Transpirational Pull

What is Transpiration Pull ?

Transpiration is the process by which plants lose water through small pores called Stomata on their leaves. While Transpiration is essential for plants to maintain their water balance and grow, it also poses a risk of dehydration in arid environments.

Transpiration is driven by a phenomenon called Transpiration Pull, which is the force responsible for moving water from the roots to the leaves of a plant. This pull is caused by the evaporation of water from the leaves, which creates a negative pressure gradient in the plant. As water is lost from the leaves, the negative pressure gradient draws water up from the roots through the Xylem tissue. This movement of water is essential for transporting nutrients and maintaining cell turgor pressure in the plant.

In this article, we will provide a comprehensive overview of Transpiration Pull in plants. We will define Transpiration and explain why it is important for plant survival. We will also discuss the phenomenon of Transpiration Pull and its role in moving water from the roots to the leaves of a plant. Finally we will explore the impact of environmental factors on Transpiration Pull and Plant Function.

What is Transpiration?

Transpiration is the process by which plants lose water vapor from their leaves and other aerial parts. It is a passive process that occurs as a result of water evaporation from the surfaces of plant tissues. Transpiration  is a natural and essential process for plant growth and survival, as water is required for various physiological processes, including photosynthesis, cell expansion and nutrient uptake. Transpiration is one of the primary ways in which water moves through the plant, from the roots to the leaves and then into the atmosphere. It also regulates their internal water balance and temperature.

How does Transpiration occur in plants?

Transpiration occurs mainly through the Stomata, which are tiny pores found on the surface of leaves and stems. Stomata are surrounded by guard cells, which can open and close to regulate the rate of water loss from the plant. When the Stomata are open water vapour diffuses out of the leaf and into the surrounding air, creating a negative pressure gradient that pulls water up from the roots through the Xylem tissue.

Types of Transpiration

There are three types of Transpiration

Cuticular Transpiration

Cuticular Transpiration occurs through the waxy cuticle layer covering the epidermis of leaves, stems and other above-ground plant parts. This type of Transpiration accounts for only a small fraction of total water loss in most plants.

Stomatal Transpiration 

Stomatal Transpiration is the most important form of Transpiration in plants. It occurs through the Stomata, small openings on the leaf surface that allow for gas exchange. Water is lost from the plant when the Stomata opens to release excess water vapour.

Lenticular Transpiration

Lenticular Transpiration occurs through the small openings in the bark of woody stems, known as lenticels. This type of Transpiration is less significant than the other two and is mainly limited to woody plants.

Factors affecting Transpiration rate

The rate of Transpiration is influenced by various environmental factors they are as follows – 

  1. Temperature – As temperature increases, so does the rate of Transpiration. This is because higher temperatures increase the rate of evaporation of water from the plant’s surface.
  2. Humidity –  High humidity reduces the rate of Transpiration, as there is less of a concentration gradient between the plant and the surrounding air.
  3. Wind – Wind increases the rate of Transpiration by removing the water vapor surrounding the plant and replacing it with drier air.
  4. Light intensity – Light stimulates the opening of Stomata, leading to increased Transpiration rates.
  5. Soil water availability – Plants require a sufficient amount of water in the soil to maintain Transpiration rates. If soil water availability is limited, Transpiration rates will decrease.

Also Check – 10 Important Factors that Affect the Rate of Transpiration

Transpiration Pull in Plants

Transpiration Pull is the process by which water is transported from the roots to the leaves of a plant against gravity. This process is driven by a combination of physical forces that work together to create a negative pressure gradient or tension that pulls water up through the plant’s vascular system.

Transpiration Pull is driven by a combination of physical forces and the plant’s anatomy, specifically the cohesion-tension theory.

How Transpirational Pull Works

What is Transpiration pull
Transpiration pull

The process of Transpiration Pull begins with the loss of water from the plant through the Stomata on the leaves. As water evaporates from the plant, it creates a negative pressure gradient, or tension, that pulls water up from the roots. This negative pressure gradient is created by a combination of forces, including adhesion, cohesion and tension.

Adhesion refers to the attractive forces between water molecules and the surfaces of the plant’s Xylem vessels. Cohesion refers to the attractive forces between water molecules themselves. Together, these forces help water to form a continuous column within the Xylem vessels, allowing it to be transported through the plant.

Tension is the force created by the negative pressure gradient, which pulls water up through the plant against the force of gravity. The strength of this tension is determined by the rate of Transpiration, which is regulated by the Stomata on the leaves.

Role of Stomata

Stomata play a crucial role in regulating the rate of Transpiration and therefore the strength of the Transpiration Pull. When the plant needs to release excess water, the guard cells surrounding the Stomata open, allowing water vapour to escape. This release of water vapour increases the tension within the Xylem vessels, which in turn strengthens the Transpiration Pull.

Stomata also play a role in limiting water loss from the plant. They can conserve water during periods of drought or high temperatures, reducing the rate of Transpiration and the strength of the Transpiration Pull.

Forces contributing to Transpiration Pull

Transpiration Pull is created by a combination of physical forces –  Adhesion, Cohesion and Tension. 

Adhesion and cohesion work together to create a continuous column of water within the Xylem vessels, while tension pulls water up through the plant against gravity.

Other factors that can affect the strength of the Transpiration Pull include the rate of Transpiration, the availability of water in the soil and the environmental conditions surrounding the plant.

Also Check – Why is Transpiration Important for Plants

The Role of Transpirational Pull in Plants

Transpiration Pull is an essential process for plants to maintain their water balance and survive in their environment.Following are few role  of Transpiration Pull in Plants – 

Maintaining Water Balance

Water is essential for plants to carry out photosynthesis, transport nutrients and maintain their structure. Plants are constantly losing water through Transpiration and they must replace this water to avoid dehydration.

Transpiration Pull helps plants maintain their water balance by pulling water from the soil and distributing it throughout the plant. As water is lost through the leaves, the Transpiration Pull created by the tension within the Xylem vessels draws water up from the roots, replenishing the plant’s water supply.

Nutrient Transport and Plant Growth

Transpiration Pull also plays a critical role in nutrient transport and plant growth. As water is transported through the plant. It carries with it essential nutrients such as nitrogen, phosphorus and potassium.

These nutrients are absorbed by the roots and transported through the plant along with the water. As the water is lost through Transpiration, the nutrients are left behind, allowing the plant to use them for growth and development.

Environmental Factors

The rate of Transpiration and the strength of the Transpiration Pull can be influenced by a range of environmental factors, including temperature, humidity, wind and soil moisture.

High temperatures and low humidity can increase the rate of Transpiration, creating a stronger Transpiration Pull. This can be beneficial for the plant in certain circumstances, such as during periods of water stress when the plant needs to pull water from the soil to maintain its water balance.

Environmental stressors can also have negative impacts on plant function. For example – high temperatures and low humidity can cause excessive water loss through Transpiration, leading to dehydration and reduced growth. Wind can also increase water loss through Transpiration.

Also Check – Transpiration- Its Role in the Hydrologic Cycle

Transpirational Pull Theory

According to Root Pressure Theory, the roots absorb water and exert a pressure, the root pressure, which pushes the water upward. The root pressure develops in the tracheary element of xylem (i.e., tracheids and vessels) as a result of metabolic activities of roots. The root pressure theory is applicable in small herbs but not in tall trees.

The major events of upward movement of water and minerals are as follows:

  1. The water filled in xylem tracheids and vessels is not pure. It has several mineral elements dissolved in it. The minerals needed by plants are absorbed from the soil through root hairs and other epidermal cells of root tip. They are taken up in the inorganic form, such as nitrates, phosphates, etc.
  2. The water and minerals, absorbed by root hair and other epidermal cells, move through the root cortex, endodermis, pericycle and reach the xylem.
  3. Water and minerals (i.e., the sap) move upward in the tracheids and vessels of xylem. Vessels are found in the xylem of only flowering plants. That means, the flowering plants (Angiosperms) possess both tracheids and vessels. The non-flowering plants (such as Cycas, fern, etc.) possess only tracheids which serve as the main conducting tissues.
  4. Xylem tracheids and vessels are dead cells and thick-walled. The tracheids are long, thin, spindle-shaped cells with tapering ends. They have pits in their thick cell walls through which the water flows from one tracheid to another. The xylem vessels are tubular structures extending from roots to the top of the plants.
  5. The cells are placed one above the other with their end walls perforated forming a continuous tube. Thus, the lumen of tracheids and vessels are filled with sap and this sap moves upward from one tracheid to another and from one vessel to another through pits and perforations. 
  6. The xylem of the root is connected with that of stem, petiole and veins of leaves. All these cells make a continuous network of tubes through which the sap can move from roots to the leaves.
  7. The water and minerals (sap) filled inside the xylem capillaries forms a continuous column which cannot be broken or pulled away from the xylem walls. Thus, the unbroken water column is just like steel rope which extends from leaves to the roots. If this column is pulled from the top, the entire rope of sap moves upward. In plants, the pull is generated by the process known as transpiration.
  8. Loss of water in the form of vapors through transpiration is replenished by movement of fresh supply of water to the leaf. This creates a suction which pulls the water column (sap) upward.

Cohesion Hypothesis

The Cohesion Hypothesis also known as the Transpiration-Cohesion Hypothesis explains the ascent of cell sap in living vascular plants. The theory was proposed by Botanists Dixon and Joly in 1894 and later by Askenasy in 1895 and has been widely supported by Renner (1911 & 1915), Curtis and Clark (1951), Bonner and Galston (1952) and Gramer and Kozlowski (1960). 

According to the theory the Transpirational Pull is responsible for the movement of water in trees due to continuous columns of water in the Xylem vessels which run through the entire length of the plant from roots to leaves.

Cohesion theory suggests that water and their unique interaction with the walls of the Xylem vessels give rise to the strong pull needed to transport water against the gravitational forces. On a molecular level, cohesive and adhesive properties of water interact with the walls of the Xylem vessels, forming a thin column of water with a tensile strength of around 30 atmospheres or 440 pounds per square inch of the area. This strong force can lift a water column without breaking, thus allowing water to be transported against gravity to the higher leaves of large plants.

It is important to note that although the theory was undisputed for a long time in botanical history it is now known that there are other underlying mechanisms that contribute to water transport. The Transpirational Pull or Cohesion-Tension theory is not exclusively applicable for water and mineral transportation in all vascular plants of all species. Cohesive forces and Transpiration Pull are responsible only for the maintenance of cell sap.

Frequently asked Questions on Transpirational Pull

  1. What is Transpiration Pull in Plants?

Answer – Transpiration Pull is the process by which water is transported from the roots to the leaves of a plant against gravity.

  1. What physical forces drive Transpiration Pull in Plants?

Answer – Transpiration Pull is driven by a combination of physical forces that work together to create a negative pressure gradient or tension that pulls water up through the plant’s vascular system. These forces include adhesion, cohesion and tension.

  1. What is the role of Stomata in regulating Transpiration Pull?

Answer – Stomata play a crucial role in regulating the rate of Transpiration and therefore the strength of the Transpiration Pull. When the plant needs to release excess water, the guard cells surrounding the Stomata open, allowing water vapour to escape. This release of water vapour increases the tension within the Xylem vessels, which in turn strengthens the Transpiration Pull.

  1. How does Transpiration Pull help plants maintain their water balance?

Answer – Transpiration Pull helps plants maintain their water balance by pulling water from the soil and distributing it throughout the plant. As water is lost through the leaves, the Transpiration Pull created by the tension within the Xylem vessels draws water up from the roots, replenishing the plant’s water supply.

  1. What is the role of Transpiration Pull in nutrient transport and plant growth?

Answer – Transpiration Pull plays a critical role in nutrient transport and plant growth. As water is transported through the plant, it carries with it essential nutrients such as nitrogen, phosphorus and potassium.

  1. How do environmental factors influence Transpiration Pull?

Answer – The rate of Transpiration and the strength of the Transpiration Pull can be influenced by a range of environmental factors, including temperature, humidity, wind and soil moisture.

  1. What is the Root Pressure Theory related to Transpiration Pull?

Answer – According to Root Pressure Theory, the roots absorb water and exert a pressure, the root pressure, which pushes the water upward. The root pressure theory is applicable in small herbs but not in tall trees.

  1. Why is Transpiration Pull important for plants?

Answer – Transpiration Pull is an essential process for plants to maintain their water balance and survive in their environment.

  1. How do adhesion and cohesion work together in Transpiration Pull?

Answer – Adhesion and cohesion work together to create a continuous column of water within the Xylem vessels, allowing water to be transported through the plant.

  1. What is tension in a Transpiration Pull?

Answer – Tension is the force created by the negative pressure gradient, which pulls water up through the plant against the force of gravity.

  1. How do Stomata help limit water loss from the plant?

Answer – Stomata can conserve water during periods of drought or high temperatures, reducing the rate of Transpiration and the strength of the Transpiration Pull.

  1. What factors can affect the strength of the Transpiration Pull?

Answer – Other factors that can affect the strength of the Transpiration Pull include the rate of Transpiration, the availability of water in the soil and the environmental conditions surrounding the plant.

  1. What is the difference between vessels and tracheids in the Xylem?

Answer – Vessels are found in the Xylem of only flowering plants, while non-flowering plants possess only tracheids which serve as the main conducting tissues.

  1. How do plants use the minerals absorbed by the roots during the Transpiration Pull?

Answer – The water and minerals, absorbed by root hair and other epidermal cells, move through the plant and as the water is lost through Transpiration, the nutrients are left behind, allowing the plant to use them for growth and development.

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