A KINETIC STUDY OF THE ABSORPTION OF ALKALI CATIONS BY BARLEY ROOTS

Abstract

Seeds of barley, var. Sacramento, were germinated in aerated, dist. water for 24 hrs. and the seedlings grown for 5 days over aerated 10-4 [image] CaSO4 in the dark at 24[degree]C. The roots were excised and used in 3-hr. absorption expts. under the following conditions: Roots, 1.00 g. fresh wt. per sample; vol., 50 ml.; temp., 30[degree]C salts, chlorides. The absorption of alkali cations (primarily Rb) was detd., using Rb86, K42, and Na24 as tracers. The independent variables were the concn. of the ion whose absorption was being measured (the substrate ion) and the concn. of another alkali cation (the interfering ion). The results are interpreted on the basis of the hypothesis that absorption involves the formation and subsequent breakdown of a labile complex, MR, of the cation, M, and a binding compound or carrier, R, in analogy to the combination of substrate, S, with the enzyme, E, in enzymatic reactions. The interfering ions assume the role of inhibitors or alternate substrates. K and Cs were found to interfere competitively with Rb uptake, indicating identical binding sites on the carrier, R, for these 3 ions. Na (except at concns. of either Rb or Na above 0.01 [image]) did not interfere competitively with Rb absorption. At Rb concns. of 0.001 [image] and below, Na up to 0.01 [image] entirely failed to interfere with Rb uptake. At relatively low concns. of Rb and Li, Li increased the rate of Rb absorption. The interference by Li at higher concns. was not competitive. It is concluded that Na and Li are bound by sites other than those which bind K, Rb, and Cs.