Absorption of Cations by Roots. Effects of Hydrogen Ions and Essential Role of Calcium

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Abstract
Experiments are described in which excised barley roots absorbed Rb from solutions of RbCI ranging in concentration from 0.002 to 0.2 mM, in the presence or absence of CaCl2, both at low pH values (3.9 - 4.1) and at pH 5.7. In the absence of Ca, the rate of Rb absorption from solutions at pH 3.9 declined with time, and fell to nil in about 1 hour. In the presence of 0.5 mM Ca the rate of Rb absorption remained constant for at least 2 hours. In the presence of 0.5 mM Ca, absorption of Rb was a function of the external concentration according to Michaelis-Menten kinetics. H ions competitively inhibit Rb absorption. The Michaelis constant Rb absorption, Km, is 0.016 mM, the inhibitor constant of H ions, Ki, is 0.038 mM, and the theoretical maximum velocity at non-limiting external concentration of Rb is 7.2 [mu]mole/g/ hour. It is concluded that H ions have 2 distinct effects on the absorption mechanisms. In the absence of Ca, H ions quickly bring about nonphysiological conditions and the absorption mechanism is progressively impaired. In the presence of Ca, H ions inhibit Rb absorption by competing with Rb for identical carrier sites. This competition need not induce progressive damage to the absorption mechanism. The findings reported here on absorption of Rb are believed to apply equally to the absorption of K.