Hormone Signals from Roots to Shoots of Sunflower (Helianthus annuusL.). Moderate Soil Drying Increases Delivery of Abscisic Acid and Depresses Delivery of Cytokinins in Xylem Sap

We quantified abscisic acid and a cytokinin trans zeatin riboside as potential positive or negative signals in root to shoot communication in sunflower plants ( Helianthus annuus L.) growing in drying soil. Delivery rates rather than concentration values were used because the former are less subject to change as a result of differences in sap flow through dilution. ABA concentration and delivery rate increased under the mild drought stress. In contrast, t-ZR concentration did not change under mild stress although delivery rates decreased significantly. With more severe drought stress, both delivery rates and concentration of t-ZR decreased considerably while ABA concentration and delivery were enhanced markedly. Root ABA contents mirrored those of xylem ABA.