Dt. Clarkson et al., NITRATE AND AMMONIUM INFLUXES IN SOYBEAN (GLYCINE-MAX) ROOTS - DIRECTCOMPARISON OF N-13 AND N-15 TRACING, Plant, cell and environment, 19(7), 1996, pp. 859-868
We compared influxes and internal transport in soybean plants (Glycine
max cv. Kingsoy) of labelled N from external solutions where either a
mmonium or nitrate was labelled with the stable isotope N-15 and the r
adioactive isotope N-13. The objective was to see whether mass spectro
metric determinations of tissue N-15 content were sufficiently sensiti
ve to measure influxes accurately over short time periods. Our finding
s were as follows. (1) There was a close quantitative correspondence b
etween estimates of N influx of individual plants using N-15 or N-13 m
easurements 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 ab
sorbed 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 betw
een root and shoot could be made only with N-13. (3) Influx of NO3- in
to root tissue could be measured by N-15 enrichment after 5-10 min at
concentrations approaching the probable KM of the high-affinity transp
ort system. (4) There was some indication of isotope discrimination, e
specially with respect to the movement of labelled N to the shoot, whe
n NO3- is the N source. For many purposes, N-15 tracing can be used sa
tisfactorily to estimate influxes of both NO3- and NH4+ in soybean roo
ts. Use of the short-lived radionuclide N-13 remains the method of cho
ice for more refined measurements of internal distribution and assimil
ation.