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

Abstract

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 (< 1 m tall) and hour‐long equilibration times promoted steady‐state flow conditions. We found that Ψ leaf remained constant near − 1·5 MPa except at the extreme 99% reduction of k when Ψ leaf fell to − 2·1 MPa. Transpiration, g s , A and K L all declined with decreasing k ( P < 0·001). As a result of the near homeostasis in bulk Ψ leaf , g s and A were directly proportional to K L ( R 2 > 0·90), indicating that changes in K L may affect plant carbon gain.