Unraveling the effects of plant hydraulics on stomatal closure during water stress in walnut

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 X 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 (Psi(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 (Psi(leaf)) above - 1.6 MPa. The same dependence of E-plant and g(s) on P-rachis or Psi(leaf) was always observed. This suggested that stomata were not responding to changes in Psi(soil) R-root or R-shoot per se but rather to their impact on P-rachis and/or Psi(leaf). Leaf rachis was the most vulnerable organ, with a threshold P-rachis for embolism induction of - 1.4 MPa. The minimum Psi(leaf) values corresponded to leaf turgor loss point. This suggested that stomata are responding to leaf water status as determined by transpiration rate and plant hydraulics and that P-rachis might be the physiological parameter regulated by stomatal closure during water stress, which would have the effect of preventing extensive developments of cavitation during water stress.