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1.
Photosynthetic oxygen evolution within Sesbania rostrata stem nodules   总被引:1,自引:0,他引:1  
The tropical wetland legume, Sesbania rostrata Brem. forms N2-fixing nodules along its stem and on its roots after infection by Azorhizobium caulinodans . The N2-fixing tissue is surrounded by a cortex of uninfected cells which, in the stem nodules (but not the root nodules), contain chloroplasts. The photosynthetic competence of these chloroplasts was assessed through a novel technique involving image analysis of chlorophyll a fluorescence. Calculation of the quantum efficiency of photosystem II (PS II) photochemistry from these images indicated that most of the chloroplasts with potential for non-cyclic photosynthetic electron transport were concentrated within the mid- and inner-cortex, close to the edge of the N2-fixing tissue. PS II activity in the cortical cells was confirmed in vivo using O2-specific microelectrodes which showed that the concentration of O2 (pO2) in the outer cortex could rise from less than 1% up to 23.4% upon increased irradiance of the nodule, but that the pO2 of the inner cortex and infected tissue remained less than 0.0025%. Nitrogenase activity of stem nodules, as measured using a flow-through acetylene reduction assay (no H2 evolution was evident), showed a reversible increase of 28% upon exposure of the nodules to supplemental light. This increase resembled that obtained with stem nodules upon their exposure to an external pO2 of 40%.  相似文献   

2.
Semi-solid medium was used to isolate an aerobic, N2-fixing (C2H2-reducing), H2-utilizing bacterium from the roots of kallar grass ( Leptochloa fusca ). The organism was identified by morphological, cultural and biochemical characteristics. The N2-fixing, zoogloeal floc-forming isolate described here is a new species.  相似文献   

3.
The symbiotic interactions between Frankia strains and their associated plants from the Casuarinaceae under controlled conditions are well documented but little is known about these interactions under natural conditions. We explored the symbiotic interactions between eight genotypically characterized Frankia strains and five Casuarinaceae species in long-term field trials. Characterization of strains was performed using the polymerase chain reaction (PCR) and restriction fragment length polymorphism (RFLP) for the nifD – nifK intergenic transcribed spacer (ITS) and 16S–23S ITS. Assessments of the symbiotic interactions were based on nodulation patterns using nodule dry weight and viability, and on actual N2 fixation using the δ15N method. The PCR–RFLP patterns showed that the analyzed strains belonged to the same genotypic group (CeD group), regardless of the host species and environment of origin. The nodule viability index is introduced as a new tool to measure the viability of perennial nodules and to predict their effectiveness. The host Casuarinaceae species was a key factor influencing both the actual N2-fixing activity of the associated Frankia strain and the viability of nodules within a location. This is the first study providing information on the symbiotic interactions between genotypically characterized Frankia strains and actinorhizal plants under natural conditions. The results revealed a way to improve a long-term management of the Casuarinaceae symbiosis.  相似文献   

4.
Interactive effects of elevated atmospheric CO2 and arbuscular mycorrhizal (AM) fungi on biomass production and N2 fixation were investigated using black locust ( Robinia pseudoacacia ). Seedlings were grown in growth chambers maintained at either 350 μmol mol−1 or 710 μmol mol−1 CO2. Seedlings were inoculated with Rhizobium spp. and were grown with or without AM fungi. The 15N isotope dilution method was used to determine N source partitioning between N2 fixation and inorganic fertilizer uptake. Elevated atmospheric CO2 significantly increased the percentage of fine roots that were colonized by AM fungi. Mycorrhizal seedlings grown under elevated CO2 had the greatest overall plant biomass production, nodulation, N and P content, and root N absorption. Additionally, elevated CO2 levels enhanced nodule and root mass production, as well as N2 fixation rates, of non- mycorrhizal seedlings. However, the relative response of biomass production to CO2 enrichment was greater in non-mycorrhizal seedlings than in mycorrhizal seedlings. This study provides strong evidence that arbuscular mycorrhizal fungi play an important role in the extent to which plant nutrition of symbiotic N2-fixing tree species is affected by enriched atmospheric CO2.  相似文献   

5.
Numerous biochemical and physiological studies have demonstrated the importance of ascorbate (ASC) as a reducing agent and antioxidant in higher plant metabolism. Of special note is the capacity of ASC to eliminate damaging activated oxygen species (AOS) including O2· and H2O2. N2-fixing legume nodules are especially vulnerable to oxidative damage because they contain large amounts of leghaemoglobin which produces AOS through spontaneous autoxidation; thus, ASC and other components of the ascorbate–reduced glutathione (ASC–GSH) pathway are critical antioxidants in nodules. In order to establish a meaningful correlation between concentrations of ASC and capacity for N2 fixation in legume root nodules, soybean ( Glycine max ) plants were treated with excess ASC via exogenous irrigation or continuous intravascular infusion through needles inserted directly into plant stems. Treatment with ASC led to striking increases in nitrogenase activity (acetylene reduction), nodule leghaemoglobin content, and activity of ASC peroxidase, a key antioxidant enzyme. The concentration of lipid peroxides, which are indicators of oxidative damage and onset of senescence, was decreased in ASC-treated nodules. These results support the conclusion that ASC is critical for N2 fixation and that elevated ASC allows nodules to maintain a greater capacity to fix N2 over longer periods.  相似文献   

6.
The activities of nitrate reductase and glutamine synthetase were evaluated in young plants of Faidherbia albida , a tropical woody legume, fed with different N sources under hydroponic conditions. Results showed that assimilation of both NO3 and NH4+ preferentially took place in shoots. A basal amount of nitrate reductase activity was detected in shoots of plants grown with an NO3-free solution or placed under N2-fixing conditions, and also in nodules of N2-fixing plants. This strongly suggests that constitutive nitrate reductase activity is present in these organs. Analyses of the soluble nitrogenous content showed that the major form of N in the different organs was α-amino acids (particularly amides), irrespective of the N status of the culture conditions. The same result was obtained for nodulated plants grown in local sandy soil. In this case, amide-N generally accounted for more than 40% of the total soluble N. This was especially true in nodules. Ureide-N never exceeded 9% of the total soluble N and did not appear to increase with increasing nodule nitrogenase activity. Amides were also predominant in three N2-fixing Sahelian acacias ( Acacia seyal , A. nilotica and A. tortilis ), showing that F. albida does not differ from Sahelian Acacia in terms of the metabolism of fixed N. However, like another Sahelian acacia growing preferentially near water ( A. nilotica ), F. albida can be distinguished from acacias growing strictly in arid zones ( A. seyal and A. tortilis ) in terms of initial growth, water and nitrate management.  相似文献   

7.
The effects of Bradyrhizobium japonicum inoculation and pre-plant additions of N fertilizer on soybean ( Glycine max L. Merrill) yields and levels of N2 fixation were studied under field conditions at two sites in Thailand. Bacterial inoculants were composed of B. japonicum strains selected for high N2 fixation levels in Thai soils. Nitrogen fertilizer addition rates used were from 0 to 250 kg N/ha in 50 kg N/ha increments. At the Chiang Mai site in northern Thailand, bacterial inoculation increased nodule weights on plants receiving 100 kg N/ha or less. Increases in nodule parameters due to inoculation were evident at 45 d after planting (DAP) but disappeared by 60 DAP. Addition of N fertilizers decreased the incidence of nodulation and sap ureide contents and decreased the contribution of N2 fixation to the N content of plants at maturity as measured by N-15 isotope dilution methods. At the Kampang Saen site in central Thailand, bacterial inoculation had significant positive effects on nodule numbers and weights, ARA, sap ureide contents and levels of N2 fixed as measured by N-15 isotope dilution methods. Addition of N fertilizers at this site also reduced the effectiveness of N2-fixing symbioses. It was concluded that small additions of N fertilizer added before planting did not significantly decrease N2 fixation levels, but did have a significant positive effect on plant growth. Larger N additions would reduce N2 fixation levels in excess of the benefits of adding more N in chemical form.  相似文献   

8.
Oxygen and the regulation of nitrogen fixation in legume nodules   总被引:3,自引:0,他引:3  
In N2-fixing legume nodules, O2 is required in large amounts for aerobic respiration, yet nitrogenase, the bacterial enzyme that fixes N2, is O2 labile. A high rate of O2 consumptition and a cortical barrier to gas diffusion work together to maintain a low, non-inhibitory O2 concentration in the central, infected zone of the nodule. At this low O2 concentration, cytosolic leghemoglobin is required to facilitate the diffusion of O2 through the infected cell to the bacteria. The resistance of the cortical diffusion barrier is variable and is used by legume nodules to regulate the O2 concentration in the infected cells such that it limits aerobic respiration and N2 fixation at all times. The resistance of the diffusion barrier and therefore the degree of O2 limitation seems to be regulated in response to changes in the O2 concentration of the central infected zone, the supply of phloem sap to the nodule, and the rate of N assimilation into the end products of fixation.  相似文献   

9.
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   

10.
11.
Plant regulated aspects of nodulation and N2 fixation   总被引:1,自引:0,他引:1  
Abstract. Root nodule organogenesis is described. Plant regulated aspects of nodulation and N2 fixation are reviewed and discussed. Since the effective N2 fixing symbiosis requires the interaction of the host plant and bacterium in an appropriate environment (the rhizosphere and the root nodule) it is essential that research aimed at improving N2 fixation involve a knowledge and understanding of the plant genes that affect nodule development, growth, and function. Current knowledge of host plant genes involved in N2 fixation is summarized. Various experimental approaches to the study of the host plant's contribution to nodulation are noted. The functions of nodule specific proteins (nodulins) in symbiosis are delineated. Future areas of research are suggested.  相似文献   

12.
Abstract: Long-term (14 days) carbon costs of N2 fixation were studied in pot trials. For this purpose the CO2 release from the root space of nodulated and non-nodulated (urea nourished) Vicia faba L. and Pisum sativum L. plants was compared and related to the amount of fixed or assimilated N. Additional measurements of shoot CO2 exchange and dry matter increment were carried out in order to calculate the overall carbon balance. The carbon costs for N2 fixation in Vicia faba 1. (2.87 mg C/mg NfiX) were higher than in Pisum sativum L. (2.03 mg C/mg Nfix). However, the better carbon efficiency in Pisum sativum 1. did not lead to a better growth performance compared to Vicia faba L. Vicia faba L. compensated for the carbon and energy expenditure by more intensive photosynthesis in the N2-fixing treatment. This was not the case with Pisum sativum L., where the carbon balance indicates that the carbon costs of N2 fixation restricted root growth. It is proposed that low carbon costs for N2 fixation indicate an adaptation to a critical carbon supply of roots and nodules, e.g., during the pod-filling of grain legumes.  相似文献   

13.
The influence of P on N2 fixation and dry matter production of young pea ( Pisum sativum L. cv. Bodil) plants grown in a soil-sand mixture was investigated in growth cabinet experiments. Nodule dry weight, specific C2H2 reduction and P concentration in shoots responded to P addition before any growth response could be observed. The P concentration in nodules responded only slightly to P addition. A supply of P to P-deficient plants increased both the nodule dry weight, specific C2H2 reduction and P concentration in shoots relatively faster than it increased shoot dry weight and P concentration in nodules. Combined N applied to plants when N2 fixation had commenced, increased shoot dry weight only at the highest P levels. This indicates that the smaller plant growth at the low P levels did not result from N deficiency. The reduced nodulation and N2 fixation in P-deficient plants seem to be caused by impaired shoot metabolism and not by a direct effect of P deficiency of the nodules.  相似文献   

14.
Nodule phosphoenolpyruvate carboxylase: a review   总被引:3,自引:0,他引:3  
Recent data concerning the fixation of CO2 and the functioning of phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) in legume, nodules are reviewed. The activites of N2 fixation (acctylene reduction) and PEP carboxylase are correlated Activities of PEP carboxylase are always higher in nodules than in roots. PEP carboxylase is located in the cytosol of the plant part of the nodules. When nodules are fed with 14CO2, radioactivity appears predominantly in malate and aspartate. The resolution of isoenzymes of PEP carboxylase shows one more band in nodules than in related roots. The role of PEP carboxylase in nodule metabolism is discussed.  相似文献   

15.
Abstract Polyamines were analyzed in 12 of N2-fixing aerobic eubacteria and other eubacteria, cyanobacteria, algae and ferns. sym -Homospermidine (homospermidine) was found to be widely distributed as a major polyamine in various N2-fixing eubacteria which belong to Azospirillum, Agromonas, Beijerinckia, Bradyrhizobium, Rhizobium and Xathnbacter . 3 species of Azotobater contained spermidine but not homospermidine, though they are N2-fixing eubactera. Homospermidine is also distributed in some eubacteria, i.e., the photosynthetic Rhodopseudomanas rutila and the sulfur-oxidizing Thiobacillus denitrificans , a cyanobacterium, Synechococcus sp., and in the cyanobacterium-symbiotic ferns, Azolla imbircatta and Azolla japonica .  相似文献   

16.
广西含羞草科和云实科一些植物结瘤状况的调查   总被引:1,自引:0,他引:1  
柏学亮  唐东阶  徐雪华   《广西植物》1987,(1):61-65+95
调查了广西一些地区含羞草科植物34种,云实科植物39种,发现有瘤的植物分别为27种和6种,其中格木是首次报道有瘤,电子显微镜切片表明这些瘤是根瘤菌侵入所形成。  相似文献   

17.
The effects of potassium (K) deficiency on growth, N2-fixation and photosynthesis in white clover ( Trifolium repens L.) were investigated using natural occurring gas fluxes on the nodules in real time of plants under three contrasting relative addition rates of K causing mild K deficiency, or following abrupt withdrawal of the K supply causing strong K deficiency of less than 0.65% in dry matter. A steady-state below-optimum K supply rate led to an increase in CO2-fixation per unit leaf surface area as well as per plant leaf surface. However, nitrogenase activity per unit root weight and per unit nodule weight was maintained, as was the efficiency with which electrons were allocated to the reduction of N2 in the nodules. Abrupt K removals stimulated nodule growth strongly without delay, but as K concentrations decreased in the plant tissue a significant decline in nitrogenase activity per unit root weight as well as per unit nodule mass occurred. Further, the rate of photosynthesis per unit leaf area was unaffected, while the CO2 acquisition for the plant as a whole increased due to an expansion of total leaf area whereas the leaf area per unit leaf weight was unaffected. The ratio between CO2-fixation and N2-fixation increased, although not statistically significant, under short-term K deprivation as well as under long-term low K supply indicating a downregulation of nodule activity following morphological and growth adjustments. This downregulation took place despite a partly substitution of the K by Na. It is concluded that N2-fixation does not limit the growth of K-deprived clover plants. K deprivation induces changes in the relative growth of roots, nodules, and shoots rather than changes in N and/or carbon uptake rates per unit mass or area of these organs.  相似文献   

18.
Abstract A new N2-fixing unicellular cyanobacterium identified as a Synechococcus sp. was isolated and purified as an axenic culture. It fixed N2 aerobically either under continuous illumination or in alternating light-dark cycle. The N2-fixing properties of the new isolate and Gloeocapsa are discussed.  相似文献   

19.
δ15N and total nitrogen content of above- and belowground tissues of 13 plant species from two successional stages (open pioneer community and ruderal grass stage) of a dry acidic grassland in Southern Germany were analysed, in order to evaluate whether resource use partitioning by niche separation and N input by N2-fixing legumes are potential determinants for species coexistence and successional changes. Within each stage, plants from plots with different legume cover were compared. Soil inorganic N content, total plant biomass and δ15N values of bulk plant material were significantly lower in the pioneer stage than in the ruderal grass community. The observed δ15N differences were rather species- than site-specific. Within both stages, there were also species-specific differences in isotopic composition between above- and belowground plant dry matter. Species-specific δ15N signatures may theoretically be explained by (i) isotopic fractionation during microbial-mediated soil N transformations; (ii) isotopic fractionation during plant N uptake or fractionation during plant–mycorrhiza transfer processes; (iii) differences in metabolic pathways and isotopic fractionation within the plant; or (iv) partitioning of available N resources (or pools) among plant groups or differential use of the same resources by different species, which seems to be the most probable route in the present case. A significant influence of N2-fixing legumes on the N balance of the surrounding plant community was not detectable. This was confirmed by the results of an independent in situ removal experiment, showing that after 3 years there were no measurable differences in the frequency distribution between plots with and without N2-fixing legumes.  相似文献   

20.
Expression of Bradyrhizobium japonicum wild-type strain USDA110 nirK , norC and nosZ denitrification genes in soybean root nodules was studied by in situ histochemical detection of β -galactosidase activity. Similarly, PnirK- lacZ , PnorC- lacZ , and PnosZ- lacZ fusions were also expressed in bacteroids isolated from root nodules. Levels of β -galactosidase activity were similar in both bacteroids and nodule sections from plants that were solely N2-dependent or grown in the presence of 4 m M KNO3. These findings suggest that oxygen, and not nitrate, is the main factor controlling expression of denitrification genes in soybean nodules. In plants not amended with nitrate, B. japonicum mutant strains GRK308, GRC131, and GRZ25, that were altered in the structural nirK , norC and nosZ genes, respectively, showed a wild-type phenotype with regard to nodule number and nodule dry weight as well as plant dry weight and nitrogen content. In the presence of 4 m M KNO3, plants inoculated with either GRK308 or GRC131 showed less nodules, and lower plant dry weight and nitrogen content, relative to those of strains USDA110 and GRZ25. Taken together, the present results revealed that although not essential for nitrogen fixation, mutation of either the structural nirK or norC genes encoding respiratory nitrite reductase and nitric oxide reductase, respectively, confers B. japonicum reduced ability for nodulation in soybean plants grown with nitrate. Furthermore, because nodules formed by each the parental and mutant strains exhibited nitrogenase activity, it is possible that denitrification enzymes play a role in nodule formation rather than in nodule function.  相似文献   

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