Increase In Nitrate Reductase Activity Research Articles

Activity of nitrate reductase and glutamine synthetase in shoot and root of mycorrhizal Allium cepa: Effect of drought stress

The effect of arbuscular-mycorrhizal (AM) fungus Glomus fasciculatum on growth and N form assimilation was measured on onion ( Allium cepa) grown under well-watered (−0.04 MPa) or drought conditions (−0.17 MPa). Two uninoculated control treatments, one provided with phosphate, were also addressed. These three treatments were supplemented with 2.0 mM nitrogen as nitrate and ammonium in a 1/1 ratio. Shoots and root weights, percentage of root colonized and glutamine synthetase (GS) (EC 6.3.1.2.) and nitrate reductase (NR) (EC 1.6.6.1) activity in shoot and root tissue were determined when water was maintained at (−0.04 MPa) or (−0.17 MPa) in the growth medium. The growth of G. fasciculatum-colonized plants was comparable to that of uncolonized P-supplemented plants under well-watered or drought conditions but mycorrhizal plants reached a higher specific and total GS activity in shoots and roots than P-fertilized plants growth at −0.04 MPa. The mycorrhizal effect on GS activity under water stress (−0.17 mPa) was evident only in roots being comparable to that found in P-fertilized plants. The proportion of GS in roots was increased in AM plants under whatever soil water conditions. The most marked increasing effect of AM-colonization on NR activity was in root tissue. Under water limitations the effectiveness of G. fasciculatum increasing NR activity in plant was enhanced. The proportion of nitrate assimilation into root was increased in AM plants particularly under well-watered conditions. Mycorrhizal modifications in the GS and NR distribution into root and shoot compartments may account for some physiological effect from mycorrhizal colonization. These results are further evidence of a direct effect on absorption, translocation and assimilation of both N forms by the endomycorrhizal system. That mycorrhizal plants can utilize nitrate form more efficiently than ammonium under drought conditions is consistent with more recent studies on the AM effect on N uptake from a neutral-alkaline soil. Results here presented suggest that either AM fungi increase the nitrogen forms assimilation in the host plant (regardless of P content) or the AM fungi have such enzymatic activities per se. This last assumption is supported by the relative high increase of NR and GS activities found in the roots of mycorrhizal plants. Nevertheless while NR was maintained increased in mycorrhizal roots under water stress the GS activity was not affected. This suggests the AM ability to provide an active nitrate acquisition in particular in water stressed environment. The different proportion of nitrate and ammonium assimilation into shoot and root compartments may account to modify physiological mycorrhizal responses related to plant sensitivity to drought.

Light-mediated Regulation of Nitrate Reductase in Higher plants

Regulation of nitrate reductase (NR) by light is a complex process and is manifested through gene expression, both at the level of transcription and translation, covalent modification of the enzyme and supply of reductant. Light induces nia gene transcription by some still unknown mechanism. Light induction of nia mRNA is also mediated via phytochrome. By using protein synthesis inhibitors it has been observed that light enhances translation of NR mRNA. Light induction of NR synthesis follows the circadian rhythmicity. Cytokinin increases NR activity by regulating NR expression at the transcriptional level. In response to light-dark transition, NR is quickly inactivated/activated by phosphorylation/dephosphorylation. Inactivation of NR involves phosphorylation of the serine-543, located at the hinge 1 region connecting MoCo domain and cyt b domain of NR, by a Mg2+ dependent protein kinase and subsequent binding of an inhibitor protein. A type 2A protein phosphatase dephosphorylates/activates NR in response to light signal. Light also regulates supply of reductant via photosynthesis for activity of NR. Whether light is absolutely necessary for NR activity is also discussed.

Nitrate reductase activity during desiccation and rehydration of the desiccation tolerant moss Tortula ruralis and the leafy liverwort Porella platyphylla

Nitrate reductase (NR) activity in vivo was detected in eight of nine bryophytes investigated, at levels generally lower than those reported for nitrate-using higher plants. In three species tested, pre-treatment with 5 mol m −3 KNO3 increased NR activity by a factor of three in Tortula ruraliformis and by a factor of six in Dicranum majus and Hookeria lucens. Water stress imposed osmotically by PEG or sorbitol solutions depressed NR activity to 20% of its full-turgor value at −4.5 MPa. No significant differences in the proportional change were found between the desiccation-tolerant and non-desiccation tolerant species. The original hydration level of the tissue influenced the pattern of response during subsequent water stress. Measurements on field-collected material of Tortula ruralis indicate that continuous exposure to NO3− can result in the loss of NR inducibility. In NO3− free conditions there was some indication of reduced NR activity in bryophytes that were water stressed by air drying, but the results were variable and not statistically significant. During the first hour of rehydration of fully-desiccated material (air dry for 30 d), NR activity in Tortula ruralis declined markedly in both dark and light (more steeply in the light) while in Porella platyphylla it remained at a relatively constant low level in the light and increased in the dark. After four days of dark starvation Porella platyphylla showed reduced but measurable NR activity, which was increased by supply of exogenous sugars. The possible physiological basis of these findings is discussed.

Effect of Azotobacter strains on sugar beet callus proliferation and nitrogen metabolism enzymes

The effect of five Azotobacter chroococcum strains and nitrogen content in nutrient media on callus growth of two Beta vulgaris L. cultivars were investigated, as well as the activity of nitrate reductase (NR), glutamine synthetase (GS) and glutamate dehydrogenase (GDH) in inoculated callus tissue. On medium with full nitrogen content (1 N) the inoculation with A. chroococcum strain A2 resulted in the highest calli mass, while strains A8 and A14 maximally increased NR activity. On media with 1/8 N the highest effect on calli growth, GS and GDH activity had the strain A8. The strain A2/1 significantly increased callus proliferation on medium without N. Asymbiotic association between sugar beet calli and Azotobacter depended on genotype/strain interaction and was realised in presence of different nitrogen levels.

Molecular and Regulatory Properties of the Nitrate Reducing Systems of Rhodobacter

Phototrophic bacteria of the genus Rhodobacter possess several forms of nitrate reductase including assimilatory and dissimilatory enzymes. Assimilatory nitrate reductase from Rhodobacter capsulatus E1F1 is cytoplasmic, it uses NADH as the physiological electron donor and reduced viologens as artificial electron donors, and it is coupled to an ammonium-producing nitrite reductase. Nitrate reductase induction requires a high C/N balance and the presence of nitrate, nitrite, or nitroarenes. A periplasmic 47-kDa protein facilitates nitrate uptake, thus increasing nitrate reductase activity. Two types of dissimilatory nitrate reductases have been found in strains from Rhodobacter sphaeroides. One of them is coupled to a complete denitrifying pathway, and the other is a periplasmic protein whose physiological role seems to be the dissipation of excess reducing power, thus improving photoanaerobic growth. Periplasmic nitrate reductase does not use NADH as the physiological electron donor and is a 100-kDa heterodimeric hemoprotein that receives electrons through an electron transport chain spanning the plasma membrane. This nitrate reductase is regulated neither by the intracellular C/N balance nor by O2 pressure. The enzyme also exhibits chlorate reductase activity, and both reaction products, nitrite and chlorite, are released almost stoichiometrically into the medium; this accounts for the high resistance to chlorate or nitrite exhibited by this bacterium. Nitrate reductases from both strains seem to be coded by genes located on megaplasmids.

Effect of earthworms on molybdenum-depending activities

Pot experiments performed in an alkaline, silty clay soil showed that the presence of Allolobophora caliginosa increased nitrate reductase activity in maize seedlings and nitrogen fixation in the soil, but did not affect photosynthesis and biomass of maize seedlings and oxygen consumption in the soil. The increase in molybdenum-depending activities is ascribed to the greater availability of molybdenum recorded in the presence of earthworms.

Nitrate reductase expression in maize leaves (Zea mays) during dark‐light transitions. Complex effects of protein phosphatase inhibitors on enzyme activity, protein synthesis and transcript levels

The effects of cytoplasmic protein synthesis and protein phosphatase activity on NADH:nitrate reductase (NR) activity, protein and transcript were examined in maize (Zea mays L.) seedling leaves. A rapid increase in NR activity, measured in the presence of 5 mM Mg2+, was found upon exposure of excised leaves to light. Inhibitors of protein phosphatase activity (okadaic acid [OKA] and microcystin [MC]‐LR) completely prevented the increase in NR activity. The cytoplasmic protein synthesis inhibitor, cycloheximide (CHX), did not affect Mg2+ inhibition of NR activity during the dark‐to‐light transition. Vmax NR activity, measured in the presence of 5mM Mg2+, was found upon exposure of excised leaves to light. Inhibitors of protein phosphatase activity (okadaic acid [OKA] and microcystin [MC]‐LR) completely prevented the increase in NR activity. The cytoplasmic protein synthesis inhibitor, cycloheximide (CHX), did not affect Mg2+ inhibition of NR activity during the dark‐to‐light transition. Vmax NR activity, measured in the presence of Pi and EDTA, remained constant or increased slightly in maize leaves during the first 2 h of the light period. OKA, MC‐LR or CHX treatment caused a 40 to 50% reduction in Vmax NR activity during this time. Incorporation of 35S‐Met into NR protein was reduced more than 90% by CHX and 80% by OKA. The inhibition of NR protein synthesis by CHX and OKA correlated with a 50 to 60% decrease in 35S‐Met incorporation into total soluble protein over the treatment period. The increase in NR mRNA levels early in the light period was prevented by OKA and MC‐LR, but not by CHX. OKA had a similar effect on sucrose phosphate synthase mRNA levels, but did not affect Catalasel or Catalase3 mRNA accumulation. The data suggest that light‐induced decreases in Mg2+ inhibition of NR activity and transcript levels are independent of new protein synthesis. The effects of OKA and MC‐LR indicate that protein phosphatase activities could be involved, directly or indirectly, in the regulation of NR activity, protein synthesis and transcript accumulation.

Nitrogen Assimilation Enzyme Activities in Witchweed (Striga) in Hosts Presence and Absence

N fertilizers suppress witchweed plant growth and development, thus reducing the severity of parasite attack and increasing host yield simultaneously. However, the underlying physiological mode of N action occurring within the parasite cells remains largely unknown. This study aims at screening for the effects of N forms and different growth conditions on some N assimilation enzymes in witchweed seedlings grown aseptically without host plant, and in pots with host plants. Results show that supply of N in NH4+or urea forms resulted in 83 to 92% reduction in nitrate reductase activity (NRc), compared with control. Increasing NO3−concentrations from 0 mM to 100 mM, led to a corresponding increase in NRc in giant witchweed. NRc of giant witchweed seedlings grown under light and dark cycles were about 270 times higher than seedlings grown in continuous darkness. A combination of NH4+and NO3−, resulted in increased giant witchweed NRc, compared with NH4+or NO3−supplied singly. Highest shoot development and NRc was at NH4+and NO3−ratio 1:1, followed by ratios 1:3, 3:1, 0:1, and 1:0, respectively. Addition of N in soils resulted in increased NRc, followed by rapid deterioration and death of giant witchweed plants. NRc, GSc, and GDHc in witchweed, maize, cowpea, and tobacco were affected by diurnal fluctuations with higher enzyme activities occurring during the day than at night. Higher GSc than GDHc suggests that NH4+assimilation occurs mainly through the GS pathway in witchweed plants. NRc and GDHc were two and four times higher in giant witchweed grown in aseptic media without host plant, than that grown in potted soils with host plants. These findings provide insight into the physiological mode of N action and their implications on witchweed control.

Stimulation of growth and nitrate assimilation inLeucaena leucocephala seedlings in response to spermidine supply

Supply of 100 μM spermidine (Spd) in the nutrient solution containing 10 mM nitrate as the sole nitrogen source, increased growth of roots and shoots, total nitrogen content andin vivo orin vitro nitrate reductase (NR) activity of leaves of 10-d oldLeucaena leucocephala seedlings. Spd and the cytokinins benzyladenine or kinetin also increased growth, total nitrogen andin vivo NR activity of isolated cotyledons. The synergistic effects of nitrate, kinetin and Spd in increasing NR activity, indicate that the Spd acted at different level than the nitrate or cytokinin.

Nitrate Reductase Activity is Endogenously Induced by Zeatin Riboside in Habituated Suspension Cultured Chenopodium rubrum Cells

Summary The natural cytokinins IP, IPA, Z, ZR, DHZ and DHZR of suspension cultured photoautotrophic and phytohormone-independent cells of Chenopodium rubrum were examined for their ability to induce nitrate reductase (EC 1.6.6.1) activity in resting cells of the batch culture. Each of the applied cytokinins was readily taken up from the medium and nitrate reductase activity was rapidly enhanced by all of them, ZR being a little more effective than the others. Nitrate reductase activity was also enhanced by adding ammonium to the cell culture in the G o phase. Subsequently, the endogenous level of ZR increased transitorily up to 2.5-fold, peaking about 4 h after addition of ammonium. The levels of the other endogenous cytokinins remained more or les unchanged. Supply of cytokinins to the stationary cell culture did not result in resumption of cell division. Induction of nitrate reductase activity by both ZR and ammonium could be inhibited with cycloheximide, indicating that protein de-novo -synthesis was involved in the enhancement of the enzyme activity. The time-span between the rise of the cytokinin level and the increase of nitrate reductase activity was about 2 h, irrespective of whether induction was triggered with ZR or ammonium nitrate. From these results, ZR was considered to be the endogenous cytokinin species that is involved in the induction of nitrate reductase in Chenopodium rubrum .

A hypothesis of plant susceptibility to atmospheric pollution based on intrinsic nitrogen metabolism: Why acidity really is the problem

Several hypotheses have been put forward to explain the causes of damage to plants by atmospheric pollution. None however, fully explain why some species are more prone to damage than others. In the literature, certain higher plants are more commonly reported as suffering damage from atmospheric pollution. These include climax species such as conifers, some broadleaf species such as oak and beech, and ericaceous species. Pioneer species such as highly nitrophillous woody species and ruderal herbaceous species are seldom reported in connection with damage from atmospheric pollutants. A screening of 18 species (woody and herbaceous) for 5 physiological and metabolic parameters and subsequent regression analysis of the data, showed that plants could be grouped according to their tolerance to acidity. There was a marked positive correlation between leaf nitrate reductase (NR) activity and the buffering capacity index (BCI) of the leaf Foliar mistings and soil applications of 3 mol m -3 KNO 3 to 6 assorted plant species also revealed that pioneer species, can induce NR activity when NO 3 - is applied directly to the foliage or soil. However, climax species have a much lower ability to increase NR activity in response to foliar and soil NO 3 - inputs. Leaf NO 3 + assimilation carries with it the direct generation of OH - , involves greater uptake of base cations and confers greater physiological buffering capability through the close interaction of carbon and nitrogen acquisition in the leaf. The hypothesis states that leaf nitrate assimilators are physiologically more competent at buffering acidic inputs whether directly as H + , or indirectly through uptake of acidic pollutants.

Influence of some growth regulators and divalent cations on the inhibition of nitrate reductase activity by lead in maize leaves

Substrate induction of in vivo nitrate reductase (NR) activity in excised maize leaf segments from seedlings grown with nutrient containing no nitrogen, was substantially inhibited by 0.5 mM Pb acetate, during a 24 h incubation of leaf segments. Inclusion of IAA, GA 3, kinetin and salicylic acid in the incubation medium increased NR activity in the absence of Pb, but only GA and salicylic acid did so in the presence of Pb. Salicylic acid negated the inhibitory effect of the Pb, which was related to its concentration. The phenolic acid had little effect on in vitro Pb effects on NR activity. Among CaCl 2, MgCl 2, MnCl 2, ZnCl 2 and CuSO 4, only MnCl 2 to some extent, negated the inhibitory effect of Pb on in vivo NR activity in the leaf segments, although the chlorides of Ca 2+, Mg 2+ and Mn 2+ caused some increase in enzyme activity in the absence of Pb. The effect of Mn 2+ caused some increase in enzyme activity in the absence of Pb. The effect of Mn 2+ was concentration dependent. But in vitro supply of MnCl 2 to NR preparation prevented the inhibitory effect of Pb on its activity at each concentration, 5, 10, and 20 mM. The experiments demonstrate that the inhibitory effect of Pb on enzyme activity could be antagonised by salicylic acid and Mn.

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

Regulation by the active form of phytochrome (PFR) 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 PFR, 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 relation to growth and soil N forms in red oak and red ash planted in three different environments: forest, clear-cut and field

Experimental hardwood plantations were established in three different environments (forest with partial openings, clear-cut and abandoned agricultural field) with different soil nitrogen status (N form and nitrification rates), in order to verify if NO 3 − assimilation of planted red oak and red ash seedlings could be linked to the performance of both species in the different plantation sites (where half of the plots were sprayed with herbicide). Nitrate reductase activity (NRA) in leaves and roots as an index of NO 3 − assimilation was chosen, as this enzyme is inducible and is also the first key enzyme involved in the assimilation of NO 3 −. Results show that NRA varied in relation to time, plantation environment, herbicide application and species of tree. Red ash always had higher values (18.43 μmol NO 2 g −1 dry weight (DW) h −1) than red oak (2.06 μmol NO 2 g −1 DW h −1) in herbicide plots in open environments, especially in the field plantation in July 1990. Herbicide spraying increased NO 3 − availability in soils and acted as a test for the presence of inducible nitrate reductase in both species. An increase in NRA is found in relation to herbicide application. Red ash shows greater differences than red oak between herbicide and control treatments. Both species show different values of height and diameter increment, and relative growth rate (RGR) in relation to the plantation sites. Red oak seedlings show greater growth gains in clear-cut with herbicide treatment (RGR = 0.79 g g −1 month −1), whereas the field plantation site produced the largest red ash seedlings (RGR = 0.56 g g −1 month −1) in herbicide plots. In limited light conditions (clear-cut control plots and forest environment), red oak had higher RGR than red ash. These results suggest that NRA in the leaves of hardwood trees can provide a useful index to evaluate the physiological status of tree seedlings, which reflects their response to environmental conditions. The use of such an index could facilitate the matching of hardwood species, according to their N requirements, to plantation sites, evaluated in terms of soil N status and light conditions.

Investigations on the nature of the phytochrome-induced transmitter for the regulation of nitrate reductase in etiolated leaves of maize

A lag phase of 30 min for phytochrome-mediated stimulation in the induction of nitrate reductase (NR) in etiolated leaves of Zea mays var. 'Ganga 5', was eliminated by a 2 h pre-irradiation with red light, supplied in the absence of nitrate. Earlier, photoreversibil-ity of NR activity was found to be lost completely by 2 h and a transmitter with a life span of 12 h was proposed to mediate the phytochrome effect on NR (Sharma and Sopory, 1984). Low temperature (0°C) treatment prevented the loss of the photoreversibility. Almost complete photoreversibility was observed after 2 h, suggesting that the formation of the transmitter requires active metabolism. Tungstate, given 2 h after red light treatment, inhibited the increase in the enzyme activity by red light, suggesting that the transmitter is not inactive NR itself, which becomes activated by nitrate. Analysis of steady-state levels of the NR transcript revealed that NR mRNA is not induced in response to Pfr formation in the absence of nitrate treatment, suggesting that it could not be the transmitter mediating the phytochrome effect on NR activity. Red light was found to increase the uptake of 45Ca2+ by isolated maize protoplasts. When supplied exogen-ously calcium increased NR activity by 48% of that obtained by red light irradiation. However, exogenous addition of phorbol myristate acetate (PMA), an activator of protein kinase C, was found to increase the NR transcript level to the same extent obtained after 5 min of red light irradiation. The results suggest that phytochrome may be acting through calcium along with messengers like diacylglycerol, generated through the phosphoinositide (PI) cycle to stimulate the induction of NR.

Evidence that the glutamine-stimulated loss of nitrate reductase protein from the yeast Candida nitratophila is not the result of inducer exclusion.

Synthesis of nitrate reductase protein and increases in nitrate reductase activity occurred in cultures of the yeast Candida nitratophila when they were incubated in medium containing ammonium nitrate. Similar treatment with glutamine plus nitrate resulted in little increase in nitrate reductase activity, in cultures grown previously with reduced nitrogen compounds, and decreases in enzyme activity, in cultures adapted to nitrate. Labelling studies conducted in vivo revealed a rapid cessation of de novo nitrate reductase synthesis when glutamine was supplied to nitrate-adapted cultures in the presence of nitrate. Intracellular glutamine concentrations increased rapidly under these conditions and these cultures exhibited high glutamine: glutamate ratios. As nitrate was taken up in the presence of glutamine in these experiments, it is concluded that the glutamine-stimulated inhibition of nitrate reductase synthesis is a consequence of repression and rapid turnover of nitrate reductase mRNA and not inducer (nitrate) exclusion.

Responses of nitrogen metabolism parameters to sublethal SO 2 pollution in wheat [ Triticum aestivum cv Wilgoyne (Ciano/Gallo)] under mild NaCl stress

Wheat plants were exposed to a factorial combination of two levels of salinity (control and 50 mM NaCl), and three levels of SO 2 (<10, 231, and 441 nl l −1) in fumigation chambers with rain-exclusion tops for 4 hr day −1 for up to 51 days. Exposure to 441 nl l −1 SO 2 initially increased nitrate reductase activity (NRA) in the youngest fully expanded leaves, but it decreased later. NRA was decreased more in older leaves than in young leaves with SO 2. Nitrogen utilization, but not uptake into plants, appeared to be affected by SO 2 fumigation, resulting in an increase in total amino acid concentration and soluble protein in the old leaves. NaCl treatment decreased nitrogen uptake rate per unit root dry weight. Concentrations of total amino acids and soluble protein increased with NaCl salinity. Both NaCl and SO 2 increased concentrations of total amino acids and soluble protein in leaves. Generally, effects of SO 2 fumigation and NaCl salinity were additive on nitrogen uptake rate g −1 root dry wt, leaf NRA, total nitrogen concentration, and total amino acid concentration in plants.

Control of Phenotypic Plasticity Via Regulatory Genes

Control of Phenotypic Plasticity Via Regulatory Genes

In vivonitrate reductase activity incarex pseudocyperusL.: The influence of nitrate ‐ ammonium concentration ratios and correlation with growth

Abstract The influence of the nitrate nutritional status and increasing ammonium concentrations on the nitrate reductase activity of shoots and roots of Carex pseudocyperus L. was investigated. The activity of this enzyme was correlated with the relative growth rates of the plant. Nitrate reductase activity was determined by a modified in vivo test (1). A specially developed test system allowed a large amount of samples to be handled easily. The optimization procedure of the incubation buffers led to different assay conditions for the shoot and the root, respectively. Enzyme activity in the shoot was dependent on the length of the incubated leaf pieces. Incubation had to take place under dark, anaerobic conditions. Enzyme activity was influenced by an evident diurnal rhythm with an optimum six hours after starting illumination, so that harvesting occurred always at that day time. Increasing nitrate concentrations of up to 2.5mM NO3‐ in the nutrient solution induced an increasing nitrate reductase activity in the shoot. The enzyme activity of the root was already fully induced at 1mM NO3 −. A nitrate concentration above 5mM NO3 − inhibited enzyme activity in shoots as well as in roots. The addition of increasing amounts of ammonium to a solution containing 2mM NO3 − led to a significant inhibition of the enzyme activity in both parts of the plant. Relative growth rates of the shoot, as a function of increasing nitrate concentrations in the nutrient solution, were highly positively correlated to the corresponding nitrate reductase activity, but only a slight, negative correlation was observed between these two parameters in the root.

Increase in Nitrate Reductase Activity with Ammonium Chloride inBacillus licheniformisby Shaking Culture

In shaking culture, nitrate reductase activity in the cell-free extracts of Bacillus licheniformis increased with the addition of NH4Cl to the medium containing NaNO3 as a single nitrogen source, where amounts of nitrogen sources were sufficient for cell growth. This increase of nitrate reductase activity therefore suggests that the activity is not for nitrate assimilation but for other physiological functions containing a dissimilatory nitrate reduction.