Effect of External K, NH₄, Na, Ca, Mg, and H Ions on the Cell Transmembrane Electropotential of Avena Coleoptile

A number of studies have shown that an electropotential difference, PD, of about 100 mv (interior negative), is present across plant cell membranes (2, 3, 7, 8, 10, 11, 13, 21), similar to that across animal cell membranes. Thus, if minerals move through cell membranes as ions, they are subject not only to concentration gradients but also to electrical fields. For precise evaluation of the energy relationships in ion accumulation and active ion transport by cells the electropotential gradient as well as the chemical gradient must be known (4, 5). Active ion transport, requiring metabolic energy, is defined as transport against the electrochemical activity gradient (4, 5). Cases are known in which ion accumulation takes place without the expenditure of metabolic energy (5, 18). The use of the Nernst equation in elucidating the fundamental relationships of electropotential differences to ionic concentration gradients across plant cell membranes has been discussed recently in detail by Dainty (5) and by Briggs, Hope, and Robertson (4). The results of similar studies of animal cells (12, 16, 19) and of giant cells of algae (2, 11, 13, 21) show that the resting electropotentials have an approximate quantitative relationship to the concentration gradient at equilibrium of the ions K+, Na+, and Cl-. This relationship is more closely approached when consideration is given to the relative permeability coefficients of these ions as in Goldman's modification (9) of the Nernst equation. This suggests that the resting potential is diffusional in origin, that it may be primarily a function of the relatively few more rapidly permeating ions, and that K+, in particular, may have a predominant effect. Previous reports have shown that oat, corn, and pea seedling tissue cells, exhibit a transmembrane PD which is influenced by external K+ concentration in the direction predicted by the Nernst equation (8). However, the effect on these plant cell PD's of ions other than K+ has not been evaluated. The present report deals with a limited survey of the effect on PD of manipulating the ion concentration gradients by altering the external concentration. This paper gives details of the results mentioned in an earlier brief report (10).