[Exchange mechanism of ion transport in corn roots]

The effect of the external anion concentration on the rate of efflux of anions from corn roots was investigated by following the efflux of labeled anions in shaken solutions of different concentrations of unlabeled anions.Rates of efflux of phosphate and chloride from corn roots are rather independent of external concentrations of the anions as long as these concentrations are low.Rates of efflux of phosphate and chloride from corn roots at high external concentrations of these ions were found to be a function of the external concentrations.Stimulation of efflux is specific: phosphate efflux is stimulated only by phosphate and not by chloride in the medium and vice versa. Since it appears unlikely that this effect is due to specific changes in the permeability of the membranes, it may by concluded that the specific stimulation of efflux by external ions as observed in this investigation is caused by ion exchange at mobile membrane units (WEIGL, 1967a). Reasons for this hypothesis are discussed and alternative hypotheses such as that calling for the protoplasm to function as ion exchanger with various specific binding sites are examined. The latter hypothesis remembers to the theory that "the ion carrier is the protoplasm itself" (OVERSTREET, 1957; BANGE and MEIJER, 1966), which diminishes the importance of membranes as limiting boundaries.