Which of the following reduces transpiration?

• A. Decreasing humidity
• B. Increasing light
• C. Increasing surrounding relative humidity
• D. Increasing wind speed

Answer: (C)

Transpiration follows the water vapor gradient between the inside of the leaf and the air outside. When the surrounding air is more humid (higher relative humidity), the outside air already contains a lot of water vapor, so the gradient driving water loss from the leaf is smaller. With a smaller gradient, less water diffuses out through the stomata, so transpiration decreases.

If humidity were reduced, or if light were increased, or if wind speed were higher, transpiration would tend to rise. Lower ambient humidity increases the vapor-pressure difference, opening stomata more for photosynthesis, and wind removes the humid air near the leaf, both boosting water loss.