Water relations and hydraulic control of stomatal behaviour in bell pepperplant in partial soil drying

Two experiments, a split-root experiment and a root pressurizing experiment , were performed to test whether hydraulic signalling of soil drying plays a dominant role in controlling stomatal closure in herbaceous bell pepper p lants. In the split-root experiment, when both root parts were dried, synch ronous decreases in stomatal conductance (g(s)), leaf water potential (LWP) and stem sap flow (SFstem) were observed. The value of g(s) was found to b e closely related to soil water potential (SWP) in both compartments. Tight relationships were observed between g(s) and stem sap flow under all condi tions of water stress, indicating a complete stomatal adjustment of transpi ration. When the half-root system has been dried to the extent that its wat er uptake dropped to almost zero, declines in g(s) of less than 20% were ob served without obvious changes in LWP. The reduced plant hydraulic conducta nce resulting from decreased sap flow and unchanged LWP may be a hydraulic signal controlling stomatal closure; the results of root pressurizing suppo rted this hypothesis. Both LWP and g(s) in water-stressed plants recovered completely within 25 min of the application of root pressurizing, and decre ased significantly within 40 min after pressure release, indicating the hyd raulic control of stomatal closure. Our results are in contrast to those of other studies on other herbaceous species, which suggested that chemical m essengers from the roots bring about stomatal closure when plants are in wa ter stress.