)) may be a scalable tool to approximate WUEi for screening because it in part reflects the competing influences of A(net) and g(s) on the CO2 partial pressure (pCO(2)) inside the leaf over time. However, in C-4 photosynthesis the CO2 concentrating mechanism complicates the relationship between delta C-13(leaf) and WUEi. Despite this complicated relationship, several studies have shown genetic variation in delta C-13(leaf) across C-4 plants. Yet there has not been a clear demonstration of whether A(net) or g(s) are the causal mechanisms controlling WUEi and delta C-13(leaf). Our approach was to characterize leaf photosynthetic traits of two Zea mays recombinant inbred lines (Z007E0067 and Z007E0150) which consistently differ for delta C-13(leaf) even though they have minimal confounding genetic differences. We demonstrate that these two genotypes contrasted in WUEi driven by differences in the speed of stomatal responses to changes in pCO(2) and light that lead to unproductive leaf water loss. These findings provide support that differences in delta C-13(leaf) in closely related genotypes do reflect greater WUEi and further suggests that differences in stomatal kinetic response to changing environmental conditions is a key target to improve WUEi.