Describe biological nitrogen fixation with reference to Rhizobium and Legume.

Explanation

The process of converting Atmospheric Nitrogen into Ammonia is termed as Nitrogen fixation.
Biological nitrogen fixation :
Symbiotic bacterium like Rhizobium fixed atomspheric nitrogen. Cyanobacteria found in Lichens, Anthoceros, Azolla and coralloid roots of Cycas also fix nitrogen. Non - Symbiotic (Free living bacteria ) like Clostridium also fix nitrogen.
a. Symbiotic nitrogen fixation :
Nitrogen fixation with nodulation
(i) Rhizobium bacterium is found in leguminous plants and fix atmospheric nitrogen.
(ii) This kind of symbiotic association is beneficial for both the bacterium and plant. Root nodules are formed due to bacterial infection.
(iii) Rhizobium enters into the host cell and proliferates, it remains separated from the host cytoplasm by a membrane
b. Stages of Root nodule formation :
1. Legume plants secretes phenolics which attracts Rhizobium.
2. Rhizobium reaches the rhizosphere and enters into the root hair, infects the root hair and leads to curling of root haris.
3. Infection thread grows inwards adn separates the infected tissue from normal tissue.
4. A membrane bound bacterium is formed inside the nodule and is called bacteroid.
5. Cytokinin from bacteria and auxin from host plant promotes cell division and leads to nodule formation.
Nitrogenase enzyme complex, Minerals (Mo, Fe and S ), anarobic condition, ATP, electron and glucose 6 phosphate as `H^(+)` donor. Nitrogenase enzyme is active only in anaerobic condition . To create this anaerobic condition a pigment known as leghaemoglobin is synthesized in the nodules which acts as oxygen scavenger and removes the oxygen. Nitrogen fixing bacteria in root nodules appears pinkish due to the presence of this leghaemoglobin pigment.