Fig. 10.

Model diagram showing the coordination between transpiration rate (E), the difference between stem and leaf water potential (ΔΨ_stem–leaf), leaf hydraulic conductance (k _leaf), stomatal conductance (g _s, where boundary layer conductance g _b is sufficiently high for g _s to dominate), and CO₂ assimilation rate (A). The blue lines represent the hydraulic feedback loop between g _s and E. Solid lines represent a positive relationship between parameters and dotted lines represent a negative relationship. The positive relationship between E and k _leaf is the predicted relationship based on data gathered from H. annuus plants grown under low, medium, and high CO₂ concentrations, and three gymnosperm and angiosperm tree species growing in a common garden (see Methods for more detail). The black box indicates the boundary between leaf processes and external environmental variables. (VPD, leaf-to-air vapour pressure difference; PAR, photosynthetically active radiation; C _a, atmospheric CO₂ concentration, C _i, leaf internal CO₂ concentration, Ψ_π,g, is the guard cell osmotic pressure, and Ψ_P,g, guard cell turgor pressure).