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 the options provided and understand the role of specific bacteria in nitrogen fixation.

### Step-by-Step Solution:

1. **Understand Nitrogen Fixation**:
- Nitrogen fixation is the process of converting atmospheric nitrogen (N₂) into a form that plants can use, such as ammonia (NH₃). This process is primarily carried out by certain bacteria.

2. **Identify Nitrogen-Fixing Organisms**:
- The main organisms responsible for nitrogen fixation are:
- **Rhizobium**: These bacteria form symbiotic relationships with leguminous plants (like peas and beans) and fix nitrogen in their root nodules.
- **Cyanobacteria**: Also known as blue-green algae, these can fix nitrogen independently in aquatic environments and some terrestrial ecosystems.

3. **Evaluate the Options**:
- **Option 1: Cycas Revoluta**: This plant is known to associate with nitrogen-fixing organisms and can utilize fixed nitrogen.
- **Option 2: Citrus Plant**: Citrus plants do not have symbiotic relationships with nitrogen-fixing bacteria like Rhizobium or cyanobacteria, so they cannot fix atmospheric nitrogen.
- **Option 3: Cicer arietinum (Chickpea)**: This is a leguminous plant that hosts Rhizobium bacteria in its root nodules, allowing it to fix nitrogen.
- **Option 4: Cassia**: This plant is also known to associate with nitrogen-fixing bacteria and can fix atmospheric nitrogen.

4. **Conclusion**:
- The plant that cannot fix atmospheric nitrogen is **Citrus Plant (Option 2)**, as it lacks the necessary nitrogen-fixing bacteria.

### Final Answer:
The plant that cannot fix atmospheric nitrogen is the **Citrus Plant**.

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