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The effects of soil drying on the activity of nitrate reductase (NR; EC 1.6.6.6) were studied in Helianthus annuus L. and non‐nodulated Lupinus albus L. plants growing under two nutrient supply regimes. NR activity was assessed in leaf and root extracts by measuring the activity of the unphosphorylated active form (NR_act), the maximal extractable activity (NR_max) and the activation state. To obtain an insight into potential signalling compounds, nitrate, free amino acids and soluble sugars were also quantified. In both species, foliar NR_act and NR_max were negatively affected by soil drying and a decreased supply of nutrients, the observed changes in NR activity being linearly correlated with the depletion of nitrate. Similar results were obtained in the roots of sunflower. Conversely, in white lupin roots, NR_max was found to be independent of tissue nitrate concentration. Regardless of the species and organ, the …