Which of the following is not a plant factor regulating the photosynthesis ?

Options

A. Age of leaf
B. Number of mesophyll cells
C. Atmospheric `CO_(2)` concentration
D. Amount of chlorophyll

Answer

To determine which of the following is not a plant factor regulating photosynthesis, we need to analyze the options provided and identify whether they are internal or external factors affecting the process of photosynthesis.

### Step-by-Step Solution:

1. **Understanding Photosynthesis**:
- Photosynthesis is the process by which plants convert light energy into chemical energy, producing food. It primarily occurs in the chloroplasts of plant cells.

2. **Identifying Plant Factors**:
- Plant factors that regulate photosynthesis can be categorized into internal and external factors.
- Internal factors are those that are part of the plant's body, while external factors are those that exist outside the plant.

3. **Internal Factors**:
- **Chlorophyll**: The amount of chlorophyll present in the plant is crucial as it absorbs light. More chlorophyll means more light absorption and, consequently, more photosynthesis.
- **Age of the Leaf**: As leaves age, the chlorophyll content decreases, leading to reduced photosynthesis.
- **Number of Mesophyll Cells**: These cells contain chloroplasts, which house chlorophyll. More mesophyll cells mean a greater capacity for photosynthesis.

4. **External Factors**:
- **Carbon Dioxide Concentration**: This is an external factor as CO2 is present in the atmosphere outside the plant. The concentration of CO2 affects the rate of photosynthesis, with optimal levels enhancing the process.

5. **Conclusion**:
- Among the factors discussed, the age of the leaf, the number of mesophyll cells, and the amount of chlorophyll are all internal factors. In contrast, the concentration of carbon dioxide is an external factor that influences photosynthesis but is not part of the plant's body.

6. **Final Answer**:
- Therefore, the factor that is **not a plant factor regulating photosynthesis** is **Carbon Dioxide Concentration**.