Transpiration in plants | Definition, Types, Stomatal Movements, Factors affecting, Advantages

Transpiration is an important part of plant transport and is one of the most important topic for NEET. Let us do a quick revision of this topic.

Transpiration in plants | Definition, Types, Stomatal Movements, Factors affecting, Advantages

Plants use a unique mechanism to excrete or discharge the excess amount of water from their body i.e. transpiration. Firstly, we will discuss its definition. Further, we will discuss its type, movements, factors affecting transpiration and finally, its significance.

What is Transpiration?

Transpiration is the evaporative loss of water from living tissues of aerial parts of the plant. It occurs mainly through stomata. Plants lose around 95% of absorbed water during transpiration. As a result, it causes unnecessary loss of water. As a result, it is also called a necessary evil.

Transpiration process diagram
Transpiration process- labelled diagram.

Types of Transpiration

Transpiration in plants can be of three types, on the basis of the organ performing it, such as:

  1. Stomatal transpiration: It is the evaporative loss of water through stomata. Stomata are the pores found in epidermal regions of plants. In this case, 80-90% of the total water is lost.
  2. Cuticular transpiration: It is the loss of water through cuticle. Cuticle is the protective layer covering the epidermis of aerial plant organs. It accounts for 3-9% of water loss.
  3. Lenticular transpiration: It is the evaporation of water through lenticels. Lenticels are pores on the stem or bark of woody pants. In this case, 0.1-1% of water is lost.

Stomata

Stomata (singular: stoma) are pores present on the epidermis of leaves, stems and other organs. It is responsible for the exchange of gases. Besides, it also controls transpiration and the exchange of oxygen and carbon dioxide in the leaf.

Furthermore, the stomata are bordered with a pair of guard cells, which are responsible for opening and closing of stomata. The increase or decrease in turgidity within the two guard cells changes its shape. As a result of which, the stomata open and close.

stomata guard cells
Stomatal aperture with guard cells. [Source: NCERT]

Stomatal Movement

As discussed above, the two bean-shaped or kidney-shaped structures control the opening and closing of stomata. This opening and closing is known as stomatal movements. The guard cells contain chloroplasts and radial thickenings of cellulose microfibrils in their walls that face towards the stomatal aperture. The inner wall towards this aperture is thick and elastic.

Further, when the turgidity increases within the two guard cells, the outer walls bulge out and apply force on the inner walls. As a result, the inner walls form into a crescent shape. Consequently, the stomata open. When the turgidity decreases, the inner walls regain their original shape. Thus, the guard cells become flaccid. As a result, the stomata close.

In addition, the microfibrils aid the movements by radiating inwards or outwards according to the change in turgor pressure. Following is an aid to memory:

TURGIDITY IN STOMATA
AID TO MEMORY: Turgidity in guard cells and stomatal movements.

Factors Affecting Transpiration in plants

There are various external and internal factors affecting transpiration, such as:

External factors

  1. Temperature
    • Increase in temperature causes increase in transpiration.
  2. Light
    • Increase in light intensity causes increase in transpiration.
  3. Wind
    • Increased movement of air or wind causes increased transpiration.
  4. Soil water
    • Increase in soil water causes increase in transpiration.
  5. Relative humidity
    • Increase in relative humidity causes decrease in transpiration.

Internal factors

  1. Sunken stomata
    • Sunken stomata reduce the rate of transpiration.
  2. Thick cuticle
    • Thicker the cuticle, lower the rate of transpiration.
  3. Leaf surface area
    • Higher the surface area of leaves, higher is the transpiration rate.
  4. Mesophyll
    • Spongy mesophyll tissues should be packed loosely for efficient working of stomata.

Advantages of Transpiration

Finally, let us discuss the advantages of transpiration:

  1. It helps in upward movement of the water, i.e. ascent of sap.
  2. It aids in absorption of mineral salts along with its translocation.
  3. Cooling effect: The rapid evaporation through transpiration brings down the temperature. As a result, it prevents the plant from excessive heating. This is known as cooling effect.

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