1. Rhizobium has symbiotic relationship with roots of leguminous plants.
2. It infects root cortex and form root nodules.
3. Root nodules are the site of nitrogen fixation.
4. Enzyme nitrogenase which catalyzes nitrogen fixation, gets inhibited by oxygen.
5. But root nodule contain a pigment called leghaemoglobin which acts as oxygen scavanger and protects nitrogenase from getting inhibited.
6. Leghaemoglobin is pink in colour.
7. Hence, healthy root nodules are pink in colour.
