Why does water stress lead to alkalinity of xylem sap in plants?

Question

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.

Answer 1

According to recent studies, drought stress does not consistently lead to increases in the pH of xylem sap. One article which discusses interspecies differences is Vit Gloser, et al., "The dynamics of xylem sap pH under drought: a universal response in herbs?," Plant and Soil 409, nos. 1-2 (2016): 259-272.

As to what may have prompted the textbook authors to assert this, see an earlier article by two of the coauthors of the Gloser essay: Sally Wilkinson and William J. Davies, "Xylem sap pH increase: a drought signal received at the apoplastic face of the guard cell that involves the suppression of saturable abscisic acid uptake by the epidermal symplast," Plant Physiology 113 (1997): 559-573. Wilkinson and Davies summarize an earlier study: "Hartung and Radin (1989) found that the pH of xylem sap from water-stressed Phaseolus vulgaris roots increased from control levels of 6.3 to 7.2." (560)