Nitrate and ammonium influxes in soybean (Glycine max) roots:Direct comparison of N-13 and N-15 tracing

Abstract

We compared influxes and internal transport in soybean plants (Glycine max cv. Kingsoy) of labelled N from external solutions where either ammonium or nitrate was labelled with the stable isotope N-15 and the radioactive isotope N-13. The objective was to see whether mass spectrometric determinations of tissue N-15 content were sufficiently sensitive to measure influxes accurately over short time periods. Our findings were as follows. (1) There was a close quantitative correspondence between estimates of N influx of individual plants using N-15 or N-13 measurements with either NO3- or NH4+ at 4 or 2 mol m(-3), respectively in the external solution. (2) Transport to the shoot of N from NO3 absorbed over a 5-15 min period could be monitored when the external NO3- concentration ranged from 0.05 to 4 mol m(-3). NH4+ as the N source labelled shoot tissue more slowly, and estimates of the transport between root and shoot could be made only with N-13. (3) Influx of NO3- into root tissue could be measured by N-15 enrichment after 5-10 min at concentrations approaching the probable KM of the high-affinity transport system. (4) There was some indication of isotope discrimination, especially with respect to the movement of labelled N to the shoot, when NO3- is the N source. For many purposes, N-15 tracing can be used satisfactorily to estimate influxes of both NO3- and NH4+ in soybean roots. Use of the short-lived radionuclide N-13 remains the method of choice for more refined measurements of internal distribution and assimilation