Response of carbon isotope discrimination and oxygen isotope composition to mild drought in slow- and fast-wilting soybean genotypes

Drought restrains soybean (Glycine max L. [Merr.]) growth and production, but few tools are available to evaluate differences in drought tolerance among soybean genotypes. Carbon isotope discrimination (Δ13C) and oxygen isotope composition (δ18O) have been used as surrogate measures of water-use efficiency (WUE) and transpiration, respectively. This three-year study evaluated drought effects during reproductive stages on five fast- and five slow-wilting genotypes under fully irrigated conditions and two levels of drought severity. Leaves and seeds were analyzed for Δ13C, and seeds were analyzed for δ18O. The Δ13C values generally decreased in both leaves and seeds with decreasing water availability (i.e., higher WUE). In contrast, as water availability decreased, the δ18O values generally increased, indicating decreased transpiration, but δ18O values were not consistent between wilting groups or among genotypes within wilting groups across years. Values of Δ13C and δ18O from seed were positively correlated for the fully-irrigated and the mild-drought levels, which is consistent with increased WUE being associated with decreased transpiration. For the fully-irrigated and mild-drought levels grain yield was also negatively correlated with Δ13C. The results indicate that Δ13C measurement is a promising tool in identifying drought-tolerant soybean genotypes and in understanding the relationship between WUE and transpiration.