Nitrate reductase activity in phytoplankton from the Oslo-fjord, Norway

The activity of the enzyme nitrate reductase (NRA) was measuredin natural populations of phytoplankton in the Oslofjord, Norway,and in culture experiments with 10 different species of phytoplankton.The NRA values from the culture experiments and from the naturalsamples were of the same order of magnitude. The NRA values,when expressed on a chlorophyll a basis, were highly species-variable.In general they were correlated to the nitrogen nutrient concentrationsin the water. In order to use NRA as a guide to an understandingof the assimilation of nitrate, it is important to know thespecies composition of the phytoplankton.     

Nitrate reductase activity in tomato fruits grown in vivo and in vitro

The activity of nitrate reductase in tomato fruits (Lycopersicon esculentum Mill.) grown in vivo and in vitro was similar throughout development. Enzyme activity was directly correlated with fruit size. As has been shown in vivo, nitrate reductase activity was also inducible in fruits grown in vitro.     

Nitrate reductase activity,biomass, yield,and quality in cotton in response to nitrogen fertilization

In the production of cotton (Gossypium hirsutum L.), nitrogen fertilization is one of the most costly crop practices, but important to reach high yields. However, high nitrogen (N) content in plants does not always translate into a high fibre production. One way of assessing the efficiency of the N fertilizer is through the enzymatic activity of the nitrate reductase (NR). This is a key enzyme in N assimilation, whose activity is regulated by a number of endogenous and exogenous factors that determine yield. The aim of this study was to assess the effect of N fertilization on yield, fibre quality, biomass, and NR enzymatic activity in vivo in the cotton variety Fiber Max 989. The evaluated application rates were 0, 50, 100, and 150 kg/ha of N, using urea as a source (46% N) in a randomizedblock design with three replicates. At harvest, the maximum yield of seed cotton and the greatest accumulation of total foliar biomass through time was reached after applying 150 kg N/ha. The different N-application rates did not affect the components of cotton-fibre quality. The activity of endogenous NR was greater on plants where 150 kg N/ha were applied. The highest cotton yield and N contents were obtained on these plants. Therefore, the NR activity in vivo could be used as a bioindicator of the N nutritional level in cotton.     

Nitrate reductase activity, nitrogenase activity and photosynthesis of black alder exposed to chilling temperatures

Actinorhizal ( Frankia -nodulated) black alder [ Alnus glutinosa (L.) Gaertn.] seedlings fertilized with 0.36 m M nitrate (low nitrate fertilizer treatment) or 7.14 m M nitrate (high nitrate fertilizer treatment) and acclimated in a growth chamber for 2 weeks were exposed to 2.5 h of night-time chilling temperatures of −1 to 4°C. Cold treatment decreased nitrogenase activity (acetylene reduction activity) 33% for low nitrate fertilized plants and 41% for high nitrate fertilized plants. Recovery of nitrogenase activity occurred within 7 days after chilling treatment. In contrast, in vivo nitrate reductase (NR) activities of leaves and fine roots increased immediately after chilling then decreased as nitrogenase activities recovered. Fine roots of alder seedlings exhibited NR activities proportional to the amounts of nitrate in the rooting medium. In contrast, the NR activities of leaves were independent of substrate and tissue nitrate levels and corresponded to nitrogenase activity in the root nodules. In a separate experiment, net photosynthesis (PS) of similarly treated black alder seedlings was measured before and after chilling treatments. Net PS declined in response to chilling by 17% for plants receiving low nitrate fertilizer and 19% for plants receiving high nitrate fertilizer. After chilling, stomatal conductance (g_s) decreased by 39% and internal CO₂ concentration (c_i) decreased by 5% in plants receiving the high nitrate fertilizer, whereas plants receiving the low nitrate fertilizer showed no change in g_s and a 13% increase in c_i. Results indicate that chilling stimulates stomatal closure only at the high nitrate level and that interference with biochemical functions is probably the major impact of chilling on PS.     

Genetic variation in populations of the arctic perennial Pedicularis dasyantha (Scrophulariaceae), on Svalbard,Norway

Isozyme variability was examined in 13 geographically isolated populations of the endemic arctic hairy lousewort (Pedicularis dasyantha) in the Svalbard Archipelago, 80° N latitude, Norway. Of the 23 enzyme systems screened on five buffer systems 18 were interpretable. Of the 31 reliable loci, only 6-phosphogluconate dehydrogenase (6-Pgd), was polymorphic. However, no heterozygotes were detected. Frequencies for allele 1 among the populations varied from 1.00 in the north to 0.00 in the south and 0.53 in the central “overlap” region. At the species level the mean number of alleles per locus (A) was 1.03. Percentage of polymorphic loci (P) was 3%. Expected heterozygosity (H_e) was 0.016. At the population level the mean number of alleles per locus was 1.01, and 1.1% of the loci were polymorphic. H_e was 0.004. These values are low compared to endemic, widespread, selling, and outcrossed species. Flower color morphs were distinct. They varied within and among the 13 populations. The frequency of color morphs coincided with allele frequencies of 6-PGD: allele 1 was found in dark purple morphs, and allele 2 was found in light morphs. This species shows more isozyme genetic variability than the five other species reported in the genus but generally less variation than other species with limited regional distributions. Low-level genetic variation in this diploid species may be a result of colonization events coupled with genetic drift, founder effects, and strong natural selection. Additional factors include the self-compatible reproductive system and the long-lived perennial habit.     

Nitrate reductase and nitrite reductase activity in free-living cells and bacteroids of Rhizobium loti

Cells of Rhizobium loti strains T1 and U226 cultured in defined medium with glutamate as the only nitrogen source and bacteroids isolated from root nodules of Lotus corniculatus, L. pedunculatus and L. tenuis did not show constitutive (non-nitrate induced) nitrate reductase activity (NRA). In contrast, nitrite reductase activity (NiRA) was present in both free-living cells and bacteroids of either strain T1 or U226. Constitutive NRA and NiRA were detected in the cytosol fraction from nodules of all three symbioses examined. An induced NRA was expressed in bacteroids after a 10 h incubation in the presence of nitrate.     

Nitrate reductase in cotyledons of cucumber seedlings as affected by nitrate, phytochrome and calcium

Regulation by the active form of phytochrome (P_FR) and the effect of Ca²⁺ was examined with nitrate reductase (NR) in etiolated cucumber ( Cucumis sativus cv. Beilpuig). Nitrate reductase activity (NRA) was studied in excised cotyledons of cucumber seedlings grown in distilled water and in darkness for seven days at 24 ± 0.5°C. All experiments were performed in the dark and a dim green safelight was used during analyses. In etiolated cucumber cotyledons NRA was induced by nitrate and a brief irradiation (15 min) with red light (R) resulted in 62% increase in NRA. This effect was nullified when R was followed immediately by a brief (5 min) far-red light (FR). NRA also showed a semidian (12 h) rhythmicity. Both P_FR, and nitrate effects were age dependent. Calcium seemed to be involved since the phytochrome effect was only observed when calcium was supplied in the external solution. The effect of R on NRA depended on the period of calcium nitrate incubation. An external supply of calcium ionophore mimicked the effect of R and, if supplied to R-irradiated cotyledons, produced a higher NR level than that caused by R alone. This suggested that intracellular free calcium was involved.     

Nitrate reductase activity in nodules of pea inoculated with hydrogenase positive and hydrogenase negative strains of Rhizobium leguminosarum

The nitrate reductase (NR, EC 1.6.6.1) activity in root nodules formed by hydrogenase positive (Hup⁺) and hydrogenase negative (Hup⁻) Rhizobium leguminosarum strains was examined in symbioses with the pea cultivar Alaska ( Pisum sativum L.), Rates of activity were determined by the in vivo assay in nodules from plants that were only N₂-dependent or grown in the presence of 2 m M KNO₃. The rates varied widely among strains, regardless of the Hup phenotype of the R. leguminosarum strain used for inoculation, but the overall results indicated that nodules formed by Hup⁻ strains accumulated more nitrite in the incubation medium than did those with Hup⁻ phenotypes. Total plant dry weight and reduced nitrogen content of pea plants grown in the presence of 2 m M KNO₃ and inoculated with single Hup⁺ and Hup⁻ R. leguminosarum strains were statistically different among some strains. These observations suggest that the possible advantages derived from the presence of the Hup system on whole plant growth may be counteracted by the higher rates of NR activity in the Hup⁻ strains in the R. leguminosarum -pea symbiosis.     

Nitrate reductase activity in high-mountain plants: a test across species,growth form and habitat type

Aims For many terrestrial plants, nitrate is the most important form of available soil nitrogen for growth. However, many plant species, which grow on acidic, ammonium-dominated soils, exhibit a constantly low level of nitrate reductase activity (NRA). Little is known about NRA in high-mountain vascular plants in similar conditions. We tested the hypothesis that high-mountain vascular plants in acidic and ammonium-dominated habitats have low levels of NRA.     

Nitrate reductase activity in leaves and roots ofAlnus glutinosa (L) Gaertner

Summary Thein vivo nitrate reductase activity (NRA) was determined inAlnus glutinosa plants grown nonsymbiotically on ammonium, nitrate, a combination of both, or symbiotically with atmospheric nitrogen as the only nitrogen source. Root NRA was absent when ammonium or atmospheric nitrogen was the nitrogen source. With nitrate in the culture solution the roots showed a high NRA. However, the leaf NRA behaved quite differently: with negligible activities on all nitrogen sources except atmospheric nitrogen. The foliar NRA measured, however, is likely not due to the activity of the plant but of microbial origin. Methods commonly used to facilitate produced nitrite to leak out of the tissue, such as addition of propanol and cutting the plant material, did not increase the nitrite release from the leaves. A turbidity developed when testing the samples for nitrite which was positively correlated with the NRA. Populations of microorganisms in the phyllosphere did not differ between the nutritional treatments. Bacteria, able to grow on a low-nitrogen medium, were present on the leaves. Nitrifiers could not be detected. The bacteria on the leaves appear to produce nitrite when incubated with leaf material. Grassland Species Research Group, Publication no. 106     

Nitrate uptake,nitrate reductase distribution and their relation to proton release in five nodulated grain legumes

Nitrate uptake, nitrate reductase activity (NRA) and net proton release were compared in five grain legumes grown at 0.2 and 2 mM nitrate in nutrient solution. Nitrate treatments, imposed on 22-d-old, fully nodulated plants, lasted for 21 d. Increasing nitrate supply did not significantly influence the growth of any of the species during the treatment, but yellow lupin (Lupinus luteus) had a higher growth rate than the other species examined. At 0.2 mM nitrate supply, nitrate uptake rates ranged from 0.6 to 1.5 mg N g(-1) d(-1) in the order: yellow lupin > field pea (Pisum sativum) > chickpea (Cicer arietinum) > narrow-leafed lupin (L angustifolius) > white lupin (L albus). At 2 mM nitrate supply, nitrate uptake ranged from 1.7 to 8.2 mg N g(-1) d(-1) in the order: field pea > chickpea > white lupin > yellow lupin > narrow-leafed lupin. Nitrate reductase activity increased with increased nitrate supply, with the majority of NRA being present in shoots. Field pea and chickpea had much higher shoot NRA than the three lupin species. When 0.2 mM nitrate was supplied, narrow-leafed lupinreleased the most H+ per unit root biomass per day, followed by yellow lupin, white lupin, field pea and chickpea. At 2 mM nitrate, narrow-leafed lupin and yellow lupin showed net proton release, whereas the other species, especially field pea, showed net OH- release. Irrespective of legume species and nitrate supply, proton release was negatively correlated with nitrate uptake and NRA in shoots, but not with NRA in roots.     

Nitrate reductase activity (in vivo and in vitro) of ditelosomic stocks of wheat (Triticum aestivum L.)

The nitrate reductase activities (NRA) of 31 ditelosomic stocks were compared with that of the control plant [Chinese Spring (CS) euploid], using in vivo and in vitro assay procedures that had been optimized with respect to the euploid. Fourteen stocks exhibited significant differences in in vivo NRA from that of the euploid; the effect of removal of a chromosome arm was always to increase NRA. Eight of these stocks showed similar effects in vitro, although in three, a casein-sensitive factor had to be eliminated before the difference was expressed. Homoeologous group effects were evident among ditelosomics of groups 2, 4, and 7, while for three chromosomes (2D, 7A, and 7B), removal of either arm resulted in a similar increase in NRA in vivo and probably in vitro.P. W. Jones was supported by a Science Research Council C.A.S.E. award with the Plant Breeding Institute, Cambridge, U.K.     

Effect of chlorate, nitrogen source, and light on chlorate toxicity and nitrate reductase activity in soybean leaves

Growth chamber studies were conducted to assess the relationship between nitrate reductase (NR) activity and development of chlorate (KClO₃) toxicity symptoms in leaflets of soybeans [Glycine max (L.) Merr.]. Fourteen day-old soybean seedlings, grown in NO₃ - or urea-nutrient solutions, were exposed to various KClO₃ concentrations (0 to 2.0 mM) and light levels (100, 67, 33 and 0% of full light which was 750 μE m^?2s^?1) for 24 h. Visual KClO₃ toxicity symptoms were noted and NR activity was measured. Toxicity symptoms (interveinal chlorosis) were evident within 24 h following addition of 0.5 mM KClO₃ to the nutrient solution, regardless of N nutrition, and symptom severity increased with increased KClO₃ concentration (up to 2.0 mM). Leaflet NR activity was lower following 24 h KClO₃ treatments at concentrations of 0.5 mM and higher, indicating that ClO₃ - or some reduction product of ClO₃ - likely ClO₂ - was detrimental to enzyme functionality. The light study supported involvement of NR activity in KClO₃ toxicity in that comparison of control and KClO₃ treated plants exposed to decreased light levels revealed a decrease in NR activity of control plants parallel to a decrease in severity of KClO₃ toxicity symptoms of treated plants. Urea-grown plants, which have an apparent constitutive NR enzyme, were used to verify that the KClO₃ toxicity symptoms were not simply N starvation symptoms due to competition of ClO₃ - and NO₃ - for uptake and reduction. In vivo NR assays also ruled out that ClO₃ - was decreasing NR activity through competition with NO₃ - for reduction sites. The close relationship between KClO₃ toxicity symptoms and NR activity, in response to light treatments, suggested that KClO₃ toxicity symptoms were associated with reduction of ClO₃ - to ClO₂ - by the NR enzyme. However, the possibility that a more direct photochemical reaction occurred in the presence of KClO₃ to produce the toxicity symptoms could not be ruled out.     

Tree decline in southeastern Australia: Nitrate reductase activity and indications of unbalanced nutrition in Eucalyptus ovata (Labill.) and E. camphora (R.T. Baker) communities at Yellingbo,Victoria

The decline of riparian Eucalyptus camphora/E. ovata stands is examined in relation to an increase in nitrogen availability and to rising salinity in low-lying areas. There are several indications that declining stands are abnormally rich in N: (i) Nitrogen availability in declining stands was greater than that recorded in other Australian forests, was dominated by nitrification and was extremely variable. (ii) Nitrate concentrations in free soil water from declining forest on a dry site were many times those in the corresponding healthy forest. (iii) N/Mg ratios in foliage of declining trees on drier sites were at the extreme end of the recorded range and similar to those found in eucalypt plantations on agricultural soils. In addition, sites where the overstorey eucalypts are declining have been invaded by a variety of herbaceous weeds, most of which display characteristics of nitrophilous plants, e.g. nitrate reductase activity was greater in herbaceous weeds than in native overstorey or understory species in declining stands of E. camphora/E. ovata and was directly related to the concentration of nitrogen in foliage. These observations are consistent with recent suggestions that forest ecosystems may become N-saturated.     

Comparative studies of diurnal variations of nitrate reductase activity in wheat, oat and barley

Diurnal variations of in vitro and in vivo (intact tissue assay) nitrate reductase (EC 1.6.6.1) activity and stability were examined in leaves of wheat ( Triticum aestivum L. cv. Runar), oat ( Avcna saliva L. cv. Mustang) and barley ( Hordeum vulgure L. cv. Agneta and cv. Gunillu). Nitrate reductase activity was generally higher for wheat than for oat and barley. However, the diurnal variations of nitrate reductase activity and stability were principally the same for all species, e.g. the high activity during the photoperiod was associated with low stability. All species showed a rapid (30-60 min) increase in the in vitro and in vivo activity when the light was switched on. When light was switched off the in vitro activity decreased rapidly whereas decrease in in vivo activity was slower. These experiments support the hypothesis that an activation/ deactivation mechanism is involved in the regulation of diurnal variations in nitrate reductase activity. Red light enhanced nitrate reductase activity in etiolated wheat and barley leaves. In green leaves, however, the daily increase in nitrate reductase activity was not induced by a brief red light treatment. Indications of different regulation mechanisms for the diurnal variations of nitrate reductase activity among the cereals were not found.     

Phosphorus ameliorates crop productivity,photosynthesis, nitrate reductase activity and nutrient accumulation in coffee senna (Senna occidentalis L.) under phosphorus-deficient soil

Abstract

Coffee senna (Senna occidentalis L.) plants were subjected to five phosphorus levels: 0, 25, 50, 75 and 100 mg P per kg soil (P₀, P₁, P₂, P₃ and P₄, respectively). A pot culture experiment was conducted in a net house, AMU, Aligarh, India, under phosphorus-deficient soil. The present data indicates that soil-applied phosphorus significantly ameliorates most of the attributes studied. Out of five phosphorus levels, 75 mg P per kg soil (P₃) proved best and enhanced fresh and dry weights, total chlorophyll and carotenoid content, nitrate reductase activity and leaf-NPK and Ca content, analyzed at 120, 270 and 300 days after sowing (DAS). The number of pods, seed-yield per plant and seed-protein content (330 DAS) were significantly enhanced by the P₃ level, except the number of seeds per pod, 100-seed weight and total anthraquinone glycosides content, respectively. Transpiration rate, stomatal conductance and net photosynthetic rate were also enhanced by this treatment.