Soybean water-use efficiency increased over 80 years of breeding

Breeders successfully increased US soybean [Glycine max (L.) Merr.] yields over the past nearly 100 years and altered various plant characteristics underpinning the yield gains. However, the impact of breeding on plant-level water-use efficiency (WUEp) has not been examined yet. This study, conducted across eight environments using maturity group IV cultivars released between 1930 and 2005, aimed to (1) determine if soybean WUEp, assessed using C isotope composition (delta 13C) measurements on shoot biomass sampled at early seed filling (R5), has changed with cultivar year of release (YoR), and (2) assess how canopy temperature (CT) and WUEp relate to each other and to seed yield. Across environments and cultivars, delta 13C ranged from -27.52 parts per thousand to -28.24 parts per thousand and the correlation between cultivar YoR and WUEp was positive in four individual environments (p <= 0.07), as well as across the eight environments (p = 0.0083), with an average increase of shoot delta 13C of 0.004 parts per thousand per year of soybean breeding. Lower average delta 13C values for specific environments were associated with higher precipitation prior to biomass sampling, which is consistent with a lower WUEp when more water was available. Interestingly, across environments, midday CT at early seed filling was negatively correlated with YoR and WUEp, suggesting that intrinsic WUE of more recently released cultivars was lower during high demand periods. Further studies are needed to understand the mechanisms underlying these relationships between WUEp and CT and to identify physiological mechanisms that can be targeted for breeding high-yielding cultivars while increasing or maintaining WUEp.