Transpiration

Definition and Overview

Transpiration is the process by which water is transported from the roots to the leaves in plants and then out into the atmosphere.
It contributes to the upward movement of water and minerals through the xylem vessels.
Transpiration is a passive process mainly driven by the evaporation of water from the leaves.

Process

Water is absorbed by root hair cells and moves across the root cortex to the xylem vessels.
Driven by the cohesive and adhesive properties of water, it moves upward through the xylem vessels in the stem.
Upon reaching the leaves, water evaporates from the cell walls into the air spaces within the leaf. This is referred to as transpiration pull.
The evaporation causes the pressure in the xylem vessels to decrease. As a result, more water is drawn from the roots, maintaining a continuous flow.

Role of Stomata

Stomata, small openings on the leaf surface, play a crucial role in transpiration.
Evaporated water exits the leaf through stomata in the form of water vapour.
Control of stomatal opening and closing, through the guard cells, regulates the rate of transpiration.

Factors Affecting Transpiration

Light intensity: More stomata open in the presence of light, increasing transpiration.
Air temperature: Higher temperatures increase the rate of evaporation and thus transpiration.
Wind speed: Wind removes water vapour from around the leaf, driving more evaporation.
Humidity: Lower humidity creates a larger concentration gradient, increasing transpiration rate.
Water availability: In conditions of water scarcity, plants may close stomata to prevent excess water loss.

Importance of Transpiration

It helps transport water and minerals from roots to different parts of the plant.
Transpiration aids nutrient absorption as water uptake from the soil is enhanced.
The evaporative cooling effect of transpiration can help to regulate the plant’s temperature.
Transpiration helps maintain the plant’s turgidity and structure.
It drives the flow of water carrying hormones and other dissolved substances within the plant.