Water-Use Efficiency of Soybean (Glycine max) Genotypes Contrasting for Carbon Isotope Discrimination.

Genetic variation for water use efficiency (WUE) has been discovered in a range of crop species. However, little research has been conducted to determine the extent of variation of WUE in soybean. Carbon isotope discrimination has been shown to be negatively correlated with water-use efficiency in a several crop species and hence may serve as a surrogate measure of WUE for field-grown soybean genotypes. Six plant introductions were selected for high and low carbon isotope discrimination based on a multi-environment study.  The genotypes were grown under well-watered and water deficit conditions in the greenhouse and field conditions. A 31% variation in WUE was observed in the greenhouse with the three low discriminating genotypes exhibiting a higher WUE as a group. A 33% variation in WUE was observed in field experiments with the genotypes selected for low discrimination exhibiting significantly higher WUE. Tissue analyses for carbon isotope composition were conducted for the field experiment and data analysis revealed a strong, negative correlation was observed between carbon isotope discrimination (whole plant, and seed) and whole plant WUE, but not grain yield.  In the field, genotypic differences in biomass were observed and genotypes with higher WUE also had a significantly higher biomass. In the greenhouse, little genotypic difference was observed for biomass, but significant variation was observed for cumulative water transpired (CWT) and mean transpiration rate (MTR). Negative correlations were observed between whole plant WUE and CWT, and whole plant WUE and MTR in the greenhouse. The selection of genotypes based on carbon isotope discrimination from a multi-environment field study allowed for the identification of lines contrasting in WUE in both the field and greenhouse.