15.2.4 Leaf to air vapour pressure difference

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Figure 15.9 Sromata are responsive to atmospheric vapour pressure deficit, so that potentially faster transpiration at low atmospheric humidity is constrained by partial closure of stomata (hence decreased gs). Patterns of response differ between species; two common forms of gs response to vapour pressure deficit are shown here for leaves with initially high rates of gas exchange.

Within the soil–plant–atmosphere continuum, water flux is driven by gradients in water potential, and the single biggest fall in water potential occurs between leaf air (Ψla) and ambient air (Ψaa). Stomata have to endure huge differences under even mild conditions. At 20ºC, and assuming a relative humidity inside leaves of 99.6%, Ψla would be -0.54 MPa. On a humid day, with ambient air at 96%, Ψaa would already be down to -5.51 MPa, a leaf to air difference of almost 5 MPa. On a drier day with ambient relative humidity of only 50%, the leaf to air difference would be 93 MPa! Leaf to air vapour pressure difference is thus a potent factor in transpiration, so that mechanisms enabling stomata to sense evaporative con-ditions and reduce conductance would have a strong selective advantage. Stomatal conductance (gs) commonly declines with increased transpiration occasioned by a decrease in atmospheric water content (expressed as an atmospheric water vapour deficit in Figure 15.9). This response may be linear or curvilinear, and sensitivity of gs to vapour pressure deficit is influenced by leaf water status and leaf age. Such regulation of water status can be highly effective because a favourable leaf water status can be maintained despite large increases in evaporative demand.

Do stomata respond directly to transpiration, or do they respond to vapour pressure deficit per se? To answer this question an artificial gas mix called HELOX has been used, in which the nitrogen in the atmosphere has been replaced by helium. Water vapour diffuses through HELOX 2.33 times faster than through air at the same vapour pressure deficit. Rates of transpiration can then be varied independently of changes in vapour pressure deficit. These studies showed that changes in gs represent stomatal responses to increased rates of transpiration rather than atmospheric water content (Mott and Parkhurst 1991).

One further possibility for explaining stomatal response to transpirational flux relates to root signals. Plants encountering soil moisture deficit have a heightened sensitivity to vapour pressure deficit. Roots are known sources of shoot-inhibitory substances such as abscisic acid, and levels increase with soil moisture stress. Conceivably, faster transpiration could then sweep greater concentrations of these substances into transpiring leaves and trigger a closing response. Intuitively, such feed-forward control would forestall excess water loss during drought stress and would be selectively useful for any vascular plant.