If `CO_(2)` conc, increases upto 0.05% the rate of photosynthesis -

Options

A. decreases
B. increases for short terms
C. first decreases and then increases
D. becomes zero

Answer

### Step-by-Step Solution:

1. **Understanding the Role of CO2 in Photosynthesis**:
- Carbon dioxide (CO2) is one of the essential raw materials for photosynthesis. It is absorbed by plants through small openings called stomata.

2. **Effect of Increased CO2 Concentration**:
- When the concentration of CO2 increases, up to a certain point (in this case, 0.05%), the rate of photosynthesis also increases. This is because more CO2 is available for the plants to convert into glucose during the photosynthesis process.

3. **Threshold Concentration**:
- The increase in the rate of photosynthesis continues as CO2 concentration rises until it reaches the threshold of 0.05%. Beyond this concentration, the benefits start to diminish.

4. **Stomatal Closure**:
- If CO2 concentration exceeds 0.05%, plants may respond by closing their stomata to prevent excessive water loss. This closure limits gas exchange, meaning that even though CO2 is available, it cannot enter the plant.

5. **Impact on Photosynthesis**:
- With the stomata closed, the gas exchange stops, leading to a halt in photosynthesis. Therefore, while an increase in CO2 concentration up to 0.05% enhances photosynthesis, any further increase can lead to a decrease in the rate of photosynthesis due to stomatal closure.

6. **Conclusion**:
- In summary, increasing CO2 concentration up to 0.05% will enhance the rate of photosynthesis, but exceeding this concentration can lead to stomatal closure and ultimately stop photosynthesis.