Water relations and hydraulic control of stomatal behaviour in bell pepper plant 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 plants. In the split-root experiment, when both root parts were dried, synchronous decreases in stomatal conductance (g_s), leaf water potential (LWP) and stem sap flow (SF_stem) were observed. The value of g_s was found to be closely related to soil water potential (SWP) in both compartments. Tight relationships were observed between g_s and stem sap flow under all conditions of water stress, indicating a complete stomatal adjustment of transpiration. When the half-root system has been dried to the extent that its water uptake dropped to almost zero, declines in g_s of less than 20% were observed without obvious changes in LWP. The reduced plant hydraulic conductance resulting from decreased sap flow and unchanged LWP may be a hydraulic signal controlling stomatal closure; the results of root pressurizing supported this hypothesis. Both LWP and g_s in water-stressed plants recovered completely within 25 min of the application of root pressurizing, and decreased significantly within 40 min after pressure release, indicating the hydraulic control of stomatal closure. Our results are in contrast to those of other studies on other herbaceous species, which suggested that chemical messengers from the roots bring about stomatal closure when plants are in water stress.