The growth rate and activities of nitrate reductase enzymes in tobacco plants are influenced by the amount of nitrate in the soil. Cytosolic nitrate reductase (cNR) activity in roots is highest at low nitrate levels, while plasma membrane-bound nitrate reductase (PM-NR) activity increases with higher nitrate levels. High PM-NR activity is linked to slow growth, possibly due to excess nitrate. Tissue nitrate content is lowest at intermediate nitrate levels in both roots and shoots. As PM-NR activity decreases, tissue nitrate content increases. In leaves, both cNR and PM-NR activities are related to internal nitrate levels, but respond differently to low nitrate levels.