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

Summary 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 K¹⁵NO₃ (99 A% ¹⁵N) 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 ¹⁵N]- and ¹⁴N]nitrate in the solution were determined by nitrate reductase and mass-spectrometric procedures and potassium by flame photometry. Influx of ¹⁵N]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 ¹⁴N]nitrate efflux. Prior growth at 15 mM further restricted ¹⁵N]nitrate influx which, together with a substantial increase in ¹⁴N]nitrate efflux, resulted in no net nitrate uptake during the course of the experiment. Efflux of ¹⁴N]nitrate occurred to solutions containing no nitrate but it was significantly enhanced upon exposure to ¹⁵N]nitrate in the external solution. Influx of ¹⁵N]nitrate was more restricted at 5°, relative to 23°, than was ¹⁴N]nitrate efflux. The nitrate concentrations of the root tissue immediately before exposure to the K¹⁵NO₃ solutions did not give a precise indication of the subsequent ¹⁵N]nitrate influx rates nor of the ¹⁴N]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 ¹⁵N]nitrate influx and ¹⁴N]nitrate efflux are largely independent processes, subject to different controls, and that net nitrate uptake provides the driving force for net potassium uptake.Paper No. 4884 of the Journal Series of the North Carolina Agricultural Experiment Station, Raleigh, NC, USA. This investigation was supported in part by the U.S. Energy Research and Development Administration, Contract No. AT-(40-1)-2410