Factors affecting transpiration:
Several factors influence transpiration. These factors can be external as well as internal.
External factors:
Light: Plant transpire more rapidly in the light than in the dark. This is largely because light stimulates the opening of stomata. Light also increases transpiration rate by warming the leaf.
Temperature: Plants transpire more rapidly at higher temperatures because water evaporates more rapidly as the temperature rises.
Humidity: Increased humidity decreases transpiration as the air is already saturated with water. As the water potential gradient is smaller, less water is lost by transpiration.
Wind: When there is no wind, the air surrounding a leaf becomes increasingly humid thus reducing the rate of transpiration. When wind is present, the humid air is carried away and replaced by drier air.
Soil water: A plant cannot continue to transpire rapidly if its water loss is not made up by replacement from the soil. When absorption of water by the roots fails to keep up with the rate of transpiration, loss of turgor occurs and the stomata close. This immediately reduces the rate of transpiration.
Internal Factors:
Internal factors that affect transpiration are the number of stomata, leaf area, thickness of cuticle(thick waxy cuticles prevent water loss, whilst thinner and less waxy cuticles allow more water to be lost). Distribution of stomata also affects transpiration-most plants have most of the stomata on the lower surface of the leaf, where the stomata are less exposed to environmental conditions.