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1.
Dry matter accumulation, nitrate reductase activity of various organs, nitrate accumulation, nitrogen derived from nitrate, and nitrogen content were studied during 17 days of vegetative regrowth of harvested (detopped) alfalfa ( Medicago sativa L.). Seedlings were grown in the glasshouse and treated with 0, 40, and 80 kilograms N per hectare applied as K 15NO 3 to determine whether reduced nitrogenase activity after shoot harvest limited vegetative regrowth. The role of nodules in reducing NO 3− during this period of low nitrogenase activity was also investigated. 相似文献
2.
In vivo CO 2 fixation activity and in vitro phosphoenolpyruvate carboxylase activity were demonstrated in effective and ineffective nodules of alfalfa ( Medicago sativa L.) and in the nodules of four other legume species. Phosphoenolpyruvate carboxylase activity was greatly reduced in nodules from both host and bacterially conditioned ineffective alfalfa nodules as compared to effective alfalfa nodules. Forage harvest and nitrate application reduced both in vivo and in vitro CO2 fixation activity. By day 11, forage harvest resulted in a 42% decline in in vitro nodule phosphoenolpyruvate carboxylase activity while treatment with either 40 or 80 kilograms nitrogen per hectare reduced activity by 65%. In vitro specific activity of phosphoenolpyruvate carboxylase and glutamate synthase were positively correlated with each other and both were positively correlated with acetylene reduction activity. The distribution of radioactivity in the nodules of control plants (unharvested, 0 kilograms nitrogen per hectare) averaged 73% into the organic acid and 27% into the amino acid fraction. In nodules from harvested plants treated with nitrate, near equal distribution of radioactivity was observed in the organic acid (52%) and amino acid (48%) fractions by day 8. Recovery to control distribution occurred only in those nodules whose in vitro phosphoenolpyruvate carboxylase activity recovered. The results demonstrate that CO2 fixation is correlated with nitrogen fixation in alfalfa nodules. The maximum rate of CO2 fixation for attached and detached alfalfa nodules at low CO2 concentrations (0.13-0.38% CO2) were 18.3 and 4.9 nanomoles per hour per milligram dry weight, respectively. Nodule CO2 fixation was estimated to provide 25% of the carbon required for assimilation of symbiotically fixed nitrogen in alfalfa. 相似文献
3.
Pigeon peas [ Cajanus cajan (L.) Millsp.] were grown in soil columns containing 15N-enriched organic matter. Seasonal N 2 fixation activity was determined by periodically assaying plants for reduction of C 2H 2. N 2 fixation rose sharply from the first assay period at 51 days after planting to a peak of activity between floral initiation and fruit set. N 2 fixation (acetylene reduction) activity dropped concomitantly with pod maturation but recovered after pod harvests. Analysis of 15N content of plant shoots revealed that approximately 91 to 94% of plant N was derived from N 2 fixation. The effect of inoculation with hydrogenase-positive and hydrogenase-negative rhizobia was examined. Pigeon peas inoculated with strain P132 (hydrogenase-positive) yielded significantly more total shoot N than other inoculated or uninoculated treatments. However, two other hydrogenase-positive strains did not yield significantly more total shoot N than a hydrogenase-negative strain. The extent of nodulation by inoculum strains compared to indigenous rhizobia was determined by typing nodules according to intrinsic antibiotic resistance of the inoculum strains. The inoculum strains were detected in almost all typed nodules of inoculated plants. Gas samples were taken from soil columns several times during the growth cycle of the plants. H2 was never detected, even in columns containing pigeon peas inoculated with hydrogenase-negative rhizobia. This was attributed to H2 consumption by soil bacteria. Estimation of N2 fixation by acetylene reduction activity was closest to the direct 15N method when ethylene concentrations in the gas headspace (between the column lid and soil surface) were extrapolated to include the soil pore space as opposed solely to measurement in the headspace. There was an 8-fold difference between the two acetylene reduction assay methods of estimation. Based on a planting density of 15,000 plants per hectare, the direct 15N fixation rates ranged from 67 (noninoculated) to 134 kilograms per hectare, while grain yields ranged from 540 to 825 kilograms per hectare. Grain yields were not increased with N fertilizer. 相似文献
4.
The effects of N source (6 m m nitrogen as NO 3− or urea) and tungstate (0, 100, 200, 300, and 400 μ m Na 2 WO 4) on nitrate metabolism, nodulation, and growth of soybean ( Glycine max [L.] Merr.) plants were evaluated. Nitrate reductase activity and, to a lesser extent, NO 3− content of leaf tissue decreased with the addition of tungstate to the nutrient growth medium. Concomitantly, nodule mass and acetylene reduction activity of NO 3−-grown plants increased with addition of tungstate to the nutrient solution. In contrast, nodule mass and acetylene reduction activity of urea-grown plants decreased with increased nutrient tungstate levels. The acetylene reduction activity of nodulated roots of NO 3−-grown plants was less than 10% of the activity of nodulated roots of urea-grown plants when no tungstate was added. At 300 and 400 μ m tungstate levels, acetylene reduction activity of nodulated roots of NO 3−-grown plants exceeded the activity of comparable urea-grown plants. 相似文献
5.
Previously, we reported (a) a positive correlation between the nitrate concentrations in growth medium and ethylene evolved from uninoculated and inoculated alfalfa ( Medicago sativa) roots and (b) a negative correlation between ethylene evolution and nodulation. Here, we report that the inhibitory effect of NO 3− on nodulation of alfalfa can be eliminated by the ethylene inhibitor aminoethoxyvinylglycine (AVG). This effect was probably related to the strong inhibition (90%) of ethylene biosynthesis caused by AVG in these inoculated and NO 3−-treated roots. These results support our hypothesis that the inhibitory effect of NO 3− is mediated through the phytohormone ethylene. A possible role of endogenous ethylene in the autoregulation of nodulation also is discussed. AVG at 10 micromolar significantly (P < 0.05) increased total nitrogenase activity (acetylene reduction) in 2.5 and 5 millimolar NO 3−-fed plants probably as a result of the very high stimulation of nodulation. 相似文献
6.
An experiment was conducted to investigate alterations in uptake and assimilation of NO 3− by phosphorus-stressed plants. Young tobacco plants ( Nicotiana tabacum [L.], cv NC 2326) growing in solution culture were deprived of an external phosphorus (P) supply for 12 days. On selected days, plants were exposed to 15NO 3− during the 12 hour light period to determine changes in NO 3− assimilation as the P deficiency progressed. Decreased whole-plant growth was evident after 3 days of P deprivation and became more pronounced with time, but root growth was unaffected until after day 6. Uptake of 15NO 3− per gram root dry weight and translocation of absorbed 15NO 3− out of the root were noticeably restricted in −P plants by day 3, and effects on both increased in severity with time. Whole-plant reduction of 15NO 3− and 15N incorporation into insoluble reduced-N in the shoot decreased after day 3. Although the P limitation was associated with a substantial accumulation of amino acids in the shoot, there was no indication of excessive accumulation of soluble reduced- 15N in the shoot during the 12 hour 15NO 3− exposure periods. The results indicate that alterations in NO 3− transport processes in the root system are the primary initial responses limiting synthesis of shoot protein in P-stressed plants. Elevated amino acid levels evidently are associated with enhanced degradation of protein rather than inhibition of concurrent protein synthesis. 相似文献
7.
An acetylene inhibition method was satisfactorily used for the in situ measurement of denitrification in two sediment-water systems incubated for not more than 22 h. In the presence of added nitrate, denitrification acted as a source of nitrous oxide in a drainage pond, but acted as a sink in its absence. The averaged rates of nitrous oxide accumulation with nitrate enrichment in the absence and presence of acetylene were 0.15 and 0.30 mg of N m −2h −1, respectively. Acetylene reduction at an average rate of 0.07 mmol of C 2H 4 formed m −2h −1 was simultaneously measured in the absence of added nitrate. In a small eutrophic lake where nitrogen was nonlimiting, the in situ rates of sediment denitrification were 0.09 and 0.11 mg of N m −2h −1 in the presence and absence of macrophytes, respectively, and no acetylene reduction activity was found. 相似文献
8.
An experiment was conducted to determine the extent that NO 3− taken up in the dark was assimilated and utilized differently by plants than NO 3− taken up in the light. Vegetative, nonnodulated soybean plants ( Glycine max L. Merrill, `Ransom') were exposed to 15NO 3− throughout light (9 hours) or dark (15 hours) phases of the photoperiod and then returned to solutions containing 14NO 3−, with plants sampled subsequently at each light/dark transition over 3 days. The rates of 15NO 3− absorption were nearly equal in the light and dark (8.42 and 7.93 micromoles per hour, respectively); however, the whole-plant rate of 15NO 3− reduction during the dark uptake period (2.58 micromoles per hour) was 46% of that in the light (5.63 micromoles per hour). The lower rate of reduction in the dark was associated with both substantial retention of absorbed 15NO 3− in roots and decreased efficiency of reduction of 15NO 3− in the shoot. The rate of incorporation of 15N into the insoluble reduced-N fraction of roots in darkness (1.10 micromoles per hour) was somewhat greater than that in the light (0.92 micromoles per hour), despite the lower rate of whole-plant 15NO 3− reduction in darkness. A large portion of the 15NO3− retained in the root in darkness was translocated and incorporated into insoluble reduced-N in the shoot in the following light period, at a rate which was similar to the rate of whole-plant reduction of 15NO3− acquired during the light period. Taking into account reduction of NO3− from all endogenous pools, it was apparent that plant reduction in a given light period (~13.21 micromoles per hour) exceeded considerably the rate of acquisition of exogenous NO3− (8.42 micromoles per hour) during that period. The primary source of substrate for NO3− reduction in the dark was exogenous NO3− being concurrently absorbed. In general, these data support the view that a relatively small portion (<20%) of the whole-plant reduction of NO3− in the light occurred in the root system. 相似文献
9.
The effects of nitrogen source NO 3− or NH 4+ on nitrogen metabolism during the first 2 weeks of germination of the rice seedling ( Oryza sativa L., var. IR22) grown in nutrient solution containing 40 μg/ml N were studied. Total, soluble protein, and free amino N levels were higher in the NH 4+-grown seedling, particularly during the 1st week of germination. Asparagine accounted for most of the difference in free amino acid level, in both the root and the shoot. Nitrate and nitrite reductase activities were present mainly in the shoot and were higher in the NO 3−-grown seedling, whereas the activity of glutamate dehydrogenase and glutamine synthetase in the root tended to be lower than that of the NH 4+-grown seedling during the 1st week of germination. Glycolate oxidase and catalase activities were present mainly in the shoot. Maximum activity of the above five enzymes occurred 7 to 10 days after germination. Differences in the zymograms of nitrate reductase, glutamate dehydrogenase, and catalase were mainly between shoot and root and not from N source. Nitrite reductase bands were observed only in plants grown in plants grown in NO 3−. 相似文献
10.
The objective of this study was to identify factors which limit leaf nitrate reductase (NR) activity as decline occurs during flowering and beginning seed development in soybean ( Glycine max [L.] Merr. cv Clark). Level of NR enzyme activity, level of reductant, and availability of NO 3− as substrate were evaluated for field-grown soybean from flowering through leaf senescence. Timing of reproductive development was altered within one genotype by (a) exposure of Clark to an artificially short photoperiod to hasten flowering and podfill, and (b) the use of an early flowering isoline. Nitrogen (N) was soil-applied to selected plots at 500 kilograms per hectare as an additional variable. Stem NO 3− concentration and in vivo leaf NR activity were significantly correlated ( R2 = 0.69 with nitrate in the assay medium and 0.74 without nitrate in the medium at P = 0.001) across six combinations of reproductive and soil N-treatment. The supply of NO 3− from the root to the leaf tissue was the primary limitation to leaf NR activity during flowering and podfill. Levels of NR enzyme and reductant were not limiting to leaf NR activity during this period. 相似文献
11.
Enhanced nitrogen (N) availability is one of the main drivers of biodiversity loss and degradation of ecosystem functions. However, in very nutrient-poor ecosystems, enhanced N input can, in the short-term, promote diversity. Mediterranean Basin ecosystems are nutrient-limited biodiversity hotspots, but no information is available on their medium- or long-term responses to enhanced N input. Since 2007, we have been manipulating the form and dose of available N in a Mediterranean Basin maquis in south-western Europe that has low ambient N deposition (<4 kg N ha −1 yr −1) and low soil N content (0.1%). N availability was modified by the addition of 40 kg N ha −1 yr −1 as a 1∶1 NH 4Cl to (NH 4) 2SO 4 mixture, and 40 and 80 kg N ha −1 yr −1 as NH 4NO 3. Over the following 5 years, the impacts on plant composition and diversity (richness and evenness) and some ecosystem characteristics (soil extractable N and organic matter, aboveground biomass and % of bare soil) were assessed. Plant species richness increased with enhanced N input and was more related to ammonium than to nitrate. Exposure to 40 kg NH 4
+-N ha −1 yr −1 (alone and with nitrate) enhanced plant richness, but did not increase aboveground biomass; soil extractable N even increased under 80 kg NH 4NO 3-N ha −1 yr −1 and the % of bare soil increased under 40 kg NH 4
+-N ha −1 yr −1. The treatment containing less ammonium, 40 kg NH 4NO 3-N ha −1 yr −1, did not enhance plant diversity but promoted aboveground biomass and reduced the % of bare soil. Data suggest that enhanced NH y availability affects the structure of the maquis, which may promote soil erosion and N leakage, whereas enhanced NO x availability leads to biomass accumulation which may increase the fire risk. These observations are relevant for land use management in biodiverse and fragmented ecosystems such as the maquis, especially in conservation areas. 相似文献
12.
An experiment was conducted to test the hypothesis that, when nitrogenase and nitrate reductase both contribute to the nitrogen nutrition of a nodulated legume, nitrogenase activity is inversely proportional to the rate of accumulation of organic nitrogen derived from the reduction of nitrate. Trifolium subterraneum L. plants, inoculated with Rhizobium trifolii and sown as small swards, were allowed to establish a closed canopy and steady rates of growth, dinitrogen fixation, and nitrogen accumulation. Swards were then supplied with nutrient solutions of 0, 0.5, 1.0, or 2.5 m m NO 3− with a 29.69% enrichment of 15N and allowed to grow for a further 33 days. Harvests were made to measure dry weight, nitrogen accumulation, 15N accumulation, NO 3− content and nitrogenase activity by acetylene reduction assay. Since the 15N of the plant organic matter could have been derived only from the NO 3− of the nutrient solution, its rate of accumulation provided a measure of the rate of NO 3− reduction. It was found that as this rate increased in response to external NO 3− concentration the rate of nitrogenase activity decreased proportionately. It is concluded that the reduction of nitrate and the reduction of dinitrogen act in a complementary manner to supply a plant with organic nitrogen for growth. 相似文献
13.
The synthesis and accumulation of nitrite has been suggested as a causative factor in the inhibition of legume nodules supplied with nitrate. Plants were grown in sand culture with a moderate level of nitrate (2.1 to 6.4 millimolar) supplied continuously from seed germination to 30 to 50 days after planting. In a comparison of nitrate treatments, a highly significant negative correlation between nitrite concentration in soybean ( Glycine max [L.] Merr.) nodules and nodule fresh weight per shoot dry weight was found even when bacteroids lacked nitrate reductase (NR). However, in a comparison of two Rhizobium japonicum strains, there was only 12% as much nitrite in nodules formed by NR −R. japonicum as in nodules formed by NR +R. japonicum, and growth and acetylene reduction activity of both types of nodules was about equally inhibited. In a comparison of eight other NR + and NR −R. japonicum strains, and a comparison of G. max, Phaseolus vulgaris, and Pisum sativum, the concentration of nitrite in nodules was unrelated to nodule weight per plant or to specific acetylene reduction activity. The very small concentration of nitrite found in P. vulgaris nodules (0.05 micrograms NO 2−-N per gram fresh weight) was probably below that required for the inhibition of nitrogenase based on published in vitro experiments, and yet the specific acetylene reduction activity was inhibited 83% by nitrate. The overall results do not support the idea that nitrite plays a role in the inhibition of nodule growth and nitrogenase activity by nitrate. 相似文献
14.
Effects of Na application on the capacity of NO 3− assimilation were studied in Na-deficient Amaranthus tricolor L. cv Tricolor plants. On day 30 after germination, Na-deficient A. tricolor plants received either 0.5 millimolar NaCl or KCl. The level of nitrate reductase activity doubled within 24 hours by the addition of Na and the enhanced level was maintained thereafter. When the plants were exposed to 2 millimolar 15NO 3−, total 15N taken up by the plants was greater in the Na-treated plants than in the K-treated plants within 24 hours of the Na treatment. Incorporation of 15N into the 80% ethanol-insoluble nitrogen fraction of the Na-treated plants in the light period was about 260% of those of the K-treated plants indicating greater capacity of NO 3− assimilation in the Na-treated plants. From these results, it was demonstrated that Na application to the Na-deficient A. tricolor plants promoted NO 3− reduction and its subsequent assimilation into protein, resulting in growth enhancement. 相似文献
15.
When adequate levels of soil NO 3− are available, concurrent NO 3− absorption and assimilation, and mobilization of vegetative N reserves accumulated prior to anthesis, may be used to supply N to developing wheat ( Triticum aestivum L.) kernels. Vegetative wheat components (stems, leaves, spike) are known to possess NO 3− reductase activity, but the in situ utilization of NO 3− translocated to the shoot has not been studied. Assimilation and partitioning of 15N was determined in winter wheat `Doublecrop.' At 7 days after anthesis, the stem immediately above the peduncle node was heat girdled to block phloem export from the flag leaf. Control plants were not girdled. One day later, 50 micromoles of 15NO 3− (98 atom percent 15N) was injected into the penultimate internodal lacuna, after which 15NO 3− utilization was determined sequentially over a 5 day period. Based on differences in spike accumulation of reduced 15N excess between treatments and the amount of reduced 15N excess remaining in the flag leaf, it was estimated that the flag leaf contributed 37% of the total reduced 15N excess in the injected shoot. The lower shoot contribution was 18% and that of the peduncle plus spike was 45%. 相似文献
16.
In soybean ( Glycine max L. Merr. cv Kingsoy), NO 3− assimilation in leaves resulted in production and transport of malate to roots (B Touraine, N Grignon, C Grignon [1988] Plant Physiol 88: 605-612). This paper examines the significance of this phenomenon for the control of NO 3− uptake by roots. The net NO 3− uptake rate by roots of soybean plants was stimulated by the addition of K-malate to the external solution. It was decreased when phloem translocation was interrupted by hypocotyl girdling, and partially restored by malate addition to the medium, whereas glucose was ineffective. Introduction of K-malate into the transpiration stream using a split root system resulted in an enrichment of the phloem sap translocated back to the roots. This treatment resulted in an increase in both NO 3− uptake and C excretion rates by roots. These results suggest that NO 3− uptake by roots is dependent on the availability of shoot-borne, phloem-translocated malate. Shoot-to-root transport of malate stimulated NO 3− uptake, and excretion of HCO 3− ions was probably released by malate decarboxylation. NO 3− uptake rate increased when the supply of NO 3− to the shoot was increased, and decreased when the activity of nitrate reductase in the shoot was inhibited by WO 42−. We conclude that in situ, NO 3− reduction rate in the shoot may control NO 3− uptake rate in the roots via the translocation rate of malate in the phloem. 相似文献
17.
Soybeans ( Glycine max L. Merr. cv Tracy and Ransom) were grown under N 2-dependent or NO 3−-supplied conditions, and the partitioning of photosynthate and dry matter was characterized. Although no treatment effects on photosynthetic rates were observed, NO 3−-supplied plants in both cultivars had lower starch accumulation rates than N 2-dependent plants. Leaf extracts of NO 3−-supplied plants had higher activities of sucrose phosphate synthase (SPS) and cytoplasmic fructose-1,6-bisphosphatase (FBPase) than N 2-dependent plants. The variation in starch accumulation was correlated negatively with the activity of SPS, but not the activity of FBPase, UDP-glucose pyrophosphorylase, or ADP-glucose pyrophosphorylase. These results suggested that starch accumulation is biochemically controlled, in part, by the activity of SPS. Leaf starch content at the beginning of the photoperiod was lower in NO 3−-supplied plants than N 2-dependent plants in both cultivars which suggested that net starch utilization as well as accumulation was affected by N source. Total dry matter accumulation and dry matter distribution was affected by N source in both cultivars, but the cultivars differed in how dry matter was partitioned between the shoot and root as well as within the shoot. The activity of SPS was correlated positively with total dry matter accumulation which suggested that SPS activity is related to plant growth rate. The results suggested that photosynthate partitioning is an important but not an exclusive factor which determines whole plant dry matter distribution. 相似文献
18.
It is unclear if the relative content of NO 3− and reduced N in xylem exudate provides an accurate estimate of the percentage reduction of concurrently absorbed NO 3− in the root. Experiments were conducted to determine whether NO 3− and reduced N in xylem exudate of vegetative, nonnodulated soybean plants ( Glycine max [L.] Merr., `Ransom') originated from exogenous recently absorbed 15NO 3− or from endogenous 14N pools. Plants either were decapitated and exposed to 15NO 3− solutions for 2 hours or were decapitated for the final 20 minutes of a 50-minute exposure to 15NO 3− in the dark and in the light. Considerable amounts of 14NO 3− and reduced 14N were transported into the xylem, but almost all of the 15N was present as 15NO 3−. Dissimilar changes in transport of 14NO 3−, reduced 14N and 15NO 3− during the 2 hours of sap collection resulted in large variability over time in the percentage of total N in the exudate which was reduced N. Over a 20-minute period the rate of 15N transport into the xylem of decapitated plants was only 21 to 36% of the 15N delivered to the shoot of intact plants. Based on the proportion of total 15N which was found as reduced 15N in exudate and in intact plants in the dark, it was estimated that 5 to 17% of concurrently absorbed 15NO 3− was reduced in the root. This was much less than the 38 to 59% which would have been predicted from the relative content of total NO 3− and total reduced N in the xylem exudate. 相似文献
19.
An experiment was conducted to investigate the relative changes in NO 3− assimilatory processes which occurred in response to decreasing carbohydrate availability. Young tobacco plants ( Nicotiana tabacum [L.], cv NC 2326) growing in solution culture were exposed to 1.0 millimolar 15NO 3− for 6 hour intervals during a normal 12 hour light period and a subsequent period of darkness lasting 42 hours. Uptake of 15NO 3− decreased to 71 to 83% of the uptake rate in the light during the initial 18 hours of darkness; uptake then decreased sharply over the next 12 hours of darkness to 11 to 17% of the light rate, coincident with depletion of tissue carbohydrate reserves and a marked decline in root respiration. Changes also occurred in endogenous 15NO 3− assimilation processes, which were distinctly different than those in 15NO 3− uptake. During the extended dark period, translocation of absorbed 15N out of the root to the shoot varied rhythmically. The adjustments were independent of 15NO 3− uptake rate and carbohydrate status, but were reciprocally related to rhythmic adjustments in stomatal resistance and, presumably, water movement through the root system. Whole plant reduction of 15NO 3− always was limited more than uptake. The assimilation of 15N into insoluble reduced-N in roots remained a constant proportion of uptake throughout, while assimilation in the shoot declined markedly in the first 18 hours of darkness before stabilizing at a low level. The plants clearly retained a capacity for 15NO 3− reduction and synthesis of insoluble reduced- 15N even when 15NO 3− uptake was severely restricted and minimal carbohydrate reserves remained in the tissue. 相似文献
20.
The influence of nutrient nitrate level (0-20 millimolar) on the effects of NO 2 (0-0.5 parts per million) on nodulation and in vivo acetylene reduction activity of the roots and on growth and nitrate and Kjeldahl N concentration in shoots was studied in bean ( Phaseolus vulgaris L. cv Kinghorn Wax) plants. Exposing 8-day old seedlings for 6 hours each day, for 15 days, to 0.02 to 0.5 parts per million NO 2 decreased total nodule weight at 0 and 1 millimolar nitrate, and nitrogenase (acetylene reduction) activity at all concentrations of nitrate. The pollutant had little effect on root fresh or dry weights. Shoot growth was inhibited by NO 2. The NO 2 exposure increased nitrate concentration in roots only at 20 millimolar nutrient nitrate. Exposure to NO 2 markedly increased Kjeldahl N concentration in roots but generally decreased that in shoots. The experiments demonstrated that nutrient N level and NO 2 concentration act jointly in affecting nodulation and N fixing capability, plant growth and composition, and root/shoot relationships of bean plants. 相似文献
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