Question

Casuarina equisetifolia fixes atmospheric nitrogen by forming symbiotic mutualistic association with:

• A. Rhizobium
• B. Frankia
• C. Azotobacter
• D. Bacillus

Hint:

Remember:
- Legumes $\leftrightarrow$ *Rhizobium*
- Actinorhizal plants (e.g., *Casuarina*, *Alnus*) $\leftrightarrow$ *Frankia*
This distinction is a favorite topic in botany and forestry exams.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Biological Nitrogen Fixation (BNF) is a process where atmospheric nitrogen is converted into ammonia by specialized microorganisms.
While legumes typically associate with *Rhizobium*, non-leguminous actinorhizal plants form symbiotic associations with other microbes.

Step 2: Detailed Explanation:
*Casuarina equisetifolia* is a fast-growing, evergreen, non-leguminous tree species native to Southeast Asia and northern Australia.
It is widely planted in coastal areas of India for shelterbelts, windbreaks, and dune stabilization.
To thrive in nutrient-poor coastal sandy soils, *Casuarina* forms a symbiotic association with a nitrogen-fixing actinomycete bacterium belonging to the genus *Frankia*.
This association leads to the development of woody root nodules where *Frankia* fixes atmospheric nitrogen.
This provides the tree with a reliable nitrogen source while the plant supplies the bacterium with carbohydrates.
Leguminous trees (e.g., *Acacia*, *Leucaena*) form relationships with *Rhizobium*, whereas actinorhizal plants like *Casuarina*, *Alnus*, and *Myrica* associate with *Frankia*.

Step 3: Final Answer:
*Casuarina equisetifolia* fixes nitrogen in association with *Frankia*.
This matches Option (B).