Rhodotorulic acid enhances root colonization of tomato plants by arbuscular mycorrhizal (AM) fungi due to its stimulatory effect on the pre-symbiotic stages of the AM fungi

Exudates of Rhodotorula mucilaginosa, a yeast commonly found in the rhizosphere, increased hyphal length of the arbuscular mycorrhizal (AM) fungi Gigaspora rosea and Gigaspora margarita. Rhodotorulic acid (RA), a siderophore compound obtained from R. mucilaginosa exudates, increased hyphal length and branching. Thus, the increase in the number of entry points and the higher AM root colonization of tomato plants in the presence of RA can at least partially be explained by the positive effect of RA on the pre-symbiotic stages of the AM fungi.     

Effects of chitin amendment of soil on microorganisms,nematodes, and growth of white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.)

Effects of soil amendment with crabshell chitin on the growth of white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.), and on populations of soil bacteria, fungi, and plant-parasitic and free-living nematodes were investigated in a pot trial. Five soil samples were collected from Te Puke (Paengaroa Shallow Sand, a Typic Hapludand) and five from Hamilton (Bruntwood silt loam, an Aquic Hapludand), New Zealand. Subsamples of each soil were either amended with chitin or unamended and planted with white clover and ryegrass. The ryegrass shoot weight in amended soil was greater (P<0.01), most probably due to N mineralised from chitin. A significantly lower (P<0.01) root: shoot ratio of ryegrass in the amended soil also suggested improved N availability, and therefore less root mass was needed to support a given shoot mass. A reduction in nodulation was observed in 12-day-old white clover seedlings (P<0.05) and also in 6-week-old seedlings (P<0.01). The shoot weight of white clover was significantly lower (P<0.05) in amended soil, possibly due to phytotoxic effects of chitin. Chitin increased (P<0.01) the populations of bacteria and fungi by 13-fold and 2.5-fold, respectively. The cyst nematode of white clover, Heterodera trifolii, was significantly reduced in chitin-amended soil, possibly due to increased levels of chitinase produced by rhizosphere microorganisms. Two other plant-parasitic nematodes, Pratylenchus spp. and Tylenchus spp., were also reduced in ryegrass roots and in soil as a result of the chitin amendment. However, the total number of free-living nematodes increased 5.4-fold in amended soil.     

Nitrogen and oxygen isotope ratios of nitrous oxide emitted from soil and produced by nitrifying and denitrifying bacteria

The isotopic composition at natural abundance levels of nitrous oxide emitted from a sandy loam, neutral pH soil under a range of soil water contents (matric potentials of-0.1,-1.0 and-5.0 kPa), from soil amended with sodium succinate and sodium ethanoate, and produced by pure cultures of the nitrifying bacteria Nitrosomonas europaea and Nitrosolobus multiformis, and by the denitrifying bacterium Pseudomonas putida, has been determined in laboratory experiments. N₂O from all sources was depleted in the ¹⁵N and ¹⁸O isotopes relative to the conventional references [atmospheric N₂ and standard mean ocean water (SMOW), respectively]. N₂O from soil was depleted in ¹⁵N and ¹⁸O to increasing extents with increasing soil water content. The isotopic composition of N₂O produced by N. europaea and N. multiformis was similar to that emitted from drier soil (matric potential of-1.0 kPa) and the N₂O produced by P. putida was similar to that emitted from wetter soil (matric potential of-0.1 kPa). N₂O emitted from the wetter soil was enriched in ¹⁵N and ¹⁸O compared with that emitted from the drier soil. The differences in isotopic composition between N₂O from the wetter and drier soil were attributed principally to isotopic fractionation during N₂O reduction to N₂ in the terminal step of denitrification. The effect of both sodium succinate and sodium ethanoate amendment was to increase the overall rate of N₂O emission, much of which arose from denitrification, as revealed by incubation in 100 kPa O₂. In addition, in the sodium ethanoate amended soil N₂O reduction to N₂ did not occur, as revealed by incubation in 10 kPa C₂H₂. The N₂O from the sodium ethanoate amended soil was depleted in ¹⁵N to a greater extent than the sodium succinate amended soil, which is consistent with the observation that N₂O reduction to N₂ leaves residual N₂O relatively enriched in ¹⁵N.     

Suppression of wheat take-all and ophiobolus patch by fluorescent pseudomonads from a fusarium-suppressive soil

Fluorescent pseudomonads isolated from a soil suppressing Fusarium wilt significantly reduced take-all (Gaeumannomyces graminis var. tritici) in wheat and Ophiobolus patch (G. graminis var. avenae) in Agrostis turfgrass. The bacteria were mixed into a conducive soil at a concentration of 10⁷ colony-forming units (cfu)g^?1 soil at sowing. There were significantly fewer (P ? 0.05) diseased wheat roots in the treatments with the bacteria and pathogen than in those with the pathogen alone. Dry weights of the tops of wheat and Agrostis turfgrass were significantly greater (P ? 0.01) in treatments inoculated with the bacteria in the presence of the pathogens compared to controls with the pathogens alone. Dry weights of the tops of plants from treatments inoculated with the bacteria alone were not significantly different to those of healthy wheat non-inoculated with the bacteria, showing that the fluorescent pseudomonads did not stimulate plant growth. At the end of the experiments, the bacterial isolates (genetically-marked with rifampicin resistance) were recovered from wheat roots and rhizosphere soil at concentrations of 10⁵–10⁷cfu g^?1 fresh weight of roots or oven-dried rhizosphere soil.Many of the fluorescent pseudomonads and some non-fluorescent pseudomonads showed in vitro antibiosis on quarter-strength potato dextrose agar (QPDA) against the pathogens. However, there was no correlation between in vitro antibiosis on agar plates and suppression of disease in pot experiments. Further, while some isolates of G. graminis var. tritici and var. avenae were inhibited by certain bacterial isolates, other isolates of the same fungus were not similarly inhibited by the same isolates of bacteria. Most of the fluorescent pseudomonads that produced inhibition zones (>5mm) against G. graminis var. tritici on QPDA did not do so on King's medium B, where fluorescent siderophores were formed. In vitro antibiosis is, therefore, a poor criterion for selecting effective bacterial antagonists of the wheat take-all fungus. All of the fluorescent pseudomonads tested produced siderophores in low-Fe media while a non-fluorescent pseudomonad and the fungal pathogens did not produce siderophores of comparable activity. The addition of 500 μg FeEDTA g^?1 with a lower stability constant did not. The evidence suggests that iron competition at the rhizoplane or in the rhizosphere is one mechanism of suppression.     

Responses by iron-efficient and inefficient oat cultivars to inoculation with siderophore-producing bacteria in a calcareous soil

Rhizosphere bacteria may enhance plant uptake of Fe by producing siderophores that chelate sparingly soluble Fe³⁺ in calcareous soils. To evaluate the extent to which plants benefit from colonization of the roots by prolific siderophore-producing bacteria, we inoculated two oat cultivars with six strains of bacteria that produced high concentrations of siderophores under Felimiting conditions in vitro. Oat cv Coker 227, an Fe-efficient cultivar, which produces the phytosiderophore avenic acid, and cv TAM 0-312, and Fe-inefficient cultivar, which does not produce the phytosiderophore, were grown in a calcareous soil (Weswood silt loam) on a light bench in the laboratory. Half of the plants were fertilized with a nutrient solution containing 5 mM Fe and half with a nutrient solution containing no Fe. After 6 weeks of growth, we compared colonization of the roots by the inoculant bacteria and the dry weight and Fe content of roots and shoots. Three species of Pseudomonas colonized the roots of both oat cultivars in high numbers (10⁶ cells g^-1 root dry weight), whereas the remaining bacteria colonized the roots in substantially lower numbers (10⁴ cells g^-1 root dry weight). Plants fertilized with 5 mM Fe were larger and supported greater numbers or rhizosphere bacteria per gram of root than plants not supplied with Fe. Comparisons of the Fe content and dry weight of roots and shoots revealed few significant differences between inoculated and uninoculated plants, or among the plants inoculated with the different strains of siderophore-producing bacteria. The differences that were observed revealed no consistent response to inoculation. We conclude that inoculation of the roots of the two oat cultivars with bacteria that produce high concentrations of siderophores in response to an Fe deficiency had little or no effect on Fe acquisition by the plants.     

Effect of phosphonate on the rhizosphere microflora and the development of root rot (Phytophthora cinnamomi) in avocado (Persea americana) and pepper-corn (Schinus molle) tree seedlings

Summary Application to soil of 1 g (recommended rate) or 10 g l^-1 of phosphonate did not affect the numbers of bacteria and fungi nor the proportions of actinomycetes and fungi antagonistic to Phytophthora cinnamomi. Foliar phosphonate applications to avocado seedlings (Persea americana) did not affect microbial numbers or the proportions of microbes in the rhizosphere capable of antagonizing P. cinnamomi. Mycelium of P. cinnamomi and zoospores of P. palmivora did not appear to respond to diffusates from excised roots of phosphonate-treated avocado and pepper-corn tree (Schinus molle) seedlings, respectively. However, less extensive lesions were observed on the roots of fungicide-treated avocado and pepper-corn tree seedlings exposed to P. cinnamomi and P. palmivora, respectively. The reduction in P. cinnamomi infection on pepper-corn tree seedlings appears to be brought about by additive rather than interactive effects of the resident soil microflora and foliar-applied phosphonate.     

Use of chrome azurol S reagents to evaluate siderophore production by rhizosphere bacteria

Summary Siderophores produced by rhizosphere bacteria may enhance plant growth by increasing the availability of Fe near the root or by inhibiting the colonization of roots by plant pathogens or other harmful bacteria. To examine the populations of siderophore-producing bacteria colonizing the roots of two grass species that differed in their susceptibility to Fe deficiency, we inoculated serial dilutions of root samples onto chrome azurol S (CAS) agar and several other selective and non-selective culture meida. CAS agar effectively differentiated bacteria that were capable of excreting large amounts of siderophore, but the composition of the medium limited its usefulness for ecological studies. A large proportion (71–79%) of the bacterial population that grew on a non-selective medium (tryptic soy agar) failed to grow on CAS agar, and several isolates that showed no sign of siderophore production on CAS agar produced siderophore in liquid culture. Similar populations of siderophore-producing bacteria were observed on roots of St. Augustine grass, which frequently exhibits Fe chlorosis, and bermuda grass, which does not. Roots of both grasses were colonized by bacteria that produced siderophore in vitro at concentrations ranging from 100 to 230 M. The CAS assay solution was also used to compare siderophore production by Pseudomonas fluorescens Q6, an isolate from bermuda grass, and by P. putida B 10, a plant growth-promoting pseudomonad. P. fluorescens Q6 produced 2.4 times more siderophore in vitro than P. putida B 10.     

Growth response of field-grown Siratro (Macroptilium atropurpureum Urb.) and Aeschynomene american L. to inoculation with selected vesicular-arbuscular mycorrhizal fungi

Summary The effect of inoculation with a selected isolate of Glomus etunicatum Becker and Gerdemann and one of G. intraradices Schenck and Smith on the growth and nutrient content of Macroptilium atropurpureum Urb. cv. Siratro and Aeschynomene americana L., at applied P levels of 10, 30, 60, and 120 kg ha^-1, was studied under field conditions. At all P levels and for all harvests, the shoot dry mass of Siratro and A. americana were greater for the plants inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungi than the control plants. Differences between the VAM fungus-inoculated and the control plants were most marked between 30 and 90 kg ha^-1 of applied P and diminished at 120 kg ha^-1. At the first harvest of Siratro, the plants inoculated with G. etunicatum had a greater shoot dry mass than those inoculated with G. intraradices, for all levels of applied P. However, for subsequent harvest of Siratro and for the one harvest of A. americana the response of shoot dry mass to the two VAM fungi was equivocal. Fungal inoculation gave at least a 30% saving in the amount of P fertilizer required (40 kg ha^-1) for the maximum yield. The plants inoculated with VAM fungi had a greater tissue concentration and total content of P and N than the control plants at low and intermediate levels of applied P. The percentage of root colonized by VAM fungi for the inoculated plants of the two legumes increased linearly with P additions up to 60 kg ha^-1. The conclusion is that under amended (limed and fertilized) soil conditions, inoculation with selected VAM fungi can improve the establishement and growth of forage legumes in fields that contain ineffective populations of native VAM fungi.     

Determinants of trophic modes of the nematophagous fungus Arthrobotrys oligospora interacting with bacterivorous nematode Caenorhabditis elegans

The determinants of saprotrophic or predatory modes of the nematophagous fungus Arthrobotrys oligospora were investigated in soil microcosms and on solid nutrient media. A sterilized soil amended with 1% w/w alfalfa meal (C:N=32) and inoculated with conidia of A. oligospora, showed lower mycelium biomass and higher specific rate of conidia production in the presence of the bacterivorous nematode Caenorhabditis elegans than in its absence. As few as 10 nematodes g⁻¹ soil were sufficient to enhance spore formation by the vegetative mycelium. Given that the fungus was not limited by available carbon and nitrogen, this indicates that nematodes provide essential growth factors regulating the development of A. oligospora. Carbon mineralisation by A. oligospora, measured as the rate of CO₂ production, was found to be 25–35% lower in the presence of 20–60 C. elegans g⁻¹ soil compared to soil without nematodes. This showed that A. oligospora had lower saprotrophic activity in the predaceous phase. Trap formation and nematophagous activity of A. oligospora were observed only where conidia were inoculated on nutrient poor medium (water agar), on low-nitrogen medium (Yeast Carbon Base agar) or on medium containing no amino-acids or vitamins (Czapek-Dox agar). A. oligospora did not form trapping structures when grown on nutrient-rich media containing three amino-acids (l-histidine monohydrocloride, dl-methionine and dl-tryptophan) and vitamins (biotin, calcium pantothenate, folic acid, inositol, niacin, p-aminobenzoic acid, pyridoxine hydrochloride, riboflavine, thiamine hydrochloride). It is concluded that predaceous behaviour of A. oligospora can be regulated either by nitrogen sources or by physiologically active compounds (amino-acids or vitamins) present in nematodes.     

Selective enrichment of a pyrene degrader population and enhanced pyrene degradation in Bermuda grass rhizosphere

Rhizosphere soil has a more diverse and active microbial community compared to nonvegetated soil. Consequently, the rhizosphere pyrene degrader population (PDP) and pyrene degradation may be enhanced compared to nonvegetated bulk soil (NVB). The objectives of this growth chamber study were to compare (1) Bermuda grass (Cynodon dactylon cv. Guymon) growth in pyrene-contaminated and noncontaminated soils and (2) pyrene degradation and PDP among NVB, Bermuda grass bulk (BB), and Bermuda grass rhizosphere soil (BR). Soils were amended with pyrene at 0 and 500 mg kg^–1, seeded with Bermuda grass, and thinned to two plants per pot 14 days after planting (DAP). Pyrene degradation was evaluated over 63 days. The PDP was enumerated via a most probable number (MPN) procedure at 63 DAP. Bermuda grass root growth was more sensitive to pyrene contamination than shoot growth. Pyrene degradation followed first-order kinetics. Pyrene degradation was significantly greater in BR compared to BB and NVB with rate constants of 0.082, 0.050, and 0.052 day^–1, respectively. The PDPs were 8.01, 7.30, and 6.83 log₁₀ MPN g^–1 dry soil for BR, BB, and NVB, respectively. The largest PDP was in soil with the most rapid pyrene degradation. These results indicate that Bermuda grass can grow in pyrene-contaminated soil and enhance pyrene degradation through a rhizosphere effect.     

Effect of manganese-reducing rhizosphere bacteria on the growth of Gaeumannomyces graminis var. tritici and on manganese uptake by wheat (Triticum aestivum L.)

Summary Five bacterial strains capable of Mn reduction were isolated from the rhizosphere of plants growing in different South Australian soils. They differed in their Mn-reducing capacity. The antagonism of these strains compared to the imported strain 2–79 (from the United States) against Gaeumannomyces graminis var. tritici was tested in agar and in a soil sandwich experiment at different Mn²⁺ concentrations in the soil. In addition, wheat seeds were coated with the different strains and with MnSO₄ or with MnSO₄ only in order to investigate their effect on plant growth and Mn uptake. With one exception, all strains inhibited the growth of G. graminis in agar, but to different degrees. In contrast, only two strains significantly inhibited the growth of the fungus in the soil. The hyphal density was decreased more than the hyphal length. The Mn²⁺ concentration in the soil also had a marked effect on fungal growth; low Mn concentrations slightly increased while high Mn concentrations strongly decreased the fungal growth. Seed treatment with MnSO₄ only (+Mn) increased Mn uptake above that of the control (no seed treatment). Only the weakest Mn reducer on agar significantly increased plant growth and Mn uptake from soil in comparison with the Mn treatment. One strain was tested as seed coating without adding MnSO₄; it increased the plant growth to an extent similar to the Mn treatment. Increasing the Mn uptake by plants may be one of the growth-promoting effects exerted by rhizosphere bacteria.     

灰黄青霉对瓜列当的防效及对番茄根区土壤微生物的影响

根寄生杂草瓜列当（Orobanche aegyptiaca）严重危害番茄（Solanum lycopersicum）等多种经济作物的产量和品质。如何有效防除仍是当今瓜列当研究重点之一。真菌是列当的生防因子之一,但目前对农作物无致病性的列当生防真菌的研究尚少。本研究通过培养皿试验研究1株灰黄青霉（Penicillium griseofulvum,CF3）的无细胞发酵滤液对瓜列当种子萌发和发芽管生长的影响,通过盆栽试验研究CF3粉状制剂对瓜列当的防除效果及对寄主番茄生长和根区土壤微生物的影响。结果表明：1）培养皿试验中,CF3发酵液抑制了瓜列当种子萌发和发芽管生长。其中,在放有瓜列当种子与番茄幼苗的培养皿中,加入CF3发酵液后培养6 d,瓜列当种子的萌发均被完全抑制;添加CF3发酵液与霍格兰德营养液体积比为1∶2、1∶4、1∶6和1∶8的混合液培养8 d后,瓜列当种子的萌发率与对照相比分别减少80.26%、70.26%、68.10%和47.51%。CF3发酵液原液、10倍稀释液和100倍稀释液处理后使瓜列当发芽管长度与对照相比分别缩短100.00%、68.84%和19.24%。2）盆栽试验中,CF3菌剂抑制了瓜列当的出土和单株瓜列当的生长,并使番茄增产。施加1.0 g·kg^-1 CF3菌剂130 d后,瓜列当的出土数量、出土率和单株瓜列当干重分别降低76.19%、85.30%和28.48%,番茄果实鲜重增加51.57%。此外,灰黄青霉菌剂还调整了番茄根区土壤的微生物区系结构,使施加菌剂130 d后番茄根区土壤中除接入CF3外真菌数量与对照相比降低75.60%,细菌与真菌的数量之比增加117.57%。平均来看,CF3使番茄根区土壤中除CF3外真菌数量降低42.81%,放线菌总数增加84.15%。本研究表明,灰黄青霉CF3具有防除番茄上寄生瓜列当的能力,适宜作为瓜列当的生防真菌。     

Nodulation competition among Bradyrhizobium japonicum strains as influenced by rhizosphere bacteria and iron availability

Summary Bacteria isolated from the root zones of field-grown soybean plants [Glycine max (L.) Merr.] were examined in a series of glasshouse experiments for an ability to affect nodulation competition among three strains of Bradyrhizobium japonicum (USDA 31, USDA 110, and USDA 123). Inocula applied at planting contained competing strains of B. japonicum with or without one of eleven isolates of rhizosphere bacteria. Tap-root nodules were harvested 28 days after planting, and nodule occupancies were determined for the bradyrhizobia strains originally applied. Under conditions of low iron availability, five isolates (four Pseudomonas spp. plus one Serratia sp.) caused significant changes in nodule occupancy relative to the corresponding control which was not inoculated with rhizosphere bacteria. During subsequent glasshouse experiments designed to verify and further characterize these effects, three fluorescent Pseudomonas spp. consistently altered nodulation competition among certain combinations of bradyrhizobia strains when the rooting medium did not contain added iron. This alteration typically reflected enhanced nodulation by USDA 110. Two of these isolates produced similar, although less pronounced, effects when ferric hydroxide was added to the rooting medium. The results suggest that certain rhizosphere bacteria, particularly fluorescent Pseudomonas spp., can affect nodulation competition among strains of R. japonicum. An additional implication is that iron availability may be an important factor modifying interactions involving the soybean plant, B. japonicum, and associated microorganisms in the host rhizosphere.Paper No. 10648 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC 27695-7601, USA     

Manganese-oxidizing ability of isolates of the take-all fungus is correlated with virulence

Two successive experiments described here show that the Mn-oxidizing ability of isolates of the take-all fungus Gaeumannomyces graminis var. tritici is positively correlated with the isolate virulence. Isolates of G. graminis var. tritici were screened for virulence on wheat in soil culture pot tests and visually scored for Mn-oxidizing capacity on agar plates. A comparison between experiments showed that the Mn-oxidizing ability of an isolate may be attenuated, as is virulence, with subculturing. Using a solution culture system, we calculated the rate of oxidation of Mn²⁺ by a virulent isolate, Ggt 500, as 4 nmol g^-1 (dry weight hyphae) h^-1. The significance of these results to the epidemiology of the take-all disease of wheat is discussed.     

Using the spermosphere model technique to describe the dominant nitrogen-fixing microflora associated with wetland rice in two Egyptian soils

Summary This study is an attempt to describe the dominant N₂-fixing microflora associated with the roots of wetland rice. Rice cultivar Giza 171 was grown in a phytotron on two alluvial Egyptian soils for 8 days, a stage when the nitrogenase activity of undisturbed plants reached a level of 245 × 10^–6 mol C₂H₄ h^–1 g^–1 dry weight of leaf. The roots and rhizosphere soils were then used for counting and isolating dominant diazotrophs. Counts and initial enrichment steps were carried out on a selective medium made of an axenic rice plantlet, the spermosphere model, incubated under 1 % acetylene. The counts were very high, exceeding 10⁸ bacteria g^–1 dry weight of rhizosphere soil. Enterobacteriaceae were dominant; most isolates were Enterobacter cloacae belonging to different biotypes in the two soils. Enterobacter agglomerans, Citrobacter freundii and Klebsiella planticola were also present as members of the dominant microflora. Azospirillum brasilense and Azospirillum lipoferum were present as well, but less abundant.     

Ecology of Arthrobacter JG-9-detectable hydroxamate siderophores in soils

Arthrobacter JG-9-detectable hydroxamate siderophores were monitored in a series of soils subjected to different treatments in the laboratory and in a series of field soils subjected to different regimes. Concentrations of soil siderophores were found to be strongly related to the quantity of organic substrates available for microbial growth in the soil. It was possible to specifically stimulate siderophore production using L-ornithine as a precursor. Soil pH, water activity and iron availability also influenced the production of siderophores in soil. For two series of field samples, siderophores concentration in soil was correlated with grass production. A model is proposed in which the rhizosphere constitutes the most important microsite in soil for siderophore production. Siderophores are thereby conceived as vehicles for iron, increasing the mass flow of iron from the soil-humus complex to the growing microbial, particularly fungal, biomass.     

Effect of passage through the intestinal tract of detritivore earthworms (Lumbricus spp.) on the number of selected Gram-negative and total bacteria

Laboratory experiments were carried out to investigate the fate of bacteria during and after passage through the intestinal tract of detritivore earthworms. Earthworms (Lumbricus spp.) were fed with cattle dung inoculated 7 days previously with one of five different Gram-negative bacteria. Bacterial concentrations were determined 2 days later in dung and soil, and in gut material from different parts of the earthworm intestinal tract. A high percentage (28–82%) of the total bacteria (epifluorescence direct counts) in the earthworm gut content was culturable. The concentration of total heterotrophic aerobic bacteria did not vary significantly among the five different bacterial additions and the non-inoculated control. In earthworm casts the number of total heterotrophs per gram dry matter (2.1×10⁹) was higher than in soil (1.7×10⁸), but lower than in the dung (1.5×10¹⁰). The test-bacteria, however, showed different survival patterns along the earthworm intestinal tract. The concentrations of Escherichia coli BJ 18 and Pseudomonas putida MM 1 and MM 11 in earthworm casts were lower than in the ingested dung, while concentrations of Enterobacter cloacae A 107 and Aeromonas hydrophila DMU 115 in dung and casts were similar. Ent. cloacae, and to aminor extent E. coli, were reduced in numbers by several orders of magnitude in the pharynx and/or crop. In the hind gut, however, the concentration of Ent. cloacae had increased to the same level as in the ingested dung, while the concentration of E. coli remained low. Our observations indicate that the bacterial flora of ingested food materials changes qualitatively and quantitatively during gut transit.     

羟基磷灰石–植物联合修复对Cu/Cd污染植物根际 土壤微生物群落的影响

针对江西贵溪Cu、Cd重金属污染土壤,通过田间试验,比较无机生物材料羟基磷灰石及3种植物(海州香薷、巨菌草、伴矿景天)与羟基磷灰石联合修复对土壤总Cu、Cd的吸收及对活性Cu、Cd的钝化吸收能力差异。采用磷脂脂肪酸(PLFA)分析法,比较不同修复模式对土壤微生物群落结构的影响,以评估土壤微生态环境对不同修复措施的响应。研究结果表明:羟基磷灰石的施加可显著提高土壤pH,并有效钝化土壤活性Cu、Cd含量,但对土壤总Cu、Cd的含量影响较小。植物与羟基磷灰石的联合修复在显著降低土壤活性Cu、Cd(P0.05)的同时,减少了植物根际土壤总Cu、Cd的含量(P0.05)。不同修复措施对土壤微生物群落组成影响差异明显。单独施加羟基磷灰石与土壤真菌群落呈显著正相关,使土壤真菌生物量提高,从而引起真菌/细菌(F/B)的升高。植物与羟基磷灰石的联合修复可有效缓解土壤真菌化的趋势,其中巨菌草与羟基磷灰石的联合修复可有效提高土壤革兰氏阳性、革兰氏阴性细菌生物量及多样性,降低F/B值,从而降低土壤真菌病害的风险。不同植物根系活性代谢引起有机质的积累促进植物与羟基磷灰石处理中根际有机碳含量显著提高。聚类增强树(Aggregated boosted tree,ABT)分析结果表明:不同修复模式是影响土壤微生物群落的重要因素,其次土壤pH和Cu的含量及活性也是改变重金属污染区域微生物群落的因子。该研究从微生物群落结构角度解释了植物与羟基磷灰石联合修复对土壤微生态体系的作用,为开展Cu、Cd等重金属污染地植物与无机生物材料的联合修复方式的筛选及实施提供可靠的理论依据。     

Reduction in dose of chemical fertilizers and growth enhancement of sesame (Sesamum indicum L.) with application of rhizospheric competent Pseudomonas aeruginosa LES4

Pseudomonas aeruginosa LES4, an isolate of tomato rhizosphere was found to be positive for several plant growth-promoting attributes like production of indole acetic acid, HCN and siderophore, solubilization of inorganic phosphate along with urease, chitinase and β-1-3-glucanase activity. In addition, it showed strong antagonistic effect against Macrophomina phaseolina and Fusarium oxysporum. P. aeruginosa LES4 caused halo cell formation and other morphological deformities in mycelia of M. phaseolina and F. oxysporum. Root colonization was studied with Tn5 induced streptomycin resistant transconjugants of spontaneous tetracycline-resistant LES4 (designated LES4^tetra+strep+) after different durations. The strain was significantly rhizospheric competent, as 17.4% increase in its population was recorded in sesame rhizosphere. Seed bacterization with LES4 resulted in significant increase in vegetative growth parameters and yield of sesame over non-bacterized seeds. However, application of LES4 with half dose of fertilizers resulted in growth equivalent to full dose treatment, without compromising with the growth and yield of sesame. Moreover, the oil yield increased by 33.3%, while protein yield increased by 47.5% with treatment of half dose of fertilizer along with LES 4 bacterized seeds, as compared to full dose of fertilizers.     

Influence of humic acids on laurel growth,associated rhizospheric microorganisms,and mycorrhizal fungi

Summary Increasing concentrations of humic acids were tested in order to determine their effects on the microbial rhizosphere and the growth of laurel (Laurus nobilis L.). Plants that were treated with 300 mg kg^-1 of humic acids had the heaviest weights of both fresh and dry shoots; however, doses of 3000 mg kg^-1 were highly phytotoxic and inhibited the growth of laurel. Total aerobic bacteria and actinomycetes were stimulated by doses of 1500 and 3000 mg kg^-1 of humic acids at the first harvest. Nevertheless, at the end of the experiment no significant differences were found among the different doses. The number of fungi living in the laurel rhizosphere was not affected by any concentration of humic acids. Vesicular-arbuscular mycorrhizal (VAM) colonization was only slightly affected by the addition of increasing concentrations of humic substances to the soil, while the hyphal growth of Glomus mosseae was reduced.