Nitrogen Fixation in the Rhizosphere of Grasses and Weeds (Associative Symbiosis)

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Nitrogen Fixation in Free-Living and Associative Symbiotic Bacteria Nitrogen Fixing Bacteria Nitrogenase Genetics Mechanism of Nitrogen Fixation Response of Plants to Azotobacter Inoculation Nitrogen Fixation in the Root Zone of Rice

Nitrogen Fixation in the Rhizosphere of Grasses and Weeds (Associative Symbiosis)

Nitrogen Fixation in the Rhizosphere of Grasses and Weeds (Associative Symbiosis)
During the last decade, a group of Brazilian workers observed specific and abundant colonization of the rhizosphere of the genus Paspalum (grasses) with Azotobacter paspali, a powerful nitrogen fixer. Sizeable amounts of acetylene were reduced (nitrogenase activity) by the rhizosphere soil samples of Paspalum notatum indicating the role of A. paspali in fixing nitrogen ion situ in the vicinity of an actively growing plant.

Quite recently, nitrogen fixation in the rhizospheres of many forage grasses such as Pennisetum purpureum, Brachiaria mutica, Brugulosa, Digitaria decumbens, Panicum maximum, Melinis minutiflora and Hyparrhenia rufa and weeds such as Heracleum sphondylium, Anthriscus sylvestris, Mercurialis perennis, Rumex acetosa, Convolvulus arvensis, Viola caniana and Stachys sylvatica have been reported

.

Although Spirillum lipoferum as a nitrogen-fixer was known since 1963, it was Dobereiner and associates in Brazil who in 1975 highlighted and attributed the nitrogen-fixation potential of some tropical forage grasses such as Digitaria, Panicum, Brachiaria, maize, sorghum, wheat and rye to the activity of S.

Llipoferum in their roots. Subsequently, the bacterium has been isolated from many tropical countries and found to reside inside the roots and aerial parts of plants.

Dobereiner coined the name' Associate Symbiosis' to denote the occurrence of nitrogen-fixing Spirillum in plants which has now been recently enlarged to encompass other possible bacterial associations by adopting the terminology 'Diazotrophic biocoenocis'.

The taxonomy of Sprillum has been re-examined and the genus has been named as Azospirillum with four species-A. lipoferum, A. brasilense, A. amazonense and A. seropedica. Due-to its-microaerophilic nature, Azospirillum can be isolated on a semi-solid malate containing medium by enrichment procedures.

The development of white, dense and undulating fine pellicles on a semi-solid malate medium is a characteristic feature of Azospirillum. During enrichment, the dominant organisms on a sodium malate medium are characteristically curved rods of various sizes with prominent refractive fat droplets. This organism is Gram-negative and contains poly-β-hydroxy butyrate granules.

Microscopic examination reveals polymorphism and spirillar movement. For fixation of molecular nitrogen, the bacterium needs microaerophilic surroundings (low O₂ conditions) but can grow profusely on ammonium containing medium without fixing nitrogen. The ability of the organism to fix nitrogen has been verified by the acetylene reduction test and uptake of 15N₂ gas. In terms of mg N₂ gas. In terms of mg N₂ fixed/g of substrate added, the range of fixation, as measured by Kjeldahl method, varies from 12 to 36 among different isolates of A. brasilense. The organisms is also known to produce growth substances such as IAA, kinetins and gibberellins .

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Nitrogen Fixing Activity of Paddy Soil in Pots under Various Treatments Nitrogen Fixation in the Rhizosphere of Grasses and Weeds Associative Symbiosis Some New Nitrogen Fixing Bacterial Associations Several Nitrogen Fixing Bacteria Response of Plants Azospirillum Inoculation The Concept of C4 Dicarboxylic Acid Pathway Nitrogen Fixation by Rhizobium in a Free Living State

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