Factors affecting the rate of transpiration
Many environmental factors, such as intensity of light, temperature, atmospheric humidity, wind, availability of soil water, etc., greatly affect the rate of transpiration. The external and internal factors affecting the rate of transpiration are as follows:
Light affects transpiration because it is directly involve in the opening and closing of stomata. Stomata usually open in the light and close in the dark. Therefore, bulk of transpiration takes place during day time. At night only small amount of water is lost by cuticular or lenticular transpiration.
Besides this, light also affects the rate of transpiration indirectly by affecting the temperature and membrane permeability of the cell. High intensity and long duration of light increases the temperature and hence, there increase in the rate of transpiration.
High intensity of light also increases the permeability of the cell membrane resulting into easier diffusion of water vapours into the atmosphere.
The rate of transpiration depends upon the difference in the vapour pressure between the internal atmosphere of the leaf and air outside the leaf. Since relative humidity is the actual amount of water vapours in the atmosphere at any given time, the rate of transpiration is mainly dependent on it.
Low humidity (i.e., low vapour pressure) outside the leaf favours transpiration because in such a situation, vapour pressure gradient between the internal leaf atmosphere and the external atmosphere is very sharp.
As the concentration of water vapours in the external atmosphere rises, the vapour pressure gradient becomes less steep and the rate of transpiration also decreases.
The vapour pressure of the atmosphere decreases with altitude just like atmospheric pressure (it means the air is drier at high altitudes). Therefore, high altitude plants often show xeromorphic adaptations to reduce transpiration.
The rate of transpiration increases with rise in the temperature. This is because at higher temperature there is more evaporation of water from mesophyll cells and, therefore, there is greater saturation of the leaf atmosphere with water vapour.
At the same time, a rise in temperature lowers the relative humidity of the air outside leaf.As a result there is rapid diffusion of water vapours from humid atmosphere inside the leaf to outer dry atmosphere.
In still air water vapours form a highly saturated atmosphere around the leaves. This reduces the steepness of vapour pressure gradient and the rate of transpiration also decreases. Breeze removes humid air present around the leaves and the rate of transpiration increases.
Stronger winds cause bending and fluttering of the leaves, forcing water vapours present in the intercellular spaces to come out. Therefore the rate of transpiration increases.
Winds of very high velocity increase the rate of transpiration initially. Due to excessive loss of water the guard cells become flaccid. As a result, the stomata close at a later stage and transpiration stops.
Available soil water:
In dry soil, the soil solution becomes more concentrated, therefore, there is less tendency for water to enter by osmosis. It means there is less uptake of water by roots, hence, the rate of transpiration is lowered.
Plants show many external and internal features which enable them to reduce transpiration. Such features are called xeromorphic and are found in xerophytes, the plants growing in dry habitats. Plants growing in soils with adequate water supply are known as mesophytes. Such plants can, however, also show some xeromorphic features.
Structure of leaf surface:
The rate of transpiration also depends upon the structure of leaf surface. Presence of thick cuticle, wax layer or trichomes on the leaf reduce the rate of transpiration. The influence of these features on water loss may be easily shown by allowing fresh detached xerophytic and mesophytic leaves to dry together under the same conditions. Visible wilting is observed in the mesophytic leaves long before it is observed in the xerophytic leaves.
Area of transpiring surface:
Leaves are the main organs of transpiration. Therefore any decrease in leaf surface area will reduce transpiration and conserve water absorbed by roots. Many grasses roll up their leaves during dry conditions, thus exposing less surface area to the air than do fully expanded leaves.
A corn plant loses 3-4 litres of water per day, whereas a tree sized desert cactus with no foliage loses less than 25 ml of water per day.
The number, distribution and structure of stomata greatly affect transpiration. The greater the number of stomata per unit area, the greater is rate of stomatal transpiration. However the distribution of stomata is also important. Generally, loss of water is more from leaves that have more stomata on the lower surface than those having equal number on both the surface.
In xerophytic plants stomata occur below the general level of the leaf surface. Such stomata are termed as sunken. In this condition water vapours diffusing through stomata accumulate in the pit where stoma is located. It reduces diffusion rate, and this helps in reducing the rate of transpiration.
Significance of transpiration
Transpiration is regarded as a “necessary evil” Curtis (1926) or unavoidable evil (Steward 1959) because it cannot be avoided though potentially harmful. The stomata remain open for exchange of gases which also results in the loss of water as vapours.
The loss of water leads to wilting, serious desiccation and often death of a plant. Despite the harmful effects, there are certain advantages of transpiration.
Advantages of transpiration:
- It helps in rapid upward movement of sap: The absorption of water and ascent of sap to various parts of the plant body is mostly due to transpiration. Transpiration pull in upward direction is responsible for mass movement of water in the upward direction.
- It helps in absorption and translocation of solute: Transpiration helps in the absorption of water and ascent of sap. The solute uptake is high in actively transpiring plants.
- It regulates plant temperature: Since transpiration involves the evaporation of water, it has a significant role in cooling of leaves.
Leaves also exchange infrared energy with their surroundings, both absorbing and radiating infrared radiation. But a leave radiates more infrared energy than it gains, thus there is a negative net infrared exchange.
One important method of dissipating the heat load is by evaporation of water from the leave surface or transpiration. Transpiration, in fact, accounts for dissipation of approximately half of the net radiation balance.
- Mechanical tissue: The increase in transpiration favours the development of mechanical tissue which is essential for providing rigidity and strength to the plant.
- Root system: Transpiration helps in better development of the root system.
- Quality of fruits: The ash and sugar contents of the fruits increase with the increase in transpiration.
- Resistance: Excessive transpiration induces hardening and resistance to moderate drought.
Disadvantages of transpiration:
- Wilting: The drooping of leaves due to loss of turgidity is termed as wilting. It occurs when transpiration exceeds the amount of water absorbed by the plant. Wilting or loss of turgidity is quite common during noon due to transpiration. Wilting reduces photosynthesis and other metabolic activities
- Reduced growth: Transpiration reduces availability of water inside the plant. Deficiency of water leads to decreased growth and hence, the plant gives a stunted appearance.
- Reduced yield: According to Tumarov (1925) a single wilting reduces yield by 50%. It is because decreased availability of water inside the plant checks meristematic activity and hence, the formation of flowers, fruits and seeds.
- Abscisic acid: Water stress produces abscisic acid which prevents several plant processes and promotes abscission of leaves, flowers and fruits.
- Wastage of energy: 98-99% of water is lost through transpiration. Hence, the energy used in absorption and conduction of water goes waste.