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 (Anet) and stomatal conductance (gs) is an intrinsic measurement of leaf water use efficiency (WUEi); however, its measurement can be challenging for large phenotypic screens. Measurements of the leaf carbon isotope composition (δ13Cleaf) may be a scalable tool to approximate WUEi for screening because it in part reflects the competing influences of Anet and gs on the CO2 partial pressure (pCO2) inside the leaf over time. However, in C4 photosynthesis, the CO2-concentrating mechanism complicates the relationship between δ13Cleaf and WUEi. Despite this complicated relationship, several studies have shown genetic variation in δ13Cleaf across C4 plants. Yet there has not been a clear demonstration of if Anet or gs are the causal mechanisms controlling WUEi and δ13Cleaf. Our approach was to characterize leaf photosynthetic traits of two Zea mays recombinant inbred lines (Z007E0067 and Z007E0150) which consistently differ for δ13Cleaf 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 pCO2 and light that lead to unproductive leaf water loss. These findings provide support that differences in δ13Cleaf in closely related genotypes do reflect greater WUEi and further suggest that differences in stomatal kinetic response to changing environmental conditions is a key target to improve WUEi.