Arabidopsis leaf hydraulic conductance is regulated by xylem sap pH, controlled, in turn, by a P-type H+ -ATPase of vascular bundle sheath cells.

The leaf vascular bundle sheath cells (BSCs) that tightly envelop the leaf veins, are a selective and dynamic barrier to xylem sap water and solutes radially entering the mesophyll cells. Under normal conditions, xylem sap pH below 6 is presumably important for driving and regulating the transmembranal solute transport. Having discovered recently a differentially high expression of a BSC proton pump, AHA2, we now test the hypothesis that it regulates the xylem sap pH and leaf radial water fluxes. We monitored the xylem sap pH in the veins of detached leaves of wild-type Arabidopsis, AHA mutants and aha2 mutants complemented with AHA2 gene solely in BSCs. We tested an AHA inhibitor (vanadate) and stimulator (fusicoccin), and different pH buffers. We monitored their impact on the xylem sap pH and the leaf hydraulic conductance (Kleaf ), and the effect of pH on the water osmotic permeability (Pf ) of isolated BSCs protoplasts. We found that AHA2 is necessary for xylem sap acidification, and in turn, for elevating Kleaf . Conversely, AHA2 knockdown, which alkalinized the xylem sap, or, buffering its pH to 7.5, reduced Kleaf , and elevating external pH to 7.5 decreased the BSCs Pf . All these showed a causative link between AHA2 activity in BSCs and leaf radial hydraulic water conductance.

Effects of throttling window on flow rate through feed-water valves.

Feed-water valves are widely used in nuclear power plants to regulate the flow rate of the water supplying to steam generators, and thus holding the water level in steam generators at desired values. The flow rate characteristics of feed-water valves have important effects on the management quality of the water level in steam generators and therefore influence the safety and the efficiency of nuclear power plants. In this paper, the effects of throttling window shapes on the flow rate characteristics through feed-water valves are investigated in two aspects, the overall performances and the fluid dynamics. First of all, a dimensionless parameter defining throttling window shapes is proposed for the quantitative assessment. Then, for the analysis of overall performances, the rated flow coefficient, the loss coefficient, and the inherent valve characteristics are investigated. A revised fitting function is obtained to predict the inherent valve characteristics with different throttling window shapes for engineering applications. Finally, for the observation of the fluid dynamics, the velocity characteristics and the pressure characteristics are both discussed. The wear conditions are predicted at different relative travels and throttling window shapes. This paper provides a reference for researchers dealing with the design work of feed-water valves and is beneficial for the improvement of the whole water level control system.