Effects of water deficit on the activity of nitrate reductase and content of sugars, nitrate and free amino acids in the leaves and roots of sunflower and white lupin plants growing under two nutrient supply regimes

Abstract

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

Contribution of mineral nutrition to the response of barley seedlings to polyethylene glycol–induced mild water stress

“…An increase in cellular osmolytes under stress conditions has been observed by other authors as well (Carillo et al, 2005;Sharma and Dubey, 2005), and these solutes were supposed to play the role of enzyme protectants. Sugars were shown to act as major osmotic solutes in a number of plant species subjected to dehydration (Irigoyen et al, 1992;Kameli and Lösel, 1995;Bajji et al, 2000;Correia et al, 2005). In the present study, reducing sugars showed a significant increase in response to water stress (Fig.…”

“…Nitrate reductase (NR) is the first and rate-limiting enzyme in the nitrate-assimilation pathway (Lillo et al, 1996). Its activity was shown to decline in water-stressed leaves in a wide range of plant species (Bandurska, 1993;Flores et al, 2000;Correia et al, 2005). The regulation of this particular enzyme is related to several internal and external factors (Vincentz et al, 1993;Lillo et al, 1996).…”

“…The appropriate provision with carbohydrates and amino acids in the required amounts must be achieved by efficient communication, regulation, and coordination of N and C metabolism (Foyer et al, 1998). The foliar concentrations of major cellular osmolytes such as free amino acids and sugars have been measured in plants grown under different N supply in order to gain insight into their possible action as signaling compounds (Man et al, 1999;Correia et al, 2005). In the present study, water deficit was imposed to plants grown under two mineral-supply levels to evaluate whether the relationships between N nutrition and the quantities of C-and N-containing compatible solutes were merely correlative or could be varied by causes other than dehydration.…”

Comparative analysis of some biochemical responses of three indica rice varieties during polyethylene glycol-mediated water stress exhibits distinct varietal differences

“…We also estimated the activity of NR since it is the ratelimiting enzyme of nitrogen assimilation and that the maximal foliar extractable NR activity has been found to decrease in situations of water deprivation (Larsson et al 1989;Correia et al 2005), and the decline was correlated with low photosynthetic activity (Baki et al 2000) rather than to water stress itself. The NR activity under drought stress was also modified via changes (i.e., decrease) in NR transcript levels or through phosphorylation-dependent inactivation and proteolytic degradation (Lejay et al 1997).…”

Combined use of δ13C, δ18O and δ15N tracks nitrogen metabolism and genotypic adaptation of durum wheat to salinity and water deficit

“…In relation to this, NR and GS activities also decreased under water deficit, salinity and in combination, but overall, tolerant genotypes exhibited higher GS and NR activities. A number of studies have reported decreased NR activity in response to water stress (Foyer et al, 1998;Correia et al, 2005) and salinity (Wang et al, 2007;Carillo et al, 2008). Salt was observed to inhibit nitrate transport to the leaf mainly because of nitrate ⁄ chloride competition, consequently affecting NR activity (Rao & Gnanam, 1990;Abd-El Baki et al, 2000).…”