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 fixes atmospheric 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
1. Rhizobium bacterium is found in leguminous plants and fix atmospheric nitrogen .
2. This kind of symbiotic association is beneficial for both the bacterium and plnat. root noduls are formed due to bacterial infection.
3. Rhizobium enters into the host cell and proliferates, it remains separated from the host cytoplasm by a membrane
b. Stages of root noduls formation.
1. Legume plants secrets 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 hairs.
3. Indection thread grows inwards and separates the infeacted tissue from normal tissue.
4. A membrane bound bacterium is formed inside the nodule and is called bacteriod.
5. Cytokinin from bacteria an auxin from host plant promotes cell division and leads to nodule formation
6. Nitrogenase enzyme complex, minerals (Mo,Fe andS) anaerobic condition ,ATP ,electron an glucose 6 phosphate as `H^+` donor.
7. Nitrogenase enzyme is active only in anaerobic condition .
8. To crate this anaerobic condition a pigment known as leghaemoglogin is synthesized in the nodules which acts as oxygen scavenger and removes the oxygen.
9. Nitrogen fixing bacteria in root nodules appears pinkish due to the presence of this leghaemoglobin pigment.