Rhizobium which lives in the root nodules of leguminous plants is an example of the association known as

Step 1: Analyze the question The question asks what type of association exists between Rhizobium bacteria and leguminous plants (e.g., beans, groundnut, cowpea) in the root nodules.

Step 2: Explain the concept

• Commensalism: One organism benefits and the other is neither helped nor harmed (e.g., an orchid on a tree).
• Epiphytism: A plant grows on another plant only for support, not food (e.g., some ferns and orchids on tree trunks).
• Mutualism: Both organisms benefit from the relationship.
• Saprophytism: An organism feeds on dead, decaying organic matter (e.g., mushrooms, bread mould).

Step 3: Apply it to Rhizobium

• Rhizobium bacteria live in nodules on the roots of legumes.
• They fix atmospheric nitrogen into nitrogen compounds (ammonia/nitrates) that the plant can use to make proteins.
• In return, the legume supplies the bacteria with carbohydrates and a protected home in the nodules.

Both partners benefit, so this is mutualism (a type of symbiosis).

Step 4: Eliminate the wrong options

• A: The legume also benefits, so it is not commensalism.
• B: The bacteria are not merely growing on the plant for support.
• D: The bacteria are not feeding on dead matter.

Answer: C. mutualism

Exam tip: Whenever you see "legume + root nodules + nitrogen fixation", think mutualism. Remember: Rhizobium gives nitrogen, the legume gives sugar. Both gain!