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

Abstract

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 modelled leaf water potential (Ψl) 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 modelled Ψl did not decline below a threshold close to turgor loss point (Ψtlp), but when leaves were exposed to a large increase in VPD, stomata closed while modelled Ψl declined below Ψtlp. Leaf ABA levels were higher in leaves exposed to a step-increase in VPD that caused Ψl to transiently decline below Ψtlp and in which stomata closed compared to leaves in which stomata did not close. We conclude that the stomata of S. minimus are insensitive to VPD until Ψl declines to a threshold that triggers the biosynthesis of ABA and that this mechanism might be common to angiosperms.

Stomatal responses to VPD utilize guard cell intracellular signaling components

“…Some work suggests that stomatal closure is driven by the rapid up-regulation of foliar ABA levels following a VPD-induced loss of leaf turgor ( Bauer et al., 2013 ; McAdam and Brodribb, 2015 ). For example, in the dicot herb, Senecio minimus , small increases in VPD had no effect on stomatal conductance, while step increases in VPD calculated to be large enough to reduce leaf turgor both increased leaf ABA content and triggered decreases in stomatal conductance ( Binstock et al., 2023 ). However, the observations that stomata in isolated epidermis show a VPD response ( Lösch, 1977 ), and that ABA-deficient mutants exhibit a reduction in a stomatal aperture in response to VPD, just like wild type ( Assmann et al., 2000 ; Merilo et al., 2018 ), raises a debate regarding the essentiality of ABA in the VPD response.…”

Passive stomatal closure under extreme drought in an angiosperm species

“…There are a few reasons why passive stomatal closure during drought in angiosperms is not widely accepted, the first is that mutants of ABA biosynthesis and signalling do not close effectively during drought or when leaf water status changes (Brodribb et al 2021;Tulva et al 2023), suggesting that if there is a passive regulation of stomatal response to leaf water status in species from this group of land plants it is minor or ineffective. Stomatal responses to short-term changes in leaf water status induced by vapour pressure difference between the leaf and the atmosphere (VPD) in angiosperms are not predictable by a passive-hydraulic model whereby guard cell turgor is linked to leaf turgor (Binstock et al 2023;Cardoso et al 2020). This contrasts with the stomatal responses to VPD in most non-angiosperm species (Brodribb and McAdam 2011).…”