Multiple Site Uptake of Individual Cations by Roots as Affected by Hydrogen Ion.

Abstract
The uptake of Rb, K, Na and Sn by excised barley roots under steady state conditions indicates at least two distinct carrier sites of absorption for each of the cations. One site dominates at higher ion concentrations while the second dominates at lower ion concentrations. The apparent dissociation constants were determined of each of the 2 sites for each of the 4 cations studied. They were similar in magnitude for K, Rb and Sn and slightly higher for Na. H ion was shown to compete directly for each of the sites that dominates at lower concentrations of all cations tested. The apparent dissociation constant for H ion at the Sn, Rb and K site that dominates at lower concentrations was 1 x 10"5. The comparable dissociation constant, Ki, at the Na site was 2 x 10"6. The apparent dissociation constants for H were similar in magnitude to the apparent dissociation constants of the ions themselves. Rb and K were shown to compete for the same site.