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

Abscisic acid (ABA), conjugated abscisic acid, phaseic acid (PA), and conju gated phaseic acid were determined by enzyme-linked immunosorbent assay (EL ISA) and gas chromatography (GC) in xylem sap of of well-watered and drough t-stressed sunflower plants. Conjugated ABA and conjugated PA were determin ed indirectly after chemical or enzymatic hydrolysis. Conjugated ABA was fo und to be the predominant ABA metabolite in xylem sap. In xylem sap from we ll-watered plants at least five, and in sap from drought-stressed plants at least six alkaline hydrolysable ABA conjugates were found. One of them cor responds chromatographically (HPLC) with abscisic acid glucose ester (ABAGE ). Under drought conditions the concentrations of ABA, alkaline hydrolysabl e ABA conjugates, beta-glucosidase hydrolysable ABA conjugates, PA, and con jugated 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.