Abstract: When plants experience a range of stress conditions, their xylem sap pH often rises. The higher pH could have induced stomatal opening,resulting in excessive water loss,which is harmful to the plants. However,the higher xylem sap pH also raises the oncentration of ABA which is ubiquitous in all land plants, which reduces the stomatal opening and the transpiration rate. In this review, we summarize the progresses of the studies related to 1) xylem sap pH ch anges in plants under various stress conditions; 2) the mechanism by which higher pH acts on the accumulation of ABA to close stomata; 3) and the potential mechanism(s) that might control xylem sap pH under stress conditions.