Characterizing the breakpoint of stomatal response to vapor pressure deficit in an angiosperm

Vapor pressure difference between the leaf and atmosphere (VPD) is the most important regulator of daytime transpiration, yet the mechanism driving stomatal responses to an increase in VPD in angiosperms remains unresolved. Here, we sought to characterize the mechanism driving stomatal closure at high VPD in an angiosperm species, particularly testing whether abscisic acid (ABA) biosynthesis could explain the observation of a trigger point for stomatal sensitivity to an increase in VPD. We tracked leaf gas exchange and modeled leaf water potential (psi l) in leaves exposed to a range of step-increases in VPD in the herbaceous species Senecio minimus Poir. (Asteraceae). We found that mild increases in VPD in this species did not induce stomatal closure because modeled psi l did not decline below a threshold close to turgor loss point (psi tlp), but when leaves were exposed to a large increase in VPD, stomata closed as modeled psi l declined below psi tlp. Leaf ABA levels were higher in leaves exposed to a step-increase in VPD that caused psi l to transiently decline below psi tlp and in which stomata closed compared with leaves in which stomata did not close. We conclude that the stomata of S. minimus are insensitive to VPD until psi l declines to a threshold that triggers the biosynthesis of ABA and that this mechanism might be common to angiosperms. Stomata in angiosperms are insensitive to an increase in vapor pressure deficit until leaf water potential drops sufficiently to trigger abscisic acid biosynthesis.