Factors Affecting Transpiration

Factors that affect transpiration are:

a) Temperature

b) Light

c) External Humidity

d) Air Circulation

e) Soil moisture

f) Carbon-dioxide Concentration

Let's discuss all the above factors in detail.

Temperature

When there is an increase in temperature, the capacity of the air to hold the water decreases, thus water vapors from the leaves can diffuse easily. The rate of water evaporation doubles for every temperature rise of about 10 degree Celsius. This increase in transpiration with increasing temperature is up to certain point. If temperature exceeds from 30 to 45 degrees Celsius the stomata are closed.

Light

Light affects transpiration by opening the stomata. In the dark the stomata become closed and rate of transpiration decreases. Light absorbed by the mesophyll cells increases the internal temperature of the leaf. This increase in temperature causes increase in the rate of transpiration. K+ actively enters the guard cells when light is available and water follows and guard cells become turgid and stomata opens.

External Humidity

The difference in the water content of the plant and that of the atmosphere affects the rate of transpiration. When the atmospheric air is fully saturated with water vapors, there is no possibility of more water vapors moving into it. Transpiration takes place when the atmosphere is partially unsaturated or dry. In greenhouse the floor and wall are watered to increase humidity and to reduce transpiration from the plants. This reduces the possibility of wilting and results in better growth.

Air Circulation

When the air is still, the air surrounding a leaf becomes saturated thus transpiration is reduced. When the air surrounding the leaves is in motion, it is carried away before it can become saturated, so water vapors can diffuse outwards continuously.

Soil Moisture

When the amount of soil water is low, less water is absorbed by the plant. The amount of water in the guard cells falls, they become flaccid and close up the stomata pores and transpiration decreases. The opposite takes place when an excess of soil water is available to the plant.

CO₂ Concentration

Low CO₂ Concentration stimulates the active transport of potassium ions into guard cells. This transport causes stomata to open and allow CO2 to diffuse in the mesophyll cells of leaves. At night cellular transpiration in the absence of photosynthesis raises CO2 levels. This stops the inward transport of K+ (potassium) ions and thus of water, allowing the guard cells to close.