Plant growth stimulation of wheat (Triticum aestivum L.) by inoculation of salinity tolerant Azotobacter strains.

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 novel technique to collect root exudates from mustard (Brassica juncea).

A very simple, novel, cost effective, easy to use technique has been developed for the collection of root exudates from small seeded plants, under laboratory conditions. 200-1000 µl micro tips (Tarsons), kept in 100 ml glass beakers, were used as holders for the small seeds of Indian mustard (Brassica juncea) and the exudates were trapped in liquid culture medium. The exudates, so obtained, were authenticated and analyzed for organic compounds such as sugars, amino acids and organic acids, as well as chemotactic response towards rhizobacteria. Method was found to be suitable and easy to handle for small seeds.

NifH-based studies on azotobacterial diversity in cotton soils of India.

In order to promote the use of Azotobacter inoculants for cotton crop, a complete characterization of soil isolates of Azotobacter, isolated and screened on the basis of physiological properties, from four different cotton-wheat cropping regions of India was carried out, and their genetic diversity determined by RFLP (restriction fragment length polymorphism) analysis of the functional gene nifH. Genetic analysis of these isolates depicted a similarity coefficient of > or = 80% among them, suggesting that though the isolates were obtained from different cotton soils of India, still they have large commonality in the nifH gene and constituted a homogeneous nifH population.

Diversity studies of Azotobacter spp. from cotton-wheat cropping systems of India.

Azotobacter are the favored bioinoculants being promoted for cotton crop in India. In order to develop bioinoculants, both metabolic fingerprinting and genetic fingerprinting have been used to study the diversity among Azotobacter spp. isolated from four different cotton-wheat cropping regions of India. On the basis of acetylene reduction, indole acetic acid production and ammonia excretion, from 76 free-living diazotrophs isolated from the rhizospheric soil of cotton, 20 efficient isolates were selected for further studies. Morphological characterization indicated a close resemblance of these isolates to Azotobacter spp. BIOLOG cataloguing divided them into two main groups, but amplified ribosomal DNA restriction analysis clustered the isolates from the four regions having different soil types into four separate sub-clusters. Metabolic fingerprinting was not able to detect subtle differences among the isolates as achieved with genetic fingerprinting. However, 16S rRNA is a highly conserved locus. Variations observed could be due to domestication of the isolates in different agro-ecological niches.

Establishment of Azotobacter on plant roots: chemotactic response, development and analysis of root exudates of cotton (Gossypium hirsutum L.) and wheat (Triticum aestivum L.).

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.

Rhizosphere microflora of plants used for the phytoremediation of bitumen-contaminated soil.

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.