Ion Uptake by Soybean Root Tissue Depleted of Calcium by Ethylenediaminetetraacetic Acid

Abstract

Calcium has a fundamental role in the cellular processes of ion absorption and retention. The ion is generally credited with having beneficial effects on the semipermeability of membranes and structure of protoplasm (9,12). The electron microscope shows Ca deficiency to result in disorganization of membrane structure (19). In addition, the presence of calcium in solution was found by Viets (25) to increase the uptake of other ions, a phenomenon widely confirmed, but with variations in extent and conditions of response. Calcium is important in cation selectivity. The K/Na (or Rb/Na) absorption ratio increases in the presence of calcium (14,26), and the inhibitory effect of Na on K uptake is practically eliminated (7). The importance of calcium is sufficiently great in salt uptake studies that the root-to-solution ratio can affect K/Na absorption due to variable dissociation of calcium from the root tissue (15). In the presence of adequate calcium there is no initial exchange adsorption of the rubidium ion (8). Epstein (7, 8) notes that Ca in the solution around root tissue represents the normal physiological condition, and the ion is essential for the integrity of the selective ion transport mechanism. The selectivity is thought to reside in specific ion carriers. Jacobson et al. (13), on the other hand, conclude that the stimulation of K absorption by Ca results from blocking an interfering cation such as H or Li. Waisel (26) maintains that Ca increases the diffusion of K across the outer cell membrane, which he considers to be the rate-limiting step of metabolic accumulation. Tanada (23) has recently reiterated his suggestion that a ribonucleoprotein complex containing free -SH groups is involved in the Ca activation of Rb uptake by mung bean root tips. In an initial investigation of ribonucleoprotein as a carrier moiety, an EDTA-initiated degradation of RNA in soybean roots resulted in impaired respiration, ion accumulation, and ion retention (10). This result was attributed to removal of Ca and Mg by EDTA.3 However, treatment with EDTA in the cold, which would limit degradative metabolism, increased Rb88 uptake, a result similar to that reported by Tanada for ultraviolet treatment (20), ribonuclease treatment (21), sulfhydryl inhibition (22), and HCl or NaCl treatment (20). Rb86 uptake may be increased and P3204 uptake decreased by EDTA, citric acid, or alkaline phosphate pretreatment (20). We have now explored in some detail the effect of a short-term pretreatment with EDTA on ion uptake by soybean root tissue. Pretreatment removes much of the root Ca, and increases the subsequent metabolic uptake of Rb86-labeled K while decreasing that of phosphate. These effects are reversed by addition of Ca. Calcium removal appears to affect accumulation of the ions into the cytoplasm.