Basis of slow-wilting phenotype in soybean PI 471938

To increase soybean [Glycine max (L.) Merr.] productivity, it will be necessary to improve yields in water-deficit regions. Genotype PI 471938, which exhibits a slow-wilting phenotype under water-deficit conditions, has proven to be a good genetic resource in developing droughtresistant progeny even though the physiological basis for this advantage is not known. The objective of this study was to investigate the involvement of four water-saving, physiological mechanisms as candidates contributing to drought tolerance of PI 471938. (i) In response to soil drying, the soil water content at which leaf gas exchange began to decrease in PI 471938 was not different from the other tested genotypes. (ii) Measurement of leaf photosynthetic capacity failed to show that PI 471938 had a high capacity allowing high CO ₂ assimilations even with partial stomata closure. (iii) Plant Introduction 471938 failed to exhibit a limited transpiration rate with increasing vapor pressure deficit (VPD), which would have allowed this genotype to conserve soil water during midday periods of high VPD. (iv) Finally, PI 471938 did not show an ability to maintain high leaf water potential (Ψ _Leaf) when VPD was increasing. In fact, there was a dramatic decrease in Ψ _Leaf with increasing VPD, which usually implies a decrease in leaf turgor pressure. Overall this study resulted in the rejection of four major hypotheses to explain the slow-wilting phenotype exhibited by PI 471938, and the basis for its drought resistance remains unknown.