Mineral Nutrition

Symbiotic nitrogen fixation in legume root nodules is described in NCERT Class 11, Chapter 12 (Mineral Nutrition). The bacterium Rhizobium living in the nodules contains nitrogenase, the enzyme that reduces atmospheric dinitrogen to ammonia. Nitrogenase is extremely sensitive to oxygen and is irreversibly inactivated in its presence, so the nodule must maintain a low-oxygen, anaerobic micro-environment. To achieve this, nodules produce leghaemoglobin, a pink, iron-containing pigment that acts as an oxygen scavenger, binding free oxygen and keeping its concentration around nitrogenase very low while still permitting respiration of the bacteroids. Phytochrome is a light-absorbing pigment involved in photoperiodism, not oxygen regulation. Cytochrome oxidase is a respiratory electron-transport enzyme, not an oxygen-buffering pigment in nodules. Plastocyanin is a copper protein of the photosynthetic electron transport chain in chloroplasts and has no role in nodules. A plausibility check supports the answer: since nitrogenase activity demands an oxygen-poor setting, the nodule must contain a dedicated oxygen-binding molecule, and the characteristic pink colour of active nodules is itself direct visual evidence of the leghaemoglobin present, which clearly fits the physiological requirement for a dedicated oxygen-binding molecule near the enzyme.