THE ABSORPTION OF ALKALINE EARTH CATIONS BY BARLEY ROOTS: KINETICS AND MECHANISM

Abstract

THE MECHANISM of the absorption of ions by plant root cells may be visualized as follows (cf. fig. 1). At the outer boundary of a barrier not permeable to the free ions, the ions combine with binding compounds or carriers., The complex thus formed traverses the barrier and through a chemical change in the complex breaks down at the inner surface, releasing the ions to the region beyond. The carriers possess a number of distinct reactive centers characterized by differential affinities for various ions or groups of ions, thus providing the chemical basis for selectivity in the transport of ions. This mechanism of active transport involving carriers is analogous to the mechanism of catalysis mediated by enzymes. In each case, the substance acted upon (the ion or substrate, respectively) combines with the agent (carrier or enzyme) to form an intermediate labile complex which subsequently breaks down again, the process in question (transport or catalysis) having taken place. Interfering chemical species may change the kinetics of the system by combining with the agent at the same reactive centers which bind the substances being acted on (competitive interference) or at other centers (non-competitive or un-competitive interference) . Epstein and Hagen (1952) and Epstein (1953) have presented evidence that the above picture of the process of active ion transport applies to the absorption of alkali cations and monovalent anions by barley roots. In the present investigation, this work has been extended to the alkaline earth cations. Active transport of these ions in barley roots will be shown to exhibit the same basic features as to kinetics and selectivity which were found to apply to monovalent ions. MATERIALS AND METHODS.-Water.-Double-distilled water stored in a tin-lined tank and piped into the laboratory through tin-lined copper pipes was passed through a column containing cation and anion exchange resins2 to remove heavy metal contaminants. The water was stored in Pyrex glass. Radioactive strontium.-Strontium containing Sr89 was obtained from the Oak Ridge National Laboratorv in the form of Sr89 (NO3) 2The salt was converted to the chloride by precipitation as the carbonate which was dissolved in excess HCl,