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1.
We compared influxes and internal transport in soybean plants ( Glycine max cv. Kingsoy) of labelled N from external solutions where either ammonium or nitrate was labelled with the stable isotope 15N and the radioactive isotope 13N. The objective was to see whether mass spectrometric determinations of tissue 15N content were sufficiently sensitive to measure influxes accurately over short time periods. Our findings were as follows. (1) There was a close quantitative correspondence between estimates of N influx of individual plants using 15N or 13N measurements with either NO 3/? or NH 4+ at 4 or 2 mol ?3, respectively in the external solution. (2) Transport to the shoot of N from NO 3 absorbed over a 5–15 min period could be monitored when the external NO 3? concentration ranged from 0–05 to 4 mol m ?3. NH 4+ as the N source labelled shoot tissue more slowly, and estimates of the transport between root and shoot could be made only with 13N. (3) Influx of NO 3? into root tissue could be measured by 15N enrichment after 5–10 min at concentrations approaching the probable KM of the high-affinity transport system. (4) There was some indication of isotope discrimination, especially with respect to the movement of labelled N to the shoot, when NO 3? is the N source. For many purposes, 15N tracing can be used satisfactorily to estimate influxes of both NO 3? and NH 4+ in soybean roots. Use of the short-lived radio nuclide 13N remains the method of choice for more refined measurements of internal distribution and assimilation. 相似文献
2.
We assessed the effects of doubling atmospheric CO 2 concentration, [CO 2], on C and N allocation within pedunculate oak plants ( Quercus robur L.) grown in containers under optimal water supply. A short-term dual 13CO 2 and 15NO 3? labelling experiment was carried out when the plants had formed their third growing flush. The 22-week exposure to 700 μl l ?1 [CO 2] stimulated plant growth and biomass accumulation (+53% as compared with the 350 μl l ?1 [CO 2] treatment) but decreased the root/shoot biomass ratio (-23%) and specific leaf area (-18%). Moreover, there was an increase in net CO 2 assimilation rate (+37% on a leaf dry weight basis; +71% on a leaf area basis), and a decrease in both above- and below-ground CO 2 respiration rates (-32 and -26%, respectively, on a dry mass basis) under elevated [CO 2]. 13C acquisition, expressed on a plant mass basis or on a plant leaf area basis, was also markedly stimulated under elevated [CO 2] both after the 12-h 13CO 2 pulse phase and after the 60-h chase phase. Plant N content was increased under elevated CO 2 (+36%), but not enough to compensate for the increase in plant C content (+53%). Thus, the plant C/N ratio was increased (+13%) and plant N concentration was decreased (-11%). There was no effect of elevated [CO 2] on fine root-specific 15N uptake (amount of recently assimilated 15N per unit fine root dry mass), suggesting that modifications of plant N pools were merely linked to root size and not to root function. N concentration was decreased in the leaves of the first and second growing flushes and in the coarse roots, whereas it was unaffected by [CO 2] in the stem and in the actively growing organs (fine roots and leaves of the third growth flush). Furthermore, leaf N content per unit area was unaffected by [CO 2]. These results are consistent with the short-term optimization of N distribution within the plants with respect to growth and photosynthesis. Such an optimization might be achieved at the expense of the N pools in storage compartments (coarse roots, leaves of the first and second growth flushes). After the 60-h 13C chase phase, leaves of the first and second growth flushes were almost completely depleted in recent 13C under ambient [CO 2], whereas these leaves retained important amounts of recently assimilated 13C (carbohydrate reserves?) under elevated [CO 2]. 相似文献
3.
Approximately 26.0% of the [ 15N] glutamate and [alpha 15N] glutamine formed in organotypic cerebellar explants was derived from [ 15N] leucine. Approximately 14.0% of the 15NH 3 and [amide 15N] glutamine synthesized came from leucine nitrogen. Another 4.0% of the alpha nitrogen of both glutamate and glutamine was derived from [ 15N] valine. These results suggest that branched-chain amino acids, particularly leucine, may be important for the synthesis of glutamic acid by the brain. 相似文献
4.
Studies of the variation in δ 15N values for plants from a fire-prone Banksia woodland in South West Australia showed that pioneer herbaceous, non-mycorrhizal species which were active in nitrate reduction and storage, had the highest values (1.81%c). A detailed study of one such species Ptilotus polystachus demonstrated a close correspondence between the δ 15N values of soil nitrate, xylem nitrate and leaf total nitrogen, suggesting an exclusive reliance on nitrate ions as nitrogen source. These pioneer species also showed a preponderance of the chloroplastic isoform of glutamine synthetase while woody species generally had higher activity associated with the cytosolic isoform. The group comprising monocotyledonous hemicryptophytes and geophytes contained species with slightly positive δ 15N values and moderately active in nitrate reduction and storage. Nitrogen-fixing species had the lowest δ 15N values (–0.36‰), irrespective of their apparent utilisation of nitrate. However, woody resprouter species which had low levels of nitrate reduction and storage had δ 15N values which fell within the range of values obtained for the miscellaneous assemblage of N 2-fixing species. Consequently, 15N abundance values failed to distinguish N 2 fixing from non-fixing woody species, and therefore, could not be used in the ecosystem to determine the dependence of putative nitrogen fixing species on N 2 fixation. The study demonstrated complex patterns of nitrogen utilization in the ecosystem in which exploitation of different nitrogen resources related to plant life form and the physiological attributes of nitrogen assimilation by component species. 相似文献
5.
Understanding ecosystem carbon (C) and nitrogen (N) cycling under global change requires experiments maintaining natural interactions among soil structure, soil communities, nutrient availability, and plant growth. In model Douglas-fir ecosystems maintained for five growing seasons, elevated temperature and carbon dioxide (CO 2) increased photosynthesis and increased C storage belowground but not aboveground. We hypothesized that interactions between N cycling and C fluxes through two main groups of microbes, mycorrhizal fungi (symbiotic with plants) and saprotrophic fungi (free-living), mediated ecosystem C storage. To quantify proportions of mycorrhizal and saprotrophic fungi, we measured stable isotopes in fungivorous microarthropods that efficiently censused the fungal community. Fungivorous microarthropods consumed on average 35% mycorrhizal fungi and 65% saprotrophic fungi. Elevated temperature decreased C flux through mycorrhizal fungi by 7%, whereas elevated CO 2 increased it by 4%. The dietary proportion of mycorrhizal fungi correlated across treatments with total plant biomass ( n= 4, r2= 0.96, P= 0.021), but not with root biomass. This suggests that belowground allocation increased with increasing plant biomass, but that mycorrhizal fungi were stronger sinks for recent photosynthate than roots. Low N content of needles (0.8–1.1%) and A horizon soil (0.11%) coupled with high C : N ratios of A horizon soil (25–26) and litter (36–48) indicated severe N limitation. Elevated temperature treatments increased the saprotrophic decomposition of litter and lowered litter C : N ratios. Because of low N availability of this litter, its decomposition presumably increased N immobilization belowground, thereby restricting soil N availability for both mycorrhizal fungi and plant growth. Although increased photosynthesis with elevated CO 2 increased allocation of C to ectomycorrhizal fungi, it did not benefit plant N status. Most N for plants and soil storage was derived from litter decomposition. N sequestration by mycorrhizal fungi and limited N release during litter decomposition by saprotrophic fungi restricted N supply to plants, thereby constraining plant growth response to the different treatments. 相似文献
6.
Nitrogen-starved cells of Frankia strain HFPArl3 incorporated [ 13N]-labeled ammonium into glutamine serine (glutamate, alanine, aspartate), after five-minute radioisotope exposures. High initial endogenous pools of glutamate were reduced, while total glutamine increased, during short term NH
inf4
sup+
incubation. Preincubation of cells in methionine sulfoximine (MSX) resulted in [ 13N]glutamine reduced by more than 80%, while [ 13N]glutamate and [ 13N]alanine levels increased. The results suggest that glutamine synthetase is the primary enzyme of ammonium assimilation, and that glutamate dehydrogenase and alanine dehydrogenase may also function in ammonium assimilation at low levels. Efflux of [ 13N]serine and lesser amounts of [ 13N]glutamine was detected from the Frankia cells. The identity of both Ser and Gln in the extracellular compartment was confirmed with gas chromatography/mass spectrometry. Serine efflux may be of significance in nitrogen transfer in Frankia.Abbreviations Pthr
phosphothreonine
- Aad
-amino-adipate
- MSX
methionine sulfoximine 相似文献
7.
15N kinetic labeling studies were performed on seedlings of Hordeum vulgare L. var. Golden Promise growing under steady state conditions. Patterns of label incorporation in the pools of nitrogen compounds of roots fed [ 15N]ammonium were compared with computer-simulated labeling curves. The data were found to be quantitatively consistent with a three-compartment model in which ammonium is assimilated solely into the amide-N of glutamine. Labeling data from roots fed [ 15N]nitrate were also found to be at least qualitatively consistent with the assimilation of ammonia into glutamine. Methionine sulfoximine almost completely blocked the incorporation of 15N label into the amino acid pools of barley roots fed [ 15N]nitrate. These observations suggest that ammonia assimilation occurs solely via the glutamine synthetase/glutamate synthase pathway in both nitrate- and ammonia-grown barley roots. 相似文献
8.
The distribution of NO 3? reduction between roots and shoots was studied in hydro-ponically-grown peach-tree seedlings ( Prunus persica L.) during recovery from N starvation. Uptake, translocation and reduction of NO 3?, together with transport through xylem and phloem of the newly reduced N were estimated, using 15N labellings, in intact plants supplied for 90 h with 0.5 m M NH 4+ and 0.5, 1.5 or 10 m M NO 3?. Xylem transport of NO 3? was further investigated by xylem sap analysis in a similar experiment. The roots were the main site of NO 3? reduction at all 3 levels of NO 3? nutrition. However, the contribution of the shoots to the whole plant NO 3? reduction increased with increasing external NO 3? availability. This contribution was estimated to be 20, 23 and 42% of the total assimilation at 0.5, 1.5 and 10 m M NO 3?, respectively. Both 15N results and xylem sap analysis confirmed that this trend was due to an enhancement of NO 3? translocation from roots to shoots. It is proposed that the lack of NO 3? export to the shoots at low NO 3? uptake rate resulted from a competition between NO 3? reduction in the root epidermis/cortex and NO 3? diffusion to the stele. On the other hand, net xylem transport of newly reduced N was very efficient since ca 70% of the amino acids synthesized in the roots were translocated to the shoots, regardless of the level of NO 3? nutrition. This net xylem transport by far exceeded the net downward phloem transport of the reduced N assimilated in shoots. As a consequence, the reduced N resulting from NO 3? assimilation, principally occurring in the roots, was mainly incorporated in the shoots. 相似文献
9.
GLU1 encodes the major ferredoxin-dependent glutamate synthase (Fd-GOGAT, EC 1.4.7.1) in Arabidopsis thaliana (ecotype Columbia). With the aim of providing clues on the role of Fd-GOGAT, we analyzed the expression of Fd-GOGAT in tobacco
( Nicotiana tabacum L. cv. Xanthi). The 5′ flanking element of GLU1 directed the expression of the uidA reporter gene in the palisade and spongy parenchyma of mesophyll, in the phloem cells of vascular tissue and in the roots
of tobacco. White light, red light or sucrose induced GUS expression in the dark-grown seedlings in a pattern similar to the GLU1 mRNA accumulation in Arabidopsis. The levels of GLU2 mRNA encoding the second Fd-GOGAT and NADH-glutamate synthase (NADH-GOGAT, EC 1.4.1.14) were not affected by light. Both
in the light and in darkness, 15NH 4+ was incorporated into [5− 15N]glutamine and [2− 15N]glutamate by glutamine synthetase (GS, EC 6.3.1.2) and Fd-GOGAT in leaf disks of transgenic tobacco expressing antisense
Fd-GOGAT mRNA and in wild-type tobacco. In the light, low level of Fd-glutamate synthase limited the [2− 15N]glutamate synthesis in transgenic leaf disks. The efficient dark labeling of [2− 15N]glutamate in the antisense transgenic tobacco leaves indicates that the remaining Fd-GOGAT (15–20% of the wild-type activity)
was not the main limiting factor in the dark ammonium assimilation. The antisense tobacco under high CO 2 contained glutamine, glutamate, asparagine and aspartate as the bulk of the nitrogen carriers in leaves (62.5%), roots (69.9%)
and phloem exudates (53.2%). The levels of glutamate, asparagine and aspartate in the transgenic phloem exudates were similar
to the wild-type levels while the glutamine level increased. The proportion of these amino acids remained unchanged in the
roots of the transgenic plants. Expression of GLU1 in mesophyll cells implies that Fd-GOGAT assimilates photorespiratory and primary ammonium. GLU1 expression in vascular cells indicates that Fd-GOGAT provides amino acids for nitrogen translocation.
The nucleotide sequence data of the GLU1 gene reported in the present study is available from GenBank with the following accession number: AY189525 相似文献
10.
Abstract: The role of voltage-sensitive Ca 2+ channels in mediating Ca 2+ influx during ischemia was investigated in NG108-15 cells, a neuronal cell line that does not express glutamate-sensitive receptor-mediated Ca 2+ channels. Concurrent 31P/ 19F and 23Na double-quantum filtered (DQF) NMR spectra were used to monitor cellular energy status, intracellular [Ca 2+] ([Ca 2+] i), and intracellular Na + content in cells loaded with the calcium indicator 1,2-bis-(2-amino-5-fluorophenoxy)ethane- N,N,N′,N′-tetraacetic acid (5FBAPTA) during ischemia and reperfusion. Cells loaded with 5FBAPTA were indistinguishable from unloaded cells except for small immediate decreases in levels of phosphocreatine (PCr) and ATP. Ischemia induced a steady decrease in intracellular pH and PCr and ATP levels, and a steady increase in intracellular Na + content; however, a substantial increase in [Ca 2+] i (about threefold) was seen only following marked impairment of cellular energy status, when PCr was undetectable and ATP content was reduced to 55% of control levels. A depolarization-induced increase in [Ca 2+] i could be completely blocked by 1 µ M nifedipine, whereas up to 20 µ M nifedipine had no effect on the increase in [Ca 2+] i seen during ischemia. These data demonstrate that voltage-gated Ca 2+ channels do not mediate significant Ca 2+ flux during ischemia in this cell line and suggest an important role for Ca 2+i stores, the Na +/Ca 2+ antiporter, or other processes linked to cellular energy status in the increase in cytosolic Ca 2+ level during ischemia. 相似文献
11.
Anabaena azollae was isolated from Azolla caroliniana by the gentle roller method and differential centrifugation. Incubation of such Anabaena preparations for 10 min with [ 13N]N 2 resulted in the formation of four radioactive compounds; ammonium, glutamine, glutamate and alanine. Ammonium accounted for 66% of the total radioactivity recovered and 58% of the ammonium was in an extracellular fraction. Since essentially no extracellular 13N-labeled organic compounds were found, it appears that ammonium is the compound most probably made available to Azolla during dinitrogen-dependent growth of the association.The kinetics of incorporation of exogenous 13NH
4
+
into glutamine and glutamate were characteristic of a precursor (glutamine)-product (glutamate) relationship and consistent with assimilation by the glutamine synthetase-glutamate synthase pathway. The results of experiments using the glutamine synthetase inhibitor, methionine sulfoximine, the glutamate synthase inhibitor, diazo-oxonorleucine, and increasing the ammonium concentration to greater than 1 mM, provided evidence for assimilation primarily by the glutamine synthetase-glutamate synthase pathway with little or no contribution from biosynthetic glutamate dehydrogenase.While showing that N 2 fixation and NH
4
+
assimilation were not tightly coupled metabolic processes in symbiotic Anabaena, these results reflect a composite picture and do not indicate the extent to which ammonium assimilatory enzymes might be regulated in filaments associated with specific stages in the Azolla-Anabaena developmental profile.Non-standard abbreviations DON
6-Diazo-5-oxo- l-norleucine
- GDH
glutamate dehydrogenase
- GOGAT
glutamate synthase
- GS
glutamine synthetase
- MSX
l-methionine- Dl-sulfoximine 相似文献
12.
The initial product of fixation of [ 13N]N 2 by pure cultures of the reconstituted symbiotic association between Anthoceros punctatus L. and Nostoc sp. strain ac 7801 was ammonium; it accounted for 75% of the total radioactivity recovered in methanolic extracts after 0.5 min and 14% after 10 min of incubation. Glutamine and glutamate were the primary organic products synthesized from [ 13N]N 2 after incubation times of 0.5–10 min. The kinetics of labeling of these two amino acids were characteristic of a precursor (glutamine) and product (glutamate) relationship. Results of inhibition experiments with methionine sulfoximine (MSX) and diazo-oxonorleucine were also consistent with the assimilation of N 2-derived NH
4
+
by Anthoceros-Nostoc through the sequential activities of glutamine synthetase (EC 6.3.1.2) and glutamate synthase (EC 1.4.7.1), with little or no assimilation by glutamate dehydrogenase (EC 1.3.1.3). Isolated symbiotic Nostoc assimilated exogenous 13NH
4
+
into glutamine and glutamate and their formation was inhibited by MSX, indicating operation of the glutamine synthetase-glutamate synthase (GS-GOGAT) pathway: However, relative to free-living cultures, isolated symbiotic Nostoc assimilated 80% less exogenous ammonium into glutamine and glutamate, implying that symbiotic Nostoc could assimilate only a fraction of N 2-derived NH
4
+
. This implication was tested by using Anthoceros associations reconstituted with wild-type or MSX-resistant strains of Nostoc incubated with [ 13N]N 2 in the presence of MSX. The results of these experiments indicated that, in situ, symbiotic Nostoc assimilated about 10% of the N 2-derived NH
4
+
and that NH
4
+
was made available to Anthoceros tissue where it was apparently assimilated by the GS-GOGAT pathway. Since less than 1% of the fixed N 2 was lost to the suspension medium, it appears that transfer of NH
4
+
from symbiont to host tissue was very efficient in this extracellular symbiotic association.Abbreviations DON
6-diazo-5-oxo- l-norleucine
- GDH
glutamate dehydrogenase
- GOGAT
glutamate synthase
- GS
glutamine synthetase
- MSX
l-methionine- dl-sulfoximine 相似文献
13.
The mean annual rainfall in southern Africa is found to explain over half of the observed variance in the stable nitrogen (N) isotopic signatures of C 3 vegetation in southern Africa ( r2=0.54, P<0.01). The inverse relationship between the stable N isotopic signatures of foliar samples from C 3 vegetation and long‐term southern African rainfall is found on a scale larger than previously observed. A modest relationship is found between stable carbon (C) isotopic signatures of C 3 vegetation and rainfall across the region ( r2=0.20, P<0.01). No such relationship is found between stable C and N isotopic signatures of C 4 vegetation and rainfall. The explanation of the relationship between 15N in C 3 vegetation and the mean annual rainfall presented here is that nutrient availability varies inversely with water availability. This suggests that water‐limited systems in southern Africa are more open in terms of nutrient cycling and therefore the resulting natural abundance of foliar 15N in these systems is enriched. The use of this relationship may be of value to those researchers modeling both the dynamics of vegetation and biogeochemistry across this region. The use of the isotopic enrichment in C 3 vegetation as a function of rainfall may provide an insight into nutrient cycling across the semi‐arid and arid regions of southern Africa. This finding has implications for the study of global change, especially as it relates to vegetation responses to changing regional rainfall regimes over time. 相似文献
14.
The extent to which increased atmospheric nitrogen (N) deposition will drive changes in plant productivity and species composition over the next century will depend on how other influential global change factors, such as climate warming, affect the N retention of ecosystems. We examined the interactive effects of simulated climate warming and N deposition on the recoveries of 15N‐labeled ammonium and 15N‐labeled nitrate tracers added as a pulse to grass‐dominated, temperate old‐field plots at spring thaw. In addition to the year‐round warming treatment, a winter‐only warming treatment was applied to a set of plots to explore the contribution of this component of climate warming to the overall warming effect. By the end of the plant growing season, there was approximately twice as much 15N enrichment in the plant roots and bulk soil from 15NH 4+‐addition plots than from 15NO 3?‐addition plots, but there were no effects of warming or N fertilization on 15N recovery. Over winter, approximately half of the excess 15N present in plant shoots was lost, which corresponded with large 15N losses from bulk soil in N fertilized plots and large 15N increases in bulk soil in nonfertilized plots. By the next spring, there was decreased 15N recovery in plants in response to N fertilization, which was largely offset by increases in plant 15N recovery in response to year‐round warming. However, 15N retention in bulk soil, where the major part of the 15N label was recovered, was approximately 40% higher in nonfertilized plots than in N fertilized plots. Overall, our results indicate that climate warming increases plant N sequestration in this system but this effect is overwhelmed by the overall effect of nitrogen deposition on ecosystem N losses. 相似文献
15.
Inorganic nitrogen metabolism in the obligate anaerobic thermophiles Chlostridium thermosaccharolyticum and Clostridium thermoautotrophicum differs in several respects. C. thermosaccharolyticum contains a nitrogenase as inferred from NH
4
+
repressible C 2H 2 reduction, a glutamine synthetase which is partially repressed by ammonium, very labile glutamate synthase activities with both NADH and NADPH, NADPH-dependent glutamate dehydrogenase, and NH
4
+
-dependent asparagine synthetase. C. thermoautotrophicum contains no nitrogenase, but glutamine synthetase, no glutamate synthase, no glutamate dehydrogenase, but a NADH-dependent alanine dehydrogenase and a NH
4
+
-dependent asparagine synthetase.Abbreviation GOGAT
glutamine-oxoglutarate amidotransferase amidotransferase (glutamate synthase) 相似文献
16.
Samples of recently produced shoot material collected in winter/spring from common plant species of mulga vegetation in eastern and Western Australia were assayed for 13C and 15N natural abundance. 13C analyses showed only three of the 88 test species to exhibit C 4 metabolism and only one of seven succulent species to be in CAM mode. Non-succulent winter ephemeral C 3 species showed significantly lower mean δ13C values (– 28·0‰) than corresponding C 3-type herbaceous perennials, woody shrubs or trees (– 26·9, – 25·7 and – 26·2‰, respectively), suggesting lower water stress and poorer water use efficiency in carbon acquisition by the former than latter groups of taxa. Corresponding values for δ15N of the above growth and life forms lay within the range 7·5–15·5‰. δ15N of soil NH 4+ (mean 19·6‰) at a soft mulga site in Western Australia was considerably higher than that of NO 3– (4·3‰). Shoot dry matter of Acacia spp. exhibited mean δ15N values (9·10 ± 0·6‰) identical to those of 37 companion non-N 2-fixing woody shrubs and trees (9·06 ± 0·5‰). These data, with no evidence of nodulation, suggested little or no input of fixed N 2 by the legumes in question. However, two acacias and two papilionoid legumes from a dune of wind-blown, heavily leached sand bordering a lake in mulga in Western Australia recorded δ15N values in the range 2·0–3·0‰ versus 6·4–10·7‰ for associated non-N 2-fixing taxa. These differences in δ15N, and prolific nodulation of the legumes, indicated symbiotic inputs of fixed N in this unusual situation. δ15N signals of lichens, termites, ants and grasshoppers from mulga of Western Australia provided evidence of N 2 fixation in certain termite colonies and by a cyanobacteria-containing species of lichen. Data are discussed in relation to earlier evidence of nitrophily and water availability constraints on nitrate utilization by mulga vegetation. 相似文献
17.
Ammonium assimilation was followed in N-starved mycelia from the ectomycorrhizal Ascomycete Cenococcum graniforme. The evaluation of free amino acid pool levels after the addition of 5 millimolar NH 4+ indicated that the absorbed ammonium was assimilated rapidly. Post-feeding nitrogen content of amino acids was very different from the initial values. After 8 hours of NH 4+ feeding, glutamine accounted for the largest percentage of free amino acid nitrogen (43%). The addition of 5 millimolar methionine sulfoximine (MSX) to NH 4+-fed mycelia caused an inhibition of glutamine accumulation with a corresponding increase in glutamate and alanine levels. Using 15N as a tracer, it was found that the greatest initial labeling was into glutamine and glutamate followed by aspartate, alanine, and ornithine. On inhibiting glutamine synthetase using MSX, 15N enrichment of glutamate, alanine, aspartate, and ornithine continued although labeling of glutamine was quite low. Moreover, the incorporation of 15N label in insoluble nitrogenous compounds was lower in the presence of MSX. From the composition of free amino acid pools, the 15N labeling pattern and effects of MSX, NH4+ assimilation in C. graniforme mycelia appears to proceed via glutamate dehydrogenase pathway. This study also demonstrates that glutamine synthesis is an important reaction of ammonia utilization. 相似文献
18.
Glasshouse experiments with Ricinus communis showed that the presence/absence of a VA mycorrhizal fungus ( Glomus clarum) changed the δ 15N value of the host by as much as 2‰ when the plants were given urea (released as NH 4+) as their only N-source. This small change in Δ 15N would create a large error in calculating sources of plant N. In particular, these results throw into doubt any models of N-cycling which assume that soil N can be treated as a single source. The correct N-source value for VAM-infected NH 4? -using plants may be the δ 15N of soil NH 4++ 2‰. Treatment effects were also found in the distribution of δ 15N and % N among plant organs. Plants with VAM had a lower N:P atom ratio and were larger in total biomass. Carbon discrimination (δ 13C) was greater in the VA-infected plants. The measured effects of VAM infection suggest that for some plants the fungus may be the primary site of N assimilation. A parallel experiment with Eucalyptus globulus and the ectomycorrhizal fungus Hydnangium carneum resulted in no significant differences in any of the variables measured for this host-fungus pair when the sole N-sources were inorganic (NO 3? and NH 4+ released from urea). Ectomycorrhizal fungi are diverse in their physiological behaviour, and these data should not be taken as being representative of the whole group. More work is required with other types of mycorrhiza and more complex sources of N. Future work will include a water balance to partition the effects of water use and nutrient supply in determining δ 13C. An on-line combustion-ANCA-MS method is described for fully automated measurement of natural abundance levels of 15/14N and 13/12C for plant materials. This method achieves the required precision while dramatically increasing sample throughout. 相似文献
19.
Reduced soil N availability under elevated CO 2 may limit the plant's capacity to increase photosynthesis and thus the potential for increased soil C input. Plant productivity and soil C input should be less constrained by available soil N in an N 2‐fixing system. We studied the effects of Trifolium repens (an N 2‐fixing legume) and Lolium perenne on soil N and C sequestration in response to 9 years of elevated CO 2 under FACE conditions. 15N‐labeled fertilizer was applied at a rate of 140 and 560 kg N ha ?1 yr ?1 and the CO 2 concentration was increased to 60 Pa pCO 2 using 13C‐depleted CO 2. The total soil C content was unaffected by elevated CO 2, species and rate of 15N fertilization. However, under elevated CO 2, the total amount of newly sequestered soil C was significantly higher under T. repens than under L. perenne. The fraction of fertilizer‐N ( fN) of the total soil N pool was significantly lower under T. repens than under L. perenne. The rate of N fertilization, but not elevated CO 2, had a significant effect on fN values of the total soil N pool. The fractions of newly sequestered C ( fC) differed strongly among intra‐aggregate soil organic matter fractions, but were unaffected by plant species and the rate of N fertilization. Under elevated CO 2, the ratio of fertilizer‐N per unit of new C decreased under T. repens compared with L. perenne. The L. perenne system sequestered more 15N fertilizer than T. repens: 179 vs. 101 kg N ha ?1 for the low rate of N fertilization and 393 vs. 319 kg N ha ?1 for the high N‐fertilization rate. As the loss of fertilizer‐ 15N contributed to the 15N‐isotope dilution under T. repens, the input of fixed N into the soil could not be estimated. Although N 2 fixation was an important source of N in the T. repens system, there was no significant increase in total soil C compared with a non‐N 2‐fixing L. perenne system. This suggests that N 2 fixation and the availability of N are not the main factors controlling soil C sequestration in a T. repens system. 相似文献
20.
On following N 2-incorporation and subsequent metabolism in the lichen Peltigera canina using 15N as tracer, it was found, over a 30 min period, that greatest initial labelling was into NH
4
+
followed by glutamate and the amide-N of glutamine. Labelling of the amino-N of glutamine, aspartate and alanine increased slowly. Pulse-chase experiments using 15N confirmed this pattern. On inhibiting the GS-GOGAT pathway using l-methionine- dl-sulphoximine and azaserine, 15N enrichment of glutamate, alanine and aspartate continued although labelling of glutamine was undetectable. From this and enzymic data, NH
4
+
assimilation in the P. canina thallus appears to proceed via GS-GOGAT in the cyanobacterium and via GDH in the fungus; aminotransferases were present in both partners. The cyanobacterium assimilated 44% of the 15N 2 fixed; the remainder was liberated almost exclusively as NH
4
+
and then assimilated by fungal GDH.Abbreviations ADH
alanine dehydrogenase
- APT
aspartate-pyruvate aminotransferase
- AOA
aminooxyacetate
- GDH
glutamate dehydrogenase
- GOT
glutamate-oxaloacetate aminotransferase
- GOGAT
glutamate synthase
- GPT
glutamate-pyruvate aminotransferase
- GS
glutamine synthetase
- HEPES
4-(2-hydroxyethyl)-1-piperazine ethanesulphonic acid
- MSX
l-methionine- dl-sulphoximine 相似文献
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