Increasing leaf hydraulic conductance with transpiration rate minimizes the water potential drawdown from stem to leaf

Leaf hydraulic conductance (k (leaf)) is a central element in the regulation of leaf water balance but the properties of k (leaf) remain uncertain. Here, the evidence for the following two models for k (leaf) in well-hydrated plants is evaluated: (i) k (leaf) is constant or (ii) k (leaf) increases as transpiration rate (E) increases. The difference between stem and leaf water potential (Delta Istem-leaf), stomatal conductance (g (s)), k (leaf), and E over a diurnal cycle for three angiosperm and gymnosperm tree species growing in a common garden, and for Helianthus annuus plants grown under sub-ambient, ambient, and elevated atmospheric CO2 concentration were evaluated. Results show that for well-watered plants k (leaf) is positively dependent on E. Here, this property is termed the dynamic conductance, k (leaf(E)), which incorporates the inherent k (leaf) at zero E, which is distinguished as the static conductance, k (leaf(0)). Growth under different CO2 concentrations maintained the same relationship between k (leaf) and E, resulting in similar k (leaf(0)), while operating along different regions of the curve owing to the influence of CO2 on g (s). The positive relationship between k (leaf) and E minimized variation in Delta Istem-leaf. This enables leaves to minimize variation in I-leaf and maximize g (s) and CO2 assimilation rate over the diurnal course of evaporative demand.