Azotobacter in soil: The presence of Azotobacter sp. in soils has beneficial effects on plants, but the abundance of these bacteria is related to many factors, soil physico-chemical (e.g. organic matter, pH, temperature, soil moisture) and microbiological properties. Its abundance varies as per the depth of the soil profile. Azotobacteria are much more abundant in the rhizosphere of plants than in the surrounding soil and that this abundance depends on the crop species.
Nitrogen fixation: Nitrogen is the component of protein and nucleic acids and chlorophyll. Thus, nitrogen supply to the plant will influence the amount of protein, amino acids, protoplasm and chlorophyll formed. Therefore, adequate supply of nitrogen is necessary to achieve high yield potential in crop.
The atmosphere comprises of ~78% nitrogen as an inert, in unavailable form. Above every hectare of ground there are ~80000 tones of this unavailable nitrogen. In order to be converted to available form it needs to be fixed through either the industrial process or through Biological Nitrogen Fixation (BNF). Without these nitrogen-fixers, life on this planet may be difficult.
Nitrogen (N) deficiency is frequently a major limiting factor for crops production. Nitrogen is an essential plant nutrient, widely applied as N-fertilizer to improve yield of agriculturally important crops. An interesting alternative to avoid or reduce the use of N-fertilizers could be the exploitation of Plant Growth-Promoting Bacteria (PGPB) capable of enhancing growth and yield of many plant species, several of agronomic and ecological significance.
Azotobacterspp. are non-symbiotic heterotrophic bacteria capable of fixing an average 20 kg N/ha/per year.Bacterization helps to improve plant growth and to increase soil nitrogen through nitrogen fixation by utilizing carbon for its metabolism.
Seed inoculationAzotobacterand nutrient uptake
Seed Inoculated with Azotobacter helps in uptake of N, P along with micronutrients like Fe and Zn, in wheat, these strains can potentially be used to improve wheat nutrition. Seed inoculation of Azotobacter profoundly contribute to increase yield by supplying nitrogen to the crops. Inoculation of seeds with Azotobacter chroococcumincreased carbohydrate and protein content of two corn varieties (Inra210 and Inra260) in greenhouse experiment. There is increment in Maize biomass with the application of manure and Azotobacter.
In nitrogen-deficient sand, seed inoculation increased plant length, dry weight, and nitrogen content in addition to a significant increase in soil nitrogen. It was found that A. chroococcumat concentration of 108 cfu ml-1 increased seed germination of Cucumber.Seeds of wheat (Triticum aestivum) were inoculated with 11 bacterial strains of A.chroococcum, Research result showed that all A. chroococcums trains had positive effect on the yield and N concentrations of wheat.
Role of Azotobacter in growth substances production and promotion
Besides, nitrogen fixation, Azotobacter produces, Thiomin, Riboflavin, Nicotin, Indol Acetic Acid and Giberalin. When Azotobacter is applied to seeds, seed germination is improved to a considerable extent. Brakel & Hilger showed that Azotobacter produced Indol-3-Acetic Acid (IAA) when tryptophan was added to the medium. Hennequin & Blachere found only small amounts of IAA in old cultures of Azotobacter to which no tryptophan was added.
Bacteria of the genus Azotobacter synthesize auxins, cytokinins, and GA-like substances, and these growth materials are the primary substance controlling the enhanced growth of tomato. These hormonal substances, which originate from the rhizosphere or root surface, affect the growth of the closely associated higher plants. Eklund et al demonstrated that the presence of Azotobacter chroococcum in the rhizosphere of tomato and cucumber is correlated with increased germination and growth of seedlings. Puertas & Gonzales report that dry weight of tomato plants inoculated with Azotobacter chroococcum and grown in phosphate-deficient soil was significantly greater than that of non inoculated plants. Phytohormones (auxin, cytokinin, gibberellin) can stimulate root development.
High Gibberillic acid production was detected in Azotobacter(71.42 %) isolates. Higher phosphate solubilization was detected in the isolates of Azotobacter (74.28 %) followed by Pseudomonas (63.00 %). Gibberellins applied in small quantities to the soil or rosettes on the leaves and shafts of certain plant produces an increase in height. In grains such as wheat and corn, they also cause an increase in length of the leaves. In some cases they also increase the fresh weight and dry weight. However it doesn’t produce any effect on the growth of the roots.
Brown & Burlinghamhave found that after treating tomato seeds or seedling roots with small amounts (0.5-0-01 pg.) of commercially produced gibberellins GA 3, the plants responded in the same way as after treatments with 14-day cultures of Azotobacter chroococcumstrain A 6. Azotobacter chroococcumstrains isolated from the sugar beet rhizosphere were also shown to produce gibberelins; the growth of pea hypocotyl was equivalent to a GA3 concentration of 0.003-0.1 μg/cm3 culture. Similarly, Cytokinins are related to nucleic acids with precursor substances that act to stimulate cell division in vegetative growth areas.
These responses suggest that Azotobacter probably influences the development of plants by producing growth-regulating substances. Therefore, Azotobacter spp. is often regarded as a member of “Plant Growth Promoting Rhizobacteria (PGPR)”.
It was also concluded that Azotobacterinoculants have a significant promoting effect on growth parameters like root, shoot length and dry mass of bamboo and maize seedlings in vitroand in pot experiments. The dual inoculation of A. chroococcumand P. indica had beneficiary response on shoot length, root length, fresh shoot and root weight, dry shoot and root weight, and panicle number that affect growth of rice plant.
Adathoda vasicaplants inoculated with A. chroococcumrevealed significantly increased root nitrogen content compared to the control plants. The Azotobacters which were inoculated with Rhizophora seedlings, increased significantly the average root biomass up to by 98.2%, the root length by 48.45%, the leaf area by 277.86%, the shoot biomass by 29.49% as compared to controls and they also increased the levels of total chlorophylls and carotenoids up to by 151.0% and 158.73%, respectively. The Azotobacter inoculated crop show better growth rate. During rosette stage of canola the Crop Growth Rate (CGR) found highest i.e. 10-12 % increment.
There is increment in dry matter accumulation in Azotobacter inoculated plants; it stimulates development of foliage, roots, branching, flowering and fruiting which is triggered by fixed nitrogen and plant growth regulator like substance produced. It also increases plant tolerance to lack of water under adverse condition. Similar result put forwarded by Sandeep et al. which revealed that there is better growth response of Azotobacter inoculated plants as compared to non-inoculated control plants. Better crop growth response ultimately results in better dry matter accumulation.