Soil and plant hydraulics constrain ecosystem productivity by setting physical limits to water transport and hence carbon uptake by leaves. While more negative xylem water potentials provide a larger driving force for water transport, they also cause cavitation that limits hydraulic conductivity. An optimum balance between driving force and cavitation occurs at intermediate water potentials, thus defining the maximum transpiration rate the xylem can sustain (denoted as Emax). The presence of this maximum raises the question as to whether plants regulate transpiration through stomata to function near Emax.
To address this question, we calculate Emax across plant functional types and climates using a hydraulic model and a global database of plant hydraulic traits.
The predicted Emax compares well with measured peak transpiration across plant sizes and growth conditions (R=0.86, P<0.001) and is relatively conserved among plant types (for a given plant size). The result that Emax is roughly conserved and scales with the product of xylem saturated conductivity and water potential at 50% cavitation is used here to explain the safety-efficiency tradeoff in plant xylem.
Stomatal conductance allows maximum transpiration rates despite partial cavitation in the xylem thereby suggesting coordination between stomatal regulation and xylem hydraulic characteristics.