Investigating the role of stomatal dynamics on agronomic traits using a slac1-2 Zea mays mutant

The production of staple food crops is becoming increasingly difficult due to the amount of freshwater needed to realize food security for a growing global population. Climate projections show hotter and drier growing seasons in traditionally productive agricultural regions, which will increase the demand for limited water resources. Thus, strategies to improve water use efficiency will only become more important for sustainable agriculture. At the leaf level, stomatal conductance has a large influence on transpirational water loss and therefore water use efficiency. The anion channel SLAC1 has been shown in several plant species to function as the primary mechanism for stomatal closure, which allows stomatal aperture to change dynamically in response to environmental stimuli. Given that slac1 is a single gene that can significantly alter stomatal conductance, it has the potential to serve as a control point for improving water use efficiency. Here we fully characterize a Zea mays slac1-2 mutant in multiple field environments. Interestingly, homozygous slac1-2 hybrids did not show improved net CO2 assimilation or increased grain yield despite having greater stomatal conductance compared to a wild-type hybrid. Net CO2 assimilation and grain yield were either lower or similar in slac1-2 compared to wild-type across environments. Furthermore, the slac1-2 hybrid did not have increased nitrogen uptake. These results suggest that the C4 carbon concentrating mechanism removes any stomatal conductance limitations to CO2 assimilation, even in highly productive wild-type Z. mays hybrids. Because the slac1-2 hybrids eliminate stomatal conductance as a major variable for modulating water loss, future studies will be able to investigate alternate regulators of plant water potential to identify novel mechanisms for increasing water use efficiency.