Gram (Chickpea)
Explanation
Symbiotic nitrogen fixation is a biological process where specific bacteria, primarily Rhizobium, form a mutualistic relationship with the roots of leguminous plants. These bacteria colonize the root hairs, leading to the formation of root nodules, where they convert atmospheric nitrogen ($N_2$) into ammonia ($NH_3$), a form usable by plants for synthesizing proteins and other compounds.
Option Analysis:
- Wheat: Incorrect. Wheat is a cereal crop belonging to the family Poaceae (grasses). It is a monocot and does not form symbiotic associations with Rhizobium for nitrogen fixation.
- Rice: Incorrect. Rice is also a cereal crop (family Poaceae). While certain free-living bacteria (like Azospirillum) or cyanobacteria (like Anabaena in association with Azolla) aid in nitrogen supply in paddy fields, rice does not form root nodules with Rhizobium.
- Gram (Chickpea) Gram (Chickpea): Correct. Gram is a leguminous crop belonging to the family Fabaceae. It possesses root nodules containing Rhizobium bacteria, which facilitate biological nitrogen fixation, enriching the soil fertility.
- Maize: Incorrect. Maize is a cereal grain (monocot) and lacks the capacity for symbiotic nitrogen fixation via Rhizobium nodulation.
Key Takeaway:
Rhizobium bacteria establish symbiotic nitrogen-fixing relationships strictly with leguminous plants (pulses like gram, peas, beans, lentils). Cereal crops (wheat, rice, maize) do not support this specific symbiosis.