Stomatal conductance and photosynthesis vary linearly with plant hydraulicconductance 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 ste ady-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 analog y leads to g(s) = K-L (Psi (soil) - Psi (leaf))/D, where D is the vapour pr essure 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 regula tion of Psi (leaf) and A in response to changes in K-L in well-watered pond erosa pine seedlings (Pinus ponderosa). To vary K-L, we systematically redu ced stem hydraulic conductivity (k) using an air injection technique to ind uce cavitation while simultaneously measuring Psi (leaf) and canopy gas exc hange in the laboratory under constant light and D. Short-statured seedling s (< 1 m tall) and hourlong equilibration times promoted steady-state flow conditions. We found that <Psi>(leaf) remained constant near - 1.5 MPa exce pt at the extreme 99% reduction of k when Psi (leaf) fell to - 2.1 MPa. Tra nspiration, 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 direc tly proportional to K-L (R-2 > 0.90), indicating that changes in K-L may af fect plant carbon gain.