ABA Uptake in Source and Sink Tissues of Sugar Beet.

The mode of abscisic acid (ABA) uptake was studied in excised leaf and root tissue discs of sugar beet (Beta vulgaris L.). Discs were incubated in buffered medium that contained 1 mm CaCl(2) and [(14)C]ABA. The sensitivity of ABA uptake to metabolic inhibitors and temperature indicated that the ABA transport system had an energy-dependent component. Energy-dependent uptake was greater in leaf than in root tissue (70% and 50%, respectively). Energy-dependent uptake by both tissues and passive uptake by root tissues were highly pH dependent. Maximal uptake was observed at pH 5.5. Leaf tissue incubated in the dark showed a 50% reduction of uptake as compared with tissue under light. The decrease was due to reduced passive uptake.The results suggest that ABA moves across membranes as the undissociated lipophilic species. As a weak acid, ABA would dissociate and accumulate in the more alkaline compartment. Therefore, the distribution of ABA within the tissue is regulated by the pH differential between any two compartments. Although diffusion may be the predominant form of transport, the uptake of ABA is dependent on metabolic energy for the establishment of a pH gradient across the membrane.