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.