What are the external factors that influence transpiration?
Velocity of the wind is a very important factor in deciding the rate of transpiration. If the air is still, the water vapours that come out of the leaf remain in the vicinity. As a result of this, the exterior of the leaf gets saturated with water vapours and there is less scope for water vapours to come out. On the other hand if there is breeze the water vapours that come out are carried away from the leaf and more water vapours can come out.
The intensity of the light is perhaps the most important single factor in deciding the rate of transpiration. Light has a direct influence on the opening and closing of the stomata. Lack of light closes the stomata and there will bre little or no transpiration
The water content in the atmosphere is an important factor, because if the atmosphere is already saturated with water, there is less scope for transpiration. Rate of transpiration is inversely proportional to the amount of atmospheric humidity in the area.
Initially an increase in temperature would increase the rate of transpiration. But this is subject to a certain limit. Usually a tempeiature range that is necessary for other physiological activities is also suitable for transpiration.