The effects of a two weeks soil drying period on the activity of nitrate reductase (NR; EC 1.6.6.6) were studied on 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 get insight into potential signalling compounds, nitrate, amino acids and soluble sugars concentrations were also quantified. On both species, foliar NR act and NR max were negatively affected by soil drying and reduced 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 activation state of the enzyme was unaffected by the nutrient supply regime. In well-watered sunflower roots only about 50% of the existing NR was unphosphorylated, but the activation state increased significantly in response to drought. In contrast, lupin roots always exhibited NR activation state values close to 80% or even higher. At the leaf level, NR activation state was hardly changed in response to soil drying. The contribution of changes in the concentrations of soluble 5 sugars and amino acids to explain the observed variations in NR activity are discussed. AbbreviationsFW, Fresh weight; NR, Nitrate reductase; NR act , activity of the unphosphorylated form of nitrate reductase; NR max , maximal extractable activity of nitrate reductase

The effects of drought on the activity of nitrate reductase (NR) were studied in Helianthus annuus L. plants subjected to soil drying and subsequent rewatering. Drought did not negatively affect the activation state of NR, but resulted in linearly correlated decreases in the activity of the unphosphorilated active form and the total activity of NR, both in roots and leaves. The concentration of nitrate in roots, xylem and leaves also decreased in waterstressed plants, whereas the concentration of total amino acids was only transiently depressed at the leaf level. In contrast, soluble sugars accumulated both in roots and leaves of waterstressed plants. Drought-induced decrease in root NR activity was correlated with the observed changes in root nitrate concentration. A higher percentage of the decrease in foliar NR activity could be explained by the decline in nitrate flux to the leaves than by leaf nitrate content. Following rewatering, the extent of recovery of NR activity was higher in roots than in leaves. The delay in the recovery of foliar NR activity did not result from the persistence of reduced flux of nitrate through the xylem. Several hypotheses to explain the after-effect of soil drying on foliar NR activity are discussed. of transgenic plants with modified capacities to synthesize ABA. Journal of Experimental Botany 53, 287-296. Brewitz E, Larsson C-M, Larsson M (1996) Response of nitrate assimilation and N translocation in tomato (Lycopersicon esculentum Mill) to reduced ambient air humidity. Journal of Experimental Botany 47, 855-861. Correia MJ, Pereira JS (1994) Abscisic acid in apoplastic sap can account for the restriction in leaf conductance of white lupins during moderate soil drying and after rewatering. Plant, Cell and Environment 17, 845-852. Correia MJ, Rodrigues ML, Osório ML, Chaves MM (1999) Effects of growth temperature on the response of lupin stomata to drought and abscisic acid. Australian Journal of Plant Physiology 26, 549-559. Crawford NM (1995) Nitrate: nutrient and signal for plant growth. The Plant Cell 7, 859-868. Dzuibany C, Haupt S, Fock H, Biehler K, Migge A, Becker TW (1998) Regulation of nitrate reductase transcript levels by glutamine accumulating in the leaves of a ferredoxindependent glutamate synthase-deficient gluS mutant of Arabidopsis thaliana, and by glutamine provided via the roots. Planta 206, 515-522. Ferrario-Méry S, Valadier MH, Foyer CH (1998) Overexpression of nitrate reductase in tobacco delays drought-induced decreases in nitrate reductase activity and mRNA. Plant Physiology 117, 293-302. Fonseca F, David M, Barrote I, Ferreira G, Correia MJ (2000) Interaction between water and nutrient deficiencies in Helianthus annuus. In 'Nitrogen in a sustainable ecosystem. From the cell to the plant'. (Eds MA Martins-Loução and SH Lips) pp. 169-175. (Backhyis Publishers, Leiden, The Netherlands) 20 Foyer CH, Valadier MH, Migge A, Becker TH. (1998) Drought-induced effects on nitrate reductase activity and mRNA and on the coordination of nitrogen and carbon metabolism...