Influence of Pseudomonas syringae R25 and P. putida R105 on the growth and N2 fixation (acetylene reduction activity) of pea (Pisum sativum L.) and field bean (Phaseolus vulgaris L.)

The effects of inoculating field peas (Pisum sativum L.) with Rhizobium leguminosarum and field beans (Phaseolus vulgaris L.) with R. phaseoli, alone or in combination with Pseudomonas syringae R25 and/or P. putida R105, were assessed under gnotobiotic conditions in growth pouches and in potted soil in a growth chamber. Inoculation of peas with P. syringae R25 or P. putida R105 alone had no effect on plant growth in pouches. In soil, however, the isolate R25 inhibited nitrogenase activity (as assessed by acetylene reduction assay) of nodules formed by indigenous rhizobia; strain R105 stimulated pea seedling emergence and nodulation. P. syringae R25 inhibited the growth of beans in either plant-growth system. P. putida R105, however, had no effects on beans in pouches, but reduced plant root biomass and nodulation by indigenous rhizobia in soil. Coinoculation of pea seeds with R. leguminosarum and either of the pseudomonads significantly (P<0.01) increased shoot, root, and total plant weight in growth pouches, but had no effect in soil. Co-inoculation of field beans with R. phaseoli and P. putida R105 had no effects on plant biomass in growth pouches or in soil, but the number of nodules and the acetylene reduction activity was significantly (P<0.01) increased in the soil. In contrast, co-inoculation of beans with rhizobia and P. syringae R25 had severe, deleterious effects on seedling mergence, plant biomass, and nodulation in soil and growth pouches. Isolate R25 was responsible for the deleterious effects observed. Although plant growth-promoting rhizobacteria may interact synergistically with root-nodulating rhizobia, the PGPR selected for one crop should be assessed for potential hazardous effects on other crops before being used as inoculants.     

Manganese modulates the responses of nitrogen-supplied and Rhizobium-nodulated Phaseolus vulgaris L. to inoculation with arbuscular mycorrhizal fungi

In Venezuela, low yields of black bean crops are attributed, in part, to the low manganese (Mn) and phosphorus (P) contents in the Quartzipsamment soils where this crop is usually sown. To test this hypothesis, black bean plants were grown in sterilized sand to simulate soil physical properties, were fertilized with increasing Mn concentrations (0.1-20 μM) and inoculated with a commercial mixture of Rhizobium leguminosarum bv phaseoli strains 127K44, 127K89, 127K105 (+Rh), in combination with arbuscular mycorrhizal fungi Scutellospora heterogama and Entrophospora colombiana (+AMF). Non-inoculated plants fertilized with 6 mM NO₃ and 2 mM P served as controls. Plants were harvested at 18, 25, 33, and 40 days after emergence. At all harvests, the greatest growth and highest P and iron (Fe) leaf concentrations occurred in control plants grown in 5 μM Mn. The growth of +AMF plants was promoted at 0.1 μM Mn and inhibited at higher than 1 μM Mn. Whereas, concentrations of 5-10 μM Mn enhanced the growth and the Mn concentrations in leaves of +Rh plants 40 days after emergence. The tripartite symbiosis (+Rh+AMF) decreased growth, nodulation and leaf ureide and chlorophyll concentrations in plants grown in less than 20 μM Mn, imputed to severe ultrastructural alterations in the leaf and nodule tissues. Only +Rh+AMF plants grown in 20 μM Mn were effectively nodulated, AMF colonized and reached the flowering stage, although with diminished growth and low chlorophyll concentrations. Results confirm the high Mn requirement of +Rh plants for growth and nodulation and question the implementation of the tripartite symbiosis to improve yields in early flowering black bean varieties planted in soils deficient in Mn and P.     

The effect of acidity on the production of signal molecules by Medicago roots and their recognition by Sinorhizobium

The Medicago sativa-Sinorhizobium symbiosis is challenged by acidity, resulting in generally poor nodulation and production. Medicago murex, however, can nodulate and grow at low pH. The effect of low pH on signal exchange in the Sinorhizobium-Medicago symbiosis was studied to gain a greater understanding of the basis for poor nodulation of M. sativa compared to M. murex. Root exudates from M. sativa and M. murex grown in buffered nutrient solution at pH 4.5, 5.8 and 7.0, were collected to measure the expression of nodB induction in Sinorhizobium. A nodB-gusA fusion was constructed and inserted into Sinorhizobium medicae strains WSM419 (acid tolerant) and CC169 (acid sensitive). We identified greater induction by root exudates from both Medicago spp. collected at pH 4.5 than at pH 5.8 and 7.0, less induction by M. murex than M. sativa and less induction of WSM419 than CC169. The same major inducing compounds, 4′,7-dihydroxyflavanone (liquiritigenin), 4′,7-dihydroxyflavone, and 2′,4′,4-trihydroxychalcone (isoliquiritigenin), were identified in exudates of M. murex and M. sativa at all pH values, although in increasing amounts at lower pH. Poor nodulation of M. sativa relative to M. murex under acid conditions is not the consequence of decreased induction of Sinorhizobium nodB by chemical inducers present in the root exudates of both species at low pH.     

Growth stimulation and fruit yield improvement of greenhouse tomato plants by inoculation with Pseudomonas putida or Trichoderma atroviride: Possible role of indole acetic acid (IAA)

Five bacteria (Pseudomonas fluorescens, P. fluorescens subgroup G strain 2, P. marginalis, P. putida subgroup B strain 1 and P. syringae strain 1) and three fungi (Penicillium brevicompactum, P. solitum strain 1 and Trichoderma atroviride) were evaluated to determine their promoting effect on the growth of mature healthy tomato plants grown under hydroponic conditions. P. putida and T. atroviride were shown to improve fruit yields in rockwool and in organic medium. The production or degradation of indole acetic acid (IAA) by the two microorganisms was investigated as possible mechanisms for plant growth stimulation. Both P. putida and T. atroviride were shown to produce IAA. The production of IAA by the two microorganisms was stimulated in vitro by the addition of l-tryptophan, tryptamine and tryptophol (200 μg ml⁻¹) in the culture medium. P. putida and T. atroviride also increased the fresh weight of both the shoot and the roots of tomato seedlings grown in the presence of increasing concentrations of l-tryptophan (up to 0.75 mM). Both microorganisms showed partial degradation of IAA in vitro when grown in a minimal medium with or without sucrose. In addition, the capacity of these microorganisms to reduce the deleterious effect of exogenous IAA was investigated using tomato seedlings. The results showed that the roots of tomato seedlings grown in the presence of increasing concentrations of IAA (0-10 μg ml⁻¹) were significantly longer when seeds were previously treated with P. putida or T. atroviride. The reduction in the detrimental effect of IAA on root elongation could be associated with a reduced ethylene production resulting from a decrease of its precursor 1-aminocyclopropane-1-carboxylic acid (ACC) by microbial degradation of IAA in the rhizosphere and/or by ACC deaminase activity present in both microorganisms.     

Growth promoting effects of corn (Zea mays) bacterial isolates under greenhouse and field conditions

Fertilizer costs are a major component of corn production. The use of biofertilizers may be one way of reducing production costs. In this study we present isolation and identification of three plant growth promoting bacteria that were identified as Enterobacter cloacae (CR1), Pseudomonas putida (CR7) and Stenotrophomonas maltophilia (CR3). All bacterial strains produced IAA in the presence of 100 mg l⁻¹ of tryptophan and antifungal metabolites to several soilborne pathogens. S. maltophilia and E. cloacae had broad spectrum activity against most Fusarium species. The only strain that was positive for nitrogen fixation was E. cloacae and it, and P. putida, were also positive for phosphate solubilization. These bacteria and the corn isolate Sphingobacterium canadense CR11, and known plant growth promoting bacterium Burkholderia phytofirmans E24 were used to inoculate corn seed to examine growth promotion of two lines of corn, varieties 39D82 and 39M27 under greenhouse conditions. When grown in sterilized sand varieties 39M27 and 39D82 showed significant increases in total dry weights of root and shoot of 10-20% and 13-28% and 17-32% and 21-31% respectively. Plants of the two varieties grown in soil collected from a corn field had respective increases in dry weights of root and shoot of 10-30% and 12-35% and 11-19% and 10-18%. In sand, a bacterial mixture was highly effective whereas in soil individual bacteria namely P. putida CR7 and E. cloacae CR1 gave the best results with 39M27 and 39D82 respectively. These isolates and another corn isolate, Azospirillum zeae N7, were tested in a sandy soil with a 55 and 110 kg ha⁻¹ of nitrogen fertility at the Delhi research Station of Agriculture and Agri-Food Canada over two years. Although out of seven bacterial treatments, no treatment provided a statistically significant yield increase over control plots but S. canadense CR11 and A. zeae N7 provided statistically significant yield increase as compared to other bacteria. The 110 kg rate of nitrogen provided significant yield increase compared to the 55 kg rate in both years.     

Indole acetic acid production by Rhizobium: Effect of 2-ketoglutaric acid

I ndole acetic acid (IAA) production from tryptophan by cell suspensions of Rhizobium trifolii, R. leguminosarum, R. phaseoli and R. lupini was studied in the presence or in the absence of 2-ketoglutaric acid. In R. lupini, production of IAA was strongly enhanced by the ketoacid, but in fast growing rhizobia it was less enhanced or unaffected. On the other hand, glutamic acid inhibits IAA production by R. meliloti, but stimulates IAA production in both R. leguminosarum and R. phaseoli. A hypothesis is proposed to explain these results.     

Regulated modulation of Trifolium subterraneum inoculated with bacteriocinproducing strains of Rhizobium trifolii

When a bacteriocin-sensitive and a bacteriocin-resistant strain of Rhizobium were added together to plant-tubes, containing Trifolium subterraneum cv. Dwalganup seedlings, the bacteriocinresistant strain occupied less than 10% of the nodules sites. But when a bacteriocin-producer was added with the inoculum, the bacteriocin-resistant strain occupied 75% of the nodule sites. In a replicate experiment, using a different bacteriocin-sensitive strain, the proportion of the nodules formed by the bacteriocin-resistant strain was increased from 17%, in the absence of the bacteriocin-producer, to 100% in its presence.Similar experiments were set up in non-sterile soil that contained ineffective, bacteriocin-sensitive rhizobia. When an effective bacteriocin-resistant strain was inoculated with a bacteriocin-producing strain, the proportion of nodules formed by the effective strain was greater than in the absence of the bacteriocin-producer. This was because nodulation by the indigenous rhizobia was reduced in the presence of the bacteriocin-producer.Such experiments suggest that bacteriocin-producing strains of R. trifolii that reduce the nodulation success of competing bacteriocin-sensitive strains could be used to increase the proportion of nodules formed by a desired bacteriocin-resistant strain.     

Low root zone temperature effects on bean (Phaseolus vulgaris L.) plants inoculated with Rhizobium leguminosarum bv. phaseoli pre-incubated with methyl jasmonate and/or genistein

Abstract

Methyl jasmonate (MeJA) has recently been shown to act as a plant-to-bacteria signal. We tested the hypothesis that pre-induction of Rhizobium leguminosarum bv. phaseoli cells with genistein and/or MeJA would at least partially overcome the negative effects of low root zone temperature (RZT) on bean nodulation, nitrogen fixation and plant growth. Otebo bean plants were grown at constant air temperature (25^oC) and two RZT regimes (25 and 17^oC) and inoculated with R. leguminosarum bv. phaseoli pre-induced with MeJA and/or genistein. Our results indicate that low RZT inhibited nodulation, nitrogen fixation and plant growth. The plants growing at low RZT began fixing nitrogen seven days later compared to those at higher RZT. The low RZT plants had fewer nodules, lower nodule weight, less N fixation, slower plant growth, fewer leaves, smaller leaf area, and less dry matter accumulation comared to plants at a higher RZT. Rhzobium leguminosarum bv. phaseoli cells induced with genistein and/or MeJA enhanced bean nodulation, nitrogen fixation and growth at both optimum and suboptimum RZTs. The results of this study indicate that MeJA improves bean nitrogen fixation and growth at both optimum and suboptimum RZTs, and can be used alone or in combination with genistein to partially overcome the low RZT induced inhibitory effects on nodulation and nitrogen fixation.     

Trophic shift of stable isotopes and fatty acids in Collembola on bacterial diets

Fatty acid (FA) analysis is a promising tool to study trophic relationships in soil food webs. We determined FA biomarkers to trace bacterial food sources (Bacillus megaterium, Pseudomonas putida, Enterobacter aerogenes) of Collembola (Heteromurus nitidus, Protaphorura fimata, Folsomia candida). In addition, δ¹⁵N, δ¹³C, C/N ratio, body weight and NLFA/PLFA ratio (neutral lipid/phospholipid fatty acids) of Collembola were assessed. These measures indicated that P. putida ranked first, B. megaterium second and E. aerogenes third in food quality. FAs specific for bacteria were found in the NLFAs of the Collembola reflecting the respective bacterial diet. Biomarker FAs for gram-positive bacteria were methyl branched i14:0, i15:0, a15:0 and i17:0. Consumption of gram-negative bacteria was reflected by the cyclic form cy17:0 (E. aerogenes, P. putida) and by 16:1ω5 (P. putida).     

Nodulation competitiveness of Rhizobium leguminosarum bv. phaseoli and Rhizobium tropici strains measured by glucuronidase (gus) gene fusion

Summary The nodulation competitiveness of 17 Rhizobium leguminosarum bv. phaseoli and 3 R. tropici strains was analysed in growth pouches, at pH 5.2 and 6.4. All 20 strains were coinoculated with a gus ⁺ strain of R. leguminosarum bv. phaseoli strain KIM5s. The gus⁺ phenotype, carrying the glucuronidase gene, was used to type nodules directly in the growth pouches. Nodule occupancy ranged from 4% for the least competitive to 96% for the most competitive R. leguminosarum bv. phaseoli strain. The R. tropici strains showed low rates of nodule occupancy at pH 6.4 but their competitiveness improved significantly under acid conditions. CIAT 895 was the only R. leguminosarum bv. phaseoli strain that was less competitive (P<0.05) at the lower pH. The competitiveness of all the other R. leguminosarum bv. phaseoli strains was unaffected by pH. Various physiological and genetic properties of the strains were analysed in search of correlations with nodulation competitiveness. Hybridisation patterns with three different DNA probes (nif KDH, common nod genes, and hup genes) and the metabolism of 53 different C sources were compared. No general correlations were found between hybridisation or growth pattern and competitiveness. The less competitive R. tropici strains had a unique DNA hybridisation pattern and were not able to use shikimate, ferulate, coumarate, or asparagine as C sources. Most of the less competitive R. leguminosarum bv. phaseoli strains could not metabolize either ferulate or coumarate. This might indicate a relationship between nodulation competitiveness and the ability to degrade aromatic compounds.     

The abundance and efficacy of Rhizobium leguminosarum bv. viciae in cultivated soils of the eastern Canadian prairie

For optimum production, the use of commercial rhizobial inoculant on pea (Pisum sativum L.) at seeding is necessary in the absence of compatible rhizobial strains or when rhizobial soil populations are low or symbiotically ineffective. Multiple site experiments were conducted to characterize the abundance and effectiveness of resident populations of Rhizobium leguminosarum bv. viciae (Rlv) in eastern Canadian prairie soils. A survey of 20 sites across a broad geographical range of southern Manitoba was carried out in 1998 and was followed by more intensive study of five of the sites in 1999 and 2000. Appreciable nodulation of uninoculated pea was observed at all sites which had previously grown inoculated pea. However, uninoculated pea grown at two sites, which had not previously grown pea, had negligible nodulation. Likewise, wild Lathyrus sp. and Vicia sp. plants collected from uncultivated areas adjacent to agricultural sites were poorly nodulated. In the more intensively studied sites, there was a tendency towards higher nodulation in pea plants receiving commercial inoculant containing Rlv strain PBC108 across all site-years (e.g., 4.7% in nodulation and 22% in nodule mass), but the effect was significant at only 2 of 10 site-years. Despite a relatively high range of soil pH (6-8), regression analysis indicated that decreasing soil pH resulted in lower nodulation rates. Likewise, electrical conductivity (EC) was correlated to nodulation levels, however the effect of EC was likely more indicative of the influence of soil texture and organic matter than salinity. As with nodulation, commercial inoculation tended to increase above-ground dry matter (DM) and fixed-N (estimated by the difference method) at the early pod-filling stage, but again the effects were significant at only 2 of 10 site-years. Specifically, above-ground DM and fixed-N levels were up to 29 and 51% greater, respectively, in inoculated compared to non-inoculated treatments at these sites. Addition of N-fertilizer at a rate of 100 kg N ha⁻¹ decreased nodulation at almost all site-years (by as much as 70% at one site), but rarely resulted in increases in above-ground DM compared to inoculated plots. The study indicates for the first time that populations of infective, and generally effective strains of Rlv occur broadly in agricultural soils across the eastern Canadian prairie, but that there is a tendency for increased symbiotic efficiency with the use of commercial inoculant.     

The diversity of rhizobia nodulating chickpea (Cicer arietinum) under water deficiency as a source of more efficient inoculants

Chickpea is an important pulse crop grown mainly in the arid and semi-arid regions. The effect of water deficiency on nodulation, biomass production, and competition for nodule occupancy was evaluated in three different soils with two chickpea cultivars, Amdoun I and Chetoui. Two watering regimes were considered; a control that was irrigated three times per week and a water-deficient treatment that was irrigated only one time a week. Results showed that water deficiency significantly decreased the nodule number and the shoot dry weight for both cultivars. Root-nodule bacteria were isolated and characterized by PCR-RFLP of 16S rDNA and nifD-K intergenic spacer. The results show that water deficiency affects the diversity of nodulating rhizobia. The nodulation by Mesorhizobium mediterraneum was reduced while inefficient nodulation by Ensifer meliloti was favoured. In both treatments, chickpea was preferentially nodulated by nifD-K type N6. Analysis for NaCl tolerance showed that most of rhizobia nodulating chickpea under water deficiency are NaCl tolerant. Inoculation with the selected salt-tolerant strain of M. mediterraneum LILM10 increased significantly nodule number and grain yield in the field.     

Synergistic effects of plant-growth promoting rhizobacteria and Rhizobium on nodulation and nitrogen fixation by pigeonpea (Cajanus cajan)

Plant‐growth promoting rhizobacteria (PGPR), in conjuction with efficient Rhizobium, can affect the growth and nitrogen fixation in pigeonpea by inducing the occupancy of introduced Rhizobium in the nodules of the legume. This study assessed the effect of different plant‐growth promoting rhizobacteria (Azotobacter chroococcum , Azospirillum brasilense, Pseudomonas fluorescens, Pseudomonas putida and Bacillus cereus) on pigeonpea (Cajanus cajan (L) Milsp.) cv. P‐921 inoculated with Rhizobium sp. (AR‐2–2 k). A glasshouse experiment was carried out with a sandy‐loam soil in which the seeds were treated with Rhizobium alone or in combination with several PGPR isolates. It was monitored on the basis of nodulation, N₂ fixation, shoot biomass, total N content in shoot and legume grain yield. The competitive ability of the introduced Rhizobium strain was assessed by calculating nodule occupancy. The PGPR isolates used did not antagonize the introduced Rhizobium strain and the dual inoculation with either Pseudomonas putida, P. fluorescens or Bacillus cereus resulted in a significant increase in plant growth, nodulation and enzyme activity over Rhizobium‐inoculated and uninoculated control plants. The nodule occupancy of the introduced Rhizobium strain increased from 50% (with Rhizobium alone) to 85% in the presence of Pseudomonas putida. This study enabled us to select an ideal combination of efficient Rhizobium strain and PGPR for pigeonpea grown in the semiarid tropics.     

Rhizosphere effect of Galega orientalis in oil-contaminated soil

Randomized lysimeters in an oil-contaminated field contained the following treatments: (1) Galega orientalis seeds inoculated with Rhizobium galegae HAMBI 540, (2) bioaugmentation with Pseudomonas putida PaW85, and (3) R. galegae -inoculated G. orientalis seeds plus bioaugmentation with P. putida PaW85. The bacterial abundance and diversity were analysed in composite samples after one growing season. A total of 208 m-toluate tolerating bacteria were isolated and screened with m-toluate tolerance and utilization tests, and the catechol test. Seventy-nine isolates were characterized with (GTG)₅-PCR genomic fingerprinting and 16S rRNA gene PCR-RFLP ribotyping. Only 10% of the isolated strains were able to degrade m-toluate. Most of the m-toluate utilizing bacteria were catechol positive indicating the existence of a TOL plasmid. Rhizosphere effect of G. orientalis was manifested in oil-contaminated soil. G. orientalis and Pseudomonas bioaugmentation increased the amount of bacteria in oil-contaminated soil. G. orientalis especially together with Pseudomonas bioaugmentation increased the numbers of m-toluate utilizing and catechol positive bacteria in the soil samples indicating an increase in degradation potential. The rhizosphere of G. orientalis increased also the diversity of bacteria. More ribotypes were found in soils treated with G. orientalis and P. putida PaW85 compared to the untreated soil, but the diversity of the m-toluate utilizing bacteria did not significantly increase.     

Phosphate-solubilizing Pseudomonas putida can influence the rhizobia-legume symbiosis

Allfalfa and soybean are the most important leguminous plants in the agricultural system of the semiarid pampas of Argentina. The possible action of phosphate solubilizing bacteria on the leguminous-rhizobia symbiosis was studied since in this region the available phosphorus distribution is not uniform. The strains used were Sinorhizobium meliloti 3DOh13, a good solubilizer of iron and phosphorus for alfalfa, Bradyrhizobium japonicum TIIIB for soybean and two phosphorus-solubilizing strains of Pseudomonas putida (SP21 and SP22) for growth promotion treatments. Modification of shoot and root system dry weights occured in soybean but not in alfalfa in presence of Pseudomonas strains.     

Rapid identification and discrimination among Egyptian genotypes of Rhizobium leguminosarum bv. viciae and Sinorhizobium meliloti nodulating faba bean (Vicia faba L.) by analysis of nodC, ARDRA, and rDNA sequence analysis

Twenty-eight Rhizobium strains were isolated from the root nodules of faba bean (Vicia faba L.) collected from 11 governorates in Egypt. A majority of these strains (57%) were identified as Rhizobium leguminosarum bv. viciae (Rlv) based on analysis of a nodC gene fragment amplified using specific primers for these faba bean symbionts. The strains were characterized using a polyphasic approach, including nodulation pattern, tolerance to environmental stresses, and genetic diversity based on amplified ribosomal DNA-restriction analysis (ARDRA) of both 16S and 23S rDNA. Analysis of tolerance to environmental stresses revealed that some of these strains can survive in the presence of 1% NaCl and a majority of them survived well at 37 °C. ARDRA indicated that the strains could be divided into six 16S rDNA genotypes and five 23S rDNA genotypes. Sequence analysis of 16S rDNA indicated that 57% were Rlv, two strains were Rhizobium etli, one strain was taxonomically related to Rhizobium rubi, and a group of strains were most closely related to Sinorhizobium meliloti. Results of these studies indicate that genetically diverse rhizobial strains are capable of forming N₂-fixing symbiotic associations with faba bean and PCR done using nodC primers allows for the rapid identification of V. faba symbionts.     

Hyperparasitism of oospores of Phytophthora megasperma var. sojae

Hyperparasites of oospores of Phytophthora megasperma Drechs. var. sojae Hildb. were present in each of 15 field soils tested. Maximum numbers of oospores parasitized ranged from 42.5 to 87.5% for flooded soils, and from 25.5 to 73.0% for soils adjusted to 50% water holding capacity; the mean for all soils was 51.5%. The frequency of hyperparasitism was not correlated with the disease potential soils for Phytophthora root-rot of soybean as determined in seedling tests on flooded soil samples. Of eight isolated hyperparasitic fungi tested in steamed soil, the most efficient parasites were Hyphochytrium catenoides, Humicola fuscoatra, and Pythium monospermum, each of which parasitized at least 76% of oospores during 3 weeks. Hyphae were not parasitized by any of the eight fungi. Parasitism by H. catenoides in sterilized soil increased as soil temperature increased from 16° to 28°C. Parasitism by P. monospermum was maximum at 20°–24°C. Oospores of P. meyasperma var. sojae race 7 were more resistant to infection by hyperparasites than were oospores of races 1 and 3. Oospores produced in culture were slightly more susceptible to hyperparasitism in soils than were oospores produced in soybean seedlings.     

Mycoparasitism by Pythium oligandrum and P. acanthicum

Isolates of the reported mycoparasites Pythium oligandrum, P. acanthicum and P. periplocum markedly reduced growth and cellulolysis by Botryotrichum piluliferum, grew rapidly across agar plates precolonized by Phialophora radicicola var radicicola (sensu Deacon) and, where tested (not P. periplocum), were non-pathogenic towards higher plants. Isolates of P. echinulatum, P. mamillatum. P. megalacanthum, P. spinosum, P. ultimum and one isolate of P. acanthicum behaved differently from the mycoparasites and could, themselves, be placed in two groupings in these tests. It is suggested that the ability or otherwise to grow on Phialophora-precolonized agar plates may help to distinguish broad biological groupings within the genus Pythium, but these groupings may cut across conventional taxonomic ones.One isolate of P. acanthicum was tested for its effects on a range of cellulolytic fungi: it reduced their growth to different extents, as did P. oligandrum.Plates of potato-dextrose agar precolonized by Phialophora radicicola were used to isolate selectively P. oligandrum and similar fungi from soils, but the use of hemp seed baits in conjunction with precolonized plates was less selective for these fungi.Straw pieces precolonized by P. oligandrum and buried in soil decomposed at the same rate as virgin straws or those precolonized by P. ultimum or Mucor hiemalis. Subsequently, Stachyholrys atra appeared to colonize straws more frequently from soil, and Fusarium spp. less so, in the presence of P. oligandrum than in its absence. In the laboratory, P. oligandrum was antagonized by Slachyholrys, whereas Fusarium spp. were frequently overgrown by the Pythium.     

Symbiosis of Trifolium subterraneum with mycorrhizal fungi and Rhizobium trifolii as affected by ammonium sulphate and nitrification inhibitors

The symbioses between Trifolium subterraneum, mycorrhizal fungi and Rhizohium are affected by (NH₄)₂SO₄ and by the nitrification inhibitors 2-chloro-6 (trichloromethyl) pyridine (N-Serve) and 2-trichloromethyl pyridine (2TMP). At 50 μg · g^?1 soil N-Serve and 2TMP had toxic effects on plant growth, measured as leaf expansion, root length and dry weight. Lower concentrations of N-Serve also produced some toxic symptoms. The addition of (NH₄)₂SO₄ to the soil at 2 and 6 m-equiv NH⁺₄ per pot, resulted in reduced root length and nodulation. Shoot dry weight was reduced at 6 m-equiv NH⁺₄ per pot. In the presence of (NH₄)₂SO₄ the toxic effects of the nitrification inhibitors on plant growth were less.Both nitrification inhibitors reduced development of mycorrhizal entry-points and extent of root colonization (% infection). Percentage infection of the root system was also reduced by (NH₄)₂SO₄. Development of nodules on the lateral roots was increased in the presence of N-Serve at 5 and 15 μ · g^?1. This effect, however, was accompanied by a marked reduction in N₂ase activity. Smaller increases in nodulation were apparent with 2TMP and were associated with variable N₂ase activity.     

Competitive interaction among strains of Rhizobium leguminosarum bv. phaseoli in the nodulation of kidney beans (Phaseolus vulgaris L.)

We examined the competitiveness of five effective Rhizobium leguminosarum biovar phaseoli strains in the nodulation of kidney beans (Phaseolus vulgaris L.), either alone or in pairwise combination, against the indigenous strains. The results showed that the introduced Rhizobium sp. strains (B2, B17, B36, T2, or CIAT 652) occupying 64–79% of the total nodules (as single inocula) were more competitive in nodulation than the native rhizobia. However, the competitiveness of the individual Rhizobium sp. strain either increased or decreased when used in a pairwise combination of double-strain inocula. For example, strain B17, although quite competitive against the indigenous population (68% nodule occupancy), became poorly competitive in the presence of strain B2 (reduced from 68 to 2.5%). A similar reduction in nodule occupancy by strain B17 was observed in the presence of B36 or CIAT 652, indicating that two competitive strains may not always be compatible. These results suggest that it is important to co-select competitive as well as compatible rhizobia for multistrain inoculant formulation.