Transpiration

Transpiration is the loss of water from the aerial parts of the plant in the form of water vapour. Transpiration may be of the following types:

Stomatal

Transpiration taking place through the stomata is called stomatal transpiration.

Cuticular

The surface of the plant is covered by a thin layer of cuticle through which water vapour is lost during transpiration. This is called cuticular transpiration.

Lenticular

The process of loss of water vapour through the lenticels is called lenticular transpiration.

Of the three types, maximum transpiration is accounted through the stomata.

Stomata

Stomata are openings, generally on the under surface of the leaves. These openings are guarded by bean-shaped cells (dumb-bell shaped in grasses) called the guard cells. The walls of the guard cells facing the stomata are thickened. Due to an increase in the concentration of K⁺ ions inside the guard cells, the water potential reduces. This results in water entering the guard cells and they become turgid. The outer thin walls bulge which pulls the thicker inner walls apart, opening the stomata. For closure the reverse occurs - loss of K⁺ ions, increased water potential, exit of water and the cells become flaccid. This brings the inner walls close together and closes the stomata. Transpiration is maximum during the day, from morning to midday. This is the time stomata remain open. They close during the evenings and at night when the transpiration rate is low.

The Rate of Transpiration

The rate of transpiration is affected by many factors such as light, temperature, availability of soil water and atmospheric humidity.

The plants also show adaptations to reduce transpiration and thereby reduce water loss. For example: Desert plants may not have leaves (stem has photosynthetic tissue and stomata) or they may be reduced with thick cuticular covering.

Importance of Transpiration

• Transpiration is responsible for uptake of water from the soil.

• It is responsible for movement of water and dissolved minerals from the roots to different parts of the plant.

• It results in cooling of the leaf surfaces, thereby protecting them from excessive heat.