Factors Affecting Rate of Transpiration

 The factors affecting the rate of transpiration can be categorized into two groups :

A. External or Environmental factors (eg. atmospheric humidity, temperature, wind velocity, light,

water supply, atmospheric pressure, sprays, and dust and vital activities)

B. Internal or Structural factors (eg. stomatal apparatus and its frequency, the water content of

mesophyll cells, and structural peculiarities of the leaf).

A. External Factors

All these factors affect the steepness of the DPD gradient and thus influence the rate of transpiration.

1. Atmospheric Humidity :

The rate of transpiration increases with the decrease in the humidity of the external atmosphere.

because the humidity influences the DPD gradient between the intercellular spaces and the outside

atmosphere.

2. Temperature

A high level of temperature increases the rate of transpiration by increasing the rate of

evaporation of water from the cell surface and thus decreasing the humidity of the external atmosphere.

3. Wind Velocity

An increase in wind velocity increases the transpiration rate by removing water vapor from the

atmosphere and lowering the relative humidity. The transpiration is faster in the mild wind. In the case of

higher wind velocity, the transpiration rate is retarded because of guard cells becoming flaccid and stomata

closed.

4. Light

Light indirectly controls the rate of transpiration in two ways: firstly, by controlling the stomatal opening

and secondly, by affecting the temperature. With the increase in the light intensity, the rate of transpiration

increases because the stomata get opened and the temperature increases. There is a close relationship

between the opening of the stomata and the presence of light.

5. Atmospheric Pressure

The reduction of atmospheric pressure increases the transpiration rate by permitting more rapid

diffusion of water. The plants growing in hills will show higher rates of transpiration because of low

atmospheric pressure.

6. Water Supply

The deficiency of soil water decreases the rate of transpiration indirectly by reducing the availability of water

and its rate of absorption.

7. Sprays and Dust

Sprays and dust affect transpiration by affecting the permeability of the cuticle and the temperature of

leaves.

8. Vital Activities

Vital activities of the plants may also influence the transpiration rate such as heat produced from

the energy provided by respiration may increase the rate of transpiration.

B. Internal Factors

1. Stomatal Frequency

Stomatal frequency means the number of stomata per unit area of the leaf surface. Salisbury used the

term Stomatal Index (SI) to represent as,

S

SI =__________ x 100

E+S