Higher Nitrate Reductase Research Articles

An Analysis of Seasonal Effects on Leaf Nitrate Reductase Activity and Nitrogen Accumulation in Maize (Zea mays L.)

The important enzyme in nitrogen (N) assimilation, nitrate reductase (NR), is an inducible enzyme influenced by many external (light, temperature, etc.) and internal (genotype) factors. The efficiency of the N assimilation system may vary with genotype and season. In the present study, the effects of season on NR activity in relation to N accumulation in maize plants were investigated. Six different cultivars of maize, namely Ganga‐11, Deccan‐103, Hi‐starch (hybrids), Arun, Manjari and Vijay (composites), were sown during the monsoon (88‐day crop duration) and in winter (150‐day crop duration). In vivo NR activity in the last fully expanded leaf (LFEL), and the N contents of the whole plant and the LFEL were estimated at seven phenological growth stages. Three different states of N metabolism in maize, namely (i) low NR activity per unit leaf area per unit time coinciding with high accumulation of N, (ii) high NR activity coinciding with low N accumulation, and (iii) low NR activity coinciding with low N accumulation, were noted. These results clearly demonstrate that the relationships between N uptake and accumulation parameters change with the season and crop growth stage and are subject to a genotypic influence. Thus it is necessary to evaluate genotypes under similar environments to select a genotype with high N use efficiency. As these relationships are growth dependent, care must be taken to evaluate them at a particular phenological stage rather than on the basis of days after sowing.

Nitrate reductase activity in some subarctic species and UV influence in the foliage of Betula pendula Roth. seedlings

Nitrate reductase (NR) activity was studied in the foliage of five subarctic species: mature trees of European white birch (Betula pubescens Erch. S.S.), Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies L. Karst), Ericaceous shrub bilberry (Vaccinium myrtillus L.), naturally growing in a forest, and seed-grown silver birch (Betula pendula Roth.) seedlings in an ultraviolet (UV) exclusion field experiment at the Pallas-Ounastunturi National Park in Finnish Lapland (68 degrees N). Mean NR activity ranged from 0 in bilberry to 1477 (S.D. = 277.7) and 1910 (S.D. = 785.4) nmol g(-1) DW h(-1) in mature trees of European white birch and silver birch seedlings, respectively. Significant differences due to UV exclosure treatments were determined for the NR activity of silver birch seedlings (F = 3.62, P= 0.025*) after three growing seasons (191 days) of UV exclusion. The ambient and control silver birch seedlings had or tended to have higher NR activity than those grown under UV exclusion. No relationship was found between the foliage NR activity and total nitrogen content, which ranged from 0.61 to 1.35% per seedling. The present study suggests large differences in NR activity between the species and the induction of NR activity in silver birch seedlings due to ambient UV radiation.

Physiological and morphological adaptations of the fruit tree Ziziphus rotundifolia in response to progressive drought stress.

The physiological basis of drought resistance in Ziziphus rotundifolia Lamk., which is an important, multipurpose fruit tree of the northwest Indian arid zone, was investigated in a greenhouse experiment. Three irrigation regimes were imposed over a 34-day period: an irrigation treatment, a gradual drought stress treatment (50% of water supplied in the irrigation treatment) and a rapid drought stress treatment (no irrigation). Changes in gas exchange, water relations, carbon isotope composition and solute concentrations of leaves, stems and roots were determined. The differential rate of stress development in the two drought treatments did not result in markedly different physiological responses, but merely affected the time at which they were expressed. The initial response to decreasing soil water content was reduced stomatal conductance, effectively maintaining predawn leaf water potential (Psi(leaf)), controlling water loss and increasing intrinsic water-use efficiency, while optimizing carbon gain during drought. Carbon isotope composition (delta13C) of leaf tissue sap provided a more sensitive indicator of changes in short-term water-use efficiency than delta13C of bulk leaf tissue. As drought developed, osmotic potential at full turgor decreased and total solute concentrations increased in leaves, indicating osmotic adjustment. Decreases in leaf starch concentrations and concomitant increases in hexose sugars and sucrose suggested a shift in carbon partitioning in favor of soluble carbohydrates. In severely drought-stressed leaves, high leaf nitrate reductase activities were paralleled by increases in proline concentration, suggesting an osmoprotective role for proline. As water deficit increased, carbon was remobilized from leaves and preferentially redistributed to stems and roots, and leaves were shed, resulting in reduced whole-plant transpiration and enforced dormancy. Thus, Z. rotundifolia showed a range of responses to different drought intensities indicating a high degree of plasticity in response to water deficits.

Host Preference of the Federally Endangered Hemiparasite Schwalbea americana L. (Scrophulariaceae)

HELTON, R. C., L. K. KIRKMAN, AND L. J. MUSSELMAN. Host preference of the federally endangered hemiparasite Schwalbea americana L. (Scrophulariaceae) J. Torrey Bot. Soc. 127:300-306. 2000.-We examined the host-preference and optimal hosts of the federally endangered hemiparasite, Schwalbea americana L. We also determined the effects of four fertilizer treatments (NO3-140ppmN, N03-280ppmN, NH4-140ppmN, and NH4280ppmN) on the formation of haustoria of S. americana to two of these host species, Panicum tenue and Pityopsis graminifolia. Results of the host preference study show that Schwalbea americana seedlings are more likely to form haustorial connections with Pityopsis graminifolia and Ilex glabra than with other common associates. We also found that size of Schwalbea americana seedlings played an important role in year to year survivorship independent of host attachment. Regardless of nitrogen form, we observed a greater rate of attachment of Schwalbea americana to Pityopsis graminifolia than to Panicum tenue. Nitrogen treatment had no effect on parasitism of Panicum tenue, although more seedlings of Schwalbea americana survived without haustoria with the NH4-280ppm treatment. Growth (leaf area and stem height) of Schwalbea americana increased with NH4-280 ppm treatment with Pityopsis graminifolia as a host. Panicum tenue had much higher nitrate reductase activity levels in the leaf tissue than Pityopsis graminifolia, and both hosts showed low levels in the root tissue. Low levels of nitrate reductase activity were found in the leaves of Schwalbea americana. Host preference of Schwalbea americana may be related to the host's ability to utilize NH4 as opposed to its ability to reduce NO3. Growth of Schwalbea americana appears to be influenced by the amount of NH4 available to the host.

Nitrogen Metabolism in Five Plant Species Characteristic of Gypsiferous Soils

Summary Nitrogen metabolism has been studied in 5 species of plants characteristic of gypsum soils: Gypsophila struthium (Gs), Helianthemum syriacum (Hsy), Helianthemum squamatum (Hsq), Helianthemum alypoides (Ha), and Lepidium subulatum (Ls). Nitrate, nitrate reductase (NR), nitrite reductase (NiR), amino acids, proteins, proline, organic N, total calcium, and population density were analysed. Our results show Ls to be the species that presented the greatest stimulation of nitrogen metabolism, with the lowest levels of NO 3 - , highest NR and NiR activities, and highest concentrations of nitrogenous compounds, a possible determining factor in this trend being the lowest Ca concentrations registered for this species. On the other hand, this species also presented the highest proline concentrations. Possibly for the highest nitrogen metabolism and the highest concentrations of proline, this species is the most prevalent of the five plants in the study zone. Finally, the opposite trend was found in the species Ha and Hsq.

Exhaustion of nitrate terminates a phytoplankton bloom in Funka Bay, Japan:change in SiO4:NO3 consumption rate during the bloom

Time series observations over 36 h were carried out in Funka Bay, Japan, to complement a 6 yr nutrient-dynamics study during a diatom spring bloom, especially focusing on nitrate and silicate as limiting nutrients. At the beginning of the study, nutrient concentrations were high (NO 3 = 10 μM, SiO 4 = 20 μM and PO 4 = 0.8 μM). Nitrate was depleted first during the night while silicate and phosphate remained at 10 and 0.3 μM, respectively. High nitrate reductase activity was observed the following morning, indicating the assimilation of nitrate in situ by the dominant phytoplankter, Chaetoceros socialis. The consumption ratio of SiO 4 :NO 3 increased by 3 times from 0.83 before the peak to 2.5 after the peak of the bloom. This change was also obvious in the elemental composition of phytoplankton, showing an increase in Si:C and Si:N ratios by a factor of 2 to 4 after the peak of the bloom, while C:N ratio remained constant at around 7. As a consequence of this change in the consumption rate of SiO 4 and NO 3 , both silicate and nitrate were finally depleted in the photic zone.

EFFECT OF N-NO3:N-NH4 RATIO ON NITRATE REDUCTASE AND GLUTAMINE SYNTHETASE ACTIVITY IN Opuntia ficus-indica (L.) Mill.

The objectives of this research were: a) to study the effect of several N-NO3:N-NH4 ratios on the activity of the enzymes nitrate reductase (NR) and glutamine synthetase (GS) and b) to identify the NO3- assimilation sites in prickly pear cactus (Opuntia ficus-indica (L.) Mill.). Five N-NO3:N-NH4 ratios (mg⋅liter-1 N-NO3:mg⋅liter-1 N-NH4) were studied [1:0 (150:0), 2:1 (100:50), 1:1 (75:75), 1:2 (50:100, and 0:1 (0:150)]. An experimental design of random complete blocks was used with four replications with a factorial ar-rangement of treatments. In shoot (cladode) and root, the activity of the enzymes nitrate reductase (NR) and glutamine synthetase (GS) were evaluated in vivo at 21 days after establishment of the experiment. High NR and GS enzyme activity was registered when the nutrient solution was supplied with a higher ratio of N-NO3:N-NH4, thus confirming the dependence of NR activity on the concentration of N-NO3 in the nutrient solution. Both enzymes had similar behavior, and more activity was detected in the roots.

Monitoring plant physiological characteristics to evaluate mine site revegetation: A case study from the wet-dry tropics of northern Australia

Biologically driven markers or monitors were used to evaluate plant and ecosystem health of uranium-mining affected sites. Plant water, nitrogen (N) and phosphorus (P) status were used to measure physiological characteristics of tree and shrub species at sites perturbed by mining activities (waste rock dumps: WRD 1, WRD 2; mine wastewater irrigated woodland) and of species at undisturbed woodland (tropical savanna). Plant water status was evaluated by measuring leaf relative water content (RWC) and carbon isotope discrimination (δ13C). Leaf RWC varied significantly (P<0.0001) between wet and dry season in species at the woodland sites with higher RWC in the wet season compared to the dry season. No seasonal differences were observed in RWC in species at the WRDs. Leaf δ13C was similar in species at woodland sites and WRD 2 (−28.8 to −28.1‰) but was significantly (P<0.05) lower in species at WRD 1 (−27.6‰). This suggests that species at WRD 1 had a lower water availability and/or lower water use compared to species at all other sites. WRD substrate had an up to 4-orders of magnitude greater availability of inorganic phosphate (Pi) compared to woodland soil as determined using in situ ion exchange resin. Pi concentrations in xylem sap of species at WRDs were 2- to 3-fold higher compared to species at woodland sites. Plant nitrate reductase (NR) activity was low in most species at woodland and WRD 1. In contrast, Eucalyptus and Acacia species had high NR activities of up to 300–700 pkat g-1 fw at WRD 2 indicating that these species had greater nitrate use than species at all other sites. Nitrate availability in the top five cm of the profile, as determined using in situ ion exchange resins, increased at all sites in the wet season, but no significant differences were observed between sites using this method. However, traditional soil analysis revealed that WRD substrate had a 2-times higher nitrate content (0 to 1000 mm depth) compared to woodland soil. Thus, it is likely that plants at WRD2 accessed nitrate from deeper parts of the profile. Proline, an indicator of plant stress, was found in appreciable quantities in leaves of herbaceous species but not in woody species. Soil and leaf δ15N were measured to investigate N-cycling and the contribution of diazotrophic N2 fixation to plant N nutrition. Soil δ15N values were highest and most variable at WRD 2 (6.2‰) compared to all other sites (irrigated woodland 3.1‰, undisturbed woodland 2.5‰, WRD 1 0.9‰). This may indicate that N-turnover and nitrification was greatest at WRD 2 leading to greater 15N enrichment of soil N. At all sites, Acacia species were nodulated and putatively fixing N2. With the exception of WRD 2 where leaf δ15N of Acacia species averaged 0.9‰, Acacia species had 15N depleted values characteristic of species that receive N derived from N2 fixation (−0.8 to −0.6‰).

Identification and characterization of the Staphylococcus carnosus nitrate reductase operon.

Physiological and genetic characterization of Staphylococcus carnosus nitrate reductase-negative mutants led to the identification of the nitrate reductase operon, narGHJI. Transcription from the nar promoter was stimulated by anaerobiosis, nitrate, and nitrite. This is in accordance with the nitrate reductase activities determined with benzyl viologen as electron donor. However, in the presence of oxygen and nitrate, high transcriptional initiation but low nitrate reductase activity was observed. Since the alphabeta complex of the nitrate reductase formed during anaerobic growth was insensitive to oxygen, other oxygen-sensitive steps (e.g., post-transcriptional mechanisms, molybdenum cofactor biosynthesis) must be involved. The nitrate-reducing system in S. carnosus displays similarities to the dissimilatory nitrate reductases of Escherichia coli. However, in the S. carnosus nar promoter, no obvious Fnr and integration host factor recognition sites are present; only one site that is related to the E. coli NarL consensus sequence was found. Studies to determine whether the E. coli proteins NarL and Fnr are functional at the S. carnosus narGHJI promoter indicated that the promoter is not functional in E. coli.

Intraspecific differences in nitrogen assimilating enzymes in spinach under contrasting forms of nitrogen supply1

Intraspecific differences in the activities of nitrate reductase (NR), glutamine synthetase (GS), NADH dependent glutamate synthase (NADH‐GOGAT), and glutamate dehydrogenase (GDH) under contrasting forms of nitrogen (N) supply were studied in tissues of three spinach (Spinacia oleracea L.) cultivars. The varieties (Viroflay, Butterflay, and Giant) were smooth, curly and semicurly leaved, respectively. The plants were grown in nutrient solutions containing NO3 as the sole source of N (100:0) and NO3 plus NH4 (80:20). Giant, the NH4 tolerance of which had been evaluated in growth and on the basis of nutrient uptake, had much higher GS and GDH activities in the roots and higher NR and NADH‐GOGAT levels in the leaves of plants grown on NO3 and NH4 than that grown on NO3 alone. On the level of N assimilating enzymes of Butterflay, mixed N nutrition caused an increase of GDH and NADH‐GOGAT in leaves and roots and at the same time a decrease of GS in the roots and NR in the leaves. An inverse relationship between GS and GDH activities was detected in the leaves and foots of Virofiay grown with both N sources. Finally, Viroflay gave the highest levels of GDH irrespective of the NO3:NH4 assayed, whereas the leaves of Giant were GDH deficient in comparison with the other cultivars. In addition, the GS activity approached zero in the roots of spinach cultivars characterized by hardly any NH4 tolerance, whereas in those of Giant it increased remarkably with the supply of NO3 plus NH4.

Soil Chemistry and Plants in Fennoscandian Boreal Forest as Exemplified by a Local Gradient

In Fennoscandian boreal forests, in which productivity in general is N limited, there are regular, topographically related variations in forest productivity and plant community composition. Regional surveys have demonstrated strong correlations among soil pH, N content, and base saturation on the one hand and plant productivity and community composition on the other, but the nature of these relationships is poorly understood. We studied in detail the variation in and controls of soil acidity, availability of N and P, and changes in community composition and plant nutrition along a short (only 90 m long) but extreme forest productivity gradient in northern Sweden, which ranged from a groundwater recharge area with low productivity to a very productive discharge area. The pH in the soil solution of the mor layer ranged from 3.5 in the recharge area to 6.4 in the discharge area, and it was strongly correlated with the base saturation of the exchange complex. Neither the acid strength of organic matter, the ionic strength of the soil solution, nor the quantity of acids could explain more than a minor part of this variation in pH. There were strong correlations between total N in the mor layer and soil solution pH (r = 0.97) and base saturation of the exchange complex (r = 0.88). At the poor end of the transect the concentration of inorganic N was very low in the mor, and plants with either ectomycorrhizae (ECM) or ericoid mycorrhizae (EM) dominated. With increasing pH, there was an increase in NH4 concentrations, while plants that potentially have arbuscular mycorrhizae (AM) became prominent along with ECM and EM species. In the discharge area, which comprised only the last 10 m of the transect, NO3 dominated over NH4 in the soil solution, the soil had a high capacity for net nitrification, and the vegetation was totally dominated by potentially AM or nonmycorrhizal herbs, some of which had high foliar nitrate reductase activity. Foliar and root N concentrations increased steeply towards the discharge area, but foliar P/N ratios declined below critical levels at the end of the transect. Root 32P uptake bioassays also indicated a P deficiency in the discharge area, where the soil total P content was high, while the concentration of PO4 in the soil solution was very low. The high capacity of the mor in the discharge area to adsorb PO4, due to the presence of organically complexed Fe and Fe-oxihydroxides, may explain the low PO4 concentrations. Our data indicate that the underlying factors influencing both productivity and community composition are pH and supply of base cations. Fundamental differences in exchange characteristics of soil and soil water underlie other related nutrient supply features, in particular the amount and availability of N. Our study of a single short topographic transect supported a previous suggestion based on a regional survey in Norway that variability in soil pH and the supply of base cations affects plant productivity and community composition via effects on N supply. Our data also encompass the interrelations between soil pH, soil N turnover, and the mycorrhizal type of dominant plant species, which, according to Read (1991), occur along long latitudinal or altitudinal climatic gradients. Through millennia discharge areas like the one observed by us have probably provided a relatively stable environment for plants demanding high soil pH and N supply, at the same time as surrounding recharge areas have been acidified naturally through podzolization.

SOIL CHEMISTRY AND PLANTS IN FENNOSCANDIAN BOREAL FOREST AS EXEMPLIFIED BY A LOCAL GRADIENT

In Fennoscandian boreal forests, in which productivity in general is N limited, there are regular, topographically related variations in forest productivity and plant community composition. Regional surveys have demonstrated strong correlations among soil pH, N content, and base saturation on the one hand and plant productivity and community composition on the other, but the nature of these relationships is poorly understood. We studied in detail the variation in and controls of soil acidity, availability of N and P, and changes in community composition and plant nutrition along a short (only 90 m long) but extreme forest productivity gradient in northern Sweden, which ranged from a groundwater recharge area with low productivity to a very productive discharge area. The pH in the soil solution of the mor layer ranged from 3.5 in the recharge area to 6.4 in the discharge area, and it was strongly correlated with the base saturation of the exchange complex. Neither the acid strength of organic matter, the ionic strength of the soil solution, nor the quantity of acids could explain more than a minor part of this variation in pH. There were strong correlations between total N in the mor layer and soil solution pH (r = 0.97) and base saturation of the exchange complex (r = 0.88). At the poor end of the transect the concentration of inorganic N was very low in the mor, and plants with either ectomycorrhizae (ECM) or ericoid mycorrhizae (EM) dominated. With increasing pH, there was an increase in NH4 concentrations, while plants that potentially have arbuscular mycorrhizae (AM) became prominent along with ECM and EM species. In the discharge area, which comprised only the last 10 m of the transect, NO3 dominated over NH4 in the soil solution, the soil had a high capacity for net nitrification, and the vegetation was totally dominated by potentially AM or nonmycorrhizal herbs, some of which had high foliar nitrate reductase activity. Foliar and root N concentrations increased steeply towards the discharge area, but foliar P/N ratios declined below critical levels at the end of the transect. Root 32P uptake bioassays also indicated a P deficiency in the discharge area, where the soil total P content was high, while the concentration of PO4 in the soil solution was very low. The high capacity of the mor in the discharge area to adsorb PO4, due to the presence of organically complexed Fe and Fe-oxihydroxides, may explain the low PO4 concentrations. Our data indicate that the underlying factors influencing both productivity and community composition are pH and supply of base cations. Fundamental differences in exchange characteristics of soil and soil water underlie other related nutrient supply features, in particular the amount and availability of N. Our study of a single short topographic transect supported a previous suggestion based on a regional survey in Norway that variability in soil pH and the supply of base cations affects plant productivity and community composition via effects on N supply. Our data also encompass the interrelations between soil pH, soil N turnover, and the mycorrhizal type of dominant plant species, which, according to Read (1991), occur along long latitudinal or altitudinal climatic gradients. Through millennia discharge areas like the one observed by us have probably provided a relatively stable environment for plants demanding high soil pH and N supply, at the same time as surrounding recharge areas have been acidified naturally through podzolization.

Foliar application of molybdenum in common beans. I. Nitrogenase and reductase activities in a soil of high fertility

Although common bean (Phaseolus vulgaris L.) has a good potential for N2 fixation, poor nodulation following inoculation, principally under field conditions, has led to increased nitrogen (N) fertilizer use in this crop. In the face of the negative environmental effects of N fertilizer, alternative methods have been studied to minimize the amount to be applied. In this sense, foliar application of molybdenum (Mo) has been cited as a promising method. Several papers show that high bean yields (1,500–2,500 kg ha‐1), may be obtained in the southeasten region of Brazil, when there is an application of N as side dressing or Mo spray 25 days after plant emergence. A field experiment was carried out to verify the effect of Mo foliar application on nitrogenase and nitrate reductase activities and on bean yield. Treatments included Rhizobium inoculation (with and without), foliar application of Mo (0 and 40 g ha‐1), N at planting (0 and 20 kg ha‐1) and N applied as side dressing (0 and 30 kg ha‐1). Molybdenum and N as side dressing were used 25 days after plant emergence. Molybdenum increased greatly the nitrogenase activity and extended the period of high nitrate reductase activity, with a consequent increase in total shoot N. Increase of nitrogenase activity did not depend on inoculation, showing that soil native rhizobia may increase in effectiveness when appropriately handled. Bean yield did not differ significantly when fertilized with either Mo or N as side dressing.

A high sensitivity nitrate reductase assay and its application to vertically migrating Rhizosolenia mats

A high sensitivity assay for nitrate reductase (NR) was developed that permitted determination on samples as small as 0.2 pg chlorophyll a. We combined a revised NR assay chemistry with chemiluminescent determination of nitrite to improve sensitivity approximately 50-fold over colorimetric techniques. Results from colorimetric and chemiluminescent techniques were linearly related (r2 = 0.96, p = 0.0001). The NR assay reagents did not interfere with chemiluminescent nitrite determinations although the reagents did contain detectable amounts of nitrite. The assay chemistry and incubation temperature required optimization for our species. We used the assay to measure NR rates in macroscopic Rhizosolenia mats from the North Pacific Ocean. Rates were adequate to support calculated nitrogen demands of mats. Floating mats had significantly higher NR activity and internal nitrate pools than sinking mats, providing more evidence to support the hypothesis that these pelagic diatoms can migrate below the nutricline to exploit deep nitrate pools and then return to the surface for photosynthesis.

Biochemical tests for the selection of Staphylococcusstrains as potential meat starter cultures

A fast screening procedure has been applied for the isolation of staphylococcal strains with possible value as starter cultures in fermented meat products. Colonies isolated in MSA (mannitol-salt phenol red-agar) from a naturally fermented spanish ‘salchichón’ were stabbed into KNO 3-containing agar plates. Then, colonies showing very high and low nitrate reductase activity were selected and used in further biochemical tests. All strains were identified as Staphylococcus xylosus, with the exception of one Staphylococcus epidermidisisolate. In addition to the APIZYM Rgalleries, other activities were quantified, such as, nitrate reductase, catalase, proteases and lipases. High nitrate reductase was found in some of the strains. Lipase activity was found in different strains, and all of them had an interesting cell-bound proteinase activity when assayed against β-casein. Another study should be undertaken to test these strains of S. xylosusfor the production of fer?mented meat products, and their activities correlated to the development of desirable physicochemical and organoleptic characteristics.

Effect of NaCI on nitrate reductase, glutamate dehydrogenase and glutamate synthase inVigna radiata calli

The effect of NaCI stress on the activities of nitrate reductase (NR), glutamate dehydrogenase (GDH) and glutamate synthase (GOGAT) in callus lines ofVigna radiata which differ in salt resistance, was studied at weekly intervals upto 28 d of growth. After 28 d, the NaCI resistant callus (selected at 300 mM NaCI) at NaCI concentrations higher than 200 mM maintained higher NR activity than non-selected line. NaCI stress also affects aminating and deaminating activities of GDH. The NADH-GDH activity in the presence of NaCI was higher in the resistant than non-selected line. On the other hand, NAD-GDH activity in both the lines was completely inhibited after 7 d of growth. The increased activity of NADH-GDH in resistant calli may play a vital role in protecting the cells from toxic effect of increased endogenous level of ammonia which probably accumulates due to efficient NO3 - reduction. NADH-GOGAT activity was found to decrease under salt stress in both the callus lines. Nitrogen assimilation in salt-resistant calli under salt stress was found to be characterized by high NR and NADH-GDH activities, concomitantly with low GOGAT activity.

Effect of Split Nitrogen Application on Growth and Yield of Wheat (T. aestivum L.) Genotypes with Different N‐Assimilation Potential

AbstractA field experiment was conducted with different nitrogen regimes to assess the growth and yield performance of wheat genotypes which differ in nitrate assimilation potential. Genotypic differences in biomass accumulation were observed at different growth stages. The nitrogen treatment had little effect on biomass accumulation at early stages of growth, while at later stages of growth there was enhanced biomass accumulation when N was applied in more than two splits. On an average, genotypes with high nitrate reductase activity (the ‘HNR’ genotypes) accumulated 14.2 % more biomass than the genotypes with low nitrate reductase activity (‘LNR’ genotypes) when an extra dose (40 kg N ha−1) of nitrogen was given at the time of anthesis. The application of nitrogen in more than two splits increased grain yield of both ‘HNR’ and ‘LNR’ genotypes mainly by increasing grain weight per ear. The application of an extra dose of nitrogen (40 kg N ha−1) at the time of anthesis increased grain yield of ‘HNR’ genotypes by 38.5 % as compared to ‘LNR’ genotypes.

Nitrate reductase activity as an indicator of ponderosa pine response to atmospheric nitrogen deposition in the San Bernardino Mountains

Determinations of nitrate reductase (NR) activity in ponderosa pine ( Pinus ponderosa Dougl. ex. Laws.) needles were performed during summer 1994 in two areas (consisting of six different sites) with different nitrogen (N) deposition levels in the San Bernardino Mountains, southern California. Nitrate reductase activity was used as an integrative indicator of atmospheric nitrogen deposition to pine trees (direct uptake of N species from the atmosphere and N transported from the soil). Deposition of nitrate (NO 3 −) to pine branches was measured in order to determine dry atmospheric inputs of the oxidized N species to tree foliage. High NR activity was detected in all of the experimental sites. Activity of the enzyme was significantly higher at the locations characterized by higher NO 3 − deposition to branches—slight positive correlation between branch deposited NO 3 − and NR activity was found. However, high variability of NR in time and between the experimental sites discredit the NR assay as a reliable indicator of N deposition for ponderosa pine in the field conditions. This could be caused by substantial interference from other abiotic and biotic factors with tropospheric ozone as probably the most important one.

Preservation of nitrifying capacity and nitrate availability in waterlogged soils by radial oxygen loss from roots of wetland plants

The effects of radial O2 loss from roots on nitrification and NO sup-inf3 availability were studied. Plants of the flooding-resistant species Rumex palustris and the flooding-sensitive species Rumex thyrsiflorus were grown on drained and waterlogged soils with an initially high nitrifying capacity. Nitrate reductase activity in the plant leaves was used as an indicator of NO sup-inf3 availability to the plants. In a separate experiment these species were shown to have higher levels of nitrate reductase activity when NO sup-inf3 was added to the soils compared to when only NH sup+inf4 was provided. In drained soils nitrification was maintained and both plant species showed relatively high nitrate reductase activities in their leaves. In the water-logged series planted with R. thyrsiflorus, nitrification was inhibited, NH sup+inf4 accumulated, and the plants grew less well compared to those on drained soils. In contrast, waterlogged soils planted with R. palustris had a redox potential high enough for O2 to be continuously replenished. Furthermore, the nitrifying capacity of these latter soils was maintained at a high level. R. palustris grew well and NO sup-inf3 must have been available to the plant, since a high level of nitrate reductase activity was observed in the leaves.

Seasonal variation of leaf glutamine synthetase isoforms in temperate deciduous trees strongly suggests different functions for the enzymes

ABSTRACTChanges in the activities of leaf glutamine synthetase (GS) isoforms were followed in four temperate deciduous trees from full leaf expansion to senescence (May to November). In the early part of the season, total GS activity was high in all species, with values ranging from 90 to 200 μmol h−1 g−1 fw. During this early period this activity comprised only the activity of the chloroplastic (GS2) isoform in all species. These high GS2 activities are consistent with the role of GS2 in the re‐assimilation of photorespired ammonia. The early high values also coincided with high nitrate reductase activity in one of the species, the highly nitrophilous species Sambucus nigra, with values of up to 16μmol h−1 g−1 fw. This indicates that GS2 is also important in the assimilation of ammonia produced from nitrate reduction. From mid‐ to late‐season, the cytosolic isoform (GS1) was detected in all four species and became increasingly more active in comparison to GS2. By the time of senescence it was the dominant enzyme of the two forms in both S. nigra and Carpinus betulus. The results provide strong support for recent findings that GS1 is an important enzyme for the mobilization of nitrogen for translocation or storage.