Which of the following factors, besides being one of the reactants in the process of photosynthesis, indirectly affects its rate?

Options

A. Oxygen
B. Carbon dioxide
C. Water
D. Chlorophyll

Explanation

To determine which factor, besides being one of the reactants in photosynthesis, indirectly affects its rate, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Reactants of Photosynthesis**:
The process of photosynthesis involves the following equation:
\[ 6 \text{CO}_2 + 12 \text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6 \text{O}_2 + 6 \text{H}_2\text{O} \]
From this equation, we can see that the primary reactants are carbon dioxide (CO2) and water (H2O).

2. **Determine Direct vs. Indirect Effects**:
- **Carbon Dioxide (CO2)**: This reactant is directly involved in the photosynthesis process as it is reduced to form glucose. Therefore, it directly affects the rate of photosynthesis.
- **Water (H2O)**: While water is also a reactant, its availability can indirectly affect the rate of photosynthesis.

3. **Analyze the Role of Water**:
- When water availability is low, plants experience water stress.
- Under water stress conditions, the stomata (tiny openings on the leaf surface) tend to close to prevent water loss.
- When stomata are closed, the uptake of CO2 is reduced, leading to a decrease in the rate of photosynthesis.

4. **Conclusion**:
Based on the analysis, while both CO2 and H2O are reactants, water indirectly affects the rate of photosynthesis by influencing stomatal behavior and CO2 uptake. Therefore, the answer to the question is **water**.