Abstract
When intact plants of Xanthium strumarium L. were water stressed, the youngest leaves accumulated the highest levels of abscisic acid (ABA). On the other hand, when leaves of different ages were detached and then stressed, the capacity to produce ABA was highest in the mature leaves. Radioactive ABA was transported from mature leaves to the shoot tips and young leaves, as well as to the roots, as evidenced by the presence of radioactive ABA and phaseic acid in the xylem exudate coming from the roots. Thus, ABA was recirculated in the plant, moving down the stem in the phloem and back up in the transpiration stream to the mature leaves. Phloem exudate collected by the use of the EDTA technique had a high concentration of ABA and phaseic acid which increased several-fold after water stress. The high ABA levels in immature leaves and apical buds are, therefore, mainly due to import from older leaves, rather than to in situ synthesis.In Ricinus communis L. cv gibsonii labeled ABA was rapidly exported from a mature leaf as measured by the appearance of radioactive ABA in the phloem collected from incisions in the stem below the treated leaf. After 2 h, small amounts of radioactive phaseic acid and dihydrophaseic acid were also present in the phloem exudate. The glucosyl ester of abscisic acid, beta-d-glucopyranosyl abscisate, was hydrolyzed before entering the phloem and moved as free ABA. It is concluded that the glucosyl ester of ABA itself cannot cross cell membranes and is, therefore, not transported.