Recent work has shown that stomatal conductance (gs) and assimilation (A) are responsive to changes in the hydraulic conductance of the soil to leaf pathway (KL), but no study has quantitatively described this relationship under controlled conditions where steady-state flow is promoted. Under steady-state conditions, the relationship between gs, water potential (Ψ) and KL can be assumed to follow the Ohm's law analogy for fluid flow. When boundary layer conductance is large relative to gs, the Ohm's law analogy leads to gs = KL (Ψsoil−Ψleaf)/D, where D is the vapour pressure deficit. Consequently, if stomata regulate Ψleaf and limit A, a reduction in KL will cause gs and A to decline. We evaluated the regulation of Ψleaf and A in response to changes in KL in well-watered ponderosa pine seedlings (Pinus ponderosa). To vary KL, we systematically reduced stem hydraulic conductivity (k) using an air injection technique to induce cavitation while simultaneously measuring Ψleaf 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 Ψleaf remained constant near − 1·5 MPa except at the extreme 99% reduction of k when Ψleaf fell to − 2·1 MPa. Transpiration, gs, A and KL all declined with decreasing k (P < 0·001). As a result of the near homeostasis in bulk Ψleaf, gs and A were directly proportional to KL (R2 > 0·90), indicating that changes in KL may affect plant carbon gain.