Factors Affecting Rate of Transpiration
The process of transpiration is influenced by a number of factors which may be broadly classified as External factors and Internal factors.
These include conditions of the environment which affect the rate of transpiration. The external factors are humidity, wind, atmospheric pressure, temperature, light and water.
Humidity refers to the amount of water vapour present in the atmosphere. If humidity is high, rate of evaporation is low and so the rate of transpiration is slow.
Wind is air in motion which enhances the rate of evaporation. Wind increases the rate of transpiration. But winds at high velocity bring about closure of stomata and thus reduce the transpiration rate.
Low atmospheric pressure increases the rate of transpiration. Water vapour from transpiring surfaces rapidly moves into the atmosphere which is at low pressure.
Increase in temperature increases the rate of transpiration as high temperature causes the water in intercellular spaces to vaporize at a faster rate.
Light influences opening of stomata and so rate of transpiration is high in light and less in darkness.
Less amount of soil water decreases the rate of transpiration. If the rate of transpiration exceeds the rate of absorption, the stomata get closed the cells lose their turgidity and the plant wilts. If the plant regains the turgidity, it will regain its original position and this is called incipient wilting. If the wilting is irreversible it is called permanent wilting.
These are factors prevailing within the plant which are inherent properties of the plant itself and include leaf structure, root-shoot ratio and age of plants.
In xerophytes, the rate of transpiration is reduced due to structural modifications such as less surface area, thick cuticle with hard and leathery surface, leaf rolling, sunken stomata, waxy coating, lower stomatal frequency, hairy covering and development of mechanical tissue. In the case of the plants such as Opuntia and Asparagus the leaf is modified into thorns and the stem becomes flattened and green to perform the function of the leaf. Such a structure is called a Cladode.
Root - Shoot Ratio
Transpiration shows a direct relation with the amount of water absorbed by the roots and the water lost through leaves. Therefore the increase in the root-shoot ratio will also increase the rate of transpiration.
Age of Plants
Germinating seeds generally show a slow rate of transpiration. It increases with age and becomes maximum at maturity. But rate of transpiration decreases during senescence.
Plants absorb water through the root system and only 2% is needed by the plant for the various metabolic activities. The rest of the water is lost through the aerial parts of the plant by a process called transpiration.
The loss of water in the form of vapour from the aerial parts of the plant is referred to as transpiration.
Types of Transpiration
Transpiration in plants is essentially of three types.
a. Cuticular Transpiration
Cuticular transpiration takes place through outer covering of the epidermis called cuticle made up of substance called cutin. Only a very little part of transpiration takes place by this process.
b. Lenticular Transpiration
Lenticels are regions on the bark having loosely arranged cells called complementary cells. A very little amount of water is lost by transpiration through lenticels.
c. Stomatal Transpiration
Stomata are minute openings on the epidermis of leaves and stems. Most of the water lost by transpiration (about 95%) takes place through the stomata.
Structure of Stoma
A stoma is a minute pore on the epidermis of aerial parts of plants through which exchange of gases and transpiration takes place.
Each stoma is surrounded by a pair of kidney shaped guard cells. Each guard cell is a modified epidermal cell showing a prominent nucleus, cytoplasm and plastids. The wall of the guard cell is differentially thickened. The inner wall of each guard cell facing the stoma is concave and is thick and rigid. The outer wall is convex and is thin and elastic.
The guard cells are surrounded by a variable number of epidermal cells called subsidiary cells.