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 each option provided:

1. **Age of Leaf**: The age of a leaf is an internal factor that can affect photosynthesis. Younger leaves typically have a higher rate of photosynthesis compared to older leaves, which may have reduced chlorophyll content and overall efficiency.

2. **Number of Mesophyll Cells**: Mesophyll cells are responsible for photosynthesis because they contain chlorophyll. However, the number of mesophyll cells itself does not directly regulate the rate of photosynthesis. Instead, it is the amount of chlorophyll within those cells that plays a significant role. Therefore, the number of mesophyll cells is not a limiting factor for photosynthesis.

3. **Atmospheric Carbon Dioxide Concentration**: Carbon dioxide is a crucial component of photosynthesis. The concentration of atmospheric CO2 directly influences the rate of photosynthesis, especially if it is below the optimal level. Thus, this is a plant factor that regulates photosynthesis.

4. **Amount of Chlorophyll**: Chlorophyll is essential for capturing light energy during photosynthesis. The amount of chlorophyll present in the plant directly affects the photosynthetic rate. Therefore, this is also a plant factor regulating photosynthesis.

After analyzing all the options, we conclude that the **number of mesophyll cells** is not a plant factor regulating photosynthesis, as it is the chlorophyll content within those cells that matters.

### Final Answer:
The option that is not a plant factor regulating photosynthesis is **number of mesophyll cells**.