Genetic variability of transpiration response to vapor pressure defi cit among soybean cultivars

Simulation studies have demonstrated that the existence of a limitation on maximum transpiration rate (TR) at high air vapor pressure defi cit (VPD) in water-limited conditions could result in signifi cant yield increases. A genotype of soybean [Glycine max (L.) Merr.] (PI 416937) has been identifi ed both with a slow-wilting phenotype in the fi eld and limited TR above a VPD breakpoint (BP) of 2 kPa. Until now, there has been no full documentation of any other soybean genotype that has a restriction on TR at high VPD. The objective of this study was to extend the observations on TR response to VPD to a wider genetic base. Seven soybean genotypes were studied: three cultivars of broad genetic background and four new lines of diverse backgrounds with PI 416937 in their pedigrees. Data collected across a VPD range of ̃1 to ̃3.5 kPa for each genotype showed a two-segment TR response to VPD for two cultivars, neither of which was derived from PI 416937. Since three progeny of PI 416937 expressed slow-wilting in the fi eld, these results indicate that the slow-wilting phenotype may also result from a mechanism other than the TR response to VPD. Identifi cation of two new genetic selections with limitation of TR at high VPD indicates there is more than one genetic source for this potentially benefi cial trait for water-limited environments.