Effect of osmotic stress on abscisic acid levels in xylem sap of sunflower (Helianthus annuus L.).

Addition of an osmoticum (-12 bars) to the rooting medium of sunflowers (Helianthus annuus L.) caused an increase in the level of abscisic acid (ABA) present in xylem exudate subsequently collected from cut shoots. Using tall and dwarf plants it was shown that there was a time lag in the appearance of increased levels of ABA in tall plants when compared with dwarf plants. The results indicate that the leaves, rather than the roots are the site of synthesis of ABA present in the xylem sap of osmotically stressed sunflower plants.