Changes of free and conjugated abscisic acid and phaseic acid in xylem sap of drought-stressed sunflower plants

Abscisic acid (ABA), conjugated abscisic acid, phaseic acid (PA), and conjugated phaseic acid were determined by enzyme-linked immunosorbent assay (ELISA) and gas chromatography (GC) in xylem sap of of well-watered and drought-stressed sunflower plants. Conjugated ABA and conjugated PA were determined indirectly after chemical or enzymatic hydrolysis. Conjugated ABA was found to be the predominant ABA metabolite in xylem sap. In xylem sap from well-watered plants at least five, and in sap from drought-stressed plants at least six alkaline hydrolysable ABA conjugates were found. One of them corresponds chromatographically (HPLC) with abscisic acid glucose ester (ABAGE). Under drought conditions the concentrations of ABA, alkaline hydrolysable ABA conjugates, b-glucosidase hydrolysable ABA conjugates, PA, and conjugated PA increased. After rewatering the drought-stressed plants, the ABA and the conjugated ABA content decreased. The possible function of the ABA conjugates in the xylem sap as a source of free ABA is discussed.