title: Nitrate influx and efflux by intact wheat seedlings: Effects of prior nitrate nutrition.
abstract: Wheat (Triticum vulgare L., cv. Blueboy) seedlings, grown with 0.25, 1.0 and 15 mM nitrate in complete nutrient solutions, were transferred 10 days after germination to 1.0 mM K(15)NO3 (∼99 A% (15)N) plus 0.1 mM CaSO4 at pH 6.0. The solutions were replaced periodically over a 6-h period (5 mW cm(-2); 23°). Changes in the [(15)N]- and [(14)N]nitrate in the solution were determined by nitrate reductase and mass-spectrometric procedures and potassium by flame photometry. Influx of [(15)N]nitrate was depressed in plants grown at 1.0 mM nitrate relative to those grown at 0.25 mM, but there was no appreciably difference in [(14)N]nitrate efflux. Prior growth at 15 mM further restricted [(15)N]nitrate influx which, together with a substantial increase in [(14)N]nitrate efflux, resulted in no net nitrate uptake during the course of the experiment. Efflux of [(14)N]nitrate occurred to solutions containing no nitrate but it was significantly enhanced upon exposure to [(15)N]nitrate in the external solution. Influx of [(15)N]nitrate was more restricted at 5°, relative to 23°, than was [(14)N]nitrate efflux. The nitrate concentrations of the root tissue immediately before exposure to the K(15)NO3 solutions did not give a precise indication of the subsequent [(15)N]nitrate influx rates nor of the [(14)N]nitrate efflux rates. Net K(+) uptake was related to the magnitude of the net nitrate uptake, not to the initial K(+) concentration in the roots. The data are interpreted as indicating that [(15)N]nitrate influx and [(14)N]nitrate efflux are largely independent processes, subject to different controls, and that net nitrate uptake provides the driving force for net potassium uptake.
authors.affiliation: Department of Soil Science, North Carolina State University, 27607, Raleigh, NC, USA.
authors.forename: W A
authors.initials: WA
authors.lastname: Jackson
authors.forename: K D
authors.initials: KD
authors.lastname: Kwik
authors.forename: R J
authors.initials: RJ
authors.lastname: Volk
authors.forename: R G
authors.initials: RG
authors.lastname: Butz