Higher rates of leaf gas exchange are associated with higher leaf hydrodynamic pressure gradients

ABSTRACT

Steady-state leaf gas-exchange parameters and leaf hydraulic conductance were measured on 10 vascular plant species, grown under high light and well-watered conditions, in order to test for evidence of a departure from hydraulic homeostasis within leaves as hydraulic conductance varied across species. The plants ranged from herbaceous crop plants to mature forest trees. Across species, under standardized environmental conditions (saturating light, well watered), mean steady-state stomatal conductance to water vapour (g_w) was highly correlated with mean rate of CO₂ assimilation (A) and mean leaf hydraulic conductance normalized to leaf area (k_leaf). The relationship between A and g_w was well described by a power function, while that between A and k_leaf was highly linear. Non-linearity in the relationship between g_w and k_leaf contributed to an increase in the hydrodynamic (transpiration-induced) water potential drawdown across the leaf (ΔΨ_leaf) as k_leaf increased across species, although across the 10 species the total increase in ΔΨ_leaf was slightly more than twofold for an almost 30-fold increase in g_w. Higher rates of leaf gas exchange were therefore associated with higher k_leaf and higher leaf hydrodynamic pressure gradients. A mechanistic model incorporating the stomatal hydromechanical feedback loop is used to predict the relationship between ΔΨ_leaf and k_leaf, and to explore the coordination of stomatal and leaf hydraulic properties in supporting higher rates of leaf gas exchange.