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

Abstract

When part of the root system of rice plants is exposed to drought stress, the ABA concentration in the Abscisic acid (ABA), conjugated abscisic acid, phaseic xylem sap increases and the leaf conductance decreases acid (PA), and conjugated phaseic acid were deter- (Bano et al., 1993). It is probable that this ABA accumu- mined by enzyme-linked immunosorbent assay (ELISA) lation in the xylem sap is the result of ABA biosynthesis and gas chromatography (GC) in xylem sap of of well- in the roots (Parry and Horgan, 1992). It has been shown watered and drought-stressed sunflower plants. that substantial amounts of ABA can also be imported Conjugated ABA and conjugated PA were determined from the soil solution into xylem vessels (Freundl et al., indirectly after chemical or enzymatic hydrolysis. 1998). In several recent papers it has been proposed that Conjugated ABA was found to be the predominant ABA root-borne ABA is transported via the xylem into the metabolite in xylem sap. In xylem sap from well- leaves where this phytohormone is responsible for watered plants at least five, and in sap from drought- stomatal closure (Gollan et al., 1992; Schurr et al., 1992; stressed plants at least six alkaline hydrolysable ABA Tardieu et al., 1996) and other adaptive processes. There conjugates were found. One of them corresponds chro- is a considerable capacity in leaves of maize and matographically (HPLC) with abscisic acid glucose Commelina to metabolize xylem delivered ABA (Jia et al., ester (ABAGE). Under drought conditions the concen- 1996). Therefore, the rapid metabolism of root-borne trations of ABA, alkaline hydrolysable ABA conjugates, ABA in the leaves may be a further mechanism of b-glucosidase hydrolysable ABA conjugates, PA, and stomatal control. conjugated PA increased. After rewatering the Recently, it has been demonstrated that, apart from drought-stressed plants, the ABA and the conjugated ABA and cytokinins, ABA metabolites are occurring in ABA content decreased. The possible function of the the xylem sap of drought-stressed sunflower plants (Bano ABA conjugates in the xylem sap as a source of free et al., 1994). These metabolites are phaseic acid, conjug- ABA is discussed. ated phaseic acid and conjugated ABA. ABA conjugates were also found in the xylem sap of plants of castor bean