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Articles

Chloride Accumulation by Mung Bean Root Tips

A Low Affinity Active Transport System at the Plasmalemma ¹

^a Department of Biology, McGill University, P. O. Box 6070, Montreal 101, Quebec, Canada

Net uptake of Cl^– into root tips of mung bean (Phaseolus aureus) increases steadily with increasing external concentrations from 1 to 60 mM. Membrane potentials were measured to determine the equilibrium concentration of Cl^– in the tissue which could be due to diffusion. This concentration was readily exceeded in both the relatively nonvacuolate tips (0 to 1 mm) and the vacuolate, mature upper sectons (1 to 11 mm) of the roots. The activity coefficient of both cytoplasmic and vacuolar Cl^–, measured with Cl^– sensitive microelectrodes, was approximately the same as that of a pure KCl solution of the same concentration. It is concluded that the "second mechanism" of ion uptake involves a large increase in the rate of active transport at the plasmalemma as the external concentration is increased above 1 mM.

¹ This work was supported by a grant from the National Research Council of Canada.