The fixation of nitrogen in the root nodules is an example of -

Options

A. associative symbiosis
B. obligatory symbiosis
C. non-symbiotic `N_(2)` -fixation
D. Phyllosphere association

Answer

**Step-by-Step Text Solution:**

1. **Understanding Nitrogen Fixation**: Nitrogen fixation is the process through which nitrogen gas (N₂) from the atmosphere is converted into ammonia (NH₃) or related compounds in soil and water. This process is crucial for plant growth as nitrogen is an essential nutrient.

2. **Role of Root Nodules**: In many legumes, nitrogen-fixing bacteria, primarily from the genus Rhizobium, form symbiotic relationships with the plant roots, leading to the formation of root nodules. These nodules are specialized structures that house the bacteria.

3. **Types of Symbiosis**: Symbiosis can be classified into several types:
- **Obligatory Symbiosis**: Both organisms depend on each other for survival.
- **Non-symbiotic Nitrogen Fixation**: Free-living bacteria fix nitrogen independently without forming nodules.
- **Associative Symbiosis**: Both organisms benefit from the relationship but do not necessarily depend on each other for survival.
- **Filarial Association**: This is not relevant in the context of nitrogen fixation.

4. **Analyzing the Options**:
- **Option 1: Associative Symbiosis**: This is correct because both the plant and the bacteria benefit from the relationship. The plant receives usable nitrogen, while the bacteria gain shelter and energy.
- **Option 2: Obligatory Symbiosis**: Incorrect, as Rhizobium can survive independently without the plant.
- **Option 3: Non-symbiotic Nitrogen Fixation**: Incorrect, as this does not involve root nodules.
- **Option 4: Filarial Association**: Incorrect, as this term does not apply to nitrogen fixation.

5. **Conclusion**: The fixation of nitrogen in root nodules is an example of associative symbiosis, where both the plant and the bacteria benefit from the relationship.

**Final Answer**: Associative symbiosis.

---