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Showing papers on "Nitrogen fixation published in 1989"


Journal ArticleDOI
TL;DR: A new species of nitrogen-fixing bacteria, Azospirillum irakense, was found associated with roots and the rhizosphere of rice in the region of Diwaniyah (Qadisya), Iraq and the phenotypic characters were found to be very close to those of A. amazonense.

195 citations


Journal ArticleDOI
TL;DR: All strains of Rhizobium isolated from ineffective nodules on white clover formed in metal-contaminated soils from a field experiment were demonstrated to be wholly ineffective in nitrogen fixation in plant infection tests on N-free nutrient agar, indicating thatwhite clover rhizobia are unable to survive (or at least unable to remain effective) in the presence of concentrations of heavy metals close to the current Commission of the European Communities guidelines for environmental protection.
Abstract: All strains of Rhizobium isolated from ineffective nodules on white clover (> 50 separate isolations) formed in metal-contaminated soils from a field experiment were demonstrated to be wholly ineffective in nitrogen fixation in plant infection tests on N-free nutrient agar. The plasmid profiles of these isolates were all very similar indicating a lack of genetic diversity in the population surviving in high concentrations of heavy metals. Isolates from comparable field plots of uncontaminated soil had a wide diversity of plasmid profiles. Inoculation of white clover at sowing with a large inoculum of effective Rhizobium leguminosarum bv. trifolii on metal contaminated soil resulted in effective N2-fixation. However, if the inoculated soils were held for 2 months in a moist condition before sowing with white clover, N2-fixation was not detected with inoculum doses of 107 cells pot−1 or less but was obvious where a very large inoculum of 1010 cells pot−1 was added. This indicates that white clover rhizobia are unable to survive (or at least unable to remain effective) in the presence of concentrations of heavy metals close to the current Commission of the European Communities guidelines for environmental protection. A rapid method of assessment of the toxic effects of pollutants on populations of Rhizobium spp is described.

187 citations


Journal ArticleDOI
TL;DR: In this article, the molybdenum effect was found to increase the activity of nitrogenase in forest litter, decaying wood and lichen Lobaria in Pacific Northwest forests.
Abstract: Nitrogenase activity as measured by acetylene reduction and nitrogen fixation as measured by 15N2 uptake is widespread in forest litter, decaying wood and the lichen Lobaria in Pacific Northwest forests In all cases the activity of nitrogenase is enhanced by addition of plant nutrient solutions and this is shown to be a specific molybdenum effect A wide range of forest species have litter which supports nitrogen fixation during decay and the majority show molybdenum stimulation The effect is most prominent in the acid soils situated between the Coast Range and the Cascade Range and extends from Oregon through Washington to British Columbia Nitrogenase is increased quantitatively by Mo additions and the effect, though best seen in the laboratory, is also measurable in the field The results are discussed on the basis of geographical distribution of the effect, plant species and the likely role of asymbiotic nitrogen fixation in contributing to long-term nutrient cycling

126 citations


Journal ArticleDOI
TL;DR: Proposed effects of fire on nonsymbiotic nitrogen fixation in Tallgrass prairie support arguments from the marine and terrestrial literature that P availability is central to regulation of ecosystem N budgets.
Abstract: Prescribed burning is a major control over element cycles in Tallgrass prairie (Eastern Kansas, USA) In this paper we report potential effects of fire on nonsymbiotic nitrogen fixation Fire resulted in additions of available P in ash, which may stimulate nitrogen fixation by terrestrial cyanobacteria Cyanobacterial nitrogenase activity and biomass responded positively to additions of ash or P in laboratory assays using soil Further assays in soil showed that cyanobacteria responded to changes in available N:available P ratio (aN:P) across a range of concentrations Nitrogen fixation rate could be related empirically to aN:P via a log-linear relationship Extrapolation of laboratory results to the field yielded a maximal estimate of 21 kg N ha-1 y-1 Results support arguments from the marine and terrestrial literature that P availability is central to regulation of ecosystem N budgets

122 citations


Journal ArticleDOI
TL;DR: The results obtained do not agree completely with other phylogenies for these bacteria and thus make probable occurrences of lateral transfer of thenif genes, as well as a number of methods used to infer phylogenies of these nitrogen fixers, based onnifH amino acid and nucleotide sequences.
Abstract: The complete nucleotide sequence of a nitrogenase (nifH) gene was determined from a second strain (HRN18a) ofFrankia, an aerobic soil bacterium. The open reading frame is 870 bp long and encodes a polypeptide of 290 amino acids. The amino acid and nucleotide sequences were compared with 21 other published sequences. The twoFrankia strains were 96% similar at the amino acid level and 93% similar at the nucleotide level. A number of methods were used to infer phylogenies of these nitrogen fixers, based onnifH amino acid and nucleotide sequences. The results obtained do not agree completely with other phylogenies for these bacteria and thus make probable occurrences of lateral transfer of thenif genes. The time of divergence of the twoFrankia strains could be estimated at about 100 million years. The vanadium-dependent (Type 2) nitrogenase present inAzotobacter spp. appears to be a recent derivation from the conventional molybdenum-dependent (Type 1) enzyme, whereas the iron-dependent (Type 3) alternative nitrogenase would have a much older origin.

120 citations


Journal ArticleDOI
TL;DR: In this paper, the interaction between soil pH and inoculation with rhizobia and vesicular-arbuscular mycorrhizae (VAM) was studied in an industrially polluted soil contaminated with high levels of Zn and Cd.
Abstract: The interaction between soil pH and inoculation with rhizobia and vesicular-arbuscular mycorrhizae (VAM) was studied in an industrially polluted soil contaminated with high levels of Zn and Cd. A silt loam soil (pH 6.7) was amended with Ca(OH)2 or elemental S to adjust the soil pH to 4.3, 5.3, 6.0, and 7.2. Alfalfa (Medicago sativa L.) was planted in each treated soil an subsequently inoculated with Rhizobium meliloti and/or a mixed VAM spore population. Alfalfa growing in soils at a pH of 4.3 and 5.3 failed to survive as a result of soil acidity and heavy metal toxicity. At the three higher pH values, growth and foliar N and P were significantly increased by inoculation with rhizobia or VAM. The greatest increase was observed when both VAM and rhizobia were inoculated together into the soil. With a soil pH of 6.0 and 6.7, the available heavy metal concentration in the soil was high and the VAM significantly decreased heavy metal uptake from these soils. The foliar concentration of Zn was reduced from 455 to 306 μg g−1 by inoculation with VAM (pH 6.0). At the highest soil pH (7.2), however, available heavy metal concentrations were generally lower and NAM significantly increased the heavy metal uptake. The influence of VAM on heavy metal uptake thus appears to be partly a function of the available heavy metal content in the soil.

104 citations


Journal ArticleDOI
TL;DR: There appears to be a very strong case for improving inoculation methods and for selecting rhizobia not only for effectiveness but also for migration in soil and along the root in a range of conditions.
Abstract: Greenhouse and field experiments were made to investigate the influence of root nodule position on nitrogen fixation in soybean. The N-15 isotope dilution method was used to quantify N 2 fixed. Differences in the location of nodules on roots were induced through varying the inoculation procedure, timing or depth of inoculum placement in the soil. Movement of Bradyrhizobium japonicum in soil and rhizosphere was very limited, and nodule formation was restricted largely to the vicinity within which the bradyrhizobia were placed. With seed inoculation, most of the nodules occurred on the crown of the roots (0–5 cm segment of root), in contrast to the profuse and well distributed nodules when the inoculum was applied throughout the soil. Nodules were mostly on the lower portions of the roots when inoculum was below the seed. Nodule position affected N 2 fixation, with nodules formed on the lower part of the root system fixing significantly more N 2 than the crown nodules, and the later-formed nodules contributed also substantially to the total amount of N 2 fixed by the soybean. There appears to be a very strong case for improving inoculation methods and for selecting rhizobia not only for effectiveness but also for migration in soil and along the root in a range of conditions.

92 citations


Journal ArticleDOI
TL;DR: It is concluded that growing winter cereals on land newly broken from pasture, coupled with high rates of inoculation of the following soybeans, may be a profitable way of diminishing plant-available soil nitrogen, thus maximizing the contribution of nitrogen from N2 fixation with benefits in seed yield and protein content.
Abstract: Nitrogen fixation by irrigated soybeans (Glycine max (L.) Merr. cv. Forrest) was studied in a field experiment on a grey clay soil at Trangie, N.S.W. during the summer of 1985-86. Cropping with oats during the previous winter diminished the concentration of plant-available nitrogen in the soil from 37.6 to 18.5 mg N kg-1 and induced differences in the natural abundance of 15N (S15N) in this nitrogen. Four rates of liquid inoculation with Bradyrhizobium japonicum strain CB 1809, interacted with soil nitrogen to produce a wide range of nodulation of the soybeans. The following main effects on growth and N2 fixation resulted: (a) Initially, growth and accumulation of plant nitrogen was lower in pre-cropped than in prefallowed soil but N2 fixation was higher. (b) Nitrogen fixation during seed development was high in pre-cropped soil and greatest at the highest rate of inoculation. It resulted in high yields of seed (3.5 t ha-1 with 100 times the normal inoculation) with significantly higher concentration of seed nitrogen than from plants grown in prefallowed soil. (c) With increasing rates of inoculation on the pre-fallowed soil, more uniform nodulation was associated with smaller variances in most of the parameters studied. Other findings included further validation of the S15N method of calculating the proportion (p) of plant nitrogen derived from N2 fixation, with good agreement between treatment effects based on such estimates and those based on the relative concentrations of ureides in vacuum-extracted xylem sap. The values of p from S15N measurements on shoot nitrogen were affected little by inclusion of root nitrogen, and similar values were obtained when uninoculated, unnodulated Forrest soybeans, a nonnodulating genotype (non-nod Clark 63) or extractable mineral nitrogen of soil were used to estimate the S15N of plant N assimilated from soil. More dry matter (flowers, young pods and older leaves) containing more nitrogen (23-26 kg N ha-1) fell from the canopy of plants during seed maturation on pre-fallowed soil (high nitrogen) than on pre-cropped soil (13-15 kg N ha-1). Several correlations between the various quantities measured were noted and are discussed. It is concluded that growing winter cereals on land newly broken from pasture, coupled with high rates of inoculation of the following soybeans, may be a profitable way of diminishing plant-available soil nitrogen, thus maximizing the contribution of nitrogen from N2 fixation with benefits in seed yield and protein content.

90 citations


Journal ArticleDOI
01 Feb 1989-Ecology
TL;DR: An annual replacement of eucaryotic algae by nostocalean cyanobacteria was studied by monitoring the algal and zooplankton communities as well as various nutrient components during three growing seasons in Pleasant Pond, Minnesota.
Abstract: An annual replacement of eucaryotic algae by nostocalean cyanobacteria was studied by monitoring the algal and zooplankton communities as well as various nutrient components during three growing seasons in Pleasant Pond, Minnesota. Short-term factorial experiments consisting of nitrogen and phosphorus enrichments and macrograzer removal were performed, and algal population growth was observed to determine whether nitrogen, phosphorus, or both were potentially limiting to different phytoplankton taxa. The algal taxa that were consumed by the dominant grazer, Daphnia pulex, were also determined. Competition for nitrogen together with grazing on the inferior competitors occurred. Several other potential mechanisms -a changing nitrogen-to-phosphorus supply ratio, in- creasing grazing pressure, and competition for silicon-did not appear to be important. Nitrogen was more limiting than phosphorus. At least some nonheterocystous species were nitrogen limited in every experiment. During the transition to dominance by Nos- tocales, Nostocales did not usually respond to nitrogen additions, whereas many other species did. This taxonomic difference in nutrient limitation suggests that some of the species unable to produce heterocysts were being outcompeted for nitrogen. Daphnia grazers consumed Anabaenaflos-aquae, Fragilaria sp., gelatinous greens, pen- nate diatoms, and Scenedesmus spp. Algal responses to added nitrogen and to grazers were often interdependent. A behavioral indirect effect via the grazers' functional response could explain this dependence.

88 citations


Journal ArticleDOI
TL;DR: Variability of N2 fixation under low available P was observed, and high N2 fixing and high yielding progeny lines were detected.
Abstract: Phosphorus is one of several factors which affect N2 fixation and along with N, is a principal yield-limiting nutrient in many regions of the world. Since the legume plant is an essential partner in symbiotic N2 fixation, knowledge of host genotype variability for this process at different levels of P availability will be useful when breeding bean cultivars for enhanced N2 fixation. The objective of this study was to obtain common bean (Phaseolus vulgaris L.) lines able with enhanced ability to support biological N2 fixation under different levels of available phosphorus. Experiments were conducted in a growth room using a sand-alumina system to provide different levels of available P and in the field on a low-N soil. In the growth room studies, P availability strongly affected plant growth and traits related to N2 fixation. No significant interaction was detected for P levels × bean lines, indicating that bean lines performed similarly at both high and low P levels. Total shoot N was used as a direct and indirect measure of N2-fixation potential under growth room (N-free media) and field (low-N soil) conditions, respectively. Based on this criterion, two of the 41 and 54 inbred backcross lines of a segregating population evaluated in the growth room and the field, respectively, contained greater shoot N content than the recurrent parent and N shoot contents similar to the donor parent. Variability of N2 fixation under low available P was observed, and high N2 fixing and high yielding progeny lines were detected.

85 citations


Book
30 Nov 1989
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Journal ArticleDOI
TL;DR: The results indicate the importance of rhizobial distribution or movement into soil for nodulation, nodule distribution, strain competitiveness, and N(2) fixation in soil-grown legumes and suggest that bradyrhizobials migration from the initial point of placement was very limited.
Abstract: The influence of seed and soil inoculation on bradyrhizobial migration, nodulation, and N2 fixation was examined by using two Bradyrhizobium japonicum strains of contrasting effectiveness in N2 fixation. Seed-inoculated strains formed fewer nodules on soybeans (mostly restricted to the tap and crown roots within 0 to 5 cm from the stem base) than did bradyrhizobia distributed throughout the soil or inoculated at specific depths. Nodulation was greater below the depths at which bradyrhizobial cells were located rather than above, even though watering was done from below to minimize passive bradyrhizobial migration with percolating water. The most profuse nodulation occurred within approximately 5 cm below the point of placement and was generally negligible below 10 cm. These and other results suggest that bradyrhizobial migration from the initial point of placement was very limited. Nevertheless, the more competitive strain, effective strain THA 7, migrated into soil to a greater extent than the ineffective strain THA 1 did. Nitrogen fixation resulting from the dual-strain inoculations differed depending on the method of inoculation. For example, the amount of N2 fixed when both strains were slurried together onto the seed was about half that obtained from mixing the effective strain into the soil with the ineffective strain on the seed. The results indicate the importance of rhizobial distribution or movement into soil for nodulation, nodule distribution, strain competitiveness, and N2 fixation in soil-grown legumes.

Journal ArticleDOI
TL;DR: The interaction between phosphorus and nitrogen treatments (inoculated and nitrogen fertilized) demonstrated that there was a greater requirement of phosphorus for symbiotic nitrogen fixation than for plant growth when soil phosphorus was low.
Abstract: To examine how soil phosphorus status affects nitrogen fixation by the Casuarinaceae —Frankia symbiosis,Casuarina equisetifolia and two species ofAllocasuarina (A torulosa andA littoralis) inoculated or fertilized with KNO3 were grown in pots in an acid soil at 4 soil phosphate levels InoculatedC equisetifolia nodulated well by 12 weeks after planting and the numbers and weight of nodules increased markedly with phosphorus addition Growth ofC equisetifolia dependent on symbiotically fixed nitrogen was more sensitive to low levels of phosphorus (30 mg kg−1 soil) than was growth of seedings supplied with combined nitrogen; at higher levels of phosphorus, the growth response curves were similar for both nitrogen fertilized and inoculated plants The interaction between phosphorus and nitrogen treatments (inoculated and nitrogen fertilized) demonstrated that there was a greater requirement of phosphorus for symbiotic nitrogen fixation than for plant growth when soil phosphorus was low

Journal ArticleDOI
TL;DR: Although these effects were small, they indicate reduced precision in estimates of isotopic fractionation, and therefore of N 2 fixation using natural 15 N abundance using natural15 N abundance.
Abstract: White clover ( Trifolium repens L.) and red clover ( T. pratense L.) were grown in a medium free of combined nitrogen (N-free) with Rhizobium leguminosarum by. trifolii strain PDD 2668 or a mixture of field isolates of rhizobia. The natural abundance of 15 N was always lower in shoots than in roots and nodules. Whole plant N was depleted in 15 N (−0.44 and −0.15% for white and red clovers, respectively) and unaffected by inoculation treatment. However, shoots were more depleted in 15 N and roots and nodules were more enriched with the field isolates. With white clover, other factors examined were molybdenum (Mo) and phosphorus nutrition (low vs high levels), watering (regular vs less frequent) and water-logging. Whole plant N was more depleted in 15 N with low Mo nutrition or with less frequent watering. Although these effects were small, they have not been accounted for in the past and indicate reduced precision in estimates of isotopic fractionation, and therefore of N 2 fixation using natural 15 N abundance.

Journal ArticleDOI
TL;DR: It is demonstrated that bacterial genes determining early nodulation functions (nodABC) as well as the regulatory gene nodD3 are under nitrogen (NH4+) control, and the results suggest that the gene product of nodD 3 has a role in mediating the ammonia regulation of early nod genes.
Abstract: The presence of combined nitrogen in the soil suppresses the formation of nitrogen-fixing root nodules by Rhizobium. We demonstrate that bacterial genes determining early nodulation functions (nodABC) as well as the regulatory gene nodD3 are under nitrogen (NH4+) control. Our results suggest that the gene product of nodD3 has a role in mediating the ammonia regulation of early nod genes. The general nitrogen regulatory (ntr) system as well as a chromosomal locus mutated in Rhizobium meliloti were also found to be involved in the regulation of nod gene expression. A R. meliloti mutant with altered sensitivity to ammonia regulation was isolated, capable of more efficient nodulation of alfalfa than the wild-type strain in the presence of 2 mM ammonium sulfate.

Journal ArticleDOI
TL;DR: A strain defective in both V‐ Nitrogenase and Mo‐nitrogenase structural genes showed no residual nitrogen fixing capacity arguing against the presence of a third nitrogen fixation system in this organism.
Abstract: Structural genes for the VFe-protein (Ac1V) of the vanadium nitrogenase from Azotobacter chroococcum were cloned and sequenced. The VFe-protein contains three subunit types with Mr of 53,793 (alpha), 52,724 (beta) and 13,274 (delta). alpha and beta subunits show 18 and 15% sequence identity respectively, with alpha and beta subunits of the MoFe-protein of A.chroococcum molybdenum nitrogenase. The genes for the three subunits vnfD (alpha), vnfG (delta) and vnfK (beta) are contiguous and form an operon whose transcription is repressed in response to ammonia. The Fe-protein component of the V-nitrogenase (Ac2V) is the product of nifH* that we have previously cloned and sequenced. This gene was located 2.5 kb upstream of vnfD. A deletion in the vnfD, G and K gene cluster prevents V-dependent nitrogen fixation. A strain defective in both V-nitrogenase and Mo-nitrogenase structural genes showed no residual nitrogen fixing capacity arguing against the presence of a third nitrogen fixation system in this organism.

Book ChapterDOI
TL;DR: The utility of the natural 15N abundance approach to assess symbiotic N2 fixation is discussed and is analogous to isotopic dilution methods widely used in agricultural studies except that it takes advantage of small deviations in the natural15N abundance of soil from that of the atmosphere and, therefore, does not require isotope application to the soil.
Abstract: Deeply rooted woody plants capable of symbiotic N2 fixation are often dominant components of plant communities. The importance of symbiotic N2 fixation to the N economy of these plants and to overall ecosystem productivity is poorly understood. This is a consequence of technical difficulties in detecting and/or measuring N2 fixation under field conditions using conventional approaches (i.e., acetylene reduction assay). The utility of the natural 15N abundance approach to assess symbiotic N2 fixation is discussed by Shearer and Kohl in Chapter 20 of this volume. This approach is analogous to isotopic dilution methods widely used in agricultural studies except that it takes advantage of small deviations in the natural 15N abundance of soil from that of the atmosphere and, therefore, does not require isotope application to the soil.

Journal ArticleDOI
TL;DR: All grass species benefitted similarly from alfalfa, although earlier maturing species with greater competitive ability were slightly more responsive.
Abstract: Fixation and transfer of nitrogen (N) from alfalfa (Medicago sativa L.) to different grass species including timothy (Phleum pratense L.) and bromegrass (Bromus inermis Leyss) were studied under field conditions, using the15N dilution technique. The percentage of alfalfa N derived from fixation (%NF) increased throughout the growing seasons and ranged from 62 to 83%. Nitrogen transfer (NT) from alfalfa to associated grasses was evident and contributed 26,46 and 38% of the total annual N yield of associated grasses or represented absolute amounts of 5, 20 and 19 kg N ha−1 during the first, second and third year, respectively. The gradual and consistent percentage of NT that occurred before first harvest indicated that this transfer is a result of a direct excretion of N compounds from alfalfa root systems. Decomposition of root and nodule debris seems to contribute to the NT from alfalfa to associated grasses in the later cuts. All grass species benefitted similarly from alfalfa, although earlier maturing species with greater competitive ability were slightly more responsive.


Journal ArticleDOI
TL;DR: In mixed cropping the various interactions acting on N nutrition of the grass probably mask observations of the actual mechanisms involved, but there was clear isotopic evidence of N-transfer from the legume to the grass in non-mycorrhizal, P-supplemented plants.
Abstract: The legume Medicago sativa (alfalfa)(+ Rhizobium meliloti ) and Lolium perenne (ryegrass) were grown, in a greenhouse, either alone or together in a soil and supplied with increasing amounts of soluble phosphate (P) with or without a vesicular-arbuscular inoculum (VAM). A small amount of 15 N-labelled ammonium sulphate was added to each pot to distinguish the sources of N in the plants. The more mycotrophic legume enhance VAM formation by the grass in the mixture at all rates of P additions. Regardless of the cropping system and the P concentration in soil VAM improved, in most cases, dry matter production and the competitive ability of the legume. In spite of that competition from ryegrass reduced alfalfa development with increasing P concentration in the soil. In general VAM increased nodulation and the concentrations of N and P in alfalfa. The 15 N enrichment of plant shoots indicated that VAM improved N 2 -fixation in alfalfa at all rates of P. In mixed cropping, alfalfa derived almost all its N from fixation, but the total amount fixed was decreased by competition from ryegrass in the same pot. The apparent soil N pool size ( A -value) for the grass growing alone was significantly higher in mycorrhizal pots and VAM actually increased the total amount of N that the grass derived from soil, supporting a role of VAM in N-uptake. In mixed cropping the various interactions acting on N nutrition of the grass probably mask observations of the actual mechanisms involved, but there was clear isotopic evidence of N-transfer from the legume to the grass in non-mycorrhizal, P-supplemented plants. Apparently VAM enhanced N-transfer in one out of four cases.

Journal ArticleDOI
TL;DR: In this article, surface soil nitrogen changes along a subarctic vegetation chronosequence on a floodplain in central Alaska resulted from interactions between stochastic flooding and the influence of vegetation.
Abstract: Surface soil nitrogen changes along a subarctic vegetation chronosequence on a floodplain in central Alaska resulted from interactions between stochastic flooding and the influence of vegetation. River alluvium initially contributed 400 kg ha-1 of nitrogen to the top 200 mm of time-zero soils. Subsequent nitrogen accumulations were in part due to nitrogen fixers such as alder (Alnus tenuifolia). Kjeldahl nitrogen levels reached 1696 kg ha-~ in surface mineral soils of 30-yr-old alder stands. Extractable forms of nitrogen also increased four-fold in 30 yr. However, soil nitrogen increases also resulted from frequent floods that deposited additional nitrogen-rich alluvium. Frequently flooded low terraces had high silt content and relatively high nitrogen levels. Frequent flooding of some forested upper terraces resulted in poorer development of forest floors, higher sand content, and dilution of nitrogen levels compared with upper terraces that were less often flooded. Although nitrogen levels were primarily determined by biotic factors, periodic flooding and associated changes in particle size also affected concentration and pool sizes of nitrogen in soils of the vegetation chronosequence.


Journal ArticleDOI
TL;DR: The effect of oxygen, ammonium ion, and amino acids on nitrogenase activity in the root-associated N2-fixing bacterium Herbaspirillum seropedicae was investigated and there is no clear evidence that ADP-ribosylation of the dinitrogenase reductase is involved in the ammonium inhibition of H. seropedICae.
Abstract: The effect of oxygen, ammonium ion, and amino acids on nitrogenase activity in the root-associated N2-fixing bacterium Herbaspirillum seropedicae was investigated in comparison with Azospirillum spp. and Rhodospirillum rubrum. H. seropedicae is microaerophilic, and its optimal dissolved oxygen level is from 0.04 to 0.2 kPa for dinitrogen fixation but higher when it is supplied with fixed nitrogen. No nitrogenase activity was detected when the dissolved O2 level corresponded to 4.0 kPa. Ammonium, a product of the nitrogenase reaction, reversibly inhibited nitrogenase activity when added to derepressed cell cultures. However, the inhibition of nitrogenase activity was only partial even with concentrations of ammonium chloride as high as 20 mM. Amides such as glutamine and asparagine partially inhibited nitrogenase activity, but glutamate did not. Nitrogenase in crude extracts prepared from ammonium-inhibited cells showed activity as high as in extracts from N2-fixing cells. The pattern of the dinitrogenase and the dinitrogenase reductase revealed by the immunoblotting technique did not change upon ammonium chloride treatment of cells in vivo. No homologous sequences were detected with the draT-draG probe from Azospirillum lipoferum. There is no clear evidence that ADP-ribosylation of the dinitrogenase reductase is involved in the ammonium inhibition of H. seropedicae. The uncoupler carbonyl cyanide m-chlorophenylhydrazone decreased the intracellular ATP concentration and inhibited the nitrogenase activity of whole cells. The ATP pool was not significantly disturbed when cultures were treated with ammonium in vivo. Possible mechanisms for inhibition by ammonium of whole-cell nitrogenase activity in H. seropedicae are discussed.

Journal ArticleDOI
TL;DR: Evidence presented here suggests a need to select soybean cultivars that are tolerant to salt stress, and then to match them with tolerant and effective Bradyrhizobium strains, to improve symbiotic nitrogen fixation potential of soybean under NaCl and KCl salt stress.
Abstract: This paper examines the importance of salt tolerance of host cultivars, Bradyrhizobium strains, and host-Bradyrhizobium combinations on the symbiotic nitrogen fixation potential of soybean under NaCl and KCl salt stress. Plants were grown in a soil medium, and the experiments were conducted under controlled environment growth room conditions. Bradyrhizobium growth was examined in yeast-mannitol broth andB. japonicum strains tolerant of NaCl and KCl (80 mM) stress were identified. Soybean cultivar Williams, which was sensitive to salt stress, performed poorly both in growth and symbiotic nitrogen fixation, irrespective of whether it was matched with a tolerant or sensitive Bradyrhizobium strain. Tolerant cultivar Manchu sustained nodulation and nitrogen fixation, irrespective of whether it was matched with a tolerant or sensitive Bradyrhizobium strain. Evidence presented here suggests a need, first to select soybean cultivars that are tolerant to salt stress, and then to match them with tolerant and effective Bradyrhizobium strains.

Journal ArticleDOI
TL;DR: A polymer-producing strain of unicellular cyanobacteria, Synechococcus sp.
Abstract: A polymer-producing strain of unicellular cyanobacteria, Synechococcus sp., was isolated from a coastal lagoon in Florida. This strain, designated BG0011, excreted a highly viscous polysaccharide. Maximum observed growth rates for BG0011 were 2.5 div. day-2. BG0011 also exhibited nitrogen fixation (nitrogenase) activity under aerobic conditions and grew at near maximum rates in medium lacking reduced nitrogen. Growth and carbohydrate production were enhanced by carbon dioxide enrichment. Rheological study of the extracellular polysaccharide revealed a viscosity versus shear rate curve similar in shape to that of xanthan gum. Maximum observed rate of carbohydrate production was 1 g dry weight liter-1 month-1.

Journal ArticleDOI
TL;DR: In general, the excess bases and the excess Base/nitrogen ratio varied considerably among harvests, and significant positive relationships between total N and proton efflux excess base excretion and H+ excretion were observed.
Abstract: Red clover (Trifolium prateuse L.), sweet clover (Meliotus alba Medik.). alfalfa (Medicago sativa L.), and crown vetch (Vica villosa Roth) were grown in nitrogen-free Hoagland's solution in a declining nutrient condition. Increases in acidity of the nutrient solutions were recorded over the experimental period. In general, the excess bases and the excess bases/nitrogen ratio varied considerably among harvests. Significant positive relationships between total N and proton efflux excess bases and H+ excretion, and dry weight and H+ efflux were observed (...)

Journal ArticleDOI
TL;DR: The results suggest that P plays an important role in dinitrogen fixation through an effective translocation of P to the leaf when P supply is limited, and efficient cultivars obtained reasonably high yield through aneffective translocations of the absorbed P toThe leaf.
Abstract: The effect of P supply on absorption and utilization efficiency of P in relation to dry matter production and dinitrogen fixation was examined in 8 pigeon pea cultivars with different growth duration and a soybean cultivar under field conditions.

Journal ArticleDOI
TL;DR: The amount of nitrogen fixed by leucaena leucocephala (Lam.) de Wit was assessed on an Alfisol at the International Institute of Tropical Agriculture located in southwestern Nigeria and estimates with the15N dilution method gave nitrogen fixation of 134 kg ha−1 in six months.
Abstract: The amount of nitrogen fixed byLeucaena leucocephala (Lam.) de Wit was assessed on an Alfisol at the International Institute of Tropical Agriculture located in southwestern Nigeria. Estimated by the difference method, nitrogen fixation of leucaena inoculated with Rhizobium strain IRc 1045 was 133 kg ha−1 in six months. Inoculation with Rhizobium strain IRc 1050 gave a lower nitrogen fixation of 76 kg ha−1. Fertilization with 40 and 80 kg N ha−1 inhibited nitrogen fixation by 43–76% and 49–71%, respectively. Estimates with the15N dilution method gave nitrogen fixation of 134 kg ha−1 in six months when leucaena was inoculated with Rhizobium strain IRc 1045 and 98 kg ha−1 for leucaena inoculated with Rhizobium strain IRc 1050. This nitrogen fixation represented 34–39% of the plant nitrogen. Inoculated leucaena derived 5–6% of its nitrogen from applied fertilizer and 56–54% from soil.

Journal ArticleDOI
TL;DR: Comparison of estimates of N2 fixation derived from the 15N-dilution technique with those based on relative ureide content of xylem sap indicated that the latter offered a simple and reliable procedure for evaluating the symbiotic performance of supernodulating plants.
Abstract: Growth and symbiotic performance of soybean (Glycine max (L.) Merrill) cv. Bragg and three of its induced nodulation mutants (nod49, non-nodulating; ntsl 116, intermediate supernodulator; nts1007, extreme supernodulator) were compared throughout development under different nitrogen regimes (0, 2, 5 and 10 mol nitrate m−3). Nitrogen fixation was assessed using 15N-isotope dilution and xylem sap analysis for ureide content. Both techniques confirmed a complete lack of N2 fixation activity in nod49. Plant reliance on nitrogen fixation by the other genotypes was dependent on the nitrate regime and the developmental stage. The ntsl007 and ntsl 116 mutants fixed more nitrogen than the parent cultivar in the presence of 10 mol m−3 nitrate in the nutrient solution, but higher input of symbiotically derived nitrogen was still insufficient to offset the amount of nitrogen removed in the harvested seed. However, the mutants utilized less nitrate for growth than Bragg. Comparison of estimates of N2 fixation derived from the 15N-dilution technique with those based on relative ureide content of xylem sap indicated that the latter offered a simple and reliable procedure for evaluating the symbiotic performance of supernodulating plants.

Journal ArticleDOI
TL;DR: The findings suggest that the quality and quantity of plant carbohydrates, proteins and amino acids shift in response to the physiological changes resulting from infection by N2-fixing bacteria or endomycorrhizal fungi.
Abstract: Soybean [Glycine max (L.) Merr. cv. Amsoy 71] plants were inoculated with either the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus fasciculatum, with a strain of Bradyrhizobium japonicum. or with both endophytes together. Non-inoculated plants were fertilized with levels of N and P that resulted in plant growth similar to that following infection by Bradyrhizobium or Glomus, respectively. In general, plants colonized by Glomus contained more pinitol (1d-3-O-methyl-chiro-inositol) but less starch than corresponding P-fertilized soybeans, while nodulated soybeans contained more pinitol and sucrose in the roots than corresponding N-fertilized plants. Compared to nutrient-amended soybeans, increases or decreases in free amino acids in symbiotically grown plants tended to follow transient changes in N content and endophyte activity. The exception to this was the elevated levels of methionine in nodulated roots or aspartate and arginine in Glomus-inoculated roots regardless of N level. There were both quantitative and qualitative changes in the proteins found in inoculated soybeans, especially in infected roots where nodulins (nodule-specific proteins) and endomycorrhizins (mycorrhiza-specific proteins) were detected by polyacrylamide gel eletrophoresis. Five nodulins (11.8% of root proteins) were detected with molecular weights of 14, 34, 40, 41 and 100 kDa. Five endomycorrhizins (5.1% of root proteins) were detected with molecular weights of 16, 17, 18, 22 and 30 kDa. Differences in nutrient uptake and allocation in symbiotic plants could not be duplicated by fertilizer input alone, although nutrient addition could lead to similar growth rates and dry weight accumulation. These findings suggest that the quality and quantity of plant carbohydrates, proteins and amino acids shift in response to the physiological changes resulting from infection by N2-fixing bacteria or endomycorrhizal fungi.