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1.
We report a novel use of the 15N dilution technique to detail the translocation of amino compounds in the legume Sesbania rostrata . The conventional 15N dilution technique follows the dilution of 15N within a labelled plant, as 14N2 is fixed by symbiotic bacteria. In our experiments, stem-nodulated Sesbania rostrata were enriched by feeding with 15N ammonium nitrate for 2 weeks, followed by a 1 week period where the only N available to the plants was via nitrogen fixation of atmospheric N2. We measured the composition, concentration and 15N enrichment of amino compounds in various plant tissues, both above and below the stem nodules, using GC-MS and isotopic abundance mass spectrometry techniques. Approximately 28% of the total N in the stem nodules was derived from internal plant sources. The ureides allantoic acid and allantoin were not abundant in xylem, leaf or nodule tissues. The amides asparagine and glutamine were the major export products from stem nodules although a wide range of other amino compounds are also synthesized. Amino acids within the nodules had a low level of enrichment, demonstrating that a small fraction (≈ 11%) was derived from outside the nodules, and significant cycling of N (28% of xylem N) through the root system was revealed by measurements of 15N distribution and amino acid concentrations.  相似文献   

2.
To develop further the methods for estimation of NOx absorption by plants supplied with 15N-labelled fertilizer, we proposed a new calculation method, total N fixed method (TNF), and compared with the 15N dilution method and the classical mass balance method (MB).
Hydroponically grown soybean plants were supplied with 15N-labelled nitrate and exposed to 200–250 nl l−1 NO2 for 7 d. The proportions of the N derived from NO2 to total N in exposed plants were estimated by the three methods.
The reported rates of NO2 absorption by several plant species, estimated by the 15N dilution method, were recalculated using the TNF method. The results of the two methods were compared and showed that: (1) The 15N dilution method overestimated the content of NO2-N in exposed plants compared with the MB method whilst the TNF method produced estimations of NO2-N closer to those by the MB method when the plants were supplied with 5 m M nitrate. (2) The differences in estimations between the MB method and either the 15N dilution method or the TNF method increased with decreasing supply of 15N-labelled nitrate to roots.  相似文献   

3.
Partitioning of nitrogen by soybeans ( Glycine max L. Merr. cv. Hodgson) grown in natural conditions was studied by successive exposures of root systems to 15N2 and periodical measurements of 15N distribution. Nitrogen derived from the atmosphere was mainly found in the aerial parts of the plants, and the stage of development exerted a strong influence on the initial 15N distribution (measured one week after incorporation). Until day 69 after sowing, leaf blades contained 47 to 57% of the fixed N. After that, reproductive structures attracted increasing proportions, 10 to 60% between days 69 and 92. Around day 82, stems and petioles stored up to 30% of the newly fixed N. During pod development and pod filling and until maturity, fixed N was remobilized from vegetative tissues and pod walls to seeds. These transfers first concerned the newly incorporated N, but at maturity 80 to 90% of the total was recovered in the seeds. The high mobility of N originating from the atmosphere as compared to that coming from the soil (vegetative tissues exported only 50% of their total N) seems to indicate that fixed N was at least partially integrated in a special pool. This was certainly the case at the later stage of N2 fixation, when a large portion of fixed N accumulated in the stems and petioles, probably in the form of storage compounds such as ureides for later transfer to the developing seeds. Further research is needed in order to investigate the nature and role of this pool in the nitrogen nutrition of soybeans.  相似文献   

4.
The (C2H4+ H2(C2H2))/15N2 ratios of 15 clover- Rhizobium symbionts. soybean, and black medick symbionts were measured. Relative efficiency based on the C2H4 production and on 15N2 incorporation were compared, and in most symbionts there was little difference between the two measures of relative efficiency. Total measurable electron flux through nitrogenase during acetylene reduction and 15N2 incorporation were nearly equal for most symbionts studied. The relative efficiency and the (C2H4+ H2(C2H2))/15N2 ratio showed an inverse correlation. Use of this ratio appears preferable to use of the ratio of C2H2 reduction/N2 reduction. Some evolution of H2 was observed in the presence of C2H2.  相似文献   

5.
Mosses have been used as biomonitors of atmospheric pollution for some years, but few studies have been carried out on the effect of NOx emissions from traffic on moss tissue N. Eight species of moss (102 samples) growing on walls or roofs next to roads exposed to different traffic densities were collected from urban and rural sites in the UK. The shoots were sampled for total N, their stable isotope 15N/14N content (δ15N) and heavy metal content (Pb, Zn). There was a lack of correlation between tissue total N and traffic exposure, but a very good correlation between traffic exposure and tissue δ15N. Plants collected near motorways or busy urban roads had δ15N values ranging between +6 and −1‰, while in rural areas with hardly any traffic these ranged from −2 to −12‰. In a separate survey of mosses, the average δ15N of shoots from busy roadsides in London was +3.66‰, whereas from samples collected from farm buildings near poultry or cattle pens it was −7.8‰. This indicates that the two main atmospheric N sources, NOx and NHx, have different δ15N signatures, the former tending to be positive and the latter negative. Tissue concentrations of both Pb and Zn show a strong positive correlation with traffic exposure, with Zn in particular being greater than Pb. The results are discussed with regard to the use of moss tissue Zn as a means for monitoring or mapping pollution from vehicles, and of δ15N as an aid to distinguish between urban (NOx) and rural (NHx) forms of N pollution.  相似文献   

6.
Uptake of [15N]-ammonia was more sensitive to UV-B exposure than the total 14CO2 fixation rate of Lithodesmium variabile Takano. Short-term UV-B radiation (15 min) had practically no effect on the kinetics of [15N]-ammonia, whereas there was an effect on [14C]-bicarbonate uptake rate. A significant reduction was found after 30 and 60 min UV-B stress. The time course of photosynthetic uptake of 15NH4Cl at several wavelengths was markedly depressed at shorter wavelengths (irradiation with WG 280). A short-term (11 min) exposure to ultraviolet radiation had no influence on the [14C]-labeled photosynthetic products. However, the [15N]-label of several amino acids and the ratio of [15N]-glutamine to [15N]-glutamic acid varied after irradiation with different ultraviolet wavebands. The results are discussed with reference to UV damage to the key enzymes of nitrogen metabolism.  相似文献   

7.
In order to establish the relationship of nitrogen nutrition to root formation in vitro, efficiency of various forms of nitrogen supply was investigated in micropropagated shoots of Pinus pinaster. Nitrate (3.3 m M as N) containing medium supplemented with glutamine (2 m M as N) resulted in optimal rooting (control medium) whereas the rooting response was inhibited by the presence of glutamate (2 m M as N) in the medium. Study of 15NO3 behaviour in shoots cultured on the control medium showed a marked increase of 15N incorporation during the second half of a period of auxin treatment (total duration 16 days). The majority of the nitrogen incorporated from the medium was accumulated into protein in the rooting zone. Autoradiography of [3H]-thymidine in this reacting zone showed strong DNA synthesis at this time. Addition of glutamate both prevented the root formation process and resulted in a marked slowdown of nitrate uptake and decrease of protein synthesis.  相似文献   

8.
Red clover, Trifolium pratense L., is the dominant forage legume in Sweden and is usually harvested twice per year, once in June and once in August. Two 15N-based methods –15N isotopic dilution (ID) and 15N natural abundance (NA) – were used to study N2 fixation from spring until first harvest in late June, from first to second harvest in late August, and from second harvest until first frost in autumn in Umeå, Sweden. The material studied comprized three neighbouring fields carrying a first year ley, a second year ley and a third year ley. For the 15N ID method, small amounts of highly enriched 15N-nitrate were added to experimental plots. The non-legumes in the plots, essentially Phleum pratense L. together with Festuca pratensis L., served as reference plants for both the ID and 15N NA measurements. Dry matter, N and 15N were separately analysed in leaves (laminae), stems (including petioles), stubble and roots. The proportion of N derived from air (pNdfa) was then calculated for each plant part and for whole plants. Estimates of the proportion of N derived from N2 fixation (pNdfa) were always very high, usually ≥0.8. Generally, estimates of pNdfa obtained by the ID and NA methods were similar, but the ID method gave higher estimates of pNdfa than the NA method when the highest N2 fixation levels were recorded, at the August harvest. Regression analyses suggest that estimates of pNdfa in leaves could provide useful indications of pNdfa in shoots and whole T. pratense plants, thus avoiding the need for time-consuming root analyses.  相似文献   

9.
[15N]-depleted (NH4)2SO4 applied to the soil in 1985 resulted in residual labeling of about 16% of the storage nitrogen (N) pool of mature walnut ( Juglans regia L. cv. Serr) trees in 1987. Application of [15N]-depleted (NH4)2SO4 fertilizer to a different set of mature walnut trees in 1987 allowed monitoring of the kinetics and utilization of N from current year uptake in 1987 and resulted in >20% labeling of fruit N following completion of leaf expansion. Redistribution of storage N to the new growth predominated during the spring flush of growth although N derived from the soil during current-year uptake contributed increasingly during leaf expansion. Labeled N from current year uptake accumulated preferentially in the leaves as compared with reproductive organs during leaf expansion but subsequent to leaf expansion, fruit were more highly labeled with N derived from current-year uptake than leaves. Pistillate flower abortion was coincident with an apparent competition for N among developing vegetative and reproductive organs and preceded the period of significant N contribution from current-year uptake.  相似文献   

10.
Effects of Ketone Bodies on Astrocyte Amino Acid Metabolism   总被引:5,自引:1,他引:4  
Abstract: The effects of acetoacetate and 3-hydroxybutyrate on glial amino acid metabolism were studied in primary cultures of astrocytes. The exchange of nitrogen among amino acids was measured with 15N as a metabolic probe and gas chromatography-mass spectrometry as a tool with which to quantify isotope abundance. Addition of either acetoacetate or 3-hydroxybutyrate (5 m M ) to the incubation medium did not alter the initial rate of appearance of [15N]glutamate in the glia, but it did inhibit transamination of glutamate to [15N]aspartate. Addition of acetoacetate also inhibited formation of [2-15N]glutamine, but 3-hydroxybutyrate had a stimulatory effect. The presence in the medium of sodium acetate (5 m M ) was also associated with diminished production of [15N]aspartate and [2-15N]glutamine with [15N]glutamate as precursor. Studies with [2-15N]glutamine as precursor indicated that treatment of the astrocytes with ketone bodies did not alter flux through the glutaminase pathway. Nor did the presence of the ketone bodies reduce significantly the flux of nitrogen from [15N]GABA to [2-15N]glutamine when the former species served as a metabolic tracer. The concentration of internal citrate increased in the presence of acetoacetate, 3-hydroxybutyrate, and acetate. Studies with purified sheep brain glutamine synthetase showed that citrate inhibited this enzyme. These findings are considered in terms of the known anticonvulsant effect of a ketogenic diet.  相似文献   

11.
Assimilation of N by heterotrophic soil microbial biomass is associated with decomposition of organic matter in the soil. The form of N assimilated can be either low molecular weight organic N released from the breakdown of organic matter (direct assimilation), or NH+4 and NO3 from the soil inorganic N pool, into which mineralized organic N is released (mineralization immobilization turnover). The kinetics of C and N turnover in soil is quantifiable by means of computer simulation models. NCSOIL was constructed to represent the two assimilation schemes. The rate of N assimilation depends on the rate of C assimilation and microbial C/N ratio, thereby rendering it independent of the assimilation scheme. However, if any of the N forms is labeled, a different amount of labeled N assimilation will be simulated by the different schemes. Experimental data on inorganic N and 15N and on organic 15N dynamics in soils incubated with 15N added as NH+4 or organic N were compared with data simulated by different model schemes. Direct assimilation could not account for the amount of 15N assimilated in any of the experimental treatments. The best fit of the model to experimental data was obtained for the mineralization immobilization turnover scheme when both NH+4 and NO3 were assimilated, in proportion to their concentration in the soil.  相似文献   

12.
Diet-tissue isotopic fractionation of carbon (C) and nitrogen (N) isotopes in short- and longer-term diet integrators of diet ( i. e. , blood serum and red cells), that involve non-invasive sampling techniques was examined using three species of phocid seals (harbor seals, gray seals, and harp seals) fed a known diet. Variability in diet-tissue fractionation values within and between species was also scrutinized to determine the legitimacy of using values obtained from one species to explore trophic positions and diets of other related species. All captive seals raised on a constant diet had tissues enriched in 13C and 15N relative to their diet. Diet-tissue isotopic fractionation values were generally consistent among conspecifics and among phocid species for a given tissue. Trophic isotopic enrichment in 13C was significantly higher in red blood cells (+1.5%±) than in blood serum (+0.8%±), whereas the reverse was observed for nitrogen isotopes (+1.7%± in red cells vs . +3.1%± in serum). However, 13C-depleted lipids were not extracted from blood tissues in this study. This results in a downward bias in the diet-tissue fractionation factors for carbon for both red cells and blood serum, particularly the latter because of their significantly higher lipid contents ( ± SD = 14.6 ± 2.3%; n = 20; red blood cells 3.8 ± 0.9%±; n = 50, muscle 7.7 ± 2.0; n = 21) in marine mammals.  相似文献   

13.
Fixation of molecular nitrogen by Methanosarcina barkeri   总被引:1,自引:0,他引:1  
Abstract Methanosarcina barkeri cells were observed in ammonia-free anaerobic acetate enrichments for sulfate-reducing bacteria. The capacity of Methanosarcina to grow diazotrophically was proved with a pure culture in mineral media with methanol. The cell yields with N2 or NH4+ ions as nitrogen source were 2.2 g and 6.1 g dry weight, respectively, per mol of methanol. Growth experiments with 15N2 revealed that 84% of the cell nitrogen was derived from N2. Acetylene was highly toxic to Methanosarcina and only reduced at concentrations lower than 100 μmol dissolved per 1 of medium. Assimilation of N2 and reduction of acetylene were inhibited by NH4+ ions. The experiments show that N2 fixation occurs not only in eubacteria but also in archaebacteria. The ecological significance of diazotrophic growth of Methanosarcina is discussed.  相似文献   

14.
A method based on simultaneous short-term exposure to 14CO2 and 15N2 is described for studying nitrogen fixation and distribution in legumes relative to carbon assimilation and use. Equipment designed to accomodate experiments under natural conditions with very little disturbance of the N2 fixing association is used. It permits continuous measurement and regulation of variables such as air temperature, humidity and CO2 concentration as well as soil aeration. Measurements of distribution and use of assimilates, respiration of nodulated roots, quantitative N2 fixation and the distribution and fate of fixed N as a function of time lead to a precise estimation of C and N budgets for each labelling period. When experiments are done at several phenological stages they give a new insight into the complex C and N interrelations in legume symbiosis.
A series of trials throughout the growth period of Glycine max (L.) Merr. cv. Hodgson demonstrated the sensitivity of the method. The development of the plants from vegetative to reproductive stages was accompanied by a complete change in the distribution patterns of current assimilates and products of nitrogen fixation. Maximum sink strength moved from the leaves to the pods and seeds which ended up receiving 70% of the incoming C and 35% of the fixed N. The fact that up to 85% of fixed N in the plants was in the reproductive organs at maturity can be accounted for by remobilisation from vegetative parts.
The respiration of nodulated roots utilized 33% of carbon translocated to below-ground plant parts before nitrogen fixation started, but as much as 50% during the period of optimal fixation. The advantages and limitations of the isotopic method described are critically discussed as a prelude to future investigations.  相似文献   

15.
Ratios of nitrogen (N) isotopes in leaves could elucidate underlying patterns of N cycling across ecological gradients. To better understand global-scale patterns of N cycling, we compiled data on foliar N isotope ratios (δ15N), foliar N concentrations, mycorrhizal type and climate for over 11 000 plants worldwide. Arbuscular mycorrhizal, ectomycorrhizal, and ericoid mycorrhizal plants were depleted in foliar δ15N by 2‰, 3.2‰, 5.9‰, respectively, relative to nonmycorrhizal plants. Foliar δ15N increased with decreasing mean annual precipitation and with increasing mean annual temperature (MAT) across sites with MAT ≥ −0.5°C, but was invariant with MAT across sites with MAT < −0.5°C. In independent landscape-level to regional-level studies, foliar δ15N increased with increasing N availability; at the global scale, foliar δ15N increased with increasing foliar N concentrations and decreasing foliar phosphorus (P) concentrations. Together, these results suggest that warm, dry ecosystems have the highest N availability, while plants with high N concentrations, on average, occupy sites with higher N availability than plants with low N concentrations. Global-scale comparisons of other components of the N cycle are still required for better mechanistic understanding of the determinants of variation in foliar δ15N and ultimately global patterns in N cycling.  相似文献   

16.
Seasonal patterns of N translocation in the xylem sap of Betula pendula were studied, to determine whether specific amino acids were recovered in spring as a consequence of N remobilization. Seedlings were grown in sand culture and provided with 15NH415NO3 (at 2·2 atom percent excess) for one growing season. The following winter dormant trees were transplanted into fresh sand and given N at natural abundance thereafter. Destructive harvests were taken during bud burst and leaf growth to determine the pattern of 15N remobilization and N uptake, along with isolation of xylem sap for analysis of their amino acid profiles and 15N enrichment by GC-MS. 15N remobilization occurred immediately following bud burst, while N derived from root uptake did not appear in the leaves until 12 d after bud burst. During N remobilization there was a 10-fold increase in the concentration of N in the xylem sap, due predominantly to increases in citrulline and glutamine. The 15N enrichment of these two amino acids demonstrated the increase in their concentration in the xylem sap following bud burst was due to N remobilization. These results are discussed in relation to measuring N remobilization and storage capacity of trees in the field.  相似文献   

17.
We examined the influence of temperature and management practices on the nitrogen (N) cycling of turfgrass, the largest irrigated crop in the United States. We measured nitrous oxide (N2O) fluxes, and plant and soil N content and isotopic composition with a manipulative experiment of temperature and fertilizer application. Infrared lamps were used to increase surface temperature by 3.5±1.3 °C on average and control and heated plots were split into high and low fertilizer treatments. The N2O fluxes increased following fertilizer application and were also directly related to soil moisture. There was a positive effect of warming on N2O fluxes. Soils in the heated plots were enriched in nitrogen isotope ratio ( δ 15N) relative to control plots, consistent with greater gaseous losses of N. For all treatments, C4 plant C/N ratio was negatively correlated with plant δ 15N, suggesting that low leaf N was associated with the use of isotopically depleted N sources such as mineralized organic matter. A significant and unexpected result was a large, rapid increase in the proportion of C4 plants in the heated plots relative to control plots, as measured by the carbon isotope ratio ( δ 13C) of total harvested aboveground biomass. The C4 plant biomass was dominated by crabgrass, a common weed in C3 fescue lawns. Our results suggest that an increase in temperature caused by climate change as well as the urban heat island effect may result in increases in N2O emissions from fertilized urban lawns. In addition, warming may exacerbate weed invasions, which may require more intensive management, e.g. herbicide application, to manage species composition.  相似文献   

18.
Changes in nodule growth and activity and in the concentrations of soluble N compounds in nodules, leaves and xylem sap under conditions of altered N nutrition in the actinorhizal plant Myrica gale L. are reported. Altering the N nutrition of symbiotic plants may alter the internal regulation of combined N which in turn may regulate nodule growth and activity. Flushing nodules daily with 100% O2 caused a decline in amide concentration and an increase in nodule growth although plants had recovered some nitrogenase activity within 4 h of exposure to O2. Samples of nodules, leaves and xylem sap were derivatized and amino acids identified and quantified using either reverse phase high performance liquid chromatography or gas chromatography-mass spectrometry in single ion monitoring mode. The ratio of asparagine in the nodules to that in the xylem was much higher in plants fed N (6.7 for NH+4-fed and 8.3 for NO3-fed plants) than for N2-fixing plants (2.5). Significant amounts of 15N added as 15NH+4 or 15NO3 accumulated in nodules following accumulation in the shoot which is consistent with the translocation of N to the nodules via the phloem. The uptake of 15NH+4 led to the synthesis and subsequent translocation of glutamine in the xylem sap. These results are discussed in terms of the feedback mechanisms that may regulate nitrogen fixation in Myrica root nodules.  相似文献   

19.
This study investigated the impact of lipid extraction, CaCO3 removal and of both treatments combined on fish tissue δ13C, δ15N and C:N ratio. Furthermore, the suitability of empirical δ13C lipid normalization and correction models was examined. δ15N was affected by lipid extraction (increase of up to 1·65‰) and by the combination of both treatments, while acidification alone showed no effect. The observed shift in δ15N represents a significant bias in trophic level estimates, i.e. lipid-extracted samples are not suitable for δ15N analysis. C:N and δ13C were significantly affected by lipid extraction, proportional to initial tissue lipid content. For both variables, rates of change with lipid content (ΔC:N and Δδ13C) were species specific. All tested lipid normalization and correction models produced biased estimates of fish tissue δ13C, probably due to a non-representative database and incorrect assumptions and generalizations the models were based on. Improved models need a priori more extensive and detailed studies of the relationships between lipid content, C:N and δ13C, as well as of the underlying biochemical processes.  相似文献   

20.
Nitrate assimilation and nitrogen circulation in Austrian pine   总被引:2,自引:0,他引:2  
Nitrate uptake, reduction and translocation were examined in 5-week-old Austrian pines ( Pinus nigra Arnold var. nigricans Host.) during exposure to 5 m M NaNO3. The rate of nitrate uptake was linear during the 7 h light period. 15N-NO3 was detected in all parts of the pine except in the needles. By the 7th hour, 43% of the absorbed nitrate had been reduced, and this increased to 64% by the 24th hour. The major part of the total reduction occurred in the roots at this growth stage. Accumulation of 15N in reduced soluble and insoluble fractions was more prevalent in roots than in shoots. In the needles, the translocated nitrogen was mainly incorporated into the insoluble fraction. It is likely that most of the nitrogen from nitrate was transported from the roots to the aerial organs as organic nitrogen; however part of the upward nitrogen flux took place as nitrate ions.
An experiment in which an exposure for 24 h to 5 m M Na15NO3 was followed by 13 days exposure to Na14NO3 (pulse chase experiment) revealed a half time of about 1 day for depletion of root nitrate. A large part of this depletion was due to the loss of 15N-NO3 to the nutrient solution. The remaining pool of 15N-nitrate was partitioned between a metabolically inactive and an active pool. During the chase period, the simultaneous decrease of 15N-incorporation in the soluble N fraction and increase in the insoluble N fraction in different pine parts, particularly in the needles, suggested that protein synthesis occurred mostly in young tissues of the shoot and was the major sink of the newly absorbed 15N-NO3.  相似文献   

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