Associative Nitrogen Fixing Bacteria

Associative Nitrogen Fixing Bacteria

Associative Nitrogen Fixing Bacteria - Associative symbiosis was first described by Dobereiner of Brazil. An example is Azotobacter paspali which grows as a sheath around the root of the sand grass Paspalum notatum. No nodules are formed, but the plant is benefited greatly. Survival of A. paspali away from the plant is poor.

The acetylene test shows that the A. paspali P.notatum association fixes as much nitrogen as a moderate legume crop. Whether this also takes place under natural conditions is yet to be established. A. paspali associated with only the five tetraploid ecotypes of P.notatum out of 33 stimulated.

A. paspali lives outside the roots in the rhizosphere. This environment is ideal for heterotrophic nitrogen fixing bacteria. It is almost anaerobic, a factor of importance to the association, which though aerobic is very sensitive to oxygen.

The energy and carbon requirements of the heterotrophic free living nitrogen fixers are met with by the root cell and soluble root exudates. These organisms occur in the rhizospheres of a variety of plants. Other examples of associative symbiosis are rice with Beijerinckia, and the grass Digitaria decumbens with Spirillum lipoferum.

Mycorrhizal fungi and the symbiotic nitrogen fixing actinomycete Frankia may occupy inter and intracellular spaces inside the roots. They may establish a highly integrated mutualistic association with their plant hosts.

Helically lobed bacteria have been reported in the collapsed epidermal cells of Ammophila arenaria and within roots of Zea mays. They appear to enter by intercellular penetration of the epidermal cells.

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