Effect of External K, NH(4), 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 celtl 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 gradlient (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 memiibranes lhas been discussed recently in detail by Dainty (5) and by Briggs, Hope, an(l Robertson (4). The results of similar studlies of aninmal cells (12, 16, 19) and of giant cells of algae (2, 11, 13, 21) show that the resting electropotentials have an approximlate quantitative relationship to the concentration gra(lient at equilibrium of the ions K+, Na+, and Cl-. This relationship is mlore closely approached when consideration is given to the relative permeability coefficients of these ions as in Goldmlan's modification (9) of the Nernst equation. This suggests that the resting potential is diffusional in origin, that it nmay be primarily a function of the relatively fewv more rapidly permleatiing ions, andl that K+, in particular, may have a pre(lomlinant effect. Previous reports have shown that oat, corn, and pea see(lling tissue cells, exhibit a transmembrane PD which is influenced by external K+ concentration in the direction predicted by the Nernst equation (8). HoNever, the effect on these plant cell PD's of ions other than K+ has not been evaluate(l. The present