Numbers Of Rhizobia Research Articles

Presence of rhizobia nodulating common birdsfoot trefoil (Lotus corniculatus L.) in soils of Serbia

Common birdsfoot trefoil (Lotus corniculatus L.) is a perennial forage legume tolerant to adverse environmental conditions such as low soil pH, poor drainage, various insects and plant diseases. Owing to its ability to establish a symbiosis with nitrogen-fixing bacteria, rhizobia, thrives on soils of low fertility and is used in grass mixtures to enrich the soil with nitrogen. The aim of the work was to investigate the presence of rhizobia that nodulate birdsfoot trefoil in the soils of Serbia and to perform the isolation and evaluation of their nitrogen fixation potential. The presence and abundance of birdsfoot trefoil rhizobia in 28 soil samples from different localities and types of soil in Serbia, as well as different ways of use, was tested. From positive samples the rhizobial strains were isolated and their efficiency of nitrogen fixation was tested under controlled conditions. The presence of rhizobia that nodulate birdsfoot trefoil was recorded in 23 samples, while all 5 samples where the presence of rhizobia was not recorded are characterized by the absence of the host plant. The number of rhizobia in samples where there was no host plant was significantly less than in its presence. A total of 49 rhizobial strains were isolated, and in some samples of low abundance it was not possible to isolate strains. In controlled conditions during reinoculation, 80% of the strains were active, achieving 2.5 times greater dry above-ground mass than non-inoculated plants. The percentage of nitrogen in plants ranged from 1.6% in the case of inactive strains to 4.6% in the case of highly active strains. The results of the study indicate the necessity of inoculation, that is, the application of rhizobia on the lands where the common birdsfoot trefoil is sown for the first time due to their reduced number and activity.

Kök lezyon nematodlarının (Pratylenchus thornei) nohut bitkisinin rhizobium bakterileri üzerinde etkisi

Chickpea (Cicer arietinum) is one of the most significant legume crops and supply high-quality protein for human nutrition. legume crops such as chickpea are important agriculturally because of their symbiotic ability to nitrogen fixation with specific soil bacteria. Legumes like chickpea depend on nitrogen provided by the activity to grow, but these rhizobium bacteria are affected by plant-parasitic nematodes that cause less activity and decrease the number of nodules in the chickpea plant. The root-lesion nematodes (Pratylenchus thornei) are common and economically important pests described as one of the major limiting factors in agriculture and the growing chickpea field in the world. In this study, the effects of this nematode on the number of rhizobia (nodules) and rhizobium bacteria activity were assessed in both wild and domesticated accession of Cicer species under laboratory conditions. We inoculated all Cicer accession with the Mesorhizobium bacteria and with one species of the genus Pratylenchus (P. thornei). The result showed that P. thornei has a negative impact on the number of nodules and the activity of rhizobium bacteria. Nematode infection on chickpea caused decreased nodulation. Where, nematode infected plant formed 4-8 nodules/root and less nodule number than an uninfected plant.

Abundance and Symbiotic Rhizobia Colonizing Soybean (Glycine max) in Soils of Kakamega County, Western Kenya

Identification of effective indigenous rhizobia strain nodulating soybean (Glycine max) was conducted in Kakamega County, Western Kenya. This study was carried out to establish the abundance and symbiotic efficiency of native rhizobia strains in soil samples collected from 12 sites in four agroecological zones within the region. The MPN enumeration technique was used to estimate the indigenous rhizobia numbers and showed that 54% of the sampled sites had high abundance. Rhizobia number in the soil collected from the six regions varied from 2.5 × 102 to 3.12 × 103 cells·g− soil. Thirty isolates from the soil collected in the four regions were tested for efficiency using soybean as the trap plant in Leonard jars containing sterilized vermiculite irrigated with nitrogen-free media. Determination of the relationship between rhizobial abundance and symbiotic efficiency with respect to the soil chemical conditions was performed using ANOVA, and the means were compared using Tukey’s honest significant difference test. Inoculation with bacterial isolates significantly ( P < 0.05 ) increased the nodule number and shoot dry weight, while weight of the seeds showed variations across the groups. The soil pH across the 4 regions ranged between 5.41 and 6.52.

Response of Phosphorus on Growth and Yield of Pearl Millet, Microbial Population and System Productivity in Pearl Millet Based Cropping Systems

The present study evaluated the use of various phosphorus management practices with and without PSB on pearl millet and its residual effect on succeeding pulse crops like green gram, blackgram and chickpea. It was conducted at agricultural college farm, Bapatla during both kharif and rabi seasons of 2017-18 and 2018-19. The treatments consisted of T1: No Phosphorus, T2: 50% RDP T3: 75% RDP, T4: 100% RDP, T5: 50% RDP + seed inoculation with PSB, T6 :75% RDP + seed inoculation with PSB, T7 : 100% RDP + seed inoculation with PSB in RBD design with three replications in kharif pearl millet crop. The results indicated that, the treatment T7 (100% RDP + seed inoculation with PSB) recorded significantly the highest growth parameters like LAI, CGR, RGR and NAR at different phenological stages of crop growth period and yield of pearl millet (2996 kg ha-1 and 2876 kg ha-1 during 2017 and 2018 respectively) and it was at par with T4 (100% RDP). Similarly, the total fungi, bacteria and rhizobium counts were increased with increase in fertilizer level and PSB treated plots (T7, T6 and T5) when compared to un-inoculated PSB plots (T4, T3 and T2). The highest number of bacteria (46.2 and 49.2 cfu per gram soil), fungi (25.4 and 24.4 cfu per gram soil) and rhizobium (33.5 and 34.4cfu per gram soil) was found in the treatment T7 (100% RDP+ seed inoculation with PSB) during 2017-18 and 2018-19, respectively. Among the different kharif residual treatments, the highest pearl millet equivalent yield of rabi crops was recorded with residual effect of 100 % RDP + PSB (M7).

Alteration of soil rhizobial populations by rabbit latrines could impair symbiotic nitrogen fixation in the insular alpine ecosystem of Teide National Park

Spartocytisus supranubius (Teide broom) is the dominant legume of the high-mountain ecosystem in Teide National Park (Tenerife, Canary Islands). Herbivory by rabbits is one important cause of its declining populations. Biological nitrogen-fixing of rhizobia in symbiosis with S. supranubius is hypothesized to be the main nitrogen (N) input in this alpine ecosystem. Therefore, our main objective was to investigate whether large rabbit dropping aggregations (latrines) affect the S. supranubius-rhizobia symbiosis by disturbing the rhizobia populations in the soils. Soils were sampled in the rhizosphere of broom plants without (RS) and with latrines (RLS) as well as in non-rhizospheric bare areas without (BS) and with latrines (BLS), and assessed for N content and number of rhizobia. Rhizobial diversity was evaluated by sequencing three housekeeping genes in root-nodule bacteria isolated from broom seedlings grown in these soils. Results showed that BS had the lowest rhizobia counts and N content, whereas RS had on average the highest, which were not exceeded in soils with N-supply from latrines (BLS and RLS). Phylogenetic analysis revealed that strains from the Bradyrhizobium canariense/Bradyrhizobium lupini lineage are the main microsymbionts of S. supranubius. Bradyrhizobium rifense/Bradyrhizobium cytisi strains were scarce and only detected in latrine influenced soils, where endophytic Bosea strains seem a common cohabitant in nodules. It can be concluded that S. supranubius-Bradyrhizobia symbiosis is a key N input in Teide soils and that rabbit latrines may affect the N-fixing symbiosis by altering abundance and diversity of the soil rhizobial populations. Moreover, latrines could also interfere in symbiosis by favouring co-entrance of non-nodulating bacteria inside the root nodules.

Biological characteristics of bacteriophages infecting three typic rhizobia of legume

Bacteriophages infecting rhizobia of legume leads to a significant decrease in the number of rhizobia in soil and nodulation in legume, which finally affects nitrogen fixation and remarkably reduces crop yield. However, limited studies have focused on bacteriophages of rhizobia. In this study, three typic rhizobium strains of Bradyrhizobium diazoefficiens USDA110T, Sinorhizobium sojae CCBAU05684T and Sinorhizobium meliloti USDA1002T were used as the host bacteria. From each host, three rhizobiophages were isolated from an agricultural black soil with double-layer plate method. We isolated nine rhizobiophages and investigated their morphological feature and biological characteristics. The results showed that the phages of SMM (infecting Sinorhizobium meliloti USDA1002T) and BDM (infecting Bradyrhizobium diazoefficiens USDA110T) belonged to Myoviridae family, while phages of SSS (infecting Sinorhizobium sojae CCBAU05684T) belonged to Siphoviridae family. The optimal multiplicity of infection for nine phages ranged from 0.001 to 1.0. The one-step growth curve showed that the latent and rising periods of BDM were remarkably longer than that of SMM and SSS, but the bust size was minimal. Nine phages had the strongest infecting activity at 30-40 ℃ and at neutral pH condition. Comparative analysis showed that the biological characteristics of phages infected with the same host were different, with the differentiation being smaller than that of phages infected with different hosts.

Microbial characterization of on-farm produced bio-enhancers used in organic farming

Different bio-enhancers commonly used in organic production of various crops were prepared for microbial characterization. Panchagavya had been utilized since long time by the farmers to provide nutrients to the plants and soil micro-organisms. Microbial analysis of bio-enhancers revealed that Panchagavya contained highest number of total bacteria (6.25 × 109 cfu/ml) as compared to Jeevamrita (3.24 × 109 cfu/ml), biodynamic liquid pesticide (2.27 × 109 cfu/ml) and Amritpani (5.49 × 109 cfu/ml). Panchagavya had also higher number of Pseudomonas (4.7 × 109 cfu/ml), Rhizobium (2.43 × 109 cfu/ml), Azotobacter (1.4 × 109 cfu/ml) and Azospirillum (1.03 × 105 cfu/ml). Fungi population was highest in Jeevamrita (1.20 × 107 cfu/ml) as compared to biodynamic liquid pesticide (2.64 × 106 cfu/ml), Amritpani (0.46 × 105 cfu/ml) and Panchagavya (0.20 × 105 cfu/ml). Actinomycetes, Pseudomonas, p-solubilizing microbes, Azotobacter and Azospirillum population were highest in biodynamic liquid pesticide (1.37 × 108, 3.28 × 108, 8.50 × 106, 2.00 × 106 and 1.40 × 105 cfu/ml). Similarly, gram positive bacteria, gram negative bacteria and Rhizobium were highest in Jeevamrita (1.60 × 108, 2.20 × 108, 7.51 × 107 cfu/ml). Amritpani had higher number of actinomycetes (1.31 × 107 cfu/ml), gram negative bacteria (1.35 × 108 cfu/ml) and p-solubilizing microbes (4.80 × 106 cfu/ml). Among all bio-enhancers, Panchagavya was rated most effective bio-enhancer followed by biodynamic liquid pesticide, Jeevamrita and Amritpani. Results reveal that these bio-enhancers could play a potent source of beneficial microbes which could improve soil fertility, crop productivity and produce quality.

Assessing root nodule microsymbionts in healthy and declined rooibos (Aspalathus linearis burm f.) at a plantation in South Africa

ABSTRACTAspalathus linearis (burm f.), commonly known as rooibos, grows in nutrient and organic matter poor sandy soils that limit its growth. In this study, samples of nodules from both declined and healthy rooibos plants were collected to determine the frequency of nodule nitrogen-fixing and endophytic bacteria. Standard microbiological procedures as well as sequence analysis of the 16S rRNA revealed that more than 75% of the bacterial isolates from the healthy plants contained microsymbionts belonging to the Rhizobium group and the remaining 25% were characterized as Pseudomonas and Burkholderia spp. The nodule from the declined plants lacks a sufficient number of rhizobia and was mostly white in color, small and contains the free-living endospore-forming Bacillus and other endophytic Burkholderia and Pseudomonas spp. The results provide a baseline data on the microsymbionts of rooibos nodules in Citrusdal and highlighted the need for further investigation using additional techniques.

Growth and Yield Responses of Cowpea to Inoculation and Phosphorus Fertilization in Different Environments.

Cowpea (Vigna unguiculata) is a major source of dietary protein and essential component of the cropping systems in semi-arid regions of Sub-Saharan Africa. However, yields are very low due to lack of improved cultivars, poor management practices, and limited inputs use. The objectives of this study were to assess the effects of rhizobia inoculant and P on nodulation, N accumulation and yield of two cowpea cultivars in Mozambique. Field study was conducted in three contrasting environments during the 2013/2014 and 2014/2015 seasons using randomized complete block design with four replications and four treatments. Treatments consisted of seed inoculation, application of 40 kg P2O5 ha-1, inoculation + P, and a non-inoculated control. The most probable number (MPN) technique was used to estimate the indigenous bradyrhizobia populations at the experimental sites. The rhizobia numbers at the sites varied from 5.27 × 102 to 1.07 × 103 cells g-1 soil. Inoculation increased nodule number by 34–76% and doubled nodule dry weight (78 to 160 mg plant-1). P application improved nodulation and interacted positively with the inoculant. Inoculation, P, and inoculant + P increased shoot dry weight, and shoot and grain N content across locations but increases in number of pods plant-1, seeds pod-1, and 100-seed weight were not consistent among treatments across locations. Shoot N content was consistently high for the inoculated plants and also for the inoculated + P fertilized plants, whereas the non-inoculated control plants had the lowest tissue N content. P uptake in shoot ranged from 1.72 to 3.77 g kg-1 and was higher for plants that received P fertilizer alone. Inoculation and P either alone or in combination consistently increased cowpea grain yield across locations with yields ranging from 1097 kg ha-1 for the non-inoculated control to 1674 kg ha-1 for the inoculant + P treatment. Grain protein concentration followed a similar trend as grain yield and ranged from 223 to 252 g kg-1 but a negative correlation between grain yield and protein concentration was observed. Inoculation increased net returns by $104–163 ha-1 over that for the control. The results demonstrate the potential of improving cowpea grain yield, quality and profitability using inoculant, although the cost-benefit for using P at the current fertilizer price is not attractive except when applied together with inoculant at low P site.

Population ecology of soybean-rhizobia in diverse crop rotations in Central India

Ensuring adequate populations of effective rhizobia in soil is essential to guarantee optimum nodulation and nitrogen fixation in legumes. The population of soybean-rhizobia and the proportion of slow and fast growers were enumerated in Vertisols of central India in different crop rotations for three years during 2013-15. Soils were from soybean-based (soybean-wheat and soybean-chickpea) and cereal-based rotations (maize-wheat, rice-wheat and maize-chickpea) rotations. Soybean-based rotations stimulated the rhizobial population by 22-fold compared to cereal rotations. Lowest populations were in the summer but increased in soybean rotations by 13.3-fold after the rainy season to 2743cellsg−1 soil at maximum vegetative growth. In the cereal rotations, the increase was by 1.7-fold to 61cellsg−1 soil. Long-term integrated nutrient management, by annual application of farmyard manure in soybean along with chemical fertilizers, improved the rhizobial numbers by 1.5-fold over chemical fertilization alone and by 2.9-fold over unfertilized treatments. Increased crop growth by chemical fertilizers also stimulated rhizobial populations by 1.9-fold over unfertilized soybean. The proportion of slow-growing soybean rhizobia was lower in soybean-based rotations (38%) compared to cereal-based rotations (62%), showing that continued soybean growth led to a greater proliferation of fast-growing rhizobia. Application of farmyard manure did not change the proportion of slow growers. The proportion of slow growers was ∼15% more in winter compared with the summer populations. The slow growers were symbiotically superior, producing greater dry matter and nodule mass (+12%), with more biologically fixed nitrogen (+18%) than the fast growers. Results highlight the importance of crop rotation with cereals, application of organic amendments along with chemical fertilizers and selection of slow-growing rhizobia as inoculants.

The effects of olive wastewater on the early growth of fababean and Rhizobium nodulation

Olive processing waste (OPW) is a by-product of olive oil production. Most of the studies about olive processing waste are carried-out in cotton, sunflower, wheat etc. However, there is little research performed on the effects of OPW on legumes. Therefore, the effect of OPW on growth of fababean and Rhizobium bacteria was studied in this research. It was used in doses of 0 (control), 18, 30 and 42 grams per pot, which is equivalent to doses of 3, 5 and 7 kg m-2. Increasing concentration of OPW improved the root fresh weight, nodule area, nodule diameter, root nodules scale value and the number of Rhizobium in one gram nodules. The results showed that the optimum OPW dose was 30 g pot-1 according to plant growth, nodules and Rhizobium. It was concluded that the OPW concentrations of 3% and it’s below can be used for fababean growing.

An ent-kaurene-derived diterpenoid virulence factor from Xanthomonas oryzae pv.oryzicola.

Both plants and fungi produce ent-kaurene as a precursor to the gibberellin plant hormones. A number of rhizobia contain functionally conserved, sequentially acting ent-copalyl diphosphate and ent-kaurene synthases (CPS and KS, respectively), which are found within a well-conserved operon that may lead to the production of gibberellins. Intriguingly, the rice bacterial leaf streak pathogen Xanthomonas oryzae pv.oryzicola (Xoc) contains a homologous operon. Here, we report biochemical characterization of the encoded CPS and KS, and the impact of insertional mutagenesis on virulence and the plant defense response for these genes, as well as that for one of the cytochromes P450 (CYP112) found in the operon. Activity of the CPS and KS found in this phytopathogen was verified - that is, Xoc is capable of producing ent-kaurene. Moreover, knocking out CPS, KS or CYP112 led to mutant Xoc that exhibited reduced virulence. Investigation of the effect on marker gene transcript levels suggests that the Xoc diterpenoid affects the plant defense response, most directly that mediated by jasmonic acid (JA). Xoc produces an ent-kaurene-derived diterpenoid as a virulence factor, potentially a gibberellin phytohormone, which is antagonistic to JA, consistent with the recent recognition of opposing effects for these phytohormones on the microbial defense response.

Naturalised populations of mesorhizobia in chickpea (Cicer arietinum L.) cropping soils: effects on nodule occupancy and productivity of commercial chickpea

Chickpea rhizobia did not occur naturally in Australian cropping soils, necessitating inoculation at sowing. Now, after more than 30 years of chickpea cultivation using a single inoculant strain, CC1192, it is likely that chickpea rhizobia are established in 1.0–1.5 Mha cropping land. The aims of this study were to examine effects of the naturalised chickpea rhizobia on nodulation and productivity (total crop N, crop N fixed and grain yield) of commercial chickpea. Soil was sampled from 26 fields to estimate chickpea rhizobial numbers, relate numbers to edaphic factors and years since previous chickpea crop, determine the proportions of CC1192 and novel strains using RAPD-PCR and subject a subset of novel strains from one site to 16S rRNA analysis. Nodules were harvested from 15 inoculated, commercial chickpea crops to determine occupancy by CC1192. The symbiotic effectiveness of a second subset of novel strains was assessed. The mean number of rhizobia in the soils varied from log 0.08 to log 5.16 rhizobia g soil−1 with population size positively correlated with soil moisture content and negatively correlated with salt concentration (ECe). RAPD-PCR analysis of 570 strains of chickpea rhizobia isolated from the soils indicated only 14 % with molecular fingerprints similar to CC1192. Occupancy by CC1192 of nodules harvested from commercial crops ranged 0–100 %, with an average of 53 %. Occupancy by CC1192 declined by an average 17 % with each log unit increase in numbers of novel chickpea rhizobia. We found no evidence that the novel mesorhizobia in the chickpea soils compromised N2 fixation or productivity of commercial chickpea crops, even though individual strains had generally reduced symbiotic effectiveness relative to CC1192.

Rhizobial counts in peat inoculants vary amongst legume inoculant groups at manufacture and with storage: implications for quality standards

Inoculation of legumes at sowing with rhizobia has arguably been one of the most cost-effective practices in modern agriculture. Critical aspects of inoculant quality are rhizobial counts at manufacture/registration and shelf (product) life. In order to re-evaluate the Australian standards for peat-based inoculants, we assessed numbers of rhizobia (rhizobial counts) and presence of contaminants in 1,234 individual packets of peat–based inoculants from 13 different inoculant groups that were either freshly manufactured or had been stored at 4 °C for up to 38 months to determine (a) rates of decline of rhizobial populations, and (b) effects of presence of contaminants on rhizobial populations. We also assessed effects of inoculant age on survival of the rhizobia during and immediately after inoculation of polyethylene beads. Rhizobial populations in the peat inoculants at manufacture and decline rates varied substantially amongst the 13 inoculant groups. The most stable were Sinorhizobium, Bradyrhizobium and Mesorhizobium with Rhizobium, particularly R. leguminosarum bv. trifolii the least stable. The presence of contaminants at the 10−6 level of dilution, i.e. >log 6.7 g−1 peat, reduced rhizobial numbers in the stored inoculants by an average of 37 %. Survival on beads following inoculation improved 2–3 fold with increasing age of inoculant. We concluded that the Australian standards for peat-based rhizobial inoculants should be reassessed to account for the large differences amongst the groups in counts at manufacture and survival rates during storage. Key recommendations are to increase expiry counts from log 8.0 to log 8.7 rhizobia g−1 peat and to have four levels of inoculant shelf life ranging from 12 months to 3 years.

Effects of elevated copper levels on biological nitrogen fixation and occurrence of rhizobia in a Tanzanian coffee-cropped soil

A study was conducted to investigate the effects of increasing copper concentration in soil on rhizobial occurrence and the process of biological nitrogen fixation. Two slow-growing bradyrhizobial strains CP1 and GM8 and two fast-growing rhizobial strains PV1 and PV2 were studied by comparing their performance under increasing copper concentrations in greenhouse-based assays involving modified Leonard jar assemblies and potted-soil experiments. Additionally, field samples from soils grown to coffee and subjected to long-term use of copper-based fungicides were analyzed for their total indigenous rhizobial populations using the most probable number-plant infection technique. Results indicated that elevated copper levels in the growth medium had inhibitory effects on nodulation, biological N2 fixation and overall rhizobial numbers in soil. Significant (p= 0.05%) reductions in fresh nodule mass, fresh nodule volume and total shoot nitrogen were recorded when copper concentration was increased from 0 to 100 ppm in both modified Leonard jar assemblies and potted-soil trials. Effective decrements in all the three parameters of fresh nodule mass, fresh nodule volume and total shoot nitrogen were more pronounced with the slow-growing bradyrhizobial strains of CP1 and GM8 than with the fast-growing PV1 and PV2. The MPN-plant infection technique results showed a non-significant (p=0.05) but substantial decrement in rhizobial and bradyrhizobial numbers when the copper-contaminated field soil (82.5 mgCu/g soil) was compared to a control soil (1.8 mgCu/g soil). The study concludes, therefore, that elevated levels of copper in soil could be harmful to free-living rhizobia and their abilities to fix N2 in respective symbiotic associations with legume species. Such negative effects were more pronounced in the slow-growing bradyrhizobial than rhizobial species used.

The influence of soil water potential on the growth and survival of alfalfa rhizobia in the soil

Growth and survival of alfalfa rhizobia (<em>Rhizobium meliloti</em> Dang.) from Pakistan and Nepal were studied in vials filled with sterile soil maintained at -0.03, -1.0, and -1.5 MPa. The main effects of water level, rhizobial strains and length of exposure to desiccation and their interactions showed a highly significant (P S 0.001) effect on the number of rhizobia g<sup>-1</sup> of soil. Growth and survival of rhizobia were highest in soil at -0.03 MPa followed by soil at -1.0 and -1.5 MPa. Highest cell counts were observed for strain UL 136 followed by strain UL 222 and the lowest was for strain UL 61. Two rhizobial strains (UL 136 and UL 222) were most tolerant to desiccation and showed highest growth and survival under low water potential. These two strains probably could be used as inoculants for alfalfa production under arid and semiarid environments.

A new sand pouch-plant infection technique for enumeration of rhizobia in soil

A sand pouch-plant infection technique for counting most probable numbers of rhizobia in soil is described. Populations of <em>Rhizobium leguminosarum</em> bv. <em>trifoli</em> detected by the plant infection method performed in enclosed glass tubes or in sand pouches did not differ significantly. The described method was used to assess numbers of <em>R. leguminosarum</em> bv. <em>trifoli</em> (<em>R.l.t</em>.) and bv. viciae (<em>R.l.v.</em>) in 20 soils of Poland. Logarithms of the populations of <em>R.l.t.</em> in the tested soils ranged from not detectable level to 4.76 and those of <em>R.l.v.</em> from 2.23 to 5.84 in g1 of soil dry mass. Numbers of <em>R.l.t.</em> were significantly correlated with soil clay, C org, and total N contents but not with the soil pH (in KCI), while numbers of <em>R.l.v.</em> showed significant correlation only with the soil pH.

Effectiveness of halo-tolerant, auxin producing Pseudomonas and Rhizobium strains to improve osmotic stress tolerance in mung bean (Vigna radiata L.)

Halo-tolerant, auxin producing bacteria could be used to induce salt tolerance in plants. A number of Rhizobium and auxin producing rhizobacterial strains were assessed for their ability to tolerate salt stress by conducting osmoadaptation assay. The selected strains were further screened for their ability to induce osmotic stress tolerance in mung bean seedlings under salt-stressed axenic conditions in growth pouch/jar trials. Three most effective strains of Rhizobium and Pseudomonas containing ACC-deaminase were evaluated in combination, for their ability to induce osmotic stress tolerance in mung bean at original, 4, and 6 dS m(-1) under axenic conditions. Results showed that sole inoculation of Rhizobium and Pseudomonas strains improved the total dry matter up to 1.4, and 1.9 fold, respectively, while the increase in salt tolerance index was improved up to 1.3 and 2.0 fold by the Rhizobium and Pseudomonas strains, respectively. However, up to 2.2 fold increase in total dry matter and salt tolerance index was observed due to combined inoculation of Rhizobium and Pseudomonas strains. So, combined application of Rhizobium and Pseudomonas strains could be explored as an effective strategy to induce osmotic stress tolerance in mung bean.

Agronomic and environmental drivers of population size and symbiotic performance of Rhizobium leguminosarum bv. viciae in Mediterranean-type environments

The population size and symbiotic performance (ability to fix N2) of rhizobia (Rhizobium leguminosarum bv. viciae) capable of nodulating field pea (Pisum sativum) were assessed in 114 soils from Mediterranean-type environments of southern Australia. All soils were collected in autumn, before the growing season, and had a history of crop legumes including field pea, faba bean, lentil, or vetch. The most probable number (MPN) technique, with vetch as a trap plant, was used to estimate the numbers of pea rhizobia in soils. Of the soils tested, 29% had low numbers of pea rhizobia (<100 rhizobia/g), 38% had moderate numbers (100–1000/g), and the remaining 33% had >1000/g. Soil pH, the frequency of a host crop in the rotation, and the number of summer days with a maximum temperature >35°C were strongly correlated with the pea rhizobia population size. Symbiotic performance (SP) of pea rhizobia in soils was assessed for soils with a MPN >100 rhizobia/g. An extract of the soils was used to inoculate two field pea cultivars growing in a nitrogen-deficient potting media in the greenhouse. Plants were grown for 5 weeks after inoculation and shoot dry matter was expressed as a percentage of the dry matter of plants grown with a commercial strain R. leguminosarum bv. viciae, SU303. Symbiotic performance ranged from 25 to 125%. One-quarter of the soils assessed had suboptimal SP (i.e. <70%). Soil and climatic variables were weakly associated with SP, with pH and average annual rainfall accounting for 17% of the variance. This research highlights the complexity of factors influencing population size and symbiotic performance of pea rhizobia in soils. Options for the improved management of populations of pea rhizobia in Mediterranean environments are discussed. Specifically, our data indicate that inoculation of pea crops is likely to be beneficial where pH(H2O) <6.6, particularly when summers have been hot and dry and when a host has been absent for ≥5 years, as numbers of rhizobia are likely to be below the thresholds needed to optimise nodulation and crop growth. New inoculation technologies and plant breeding will be required to overcome large populations of pea rhizobia with suboptimal SP.

Studies on allelopathic effect of artemisinin on rhizobium

Two strains of bean rhizobia, Rhizobium vigna 01 (slow-growing Rhizobium) and Rh. vigna 03 (fast-growing Rhizobium), were adopted to study allelopathic effect of artemisinin on the rhizobia. The results showed a significant inhibition of the reproduction and growth of rhizobium by artemisinin. After about 8 hours by adding 40 mg x L(-1) artemisinin into the culture medium, the number of rhizobia was less than half of those in normal culture. The utilization of sucrose and glucose by rhizobia decreased significantly as the concentration of artemisinin increased in the culture medium, which could be one of the main reasons for the inhibition of reproduction and growth of rhizobia by artemisinin. In addition, the activities of extracellular protease and acid phosphatase released from rhizobia decreased significantly as the concentrations of artemisinin increased. Artemisinin refluxed from Artemisia annua could thus inhibit the formation of root nodules and interfered with energy supply and reception between bacteroid and host cells. y = e(-ax) + b reflected the relationships between nitrogenase activities (y) and concentrations of artemisinin (x). In the culture medium with 48 mg x L(-1) of artemisinin, nitrogenase activities were about zero, resulting in the inactivation of nitrogenase in nodules formed. In general, artemisin in A. annua grown soils may inhibit the reproduction and growth of rhizobia, nodule formation and nitrogen biofixation, leading to less nitrogen supply, poor growth and development, and low yields of beans.