Transpiration and Type of Transpiration

Contents for Tables:-

• • Introduction

• • What is Transpiration?

• • Types of Transpiration

• • Factors affecting Transpiration

• • Significance of Transpiration

• • What is Plant anti- transpiration

• • Advantage of transpiration

• • Conclusion

Introduction

Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. It is a passive process that requires no energy expense by the plant. Transpiration also cools plants, changes osmotic pressure of cells, and enables mass flow of mineral nutrients. When water uptake by the roots is less than the water lost to the atmosphere by evaporation plants close small pores called stomata to decrease water loss, which slows down nutrient uptake and decreases CO₂ absorption from the atmosphere limiting metabolic processes, photosynthesis, and growth.

Like all living organism, plants also require an excretory system to discharge excess water from their body. This process of elimination of excess water from the plant body is known as transpiration. It is generally the evaporation of water from the surface of the leaves.

During the process of transpiration, water molecules in the plant tissues are removed from the aerial parts of the plants. Only a small amount of water absorbed by the plants is utilised in growth and development. The rest is eliminated in the form of transpiration.

What is Transpiration?

• • Transpiration is a fundamental physiological process in plants involving the movement of water from the plant’s interior to its exterior, primarily through the evaporation of water vapor from the aerial parts such as leaves, stems, and flowers. This process occurs passively, meaning it does not require energy expenditure from the plant.

• • During transpiration, water absorbed by the plant roots moves upward through the vascular system and eventually evaporates into the atmosphere. This evaporation predominantly takes place through tiny openings on the leaf surfaces known as stomata. Transpiration serves multiple functions: it cools the plant, adjusts osmotic pressure within cells, and facilitates the mass flow of essential mineral nutrients from the soil to the plant.

• • When water loss from transpiration exceeds water uptake by the roots, plants respond by closing their stomata to reduce water loss. This action, while conserving water, also slows down the uptake of nutrients and decreases carbon dioxide absorption. Consequently, the plant’s metabolic processes, including photosynthesis and growth, may be hindered.

• • Transpiration, therefore, is crucial for maintaining the plant’s internal water balance and nutrient distribution, playing a key role in its overall physiological health.

Types of Transpiration

Transpiration in plants occurs through several distinct pathways, each playing a specific role in water movement and evaporation. These pathways are classified into four primary types: stomatal, cuticular, lenticular, and bark transpiration.

1. 1. Stomatal Transpiration: This is the predominant form of transpiration, accounting for approximately 50-97% of the total transpiration in most plants. Stomatal transpiration takes place through stomata, which are small pores primarily located on the surfaces of leaves. Some stomata are also present on young stems, flowers, and fruits. The stomata facilitate the diffusion of water

• • vapor from the internal plant tissues to the atmosphere. The internal air of the plant becomes saturated with water vapor, and as the external air is typically less saturated, water vapor diffuses outward. This process continues as long as the stomata remain open.

• • Cuticular Transpiration: This type of transpiration occurs through the cuticle or epidermal cells covering the plant surfaces. Although cuticular transpiration generally contributes only 3-10% of the total transpiration in most plants, it can be more significant in herbaceous plants with thin cuticles, sometimes accounting for up to 50% of total transpiration. Cuticular transpiration happens continuously, both day and night, as the cuticle allows water vapor to escape directly from the plant’s surface.

• • Lenticular or Lenticellate Transpiration: This transpiration occurs through lenticels, which are specialized structures found in the woody branches of trees. Lenticular transpiration contributes a minimal amount, about 0.1% of the total transpiration. It is a continuous process because lenticels lack closure mechanisms, allowing for ongoing water vapor exchange between the atmospheric air and the plant’s cortical tissue through intercellular spaces.

• • Bark Transpiration: Bark transpiration involves the evaporation of water through the corky covering of stems. Although the rate of bark transpiration is relatively low, it is often higher than lenticular transpiration due to the larger surface area of the bark. Like cuticular and lenticular transpiration, bark transpiration occurs throughout both day and night, contributing to the plant’s overall water regulation.

Factors affecting Transpiration

Transpiration is influenced by a variety of factors that can be classified into two primary categories: external (environmental) and internal (structural) factors.

External or Environmental Factors

1. 1. Atmospheric Humidity
      1. High Humidity: When the atmospheric humidity is high, the rate of transpiration decreases. This occurs because the air is more saturated with moisture, reducing the diffusion gradient for water vapor from the plant’s internal tissues to the atmosphere.
      1. 1. Low Humidity: In contrast, low atmospheric humidity enhances transpiration rates as the air is less saturated with moisture, facilitating a more significant diffusion of water vapor.

• • Temperature
    
    • • Increased Temperature: Higher temperatures generally increase transpiration rates. This is due to the combined effects of lower relative humidity and the enlargement of stomatal apertures. Temperatures approaching 0°C often result in closed stomata, whereas temperatures up to 30°C encourage stomatal opening and increased transpiration.

• • Wind
    • Gentle Wind: Mild wind enhances transpiration by removing the moisture-laden air near the plant’s surface, thereby accelerating the diffusion of water vapor from the leaves.
    • • Strong Wind: Excessive wind can hinder transpiration by impeding the outward movement of water vapor and potentially causing stomatal closure.

• • Light
    
    • • Exposure to Light: Light increases the rate of transpiration as it promotes stomatal opening and raises leaf temperature. In darkness, stomata close, which significantly reduces transpiration.

• • Available Soil Water
    • Sufficient Water: Adequate soil water availability supports normal transpiration rates by ensuring efficient water absorption by the roots.
    • • Water Deficiency: Insufficient soil water leads to reduced transpiration, as the plant struggles to maintain water balance, impacting overall water uptake and distribution.

• • CO2 Concentration
    
    • • Increased CO2: Higher atmospheric or internal CO2 concentrations lead to stomatal closure, thereby reducing transpiration rates.

Factor	Description
Atmospheric Humidity	– High Humidity: Decreases transpiration due to reduced water vapor diffusion. – Low Humidity: Increases transpiration by facilitating vapor diffusion.
Temperature	– Higher temperatures increase transpiration by lowering relative humidity and enlarging stomatal apertures. – Near freezing temperatures may close stomata.
Wind	– Gentle Wind: Increases transpiration by removing moisture-laden air. – Strong Wind: May decrease transpiration by hindering water vapor diffusion and closing stomata.
Light	– Light increases transpiration by promoting stomatal opening and increasing leaf temperature. – Darkness reduces transpiration due to stomatal closure.
Available Soil Water	– Sufficient Water: Supports normal transpiration. – Deficiency: Reduces transpiration due to impaired water uptake.
CO2 Concentration	– Increased CO2: Causes stomatal closure, reducing transpiration.

Internal or Structural Factors

1. 1. Internal Water Conditions
      
      1. 1. Water Availability: Internal water conditions are crucial for transpiration. A shortage of water within the plant reduces the transpiration rate. Prolonged high transpiration rates can also create an internal water deficit if water absorption does not match the rate of loss.

1. 1. Leaf Area
      
      1. 1. Leaf Size: Larger leaf areas generally increase the magnitude of transpiration. However, smaller plants may transpire at a higher rate per unit area compared to larger plants.

1. 1. Leaf Structure
      1. Cuticle Thickness: The thickness of the cuticle can influence transpiration rates. A thicker cuticle generally reduces transpiration.
      1. 1. Epidermal Hairs: The density and thickness of epidermal hairs on the leaf surface can trap a layer of air, reducing transpiration by creating a barrier to water vapor loss.

1. 1. Stomata
      
      1. 1. Stomatal Characteristics: The number, distribution, size, and periodic opening of stomata affect transpiration rates. Xerophytes, for example, often have stomata that are open at night and closed during the day to conserve water, resulting in lower daytime transpiration.

1. 1. Leaf Orientation, Size, and Shape
      
      1. 1. Leaf Position: The angle of leaf orientation relative to sunlight affects transpiration. Leaves perpendicular to light absorb more heat, increasing transpiration, while parallel leaves experience less heat absorption.

• • Leaf Modifications: Reduced leaf size and specialized structures such as spines or thorns lower transpiration rates by minimizing surface area and exposure.

• • Root-Shoot Ratio
    
    • • Ratio Impact: An increased root-shoot ratio typically enhances transpiration efficiency. Plants like sorghum often transpire more per unit leaf surface compared to plants like corn.

• • Disease
    
    • • Effect of Disease: Diseased plants may exhibit higher transpiration rates compared to healthy plants, often due to compromised water regulation mechanisms.

Factor	Description
Internal Water Conditions	– Adequate Water: Essential for maintaining transpiration rates. – Water Deficit: Reduces transpiration due to internal water shortage.
Leaf Area	– Larger Leaves: Generally increase transpiration magnitude. – Smaller Plants: May transpire more per unit area compared to larger plants.
Leaf Structure	– Cuticle Thickness: Thicker cuticles reduce transpiration. – Epidermal Hairs: Increase thickness of the stationary air layer, reducing transpiration.
Stomata	– Number, Size, Distribution: Affect transpiration rates. – Xerophytes: Stomata often closed during the day to conserve water.
Leaf Orientation, Size, Shape	– Leaf Position: Perpendicular leaves absorb more heat and increase transpiration. – Reduced Leaf Size: Decreases transpiration rates.
Root-Shoot Ratio	– Higher Ratio: Typically increases transpiration efficiency. – Sorghum vs. Corn: Sorghum often has higher transpiration per unit leaf surface.
Disease	– Diseased Plants: May have higher transpiration rates due to compromised water regulation mechanisms.

Significance of Transpiration

Transpiration is a crucial physiological process in plants with several important functions. Its significance extends to various aspects of plant health and functionality, which can be categorized into the following key areas:

1. 1. Water and Mineral Absorption
      
      1. 1. Water Uptake: Transpiration facilitates the absorption of water from the soil by creating a negative pressure within the plant’s vascular system. This pressure draws water up from the roots to the leaves, where it evaporates.

• • Mineral Transport: The water flow driven by transpiration also aids in the movement of mineral salts dissolved in the soil solution, ensuring that essential nutrients reach different parts of the plant.

• • Sap Ascent and Nutrient Translocation
    
    • • Suction Force: The process of transpiration generates a suction force that assists in the ascent of sap through the xylem vessels. This force is critical for the efficient translocation of water and dissolved minerals from the roots to the leaves and other plant tissues.

• • Cell Turgidity and Osmosis
    • Maintaining Turgidity: Transpiration helps in maintaining the optimum turgidity of plant cells. Adequate turgor pressure is essential for cell structure and growth, as it supports the plant’s overall rigidity and shape.
    • • Osmotic Balance: The removal of water through transpiration also plays a role in proper osmosis within the cell sap, which is vital for nutrient balance and cellular function.

• • Excess Water Removal
    
    • • Water Regulation: Transpiration assists in the removal of excess water from the plant body. This process helps prevent waterlogging and maintains internal water balance.

• • Cooling Effect
    
    • • Temperature Regulation: The evaporation of water from plant surfaces has a cooling effect, reducing leaf temperature. This cooling effect is important for preventing heat stress and maintaining optimal metabolic processes within the plant.

• • Stomatal Function and Photosynthesis
    
    • • Stomatal Dynamics: Transpiration influences the opening and closing of stomata, which are crucial for gas exchange. This regulation impacts photosynthesis and respiration by controlling the availability of carbon dioxide and oxygen.

What is Plant anti-transpirants?

Anti-transpirants are substances applied to plants to reduce the rate of transpiration, thereby mitigating water loss and preventing wilting. This approach can be crucial in optimizing crop yields, particularly in environments where high transpiration rates could lead to reduced plant health and productivity. Anti-transpirants are classified into two main categories based on their mechanisms of action: metabolic inhibitors and film-forming agents.

1. Metabolic Inhibitors:

• • Phenylmercuric Acetate (PMA):
    
    • • PMA is a chemical compound that impedes the normal metabolic processes in plant cells, leading to a reduction in stomatal opening. This decreased stomatal aperture limits water loss through transpiration.

• • Abscisic Acid (ABA):
    
    • • ABA is a plant hormone known for its role in regulating stomatal closure. By promoting the closing of stomata, ABA effectively reduces water vapor loss and helps in conserving water within the plant.

• • Aspirin:
    
    • • Aspirin, primarily known as a medicinal compound, also functions as an anti-transpirant by influencing stomatal behavior. Its application can reduce the rate of transpiration by affecting the plant’s internal signaling pathways.

2. Film-Forming Anti-Transpirants:

• • Colorless Plastics:
    
    • • These materials form a thin, transparent layer on the plant’s surface. They restrict the loss of water vapor while still allowing the exchange of gases

• • such as oxygen (O₂) and carbon dioxide (CO₂), which are essential for photosynthesis.

• • Silicon Emulsions:
    
    • • Silicon-based anti-transpirants create a protective coating on plant surfaces. This coating prevents excessive water loss without significantly obstructing gas exchange, thus balancing water conservation with photosynthetic needs.

Application and Benefits:

• • Anti-transpirants can be particularly advantageous in crop production by helping to maintain optimal water levels in plants, even under conditions of high transpiration. This, in turn, can lead to improved plant health and higher yields.

• • Examples of film-forming anti-transpirants include silicon oil and various waxes, which provide an effective barrier against water loss while permitting necessary gas exchanges.

Advantages of transpiration

Transpiration, the process through which plants lose water vapor from their aerial parts, plays a crucial role in the overall physiology and functioning of plants. Below are the key advantages of transpiration:

1. 1. Water Absorption Enhancement:
      
      1. 1. Transpiration creates a negative pressure within the leaf, which contributes to the upward movement of water from the roots through the xylem vessels. This tension, often referred to as the “transpiration pull,” facilitates the continuous absorption of water from the soil by the roots.

1. 1. Ascent of Sap:
      
      1. 1. The tension created by transpiration is vital for the ascent of sap. As water evaporates from the leaf surfaces, it pulls more water upward from the roots through the xylem. This process ensures that water, along with essential nutrients and minerals absorbed by the roots, is effectively transported to various parts of the plant.

1. 1. Nutrient Distribution:
      
      1. 1. Transpiration is integral to the movement of water and dissolved minerals from the roots to the other parts of the plant. This upward movement ensures that all parts of the plant receive the necessary nutrients required for growth and development.

• • Cooling Effect:
    
    • • The evaporation of water from the leaves during transpiration has a cooling effect, which helps in regulating the temperature of the plant. This cooling mechanism is particularly important in preventing heat injury to the leaves during periods of high temperature and intense sunlight. The cooling also extends to the surrounding air, contributing to the microclimate around the plant.

• • Protection from Heat Injury:
    
    • • By cooling the leaves, transpiration plays a defensive role against potential heat damage. This is especially crucial in conditions where the plant is exposed to high temperatures, as it helps maintain the structural and functional integrity of the leaves.

Conclusion

Transpiration in plants is a crucial process. In the absence of transpiration, excess water will get accumulated in the plant cells, and the cells will eventually burst. More than 10% of the earth’s moisture is from transpiration. It is known to be a part of the water cycle.

Transpiration in plants occurs through several distinct pathways, each playing a specific role in water movement and evaporation. These pathways are classified into four primary types: stomatal, cuticular, lenticular, and bark transpiration.

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