Transpiration rate mainly depends on –

Options

A. Availability of Blue light
B. Difference of vapour pressure
C. Atmospheric pressure
D. Temperature

Answer

### Step-by-Step Solution:

1. **Understanding Transpiration**:
Transpiration is the process by which plants lose water vapor through small openings in their leaves called stomata. This process is crucial for maintaining plant health and regulating temperature.

2. **Factors Influencing Transpiration**:
Several factors can influence the rate of transpiration, including:
- **Atmospheric Pressure**: The pressure exerted by the weight of the atmosphere can affect how easily water vapor can escape from the leaf surface.
- **Temperature**: Higher temperatures can increase the rate of evaporation, thereby increasing transpiration.
- **Humidity**: The amount of moisture in the air can influence the concentration gradient for water vapor, affecting transpiration rates.
- **Wind**: Wind can remove the water vapor that accumulates around the leaf surface, enhancing transpiration.
- **Light**: Availability of light can influence stomatal opening, thus affecting transpiration.

3. **Identifying the Main Factor**:
Among the options provided (availability of blue light, difference of vapor pressure, atmospheric pressure, and temperature), the main factor that primarily influences the rate of transpiration is **atmospheric pressure**. This is because it directly affects the evaporative demand of the atmosphere surrounding the leaves.

4. **Conclusion**:
Therefore, the correct answer to the question "Transpiration rate mainly depends on" is **atmospheric pressure**.