Electric potential differences in bean roots and their relation to salt uptake.

Reversible diffusion of KGl is shown to occur between an aqueous solution and young bean roots, probably in the protoplasmic phase. Evidence for this is presented from a study of electric potential difference (p.d.) changes and changes in environmental salt concentration in different samples but under comparable conditions. Characteristic changes of p.d. with time are obtained when a root tip taken from distilled ~ater is dipped in O.OIN KCl followed by O.OOOIN KCl, indicating an increase of concentration in the root tissue while in the first solution and then a decrease while in the more dilute solution. When excised root tips which have been in distilled water are immersed in O.OIN KCl, conductivity measurements show a mean initial uptake of 0.13 x 10-5 g. mol.!g. of tissue. An approximately equal amount of salt leaves the roots if they are then transferred to O.OOOIN KCl. The region of the tissue to which the electrolyte has access by diffusion, called the "apparent free space," is calculated to be 13 per cent. for the material used. Approximately 10· of the 13 per cent. is likely to be proto-plasm. The evidence supports the hypothesis that electric p.d.'s between young root· tissue and surrounding electrolytes are the result of differential mobility of cations and anions in the protoplasmic phase of the epidermal cells. The relation of the evidence to theories postulat.ing a high resistance barrier to ion diffusion at the boundary of protoplasm and environment is discussed. It is concluded that the properties of the protoplasm/environment boundary do not include high resistance to ion diffusion. It is possible that protoplasm contains a certain concentration of non-mobile anions resulting in a Donnan distribution of ions between this phase and the surrounding solution. Ways of testing this hypothesis are suggested.