Question

Explain in details the External factors that affect Transpiration.

Answer 1

External Factors or Environmental Factors for Transpiration Process
External factors refer to the environmental factors that affect the rate of transpiration which are as follows:
(i) Atmospheric Humidity
Atmospheric humidity refers to the amount of water vapor present in the air. If the air is saturated with water vapor, the rate of transpiration decreases, and as the air becomes dry, the rate of transpiration increases. Thus, the rate of transpiration is inversely proportional to atmospheric humidity.
(ii) Temperature
The rate of transpiration increases with the increase in the temperature of the atmosphere as it is directly proportional to the temperature. Increased temperature not only helps in quicker evaporation of the water molecules but also keeps the air warm to hold more water vapours than cold air. But at higher temperatures, wilting takes place in plants that results in a decrease in the rate of transpiration.
(iii) Light
Bright light is the main stimulus that causes the stomata to open, because of which the plants show a daily periodicity of transpiration rate. The mode of action of light is both direct and indirect. The increase in the intensity of light raises the temperature of the leaf cells and thus increases the rate at which the liquid water is transformed into vapours. Thus, the rate of transpiration is directly proportional to the intensity of light. But at a higher light intensity, the number of heat increases which increases the temperature, thus resulting in the wilting of plants, which causes a decrease in the rate of transpiration.
(iv) Wind Velocity
Fast-moving air currents continuously bring fresh, dry masses of air in contact with leaf surfaces that carry water vapor produced during transpiration and thus increase the rate of transpiration. Thus, the velocity of wind greatly affects the rate of transpiration.
(v) Soil Water Content
The availability of water in soil greatly affects the rate of transpiration. If little water is available to the plants, then it results in the dehydration of the leaf, causing stomatal closure in the mechanism of stomatal transpiration, which decreases the rate of transpiration. This condition usually occurs during drought or when the soil is frozen or at such a low temperature that roots are unable to absorb water.
(vi) Atmospheric Pressure
The rate of transpiration is inversely proportional to the atmospheric pressure. With the decrease in the atmospheric pressure, the rate of transpiration increases because the air can move out of the plant easily due to the development of pressure deficit.