The loss of water from the living tissue of aerial parts of the plant in the form of water vapour is termed as transpiration.
Significance of Transpiration
Transpiration is a necessary evil. It is vital and unavoidable phenomenon of plants.
Transpiration is purely incidental due to the structural arrangement of plants for exit and entry of gases. It does not serve any good purpose. Millions of plants die due to excess loss of water during transpiration.
Advantages:
1. It creates suction force and helps in the ascent of sap.
2. It affects the diffusion pressure deficit, thereby indirectly helping diffusion through the cells.
3. It affects the absorption of water and minerals by roots.
4. It helps in evaporating excess amount of water.
5. It plays a necessary role in translocation of food from one portion of plant to the other.
6. It maintains suitable temperature for the leaves.
7. It brings about the opening and closing of stomata, which indirectly influence the process of photosynthesis and respiration.
Transpiration is necessary:
It is a process, which acts for energy dispersal, and it is considered that the chief function of transpiration is the dissipation of excess energy, which the plant receives from the sun.
Mechanism
Transpiration occurs in two stages:
1. evaporation of water from the cell walls into the intercellular spaces and
2. Diffusion of these water vapors of the intercellular spaces into the outside atmosphere.
Kinds of Transpiration
Cuticular Transpiration- Cuticle is a layer of wax like covering on the epidermis of leaves and herbaceous stem. Its thickness varies from plants to plants. Though it is meant to check transpiration, it is rarely completely impermeable due to some cracks in it. Upto 20% of the total transpiration may take place through it.
Lenticular Transpiration- Lenticels are area in the bark which are filled with loosely arranged cells known as complementary cells. The water vapour lost through lenticels amounts to about 0.1% of the total loss. It is quite negligible in comparison to loss by whole plant.
Stomatal Transpiration- Stomata are minute pores in the epidermis, their opening and closing being controlled by guard cells. Maximum diffusion of water vapour takes place through these pores The loss of water vapour through stomata amount to be 80-90 of the total loss.
Plant Anti-transpirants
In transpiration, almost 98 per cent of water absorbed by the plants is lost and only insignificant amount is being utilized by the plant for own purpose. Due to it, plant has to face several problems. If, somehow, this enormous loss of water can be reduced, it will be an asset to agriculturist and also to nature. In recent years, efforts on these lines have been made to find anti transpirants substances reducing the transpiration rate without adversely affecting exchange of gases during photosynthesis and respiration and plant growth.
Some examples of anti transpirants are colorless plastics, silicone oils, low-viscosity waxes, phenylmercuric acetate, abscissic acid, CO2 etc.
Factors affecting the rate of transpiration:
External or environmental factors- that include humidity of the air, temperature, wind velocity, light, water supply atmospheric pressure, sprays and dusts and vital activities.
Internal or structural factors- that include stomatal apparatus and its frequency, water content of mesophyll cells and structural peculiarities of the leaf.
