Role of Calcium in Absorption of Monovalent Cations

Abstract

PH influences the absorption of monovalent cations by excised barley (Hordeum vulgare) roots and determines in part the relative absorption rates from mixtures of monovalent cations. The presence of Ca in the solution modifies the absorption rate of a monovalent cation in a manner determined by the pH and the particular species of monovalent cation involved. The absorption of K, Rb and Cs is enhanced at low by the presence of Ca. Li absorption is repressed almost completely in the presence of Ca at all pH values. Na occupies an intermediate position in its response to Ca. In mixtures of Li and K, the presence of Ca prevents absorption of Li but increases K absorption. The H-Ca interaction is similar to that of L''i-Ca and the presence of Ca prevents entrance of H at low pH values. It is''postulated that the presence of Ca creates a selective barrier, probably at the cell surface. This barrier which is particularly effective in blocking Li and H determines the relative amounts of the cations reaching the absorption site. The stimulating effect of Ca is considered to be essentially a blocking of interfering ions.