What is transpiration?
The process by which moisture is carried through plants from roots to small pores on the underside of leaves, where it changes to vapor and is released to the atmosphere is called Transpiration. The loss of water in the form of vapours from the aerial parts of plants is known as transpiration. Transpiration is essentially evaporation of water from plant leaves. It is not a purely physical evaporation process, but a vital phenomenon.
Types of Transpiration
Transpiration takes place from all aerial parts of the plant. The following three types of transpiration have been recognized so far.
i) Cuticular transpiration
ii) Lenticular transpiration
iii) Stomatal transpiration
Loss of water from the surfaces of epidermal cells of leaves and herbaceous stems is said to be cuticular transpiration. Since surfaces of epidermal cells are often enclosed with a cutin layer, called cuticle, such transpiration is called cuticular transpiration. Cuticular transpiration accounts for up to 10% of the total water loss from the plant.
Water loss through lenticels, present in woody stems, is called lenticular transpiration. The magnitude of lenticular transpiration is very low due to limited distribution of lenticels. It accounts for less than 1% of total water loss by the plant.
Similar to the sweat glands on our skin, plants have openings on their leaves that allow water to escape, called stomata. Loss of water vapors through stomata is known as stomatal transpiration. About 80 to 90% of the total transpiration by the plant occurs through stomata.
Significance of Transpiration
Transpiration is a very vital process in an actively growing mesophytic plant. There are two conflicting views regarding the importance of transpiration in plant's life.
Transpiration is a necessary evil. Some plant physiologists believe that transpiration is a necessary evil as it does not produce any useful effect on the plant. On the contrary, it may produce water deficit and dehydration injuries in the plant. However, plants are unable to avoid this water loss. When stomata are open, exchange of CO2 and O2, there is no way to stop the loss of water vapors through them.
On the other hand transpiration is essential for the plant. There are others who consider that transpiration is in some ways essential for the plant. It performs the following three important functions.
Maintenance of turgidity
Transpiration keeps the pressure potential of cells well below their osmotic potential. Cells are thus never fully turgid. In an atmosphere saturated with water vapors, when there is no transpiration, the plants become softer. |
Promotion of uptake and translocation of solutes
In general, higher the rate of transpiration, greater is the rate of absorption of solutes from the soil. Transpiration also creates suction force which helps in the ascent of sap.
Reduction of leaf temperature
On a hot summer season day, the temperature of the leaf may rise up to an unbearable point when it is fatal for the plant. The loss of water produces a cooling effect on the leaf and is able to reduce the temperature by several degrees and safe the tree life.