The Dynamics of Xylem Sap pH under Drought: a Universal Response in Herbs?
Gloser, V., Korovetska, H., Martin-Vertedor, A. I., Hajickova, M., Prokop, Z., Wilkinson, S., Davies, W.
PLANT AND SOIL 409: 259-272 (2016)
Long distance signals in xylem from roots to leaves are important in plant response to drought stress. Abscisic acid (ABA) plays a key role in drought signaling in plants but apoplastic pH may modulate its effect by distributing ABA into various compartments in leaves. We aimed to reveal the dynamics of changes in sap pH and its relationships with the transport of inorganic and organic ions in eight herbaceous plant species under continuously declining soil water content. We tested several hypotheses related to the mechanism of pH changes in xylem. We used a pressure chamber to collect xylem sap and to measure of leaf/stem water potential at various stages of soil drying. We measured pH and concentrations of the most abundant inorganic (NO3−, SO42−, PO43− and Cl−) and organic (malate and citrate) anions in xylem sap. Species differed considerably in the dynamics of pH changes in xylem in drying soil. Changes in xylem sap pH during drying did not relate to the nitrogen assimilation strategy but may be affected by sap flow rate. Simultaneous changes in the concentrations of inorganic and organic anions were highly species-specific. High variability among species in the observed relationships in response to drought indicates that comparisons among different studies and the generalization of results should be made with caution.