Control of stomatal conductance by leaf water potential in Hymenoclea salsola (T. & G.), a desert subshrub

The role of leaf water potential in controlling stomatal conductance ( g s ) was examined in the desert subshrub Hymenoclea salsola . For plants operating at high irradiance, stomatal closure in response to high leaf‐air humidity gradient ( D ) was largely reversed by soil pressurization. Stomatal re‐opening eliminated, on average, 89% of the closure normally induced by high D . Transpiration rates ( E ) reached under these conditions were far higher than maximal rates normally observed at any point of the D response. In situ stem psychrometry indicated that water flux at all times conformed to a simple Ohm’s‐law analogy. Under conditions of high D, E increased substantially in response to soil pressurization. Stomatal regulation did not constrain E during this treatment, but did result in nearly constant minimum leaf water potentials.