COMPARISON OF EXPORTATION AND METABOLISM OF XYLEM-DELIVERED ABA IN MAIZE LEAVES AT DIFFERENT WATER STATUS AND XYLEM SAP PH

H-3-ABA was introduced into the xylem stream of maize (Zea mays) leave s on intact plants by incubation of a semi-attached 'flap' of the shea th in solutions. The relative contribution of exportation and metaboli sm to the fate of xylem-delivered ABA was assessed in leaves which wer e either kept at different water potentials through soil drying treatm ents or subjected to different xylem pHs (pH 7.4 vs. pH 5.5) through a phosphate buffer in the feeding solutions. Xylem-delivered ABA was ra pidly metabolised in well-watered leaves with a half-life of 2.19 h in the relatively mature leaves used in this study. Re exportation of xy lem-delivered ABA from leaves was much slower than metabolism. It took 24 h for half of the fed radioactivity to disappear from the well-wat ered leaves, and very possibly this radioactivity was in the form of m etabolites of fed H-3-ABA. Although soil drying usually increases the output of ABA through phloem as reported in previous studies, it great ly reduced the re-exportation of xylem-fed ABA and/or its metabolites. Metabolism was also significantly reduced by the treatment of soil dr ying (half-life extended from 2.19 to 3.63 h), although the magnitude of change was much less than that of exportation. Manipulation of the pH in the feeding solution also had its effect on the re-exportation. A. shift of pH from 5.5 to 7.4 reduced the rate of disappearance of th e total radioactivity fed into the attached leaves, but showed no sign ificant effect on the rate of ABA metabolism. It was concluded that it was the ABA metabolism, rather than a re-exportation from leaves, whi ch was mainly responsible for the disposal of the ABA signal from the xylem and therefore preventing an accumulation in leaves. Water stress and pH increase of xylem sap would increase the time of such ABA's pr esence in the leaves. Since xylem-imported ABA is unlikely to be re-ex ported from leaves in its intact form, we believe a recycling of ABA f rom xylem to phloem through leaves plays only a minor role.