Effect of ammonium nutrition on the activity of nitrate reductase in the roots of apple seedlings

Active extracts of nitrate reductase were prepared from the roots of apple seedlings c.v. Granny Smith which were grown in nutrient solution under controlled enviromental conditions. The nutrient solutions contained various ratios of nitrate and ammonium ions but all the treatments contained a total of 112 ppm nitrogen. Maximum nitrate reductase activity in the roots was obtained when plants were supplied with nitrate as the sole source of nitrogen. Roots grown in solution containing only ammonium nitrogen had little or no activity. When plants were supplied with both forms of nitrogen in the nutrient solution, the presence of ammonium ions markedly lowered the activity of nitrate reductase in the roots. Plants supplied with 98 ppm nitrate nitrogen plus 14 ppm ammonium nitrogen had activities only half those of plants grown in nitrate alone. Plants supplied with equal amounts of ammonium and nitrate nitrogen had activities less than one sixth those of plants grown in nitrate alone.