Breeding efforts in soybean [ Glycine max (L.) Merr.] have addressed the challenge of water-limited yields by incorporating parental stocks which exhibit drought-tolerant traits. Multiple cycles of empirical selection for improved yielding ability in water-deficient field environments have produced new generations of adapted breeding lines. However, the impact of this selection process on specific putative drought-tolerant traits is unknown. The objective of this study was to determine if breeders' selection of 10 elite lines for high seed yield under dry conditions is associated with the presence of physiological expression of three putative drought-tolerant traits: (i) limited transpiration rate under high vapor pressure deficit (VPD), (ii) early decrease in transpiration rate with soil drying, and (iii) drought-tolerant N 2 fixation. Greenhouse experiments were undertaken to characterize each genotypes for their phenotype of each of these three traits. Unlike most soybean cultivars, 9 of the 10 elite lines expressed a limited transpiration rate under elevated VPD. The VPD at which transpiration rate became limited was 1.9 kPa or less. There was no difference among genotypes in the threshold for decline in transpiration rate with soil drying, although all genotypes expressed high thresholds indicating an ability to conserve soil water. All lines expressed drought tolerance in their N 2 fixation rates, which was superior to that commonly observed in soybean. This study demonstrated that mating of parents that expressed a drought trait and multiple rounds of progeny selection based on improved yield under water-limited conditions resulted in the elite lines expressing improved drought traits.
