A study of abscisic acid uptake by apical and proximal root segments of Phaseolus coccineus L

1. We investigated the pH and concentration dependence of abscisic acid uptake by short segments taken from different zones along the length of primary roots of Phaseolus coccineus L. (Runner bean). Tissue from all regions studied, up to and including the zone of lateral root initiation showed a non-saturable uptake component identifiable with passive diffusion of the undissociated species of abscisic acid. The net uptake increased through the elongation zone towards the apex, perhaps principally due to the increasing relation volume of cytoplasm (pH value 7-8; cf pH 4-6 for vacuole) acting as an anion trap. A saturable uptake component, Km=2.6±0.8 μmol dm(-3), is restricted to the apical 4-6 mm of the root (including lateral roots), is not of metabolic origin, and is likely to be a carrier. 2. No polarity of transport could be detected using donor blocks containing [2-(14)C]abscisic acid applied to 15 mm or 40 mm segments whose apical 10 mm had been removed; if the elongation zone were present in the test segments, a distribution of radioactivity that might be expected from acropetal polarity was obtained, but which may simply be accounted for by the greater uptake capacity of the elongating, relatively unvacuolated cells in the extending region of the root.