I was reading this book: Plant Physiology and Development, Sixth Edition by Lincoln Taiz, Eduardo Zeiger, Ian M. Møller, and Angus Murphy when this doubt came to my mind. Abscisic acid, the stress hormone, works to prevent water loss from a plant. Its mechanism of action requires the xylem sap of the plant to be alkaline. I can't figure out how water stress in the plant body causes alkalinisation of the xylem sap. The section in the book stated as follows:-
During the early stages of water stress,however, the pH of the xylem sap becomes more alkaline,increasing from about pH 6.3 to about pH 7.2. Stress-induced alkalinization of the apoplast favors formation of the dissociated form of abscisic acid, ABA–, which does not readily cross membranes. Hence, less ABA enters the mesophyll cells, and more reaches the guard cells via the transpiration stream.
I'd be grateful if anyone out there can help.