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

Abstract

The absorption of inorganic salts by plant roots and other biological systems depends upon metabolic processes. When absorption proceeds against a concentration gradient, an expenditure of metabolically derived energy is obviously required. In some systems, such as barley roots, even absorption along a concentration gradient is negligible under conditions of arrested metabolism; and the concentration of the salt in the sap fails to reach the level of the outside solution (12, 13, 17). The absorption process is characterized by a considerable degree of se-lectivity. The discrimination on the part of living cells between potassium and sodium is particularly striking. AMost living cells accumulate potassium in preference to sodium, even when the external environment is rich in sodium and poor in potassium. The view has come to the fore in recent years that both the role of metabolism and the specificity in the absorption process can best be accounted for by assuming that ion binding compounds are generated by metabolic processes. General reviews which make this point include those by USSING (29), ROBERTSON (25), and STEINBACH (27). OSTERHOUT (22) has succinctly stated some of the problems for whose solution such a concept would be useful. OSTERHOUT (22) and JACOBSON and OVERSTREET (14) have outlined several properties which the ion binding compounds must possess to account for characteristic features of the absorption process. These workers also stress the need for assuming that the complex is labile and breaks down again, releasing the ions. COWIE et al. (5), ROBERTS et al. (23), CONWAY et al. (4), EDDY et al. (7), and EDDY and HINSHELWOOD (8, 9) have investigated the problem of ion binding in microorganisms, arriving at similar views. ROBERTS and his co-workers (23, 24) and EDDY and co-workers (7, 8) have related the binding of alkali cations to carbohydrate metabolism, and BOLTON and ROBERTS (1) have produced evidence that the hexose phosphates are responsible for the binding of potassium in Escherichia coli. The hypothesis that the absorption of inorganic ions involves their combination with binding compounds forms the basis of the present study. Evidence will be presented for the existence of several distinct reactive sites involved in the binding of alkali cations by barley roots. It will be demonstrated that there are large differences in the affinity of these sites for various cations. Methods Roots of the Sacramento variety of barley were used as the experimental material. The method for obtaining barley roots with a high capacity for 457