Uptake of glucose, 3-O-methylglucose and sucrose into beetroot protoplasts is considerably stimulated by 10(-6)M fusicoccin. This effect is decreased in the presence of 10mM Na(+) or K(+), 2 mM Mg(2+) or Ca(2+). Whereas fusicoccin causes no change in the pH-optimum of the sugar uptake (pH 5.0), the apparent Km of this uptake which obeys a biphasic kinetics is decreased by the action of fusicoccin. In the protoplast suspension, fusicoccin induces an acidification which is suppressed by uncoupling agents. Correspondingly, uncouplers as well as vanadate and diethylstilbestrol markedly inhibit the effect of fusicoccin on sugar uptake. The present data support the view that glucose uptake into beetroot protoplasts depend on the proton-pumping activity of the plasmalemma-ATPase. cis-Abscisic acid diminishes significantly the fusicoccin-enhanced glucose uptake. By using a radioimmunoassay, the internal abscisic acid content of the protoplast was estimated to be in the range of 10(-6) M. Protoplasts isolated from bundle tissue contain twice as much abscisic acid as those derived from storage parenchyma. Because protoplasts from the bundle tissue were shown to take up sugars much faster than those from the storage cells, the observed effect of abscisic acid might reflect an involvement of this hormone in the regulation of carbohydrate partitioning in the beet.