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

Recent work has shown that stomatal conductance (g(s)) and assimilation (A) are responsive td 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 (Psi) 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 (Psi (soil) - Psi (leaf))/D, where D is the vapour pressure deficit. Consequently, if stomata regulate Psi (leaf) and limit A, a reduction in K-L will cause g(s) and A to decline. We evaluated the regulation of Psi (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 Psi (leaf) and canopy gas exchange in the laboratory under constant light and D. Short-statured seedlings (< 1 m tall) and hourlong equilibration times promoted steady-state flow conditions. We found that <Psi>(leaf) remained constant near - 1.5 MPa except at the extreme 99% reduction of k when Psi (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 <Psi>(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.