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Soybean (Glycine max (Merr.) L) is not often irrigated (<5% of the land area in the major producing countries), so the crop is vulnerable to variations in rainfall. Even short periods of soil water deficit can adversely affect soybean yields, especially because of the high sensitivity of its symbiotic nitrogen fixation to even minor decreases in soil moisture. Therefore, soil water conservation traits have the potential to result in yield increases. Curiously, little variation has been identified among soybean genotypes for early partial stomatal closure with soil drying. On the other hand, a few genotypes have been identified that express initiation of partial stomatal closure at vapor-pressure deficit as low as 2 kPa. Genotype PI 416937, in particular, has become an important genetic contributor in developing soybean cultivars for dry-land conditions. This genotype has been found to have unique properties in plant hydraulic conductivity and aquaporin expression.