Changes in root hydraulic resistance in response to alterations in nitrate supply were explored in detail as a potential mechanism that allows plants to respond rapidly to changes in their environment. Sunflower (Helianthus annuus cv. Holiday) plants grown hydroponically with limited nitrate availability (200 μmol l⁻¹) served as our model system. Experimental plants were 6–9-weeks-old with total dry mass of 2–4 g. Root pressurization of intact plants and detached root systems was used to elucidate the temporal dynamics of root hydraulic properties in sunflower plants following changes in external nitrate availability. The response was rapid, with a 20% decrease in hydraulic resistance occurring within the first hour after the addition of 5 mM nitrate and the magnitude of the effect was dependent on nitrate concentration. The change in root hydraulic resistance was largely reversible, although the temporal dynamics of the response to nitrate addition versus nitrate withdrawal was not symmetric (a gradual decrease in resistance versus its fast increase), raising the possibility that the underlying mechanisms may also differ. Evidence is presented that the observed changes in root hydraulic properties require the assimilation of nitrate by root cells. The hydraulic resistance of roots, previously stimulated by the addition of nitrate, increased more than in control plants in low nitrate under anoxia and that suggests a key role of aquaporin activity in this response. It is proposed that a rapid decrease in root hydraulic resistance in the presence of increased nitrate availability is an important trait that could enhance a plant's ability to compete for nitrate in the soil.

Soil drying combined with nitrogen (N) deficiency poses a grave threat to agricultural crop production. The rate at which nitrate (NO₃⁻) is taken up depends partly on the uptake and transpiration of water. Rapid changes in nitrate assimilation, in contrast to other N forms, may serve as a component of the plant stress response to drought because nitrate assimilation may lead to changes in xylem pH. The modulation of xylem sap pH may be relevant for stomata regulation via the delivery of abscisic acid (ABA) to guard cells. In several factorial experiments, we investigated the interactions between nitrate and water availability on nitrate fate in the plant, as well as their possible implications for the early drought-stress response. We monitored the short-term response (2–6 days) of nitrate in biomass, transport to shoot and reduction in Pisum sativum, Hordeum vulgare, Vicia faba, and Nicotiana tabacum and correlated this with sap pH and transpiration rates (TRs). Cultivation on inorganic substrate ensured control over nutrient and water supply and prevented nodulation in legume species. NO₃⁻ content in biomass decreased in most of the species under drought indicating significant decline in NO₃⁻ uptake. Hordeum vulgare had the highest NO₃⁻ concentrations in all organs even under drought and low NO₃⁻ treatment. This species can likely respond much better to the combined adverse effects of low NO₃⁻ and water scarcity. Nitrate reductase activity (NRA) was reduced in both roots and leaves of water deficient (WD) plants in all species except H. vulgare, presumably due to its high NO₃⁻ contents. Further, transient reduction in NO₃⁻ availability had no effect on sap pH. Therefore, it seems unlikely that NRA shifts from shoot root leading to the supposed alkalization of sap. We also did not observe any interactive effects of NO₃⁻ and water deficiency on transpiration. Hence, as long as leaf NO₃⁻ content remains stable, NO₃⁻ availability in soil is not linked to short-term modulation of transpiration.

• Background and Aims The regrowth dynamics after defoliation of the invasive grass Calamagrostis epigejos were studied. As nitrogen (N) reserves have been shown to play an important role during plant regrowth, the identity, location and relative importance for regrowth of N stores were determined in this rhizomatous grass. • Methods Plant growth, nitrate uptake and root respiration were followed during recovery from defoliation. Water soluble carbohydrates, nitrate, free amino acids and soluble proteins were analysed in the remaining organs. • Key Results Nitrate uptake and root respiration were severely reduced during the first days of regrowth. Roots were the main net source of mobilized N. The quantitatively dominant N storage compounds were free amino acids. Free amino acids and soluble proteins in the roots decreased by 55 and 50 %, respectively, and a substantial (∼38 %) decrease in stubble protein was also observed. Although the relative abundance of several soluble proteins in roots decreased during the initial recovery from defoliation, no evidence was found for vegetative storage protein (VSP). Furthermore, rhizomes did not act as a N storage compartment. • Conclusions Production of new leaf area was entirely reliant, during the first week after defoliation, on N stores present in the plant. Mobilized N originated mainly from free amino acids and soluble proteins located in roots, and less so from proteins in stubble. Presence of VSP in the roots was not confirmed. The data suggest that rhizomes played an important role in N transport but not in N storage.