Stomatal response in soybean during drought improves leaf-scale and field-scale water use efficiencies

Abstract

Diminishing water resources and an expected increase in frequency of extreme water stress events necessitate tools to diagnose and improve the drivers of variability in agronomic water use efficiency. The objective of this study was to determine if leaf-scale water use efficiency (measured as intrinsic water use efficiency and transpiration efficiency) is expressed at the field scale as yield or agronomic water use efficiency variability. We measured grain yield, total aboveground biomass, and carbon isotope discrimination and estimated evapotranspiration using a mass balance approach for field-grown Glycine max (soybean) over five years. We found that the high agronomic water use efficiency in years characterized by high vapor pressure deficit was caused by a large reduction in evapotranspiration and a relatively smaller reduction in yield. This has implications for developing drought tolerance in soybeans without compromising yield. We observed a positive relationship between transpiration efficiency and agronomic water use efficiency, with the leaf scale explaining 68% of the variability at the field scale. Through this analysis, we infer that increasing transpiration efficiency at the leaf scale will likely improve agronomic water use efficiency at the field scale in rain-fed soybean systems.