Which of the following plants cannot fix atmospheric nitrogen?

Options

A. Cycas revoluta
B. Citrus
C. Cicer arietinum
D. Cajanus cajan

Answer

To determine which of the following plants cannot fix atmospheric nitrogen, we need to analyze each option based on their ability to engage in nitrogen fixation.

### Step-by-Step Solution:

1. **Understand Nitrogen Fixation**:
- Nitrogen fixation is the process by which certain organisms convert atmospheric nitrogen (N₂) into a form that plants can use (like ammonia). This process often involves symbiotic bacteria, such as Rhizobium, which are commonly found in leguminous plants.

2. **Identify the Plants in the Options**:
- We need to look at the specific plants mentioned in the question. The options given are:
- Cyclus revoluta
- Citrus
- Cicer arietinum (chickpea)
- Cajanus cajan (pigeon pea)

3. **Evaluate Each Plant for Nitrogen Fixation**:
- **Cyclus revoluta**: This plant has associations with blue-green algae (cyanobacteria), which can fix atmospheric nitrogen. Therefore, it can fix nitrogen.
- **Citrus**: Citrus plants do not have any symbiotic bacteria or cyanobacteria associated with them for nitrogen fixation. They are not leguminous plants and do not participate in nitrogen fixation.
- **Cicer arietinum (Chickpea)**: This is a leguminous plant and has Rhizobium bacteria associated with it, allowing it to fix atmospheric nitrogen.
- **Cajanus cajan (Pigeon Pea)**: This is also a leguminous plant and can fix atmospheric nitrogen through its association with Rhizobium.

4. **Conclusion**:
- Among the options, **Citrus** is the only plant that cannot fix atmospheric nitrogen because it lacks the necessary symbiotic relationships with nitrogen-fixing bacteria.

### Final Answer:
The plant that cannot fix atmospheric nitrogen is **Citrus** (Option 2).

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