An association between bacteria and root nodules of legume is called

1. Analyze the question The question asks for the name of the relationship between bacteria (Rhizobium) and the root nodules of leguminous plants such as beans, groundnut, cowpea and soybean.

2. Explain the concept Symbiotic relationships come in several types:

• Commensalism: one organism benefits and the other is neither helped nor harmed.
• Mutualism: both organisms benefit.
• Parasitism: one benefits and the other is harmed.
• Saprophytism: an organism feeds on dead or decaying organic matter.

3. Step-by-step solution

• Rhizobium bacteria live in nodules on the roots of legumes.
• The bacteria fix atmospheric nitrogen (N₂) into ammonia and nitrogen compounds that the plant can use. The plant benefits.
• The legume supplies the bacteria with carbohydrates (food) and a protected home in the nodules. The bacteria benefit.
• Since both partners gain, this is mutualism.

Checking the other options:

• A: The plant is not unaffected. It gains nitrogen, so this is not commensalism.
• C: The plant is not harmed, so this is not parasitism.
• D: The bacteria live in a living plant and do not feed on dead matter, so this is not saprophytism.

4. Answer The correct option is B. Mutualism.

5. Exam tip Remember "legume + Rhizobium = nitrogen fixation = both win." Whenever you see root nodules, think mutualism. Other classic mutualism examples are lichens (alga + fungus) and mycorrhiza (fungus + plant roots).