Rate of Transpiration and Relative Humidity

Transpiration is the process by which plants lose water in the form of vapor through tiny openings called stomata on the surface of leaves. The rate of transpiration is influenced by several factors, including relative humidity.

Effect of Relative Humidity on Transpiration

Relative humidity refers to the amount of moisture present in the air compared to the maximum amount of moisture the air can hold at a specific temperature. It is expressed as a percentage.

1. Option A: Relative humidity decreases
When the relative humidity decreases, it means that the air is drier and has a lower moisture content. In such conditions, the rate of transpiration gradually increases. This is because the difference in water vapor concentration between the inside of the leaf and the surrounding air increases.

- As the air becomes drier, the water potential of the surrounding air decreases.
- This creates a larger water potential gradient between the leaf and the air.
- The larger the water potential gradient, the faster the rate of transpiration.
- Therefore, when the relative humidity decreases, the rate of transpiration gradually increases.

2. Option B: Relative humidity remains unchanged
If the relative humidity remains unchanged, there is no significant effect on the rate of transpiration. The water potential gradient between the leaf and the air remains constant, resulting in a steady rate of transpiration.

3. Option C: Relative humidity increases
When the relative humidity increases, it means that the air is more saturated with moisture. In such conditions, the rate of transpiration decreases. This is because the difference in water vapor concentration between the inside of the leaf and the surrounding air decreases.

- As the air becomes more humid, the water potential of the surrounding air increases.
- This reduces the water potential gradient between the leaf and the air.
- The smaller the water potential gradient, the slower the rate of transpiration.
- Therefore, when the relative humidity increases, the rate of transpiration gradually decreases.

4. Option D: Water potential gradient remains unchanged
If the water potential gradient between the leaf and the air remains unchanged, there is no significant effect on the rate of transpiration. The relative humidity does not directly affect transpiration unless it alters the water potential gradient.

Conclusion

In summary, the rate of transpiration of a plant gradually increases when the relative humidity decreases. This is because a lower relative humidity creates a larger water potential gradient between the leaf and the surrounding air, leading to an increased rate of transpiration.