Bradyrhizobium Populations Research Articles

Efeito de concentrações de solução açucarada na aderência do inoculante turfoso às sementes, na nodulação e no rendimento da soja

A associação simbiótica da soja [Glycine max (L.) Merrill] com bactérias do gênero Bradyrhizobium resulta no processo de fixação biológica do N2. A inoculação com estirpes selecionadas garante a maximização do processo biológico e, no Brasil, são comercializados, anualmente, quase 13 milhões de doses de inoculante, a maioria à base de turfa. A adesão da turfa às sementes, porém, é baixa, requerendo o uso de adesivos. A recomendação atual é de uso de solução açucarada (açúcar comercial) a 25%, mas essa concentração poderia reduzir o vigor e aumentar a incidência de doenças nas sementes. Neste estudo, foram testadas, então, doses inferiores a 25%. Confirmou-se que a solução açucarada favorece o processo de inoculação, incrementando em até 90% a aderência da turfa às sementes e, em casa de vegetação, aumenta a nodulação e as taxas de fixação do N2. Em quatro experimentos realizados em solos com população estabelecida de Bradyrhizobium, a reinoculação da soja com solução açucarada a 10% resultou em um incremento médio, estatisticamente significativo, de 237 kg ha-1 de grãos, ou 8,6%, em relação à reinoculação com água, e de 17,5%, em relação à população naturalizada do solo, indicando que a concentração de açúcar atualmente recomendada pode ser reduzida para 10%.

Low pH and aluminum tolerance of Bradyrhizobium strains isolated from acid soils in Indonesia

Sixty-seven strains of Bradyrhizobium isolated from soybean plants growing on acid soils in West Java and Sumatra, Indonesia, were examined for the effect of the pH and aluminum concentration on their growth in nutrient media, compared with 61 strains of Bradyrhizobium from soils in Japan. The results in this study indicated that the indigenous population of Bradyrhizobium in the soils of Indonesia showed a large difference in acid- and Al-tolerance from that of Japan. Eighty-five and 48% of the isolates from Japanese soils and Indonesian soils, respectively, were unable to grow in YEM broth at pH below 4.5. The acid-tolerance was correlated with AI-tolerance of the isolates on YEM agar plates at pH 4.4. Seventy-five percent of the isolates that grew in YEM broth at pH 4.5 were also resistant to 400 µM Al on the YEM plates. Acetylene reduction assay of the root nodules revealed that 3 of the acid- and Al-tolerant isolates from Indonesian soils showed a significantly high nitrogen fixation activity.

Using spontaneous antibiotic-resistant mutants to assess competitiveness of bradyrhizobial inoculants for nodulation of soybean

Spontaneous mutants (3/parental strain) of soybean bradyrhizobia resistant to streptomycin and erythromycin were selected from strains isolated from bradyrhizobial populations indigenous to Cape Fear and Dothan soils. These were used to evaluate (i) the validity of using antibiotic-resistant mutants to make inferences about the competitiveness of parental strains in soil environments and (ii) the recovery of strains in nodules after inoculation of soybeans grown in soils with indigenous bradyrhizobial populations. Streptomycin and erythromycin resistances of all mutants were stable after approximately 27 generations of growth in yeast extract - mannitol medium, but 33% of the mutants lost resistance to erythromycin upon passage through nodules. Only 17% of the mutants were as competitive as their parental strain when inoculated in a ratio near 1:1 in vermiculite. Four of 10 mutants, which differed in competitiveness from their parental strain in vermiculite, had competitiveness against the soil populations equal to that of their parental strain. Therefore, assessment of competitiveness of mutants and parental strains in non-soil media may not accurately reflect their competitiveness in soil systems. For both the Cape Fear and Dothan soils, recovery of a given mutant from nodules of field-grown plants was always lower than from nodules of plants grown in the greenhouse. Inoculation of the entire rooting zone in the greenhouse experiment and of only a portion of the rooting zone in the field experiments may account for this difference in recovery. Techniques that increase the volume of soil inoculated may enhance nodulation by inoculant strains.Key words: Bradyrizobium, antibiotic resistance, competition.

Lipopolysaccharide Profiles from Nodules as Markers of Bradyrhizobium Strains Nodulating Wild Legumes

To develop the use of electrophoretic lipopolysaccharide profiles for Bradyrhizobium strain identification, we studied the feasibility of using electrophoresis of whole legume nodule homogenates to obtain distinctive lipopolysaccharide profiles. The electrophoretic patterns were the same whether we used nodule extracts, bacteroids, or cultured bacteria as samples, and there was no evidence of changes in the ladder-like pattern during the nodulation process. To assess the reliability of using lipopolysaccharide profiling performed with individual nodules for studying the diversity and microdistribution of the rhizobia nodulating wild shrub legumes, we used a population of Adenocarpus foliolosus seedlings. We obtained 75 different profiles from the 147 nodules studied. There was no dominant profile in the sample, and a plant with different nodules generally produced different profiles. Electrophoresis of legume root nodules proved to be a fast and discriminating technique for determining the diversity of a bradyrhizobial population, although it did not allow the genetic relationships among the nodulating strains to be studied.

Influence of soil and crop factors on the native rhizobial populations in soils under dryland farming

Native rhizobial populations ( Bradyrhizobium sp.) in 88 soil samples from 13 important legume growing locations in India are estimated and regressed with mean annual rainfall, soil pH, organic carbon, clay content and crop history. These locations are under dryland farming with Alfisols and Vertisols as predominant soil orders and divergent cropping histories. Wide variability was found for the abundance of native Bradyrhizobium populations in samples from within a location and among the locations. The populations showed significant relationship with organic carbon and previous crop history but clay content, mean annual rainfall and soil pH had no influence. Variation in the solum depth did not have significant influence on the distribution of Bradyrhizobium sp. in the profile (0–25 cm) of a newly cleared Alfisol watershed site which was under long term fallow. The population, however, increased significantly following crop cultivation. The increase was more with pigeonpea and sorghum than with sunflower. Data from 28 inoculation trials involving three legumes were used to assess the relationship between population of native Bradyrhizobium at planting, previous crop history and the response of the legumes to inoculation. When castor bean and sunflower are cultivated in the previous year, there was a significant decline in the population of Bradyrhizobium sp. and a positive response to inoculation of legumes in the subsequent year. When pigeonpea, groundnut and sorghum were grown as preceding crops, the population remained high (> log 10 3.0 MPN/g soil) at planting and very few trials showed positive response. While stimulatory effect of legume cultivation on rhizobial population is well documented, present studies provide evidence to the suppression of the population by some non legume crops in the cropping system. The overall results revealed that crop related factors have more critical influence on the abundance of native rhizobial populations than soil or climatic factors.

Phenotypic characterization of soybean bradyrhizobia in two soils of North Carolina

Serotypic composition of nodules (480 per soil type) from five soybean cultivars grown on two (Dothan and Cape Fear) soils of the Atlantic Coastal Plain of North Carolina was characterized. Symbiotic N 2-fixation efficiency, capacity for induction of foliar chlorosis symptomatic of rhizobitoxine production and antibiotic resistances of isolates purified from these nodules were also determined. While host plant cultivar had no significant effect on the serotype distribution, soil type had a large effect on the distribution and diversity of serotypes. Forty-six serotypes were identified among nodules from the Cape Fear soil, but only serotype 46 76 (8%), 76 (11%), 94 (9%) and 122 124 (12%) occurred in more than 5% of the nodules. Thirty percent of nodule occupants were not identified with the eleven antisera used. Twenty-four serotypes were identified among nodules from the Dothan soil. Of these serotypes 31 94 (32%), 46 76 (16%), and 76 (23%) occurred in more than 15% of the nodules. Five percent of the nodule occupants were not identified. Major serotypes did not change, but their frequency changed when fields were sampled at different growth stages in the same season and at the same growth stage in different seasons. Isolates serotyped as 31 94 , 46 76 , and most of the isolates serotyped as 76 generally exhibited higher levels of resistance to streptomycin and erythromycin than isolates serotyped as 24, 94 and 122 124 . Five percent of the isolates from the Cape Fear soil (all serotyped as 31 94 ) and 18% of the isolates from the Dothan soil (serotyped as 31 94 or 76) induced foliar chlorosis when cultivar Brim was the host. Only 12–14% of the isolates from the two populations had N 2-fixation capacity equal to or greater than that of the efficient reference strain MN110. However, four improved soybean cultivars grown in the same fields and year that isolates were obtained did not exhibit a significant seed yield response to application of 150 kg N ha −1 when yields in the minus N treatment ranged from 3.2 to 3.7 Mg ha −1. A significant seed yield response by a non-nodulated cultivar indicated that these soils were N limited. Therefore, the symbiotic N 2-fixation capacity of these bradyrhizobial populations did not limit soybean seed yields despite the low percentage of isolates with high N 2-fixation efficiency and the presence of isolates with the capacity to induce leaf chlorosis symptomatic of rhizobitoxine production.

Influence of soil properties and soya bean cropping history on the Bradyrhizobium japonicum population in some French soils

SummaryAs French soils are naturally free of Bradyrhizobium japonicum, soya beans planted into new areas must be inoculated with this bacterium. Although, the B. japonicum inoculum can survive in soils for long periods of time even in the absence of a soya bean crop, re–inoculation may increase nodulation and grain yield. Thus, populations of B. japonicum can fall below optimum for plant growth. To identify the soil properties controlling survival of the inoculated bacteria samples of soil were collected from 52 sites from France that had previously grown soya beans. The samples were analysed for some physical and chemical characteristics and the B. japonicum population counted. The soil's CaCO3 content was the main factor affecting survival. The average B. japonicum numbers (per g soil) were 80 for calcareous soils and 15000 for non–calcareous soils. In the latter, silt and sand contents were correlated with the numbers of B. japonicum. The cropping frequency of soya bean and the time since the crop was last grown were other factors affecting Bradyrhizobium populations. Thus, there is a probability of enhancing economic benefit for farmers with re–inoculation of soya bean in calcareous and sandy soils.

Inheritance of Host‐Controlled Restriction of Nodulation by Bradyrhizobium japonicum Strain USDA 110

The use of soybean [Glycine max (L.) Merr.] genotypes which specifically restrict nodulation by competitive indigenous Bradyrhizobium populations has been proposed as an approach to favor increased nodulation by more effective, inoculant‐quality strains. Soybean plant introduction (PI) PI 417566 restricts nodulation by Bradyrhizobium japonicum (Kirchner, Jordan) strains in serogroup 110. In this study, the interaction of PI 417566 with strains in serogroup 110 is being used as a model system to determine whether host‐controlled nodulation restriction can be used to alter the competitiveness of indigenous bradyrhizobia. The objectives of this study were to determine the type of inheritance and the number of genes in PI 417566 which condition restriction of nodulation by strain USDA 110 and whether this gene(s) could function to prevent nodulation of serogroup 110 strains under field conditions. Inheritance of host‐controlled nodulation restriction was determined by crossing P1417566 with the nonrestricting cultivars Agassiz and Evans. The nodulation phenotype of 126 F2 plants from Agassiz × PI 417566 and 44 F2 plants from Evans × PI 417566 was assessed in growth chamber studies. In the former cross, 98 nodulating and 28 non‐nodulating plants, and in the latter, 32 nodulating and 12 non‐nodulating plants were observed. In both cases, a Chi‐squared test indicated a good fit to a 3:1 segregation ratio, indicating that a single recessive gene conditions restricted nodulation in P1 417566. A 2‐yr field study utilizing F2:4 and F2:5 progeny from Agassiz × PI 417566 and F2:4 progeny from Evans × PI 417566 demonstrated that the lines had the ability to restrict nodulation by serogroup 110 strains under field conditions and that restriction occurred even in the presence of large numbers (5 × 1O7 cells/g soil) of added USDA 110.

Ability of two Bradyrhizobium japonicum strains to persist in soil and nodulate soybeans after dual inoculation

The competitiveness of two Bradyrhizobium japonicum strains was determined under controlled conditions. The relationship between the ratio of the nodules formed by the two bradyrhizobial strains and the ratio of the two strains in the inoculum was established and used as an internal standard to evaluate the equilibrium ratio between populations of the two strains in the soil. The fluorescent antibody (FA) technique was also used for counting the bradyrhizobial populations directly in the soil. Results with the FA technique were analysed and compared with the estimate obtained through nodule occupancy. The initial ratio of the two bradyrhizobial strains in populations introduced simultaneously into the soil changed during incubation time and the two strains reached the same equilibrium levels after 53 days. Never-theless, the strain introduced into the soil as an overpopulation remained clearly dominant for the nodule occupancy. We conclude that direct counts with the FA approach are selective and these counts could be mainly of the unattached bacteria.

Enrichment of Bradyrhizobium spp populations in soil due to cropping of the homologous host legume

Host legumes can enrich their immediate soil environment with rhizobia through rhizosphere effects. The extent to which this enrichment occurs, the specificity of the process and its interaction with soil management factors remain poorly described. In a series of field trials, we measured changes in the size of indigenous populations of Bradyrhizobium in response to cropping of host and non-host legumes under two N fertilizer regimes. Uninoculated cowpea ( Vigna unguiculata) and soybean ( Glycine max) were grown with or without applied urea (900 kg N ha −1) at three field sites on the island of Maui, HI, U.S.A., not previously cropped with legumes. Using the most-probable-number plant infection method and Siratro ( Macroptilium atropurpureum) as the host, the population density of Bradyrhizobium in the bulk soil at each site was measured at planting and at grain maturity and compared to the population density in adjacent fallow soil. When the size of the initial indigenous population was low (18 and 580 cells g −1 soil), significant increases in the population density compared to fallow soil were observed only in soils cropped with cowpea receiving no applied urea. When the size of the initial indigenous bradyrhizobial population was high (5.8 × 10 4 cells g −1 soil), no significant increase in the population density was observed. These results suggested that enrichment of soil bradyrhizobial populations was host-specific, that symbiotic legumes can enrich their soil environment with microsymbionts up to a threshold level and that such enrichment can be curtailed by soil management practices that suppress nodulation.

Variation of Clonal, Mesquite-Associated Rhizobial and Bradyrhizobial Populations from Surface and Deep Soils by Symbiotic Gene Region Restriction Fragment Length Polymorphism and Plasmid Profile Analysis

Genetic characteristics of 14 Rhizobium and 9 Bradyrhizobium mesquite (Prosopis glandulosa)-nodulating strains isolated from surface (0- to 0.5-m) and deep (4- to 6-m) rooting zones were determined in order to examine the hypothesis that surface- and deep-soil symbiont populations were related but had become genetically distinct during adaptation to contrasting soil conditions. To examine genetic diversity, Southern blots of PstI-digested genomic DNA were sequentially hybridized with the nodDABC region of Rhizobium meliloti, the Klebsiella pneumoniae nifHDK region encoding nitrogenase structural genes, and the chromosome-localized ndvB region of R. meliloti. Plasmid profile and host plant nodulation assays were also made. Isolates from mesquite nodulated beans and cowpeas but not alfalfa, clover, or soybeans. Mesquite was nodulated by diverse species of symbionts (R. meliloti, Rhizobium leguminosarum bv. phaseoli, and Parasponia bradyrhizobia). There were no differences within the groups of mesquite-associated rhizobia or bradyrhizobia in cross-inoculation response. The ndvB hybridization results showed the greatest genetic diversity among rhizobial strains. The pattern of ndvB-hybridizing fragments suggested that surface and deep strains were clonally related, but groups of related strains from each soil depth could be distinguished. Less variation was found with nifHDK and nodDABC probes. Large plasmids (>1,500 kb) were observed in all rhizobia and some bradyrhizobia. Profiles of plasmids of less than 1,000 kb were related to the soil depth and the genus of the symbiont. We suggest that interacting selection pressures for symbiotic competence and free-living survival, coupled with soil conditions that restrict genetic exchange between surface and deep-soil populations, led to the observed patterns of genetic diversity.

Field Response of Soybean to Increased Dinitrogen Fixation

A number of studies have shown an association between soybean [Glycine max (L.) Merr.] seed yield and plant N accumulation, suggesting that yield could be increased by nodulation by superior strains of Bradyrhizobium japonicum. Attempts to evaluate the response of field‐grown soybean to bradyrhizobia‐mediated increases in N accumulation have been hindered by an inability of introduced strains to compete successfully with indigenous strains. Therefore, to determine the effect of increased N2 fixation on soybean growth and seed yield, an experiment was conducted using soybean seedlings that were nodulated under greenhouse conditions and then transplanted to the field. Two inoculation treatments, USDA 122 and a mixture of bradyrhizobia indigenous to the field site (control), were applied to the seed and seedlings were developed for 21 d in the greenhouse. Prenodulated seedlings were then transplanted to 1.8 m2 field microplots on a Matapeake silt loam (fine‐loamy, mixed, mesic Typic Hapludult) with a most probable number of indigenous bradyrhizobial population of 6.1 × l08 cells kg−1 soil. Relative to the control, nodulation by USDA 122 increased both N2 fixed and total shoot N by a 2‐yr average of 29%. Plants nodulated by USDA 122 also assimilated more soil N. The increase in N2 fixation and total N accumulation occurred prior to, but not during, seedfill. The USDA 122 treatment increased seed yield by an average of 31% (90 g m−2). The yield increase was associated with increases in pod number, seedfill rate, remobilized dry matter, remobilized N, and vegetative biomass.

Effects of Bradyrhizobium strain and host genotype, nodule dry weight and leaf area on groundnut (Arachis hypogaea L. ssp. fastigiata) yield

Effects of inoculating four Arachis hypogaea ssp. fastigiata cultivars with 17 Bradyrhizobium spp. strains were studied in a glasshouse experiment using a sandy soil devoid of an indigenous Bradyrhizobium population. Firstly, a wide range of parameters, indicative of symbiotic performance, were assessed for their influence on seed yield, by correlation and statistical analyses. It was found that nodule dry weight and leaf area were relevant parameters concerning seed yield. Secondly, the effects of host and strain genotype on those parameters were described.

Sludge‐Borne Salt Effects on Survival of Bradyrhizobium japonicum

AbstractThe soil population of Bradyrhizobium japonicum, the microsymbiont of soybean [Glycine max (L.) Merr.], is reduced following land application of sewage sludge. Previous research on the ability of B. japonicum to survive in soil recently amended with sewage sludge has been conflicting as to whether sludge‐borne salts or heavy metals inhibit survival. To examine this question, sewage sludge compost was added to a rhizobia‐free, Sassafras sandy loam soil (Typic Hapludult, fine loamy mixed mesic) amended with various rates of sludge compost, inoculated with B. japonicum USDA 110 ARS and the mixture was incubated for 42 d. The population of B. japonicum was reduced to nearly undetectable levels when inoculated into soil amended with sludge compost at a rate of 25%. The sludge compost was subsequently leached with 3‐, 6‐, 9‐ and 12‐column void volumes of distilled water resulting in sludge compost samples with equal concentrations of heavy metals but decreasing concentrations of soluble salts. Leached and unleached compost samples were added to soil at a rate of 25% and the mixture inoculated with B. japonicum. Soil amended with sludge compost leached with six void volumes water exhibited the highest rhizobial population, while soil amended with unleached sludge compost contained the lowest rhizobial population. In soil amended with sludge compost leached with 3‐, 9‐, and 12‐void volumes water, populations were not significantly different. Results demonstrate that sludge‐borne soluble salts, and not heavy metals, are primarily responsible for short‐term reductions in bradyrhizobial populations following the application of very high rates of sludge to soil.

Response of Soybean to Inoculation with Efficient and Inefficient Bradyrhizobium japonicum Variants

Soybean [Glycine max (L.) Merr.] plants are grown in soils containing large indigenous bradyrhizobial populations. Individual strains within these populations differ in symbiotic efficiency and competitiveness for nodule occupancy. The purpose of this study was to determine the effect of mixed inocula of an efficient and an inefficient variant of Bradyrhizobim japonicum on symbiotic performance in soybean. Lee soybean was initially inoculated with either an efficient or an inefficient USDA II0 colony morphology variant. The opposite variant was then applied after 0, 2, 4, 8, or 13 d. Delayed inoculation with the efficient variant following the inefficient variant resulted in progressively decreased symbiotic performance. In a subsequent competition experiment, soybean cultivars Lee and Ransom were inoculated with efficient and inefficient variants at ratios of 1:1, 1:10 and 10:1. More nodules were formed by the efficient variant than were expected by chance. Significantly reduced dry weight and whole plant N contents were noted when the inefficient variant was present in more than 50% of the nodules. Split‐root experiments were conducted and either type of variant was capable of inhibiting the other after a 7‐d delay in inoculation. In contrast, when both sides of the split‐root were simultaneously inoculated, nodule numbers were similar; however, nodules formed by the efficient variant were larger. This increase in size indicated a preferential partitioning of photosynthate to the nodules formed by the efficient variant. These results together indicate that N2 fixation is enhanced with increased nodule occupancy by superior variants due to more effective strain‐cultivar interactions.

Competitiveness of indigenous populations of Bradyrhizobium japonicum serocluster 123 as determined using a root-tip marking procedure in growth pouches

Soil Bradyrhizobium populations limit nodule occupancy of soybean by symbiotically-superior inoculant strains throughout much of the American midwest. In this study, the competitiveness of indigenous populations of B. japonicum serocluster 123 from Waukegan and Webster soils was evaluated in growth pouches using a root-tip marking procedure. The native rhizobia were from soils incubated 0–8 h in soybean root exudate (SRE) or plant nutrient solution (PNS) prior to inoculation. Populations of serocluster 123 strains in soil and nodule occupancy by these strains were assessed using fluorescent antibodies prepared against B. japonicum USDA 123. There were no significant differences in populations that came from SRE or PNS incubated soils: both populations increased in number over the incubation period. Nodule occupancy by both populations in growth pouches was similar to that previously encountered in field studies with these two soils. With the Waukegan soil, the serocluster 123 population dominated nodulation forming 69 and 62% of taproot nodules above and below the root tip mark, respectively. However, for the more alkaline Webster soil, serocluster 123 strains were much less competitive, producing only 9 and 13%, respectively, of the nodules formed above and below the root tip mark. In growth pouches, soil populations of bradyrhizobia from the Webster soil produced significantly more nodules than those from the Waukegan soil, but both strains and a pure culture of USDA 110 had a similar distribution of nodules.

Subgroups of the Cowpea Miscellany: Symbiotic Specificity within Bradyrhizobium spp. for Vigna unguiculata, Phaseolus lunatus, Arachis hypogaea , and Macroptilium atropurpureum

Rhizobia classified as Bradyrhizobium spp. comprise a highly heterogeneous group of bacteria that exhibit differential symbiotic characteristics on hosts in the cowpea miscellany cross-inoculation group. To delineate the degree of specificity exhibited by four legumes in the cowpea miscellany, we tested the symbiotic characteristics of indigenous cowpea bradyrhizobia on cowpea (Vigna unguiculata), siratro (Macroptilium atropurpureum), lima bean (Phaseolus lunatus), and peanut (Arachis hypogaea). The most-probable-number counts of indigenous bradyrhizobia at three sites on Maui, Hawaii, were substantially different on the four hosts: highest on siratro, intermediate on cowpea, and significantly lower on both lima bean and peanut. Bradyrhizobia from single cowpea nodules from the most-probable-number assays were inoculated onto the four hosts. Effectiveness patterns of these rhizobia on cowpea followed a normal distribution but were strikingly different on the other legumes. The effectiveness profiles on siratro and cowpea were similar but not identical. The indigenous cowpea-derived bradyrhizobia were of only moderate effectiveness on siratro and were in all cases lower than the inoculant-quality reference strain. Between 5 and 51% of the bradyrhizobia, depending on site, failed to nodulate peanut, whereas 0 to 32% failed to nodulate lima bean. No significant correlation was observed between the relative effectiveness of the bradyrhizobia on cowpea and their corresponding effectiveness on either lima bean or peanut. At all sites, bradyrhizobia that were ineffective on cowpea but that effectively nodulated lima bean, peanut, or both were found. Eighteen percent or fewer of the bradyrhizobia were as effective on lima bean as the reference inoculant strain; 44% or fewer were as effective on peanut as the reference strain. Only 18% of all cowpea-derived bradyrhizobia tested were able to form N(2)-fixing nodules on both lima bean and peanut. These results indicate the need to measure indigenous bradyrhizobial population characteristics directly with the crop of interest to obtain an accurate assessment of the need to inoculate.

Ammonium and glutamate assimilation by tryptophan‐catabolic variants of Bradyrhizobium japonicum USDA 26

Tryptophan‐catabolic variants, tan 4b and 18ac, of Bradyrhizobium japonicum USDA 26 were isolated from enrichment cultures in nitrogen (N)‐limited media containing either ammonium or glutamate. Presence of exogenous tryptophan (trp) in the medium led to sparse restricted growth and elicited selective growth of tan‐coloured variants over that of parental USDA 26. A 36% increase in cellular uptake‐accumulation of the ammonia analogue [14C]‐methylamine was found with trp‐induced tan 18ac cells over that measured with either ammonium‐induced tan 18ac or trp‐induced tan 4b. The assimilation patterns of uniformly labelled [14C]‐glutamate also differed when tan 18ac was compared with tan 4b. These studies of tan variants isolated from enrichment cultures suggest that bradyrhizobial populations can be manipulated by changing the N sources that limit their growth.

Intrinsic antibiotic resistance and serological characterization of populations of indigenous Bradyrhizobium isolated from nodules of Desmodium intortum and Macroptilium atropurpureum in three soils of S.E. Queensland

A collection of 1275 isolates of bradyrhizobia from three sites was made in the first (early) and 5th or 6th (late) growing seasons of uninoculated field plots of Desmodium intortum and Macroptilium atropurpureum. Isolates were characterized by intrinsic antibiotic resistance (IAR) and serological techniques. Non-hierarchial pattern analysis was used to group the isolates into antiobiotic inhibition response (IAR) types. Antisera were prepared for representative isolates of the most populous IAR types representing both early and late growing seasons. The data were used to test the hypothesis that under the influence of the soil root-rhizosphcre environment one or two IAR or serological types may dominate in the formation of nodules after several growing seasons. The IAR inhibition response patterns indicated a range of heterogeneity within and between sites and there was no significant increase of any IAR type in proportion of nodules formed in the late growing season samples. Most isolates at a site were accounted for by 2 or 3 of the IAR groups but the proportions of these types were different between sites. Serological grouping did not agree with the IAR groups, was more heterogeneous, and identified only 37–68% of isolates. However, there were serological types at each site that increased their proportion in the nodule population during the period of the experiment. Although the proportion of IAR types did not change between growing seasons the IAR screening provided a mechanism of selecting a representative dominant type for serological characterization and thus an avenue for selecting dominant strains that intrinsically may have the survival and competitive qualities required of a good inoculum strain.

Competition for nodule occupancy of introduced Bradyrhizobium japonicum strains in a Wisconsin soil with a low indigenous bradyrhizobia population

The population dynamics of six introduced Bradyrhizobium japonicum strains were measured over three growing seasons in a Wisconsin soil with a low incidence of indigenous B. japonicum (10 cells/gm). Four antibiotic-resistant members of the 123 serocluster (which were either spectinomycin resistant or streptomycin resistant), USDA 110, and USDA 138 were inoculated using liquid inoculum, at a rate of 1 × 108 cells per 2.5-cm row, on two soybean cultivars in 1985. Nodule occupants were identified using an enzyme-linked immunosorbent assay (ELISA), fluorescent antibodies, and antibiotic-resistant mutants. In the first growing season, 100% of the nodules were formed by the introduced strains. The nodules from the uninoculated plots were occupied by an indigenous 110 serogroup. In the second and the third season at the same site (without further inoculation), a high percentage (> 60%) of the nodules from all the plots were nodulated by the 123 serocluster (either alone or as mixed infections). However, < 25% of the nodules in the 123-inoculated plots and < 9% in the other plots were formed by any of the antibiotic-marked 123 inoculum strains introduced in 1985. The main conclusions are (i) that it is possible to successfully introduce inoculum strains in soils where the indigenous Bradyrhizobium population is low and to obtain 100% nodule occupancy in the first growing season, and (ii) that successful inoculation in one year in soils with a low incidence of Bradyrhizobium does not ensure that the introduced inoculum strains will form nodules in subsequent years. Key words: Bradyrhizobium japonicum, indigenous bradyrhizobia, interstrain competition, field trials.