A pot experiment was conducted in the green house to investigate the establishment of phosphate solubilizing strains of Azotobacter chroococcum, including soil isolates and their mutants, in the rhizosphere and their effect on growth parameters and root biomass of three genetically divergent wheat cultivars (Triticum aestivum L.). Five fertilizer treatments were performed: Control, 90 kg N ha(-1), 90 kg N + 60 kg P2O5 ha(-1), 120 kg N ha(-1) and 120 kg N + 60 kg P2O5 ha(-1). Phosphate solubilizing and phytohormone producing parent soil isolates and mutant strains of A. chroococcum were isolated and selected by an enrichment method. In vitro phosphate solubilization and growth hormone production by mutant strains was increased compared with soil isolates. Seed inoculation of wheat varieties with P solubilizing and phytohormone producing A. chroococcum showed better response compared with controls. Mutant strains of A. chroococcum showed higher increase in grain (12.6%) and straw (11.4%) yield over control and their survival (12-14%) in the rhizosphere as compared to their parent soil isolate (P4). Mutant strain M37 performed better in all three varieties in terms of increase in grain yield (14.0%) and root biomass (11.4%) over control.

No abstract

Biofertilizers contribute in N(2) fixation, P solubilization, phytohormone production and thus enhance plant growth. Beneficial plant-microbe interactions and the stability and effectiveness of biofertilizer depend upon the establishment of bacterial strains in the rhizosphere of the plant. This interaction depends upon many factors, one of them being plant exudates. Root exudates are composed of small organic molecules like carbonic acids, amino acids or sugars etc., which are released into the soil and bacteria can be attracted towards these exudates due to chemotaxis. The chemotactic behaviour of Azotobacter strains was studied using cotton (Desi HD 123 and American H 1098) and wheat (WH 711) seedlings and the root exudates of these two plants were chemically characterized. Analysis of the root exudates revealed the presence of sugars and simple polysaccharides (glucose), amino acids (glutamate, lysine) and organic acids (citric acid, succinic acid, maleic acid, malonic acid). Differences between cotton cultivars in root exudates were observed which influenced chemotactic response in Azotobacter. These results indicate colonization with rhizobacteria which implies that optimal symbionts, on the sides of both plant cultivar and bioinoculant bacteria can lead to better plant growth under cultivation conditions.

No abstract

Soil bacteria belonging to the genus Azotobacter, Pantoea and some unidentified soil isolates were tested in vitro for phytohormone production under laboratory and soil conditions. The German wheat variety Munk was inoculated by several soil bacteria with exogenously applied hormones (IAA, 2,4-D) and a flavonoid (naringenin) with a half of the amount of recommended doses of fertilizers under greenhouse conditions. Most of the soil bacteria tested were able to produce indole acetic acid (IAA), and stimulated a lateral root development and colonization by the addition of 2,4-D and IAA. A formation of paranodules on roots as a result of crack entry invasion was observed with 2,4-D as well as with IAA. We were able to reisolate the organism from the paranodules and could establish the same results. Analyses for root exudates and in vitro phytohormone production by various bacterial isolates were also carried out, revealing that 2,4-D can be replaced either by high IAA producing bacteria or by exogenous application of IAA. Bacterial survival in the rhizosphere as well as the root and shoot weight of wheat plants were positively affected also by the addition of IAA, 2,4-D and naringenin. A. brasilense (Sp7) partly due to the development of a protected niche. These paranodule structures derived from the induction of the initials of the lateral roots are quite dissimilar to root nodules particularly when colonized by non-symbiotic bacteria. Protoplasts of the bacteria (L-forms of Azotobacter, Pseudomonas syringae and Bacillus polymyxa) were suggested to have the ability to penetrate the cell wall and membrane structures of living plant cells and to colonize plant tissues (Cocking et al. 1990). Also some microorganisms colonize in the intercellular spaces as seen in apoplast of stem of sugarcane (Cocking 2003). Despite encouraging results with Azospirillum, Azotobacter etc., some controversy still exists about the mechanism of bacterial root interactions. Reports suggest that the colonization of these bacteria is caused by factors like N 2 fixation, siderophores, ammonia excretion, phytohormones (Lakshminarayana 1993) and antifungal properties etc. (Verma et al. 2001), collectively enhancing the root proliferation, increase in the lateral roots and root hair formation. Dobbelaere et al. (1999) suggested that plant growth substances are one of the key factors observed in plant growth promotion. Tien et al. (1979) reported that the morphological changes due to bacterial inoculations can be mimicked by applying a combination of plant growth substances.Keeping all these factors in mind, we made an attempt to screen different soil bacteria for the production of various phytohormones, their colonization on German winter wheat (var. Munk) with or without exogenously applied natural and synthetic plant hormones and the concomitant changes in root morphology or formation of paranodules under controlled greenhouse conditions in addition to plant growth parameters. Natural hormone like IAA, synthetic-2,4-D (2,4-dichlorophen...

The microbial communities and their degradative potential in rhizospheres of alfalfa (Medicago sativa) and reed (Phragmites australis) and in unplanted soil in response to bitumen contamination of soil were studied in pot experiments. According to the results of fluorescence microscopy, over a period of 27 months, bitumen contamination of soil reduced the total number of microorganisms more significantly (by 75%) in unplanted than in rhizosphere soil (by 42% and 7% for reed and alfalfa, respectively) and had various effects on some important physiological groups of microorganisms such as actinomycetes as well as nitrogen-fixing, nitrifying, denitrifying, ammonifying, phosphate-solubilizing, sulphur-oxidizing, cellulolytic and hydrocarbon-degrading microorganisms. The changes in the physiological structure of the microbial community under bitumen contamination were found to hinge on not merely the presence of plants but also their type. It was noted that the rhizosphere microflora of alfalfa was less inhibited by hydrocarbon pollution and had a higher degradative potential than the rhizosphere microflora of reed.

Five salinity tolerant Azotobacter strains i.e., ST3, ST6, ST9, ST17 and ST24 were obtained from saline soils. These Azotobacter strains were used as inoculant for wheat variety WH157 in earthen pots containing saline soil under pot house conditions, using three fertilizer treatment doses i.e., control (no fertilizer, no inoculation), 90 Kg N ha(-1) and 120 Kg N ha(-1). Inoculation with salinity tolerant Azotobacter strains caused significant increase in total nitrogen, biomass and grain yield of wheat. Maximum increase in plant growth parameters were obtained after inoculation with Azotobacter strain ST24 at fertilization dose of 120 kg N ha(-1) and its inoculation resulted in attaining 89.9 cms plant height, 6.1 g seed yield, 12.0 g shoot dry weight and 0.7 % total nitrogen. The survival of Azotobacter strain ST24 in the soil was also highest in all the treatments at 30, 60 and 90 days after sowing (DAS). However, the population of Azotobacter decreased on 90 DAS as compared to counts observed at 60 DAS at all the fertilization treatments.

A field experiment was conducted in a randomized block design with three replications over 2 years to evaluate the effect of wheat cultivar and dual inoculation of Azotobacter chroococcum (Azc) and arbuscular mycorrhiza fungi (AMF, Glomus fasciculatum) on root characters and AMF infection in three crosses of wheat. The experimental material comprised four wheat parents, WH‐147, WH‐157, WH‐542 and PBW‐175, and three F1 crosses, WH‐147 ×WH‐157, WH‐147 × WH‐542 and WH‐147 × PBW‐175. Comparison of treatment averages, i.e. control (mineral nutrients 60 kg N + 30 kg P2O5 + 12.5 kg ZnSO4 ha−1, as in other two treatments), AMF and AMF + Azc, revealed that inoculation of Azc led to an increase in AMF infection in roots. Maximum root biomass was obtained in F1 hybrids WH‐147 × WH‐157 in the AMF treatment and in WH‐147 × PBW‐175 receiving AMF + Azc. Total root length and AMF infection of roots was maximum in WH‐147 × PBW‐175 for all the treatments during both years. A positive association between AMF infection in roots and Azotobacter survival in the rhizosphere was apparent. Similarly, maximum A. chroococcum counts were observed 80 and 120 days after sowing in the AMF + Azc treatment in cross WH147 × PBW175.