The plasmalemma: seat of the type 2 mechanisms of ion absorption.

According to the classical view, a plant cell is a metabolic microcosm separated from the medium without by a membrane, the plasmalemma, which acts as a permeability barrier to polar solutes including inorganic ions. Many such solutes nevertheless are taken into the cell. This phenomenon is commonly thought to be due, not to diffusion of the free ions across the plasmalemma, but to the operation of mechanisms of active transport (3). The resistance of this membrane to free diffusion of ions and many other solutes, coupled with its capacity for transport, is thought to fulfill the twin requirement for the maintenance of chemical homeostasis within the cytoplasm and for the regulated transport of nutrients and metabolites into and out of the cell. This, the classical view of the function of the plasmalemma, has been repeatedly challenged. It has been claimed, specifically, that inorganic ions readily diffuse across the plasmalemma, a claim which has been countered by cogent arguments on the part of defenders of the classical view )(1, 2, 3, 4,8,9). Nevertheless, Laties and his collaborators (11, and references cited there) have once again put forward the contention that ions readily negotiate the plasmalemma by diffusion. They confirmed for corn roots and beet discs the operation of dual mechanisms of absorption of a given ion, as described by Epstein (5). They went on to contend that the type 2 mechanisms-those that become apparent at concentrations above 1 mM-are located in the tonoplast, and are reached by the ions after they have diffused across the plasmalemma, considered by them to be permeable in this range o,f concentrations. We have shown elsewhere (16) that even in the range of high concentrations (> 1 mM), where the type 2 mechanisms are operative, the plasmalemma does not become permeable to monovalent cations, and that mechanisms 1 and 2 of alkali cation absorption operate in parallel across the plasmalemma. In the present paper we confirm this conclusion by means of independent evidence, and extend it to the absorption of Cl. In the experiments, duplicate or triplicate sets of 1.00-g samples of excised roots of barley, Hordeum vulgare, Arivat, enclosed in open-weave cheesecloth "tea bags" (7) were exposed to aerated experimental solutions containing 0.5 mM CaSO4 and KCl labeled with 86Rb or NaCl labeled with 36Cl, at pH 5.5 ± 0.3 and 30°. Unless otherwise indicated, the experimental period was terminated by procedure B of Epstein et al. (7) in which readily exchangeable and diffusible ions (those in the "outer space" and on charged sites of the cell wall) are eluted by a 30-min exposure of the tissue to a 0.5 mM CaSO4, 5 mM KCl or NaCl solution (unlabeled), in the cold (about 4°). In the experiment reported in table I, the root samples were exposed to the experimental solutions either for a continuous 10-min period, or for 5 1-min pulses interrupted by 1-min periods of rinsing in unlabeled solutions. The temperature remained 30° during both the periods of absorption and the interruptions when the tissue was being rinsed in unlabeled solutions. For the 0.1 mM run, the unlabeled rinsing solution was 0.5 mM CaSO4, since nonmetabolic exchange adsorption of the labeled K is insignificant at this K concentration, in the presence of 0.5 mM Ca l(6). At the end of the experiment the samples were rinsed with 0.5 mM CaSO4 followed by water. For the run at 5 mM, where exchange adsorption is appreciable (7), the roots were rinsed, during the 1-min interruptions of the 1-min absorption pulses, with a 0.5 mM CaSO4, 5 mM KCl solution (unlabeled). The very last rin,se, just before reimmersion in the labeled experimental solutions, was with 0.5 mM CaSO4 solution. Table I shows that at both 0.1 and 5 mM K the amounts of K absorbed in five 1-min pulses were half the amounts absorbed in continuous 10-min periods. Table II presents the results of an experiment in which the roots were exposed to the labeled expt rimental solutions for either 1 or 60 min. At both concentrations (0.5 and 25 mnI) the amounts absorbed in 1 min were one-sixtieth those absorbed in 60 min.