Nitrate and ammonium influxes in soybean (Glycine max) roots: direct comparison of ¹³N and ¹⁵N 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 tbe stable isotope'-'^N and the radioactive i.sotope'''N. The objective was to see whether mass spectrometric determinations of tissue '^N content were sufficiently sensitive to measure influxes accurately over short time periods. Our findings were as follow.s. (1) There was a close quantitative correspondence between estimates of N influx of individual plants using """^N or '"'N niea.surements with either NO," or NH/ at 4 or 2 mol m"\ respectively in the external solution.(2) Transport to the shoot of N from NO, absorbed over a 5-15 min period could be monitored when the external NO," concentration ranged from 0-05 to 4 mol m""\ NH/ as the N source labelled shoot tissue more slowly, and estimates of the transport between root and shoot could be made only with '"^N. (3) Influx of NO," into root tissue could be measured by "'N enricbment after 5-10 min at concentrations approaching the probable AM of the high-aninity transport system. (4) There was some indication of isotope discrimination, especially with respect to the movement of labelled N to the shoot, when NO," is the N source. For many purposes, '"''N tracing can be used satisfactorily to estimate influxes of both NO," and NH/ in soybean roots. Use of the short-lived radionuclide ' N remains the method of cli«)ice for more refined measurements of Internal distribution and a.ssimilation.

Transcript Profiling in thechl1-5Mutant of Arabidopsis Reveals a Role of the Nitrate Transporter NRT1.1 in the Regulation of Another Nitrate Transporter, NRT2.1[W]

“…Briefly, the plants were sequentially transferred to 0.1 mM CaSO 4 for 1 min, to complete nutrient solution, pH 5.8, containing either 15 NO 3 ÿ or 15 NH 4 þ (99% atom excess 15 N) for 5 min at the concentrations indicated in the figures, and finally to 0.1 mM CaSO 4 for 1 min. Roots were then separated from shoots, and the organs dried at 708C for 48 h. After determination of their dry weight, the samples were analyzed for total N and atom percentage 15 N using a continuous-flow isotope ratio mass spectrometer coupled with a C/N elemental analyzer (model ANCA-MS; PDZ Europa, Crewe, UK) as described in Clarkson et al (1996). Each influx value is the mean of 6 to 12 replicates.…”

Regulation of Root Ion Transporters by Photosynthesis: Functional Importance and Relation with Hexokinase

“…At the end of the 15 N labeling procedure, roots were washed for 1 min in 0.1 mM CaSO 4 and separated from shoots. The organs were dried at 70ЊC for 48 h, weighed, crushed in a hammer mill, and analyzed for total 15 N content using a continuous-flow isotope ratio mass spectrometer coupled with an C/N elemental analyzer (model ANCA-MS; PDZ Europa, Crewe, UK), as described by Clarkson et al (1996).…”

Three Functional Transporters for Constitutive, Diurnally Regulated, and Starvation-Induced Uptake of Ammonium into Arabidopsis Roots

“…Roots were separated from shoots and dried for 48 hr at 70ЊC. 15 N contents were determined by mass spectrometry using a continuous-flow isotope ratio mass spectrometer coupled with an elemented analyzer (model ANCA-MS; Europa Scientific, Crewe, UK; Clarkson et al, 1996).…”