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

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 15 N and the radioactive isotope 13 N. The objective was to see whether mass spectrometric determinations of tissue 15 N 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 15 N or 13 N measurements with either NO 3/ − or NH 4 + at 4 or 2 mol −3 , respectively in the external solution. (2) Transport to the shoot of N from NO 3 absorbed over a 5–15 min period could be monitored when the external NO 3 − concentration ranged from 0–05 to 4 mol m −3 . NH 4 + as the N source labelled shoot tissue more slowly, and estimates of the transport between root and shoot could be made only with 13 N. (3) Influx of NO 3 − into root tissue could be measured by 15 N enrichment after 5–10 min at concentrations approaching the probable K M 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 NO 3 − is the N source. For many purposes, 15 N tracing can be used satisfactorily to estimate influxes of both NO 3 − and NH 4 + in soybean roots. Use of the short‐lived radio nuclide 13 N remains the method of choice for more refined measurements of internal distribution and assimilation.