Leguminous plants
Explanation
The symbiotic biological nitrogen fixation. This is a process where specific soil bacteria form a mutualistic association with plant roots to convert atmospheric nitrogen ($N_2$) into ammonia ($NH_3$), a form usable by plants.Detailed Analysis:
- Leguminous plants (Correct): Rhizobium bacteria exhibit a specific host-symbiont relationship with plants of the family Fabaceae (legumes), such as peas, beans, lentils, groundnut, and alfalfa. The bacteria infect the root hairs, leading to the formation of specialized structures called root nodules. Within these nodules, Rhizobium fixes atmospheric nitrogen.
- Cereal crops (Incorrect): Cereals like wheat, rice, and maize are monocots and do not form root nodules with Rhizobium. They may benefit from free-living nitrogen fixers (e.g., Azotobacter) or loose associations (e.g., Azospirillum), but not through nodulation.
- Gymnosperms (Incorrect): Gymnosperms do not associate with Rhizobium. For instance, Cycas has specialized coralloid roots that harbor nitrogen-fixing Cyanobacteria (blue-green algae like Anabaena or Nostoc), not Rhizobium.
- Aquatic plants (Incorrect): While certain aquatic plants like the fern Azolla are biofertilizers, they associate with the cyanobacterium Anabaena, not Rhizobium.
Key Takeaway: Rhizobium is a symbiotic bacterium strictly associated with the formation of root nodules in leguminous plants for nitrogen fixation.