Transpiration Pull in Plants

An overview of transpiration pull in plants describes the details of transpiration pull, a physiological process that helps the plants pull the water from the soil to the leaves by a force generated within the xylem. This process starts when the stomatal opening of the leaf allows water and carbon dioxide to enter the plant.

As photosynthesis happens, water vapor is released through the leaf, causing a drop in air pressure within the leaf. This negative air pressure aids the water molecules to reach the leaf through the stem. In this article, we will look into transpiration pull – meaning, phenomenon, factors affecting, and significance of transpirational pull in plants.

What is Transpiration?

Transpiration is the loss of water from the aerial parts of the plant in the form of water vapor. There are three types of transpiration based on the location through which it occurs.

• Stomatal Transpiration: Evaporation of water occurs through stomata present on the leaf of the plant. About 90% of the water is transpired through the stomata of the leaves. Water is transformed into vapor near the leaf surface and evaporates through open stomata. Turgor pressure regulates the opening and closing of the stomata.
• Cuticular Transpiration: Here, the evaporation of water occurs through the cuticle of plants which is a waxy layer over the leaf surface. About 5 – 10% of water is transpired through the cuticle. In the dry environment, when plants close their stomata, this type of transpiration takes place.
• Lenticular Transpiration: This type of transpiration occurs through lenticels which are the small openings present on the bark or stem of the plants. A very small amount of water transpired through the cuticle in comparison to stomatal transpiration. It occurs in dry environments.

What is the Transpirational Pull?

The transpirational pull definition states that it is a suction force that pulls water from soil up to the leaves of the plants. When plants transpire, i.e. when they lose water in the form of water vapour, a negative pressure gradient develops within the xylem vessels.

This pressure difference along with cohesive and adhesive forces of water develops a cumulative force that pulls the water from the ground to the topmost parts of the plant. This force or pull is called the transpirational pull. Transpirational pull is extremely important for the plant as it helps in the ascent of sap and the proper distribution of water and minerals throughout the plant body.

Cohesion Hypothesis

The cohesion-tension theory postulated by John Joly and Henry Horatio Dixon in 1894 explains the intermolecular attraction observed in the process of upward movement of water molecules through the xylem of plants against gravity. According to this theory, the transpiration pull occurs in the following ways:

• Roots absorb the water from the soil due to lower water potential in the roots than soil.
• The mesophyll cells of leaves have a continuous thin layer of existing water which evaporates from the intercellular spaces by transpiration.
• An air-water interface occurs in between cellulose microfibrils and mesophyll cells creating small spaces.
• The continuous evaporation of water creates tension in the water column. This water column does not collapse due to high tensile strength.
• This surface tension in the water column helps in drawing the water from the nearby cells.
• Thus, the water potential reduces in the roots than the atmosphere which causes the water to diffuse into the surrounding air and further creates a suction force or transpiration pull.
• So, it can be concluded that the water from the soil enters the roots due to lower water potential, and then the water is drawn up to the xylem by cohesive and adhesive forces. Then the water molecules form a column throughout the xylem vessels due to transpiration and are released through the stomata of leaves as water vapors to the atmosphere.

The Phenomenon of Transpiration Pull in Plants

The phenomenon of transpiration pull in plants is discussed below:

• During transpiration, the water molecules from the soil are combined due to cohesive force and create a column of water in the xylem.
• The adhesive force between water molecules and the xylem vessel also contributes to constructing this water column.
• A suction force or transpiration pull is then generated that helps in the upward movement of water to the leaves and other green parts of the plant where photosynthesis takes place.
• An opposing hydrostatic pressure in the mesophyll cells of the leaves also operates to maintain the pull of water from the roots through the stem towards the leaves of the plant.

Role of Transpiration Pull in Plants

Transpiration pull is an important physiological process in plants. The role of transpiration pull in plants is cited below:

• Water Absorption and Transport: The plants absorb water along with nutrients from the soil and transport them to different parts of the plant body with the help of transpiration pull.
• Water Balance: It helps to release excess water from the plant body and maintain a proper balance of water in plants.
• Photosynthesis: It helps in the upward movement of water from the roots to the leaves through xylem tissues. This water is used in photosynthesis.
• Gaseous Exchange: It helps gaseous exchange in plants during photosynthesis.
• Cell Turgidity and Osmotic Pressure: It helps in maintaining cell turgidity and osmotic pressure.
• Cooling the Plant Body: It helps in cooling the plant body by evaporation of water from the leaves.
• Growth and Development of Plants: It helps in the proper growth and development of plants by maintaining proper water balance in the body.

Conclusion – An Overview of Transpiration Pull in Plants

Transpiration is the main cause of the development of transpiration pull. The suction force or pull generated during transpiration draws the water along with nutrients from the soil to enter the roots and then transfer to the xylem vessels. A negative hydrostatic pressure works during this transfer of water from the roots to the leaves. The stronger cohesive force of water molecules helps to establish a capillary action that prevents the breakage of the water column throughout the xylem vessels. Thus, a continuous flow of water and nutrients takes place through the xylem tissue.

FAQs on Transpiration Pull in Plants

Why does Transpiration Pull the Water?

The evaporation of water during transpiration creates a negative pressure in the leaves. This, in turn, creates a transpiration pull that helps water flow towards the stem from the roots, and the roots absorb more water.

What is the meaning of Root Pressure and Transpiration Pull?

Root pressure refers to the positive hydrostatic pressure that develops in the root xylem. The process of transpiration develops a negative water vapor pressure, called transpiration pull that develops in the cells of leaves.

What is the Ascent of Sap and Transpiration Pull?

The ascent of sap is the movement of water and minerals from the roots to the aerial parts of a plant. Transpiration pull is the process that allows this movement to occur.

What are Transpiration Pull and Adhesion?

Transpiration pull is a suction force that helps move water from a plant’s roots to its leaves. Adhesion is the force that acts between water molecules and the xylem vessel.

What gave the Transpiration Pull Theory?

The transpiration pull theory, also known as the cohesion tension theory, was proposed by Dixon and Jolly in 1894. The theory explains how transpiration pull helps plants transport water.

What is Capillarity and Transpiration Pull?

Capillarity is the spontaneous movement of a liquid through a capillary. Transpiration pull is the process of water movement through a plant in an upward direction due to transpiration.

How is Pull Created During Transpiration?

Leaves lose water vapor by transpiratoin and this creates a vacuum in xylem. Water molecules stick together, pulling a continuous chain from roots to fill the gap.

What are the Factors that Affect Transpiration Pull?

Various environmental factors like temperature, humidity, wind, and light along with the plant related factors such as stomata opening, water availablity, and leaf surface area affect the transpiration pull.

What are the Different Types of Transpiration?

The different types of transpiration are – Stomatal transpiration, Cuticular transpiration, and Lenticular transpiration.