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Understanding Transpiration and Guttation: Key Processes in Plant Water Regulation

byWiredufred Published:

 Transpiration

Transpiration is a vital process in plants that involves the movement of water from the roots to the leaves, where it evaporates into the atmosphere. This process not only helps in the transportation of water and nutrients but also plays a crucial role in regulating temperature and maintaining turgor pressure in plant cells. This article explores the mechanisms, significance, and factors affecting transpiration in plants.

understanding Transpiration


What is Transpiration?

Transpiration is the loss of water vapor from the aerial parts of plants, mainly through the stomata on the leaves. It is a continuous process that drives the upward movement of water and dissolved minerals from the roots to the leaves.


Types of Transpiration and Mechanisms of Transpiration

1. Stomatal Transpiration

Transpiration takes place through stomata in the leaves. About 90% to 95 % of transpiration of plant takes place through stomata. The stomata occur in the lower epidermis of a leaf bordered by guard cells. Water vapor escapes from the leaf into the atmosphere due to changes in the turgor pressure of the guard cells.

  • Stomata: Guard cells control the opening and closing of stomata, regulating the rate of transpiration.

2. Cuticular Transpiration

There are Cutine coatings in young stems and leaves called Cuticle. At the cuticle water vapor diffuses across the cuticle of the upper epidermis of the leaf into the atmosphere along diffusion gradients.

  • Cuticle: Although it acts as a barrier to water loss, some water can still escape through it.

3. Lenticular Transpiration

Due to secondary growth in stem, small pores are developed by rupturing of the epidermal layer. These pores are called Lenticels. The cells of lenticel are loosely packed. Some portion of transpiration takes place through these lenticels.

  • Lenticels: Facilitate gas exchange and transpiration in stems.

Importance of Transpiration

  1. Water and Nutrient Transport

Transpiration creates a negative pressure in the leaf, pulling water and dissolved minerals from the roots through the xylem vessels.

  • Xylem Transport: Essential for nutrient distribution and plant growth.
  1. Temperature Regulation

Evaporation of water from the leaf surface helps cool the plant, preventing overheating.

  • Cooling Mechanism: Transpiration acts as a cooling system for plants.
  1. Maintaining Turgor Pressure

Transpiration helps maintain turgor pressure, which is crucial for maintaining the structure and rigidity of plant cells.

  • Turgor Pressure: Provides structural support to plants.
  1. Supplying Water for Photosynthesis

Transpiration brings water to the mesophyll cells in the leaves, which is essential for photosynthesis.

  • Photosynthesis: Requires water as a key reactant.
  1. Removal of Excess Water

Transpiration helps in the removal of excess water from the plant, preventing waterlogging and maintaining optimal internal conditions.

  1. Enhancing CO2 Absorption

Transpiration ensures that the walls of the spongy cells in the leaf are continuously wet, enhancing the absorption of carbon dioxide necessary for photosynthesis.

  • Carbon Dioxide Absorption: Critical for photosynthesis efficiency.


Disadvantages of Transpiration

While transpiration is essential for plant health, it also has some disadvantages:

  1. Water Loss

Excessive water loss through transpiration can lead to dehydration, especially in arid environments where water is scarce.

  • Dehydration Risk: Plants may suffer from water stress due to high transpiration rates.
  1. Energy Expenditure

The process of transpiration requires energy for the opening and closing of stomata and the transportation of water through the plant.

  • Energy Costs: Managing water loss involves significant energy expenditure by the plant.
  1. Nutrient Loss

Rapid transpiration can lead to the loss of essential nutrients, especially in nutrient-poor soils.

  • Nutrient Depletion: High transpiration rates can deplete soil nutrients quickly..


Structural Adaptations of Plants to Reduce Transpiration

1.   Curled up leaves, so it creates a humid environment around the leaf, hence less transpiration occurs.

2.  Presence of hairs and scales on the surface of the leaves, to trap escaping water molecules, to reduce the rate of transpiration.

3. Sunken stomata; stomata of some leaves e.g.  xerophytes are sunk deep into the surface to reduce water loss. 

4.     Having stomata closed during the day time.

5.  Leaf spines: Some plants have spines instead of leaves. Spines usually have thicker cuticles and a very small surface area, which decreases transpiration.

6.   Some plants that occur in dry places have a thick cuticle that reduces transpiration.

7.   Reduction of leaf size: small leaves have a smaller surface area for transpiration to occur.


Factors Affecting the Rate of Transpiration  

1. Environmental Factors

  • Temperature: Higher temperatures increase the rate of transpiration by causing more rapid evaporation of water.  At low temperatures, the air around the leaf gets saturated and this lowers the transpiration rate.

  • Humidity: Low humidity levels create a steeper gradient for water vapor, enhancing transpiration, while high humidity reduces it.

  • Wind: Wind removes the water vapor around the stomata, increasing the rate of transpiration.

  • Light Intensity: Light affects transpiration because stomata usually open in the light and closes in darkness. As the light intensity increases the degree of opening of stomata also increases, hence transpiration also increase.

2. Plant Factors

  • Leaf Area: Larger leaves have more stomata, leading to higher transpiration rates also depends upon the surface area of leaf. More surface area provides more stomata and there is more transpiration.

  • Stomatal Density: More stomata per unit area can increase the rate of transpiration. The more the stomata, the faster the rate of transpiration. Plants with more stomata on the upper surfaces of the leaves transpire at a higher rate.

  • Cuticle Thickness: Thicker cuticles reduce water loss through cuticular transpiration.

  • Internal surface of leaf: thin cuticle, thin cell walls, exposed stomata, and well-developed spongy parenchyma favor transpiration. On the other hand, leaves with thick cuticle, thick cell walls, well-developed palisade, sunken stomata etc. will have reduced transpiration rate.


Measurement of the Rate of Transpiration

Weighing method

1.   A potted plant is well watered and the pot is enclosed within a polythene bag to prevent direct evaporation of water from the soil.

2.       It is then weighed and the mass is recorded.

3.       The plant is taken outside for a few hours after which it is weighed again.

4.      The difference in weight represents the amount of water lost through transpiration.

5.     The rate of transpiration is then calculated as the amount of water lost per unit time.  

Measurement of the Rate of Transpiration

Potometer 

Potometer is the instrument used to measure the rate of water uptake in a plant. When a Potometer is used to calculate the rate of transpiration, it is assumed that all the water taken up is lost by transpiration. It measures how factors such as light, temperature, humidity, light intensity and wind will affect the rate of transpiration.


Potometer Experiments 

Procedure

1.   Use a sharp razor blade to cut a leafy shoot under water.
2.    Insert the leafy shoot through the hole of the stopper provided with the potometer.
3.   Fill the potometer with water and fit the stopper holding the leafy shoot to the apparatus.
4.  Use Vaseline to seal all the connections of the apparatus.
5.  Trap an air bubble in the capillary tube by the following procedures:
     a.   dip the end of the capillary tube into a beaker of water,
     b   close the tap of the reservoir,
     c   take away the beaker of water and allow the plant to transpire for a while, and
     d.   re-immerse the capillary tube into the beaker of water again

6  Estimate the rate of transpiration by measuring the distance moved by the air bubble per unit time.

7. Transpiration rate can be expressed in terms of water transpired per unit time per unit area of leaf surface.

 

Transpiration rate bubble potometer

Potometer

Limitations of Potometer

1. It only measures the rate of water uptake, rather than the actual transpiration rate.

2. It uses a cut shoot rather than a whole plant. Twig may receive shocks and may not function normally.

3. The presences of bubbles may prevent continuous flow of water.


2. Lysimeter

A lysimeter is an essential tool used in environmental science and agricultural research to measure the amount of water lost by plants through transpiration and soil evaporation. This device provides valuable data that helps scientists understand the water needs of plants, the efficiency of irrigation practices, and the impact of different soil types on water retention and usage.

How Does a Lysimeter Work?

A lysimeter typically consists of a large container filled with soil, in which plants are grown under natural conditions. The container is placed on a weighing scale that measures the weight loss due to water evaporation and plant transpiration. By monitoring the weight changes over time, researchers can determine the rate of water usage by the plant and the soil.

  • Weighing Mechanism: The container is periodically weighed to record the loss of water through evapotranspiration.
  • Soil and Plant Monitoring: It mimics natural soil conditions and plant growth, providing accurate field data.

Types of Lysimeters

  1. Weighing Lysimeters

    • Description: These are the most accurate and widely used lysimeters. They directly measure the weight loss of the soil-plant system.
    • Applications: Ideal for detailed studies on water balance, crop water use, and irrigation management.

  2. Non-Weighing Lysimeters

    • Description: These lysimeters do not measure weight loss directly but instead collect leachate from the soil, which is then analyzed to determine water usage.
    • Applications: Useful in studying soil leaching and nutrient loss, particularly in agricultural research.

Applications of Lysimeters

  1. Agricultural Research

    • Irrigation Management: Lysimeters help optimize irrigation practices by providing accurate data on crop water requirements.
    • Crop Water Use Efficiency: They are used to study how efficiently different crops use water under various conditions.

  2. Soil Science

    • Soil Water Retention: Researchers use lysimeters to understand how different soil types retain and release water, which is crucial for soil health and plant growth.
    • Nutrient Leaching: Lysimeters help track the movement of nutrients through the soil, aiding in the development of sustainable farming practices.

  3. Environmental Monitoring

    • Water Cycle Studies: Lysimeters contribute to understanding the water cycle by measuring evapotranspiration rates in different ecosystems.
    • Climate Change Research: Data from lysimeters helps predict how changes in climate affect water usage and availability in various regions.

Advantages of Using Lysimeters

  • Accuracy: Weighing lysimeters provide precise measurements of water loss, making them invaluable for research.
  • Realistic Conditions: Lysimeters simulate natural growing conditions, ensuring that the data collected is relevant to real-world scenarios.
  • Versatility: They can be used for a wide range of studies, from agricultural practices to environmental monitoring.

Challenges and Considerations

  • Cost and Maintenance: Setting up and maintaining lysimeters can be expensive and labor-intensive.
  • Data Interpretation: Analyzing the data from lysimeters requires careful consideration of various factors, including plant type, soil properties, and environmental conditions.
  • Lysimeter Studies: Useful for field research on plant water use.

Discover Lysimeter Research on Frontiers in Plant Science.


Practical Applications of Transpiration

  1. Agriculture

Understanding transpiration helps in optimizing irrigation practices and improving water use efficiency in crops.

  • Irrigation Management: Enhances crop productivity by regulating water supply.
  1. Horticulture

Transpiration knowledge aids in designing greenhouses and selecting plant species suitable for specific environments.

  • Greenhouse Management: Regulates humidity and temperature to control transpiration.
  1. Climate Research

Transpiration plays a significant role in the water cycle and can influence local and global climate patterns.

  • Climate Studies: Transpiration contributes to atmospheric moisture and climate regulation.

Experiment to Demonstrate Transpiration

Aim: To show that water vapor is given off during transpiration

Apparatus: A potted plant, 2 bell-jars, polythene bag, anhydrous copper sulphate


Procedure

1.   Take a potted plant and cover the pot and base of stem with polythene bag.

2.  Place the potted plant on a glass plate and invert a dry bell-jar over it. 

3. Leave the apparatus in sunlight.

4. Set up a control experiment with no plant.

Experiment to show that water vapor is given off during transpiration

Observation

After an hour, drops of colorless liquid are seen inside the bell-jar with the plant. To show that these drops are water, touch them with anhydrous copper sulphate (white) and its color changes to blue. No drops of water are found in the control experiment.

Conclusion

The water droplets on the inside of the jar containing the plant came from the leaves because the rest of the plant body and the soil were covered with polythene bag. Thus the potted plant present in the bell-jar showed the phenomenon of transpiration.


Experiment to Show that Leaves have more Stomata on their Lower Surfaces

Aim: To show that there is more transpiration from the lower leaf surface as compared to the upper.


Experiment to Show that Leaves have more Stomata on their Lower Surfaces


Procedure

1.  Take a potted plant. Water the plant and leave it for an hour.

2.  Take two equal size cobalt chloride papers and with the help of forceps place one on the upper surface and the other on the lower surface of a leaf.

3.  Place dry glass sides on the upper and the lower cobalt chloride papers and fix them with a rubber band. (The glass slides will prevent the cobalt chloride papers to come in contact with atmospheric humidity.)

4.  Note changes in the color of the two cobalt chloride papers.



Guttation in Plants: An In-Depth Look

Introduction

Guttation is a fascinating phenomenon in plants where water droplets are exuded from the tips or edges of leaves. This process occurs primarily during the night or early morning when soil moisture is high, and transpiration rates are low. Understanding guttation helps in gaining insights into plant physiology, water regulation, and overall plant health.


What is Guttation?

Guttation is the secretion of water droplets from the edges or tips of leaves through specialized structures called hydathodes or water glands. Unlike dew, which forms from atmospheric moisture condensing on the leaf surface, guttation is the result of root pressure forcing water up through the plant.

  • Hydathodes: Specialized structures located at the leaf margins where guttation occurs.


Mechanism of Guttation

1. Root Pressure

Root pressure is a key driver of guttation. When soil moisture is high, roots absorb excess water, creating positive pressure that pushes water upwards through the xylem vessels.

  • Root Pressure Explained: Essential for understanding how water moves in plants.

2. Water Movement

Water moves through the plant’s vascular system and is exuded from the hydathodes when it reaches the leaf edges or tips.

  • Vascular System: Facilitates the movement of water and nutrients.


Factors Influencing Guttation

  1. Soil Moisture

    • High soil moisture levels increase root pressure, promoting guttation.
    • Soil Moisture's Role: Crucial for water regulation in plants.

  2. Transpiration Rates

    • Low transpiration rates, typically during the night, enhance guttation as less water is lost through evaporation.
    • Transpiration and Guttation: Understand their interplay.

  3. Plant Species

    • Some plant species are more prone to guttation than others, depending on their physiology.
    • Species-Specific Guttation: Learn which plants exhibit guttation.


Importance of Guttation

  1. Water Regulation

    • Helps in expelling excess water, maintaining internal water balance in plants.
    • Internal Water Balance: Key to plant health.

  2. Nutrient Transport

    • Guttation aids in the transport of nutrients dissolved in water from the roots to the leaves.
    • Nutrient Transport Mechanisms: Crucial for growth and development.

  3. Pathogen Defense

    • Guttation droplets can contain secondary metabolites that deter pathogens and herbivores.
    • Defense Mechanism: Protects plants from external threats.

Observing Guttation

Guttation is most commonly observed in the early morning when transpiration is minimal. The droplets are often mistaken for dew, but unlike dew, guttation droplets contain dissolved minerals and organic compounds.

  • Identifying Guttation: Learn how to distinguish between dew and guttation.


Potential Issues Related to Guttation

  1. Fungal Growth

    • Excessive guttation can create a moist environment conducive to fungal growth.
    • Fungal Risks: Important for plant disease management.

  2. Nutrient Loss

    • While guttation helps in nutrient transport, it can also lead to the loss of essential nutrients if the droplets fall away from the plant.
    • Nutrient Efficiency: Balancing the benefits and drawbacks.

Conditions for Guttation

Guttation in plants occurs under specific environmental and physiological conditions that promote the buildup of root pressure. Understanding these conditions helps in recognizing when and why guttation happens, offering insights into plant water regulation and health.

1. High Soil Moisture

High soil moisture levels are a primary condition for guttation. When the soil is saturated with water, roots absorb this excess moisture, creating positive pressure within the plant’s vascular system. This pressure pushes water up through the xylem vessels to the leaves, where it is exuded through hydathodes.

  • Effect of Soil Moisture: Increased soil moisture directly correlates with higher rates of guttation.

2. Low Transpiration Rates

Guttation typically occurs when transpiration rates are low, usually during the night or early morning. Transpiration is the process by which water vapor is lost from the plant through stomata. At night, stomata are usually closed to conserve water, reducing transpiration and allowing root pressure to build up, leading to guttation.

  • Night-Time Conditions: Lower temperatures and higher humidity at night contribute to reduced transpiration.

High humidity levels reduce the rate of transpiration by decreasing the water vapor gradient between the inside of the leaf and the external environment. This condition promotes guttation by allowing the root pressure to build without being offset by water loss through transpiration.

  • Humidity's Impact: High humidity helps maintain the balance required for guttation.

4. Plant Species and Leaf Structure

Certain plant species are more prone to guttation due to their specific physiological characteristics and leaf structures. Plants with well-developed hydathodes at the leaf margins, such as grasses and some herbaceous plants, are more likely to exhibit guttation.

5. Soil Composition and Fertility

Soil composition and fertility also play a role in guttation. Soils that retain moisture well, such as loamy or clay soils, provide the conditions necessary for sustained root pressure. Additionally, nutrient-rich soils can enhance the guttation process as plants uptake water along with dissolved nutrients.


Differences between Guttation and Transpiration

Guttation

Transpiration

Water is loss in liquid state

Water is loss in gaseous state

Occurs only through large specialized pores in leaves called hydrothodes

Occurs through the stomata, lenticel and cuticle

 Hydrothodes remain open all the time

Stomata opens during the day


Similarities between Guttation and Sweating

1.  Both processes involve loss of water

2.   Both result in cooling

4.  The rate of both is affected by environmental condition
5.     Evaporation occurs in both processes

6.   Water is loss through pores


Difference between Transpiration and Sweating

Transpiration

Sweating

Occurs in plants through stomata, lenticel and cuticle

Occurs in mammal through sweat pores in the skin

Involves only loss of water

Involve loss of water, salt and nitrogenous waste

Water is lost in the form of vapor

Water is lost in liquid form

Occur during the day

Occur in the day and night

No glands involved

Special glands are involved



Conclusion

Transpiration is a critical process that supports water and nutrient transport, temperature regulation, and structural integrity in plants. By understanding the mechanisms and factors influencing transpiration, we can better manage plant health and optimize agricultural practices. For further reading on plant physiology and transpiration, visit Plant Physiology on Frontiers in Plant Science.


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