Nitrate influx and efflux by intact wheat seedlings: Effects of prior nitrate nutrition.

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.