The nitrogen-fixing bacterium Rhizobium forms a symbiotic association with:

The correct option is

The root nodules of legumes

Explanation

Nitrogen fixation is the biological process by which atmospheric nitrogen (N₂) is converted into ammonia (NH₃), making it accessible to plants. Rhizobium is a genus of soil bacteria that establishes a mutualistic symbiotic relationship with specific plants to facilitate this process.

Detailed Analysis:

• Legumes and Root Nodules: Rhizobium bacteria specifically infect the roots of leguminous plants (such as peas, beans, lentils, groundnuts, and clover). This infection leads to the formation of specialized structures called root nodules. Inside these nodules, the bacteria fix atmospheric nitrogen, providing the plant with essential nutrients for protein synthesis. In return, the plant supplies the bacteria with carbohydrates and shelter.
• Graminoids (Grasses): Cereal crops and grasses (graminoids) typically associate with free-living or associative nitrogen fixers like Azotobacter or Azospirillum, rather than forming symbiotic nodules with Rhizobium.
• Sugarcane: Nitrogen fixation in sugarcane is often associated with bacteria such as Acetobacter diazotrophicus (now Gluconacetobacter), which may colonize the internal tissues of the stem, but this is distinct from the nodule formation seen in legumes.
• Cotton: Cotton plants do not form symbiotic nitrogen-fixing root nodules with Rhizobium.

Key Takeaway:

Rhizobium is a symbiotic bacterium that fixes atmospheric nitrogen exclusively within the root nodules of leguminous plants.