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The objectives of the study were to identify the relevant hydraulic parameters associated with stomatal regulation during water stress and to test the hypothesis of a stomatal control of xylem embolism in walnut (Juglans regia× nigra) trees. The hydraulic characteristics of the sap pathway were experimentally altered with different methods to alter plant transpiration (E  _plant) and stomatal conductance (g  _s). Potted trees were exposed to a soil water depletion to alter soil water potential (Ψ_soil), soil resistance (R  _soil), and root hydraulic resistances (R  _root). Soil temperature was changed to alter R  _root alone. Embolism was created in the trunk to increase shoot resistance (R  _shoot). Stomata closed in response to these stresses with the effect of maintaining the water pressure in the leaf rachis xylem (P  _rachis) above −1.4 MPa and the leaf water potential (Ψ_leaf) above −1.6 MPa. The same dependence of E  _plant …