A limited transpiration rate under high vapor pressure deficit (VPD) could be used to conserve soil water for later use under drought conditions. Many crops show this behavior either as limited transpiration or decreases in stomatal conductance. However, little work has been done in Phaseolus. Four experiments evaluated stomatal closure across a range of VPD for well-watered plants, each experiment using varying combinations of genotypes of common (15), lima (6) and tepary beans (7 genotypes). A two-year experiment found genotypic variation in average stomatal conductance, but genotypes only had 14% stomatal closure between a VPD of 1–4kPa. In comparison, soybean, which is known to close stomata, had a 40% decrease for similar conditions in Davis, CA, USA. In a second field experiment and outdoor pot experiments, genotypes from the three species displayed, on average, a 34, 50–45% increase in stomatal conductance with increasing VPD. Six genotypes were statistically indistinguishable from a 40% decrease, but all had low probability (p<0.21) of having 40% closure, and some showed little closure in other experiments. The VPD range measured in this study was large relative to the range for hot, arid California, thus the results are generalizable: most Phaseolus beans are not expected to have appreciable stomatal closure under well-watered conditions. Thus, there is limited evidence that Phaseolus has somegenetic diversity in stomatal responses to VPD, relative to that shown in other species. However, there was constitutive genetic variation in species and genotypic stomatal conductance under low VPD conditions.
