nir1, a conditional-lethal mutation in barley causing a defect in nitrite reduction

Summary Eleven green individuals were isolated when 95000 M₂ plants of barley (Hordeum vulgare L.), mutagenised with azide in the M₁, were screened for nitrite accumulation in their leaves after nitrate treatment in the light. The selected plants were maintained in aerated liquid culture solution containing glutamine as sole nitrogen source. Not all plants survived to flowering and some others that did were not fertile. One of the selected plants, STA3999, from the cultivar Tweed could be crossed to the wild-type cultivar and analysis of the F₂ progeny showed that leaf nitrite accumulation was due to a recessive mutation in a single nuclear gene, which has been designated Nir1. The homozygous nir1 mutant could be maintained to flowering in liquid culture with either glutamine or ammonium as sole nitrogen source, but died within 14 days after transfer to compost. The nitrite reductase cross-reacting material seen in nitrate-treated wild-type plants could not be detected in either the leaf or the root of the homozygous nir1 mutant. Nitrite reductase activity, measured with dithionite-reduced methyl viologen as electron donor, of the nitrate-treated homozygous nir1 mutant was much reduced but NADH-nitrate reductase activity was elevated compared to wild-type plants. We conclude that the Nir1 locus determines the formation of nitrite reductase apoprotein in both the leaf and root of barley and speculate that it represents either the nitrite reductase apoprotein gene locus or, less likely, a regulatory locus whose product is required for the synthesis of nitrite reductase, but not nitrate reductase. Elevation of NADH-nitrate reductase activity in the nir1 mutant suggests a regulatory perturbation in the expression of the Narl gene.     

In vivo Characterization of Nitrate Reductase Activity in Cotton

The in vivo nitrate reductase activity in leaf tissue of cotton (Gossypium hirsutum L.) was characterized. Enzymatic activity was linear with time up to 60 min. The assay for nitrate reductase activity was optimized in leaf slices 400 μm wide incubated in an anaerobic system at 30°C, in a 0.02 M KNO₃ medium at pH 7.0 with 1 % propanol. In vivo activity was highest in recently matured leaves at the top of the plant. Both light and nitrate enhanced in vivo enzymatic activity. The activity was highest after 9 hours in the light and then decreased steadily for several more hours even in the presence of light. The nitrate reductase activity was more strongly correlated to the levels of NO₃-N in the culture solution than to the NO₃-N level in the tissue. The utility of this technique in nitrate reductase assay in a tissue containing large amounts of phenolic compounds is discussed.     

Nitrogen Fixation and Nitrogen Assimilation in a Temperate Saline Ecosystem

As nitrogen is known to be a limiting factor for plant growth, we were interested in the relationship between soil microbial activity and the nitrogen assimilation of 5 different halophytes from 4 saline sites near the lake “Neusiedlersee”, Austria. The following were studied between May and October 1985: nitrogen fixation (¹⁵N₂ and acetylene reduction): N-mineralization; several soil characteristics and in vivo nitrate reductase activity of roots and shoots of these plants. NO^?₃, org. N- and carboxylate contents of both roots and shoots, as well as the effect of NO^?₃-fertilization on the amounts of these substances, were determined on plants growing in the field during a 3-day period in September 1985. Fertilization led to a decrease in acetylene reduction activity at most sites, and an increase in the nitrate reductase activity of the shoots of all plants. Overall, carboxylate and organic nitrogen contents of these halophytes did not change in response to fertilization. Only in the roots of Aster tripolium and Atriplex hastata was there a marked increase in the nitrate reductase activity in response to fertilization. Species growing at the same site, such as Plantago maritima and Lepidium crassifolium showed contrasting levels of assimilatory activity. Apparent low rates of ammonification and nitrification were detected in soils from the 4 sites. The results are discussed in relation to the nitrogen and carbon economies of the microorganisms and plants.     

Nitrate reductase activity in corn seedlings as influenced byArthrobacter sp.

Summary Nitrate reductase activity has been assayed in corn seedlings grown in a nutrient solution containing different concentrations of nitrate and inoculated withArthrobacter sp. The enzymatic activity was greatly enhanced especially at the levels of NO₃-ions suboptimal for the maximum induction. The ability of synthetic phytohormones (IAA, IPA and GA₃) in inducting nitrate reductase activity has been also texted.     

Nitrate reductase activity and growth in Paul's Scarlet rose suspension cultures in relation to nitrogen source and molybdenum

Growth and nitrate reductase activity were measured in Paul's Scarlet rose cell suspensions, cultured in media purified from molybdenum and containing nitrate or urea as sole nitrogen source with or without added Mo. Urea could replace nitrate to yield 80% of the fresh weight in nitrate medium. Nitrate reductase activities were compared by in vivo and in vitro assays. The latter varied due to inactivation during extraction. Compared with activities in cells in complete NO₃ ^- medium, activity in NO₃ ^--Mo cells was reduced to 30% and, in urea-grown cells, to trace amounts. Increases in nitrate reductase activity were found when NO₃ ^- alone was added to NO₃ ^- or urea+Mo cultures. In NO₃ ^--Mo cultures, Mo alone or with NO₃ ^- caused a similar increase in activity, whereas urea-Mo cultures required both NO₃ ^- and Mo for enzyme induction.Abbreviations FAD flavin adenine dinucleotide - Mo molybdenum - NADH reduced nicotinamide adenine dinucleotide - NO₃ ^-+Mo standard MX₁ culture medium - NO₃ ^--Mo MX₁ medium purified of Mo and used for continuous subculture with nitrate - NR nitrate reductase - PSR Paul's Scarlet rose - PVP polyvinylpyrrolidone - U urea - U+Mo MX₁ medium containing urea instead of nitrate - U-Mo MX₁ medium containing urea instead of nitrate and also purified of Mo     

Preparation and characterization of a soluble nitrate reductase from Azotobacter chroococcum

Summary The assimilatory nitrate reductase of the N₂-fixing bacterium Azotobacter chroococcum has been prepared in a soluble form from cells grown with nitrate as the nitrogen source, and some of its properties (electron donors and cofactors, K _mvalues for substrates, molecular weight, inhibitors, activators, etc.) have been studied. The enzyme is of an inducible nature and can exist in two interconvertible forms, either active or inactive.Tungstate very efficiently inhibits growth of the microorganism in media with nitrate. When either nitrite or ammonia are substituted for nitrate as the nitrogen source, growth is unaffected by tungstate concentrations which otherwise completely suppress growth on nitrate. Tungstate interferes by decreasing the cellular level of nitrate reductase activity, preventing, as a consequence, utilization of nitrate.     

Biochemical genetics of further chlorate resistant, molybdenum cofactor defective, conditional-lethal mutants of barley

Summary Three plants, R9201 and R11301 (from cv. Maris Mink) and R12202 (from cv. Golden Promise), were selected by screening M₂ populations of barley (Hordeum vulgare L.) seedlings (mutagenised with azide in the M₁) for resistance to 10 mM potassium chlorate. Selections R9201 and R11301 were crossed with the wild-type cv. Maris Mink and analysis of the F₂ progeny showed that one quarter lacked shoot nitrate reductase activity. These F₂ plants also withered and died in the continuous presence of nitrate as sole nitrogen source. Loss of nitrate reductase activity and withering and death were due in each case to a recessive mutation in a single nuclear gene. All F1 progeny derived from selfing selection R12202 lacked shoot nitrate reductase activity and also withered and subsequently died when maintained in the continuous presence of nitrate as sole nitrogen source. All homozygous mutant plants lacked not only shoot nitrate reductase activity but also shoot xanthine dehydrogenase activity. The plants took up nitrate, and possessed wild-type or higher levels of shoot nitrite reductase activity and NADH-cytochrome c reductase activity when treated with nitrate for 18 h. We conclude that loss of shoot nitrate reductase activity, xanthine dehydrogenase activity and withering and death, in the three mutants R9201, R11301 and R12202 is due to a mutation affecting the formation of a functional molybdenum cofactor. The mutants possessed wild-type levels of molybdenum and growth in the presence of unphysiologically high levels of molybdate did not restore shoot nitrate reductase or xanthine dehydrogenase activity. The shoot molybdenum cofactor of R9201 and of R12202 is unable to reconstitute NADPH nitrate reductase activity from extracts of the Neurospora crassa nit-1 mutant and dimerise the nitrate reductase subunits present in the respective barley mutant. The shoot molybdenum cofactor of R11301 is able to effect dimerisation of the R11301 nitrate reductase subunits and can reconstitute NADPH-nitrate reductase activity up to 40% of the wild-type molybdenum cofactor levels. The molybdenum cofactor of the roots of R9201 and R11301 is also defective. Genetic analysis demonstrated that R9201, but not R11301, is allelic to R9401 and Az34 (nar-2a), two mutants previously shown to be defective in synthesis of molybdenum cofactor. The mutations in R9401 and R9201 gave partial complementation of the nar-2a gene such that heterozygotes had higher levels of extractable nitrate reductase activity than the homozygous mutants.We conclude that: (a) the nar-2 gene locus encodes a step in molybdopterin biosynthesis; (b) the mutant R11301 represents a further locus involved in the synthesis of a functional molybdenum cofactor; (c) mutant Rl2202 is also defective in molybdopterin biosynthesis; and (d) the nar-2 gene locus and the gene locus defined by R11301 govern molybdenum cofactor biosynthesis in both shoot and root.     

降水和氮沉降增加对草地土壤酶活性的影响

为探究降水和氮沉降增加对草地生态系统土壤酶活性的影响,于2014年生长季在内蒙古温带典型羊草草原开展了野外原位控制实验。试验共设置降水(对照,W0,自然降水;W15,增加15%的年均降水量)、施氮(对照,CK,0 kg N hm~(-2)a~(-1);低氮,LN,25 kg N hm~(-2)a~(-1);中氮,MN,50 kg N hm~(-2)a~(-1);高氮,HN,100 kg N hm~(-2)a~(-1))及其交互作用等8个不同的处理水平来模拟降水和氮沉降增加的全球变化情景,分别定量探讨了不同水、氮添加条件下草地表层土壤中与氮循环相关的蛋白酶,脲酶,硝酸还原酶,亚硝酸还原酶活性的月动态变化及其与土壤理化性质之间的相关性。研究结果表明:在自然降水条件下,不同施氮水平蛋白酶、脲酶和硝酸还原酶活性无显著差异,亚硝酸还原酶活性相比于对照显著降低;在增加降水条件下,不同施氮水平对蛋白酶和硝酸还原酶活性未产生显著性影响,高氮水平显著降低脲酶和亚硝酸还原酶活性。不同施氮水平是否添加降水对亚硝酸还原酶活性无影响,而增添降水使低氮处理的蛋白酶活性和中、高氮处理水平的硝酸还原酶活性增加、高氮处理的脲酶活性降低。降水在影响蛋白酶和硝酸还原酶活性方面具有主效应,氮沉降在影响亚硝酸还原酶活性方面具有主效应,而降水和施氮处理未表现出明显地交互作用。土壤亚硝酸还原酶活性与土壤碳氮比和NH~+_4-N含量极显著正相关,与NO-3-N含量显著正相关。     

Nitrogenase activity and nitrate respiration inAzospirillum spp.

The interaction between nitrate respiration and nitrogen fixation inAzospirillum lipoferum andA. brasilense was studied. All strains examined were capable of nitrogen fixation (acetylene reduction) under conditions of severe oxygen limitation in the presence of nitrate. A lag phase of about 1 h was observed for both nitrate reduction and nitrogenase activity corresponding to the period of induction of the dissimilatory nitrate reductase. Nitrogenase activity ceased when nitrate was exhausted suggesting that the reduction of nitrate to nitrite, rather than denitrification (the further reduction of nitrite to gas) is coupled to nitrogen fixation. The addition of nitrate to nitrate reductase negative mutants (nr^-) ofAzospirillum did not stimulate nitrogenase activity. Under oxygen-limited conditionsA. brasilense andA. lipoferum were also shown to reduce nitrate to ammonia, which accumulated in the medium. Both species, including strains ofA. brasilense which do not possess a dissimilatory nitrite reductase (nir^-) were also capable of reducing nitrous oxide to N₂.     

In vivo and in vitro studies of nitrate reductase regulation in Asperillus nidulans.

Summary Induced wildtype cells ofA. nidulans rapidly lost NADPH — linked nitrate reductase activity when subjected to carbon and or nitrogen starvation. A constitutive mutant at the regulatory gene for nitrate reductase,nirA ^c1, rapidly lost nitrate reductase activity upon carbon starvation. This loss of activity is thought to be due to a decrease in the NADPH concentration in the cells. Cell free extracts from wild-type cells grown in the presence of nitrate, rapidly lost their nitrate reductase activity when incubated at 25° C. NADPH prevented this loss of activity. Wildtype cells grown in the presence of nitrate and urea have a higher initial NADPH: NADP⁺ ratio and cell free extracts from such cells lost their nitrate reductase activity slower than extracts of cells grown with nitrate alone.The Pentose Phosphate Pathway mutant,pppB-1, had a lower NADPH concentration compared with the wildtype grown under the same conditions and cell free extracts lost their nitrate reductase activity more rapidly than the wildtype. Cell free extracts ofnirA ^c-1 and a non-inducible mutant for nitrate reductase,nirA ^--14, upon incubation lost little of their nitrate reductase activity.     

Combination of kanamycin resistance and nitrate reductase deficiency as selectable markers in one nuclear genome provides a universal somatic hybridizer in plants

Summary The combination in the nuclear genome of a dominant resistance marker (to select against unfused wild-type cells) and a recessive deficiency marker (to select against unfused mutant cells) in a cell line should provide a system for selecting fusion hybrids between the mutant line and any wild-type line. To test this idea, we fused protoplasts from a non-morphogenic cell line of Nicotiana tabacum which was kanamycin resistant (by transformation) and deficient in nitrate reductase (NR^-K⁺) with protoplasts from N. tabacum cv. Petit Havana clone SR1, which provided resistance against streptomycin as an additional selectable marker (NR⁺K^-SR⁺). Putative hybrids were selected using a culture medium containing no available reduced nitrogen source and 50 mg/l kanamycin sulphate. After regeneration into plants, the hybrid character was demonstrated from: (i) the morphological variation of the regenerants; (ii) the chromosome number; (iii) the ability to grow on medium without a reduced nitrogen source and containing kanamycin sulphate at 50 mg/l; (iv) the presence of nitrate reductase activity; (v) the presence of the gene coding for neomycin phosphotransferase, which provides resistance to kanamycin sulphate; (vi) callus formation from leaves on medium containing 1 g/l streptomycin or 50 mg/l kanamycin sulphate; (vii) F₁ plants containing nitrate reductase and the gene for neomycin phosphotransferase. Fusions between the mutant cell line (NR^-K⁺) and three wild-type tobacco species and subsequent cultivation on medium containing no available nitrogen source but 50 mg/l kanamycin sulphate resulted in callus formation with all combinations, while hybrid plants were only regenerated when N. sylvestris was the fusion partner.     

Involvement of the narJ and mob gene products in distinct steps in the biosynthesis of the molybdoenzyme nitrate reductase in Escherichia coli

The Escherichia coli mob locus is required for synthesis of active molybdenum cofactor, molybdopterin guanine dinucleotide. The mobB gene is not essential for molybdenum cofactor biosynthesis because a deletion of both mob genes can be fully complemented by just mobA. Inactive nitrate reductase, purified from a mob strain, can be activated in vitro by incubation with protein FA (the mobA gene product), GTP, MgCl₂, and a further protein fraction, factor X. Factor X activity is present in strains that lack MobB, indicating that it is not an essential component of factor X, but over-expression of MobB increases the level of factor X. MobB, therefore, can participate in nitrate reductase activation. The narJ protein is not a component of mature nitrate reductase but narJ mutants cannot express active nitrate reductase A. Extracts from narJ strains are unable to support the in vitro activation of purified mob nitrate reductase: they lack factor X activity. Although the mob gene products are necessary for the biosynthesis of all E. coli molybdoenzymes as a result of their requirement for molybdopterin guanine dinucleotide, NarJ action is specific for nitrate reductase A. The inactive nitrate reductase A derivative in a narJ strain can be activated in vitro following incubation with cell extracts containing the narJ protein. NarJ acts to activate nitrate reductase after molybdenum cofactor biosynthesis is complete.     

Nitrate content and nitrate reductase activity in Rumex obtusifolius L.

Summary The aim of this work was to investigate the effect of nitrogen starvation and subsequent fentilization with nitrate or ammonium on nitrate content and nitrate reductase activity of Rumex obtusifolius L. under natural conditions.When plants were transplanted to nitrate-poor media, endogenous nitrate was reduced within a few days. In parallel, nitrage reductase activities dropped to about 25% of the initial values. As a consequence of nitrate fertilization (1; 10 or 100 mmol KNO₃/l substrate), endogenous nitrate content of the plant abruptly increased within one day. In extreme cases, nitrate concentrations of up to 10% of plant dry weight could be observed without being lethal. High external nitrate concentrations caused an inhibition of nitrate reductase within the leaves, while low external concentrations provoked an increase in the enzyme activity of about 450% within one day. Ammonium fertilization (5 mmol (NH₄)₂SO₄/l substrate) also caused an increase in nitrate reductase activity and nitrate content within leaf blades. This observation indicates a rapid nitrification of ammonium in the substrate. When plants were fertilized with ammonium plus nitrate (2.5 mmol (NH₄)₂SO₄+ 5 mmol KNO₃/l substrate), an extremely high and long term increase in nitrate reduction could be observed. Due to an intensive enzymatic nitrate turnover, the nitrate content of leaf blades then remained relatively low. Our observations do not point to an inhibition of nitrate reductase activity in leaves of Rumex obtusifolius by ammonium. Despite temporarily high endogenous nitrate concentrations, Rumex obtusifolius may not be termed as a nitrate storage plant, since the accumulation of nitrate is a short term process only.     

Regulation of the nitrate-reducing system enzymes in wild-type and mutant strains of Chlamydomonas reinhardii

Summary Six mutant strains (301, 102, 203, 104, 305, and 307) affected in their nitrate assimilation capability and their corresponding parental wild-type strains (6145c and 21gr) from Chlamydomonas reinhardii have been studied on different nitrogen sources with respect to NAD(P)H-nitrate reductase and its associated activities (NAD(P)H-cytochrome c reductase and reduced benzyl viologen-nitrate reductase) and to nitrite reductase activity. The mutant strains lack NAD(P)H-nitrate reductase activity in all the nitrogen sources. Mutants 301, 102, 104, and 307 have only NAD(P)H-cytochrome c reductase activity whereas mutant 305 solely has reduced benzyl viologen-nitrate reductase activity. Both activities are repressible by ammonia but, in contrast to the nitrate reductase complex of wild-type strains, require neither nitrate nor nitrite for their induction. Moreover, the enzyme from mutant 305 is always obtained in active form whereas nitrate reductase from wild-types needs to be reactivated previously with ferricyanide to be fully detected. Wild-type strains and mutants 301, 102, 104, and 307, when properly induced, exhibit an NAD(P)H-cytochrome c reductase distinguishable electrophoretically from contitutive diaphorases as a rapidly migrating band. Nitrite reductase from wild-type and mutant strains is also repressible by ammonia and does not require nitrate or nitrite for its synthesis. These facts are explained in terms of a regulation of nitrate reductase synthesis by the enzyme itself.     

The mobA Gene Is Required for Assimilatory and Respiratory Nitrate Reduction but not Xanthine Dehydrogenase Activity in Pseudomonas aeruginosa

The requirement for the mobA gene in key assimilatory and respiratory nitrogen metabolism of Pseudomonas aeruginosa PAO1 was investigated by mutational analysis of PA3030 (mobA; MoCo guanylating enzyme), PA1779 (nasA; assimilatory nitrate reductase), and PA3875 (narG; respiratory nitrate reductase). The mobA mutant was deficient in both assimilatory and respiratory nitrate reductase activities, whereas xanthine dehydrogenase activity remained unaffected. Thus, P. aeruginosa requires both the molybdopterin (MPT) and molybdopterin guanine dinucleotide (MGD) forms of the molybdenum cofactor for a complete spectrum of nitrogen metabolism, and one form cannot substitute for the other. Regulation studies using a Φ(PA3030-lacZGm) reporter strain suggest that expression of mobA is not influenced by the type of nitrogen source or by anaerobiosis, whereas assimilatory nitrate reductase activity was detected only in the presence of nitrate.     

Nitrate reductase and nitrogenase activities of common beans (Phaseolus vulgaris L.) from different geographic locations

Summary Cultivars ofPhaseolus vulgaris (L.) from contrasting geographic locations were cultivated under fields conditions for measurements of nitrogenase and nitrate reductase activities. A first trial with two cultivars indicated that a tropical cultivar B-789 has a higher nitrogenase activity than a temperate one Elsa. And the converse was true for the nitrate reductase activity. While where a post flowering application was made, a renewal of nitrate reductase activity occurred. Further similar comparisons of both enzymatic activities upon eight tropical and temperate cultivars of equivalent vegetative cycles indicated, on the average, that tropical cultivars have a higher level of (C₂H₂) reduction and a lower nitrate reductase activity than temperature cultivars. These observations suggest that there exists an inverse relationship between the two enzymatic activities in common beans, and there probably exists genetic variability for a possible improvement of N-fixation ability. An early application of N-fertiliser upon the Elsa and B-789 plots promotes later nitrogenase activity while a post flowering application shows obvious a renewal of nitrate reductase activity. Thus, analyses of nitrate reductase and nitrogenase activities of a common bean crop at different physiological stages may give us an indication of the best time to apply supplementary nitrogen fertilisation to common beans to increase seed yield.     

Diverse effects of formate on the assimilatory metabolism of nitrate and nitrite inRhizobium

Two strains ofRhizobium, cowpeaRhizobium 32H1 andRhizobium japonicum CB 1809, showed a marked stimulation in growth on addition of formate to the minimal medium containing nitrate as the sole source of nitrogen. The amount of accumulated nitrite and specific nitrate reductase activity was much higher in cultures supplemented with formate than in the control medium. In contrast, growth, consumption of nitrite and specific nitrite reductase activity in minimal medium + nitrite was greatly reduced by the addition of formate. A chlorate resistant mutant (Chl-16) was isolated spontaneously which contained a nitrite reductase which was not inhibited by formate. The results suggest that formate serves as an electron donor for nitrate reductase and inhibits nitrite assimilation inRhizobium     

Chlorate toxicity in Aspergillus nidulans. Studies of mutants altered in nitrate assimilation.

Summary It had previously been held that chlorate is not itself toxic, but is rendered toxic as a result of nitrate reductase-catalysed conversion to chlorite. This however cannot be the explanation of chlorate toxicity in Aspergillus nidulans, even though nitrate reductase is known to have chlorate reductase activity. Among other evidence against the classical theory for the mechanism of chlorate toxicity, is the finding that not all mutants lacking nitrate reductase are clorate resistant. Both chlorate-sensitive and resistant mutants lacking nitrate reductase, also lack chlorate reductase. Data is presented which implicates not only nitrate reductase but also the product of the nirA gene, a positive regulator gene for nitrate assimilation, in the mediation of chlorate toxicity. Alternative mechanisms for chlorate toxicity are considered. It is unlikely that chlorate toxicity results from the involvement of nitrate reductase and the nirA gene product in the regulation either of nitrite reductase, or of the pentose phosphate pathway. Although low pH has an effect similar to chlorate, chlorate is not likely to be toxic because it lowers the pH; low pH and chlorate may instead have similar effects. A possible explanation for chlorate toxicity is that it mimics nitrate in mediating, via nitrate reductase and the nirA gene product, a shut-down of nitrogen catabolism. As chlorate cannot act as a nitrogen source, nitrogen starvation ensures.     

植被不同退化状态下尕海湿地土壤氮含量及酶活性特征

土壤中氮素的吸收、转化及含量的变化是影响植被生长的关键因素。为探讨湿地植被不同退化状态对土壤氮组分含量和相关酶活性的影响,以及土壤氮组分含量与相关酶活性之间的关系,以甘南尕海湿地不同植被退化状态样地（未退化CK、轻度退化SD、中度退化MD和重度退化HD）为研究对象,采用野外采样与室内实验相结合的方法,分析了植被不同退化状态下不同形态氮组分（全氮、铵态氮、硝态氮和微生物量氮）含量的变化特征,以及土壤氮转化酶（蛋白酶、脲酶、硝酸还原酶和亚硝酸还原酶）活性之间的相关关系。结果表明：（1）在植被退化状态下,土壤含水量逐渐减小,土壤温度呈先减小后增大的趋势;（2）随着植被退化程度的加剧,硝态氮含量呈增加趋势,而全氮、铵态氮和微生物量氮含量均随退化程度加剧呈减小趋势;土壤蛋白酶活性随退化程度的加剧而减小,脲酶活性呈先减小后增大的趋势,重度退化活性最高,轻度退化最低;硝酸还原酶活性随退化程度的加剧而增加,亚硝酸还原酶活性表现为"升-降-升"的变化趋势,即轻度退化活性最高,未退化和中度退化较低;（3）土壤蛋白酶活性与全氮、铵态氮和微生物量氮呈极显著正相关关系（P < 0.01）,与硝态氮含量呈显著负相关关系（P < 0.05）;硝酸还原酶活性与蛋白酶活性恰好相反;脲酶活性与微生物量氮含量呈极显著正相关关系（P < 0.01）,与全氮含量呈显著正相关关系（P < 0.05）;亚硝酸还原酶活性与全氮和铵态氮含量呈极显著正相关关系（P < 0.01）,与硝态氮含量呈显著负相关关系（P < 0.05）。综上,在尕海湿地植被退化条件下,土壤氮组分含量增加可以有效提高相关酶活性。     

Nodule Nitrate Reductase as a Source of Reduced Nitrogen in Soybean, Glycine max

Hydroponically grown soybeans were fed ¹⁵N-enriched NaNO₃ at nine reproductive stages of development. The stem exudates contained excess ¹⁵N in the fully reduced nitrogen fraction. The soybean nodules had high nitrate reductase activity, whereas the roots had no detectable nitrate reductase activity. Based on these results, we concluded that the nodule nitrate reductase system has the potential of contributing significantly to the nitrogen economy of the plant.