Symbiotic efficiency and compatibility of native rhizobia in northern Thailand with different soybean cultivars

Symbiotic efficiency and compatibility of 81 isolates of native bradyrhizobia from irrigated areas in northern Thailand with four soybean cultivars and one cowpea cultivar were evaluated under laboratory conditions. Effectiveness and / or compatibility of the tested isolates were compared with those of a standard strain (Bradyrhizobium japonicum CB 1809) by using plants grown on plastic seed bags. Effectiveness of the isolates was also estimated using uninoculated control plants grown in a nitrogen-free solution. Nodulation of a wide range of host plants by the majority of the tested isolates was observed, which agreed well with the results of our previous field experiment (Shutsrirung et al. 2002: Soil Sci. Plant Nutr., 48, 491–499). Up to 75% of the tested isolates induced a higher growth efficiency than that of the uninoculated control in association with one of the tested cultivars, Black soybean. Comparision with uninoculated control plants, enable to estimate the proportion of the tested isolates leading to effective growth promotion (E + e) of each cultivar, namely, Black soybean (local Thai cultivar), 75%; Cowpea, 82%; SJ5 (commercial Thai cultivar), 33%; Bragg (US cultivar), 33%; and Improved Pelican (US cultivar), 9%. These results indicated that although isolates with a high infectiveness with both “Asian-type” and “US-type” soybeans could be found, a high frequency of isolates leading to inefficient nodules was observed in the US cultivar, suggesting the presence of genetic differences in the soybean cultivars that express high-preference (efficient nodules) or low-preference (inefficient nodules) for a certain group of tested isolates. Based on the results of this laboratory experiment together with our previous field experiment, native rhizobial populations in the irrigated area of northern Thailand could be separated into three groups; Group 1: rhizobium strains showing a high effectiveness with only Asian cultivars, Group 2: strains showing a high effectiveness with only US origin cultivars, and Group 3: strains showing a high effectiveness with both Asian and US origin cultivars. The majority of the native rhizobial populations belonged to Group 1. The isolates in Group 3 may display a high potential for manipulating useful rhizobial inoculant.     

New insights into the origins and evolution of rhizobia that nodulate common bean (Phaseolus vulgaris) in Brazil

It is generally accepted that there are two major centers of genetic diversification of common beans (Phaseolus vulgaris L.): the Mesoamerican (Mexico, Colombia, Ecuador and north of Peru, probably the primary center), and the Andean (southern Peru to north of Argentina) centers. Wild common bean is not found in Brazil, but it has been grown in the country throughout recorded history. Common bean establishes symbiotic associations with a wide range of rhizobial strains and Rhizobium etli is the dominant microsymbiont at both centers of genetic diversification. In contrast, R. tropici, originally recovered from common bean in Colombia, has been found to be the dominant species nodulating field-grown common-bean plants in Brazil. However, a recent study using soil dilutions as inocula has shown surprisingly high counts of R. etli in two Brazilian ecosystems. In the present study, RFLP-PCR analyses of nodABC and nifH genes of 43 of those Brazilian R. etli strains revealed unexpected homogeneity in their banding patterns. The Brazilian R. etli strains were closely similar in 16S rRNA sequences and in nodABC and nifH RFLP-PCR profiles to the Mexican strain CFN 42^T, and were quite distinct from R. etli and R. leguminosarum strains of European origin, supporting the hypothesis that Brazilian common bean and their rhizobia are of Mesoamerican origin, and could have arrived in Brazil in pre-colonial times. R. tropici may have been introduced to Brazilian soils later, or it may be a symbiont of other indigenous legume species and, due to its tolerance to acidic soils and high temperature conditions became the predominant microsymbiont of common bean.     

Co-selection of compatible rhizobia and vesicular-arbuscular mycorrhizal fungi for cowpea in sterilized and non-sterilized soils

Summary We selected two isolates of Rhizobium for cowpea (Vigna unguiculata) with sterilized soil tests and two different isolates by non-sterilized soil testing. The four rhizobia were then paired individually with either Glomus pallidum, Glomus aggregatum, or Sclerocystis microcarpa in separate, sterilized, or non-sterilized soil experiments. The purpose of the experiments was to determine the effect of soil sterilization on the selection of effective cowpea rhizobia, and to see whether these rhizobia differed in their effects on cowpea growth when paired with various vesicular-arbuscular mycorrhizal (VAM) fungi. Our experiments showed that the rhizobia selected in sterilized soil tests produced few growth responses in the cowpea compared to the other introduced rhizobia, irrespective of pairing with VAM fungi in sterilized or non-sterilized soil. In contrast, the two rhizobia initially selected by non-sterilized soil testing significantly improved cowpea growth in non-sterilized soil, especially when paired with G. pallidum. Our results suggest that it is important to select for effective rhizobia in non-sterilized soil, and that pairing these rhizobia with specific, coselected VAM fungi can significantly improve the legume growth response.     

Edaphic factors as determinants for the distribution of intrinsic antibiotic resistance in a cowpea rhizobia population

 A large collection of cowpea rhizobia strains was obtained from soil samples collected from either a semi-arid or a tropical rain forest area located at about the same latitude in the north-eastern region of Brazil and evaluated for their intrinsic antibiotic resistance to eight commercial antibiotics. The aim of this study was to correlate antibiotic resistance of native rhizobia strains to edaphic-climatic factors as a way to establish suitable inoculants for specific areas. A large diversity regarding intrinsic antibiotic resistance was found, and 17 clusters were identified as varying from sensitive to gradually resistant up to 500 μg·ml^–1 of the antibiotics tested. Clustering analysis did not show any pattern related to the geographic region where isolates have been obtained. On the other hand, an increase in the antibiotic-resistant rhizobia population was associated with an increase in soil P and Al contents. lsolates which were sensitive to spectinomycin, ampicillin, streptomycin, chloramphenicol and tetracycline were present at higher rates in soils devoid of Al. Rhizobia bacteria producing mucus type I (fluid and capable of spreading over the solid media) were found preferentially in soil with Al concentrations up to 36 mg·kg^–1, diminishing quickly at higher levels. Received: 2 May 1997     

Symbiotic effectiveness and host ranges of indigenous rhizobia nodulating promiscuous soyabean varieties in Zimbabwean soils

Presence of indigenous rhizobia nodulating promiscuous soyabean was determined in 92, mainly sandy soils, from wetter agro-ecological zones of Zimbabwe suited to soyabean production. A total of 129 isolates were obtained from nodules of promiscuous soyabean varieties, Magoye and Hernon 147, and a specific variety, Roan grown in potted soils. Magoye nodulated in 80%, Hernon 147 in 50% and Roan in only 25% of the 92 soils tested. Rhizobia populations ranged from undetectable to 2.4×10⁴ cells g⁻¹ of soil. Twenty-one of these isolates were tested for symbiotic effectiveness on two varieties, promiscuous Magoye and specific Roan. Differences in parameters such as nodule numbers, nodule weights and total N fixed reflected diversity among the indigenous isolates. Three isolates had significantly higher N₂-fixing potential in comparison with the commercial strain MAR 1491 on promiscuous Magoye. Host ranges of 34 isolates were evaluated on nine legume species: Arachis hypogaea, Cajanus cajan, Crotalaria juncea, Glycine max, Macroptilium atropurpureum, Phaseolus vulgaris, Sesbania sesbania, Vigna subterranea, Vigna unguiculata. Of these 34 isolates, 33 formed nodules with M. atropurpureum of which 61% were moderately effective to very effective while all nodulated V. unguiculata with 58% being moderately effective to very effective. Twenty-eight isolates nodulated V. subterranea and C. cajan (short season variety) with 76 and 36% of these being moderately to very effective, respectively. None of the isolates formed nodules on Phaseolus vulgaris, Arachis hypogaea or Sesbania sesban. Our results indicate that the ability of even specific varieties of soyabean to nodulate with indigenous isolates in African soils is greater than generally assumed.     

Integrated soil fertility management enhances population and effectiveness of indigenous cowpea rhizobia in semi-arid eastern Kenya

Legume biological nitrogen fixation is an environmentally friendly and economical means that can reduce low resource farmer dependence on expensive chemical nitrogen (N) fertilizers. We investigated the effect of two cowpea (Vigna unguiculata (L.) Walp) varieties (IT95K-52-34, an international variety from IITA and Kang’au, a local variety) under an integrated soil fertility management trial on indigenous symbiotic rhizobia in a semi-arid farmer's field in eastern Kenya. The ox-ploughed field trial had the following treatments: an unamended control, manure applied at 2.5 t ha⁻¹, triple superphosphate (TSP as (P₂O₅, 0:46:0) at 15 kg ha⁻¹; and a combination of manure and TSP applied at the single rates. Soil samples were collected from each treatment during planting and harvesting of the cowpea crop and used in most probable number (MPN) plant infection assays with the two cowpea varieties as traphosts in Leonard jar growth systems and grown under glasshouse conditions. Generally, soil amendments enhanced cowpea rhizobial populations which varied from 4.89 × 10² rhizobia g⁻¹ soil to 1.074 × 10³ rhizobia g⁻¹ soil. The highest shoot biomass accumulation occurred on cowpea variety IT95K-52-34 plants inoculated with soils from the manure applied plots. We isolated 150 fast- and slow-growing cowpea rhizobia. Contrary to most previous studies, the bulk (97%) of the isolates was fast growing which grouped into 9 types on the growth characteristics on yeast extract agar (YEMA). The study indicated that ISFM was important for rhizobia population build up over a cowpea-growing season.     

Symbiotic and genomic diversity of ‘cowpea’ bradyrhizobia from soils in Botswana and South Africa

Strain CB756 is usually an effective competitor against indigenous bradyrhizobia for nodulation of peanut in South Africa. Recently, inoculation of peanut and cowpea with CB756 in loamy sand soils of Botswana or a sandy clay loam in South Africa proved unsuccessful, achieving <2% nodule occupancy. A survey of ‘cowpea’ bradyrhizobia from five soils in Botswana and one in South Africa showed that many were effective in ability to fix N₂ on peanut and cowpea. However, 15 isolates from Good Hope, Botswana were all effective on cowpea but ineffective on peanut, three failing to nodulate the latter. Selected cowpea isolates were significantly more competitive than CB756 for nodulation of cowpea in Leonard jars, but four were unsuccessful when inoculated at Roodeplaat, South Africa. When strain CB756 and two isolates were inoculated in pots containing Roodeplaat soil, at a 4:1 inoculant to soil bradyrhizobia ratio, their average nodule occupancy was 8% on cowpea compared to 40% on peanut. Significant differences in strain nodule occupancy were not detected on either cowpea or peanut. In contrast, nodule occupancy in loamy sand from Good Hope, Botswana, inoculated at a 40:1 inoculant to soil bradyrhizobia ratio, was 22.4% on cowpea and only 6.8% on peanut. In Good Hope soil, strain CB756 was the weakest competitor on cowpea but strain differences were insignificant on peanut. Whereas the Good Hope soil population was effective on cowpea, it was ineffective on peanut. DNA fingerprinting showed that isolates from Gaborone, Francistown and Roodeplaat contained several different genotypes, whereas those from Good Hope, Rasesa and Maun were more homogeneous. The dominance at Good Hope of genotypes effective on cowpea but ineffective on peanut emphasises the value of assessing the symbiotic capabilities and structures of indigenous populations.     

Intrinsic antibiotic resistance in relation to colony morphology in three populations of West African cowpea rhizobia

Rhizobia isolated from cowpeas (Vigna unguiculata (L.) Walp.) grown in three west African soils were examined for intrinsic resistance to five antibiotics and were scored for one of two colony morphologies. Half of the strains tested had a “wet”, slimy colony morphology and half had small discrete “dry”, non-slimy colonies. The populations as a whole were resistant to gentamicin (87%) but varied in their resistance to streptomycin, rifampicin, kanamycin and penicillin. Thirteen patterns of resistance were found for the 128 strains screened and strains within the same pattern usually had the same colony type. The most common pattern (32%) was resistance to all five antibiotics. Associations between resistances were random within populations and colony type. Each population was diverse, expressing from 6 to 8 patterns of resistance, but one population was relatively homogeneous, with 68% of its members exhibiting the same pattern. Correlations between intrinsic antibiotic resistance and colony type were demonstrated both for the west African cowpea rhizobia and for a broader group of cowpea miscellany rhizobia. The method was practical, rapid and reliable for identification of groups within populations.     

Effect of no-tillage on turnover of organic matter in a Rhodic Ferralsol

Abstract. Soil organic matter (SOM) is considered to be key to sustainability of agriculture in the tropics. In southern Brazil, no-tillage has been adopted widely to control soil erosion, but its impact on the dynamics of SOM is not well established. We measured soil carbon (C) and δ¹³C in two crop rotations, one of which contained C4 maize ( Zea mays L.), after 21 years of contrasting tillage (conventional tillage versus no-tillage). Adjacent sites that reflected historic land-uses were also sampled. In the tillage experiment there was no effect of tillage on the total amount of C in the 0–40 cm profile (even when contrasting bulk density was accounted for), and the concentration of C differed only in the 0–5 cm and 5–10 cm layers. However, the occasional input of C4 material in the maize rotation resulted in a significant effect of rotation on δ¹³C ( P <0.001). Using ¹³C as a tracer for the SOM formed since the start of the experiment, we estimated the abundance of 'recent' and 'old' C within each depth interval. We found the main effect of tillage was to increase the medium-term turnover of SOM, particularly in the subsoil (i.e. below 20 cm depth). Compared with no-tillage, there was almost five times more recent C in the subsoil, and 20% more recent C in the 0–40 cm profile as a whole.     

Symbiotic performance of fast-growing rhizobia isolated from the coastal sand dune legumes of west coast of India

The symbiotic efficiency of coastal sand dune rhizobial isolates on four cultivated legumes, cowpea (Vigna unguiculata), green gram (Vigna radiata), black gram (Vigna mungo) and horse gram (Macrotyloma uniflorum), was assessed. Among the isolates of Someshwara (S1–S5), inoculation of S5 resulted in the highest increase of shoot biomass in cowpea (control vs experimental, 1:6), while inoculation of P1 among the Padubidri isolates (P1–P5) induced the highest shoot biomass in cowpea (1:14.4). Inoculation of the isolate P2 induced higher shoot biomass against uninoculated controls of horse gram (12.6:1), green gram (11.2:1) and black gram (6.1:1). One-way ANOVA revealed significant difference in the shoot biomass between uninoculated and inoculated cowpea plants with ten rhizobial isolates (P <0.05). Cultivation of surface-sterilized green gram seeds on unsterilized dune sand resulted in profuse flowering as well as nodules within 6 weeks indicating possibilities for isolating efficient rhizobial strains through cultivating edible legumes on coastal sand dune soils.     

The effect of acidity on the distribution and symbiotic efficiency of rhizobia in Lithuanian soils

The distribution and symbiotic efficiency of nodule bacteria Rhizobium leguminosarum_bv. trifolii F., Sinorhizobium meliloti D., Rhizobium galegae L., and Rhizobium leguminosarum bv. viciae F. in Lithuanian soils as dependent on the soil acidity were studied in the long-term field, pot, and laboratory experiments. The critical and optimal pH values controlling the distribution of rhizobia and the symbiotic nitrogen fixation were determined for every bacterial species. The relationship was found between the soil pH and the nitrogen-fixing capacity of rhizobia. A positive effect of liming of acid soils in combination with inoculation of legumes on the efficiency of symbiotic nitrogen fixation was demonstrated.     

成都平原蚕豆高效根瘤菌的筛选及其促生功能初步评价

[目的]筛选适用于成都平原的高效广谱蚕豆根瘤菌,并对其相关促生功能进行初步评价,为成都平原高效蚕豆根瘤菌剂的研制与应用提供科学依据。[方法]供试6株根瘤菌由课题组前期分离自成都平原,其与四川主栽蚕豆‘大白蚕豆’匹配良好,采用常规方法测定了这6个菌株分泌生长素及溶磷能力。菌株与蚕豆品种匹配试验采用低氮砂培法,供试蚕豆品种为成都平原主栽品种‘成胡14’、‘成胡15’;两个品种的蚕豆种子播种后,分别接种6个菌株,以不接种为对照(CK),光照(控温22~25℃、光照强度2800 lx左右、日照时间14 h)下培养41天后收获,测定植株生物量和根瘤数。然后,对匹配性试验筛到的两株高效广谱根瘤菌进行田间验证,供试蚕豆品种为成胡15,将2个根瘤菌制备的菌剂(活菌数5.0×10^8 CFU/g以上,载体为泥炭)进行拌种,以不接菌处理的灭菌泥炭为对照。在盛花期(生育期105 d)采样测定株高、根瘤数、地上部分植株干重;收获期(生育期200 d)采样测产;测定两个时期植株样品氮、磷、钾含量。盛花期采用BOX-PCR分子标记法测定接种根瘤菌占瘤率,同时提取接种菌株SCAUf73、SCAUf76的总DNA,比较接种菌株及相应根瘤类菌体根瘤菌DNA的BOX-PCR分子指纹图谱。用多位点基因序列分析法对田间验证的优良菌株SCAU73进行分类地位研究。[结果]1)通过匹配性砂培试验,筛选到2株与2个成都平原主栽蚕豆品种均高效匹配的根瘤菌SCAUf73、SCAUf76。SCAUf76、SCAUf73能使‘成胡14’、‘成胡15’植株干重较CK显著增加40.5%~61.6%。2)通过两株菌田间接种试验发现,接种SCAUf76处理的蚕豆产量与CK差异不显著;接种SCAUf73处理蚕豆植株干重、全氮含量等指标均高于CK,籽粒鲜产比CK显著增加25.0%,并显著高于SCAUf76,其占瘤率达到33%。3)多位点基因序列分析表明,SCAUf73可能是Rhizobium的一个新类群。4)促生性试验表明,6株菌都能分泌生长素(IAA),最大分泌量为21.0 mg/L(SCAUf76);供试菌株的溶磷能力不明显。[结论]从成都平原上筛选的6个菌株中,SCAUf73具有分泌IAA能力,与蚕豆接种后,占瘤率达33%,可显著促进蚕豆氮素吸收积累,提高蚕豆籽粒产量。与成都平原的主栽蚕豆品种匹配的高效广谱根瘤菌SCAUf73,适用于成都平原的蚕豆生产。     

Population size,distribution, and symbiotic characteristics of indigenous Bradyrhizobium spp. that nodulate TGx soybean genotypes in Africa

Soils of many potential soybean fields in Africa are characterized by low levels of biological nitrogen fixation (BNF) activities and often cannot support high soybean yields without addition of inorganic N fertilizers or external application of soybean rhizobia. The most probable number (MPN) technique was used to determine the bradyrhizobial populations that nodulate TGx soybean genotypes (a cross between nonpromiscuous North American soybean genotypes and promiscuous Asian soybean genotypes), cowpea or North American soybean cv. Clark IV, in soils from 65 sites in 9 African countries. The symbiotic effectiveness of isolates from these soils was compared to that of Bradyrhizobium japonicum strain USDA110. The bradyrhizobial population sizes ranged from 0 to 10⁴ cells g⁻¹ soil. Bradyrhizobium sp. (TGx) populations were detected in 72% and B. japonicum (Clark) in 37% of the soil samples. Bradyrhizobium sp. (TGx) populations were generally low, and significantly less than that of the cowpea bradyrhizobial populations in 57% of the samples. Population sizes of less than 10 cells g⁻¹ soil were common as these were detected in at least 43% of the soil samples. B. japonicum (Clark) occurred in higher population densities in research sites compared to farmers’ fields. Bradyrhizobium sp. (TGx) populations were highly correlated with biotic but not abiotic factors. The frequent incidence of low Bradyrhizobium sp. (TGx) populations is unlikely to support optimum BNF enough for high soybean yields while the presence of B. japonicum (Clark) in research fields has the potential to compromise the selection pressure anticipated from the indigenous Bradyrhizobium spp. (Vigna) populations. Bradyrhizobium isolates could be placed in four symbiotic phenotype groups based on their effectiveness on a TGx soybean genotype and the North American cultivar Clark IV. Symbiotic phenotype group II isolates were as effective as B. japonicum strain USDA110 on both soybean genotypes while isolates of group IV were effective on the TGx soybean genotype but not on the Clark IV. The group IV isolates represent a unique subgroup of indigenous bradyrhizobia that can sustain high soybean yields when available in sufficient population densities.     

一株阿特拉津高效降解菌的分离与鉴定

在河北省宣化农药厂附近的土样中筛选到一株阿特拉津高效降解菌,高效液相色谱定量检测,证明降解率在99%以上,编号为ADX10,并对该菌进行了鉴定。结果表明,ADX10在LB培养基上产生光滑、圆形、微小、凸起的菌落,培养过程中产生柠檬绿色色素,幼体菌为短杆状(0.5~0.6μm×0.8~1.2μm),老龄培养物为球状,革兰氏染色为阳性,但极易退色,接触酶阳性,氧化酶阴性,液化明胶,脲酶阴性,不水解淀粉,不产生芽孢,37℃生长,好氧生长,对多种抗生素有抗性。16S rDNA聚类分析结合生理生化特性,将该菌株定为Arthrobactersp.。     

Effect of tillage and liming on organic matter composition in a Rhodic Ferralsol from Southern Brazil

The effects of tillage and liming on degradation of organic matter of a Rhodic Ferralsol were investigated. Samples were taken from a tillage experiment and from a trial with different levels of lime application. Organic matter was separated into the fractions of undecomposed organic matter and humic substances by density fractionation. Chemical composition of the humic substances was determined by Kononova's fractionation method. Other chemical parameters measured were total organic C, pH, exchangeable cations, and the physical property determined was aggregate stability. Tillage, and to a lesser degree, liming, substantially decreased organic matter content when compared to samples taken from a virgin forest. The proportion of humic acids C (HC) varied most, so that the ratio of fulvic acids C (FC) to HC increased from 2.5 (virgin forest), to 3.4 (lime treatments and no-tillage) and to 5.8 (conventional tillage). Content of FC was affected by pH, while no distinct factors were found to influence HC content. Aggregate stability was best related to content of HC, confirming the importance of this fraction of organic matter for aggregation. It was concluded that especially excessive tillage greatly affects soil fertility because organic matter is decomposed to a great extent. However, a further need for research seems necessary to clarify interactions conclusively between changes of pH, adsorption of humic substances and other chemical properties in the entire profile of Ferralsols.     

Symbiotic efficiency of Sesbania rostrata and S. cannabina as affected by agronomic practices

A field experiment was conducted to study the N₂ fixation efficiency of Sesbania rostrata and S. cannabina as affected by agronomic practices in semi-arid subtropical climate, Sowing seeds resulted in smaller numbers of nodules, lower dry weight, lower total biomass, less N uptake, and less N₂ fixation for S. rostrata than S. cannabina, while cut-stem planting improved the symbiotic efficiency. Flooding the soil increased the relative humidity of the crop micro-environment by 4–11% and induced early appearance of stem nodules in S. rostrata. Only 67 kg N ha^-1 was fixed by S. rostrata compared to 160 kg N ha^-1 by S. cannabina when normal agronomic practices (sowing and non-flooding) were followed. In contrast, planting stem cuttings and flooding resulted in greater biological N₂ fixation, 307 and 209 kg N ha^-1 by S. rostrata and S. cannabina, respectively. Therefore, S. rostrata can be successfully exploited as a green manure when stem cuttings are planted under flooded conditions.     

No tillage and crop rotation effects on soil aggregation and organic carbon in a Rhodic Ferralsol from southern Brazil

In Brazil, no tillage (NT) is a soil conservation practice now widely adopted by farmers, including smallholders. The effect of NT and conventional tillage (disc ploughing followed by two light disc harrowings, CT) was investigated on the aggregation properties of a clayey Rhodic Ferralsol from southern Brazil under different crop rotations. The same soil type under secondary forest was used as reference. Macro- and microaggregate classes were separated by wet sieving using a series of eight sieves (8, 4, 2, 1, 0.5, 0.25, 0.125, 0.053 mm) at four sampling layers (0–5, 5–10, 10–20, 20–30 cm). The soil in general had high structural stability. At 0–5 cm, meanweight diameter (MWD, 11.1 mm) and total organic C in macroaggregates (TOC, 39 g kg⁻¹ soil) were highest for the forest soil. Soil under NT had a more similar distribution of aggregate size classes and TOC to the forest soil than CT. The most pronounced difference between tillage systems was observed in the surface soil layer (0–5 cm). In this layer, NT had higher aggregate stability (AS_NT: 96%; AS_CT: 89%), had higher values of aggregate size distribution (MWD_NT: 7.9 mm, MWD_CT: 4.3 mm), and had on average 28% greater TOC in all aggregate size classes than CT. Soil under NT had greater TOC in macroaggregates (NT: 22 g kg⁻¹; CT: 13 g kg⁻¹). Crop rotation did not have a significant effect on soil aggregate distribution and TOC. By increasing macroaggregation NT increased organic carbon accumulation in soil.     

Characterization of a cowpea (Vigna unguiculata) rhizobiophage and its effect on cowpea nodulation and growth

A cowpea rhizobiophage (JRW 3 phage) from Jamaican soil was isolated and characterized. The phage has a polyhedral head and a non-contractile tail; maximum adsorption of the phage to the host occurred after 5 min. A one-step growth experiment revealed that the latent period, rise period and burst size of JRW3 phage were 12 h, 16 h, and 28 plaque-forming units/cell, respectively. The JRW 3 phage was highly sensitive to heat, but survived well between pH 5 and 8. The phage was stable in EDTA, though completely inactivated in sodium citrate. Host range analysis showed that 7 of the 40Rhizobium andBradyrhizobium strains tested were sensitive to phage infection. The phage significantly reduced nodule numbers and shoot dry weight of cowpea plants when inoculated with rhizobia in combination with the phage.