Influence of growth-promoting bacteria on the growth of wheat in different soils and temperatures

Plant-growth-promoting bacteria isolated from the rhizosphere, phyllosphere and soil of the root zone in different climatic regions of Germany and Uzbekistan were analysed for plant-growth-promoting effects and nutrient uptake on winter wheat on different soils and under different temperature regimes. The investigations were carried out in pot experiments using loamy sand and sandy loam soils from Müncheberg, Germany and Calcisol soil from Tashkent, Uzbekistan. The temperature and soil types were found to influence growth-promoting effects. Inoculation with bacterial strains Pseudomonas fluorescens PsIA12, Pantoea agglomerans 050309 and Mycobacterium sp. 44 isolated from Müncheberg (semi-continental climate) was found to significantly increase the root and shoot growth of winter wheat at 16 °C compared to 26 °C in loamy sand. Mycobacterium phlei MbP18 and Mycoplana bullata MpB46 isolated from Tashkent (semi-arid climate) were found to significantly increase the root and shoot growth of winter wheat in nutrient-poor Calcisol at 38 °C as well as in nutrient-rich loamy sand at 16 °C. Bacterial inoculation also resulted in significantly higher N, P, and K contents of plant components. The bacteria isolates were able to survive in the rhizosphere and in the soil of winter wheat after root and shoot inoculation.     

Halophile plant growth-promoting rhizobacteria induce salt tolerance traits in wheat seedlings (Triticum aestivum L.)

Salinity is one of the most important growth-limiting factors for most crops in arid and semi-arid regions;however,the use of plant growth-promoting rhizobacteria isolated from saline soils could reduce the effects of saline stress in crops.This study aimed to evaluate the efficiency of plant growth-promoting rhizobacteria(PGPRs),isolated from the rhizosphere of halophile plants,for the growth,Na⁺/K⁺ balance,ethylene emission,and gene expression of wheat seedlings(Triticum ...     

Plant‐growth‐promoting rhizobacteria isolated from a Calcisol in a semi‐arid region of Uzbekistan: biochemical characterization and effectiveness

Bacteria were isolated from the rhizosphere of cotton, wheat, alfalfa, and tomato grown in field locations within a semi‐arid region of Uzbekistan. Strains were identified as Pseudomonas denitrificans, P. rathonis, Bacillus laevolacticus, Bacillus amyloliquefaciens, and Arthrobacter simplex. The isolated strains produced different enzymes, phytohormone auxin and were antagonists against specific plant‐pathogenic fungi. Most of the strains are tolerant with respect to salt and temperature. All of the bacterial strains isolated in this study have been found to increase plant growth of wheat and maize in pot experiments.     

Azospirillum strains isolated from roots and rhizosphere soil of wheat (Triticum aestivum L.) grown under different soil moisture conditions

The present study deals with the isolation and characterization of Azospirillum strains isolated from roots and rhizosphere soil of wheat (at tillering and anthesis stages) plants grown under different moisture regimes in the field and in pots. The survival of Azospirillum isolates from plants of irrigated field and those from well-watered pots was higher than that of Azospirillum strains isolated from roots and rhizosphere soils of plants grown under arid and semiarid (14–8% soil moisture) field conditions and under water-stressed (8% soil moisture) conditions in pots. On the basis of carbon/nitrogen source utilization, the Azospirillum strains isolated from wheat under field and pot conditions were grouped in three groups. The unweighted pair group method with arithmetic means cluster analysis based on random amplification of polymorphic DNA showed that two groups of Azospirillum were similar. The strains isolated from plants (at tillering stage) grown under low moisture conditions either in pots or in field were genetically similar to strains isolated from plants grown under well-watered conditions in both pots and field. Inoculation of wheat with isolates from water-stressed plants induced tolerance to water stress in inoculated plants. Isolates from water-stressed conditions exhibited lower production of indole acetic acid, gibberellic acid, and trans zeatin riboside but a higher production of abscisic acid.     

接种耐铬细菌菌株对二月兰中铬积累及其生长状况的影响

Beneficial interactions between microorganisms and plants, particularly in the rhizosphere, are a research area of global interest. Four cadmium （Cd）-tolerant bacterial strains were isolated from heavy metal-contaminated sludge and their effects on Cd mobility in soil and the root elongation and Cd accumulation of Orychophragmus violaceus were explored to identify the capability of metal- resistant rhizobacteria for promoting the growth of O. violaceus roots on Cd-contaminated soils. The isolated strains, namely, Bacillus subtilis, B. cereus, B. megaterium, and Pseudomonas aeruginosa, significantly enhanced the plant Cd accumulation. The Cd concentrations in the roots and shoots were increased by up to 2.29- and 2.86-fold, respectively, by inoculation of B. megaterium, as compared with the uninoculated control. The bacterial strains displayed different effects on the shoot biomass. Compared with the uninoculated plants, the shoot biomass of the inoculated plants was slightly increased by B. megaterium and significantly decreased by the other strains. B. megaterium was identified as the best candidate for enhancing Cd accumulation in O. violaceus. Thus, this study provides novel insight into the development of plant-microbe systems for phytoremediation.     

Effect of P‐solubilizing Azotobacter chroococcum on N,P, K uptake in P‐responsive wheat genotypes grown under greenhouse conditions

Natural and mutant strains of A. chroococcum were isolated from Indian soils. Their ability to dissolve phosphate and their phytohormone production were tested under in vitro conditions. In addition the effect of bacterial inoculation of Azotobacter on N, P, K uptake by three P responsive wheat genotypes (Triticum aestivum L.) under greenhouse conditions at five nutrient levels (Control, 90 kg N ha^—1, 90 kg N + 26 kg P ha^—1, 120 kg N ha^—1 and 120 kg N + 26 kg P ha^—1) was studied. In vitro phosphate solubilization and growth hormone production by mutant strains was more than by the soil isolates. Inoculation of wheat varieties with the soil isolates and mutant strains of A. chroococcum showed greater NPK uptakes as compared with parent soil isolates. Mutant strains M15 and M37 were proved to be the most effective for all three wheat varieties with regard to NPK uptake as well as root biomass production under greenhouse conditions.     

Salt-tolerant rhizobacteria-mediated induced tolerance in wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis>) and chemical diversity in rhizosphere enhance plant growth

Salt-tolerant isolates Bacillus pumilus, Pseudomonas mendocina, Arthrobacter sp., Halomonas sp., and Nitrinicola lacisaponensis isolated from high saline habitats exhibited plant growth-promoting traits like P solubilization and indole acetic acid (IAA), siderophore, and ammonia production. These isolates were inoculated in wheat to assess microbe-mediated responses and plant growth promotion in salt affected soil. Maximum shoot and root length (33.8 and 13.6 cm) and shoot and root biomass (2.73 and 4.48 g dry weight) was recorded in plants inoculated with B. pumilus after 30 days. Total chlorophyll content was maximum in the leaves of the plants treated with Halomonas sp. (24.22 mg g⁻¹ dry weight) followed by B. pumilus (23.41 mg g⁻¹ dry weight) as compared to control (18.21 mg g⁻¹ dry weight) after 30 days. Total protein content was maximum in Arthrobacter sp. inoculated plant leaves (3.19 mg g⁻¹ dry weight) followed by B. pumilus (2.47 mg g⁻¹ dry weight) as compared to control (2.15 mg g⁻¹ dry weight) after 30 days. Total carotenoid content was maximum in plants inoculated with Halomonas sp. (1,075.45 and 1,113.29 μg g⁻¹ dry weight) in comparison to control (837.32 and 885.85 μg g⁻¹ dry weight) after 15 and 30 days. Inoculation of bacterial isolates increased presence of individual phenolics (gallic, caffeic, syringic, vanillic, ferulic, and cinnamic acids) and flavonoid quercetin in the rhizosphere soil. The concentration of IAA in rhizosphere soil and root exudates was also higher in all treatments than in control. Accumulation of phenolics and quercetin in the plants played a cumulative synergistic role that supported enhanced plant growth promotion of wheat in the stressed soil.     

Autochthonous halotolerant plant growth-promoting rhizobacteria promote bacoside A yield of Bacopa monnieri (L.) Nash and phytoextraction of salt-affected soil

Phytoremediation is a promising approach for reclamation of salt-affected soil. Phytoextraction is the most commonly used process, which exploits plants to absorb, immobilize, and accumulate salt in their shoots. In this study, halotolerant plant growth-promoting rhizobacteria (PGPR) were isolated from the rhizosphere of wild grasses growing naturally in salt-affected areas of Lucknow, Uttar Pradesh (India) and were tested for their efficacies of salt-tolerance and plant growth-promoting (PGP) abilities. Based on 16S rRNA sequences, the most efficient halotolerant isolates possessing PGP traits were identified as Pseudomonas plecoglossicida (KM233646), Acinetobacter calcoaceticus (KM233647), Bacillus flexus (KM233648), and Bacillus safensis (KM233652). Application of these isolates as bio-inoculants significantly (P < 0.05) increased the growth and bacoside A yield of a medicinal plant, Bacopa monnieri (L.) Nash, grown on natural salt-affected soil. The phytoremediation of salt-affected soil was evident by the substantial increase in shoot Na⁺:K⁺ ratio of bio-inoculant-treated plants. When compared to un-inoculated control plants, the soil physico-chemical properties of bio-inoculant-treated plants were improved. The shoot and root biomass (fresh and dry weights), soil enzymes, and soil nutrient parameters showed significant positive correlations with the shoot Na⁺:K⁺ ratio. Consequently, the halotolerant PGPR screened in this study could be useful for the reclamation of saline soils concomitant with improved plant growth and bacoside A yield.     

Effect of Piriformospora indica and Azospirillum strains from saline or non-saline soil on mitigation of the effects of NaCl

Salinity toxicity is a worldwide agricultural and eco-environmental problem. The intent of this study was to determine the salt tolerance of Piriformospora indica and strains of Azospirillum, isolated from non-saline and saline soil, as well as to determine their affect on the tolerance of wheat to soil salinity. In this study, an experiment was conducted to investigate the salt stress tolerance abilities of the endophytic fungi, P. indica, and Azospirillum strains, isolated from non-saline and saline soil, at five NaCl levels (0, 0.1, 0.2, 0.3, 0.4, 0.5 mol L^?1). Additionally, a greenhouse experiment was conducted to test the effects of these selected microorganisms under increasing salinity levels on seedling growth, solute accumulation (proline and sugars), and photosynthetic pigments (Chl a, b, ab) of seedling wheat. Azospirillum strains were isolated in Iran from the root of field-grown maize from non-saline soil with an EC = 0.7 dS m^?1 and from saline soil with an EC = 4.7 dS m^?1. Plants were irrigated with non-saline water–tap water with an electrical conductivity water (ECw) value of 0.2 dS m^?1, as well as low, moderate and severe saline water-irrigation with saline water with an ECw of 4 dS m^?1, 8 dS m^?1 and 12 dS m^?1, respectively. The upper threshold of P. indica salinity tolerance was 0.4 mol L^?1 NaCl in both liquid and solid broth medium. The upper thresholds of the salt adapted and non-adapted Azospirillum strains were 0.2 and 0.4 mol L^?1 NaCl, respectively. The results indicated a positive influence of the organisms on salinity tolerance, more with the saline-adapted Azospirillum strains than the non-adapted strains. P. indica was more effective than the Azospirillum strains. These results could be related to a better water status, higher photosynthetic pigment contents and proline accumulation in wheat seedlings inoculated with P. indica. The benefits of both isolates and P. indica depended on two factors: water salinity and growth stage of the host plant. Inoculation with the two isolates increased salinity tolerance of wheat plants; the saline-adapted Azospirillum strains showed better performance with respect to improved fresh and dry weights at 80 and 100 days after sowing under both non-saline and saline conditions. When compared to plants inoculated with non-saline-adapted Azospirillum strains, those inoculated with adapted Azospirillum strains had much better performance with respect to the presence of photosynthetic pigment (Chl a, b and ab) and proline accumulation. Overall, these results indicate that the symbiotic association between P. indica fungus and wheat plants improved wheat growth, regardless of the salinity. It is concluded that the mechanisms for protecting plants from the detrimental effects of salinity by P. indica fungus and Azospirillum strains may differ in their salinity tolerance and influence the uptake of water, photosynthetic pigment contents and proline accumulation in wheat seedlings.     

Rhizosphere effects on Cu availability and fractionation in sewage sludge‐amended calcareous soils

Rhizosphere processes have a major impact on copper (Cu) availability and its fractions in soils. A greenhouse experiment with wheat was performed to investigate availability (using seven chemical procedures) and fractionation of Cu in the rhizosphere of ten agricultural soils (Typic Calcixerepts) amended with sewage sludge (1% w/w) using rhizoboxes. The results show that available Cu concentrations in rhizosphere soils were significantly (P < 1%) lower than in bulk soils. In comparison with the bulk soils, in the rhizosphere soils the concentration of Cu associated with organic matter and residual Cu increased by 24 and 4%, respectively, whereas exchangeable Cu, Cu associated with iron‐manganese oxides, and Cu associated with carbonate decreased by 20, 14, and 12%, respectively. Dissolved organic carbon (DOC) and Cu associated with iron‐manganese oxides and Cu associated with organic matter in the rhizosphere and bulk soils were significantly correlated (P < 5%). The results show that the differences between rhizosphere and bulk soils in chemical conditions such as DOC concentrations can change the proportion of soil Cu fractions and, therefore, Cu availability for wheat in calcareous soils amended with sewage sludge. The results show that the wheat root‐induced modifications of chemical and biological soil conditions do not only lead to Cu depletion in mobile soil Cu fractions, but also to modification in soil Cu fractions which are commonly considered as more stable.     

Growth,P uptake and rhizosphere properties of wheat and canola genotypes in an alkaline soil with low P availability

The aim of the present study was to assess the role of soil type on growth, P uptake and rhizosphere properties of wheat and canola genotypes in an alkaline soil with low P availability. Two wheat (Goldmark and Janz) and two canola genotypes (Drum and Outback) were grown in a calcareous soil (pH 8.5) at two P levels [no P addition (0P) or addition of 200 mg kg⁻¹ P as Ca₃(PO₄)₂ (200P)] and harvested at flowering or maturity. Shoot and root dry weight, root length and shoot P content were greater in the two canola genotypes than in wheat. There were no consistent differences in available P, microbial P and phosphatase activity in the rhizosphere of the different genotypes. Shoot P content was significantly positively correlated with root length, pH and phosphatase activity in the rhizosphere. The microbial community composition, assessed by fatty acid methylester analysis, of the canola genotypes differed strongly from that of the wheat genotypes. The weight percentage bacterial fatty acids, the bacteria/fungi (b/f) ratio and the diversity of fatty acids were greater in the rhizosphere of the canolas than in the rhizosphere of the wheat genotypes. In contrast to the earlier studies in an acidic soil, only small differences in growth and P uptake between the genotypes of one crop were detected in the alkaline soil used here. The results confirmed the importance of root length for P uptake in soils with low P availability and suggest that the rhizosphere microbial community composition may play a role in the better growth of the canola compared to the wheat genotypes.     

Fusarial wilt suppression and crop improvement through two rhizobacterial strains in chick pea growing in soils infested with Fusarium oxysporum f.sp. ciceris

 Bacterization of chick pea seeds with a siderophore-producing fluorescent Pseudomonas strain RBT13 and an antibiotic-producing Bacillus subtilis strain AF1, isolated from tomato rhizoplane and pigeon pea rhizosphere repectively, increased the shoot height, root length, fresh weight, dry weight and yield in soils infected with Fusarium oxysporum f.sp. ciceris. Seed bacterization also resulted in a significant reduction in chick pea wilt caused by the same pathogen. Addition of iron to the soil completely eliminated disease suppression by RBT13 but not by AF1. Dual drug-resistant mutant strains derived from the rhizobacteria were used to monitor and confirm root colonization. The results indicate the potential for development of both strains for the biological control of chick pea wilt. Received: 29 April 1998     

Fractionation and Speciation of Manganese in Rhizosphere Soils of Pseudomonas sp. Rhizobacteria Inoculated Pistachio (Pistacia vera L.) Seedlings Under Salinity Stress

Enhancement of manganese (Mn) availability in saline and Mn-deficient soils is very important for plant growth. An experiment was carried out to evaluate the effect of Pseudomonas sp. rhizobacteria (P₀ (control), P₁, P₂ and P₃) and Mn (0 and 10 mg Mn kg^?1 soil) on the distribution of Mn in the rhizosphere of pistachio seedlings under salinity stress (0, 1000 and 2000 mg NaCl kg^?1 soil). The results showed that salinity decreased the dry weight, Mn uptake and chlorophyll content of the pistachio seedlings. However, inoculation with rhizobacteria increased these parameters in saline conditions. Application of rhizobacteria increased the availability of Mn in the rhizosphere soil. The use of rhizobacteria decreased the residual-Mn form in the rhizosphere. Inoculation with rhizobacteria increased the percent of Mn²⁺ and MnCl⁺ species in the soil solution. However, pistachio seedlings inoculation with rhizobacteria increased the contents of Mn available forms in the rhizosphere soil.     

Taxonomic diversity of bacteria associated with the roots of modern, recent and ancient wheat cultivars

Breeding programs for crop plants are designed to improve agronomic characteristics such as yield, fertilizer use efficiency and disease resistance. These programs do not typically consider interactions between plants and soil microflora. This study assessed the bacterial communities associated with roots of various spring wheat (Triticum spp.) cultivars of related lineage. Bacteria (n=ca. 1900) were isolated from the rhizosphere and root interior of Triticum moncoccum PI 167549 (an ancient land race that originated in Turkey), T. aestivum cv. Red Fife (historical spring wheat cultivar released in Canada ca. 1845) and T. aestivum cv. CDC Teal (modern cultivar registered in Canada in 1991) grown at two different field sites. Bacteria were identified by gas chromatography-MIDI (microbial identification software) fatty acid methyl ester analysis. Twenty-eight bacterial genera were identified as being associated with the three wheat cultivars, but only Aureobacter species differed significantly between cultivars with 16 isolates identified from the root interior of PI 167549 compared to one isolate from Red Fife and two from CDC Teal. In contrast, the bacterial endophytic community of the more modern cultivars was more diverse than that seen for the ancient land race. Increases in diversity were not limited to a single genus and some species were selected against. For example, pseudomonads were more numerous and diverse in the root interior (11 species identified in 117 isolates) compared to the rhizosphere (eight species identified in 94 isolates), but Pseudomonas fluorescens abundance decreased in the root interior compared to the rhizosphere. The fact that the roots of newer wheat cultivars were aggressively colonized by endophytic pseudomonads suggests that these bacteria might be exploited as plant growth-promoting rhizosphere bacteria or as a means to establish specific catabolic activities in these plants.     

Inoculating wheat seedlings with exopolysaccharide-producing bacteria restricts sodium uptake and stimulates plant growth under salt stress

A pot experiment was conducted to elucidate the effects of inoculating five exopolysaccharide- (EPS-) producing bacterial strains on the dry matter yield and the uptake of K⁺, Na⁺, and Ca²⁺ by wheat seedlings grown in a moderately saline soil. The bacteria were isolated from the rhizosphere soil (RS) of wheat grown in a salt-affected soil and included Aeromonas hydrophila/caviae (strain MAS-765), Bacillus insolitus (strain MAS17), and Bacillus sp. (strains MAS617, MAS620 and MAS820). The inoculation substantially increased the dry matter yield of roots (149–527% increase) and shoots (85–281% increase), and the mass of RS (176–790% increase). All the strains, except MAS617, also increased the RS mass/root mass ratio as well as the population density of EPS bacteria on the rhizoplane, and both these parameters were significantly correlated with the content of water-insoluble saccharides in the RS. Inoculation restricted Na⁺ uptake by roots, which was not attributable to the binding of Na⁺ by the RS, or to the ameliorative effects of Ca²⁺ under salinity. The decreased Na⁺ uptake by roots of inoculated than uninoculated plants was probably caused by a reduced passive (apoplasmic) flow of Na⁺ into the stele due to the higher proportion of the root zones covered with soil sheaths in inoculated treatments. Among the strains tested, MAS820 was the most efficient in all respects, whereas MAS617 was the least effective. Results suggested that inoculating selected EPS-producing bacteria could serve as a useful tool for alleviating salinity stress in salt-sensitive plants.     

Relationship between rhizosphere acidification and phytoremediation in two acacia species

Purpose

Phytoremediation is the most sustainable and economical strategy for reclamation of the salt-affected soils. In order to investigate the relationship between phytoremediation and rhizosphere acidification, two experiments (greenhouse and field) were conducted using two acacia species viz. Acacia ampliceps and Acacia nilotica.

Materials and methods

In greenhouse experiment, both the species were exposed to 100 and 200 mM NaCl concentrations in solution culture. The release of organic acids from plant roots was determined after 14 and 28 days of the salt treatment. Shoot and root ash alkalinity was determined after harvesting the plants. In field experiment, both the species were grown on a saline sodic soil for 2 years. After every 6 months, plant growth data were recorded and soil samples were collected from different soil depths for physicochemical analyses.

Results and discussion

The results of greenhouse study indicated higher rhizosphere acidification by A. ampliceps than A. nilotica in terms of release of citric acid, malic acid, and tartaric acid along with ash alkalinity. The comparison of both the species in the field indicated higher amelioration in the soil properties like pH_s, EC_e, SAR, bulk density, and infiltration rate by A. ampliceps than A. nilotica.

Conclusions

It is concluded from these studies that A. ampliceps is more suitable species than A. nilotica for the phytoremediation of the salt-affected soils due to its higher rhizosphere acidification potential.

     

盐胁迫油菜根际中含假单胞菌（Pseudomonas spp.1）的氨基环丙烷-1-羧酸 (ACC) 脱氨基酶特征研究

When exposed to biotic or abiotic stress conditions,plants produce ethylene from its immediate precursor 1-aminocyclopropane-1-carboxylate(ACC),leading to retarded root growth and senescence.Many plant growth-promoting rhizobacteria contain the enzyme ACC deaminase and this enzyme can cleave ACC to form α-ketobutyrate and ammonium,thereby lowering levels of ethylene.The aim of this study was to isolate and characterize ACC deaminase-producing bacteria from the rhizosphere of salt-stressed canola(Brassica napus L.).Out of 105 random bacterial isolates,15 were able to utilize ACC as the sole source of nitrogen.These 15 isolates were also positive for indole acetic acid(IAA) production.Phylogenetic analysis based on partial 16 S rDNA sequences showed that all isolates belonged to fluorescent Pseudomonas spp.In the canola rhizosphere investigated in this study,Pseudomonas fluorescens was the dominant ACC deaminase-producing species.Cluster analysis based on BOX-A1R-based repetitive extragenic palindromic-polymerase chain reaction(BOX-PCR) patterns suggested a high degree of genetic variability in ACC deaminase-producing P.fluorescens strains.The presence of indigenous ACC-degrading bacteria in the rhizosphere of canola grown in saline soils indicates that these bacteria may contribute to salinity tolerance.     

Inoculation of wheat with Azospirillum brasilense and Pseudomonas fluorescens: Impact on the production and culturable rhizosphere microflora

Scientific evidence recognizes that the operation of a terrestrial ecosystem depends on soil microbial activity. Some Azospirillum strains produce plant growth regulators, increase the development of roots, and fix atmospheric nitrogen (N₂). Some Pseudomonas strains are capable of producing cytokinins and solubilizing organic phosphorus. A sustainability analysis requires a detailed knowledge of the interrelationships between the microorganisms added to the system and those present in the soil. This study examines the effect of three commercial inoculants Azospirillum brasilense Az1 and Az2 as well as Pseudomonas fluorescens Pf on biomass production, grain yield and rhizosphere colonization of wheat, combined with two levels of N-addition. Plate counts of rhizosphere soil showed that the inoculation and N-addition did not affect the number of P. fluorescens, whereas it significantly affected the number of Azospirillum. N-addition and inoculation did not change the communities of actinomycetes and bacteria but they changed the number of fungi at the rhizosphere of wheat plants. Community-level physiological profiles of carbon-source utilization of rhizosphere soil microbial communities were changed after inoculation with Az1, Az2 and Pf depending on the phenological stage of the crop. Although no significant responses were observed, in average, PGPB inoculation increased aerial biomass by 12%, root biomass by 40% and grain yield by 16%. These increases represent important earnings for the farmer and they may help to obtain a greater sustainability of the agroecosystems.     

Cyanide production by rhizobacteria as a possible mechanism of plant growth inhibition

Summary Volatile metabolites from a number of rhizosphere pseudomonads prevented lettuce root growth in a seedling bioassay. One of these metabolites was identified as cyanide. Direct contact between rhizobacteria and plant roots produced, with one exception, similar responses. However, not all cyanogenic isolates were plant-growth-inhibitory rhizobacteria. When grown in liquid culture, cyanogenic strains produced an average of 37 nmol HCN ml^–1 over a 36-h period and inhibition of root growth occurred at concentrations as low as 20 nmol ml^–1. Cyanogenic strains introduced into sand or soil also produced HCN. Two cyanogenic strains ofPseudomonas fluorescens, one (5241) a plant-growth inhibitory rhizobacterium and the other (S97) a plant-growth-promotory rhizobacterium, were used to treat bean and lettuce seedlings prior to planting in soil. Lettuce dry weight was reduced by 49.2% (day 28) and 37.4% (day 49) when inoculated with S241 whereas S97 increased growth initially (+64.5% at day 28, no difference from control at day 49). Equivalent figures for inoculated bean plants were: –52.9% and –65.1% (5241); +40.7% and +23.3% (S97). A more detailed experiment using only bean plants confirmed these contrasting affects. Inhibition by S241 was related to consistently higher levels of rhizosphere cyanide in comparison with S97-treated plants and control soils. S241 also survived in the rhizosphere at higher densities and for a longer period of time than S97. The possible contribution of rhizobacterial cyanogenesis to plant growth inhibition is discussed.     

Effects of seed priming with plant growth-promoting rhizobacteria on wheat yield and soil properties under contrasting soils

In the present investigation, different strains of Plant growth-promoting rhizobacteria (PGPR), namely Bacillus megaterium, Pseudomonas fluorescens and Bacillus subtilis were evaluated for their growth-promoting effects on wheat as well as on soil properties under field conditions at two different sites having sandy loam and silt loam type of soils. PGPR strains were evaluated either singly or in consortia. Amongst all the treatments, wheat inoculated with consortia was found most effective as it increased grain yield up to 53% over control in silt loam soil, whereas, corresponding effects in sandy loam soil were less pronounced as an increase of 31% was observed in corresponding treatments, respectively. Enhanced effects on soil properties were also more intense in silt loam as there was an increase of 205% organic matter as against sandy loam soil where this value was 110%.