Indigenous Rhizobia Population Research Articles

Symbiotic effectiveness of Bradyrhizobium strains on soybean growth and productivity in Northern Mozambique

Inoculation of soybean [Glycine max (L.) Merr.] with rhizobia strains is a low-cost investment which can increase yields of smallholder farmers in Mozambique. The performance of four Bradyrhizobium strains was evaluated to identify the best strain to inoculate soybean grown in different agro-ecologies. Field experiments were conducted in three ecological zones in 2018 and 2019 using soybean variety Zamboane inoculated with Bradyrhizobium diazoefficiens strain USDA 110, B. japonicum strains USDA 136, USDA 442 and WB74, and a non-inoculated control in a randomized complete block design with four replications. Indigenous rhizobia populations at the sites ranged from 9.0 x 101 to 2.2 x 103 cells g−1 soil. All four strains increased nodulation, but USDA 110 was superior at two sites with low native rhizobia population, whereas USDA 442 and WB74 were the best at the site with relatively high native rhizobia population. On an average, the strains doubled the number of nodules and increased the dry weight up to 5.8-fold. Inoculation increased shoot dry weight and N content at podding, plant biomass, and number of pods plant−1 across sites but the effects of the strains on seeds per pod, and 100-seed weight were inconsistent. Shoot N content did not differ among inoculant strains and ranged from 15.70 g kg−1 in the control to 38.53 g kg−1 across inoculation. All four strains increased soybean grain yield across sites in 2018 but USDA 110 outperformed the other strains and was also the best at one of the two sites in 2019. Grain yield responses associated with USDA 110 ranged from 552 kg ha−1 (56%) to 1,255 kg ha−1 (76%). Positive correlations between nodule dry weight plant−1 and seed yield, and number of pods plant−1 and grain yield were observed. The gross margin ranged from $343.50–$606.80 ha−1 for the control, but it increased to $688.34–$789.36 when inoculants were applied. On an average, inoculation increased gross margin by $182.57-$395.35 ha−1 over that of non-inoculated control in 2018 but drought stress in 2019 reduced the benefit. The results demonstrate that USDA 110 was the best inoculant strain and has the potential of increasing smallholder productivity and net returns.

Root nodulation in guar: Effects of soils, Rhizobium inoculants, and guar varieties in a controlled environment

Guar (Cyamopsis tetragonolobus L. Taub.) is a leguminous crop plant that produces a seed containing galactomannan gum, which is used in many applications as a lubricant, binder, thickener, and emulsifier, among other functions. A recent spike in worldwide guar consumption has been driven largely by the industrial uses of the gum. As a legume, the primary ecosystem service agricultural producers expect from guar is nitrogen fixation, but the common perception is that guar does not nodulate effectively. The objective of this replicated and repeated greenhouse study was to test the effects of contrasting soils, representing typical alkaline soils in which guar is cultivated worldwide, and Rhizobia inoculants on nodulation and plant growth characteristics in two guar varieties. In the study, abundant nodulation was observed. The soils had contrasting effects on nodulation, with a high nodule number of low weight (497 mg/24.3 nodules) in the clay loam and low nodule number of high weight (583 mg/8.64 nodules) in the sandy loam, but no effect of the inoculants was observed. The difference in nodule characteristics between the soils may have resulted from differences in indigenous Rhizobium population sizes and/or species available to associate with guar roots. The lack of inoculant effects may have resulted from high indigenous levels of Rhizobium in the soil and/or low concentrations of Rhizobium or uncompetitive Rhizobium delivered through the seed-applied inoculants. Based on the results of this study, we expect that guar will nodulate and fix nitrogen effectively in field conditions, given that these processes are not inhibited by exogenous factors, such as water stress. Achieving increases in nodulation and nitrogen fixation above baseline levels may require a concerted effort to develop effective inoculants for guar, which may offer a unique opportunity to expand biological nitrogen fixation in agriculture in semi-arid regions.

English

Inoculation of promiscuous soybean with rhizobia has been proven to improve yield. Screenhouse pot experiments were conducted to assess the response of promiscuous soybean, TGx1448-2E to rhizobia inoculation and to determine the relative efficiency of indigenous rhizobia from soils collected from the Northern and Southern Guinea and Sudan Savanna agro-ecological zones of Nigeria. Soils were collected at 0 to 20 cm depth from 45 sites spread in 15 local government areas of Kaduna and Kano states. The treatments used were uninoculated control, mineral nitrogen and Legumefix, a commercial rhizobia inoculant which were arranged in randomized complete block design and replicated three times. Nodulation and biomass were assessed at eight weeks after planting. Response to inoculation and relative efficiency were also determined. The results showed that response to inoculation was higher in the Sudan Savanna than in the Northern Guinea Savanna. These were observed in 12 out of 21 and 9 out of 19 sites in the Sudan Savanna and Northern Guinea Savanna, respectively. Of these, inoculation response reached 62 and 90% in two sites in the Sudan Savanna while it barely reached 20% in the remaining sites. The results also showed a relative efficiency which is greater than 100% in some locations. This implies that the indigenous rhizobia populations were superior to mineral nitrogen and Legumefix inoculant. Notable among the locations were ALB 1, GRK 1, SOB 4 and KOL, which spread across all the agro-ecological zones. Strains from such locations have high potential for use as inoculants. Key words: Legumefix, mineral nitrogen, inoculation, indigenous strains, agro-ecological zones.

Isolation and Molecular Characterization of Elite Indigenous <i>Rhizobia</i> Nodulating <i>Phaseolus</i> bean (<i>Phaseolus vulgaris</i> L.)

Nowadays application of biological nitrogen fixation (BNF) through rhizobia inoculums is highly promoted as a solution to solve the problem of poor soil fertility in areas where legumes are cultivated. This is due to the fact that, rhizobia enhance nitrogen fixation, induce disease resistance, reduce heavy metal in the soil, facilitate bioavailabity of iron in soil and is environmental friendly. To get rhizobia strains which are suitable for inoculants production, isolation and molecular characterization of elite rhizobia are highly needed. Molecular characterization acts as a spark plug for discovery of many microbes including Rhizobia. Multi Locus Sequence Analysis (MLSA), 16S rRNA gene sequence analysis, DNA-DNA hybridization and SDS-PAGE analysis of the whole-cell proteins are the molecular techniques mostly used in characterizing rhizobia. But before deciding to use or not to use rhizobia inoculants in certain areas, knowing the population size of indigenous rhizobia found in that area is very important, because this is a major factor which determines inoculums responses as well gives clues on which areas need or do not need inoculation. The Most Probable Number (MPN) method is mostly used in enumerating rhizobia population of the soil. Given that, in most of the developing countries, including Tanzania, Biological Nitrogen Fixation (BNF) technology is not fully flourished; more efforts in isolation, molecular characterization of elite rhizobia and estimation of indigenous rhizobia population in various areas are required.

Effects of lime, phosphorus and rhizobia on Sesbania sesban performance in a Western Kenyan acid soil

Aluminium (Al) toxicity, phosphorus (P) deficiency and low rhizobia populations limit Sesbania (Sesbania sesban ) performance in tropical acid soils. The study determined the i) indigenous rhizobia populations that nodulate sesbania and ii) effects of lime (0 and 4 t/ha), P-fertilizer (0 and 60 kg/ha) and acid tolerant rhizobia (0 and inoculation) on soil and selected sesbania accessions performance in Western Kenya acid soil. Study site had acid soil, low available P, nitrogen (N) and rhizobia populations that nodulate Sesbania (146 cells/g soil). Lime increased soil pH, while both lime and P-fertilizer increased available P. Aluminium toxicity tolerant and P-efficient accessions (SSBSA004, SSUG3, SSUG4 and SSUG5) had faster growth, higher nodulation, shoot P, and shoot N and response to treatments than the sensitive one (SSBSA203). After 7 months of growth, SSUG3 had highest shoot length (306 cm) and dry matter (5.64 tons/ha), hence, most suitable for building poles and fuel wood. SSUG5 accumulated the highest shoot N (222 kg N/ha) and was therefore, most suitable soil N replenishment. Thus, in acid P deficient and low rhizobial population soils of Western Kenya, the use of lime, P-fertilizer, rhizobia inoculation and Al toxicity tolerant Sesbania are important for Sesbania establishment and growth. Key words: Rhizobia, Sesbania , soil acidity, aluminum toxicity, lime, phosphorus. INTRODUCTION Sesbania [Sesbania sesban (L) Merr ] is an important multipurpose tree legume that provides fuel wood, fodder for livestock and nitrogen (N)-rich biomass suitable for soil fertility replenishment. It is recommended for planting by smallholder farmers in the tropical regions of Eastern and Southern Africa, to correct soil N deficiency (Makatiani and Odee, 2007). In regions where farmers plant it, acid soils occupy 29 and 13% of the total land area in sub-Saharan Africa (SSA) and Kenya, respectively (Eswaran et al., 1997). In Kenya, most acid soils are found in the highlands East of Rift valley and

Eficiência simbiótica de estirpes de Cupriavidus necator tolerantes a zinco, cádmio, cobre e chumbo

O objetivo deste trabalho foi avaliar a tolerância de estirpes de Cupriavidus necator a zinco, cádmio, cobre e chumbo, além de determinar a eficiência simbiótica das estirpes mais tolerantes em associação a espécies leguminosas com potencial para revegetação. A tolerância foi testada em meio LB, suplementado com 2,5; 5,0; 7,5; 10; 12,5 e 15 mmol L-1 de ZnSO4.7H2O, CdSO4.8H2O, CuSO4.5H2O e PbCl2, respectivamente, em comparação ao controle sem adição de metal. Determinou-se a eficiência simbiótica das quatro estirpes de C. necator mais tolerantes aos metais avaliados (UFLA02-71, UFLA02-73, UFLA01-659 e UFLA01-663), as quais foram inoculadas nas espécies: Leucaena leucocephala, Enterolobium contortisiliquum, Acacia mangium, Mimosa caesalpiniifolia, M. pudica, M. pigra e M. acutistipula. Em vasos com solos, avaliaram-se L. leucocephala, M. pudica e M. caesalpiniifolia e as estirpes UFLA01-659 e UFLA02-71, selecionadas na avaliação de eficiência simbiótica. A estirpe UFLA02-71 proporcionou incrementos de matéria seca da parte aérea de 870% em M. caesalpiniifolia, enquanto que UFLA01-659 proporcionou 885% em M. pudica e 924% em L. leucocephala. As estirpes UFLA01-659 e UFLA02-71, além da alta tolerância a metais pesados, apresentaram eficiência em fixar nitrogênio, em simbiose com essas leguminosas, em solos com rizóbios nativos capazes de nodulá-las, e devem ser avaliadas quanto ao seu potencial de utilização em programas de recuperação de áreas degradadas.

Molecular Methods for Assessement the Bacterial Communities from Different Type of Soils in Romania

Rhizobia are soil bacteria that are capable to form nitrogen-fixing symbiosis with leguminous plants. This ability, as well as the diversity of microbial populations in the soil, and in the rhizosphere of host plants and non-host plants is influenced by several factors, including crop management. The aim of this work was the examination of the influence of some factors on indigenous populations of rhizobia in soils under different crop managements. The genetic diversity of rhizobial strains isolated directly from soil (free-living state) or from root nodules of three herbaceous perennial legumes was examined. The study was conducted in the experimental fields located in Moara Domnească area (South of Romania) and in the Brașov County. The characteristics of brown reddish soil were determined (nitrogen content, organic carbon content and pH). Counting of the rhizobia populations was done by most probable number estimation and by viable plate counts. Bacterial strains were isolated directly from soil samples or from root nodules of different plant species (Trifolium repens, T. pratense and Lotus corniculatus). The characterization of rhizobia was performed by DNA fingerprinting (ERIC PCR and BOX PCR) and the bacterial diversity of soils was examined by DGGE technique. The results revealed that the rhizobial diversity was significantly lower in soils under increased fertilization with N. A reduced intraspecific polymorphism was observed in the strains recovered from the same plant species (Trifolium spp.), whatever the origin of the plant (Moara Domnească or Brașov) but clear differences appeared to be related to the origin of nodules (red or white clover) as revealed by DNA fingerprints. However, various amplicon profiles were observed by DGGE when total DNA isolated from soils was examined, the differences being associated with the fertilization level.

Forage Production, Nitrogen Fixation, and Soil N Accumulation of White Clover in the Hill Farming System of Azad Jammu and Kashmir

Quantitative measurements of plant growth characteristics, forage production, nitrogen (N) fixation, and soil N accumulation by white clover were determined in a field experiment at the subhumid hilly region of Rawalakot, Azad Jammu and Kashmir (AJK). Three indigenous and two exotic ecotypes of white clover were used in the study. Indigenous ecotypes were collected from three different locations (i.e., Tollipir, Banjosa, and Rawalakot), whereas exotic ecotypes (NuSiral and Irrigation) were collected from New South Wales Agricultural Research and Advisory Station, Australia. Data were collected for two seasons (spring 2004–autumn 2004). Total average values for height, number of stolons, length of stolons, number of leaves, and leaf area were 13–50 cm, 9–20, 2–4 cm, 23–81, and 7–16 cm2, respectively. The morphological characteristics of exotic ecotypes were significantly higher than the indigenous ecotypes, and the percentage increase in different plant characteristics was +6% to 214%. Total herbage dry‐matter yield (DMY) in the indigenous and exotic ecotypes varied between 0.5–2.3 and 3.6–4 Mg ha−1, respectively. All the ecotypes showed substantial nodulation potential, and the number of nodules in plant roots ranged from 65 to 119, confirming the presence of indigenous Rhizobium population in the soil. The N contents of harvested herbage of white clover were 2.3–3.0% compared to 0.85% in the grass, and the estimated rates of N2 fixation were 26 kg N ha−1 in the indigenous to 79 kg N ha−1 in the exotic ecotypes. Amount of N2 fixed was strongly correlated with DMY, suggesting that crop DM can be used as an indicator of N2 fixation in white clover. Protein content of white clover was 14–19%, compared to 5% in the indigenous grass species. Total organic carbon (C) and N in control soil were 8.5 and 0.75 g kg−1, which increased significantly to 13.1 and 0.93 g kg−1 in soil under white clover. It is concluded that white clover has substantial potential for growth and establishment in the subhumid hilly regions and can be used to recuperate degraded soils because of its ability to sustain high level of pasture production and increase the N status of soil. These benefits could be of particular use for small‐scale resource‐poor farmers.

Forage Production, Nitrogen Fixation, and Soil N Accumulation of White Clover in the Hill Farming System of Azad Jammu and Kashmir

Quantitative measurements of plant growth characteristics, forage production, nitrogen (N) fixation, and soil N accumulation by white clover were determined in a field experiment at the subhumid hilly region of Rawalakot, Azad Jammu and Kashmir (AJK). Three indigenous and two exotic ecotypes of white clover were used in the study. Indigenous ecotypes were collected from three different locations (Tollipir, Banjosa, and Rawalakot), and exotic ecotypes (NuSiral and Irrigation) were collected from New South Wales Agricultural Research and Advisory Station, Australia. Data were collected for two seasons (spring 2004–autumn 2004). Total average values for height, number of stolons, length of stolon, number of leaves, and leaf area were 13–50 cm, 9–20, 2–4 cm, 23–81, and 7–16 cm2, respectively. The morphological characteristics of exotic ecotypes were significantly greater than the indigenous ecotypes, and the percentage increase in different plant characteristics was 6% to 214%. Total herbage dry‐matter yield (DMY) in the indigenous and exotic ecotypes varied between 0.5 and 2.3 and between 3.6 and 4 Mg ha−1, respectively. All the ecotypes showed substantial nodulation potential, and the number of nodules in plant roots ranging from 65 to 119 confirmed the presence of indigenous Rhizobium population in the soil. The N contents of harvested herbage of white clover were 2.3–3.0% compared to 0.85% in the grass, whereas the estimated rates of N2 fixation were 26 kg N ha−1 in the indigenous to 79 kg N ha−1 in the exotic ecotypes. Amount of N2 fixed was strongly correlated with DMY, suggesting that crop DM can be used as an indicator of N2 fixation in white clover. Protein content of white clover was 14–19% compared to 5% in the indigenous grass species. Total organic carbon (C) and N in control soil were 8.5 and 0.75 g kg−1, which increased significantly to 13.1 and 0.93 g kg−1 in soil under white clover. It is concluded that white clover has substantial potential for growth and establishment in the subhumid hilly regions and can be used to recuperate degraded soils via their ability to sustain high levels of pasture production and increase the N status of soil, and these benefits could be of particular use for small‐scale resource‐poor farmers.

Response of Rhizobial Populations to Moderate Copper Stress Applied to an Agricultural Soil

The use of pesticides in agricultural soils may affect the soil microbiota. The effect of repeated application of copper sulfate in soil on indigenous populations of rhizobia was assessed in a medium-term field experiment. Copper sulfate was applied over 8 years at two different rates, 12.5 and 50 kg of CuSO4 ha(-1) year(-1), in the field. The concentrations of total copper in soil varied between 14.0 (control plots that did not receive copper sulfate) and 91.0 mg kg(-1) (the most contaminated plots) at the time of sampling, 3 years after the end of the copper treatments. All the other physicochemical parameters were similar among the plots that also shared the same cropping history. The target rhizobia were monospecific populations of Rhizobium leguminosarum bv. viciae nodulating Vicia sativa and communities of rhizobial species nodulating Phaseolus vulgaris. The size of the vetch rhizobial populations was significantly reduced in the soils with the higher Cu content, whereas the size of the Phaseolus rhizobial populations was not significantly affected. However, the number of nodules formed on both vetches and common beans were reduced for the plants grown in the most contaminated soils, suggesting an additional toxic effect of copper on plant physiology. The diversity (Simpson's indices) of rhizobial genotypes, as characterized by polymerase chain reaction restriction fragment length polymorphism of 16S-23S rDNA intergenic spacer (IGS), was not influenced by copper application. Also, the genetic structure of the R. leguminosarum bv. viciae populations was not modified by copper treatments. By contrast, a shift was observed in the composition of the Phaseolus-nodulating communities in relation to soil copper content. The communities were composed of three 16S rDNA haplotypes: one corresponding to the R. leguminosarum (biovar phaseoli) species, the two others forming a new lineage of Phaseolus rhizobia based on 16S rDNA sequence analysis. The reduced frequency of the R. leguminosarum species in the Phaseolus-nodulating communities from the copper-treated soils was linked to its higher sensitivity to copper as compared to the higher tolerance of isolates belonging to the other rhizobial lineage. The new lineage was functionally efficient for symbiotic nitrogen fixation with P. vulgaris. Our results suggest that functional redundancy among species exhibiting variability for copper tolerance preserved the size of Phaseolus-nodulating communities. In contrast, the abundance of the vetch-nodulating rhizobia, which was a monospecific functional group mainly constituted by copper-sensitive genotypes, was adversely affected by repeated application of copper sulfate.

Water-condition effects on rhizobia competition for cowpea nodule occupancy

Two indigenous bradyrhizobia strains displaying different natural behaviours towards water regime (strain ORS 3257, nodulating more frequently in favourable-water conditions and strain ORS 3260, in limited-water conditions) were studied for their competitivity for nodulation of cowpea (Mouride cultivar) under favourable and limited water conditions in non-sterile soil. The nodule occupancy was studied by PCR-RFLP analysis. Both strains showed good competition with other indigenous rhizobia populations under favourable- and limited-water conditions. Competition between the inoculated strains in the mixture varied between water regimes. In non-limited-water conditions, strain ORS 3257 was the best competitor, whereas in limited-water conditions, strain ORS 3260 was the best competitor. Results indicated that screening of strains according to their environmental origin could ensure successful rhizobia inoculation

Effect of combined inoculation strains on growth of lupin on newly reclaimed land in Egypt

Inoculation studies with Bradyrhizobium isolates were investigated in a glasshouse and field experiment to compare the effects of single and multiple rhizobium inoculations on growth and symbiotic performance in lupin cultivation. A clear difference in host–strain compatibility was observed 120 days after sowing (greater number and dry weight of nodules, higher shoot dry weight and total N accumulation). To confirm the results from the glasshouse, combined inoculations with a different level of N fertilizer and six lupin cultivars were tested on newly reclaimed land at Tahreer Governorate, Egypt. N fertilizer application significantly decreased nodulation status of white lupin, but increased seed and straw yields compared with the non-fertilized control. Two combined inoculations were superior to the other strains in terms of infection and development of nodules in four accessions (1, 20, 30 and 32) and the cultivar Giza 1. All combinations of rhizobium strains tested enhanced the growth and total N accumulation of the lupin cultivars. Rhizobium inoculation increased seed and straw yield compared with the non-inoculated control. Reactions of the tested strain inoculations and interaction between Rhizobium strains and host plant in the glasshouse were almost identical to their reaction in the field. The results from this study suggest that the improvement through development of combined inoculation strains could be possible and would offer security for nodulation. High yields were accomplished with seed inoculation, particularly when lupin was cultivated for the first time and indigenous populations of rhizobia in the soil were low.

Response of field-grown bean ( Phaseolus vulgaris L.) to Rhizobium inoculation and nitrogen fertilization in two Cerrados soils

Most soils sown with field beans (Phaseolus vulgaris L.) contain indigenous rhizobia which might interfere with the establishment of inoculated strains. As a consequence, the benefits of bean inoculation are usually questioned, and the use of N fertilizer is gradually becoming a common practice. The present study had the objective of evaluating the effectiveness of inoculation and N fertilization in field soil with (site 1) and without (site 2) a previous bean-cropping history. At site 1, which had a rhizobial population of 7×102 cells g–1 soil, inoculation had no effect on nodulation or yield, whereas at site 2 (<10 cells g–1 soil) inoculation increased nodulation, nodule occupancy by the inoculated strain and grain yield. N fertilizer decreased nodulation at both sites, but increased grain yield at site 1 but not at site 2, indicating that the response to inoculation and N fertilization depends on the cropping history. When bean was cultivated for the first time, indigenous populations of rhizobia were low and high yields were accomplished solely with seed inoculation, with no further response to N fertilizer. In contrast, previous cultivation of bean increases soil rhizobia, preventing nodule formation by inoculated strains, and N fertilizer may be necessary for maximum yields. A significant interaction effect between N fertilizer and inoculation was detected for serogroup distribution only at site 2, with N fertilizer decreasing nodule occupancy by the inoculated strain and increasing the occurrence of indigenous strains. Consequently, although no benefits were obtained by the combination of inoculation and N fertilizer, this practice may be feasible with the selection of appropriate N-tolerant strains from the indigenous rhizobial population.

Symbiotic Nitrogen Fixation in Soyabean as Influenced by Time of Inoculation

ABSTRACT The influence of the time of inoculation with an indigenous Rhizobium strain on biological nitrogen fixation in soyabean was tested in a field experiment in Greece, in an area extremely low in indigenous Rhizobia effective on soyabean. Inoculation was either at sowing by seed inoculation or 30 days later with liquid solution. One nodulating and one non-nodulating variety of Glycine max (L.) Merr., were used, while the contribution of the low indigenous Rhizobium population was measured with uninoculated plants of the nodulating variety. Nitrogen fertilizer labelled with 15N enabled the nitrogen derived from fixation (% NdfF), from fertilizer (% Ndff) and from the soil (% Ndfs) to be estimated. The preadaptation of the inoculum to the ecological conditions of the experimental area facilitated its survival with a very satisfactory effect on the amount of fixed nitrogen in all the inoculated treatments (% NdfF 48%, N2-fixed 115 kg ha−1). The time of inoculation had no significant effect on N%, N2-yield, % NdfF and N2-fixed. On the other hand, all these parameters were considerably higher in the inoculated treatments than in the uninoculated or the non- nodulating. Either method or time of inoculation can be equally well employed.

Growth of Vicia faba as affected by inoculation with vesicular-arbuscular mycorrhizae and Rhizobium leguminosarum bv. viceae in two soils

Desert areas of Egypt are rapidly being planted with faba beans (Vicia faba) to increase the production of this economically important legume. Since indigenous populations of rhizobia or vesicular-arbuscular (VA) mycorrhizae are absent or low in this soil, the objective of the current study was to determine whether inoculation with several strains of Rhizobium leguminosarum bv. vicae and VA mycorrhizae could successfully increase the growth of faba beans. Growth was compared to that in a fertile silt loam soil from the Nile River Valley. The effect of rock phosphate and superphosphate on the faba bean was also examined. The growth of faba beans was increased by dual inoculation with VA mycorrhizae and rhizobia, to a much greater extent than can be attributed to either inoculum on a singular basis. Rhizobium leguminosarum bv. Viceae USDA strain 102 F84 was the most effective of the rhizobial strains examined. Growth, plant nutrient content, nodulation, and root colonization were invariably greater in the silt loam soil than the calcareous soil. The addition of rock or superphosphate to soil enhanced these parameters in the calcareous soil, but less than that observed in the silt loam soil. These results demonstrate that the growth of faba beans can be increased in calcareous desert soils by inoculation with rhizobia and VA mycorrhizae. However, the growth remained inferior to that observed in the fertile Nile River Valley soil.

Soil fumigation with methyl bromide as a means of increasing the occurrence of the inoculum strain in peanut nodules

A field experiment was carried out on sandy loam soil to determine the effect of different pesticides on the nodulation activity of peanut-specific local rhizobia and the recovery rate of the inoculum applied. The experiment included a pesticide-untreated control and the following six treatments: methyl bromide, Edabrom, Edigan (metham-sodium), ethoprophos, fenamiphos and a combination of Edabrom and Edigan. The ELISA (enzyme-linked immunosorbent assay) reactions of nodule antigens (38, 64, 86 and 128 days after planting) showed that despite the pesticides applied, the indigenous peanut Rhizobium population was active in nodulation. However, methyl bromide substantially decreased the number of nodules containing native rhizobial strains. The amount of nodules reacting with antibodies specific to the inoculum strain ranged from 47 to 63%, as compared with 25 to 40% in the control plots and other treatments tested. The positive effect of soil fumigation with methyl bromide on the occurrence of the introduced strain was confirmed in an additional field experiment. Serological evaluation by ELISA showed that 48% of the nodules formed on plants derived from inoculated + methyl bromide-treated plots contained the inoculum strain, as compared with 11 % in the inoculated but non-fumigated plots. The highest values of nitrogenase activity and top dry weight were recorded in the inoculated but non-fumigated plots. Although this treatment produced the maximal yield of pods, the differences between inoculated and non-inoculated methyl bromide-treated plots did not reach the threshold of significance. Seed- and pod-weights in the fumigated and inoculated plots were significantly higher than in the other treatments.

Competition studies withRhizobium trifolii in laboratory experiments

Competition studies were carried out in soil cores by comparing the nodule forming ability of antibiotic resistance marked strains, inoculated at three inoculum levels onTrifolium repens versus an effective indigenous Rhizobium population of 1.5×105/gm. It was seen that the indigenous marked isolate G1067 formed a high proportion of nodules sampled (>90%) at all three inoculum levels (105, 107 and 109 cells/seed) wherein the introduced ‘foreign’ strain G1032 formed >50% of the nodules at the highest inoculum level. In a test tube experiment, competition for nodule sites was examined by inoculating mixtures of twoR. trifolii strains at different input levels on two cultivars ofTrifolium repens var. Huia and Titan. It was seen that G1032 was less competitive than G1006 and G1067 on cv. Huia but was more competitive on cv. Titan than the other two strains.

Competition studies with Rhizobium trifolii in a field experiment

McLoughlin, T.J. Bordeleau L.M. &amp; Dunican, L.K. 1984. Competition studies with Rhizobium trifolii in a field experiment. Journal of Applied Bacteriology56, 131–135.Competition studies were carried out in a field experiment by comparing the ability of antibiotic marked Rhizobium strains inoculated at three inoculum levels in forming nodules against an effective indigenous Rhizobium population of 1.5 times 105/g. Three inoculum strains were assessed. It was seen that by increasing the level of inoculum there was a corresponding increase in the number of nodules formed by the introduced strains. G1067 formed a high proportion (85%) at the highest level, whereas the other two inoculum strains occupied less than 50% of the nodules at the same level. Persistence of the inoculum strain was measured into the second season; G1032 could not be detected, G1006 (another ‘foreign’ strain) formed 43% and G1067 formed 97% of the nodules sampled. The indigenous Rhizobium population isolated from nodules was shown to be heterogenous, and 15 different intrinsic antibiotic resistance patterns were found. Fifty per cent of the isolates fell in the same group which showed a similar pattern to that of the inoculum strain G1067.