Abstract
Dynamic variation of the stomatal pore in response to changes in leaf–air vapour pressure difference (VPD) constitutes a critical regulation of daytime gas exchange. The stomatal response to VPD has been associated with both foliage abscisic acid (ABA) and leaf water potential (Ψ l); however, causation remains a matter of debate. Here, we seek to separate hydraulic and hormonal control of stomatal aperture by manipulating the osmotic potential of sunflower leaves. In addition, we test whether stomatal responses to VPD in an ABA-deficient mutant (w-1) of sunflower are similar to the wild type. Stomatal apertures during VPD transitions were closely linked with foliage ABA levels in sunflower plants with contrasting osmotic potentials. In addition, we observed that the inability to synthesize ABA at high VPD in w-1 plants was associated with no dynamic or steady-state stomatal response to VPD. These results for sunflower are consistent with a hormonal, ABA-mediated stomatal responses to VPD rather than a hydraulic-driven stomatal response to VPD.
Highlights
The stomatal response to vapour pressure difference (VPD) has been associated with both foliage abscisic acid (ABA) and leaf water potential (Ψ l); causation remains a matter of debate
Ψ l following the VPD transition rapidly declined to the threshold Ψ l found to trigger the accumulation of foliage ABA levels in bench-dried branches (Fig. 1)
We present data that add to a growing body of evidence indicating that foliage ABA levels are major determinants of stomatal responses to VPD in angiosperms (Bauerle et al 2004; Xie et al 2006; Bauer et al 2013; McAdam and Brodribb 2015, 2016; McAdam et al 2016)
Summary
Stomata on the leaves of terrestrial plants regulate the diffusion of CO2 and water vapour between the leaf and the atmosphere, thereby controlling plant hydration and photosynthetic rate (Farquhar and Sharkey 1982). Received: 16 December 2019; Editorial decision: 25 May 2020; Accepted: 11 June 2020 The mechanism responsible for this response in angiosperms remains under debate, with some research supporting the involvement of passive changes in guard cell turgor as the primary driver of these responses (i.e. passivehydraulic regulation) (Mott et al 1997; Assmann et al 2000; Mott and Peak 2013; Peak and Mott 2011). Understanding this mechanism is of considerable importance because attempts to increase the productivity of irrigated crops have identified VPD responses as a primary target for improvement (Sinclair et al 2016)
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