Absorption of Chloride by Barley Roots: Kinetics and Selectivity

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Abstract
Excised roots of barley, Hordeum vulgare var. Arivat, adsorbed Cl labeled with Cl36 from solutions of KC1 ranging in concentration from 0.005 to 0.2 mM, Ca was usually present as 0.5 mM CaSO4. The rate of Cl absorption was constant for at least 60 minutes and labeled Cl absorbed was not isotopically exchangeable with external unlabeled CL Omission of Ca lowered the rate of Cl absorption and rendered it more sensitive to the external pH. In the presence of 0.5 mMCa, the external pH over the range 4.0-5.6 had little effect on the rate of Cl absorption. Over the concentration range 0.005-0.2 mM Cl. the rate of Cl absorption was a function of the external concentration in strict accord with Michaelis-Menten kinetics. The Michaelis constant, Km, was 0.013 mMin the presence, and 0.046 mM in the absence of Ca. At 0.1-0.2 mM Cl. the rate of Cl absorption virtually reached the theoretical maximum velocity, Vmax. Br added at 1.0 mM competitively inhibited Cl absorption. The inhibitor constant, Ki, was 0.036 mM. Neither I nor F had any effect upon the rate of Cl absorption. Lowering the temperature and addition of metabolic inhibitors sharply reduced the rate of Cl adsorption.