Transpiration

Transpiration is the process by which plants lose water in the form of vapour through stomata in their leaves. It plays a crucial role in maintaining the water cycle and aids in nutrient transport, temperature regulation, and photosynthesis. This natural phenomenon is influenced by environmental factors like light, temperature, and humidity. In this article, transpiration, transpiration in plants, the process of transpiration, types of transpiration, factors affecting transpiration, transpiration and water relations in plants, and the importance of transpiration are discussed. Transpiration is a topic of the chapter Transport in Plants in Biology.

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This Story also Contains

1. Definition of Transpiration
2. Transpiration in Plants
3. The Process of Transpiration
4. Types of Transpiration
5. Factors Affecting Transpiration
6. Transpiration and Water Relations in Plants
7. Importance of Transpiration

Definition of Transpiration

Transpiration is when plants absorb water via their roots and transport it up the plant through the xylem, losing it as water vapour through small pores called stomata, mainly situated on the undersides of leaves. This cools the plant down while also being significant in nutrient uptake and turgor in plants.

Essentially, it is the mechanism of the plant for the balance of water and the rise of minerals upward from the soil. A continuous flow of water from roots to leaves makes it possible to scatter all the essential nutrients within plants promoting the development of growth.

Transpiration in Plants

Plants drive the uptake and distribution of minerals and nutrients from the soil through a process called transpiration. There is transpirational cooling through evaporative cooling to prevent overheating and maintain the optimum internal temperature of plants. Transpiration also maintains turgor pressure, which is responsible for maintaining the shape and rigidity of plant cells.

It throws light upon the entire elaborate mechanism of transpiration, covering its mechanisms, types, factors affecting it, and its significance. The pathways of water movement, the role of stomata, and the various adaptational strategies evolved in plants to effectively regulate transpiration have been discussed in detail here.

Also Read-

• MCQ Practice on Transpiration
• Transport in Plants
• Osmosis
• Diffusion

The Process of Transpiration

Transpiration is a rather complex, yet vital activity in plant biology, involving the movement of water from the soil to the plant and finally into the atmosphere. In explaining this process, one would need to trace the various mechanisms, pathways, and structures involved.

Mechanism of Transpiration

• Water evaporates from the cell walls of the spongy mesophyll into the intercellular spaces within the leaf.

• It is then diffused out to the atmosphere through stomata, moving down a concentration gradient.

Water Movement Pathways

• Apoplastic Pathway: Water passes through the cell walls and the intercellular spaces, not crossing any membranes.

• Symplastic Pathway: The water moves from cell to cell through the cytoplasm using plasmodesmata, crossing the cell membrane once.

Stomata

• Stomata are small pores on the surface of the leaf, which are surrounded by guard cells. Guard cells close or open the stomata depending on their shape.

• The guard cells open the stomata or close them by changing their turgor pressure by the gaining or losing of potassium ions.

• Usually, stomata are open during the day to photosynthesize but close at night to avoid excessive loss of water.

Types of Transpiration

Transpiration occurs through different pathways which all add up to result in loss of water from plants.

Stomatal Transpiration

• It is the primary pathway of water loss in most plants.

• It loses the maximum amount of water vapours from the plant.

• The opening and closing of the stomata by the guard cells strike a balance between the loss of water and the intake of carbon dioxide for photosynthesis.

Cuticular Transpiration

• It occurs through the cuticle: A waxy layer which covers the epidermis on leaves and stems.

• Less significant than stomatal: Typically contributes to a minor portion of total water loss.

• Increases when stomata are closed: Becomes more significant when stomata are closed, for example during drought conditions.

Lenticular Transpiration

• Happens through lenticels: Small openings in the bark of woody stems.

• Minor contribution to total transpiration: Participates a fairly small part, when contrasted with stomatal and cuticular transpiration

• Found in woody stems: Found in trees and bushes, which helps exchange gases.

Factors Affecting Transpiration

The rate of transpiration is affected by both internal and external factors. All these combined factors play an important role in ascertaining the amount of water loss.

Internal Factors

• A larger leaf area increases the surface through which water may be lost.

• Features that reduce transpiration include a thicker cuticle or trichomes, small hairs or other outgrowths on a leaf surface.

• An increase in the number of stomata might allow for increased transpiration rates; their overall distribution and regulation would factor into this process, however.

External Factors

• Light will increase transpiration by warming the leaf, hence opening the stomata.

• The higher the temperature, the greater the rate of evaporation and diffusion of water vapour.

• A lower relative humidity will steepen the concentration gradient outside the leaf and thus raise the rate of transpiration.

• Wind removes moist air lying immediately above the leaf surface and hence improves the diffusion gradient for water vapour.

• A good supply of soil water ensures a high rate of continued transpiration, but this is lowered in times of drought.

Transpiration and Water Relations in Plants

Understanding the relationship between transpiration and water movement in plants holds a central place in plant physiology.

Water Potential and Its Components

• Water flows from an area of higher to lower water potential.

• Solute potential, osmotic potential, pressure potential, and turgor pressure are its prime components.

Transpiration Stream

• The continuous flow of water from roots to leaves is driven by the evaporation of water from leaf surfaces.

• Differences in the water potential between soil, root, and atmosphere ensure upward water movement.

Cohesion-Tension Theory

• Water molecules stick together cohesion and to the walls of xylem vessel adhesion.

• Based on cohesive properties of water and tension created by evaporation.

• The pull from evaporation at the leaf surface creates a tension that draws water upward.

Role of Transpiration in Mineral Transport

• Facilitates upward movement of minerals.

• Dissolved nutrients are carried with the transpiration stream.

• Ensures delivery of all the essential nutrients.

• Critical for plant growth and development.

Importance of Transpiration

Transpiration is not simply a water loss process; it has various crucial functions essential for the survival of plants.

Cooling Effect

• Evaporative cooling: lowers the leaf temperature.

• Prevents overheating: protects against heat damage of enzymes and other cellular structures.

Nutrient Uptake

• Nutrient uptake in mineral form: Nutrients are dissolved in the soil water and are taken up by the roots.

• Transport to various parts of plants: Distributed in the plant through the transpiration stream.

Turgor Pressure

• Maintains turgidity of cells: Integrity and functions of the cells are maintained.

• Plant structure is maintained. Wilting, leading to drooping habit or prostrate habit of the plant, is avoided.

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