THE INTERNAL CONTROL OF NITRATE UPTAKE INTO EXCISED BARLEY ROOTS WITH DIFFERING SALT CONTENTS

Summary Measurements of net NO 3 ‐ influx have been made with excised bailey roots of differing salt status. With roots grown in 0.2 mM CaSO 4 , net NO 3 ‐ influx shows partial saturation with external NO 3 ‐ concentrations greater than 0.5 mM, and is greatly reduced in the absence of external K + or Na + . Net NO 3 ‐ influx from NaNO 3 solution is as rapid as from KNO 3 solution, but when both K + and Na + are present the roots show high specificity for K + over Na + . When roots are high in Cl ‐ , net NO 3 ‐ influx is greatly reduced, and the reciprocal effect of reduction in Cl ‐ influx into roots high in NO 3 ‐ is also found. There is some decrease in net NO 3 ‐ influx and in Cl ‐ influx when roots are high in organic anions, and under these conditions net NO 3 ‐ influx is completely independent of cation influx. There is no effect of external Cl ‐ (up to 10 mM) on net NO 3 ‐ influx (1 mM solution), and very little or no effect of external NO 3 ‐ on Cl ‐ influx. The results are considered in relation to the hypothesis that influx of NO 3 ‐ and Cl ‐ is subject to feedback control from the internal NO 3 ‐ plus Cl ‐ content, but it is concluded that organic anions can also play an important part in controlling influx of NO 3 ‐ and Cl ‐ . The interactions between cation and anion fluxes are discussed, and a unifying model for ion transport proposed.