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

Calcium has a fundamental role in the cellular processes of ion absorption and retention. The ion is generally credited with lhaving beneficial effects on the semipermeability of membranes and structure of protoplasm (9,12). The electron microscope shows Ca deficiency to result in (lisorganization 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