Stomatal conductance and photosynthesis vary linearly with plant hydraulic conductance in ponderosa pine

Abstract Recent work has shown that stomatal conductance ( g s ) and assimilation ( A ) are responsive to changes in the hydraulic conductance of the soil to leaf pathway ( K L ), but no study has quantitatively described this relationship under controlled conditions where steady‐state flow is promoted. Under steady‐state conditions, the relationship between g s , water potential (Ψ) and K L can be assumed to follow the Ohm's law analogy for fluid flow. When boundary layer conductance is large relative to g s , the Ohm's law analogy leads to g s = K L (Ψ soil −Ψ leaf )/ D , where D is the vapour pressure deficit. Consequently, if stomata regulate Ψ leaf and limit A , a reduction in K L will cause g s and A to decline. We evaluated the regulation of Ψ leaf and A in response to changes in K L in well‐watered ponderosa pine seedlings ( Pinus ponderosa ). To vary K L , we systematically reduced stem hydraulic conductivity ( k ) using an air injection technique to induce cavitation while simultaneously measuring Ψ leaf and canopy gas exchange in the laboratory under constant light and D . Short‐statured seedlings ( 0·90), indicating that changes in K L may affect plant carbon gain.