Abstract: | The short-lived radio-isotope nitrogen-13 (half-life 10 min)was used as a tracer in studying fluxes of N in the roots ofintact barley plants. After supplying the plants with 13N-nitratefor 30 min, efflux of 13N into an unlabelled (wash) solutionwas followed under steady-state conditions for a further 10min. Tests with ion exchange resins suggested that all of the13N released during this period was in the form of nitrate. In addition to nitrate from a surface film of solution and fromthe free space of the roots, efflux from another compartmentwas detected, tentatively identified as the cytoplasmic nitratepool. In plants grown with nitrate as the only external N-source,efflux from this compartment decreased with a rate constantabout 0·17 min1 (half-time 4 min). Adding ammoniumsulphate to the wash solution alone did not significantly affecteither the initial rate, or the rate constant, of efflux of13N from these roots. However, 13N efflux decreased more rapidly(rate constant about 0·32 min1, half-time 2·2min) in roots grown in, and subsequently washed with, solutioncontaining ammonium nitrate. In barley plants grown with 1·5 mol m3 nitrate,the cytoplasmic nitrate pool was estimated to contain about2% of the total nitrate in the roots, corresponding to a cytoplasmicnitrate concentration 26 mol m3. Nitrate efflux was equivalentto almost 40% of nitrate influx in the roots of these plants. Key words: Ion transport, nitrate, ammonium, efflux analysis, compartmentation |