Question: why xylem sap is acidic?
Answer: Within leaves, ABA is redistributed as a function of pH. In a well-watered plant, the xylem sap is more acidic (approximately pH 6.3) and ABA occurs in its protonated form (ABAH). During drought stress, the sap becomes slightly alkaline (approximately pH 7.2), favoring the deprotonation of ABAH to ABA.