The following article highlights the two types of factors that affect the rate of transpiration.
The factors affecting the rate of transpiration can be categorized into two groups: (A) External or Environmental Factors and (B) Internal or Structural or Plant Factors.
Type # A. External or Environmental factors:
1. Atmospheric Humidity:
The rate of transpiration is roughly inversely proportional to atmospheric humidity. As the outward diffusion of water vapors through stomata is in accordance with the law of simple diffusion, the rate of transpiration is greatly reduced when the atmosphere is very humid. As the air becomes dry, the rate of transpiration also increases proportionately.
With the increase in atmospheric temperature, the rate of transpiration also increases. This is not only because evaporation occurs quickly in warmer air but also because warm air is capable of holding more water vapors than the cold air.
The rate of transpiration is roughly proportional to the intensity of light. The mode of action of light is both direct and indirect. The increasing light intensity raises the temperature of leaf cells and thus increases the rate at which liquid water is transformed into vapors. Direct effect of light is on the opening and closing of stomata. Bright light is the chief stimulus which causes stomata to open. It is simply because of this reason that all plants show a daily periodicity of transpiration rate.
4. Wind Velocity:
The velocity of wind greatly affects the rate of transpiration. Fast moving air currents continually bring fresh, dry masses of air in contact with leaf surfaces and thus maintain a high rate of transpiration.
5. Soil Water Content:
Availability of soil water greatly affects the rate of transpiration. If there is little water available, the resulting tendency for dehydration of the leaf causes stomatal closure and a consequent fall in transpiration. Such a condition usually occurs during periods of drought and when the soil is frozen or at a temperature so low that water is not absorbed by roots.
6. Atmospheric Pressure:
The rate of transpiration is inversely proportional to the atmospheric pressure.
7. Carbon Dioxide Concentration:
Reduced CO2 concentration favours opening of stomata while an increase in CO2 concentration promotes stomatal closing.
Type # B. Internal or Structural or Plant Factors:
1. Leaf Area:
If leaf area is more, transpiration is faster. However, the rate of transpiration per unit area is more in smaller leaves than in larger leaves due to higher number of stomata in the small leaf. Number of stomata per unit area of leaf is called stomatal frequency.
I = S/E+S × 100
I = Stomatal index
S = No. of stomata per unit area
E = No. of epidermal cells in same unit area
2. Structural Peculiarities of Leaf:
The anatomical features of leaves like sunken or vestigial stomata; presence of hair, cuticle or waxy layer on the epidermis; presence of hydrophilic substances such as gums, mucilage etc. in the cells; compactly arranged mesophyll cells etc. help in reducing the rate of transpiration.
3. Root Shoot Ratio:
According to Parker (1949) the rate of transpiration is directly proportional to the root-shoot ratio.
4. Orientation of Leaves:
If the leaves are arranged transversely on the shoot they lose more water because they are exposed to direct sunlight. If placed perpendicularly they transpire at slower rate.