Transpiration in Plants

What is Transpiration in Plants

Transpiration is the process by which water is lost from a plant’s leaves, stems and flowers through tiny pores called Stomata. It plays a crucial role in plant physiology, as it facilitates the uptake of nutrients and water from the soil, regulates temperature and maintains turgor pressure. Transpiration also contributes to the movement of water and nutrients throughout the plant, allowing it to grow and thrive. 

Plants absorb water through their roots and lose it from their leaves by evaporation.

The loss of water from the leaves draws more water up through the plant. 

This continual flow of water through a plant, driven by evaporation, is known as Transpiration in Plants.

Transpiration occurs in response to several factors, including environmental conditions such as temperature, humidity, wind and light intensity, as well as internal factors such as the plant’s water content and metabolic activity.

Stomata are the primary site of Transpiration in plants. These small openings, located on the underside of leaves, allow for gas exchange between the plant and its environment. During Transpiration, water vapour diffuses out of the plant’s cells and into the intercellular spaces, where it can then escape through the Stomata and into the atmosphere. As water is lost through Transpiration, it creates a negative pressure gradient that causes water to be pulled up through the plant’s Xylem vessels, allowing for the movement of water and nutrients throughout the plant.

Mechanism of the process of Transpiration

Transpiration is the process by which water is lost from the aerial parts of plants, primarily through Stomata on the leaves. There are three main types of Transpiration- 

• Cuticular Transpiration
• Stomatal Transpiration
• Lenticular Transpiration

Cuticular Transpiration

Cuticular Transpiration occurs when water is lost directly through the cuticle, which is a waxy layer covering the leaf surfaces. This type of Transpiration is usually minor and does not contribute significantly to overall water loss.

Stomatal Transpiration

Stomatal Transpiration is the most common type of Transpiration and occurs when water vapour diffuses out of the leaf through the Stomata, which are small pores on the leaf surface. Stomata are surrounded by two specialised cells called guard cells, which can change shape to open or close the Stomata. When the Stomata are open, water vapour can escape from the leaf through the Stomatal pore.

Lenticular Transpiration

Lenticular Transpiration occurs when water is lost through lenticels, which are small pores on the surface of stems, Roots and some fruits. Lenticels are surrounded by loosely packed cells and do not have specialised guard cells like Stomata.

Also Check – Transpiration- Its Role in the Hydrologic Cycle

Steps of Transpiration in Plants 

1. Water uptake – Water is taken up by plant Roots from the soil through osmosis, driven by differences in water potential between the Root and soil. This water is transported to the leaves through specialised tubes called Xylem.
2. Water movement –  Water moves through the plant from the Roots to the leaves primarily by capillary action, where water molecules are attracted to one another and to the walls of the Xylem vessels. This movement is driven by the Transpiration Pull, which is created by the loss of water from the leaves through Transpiration.
3. Stomatal regulation – The rate of Transpiration is regulated by the opening and closing of Stomata on the leaves. Stomata are small pores that allow gases, including carbon dioxide and oxygen, to enter and exit the leaf. When Stomata are open, water vapour is lost from the leaf through Transpiration. The opening and closing of Stomata is controlled by guard cells, which change shape in response to changes in water potential and environmental conditions.
4. Guard cells play an important role in regulating Transpiration. When water is plentiful, the guard cells take up water and become turgid, causing the Stomata to open. This allows for gas exchange and Transpiration to occur. However, when water is scarce, the guard cells lose water and become flaccid, causing the Stomata to close and reducing water loss through Transpiration.
5. Environmental factors – The rate of Transpiration is also influenced by environmental factors such as temperature, humidity and wind. Higher temperatures and lower humidity increase the rate of Transpiration, while high winds can increase water loss from the leaves.

Also Check – What is Transpiration Pull?

Also Check – Transpiration Stream

Factors Affecting Transpiration

The rate of Transpiration is influenced by a variety of factors, including environmental conditions, plant anatomy and physiology. The major environmental factors affecting Transpiration are temperature, humidity, wind and light.

• Temperature –  Transpiration rates increase with higher temperatures due to increased evaporation of water from the plant. This is because warm air can hold more moisture than cold air, creating a greater water vapour concentration gradient between the plant and the air. However, extremely high temperatures can cause Stomata to close, reducing Transpiration rates and conserving water.
• Humidity –  Transpiration rates decrease as humidity levels increase, as the air surrounding the plant becomes more saturated with water vapour. This reduces the water vapour concentration gradient between the plant and the air, slowing down the diffusion of water vapour out of the plant. Conversely, low humidity levels increase Transpiration rates, as the air is able to absorb more water vapour from the plant.
• Wind – Wind can increase Transpiration rates by increasing air movement around the plant, carrying away the water vapour released by the plant. This increases the concentration gradient of water vapour between the plant and the air, facilitating the diffusion of water vapour out of the plant. However, excessively high winds can also cause Stomata to close, reducing Transpiration rates.
• Light –  Transpiration rates are typically higher in the presence of light, as photosynthesis requires the absorption of carbon dioxide through Stomata, which in turn increases water loss through Transpiration. However, some plants have adaptations to reduce water loss during periods of high light intensity, such as thickened cuticles or sunken Stomata.
• Cellular Factors –  The structure and physiology of plant cells can also impact Transpiration rates. For example, the presence of aquaporins, specialised proteins that facilitate the transport of water across cell membranes, can increase the rate of water movement in and out of cells. Additionally, the presence of salt in soil or water can affect the ability of cells to take up water, which can impact Transpiration rates.
• Water Availability – The availability of water in the soil and surrounding environment can also impact Transpiration rates. Plants that are water-stressed, or that have limited access to water, will typically have lower rates of Transpiration in order to conserve water.
• Surface Area of the Leaves –  The surface area of the leaves also plays a role in Transpiration rates. Plants with larger leaves, or with a higher leaf-to-stem ratio, will typically have higher rates of Transpiration, as there is more surface area available for water to evaporate from. Conversely, plants with smaller leaves or a lower leaf-to-stem ratio will typically have lower rates of Transpiration.

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

Ascent of Sap

The Ascent of Sap is a process by which water and minerals are transported from the roots of a plant to its leaves. This process is mainly driven by Transpiration, which is the loss of water vapour from the leaves. The Ascent of Sap is possible due to several properties of water. They are

• Cohesion
• Adhesion
• Surface tension

Cohesion

Cohesion refers to the mutual attraction between molecules of water. In Xylem water molecules are held together by cohesive forces allowing them to form a continuous column that extends from the roots to the leaves. This cohesion also allows water to be pulled up the Xylem in response to Transpiration creating a negative pressure gradient that draws water and minerals up through the plant.

Adhesion

Adhesion is the attraction of water molecules towards polar surfaces. In Xylem water molecules adhere to the walls of the Xylem vessels allowing them to move upwards against the force of gravity. This adhesion, combined with cohesion, helps water and minerals to move efficiently through the Xylem.

Surface tension

Surface tension is another property of water that contributes to the Ascent of Sap. Molecules of water are more attracted to each other in the liquid phase than in the gas phase. This surface tension, along with the cohesion and adhesion of water molecules helps to maintain the continuous column of water in the Xylem and facilitate the transport of water and minerals from the roots to the leaves.

Also Check – What is Root Pressure ?

Opening and Closing of Stomata

Stomata are small openings found on the surface of leaves that allow plants to exchange gases and water vapour with their environment. Each stoma is made up of a pair of specialised cells called guard cells, with an aperture in between. During the daytime, the Stomata remain open to facilitate the exchange of gases, while at night, they close to prevent water loss.

The opening and closing of Stomata is regulated by the turgidity or water content of the guard cells. The interior wall of the guard cells facing the aperture is dense and flexible. When the turgidity of the guard cells increases the exterior walls bulge outwards and the interior walls form a crescent shape, causing the Stomata to open.

The orientation of the microfibrils, which are long, thread-like structures within the cell walls, also plays an important role in the opening and closing of Stomata. In guard cells the microfibrils are oriented radially which makes it easier for the Stomata to open when the guard cells take in water.

When the turgidity of the guard cells decreases due to water loss the interior walls of the guard cells form a crescent shape causing the Stomata to close. This closure helps to prevent excessive water loss and maintain proper water balance in the plant.

The distribution of Stomata can vary depending on the type of plant. In dicot plants, which have two cotyledons in their seeds, the lower side of leaves typically has more Stomata than the upper side. In monocot plants which have only one cotyledon in their seeds the number of Stomata is usually equal on both sides of the leaf.  Read More..

Also Check – Also Check – What is Guard Cells ? Definition,Location, Structure, Function and Diagram of Guard Cells

Evaporation and its role in Transpiration

Evaporation plays a crucial role in the process of Transpiration. When water evaporates from the surface of the leaf, it creates a negative pressure that draws water up through the plant. This is known as the Cohesion-Tension Theory.

Water primarily evaporates through tiny pores on the surface of leaves called Stomata. Carbon dioxide enters the plant through these openings, while oxygen diffuses out. When water evaporates from the moist surfaces of the mesophyll cells in the leaf, it diffuses out through the Stomata.

The mesophyll cells are surrounded by a film of moisture, which evaporates as water vapour diffuses through the Stomata. This process is driven by a difference in water vapour concentration between the inside and outside of the leaf.

As water is lost from the leaves, more water is drawn up through the Xylem tubes in the stem to replace it. The Xylem tubes are tiny channels that run through the stem, transporting water and nutrients from the Roots to the leaves.

The opening and closing of the Stomata is regulated by the plant in response to environmental conditions such as light, temperature and humidity. When a plant needs to conserve water, it can close its Stomata to reduce the rate of Transpiration.

Also Check – 15 Differences between Transpiration and Evaporation

Adaptations for Reducing Transpiration

Plants that grow in dry environments have evolved various adaptations to reduce water loss by Transpiration. Some of these adaptations are as follows – 

1. Fewer Stomata –  Plants that grow in dry environments have fewer Stomata on their leaves, which reduces the amount of water that can evaporate. This means that the plant can conserve water, but it may also limit the amount of carbon dioxide that can be absorbed for photosynthesis.
2. Thick cuticles –  The cuticle is a waxy layer that covers the surface of leaves and stems, which helps to reduce water loss by Transpiration. Plants that grow in dry environments often have thicker cuticles to prevent water loss.
3. Hairy leaves –  Some plants have hairy leaves or stems, which can help to trap moisture and reduce the rate of Transpiration.
4. Reduced leaf size – Plants that grow in dry environments may have smaller leaves, which reduces the surface area for water loss.
5. Water-storing tissues –  Some plants have specialised tissues for storing water, such as succulent plants. These tissues allow the plant to store water during times of plenty and use it during periods of drought.
6. CAM photosynthesis-  Some plants, such as cacti and succulents, use a type of photosynthesis called CAM (crassulacean acid metabolism) that allows them to conserve water by opening their Stomata at night when the air is cooler and more humid and closing them during the day.

Also Check- Adaptation in Plants to reduce Excessive Transpiration 

How is the Rate of Transpiration Measured ? 

There are a number of methods for measuring Transpiration. Some of the methods are as follows 

Weighing method

Method 1 

• A small lightweight potted plant can be weighed before and after the end of a certain period of time. 
• The soil surface and the pot should be fully covered to prevent evaporation from the surfaces other than the plant.
• The loss in weight by the plant during that time is due to the loss of water by Transpiration.
• An improvement in the weighing method can be made by using a glass bottle linked by a rubber tube to a graduated side tube, filled with water.
• The water level in the side tube falls to demonstrate loss of water through Transpiration from the leaves.
• This would indicate the volume of water loss that can be compared with the loss in weight with the help of a weighing machine.

Method 2

• Another weighing experiment can be done by using a test-tube filled with water and inserting a leafy shoot (no roots) in it and pouring some oil on the surface to prevent loss of water from the test tube by evaporation.
• Place the test tube in a  small beaker and weigh them together. 
• Remove the intact test tube and keep it straight in the test tube stand for a few hours.
• Weigh it again by keeping it in the same beaker. Any difference in weight will indicate loss of water by the shoot (due to Transpiration).
• Since there are no roots to actively absorb water, the water loss through Transpiration will be much less.

Potometer Method 

• A Potometer is a device that measures the rate of water intake by a plant, and this water intake is almost equal to the water lost through Transpiration.
• Rate of Transpiration can be estimated by using an apparatus called a potometer to measure how quickly a plant takes up water. 
• By potometer one can change light intensity, temperature, humidity, and wind speed to see how environmental factors affect the rate. 

Significance of Transpiration

Transpiration is a crucial process for plants as it helps in facilitating the uptake of nutrients and water, regulating temperature and maintaining turgor pressure. The significance of Transpiration can be explained as follows – 

• Conduction of water and minerals-  Transpiration helps in the conduction of water and minerals to different parts of the plants. This process enables the movement of water from the Roots to the leaves, where it is required for photosynthesis.
• Maintaining water balance –  Due to the continuous elimination of water from the plant body, there is a balance of water maintained within the plant. This balance ensures that the plant does not become waterlogged, which can lead to several problems.
• Osmosis and cell rigidity – Transpiration maintains osmosis and keeps the cells rigid. This process helps in the movement of water from areas of higher concentration to areas of lower concentration, which is essential for the proper functioning of cells.
• Suction force for water movement –  A suction force is created by Transpiration that helps in the upward movement of water in the plants. This force is created due to the evaporation of water from the leaves, which creates a negative pressure gradient that pulls the water upwards.
• Moisture on leaves – Certain hydrophilic salts are accumulated on the surface of the leaves, which keeps the leaves moist. This helps in protecting the leaves from drying out and also helps in maintaining a favourable environment for photosynthesis.
• Turgidity and cell division-  Transpiration maintains the turgidity of the cells and helps in cell division. Turgidity is the pressure exerted by the cell wall on the cell contents, which is essential for the proper functioning of cells.
• Proper growth –  Optimum Transpiration helps in the proper growth of the plants. This is because Transpiration is closely related to photosynthesis, which is the process by which plants produce food for growth.
• Cooling effect –  The cooling effect of a tree is due to the evaporation of water from its leaves. This helps in regulating the temperature of the plant and its surroundings.

Drawbacks Of Transpiration 

In addition to the significance, Transpiration has a few drawbacks. These include:

• Water loss – Transpiration slows down if the transpired water is not compensated by absorption from the soil. This can lead to water stress in the plant.
• Energy loss –  A lot of energy is released during Transpiration, which can be wasteful.
• Water absorption –  Plenty of unnecessary water is absorbed by the plants during the process. This can lead to waterlogging and other problems.

Also Check – How Transpiration Affects Climate

Frequently asked Questions on Transpiration in Plants

What is Transpiration in plants?

Answer- Transpiration is the process by which water is lost from a plant’s leaves, stems, and flowers through tiny pores called Stomata.

What are the functions of Transpiration in plants?

Answer- Transpiration plays a crucial role in plant physiology, as it facilitates the uptake of nutrients and water from the soil, regulates temperature and maintains turgor pressure. Transpiration also contributes to the movement of water and nutrients throughout the plant, allowing it to grow and thrive.

What are the primary sites of Transpiration in plants?

Answer- The primary sites of Transpiration in plants are Stomata, which are small openings located on the underside of leaves.

What are the three main types of Transpiration in plants?

Answer- The three main types of Transpiration in plants are Cuticular Transpiration, Stomatal Transpiration, and Lenticular Transpiration.

What is Stomatal Transpiration?

Answer- Stomatal Transpiration is the most common type of Transpiration and occurs when water vapour diffuses out of the leaf through the Stomata.

What is the role of guard cells in regulating Transpiration?

Answer- Guard cells play an important role in regulating Transpiration by controlling the opening and closing of Stomata on the leaves in response to changes in water potential and environmental conditions.

What are the major environmental factors affecting Transpiration?

Answer- The major environmental factors affecting Transpiration are temperature, humidity, wind, and light.

How does humidity affect Transpiration?

Answer- Transpiration rates decrease as humidity levels increase, as the air surrounding the plant becomes more saturated with water vapour. Conversely, low humidity levels increase Transpiration rates, as the air is able to absorb more water vapour from the plant.

What are some adaptations that plants have to reduce water loss during periods of high light intensity?

Answer- Some adaptations that plants have to reduce water loss during periods of high light intensity include thickened cuticles and sunken Stomata.

How does Transpiration contribute to the movement of water and nutrients throughout the plant?

Answer- As water is lost through Transpiration, it creates a negative pressure gradient that causes water to be pulled up through the plant’s Xylem vessels, allowing for the movement of water and nutrients throughout the plant.

 What is the Cohesion-Tension Theory?

Answer- The Cohesion-Tension Theory explains how water is drawn up through the plant due to negative pressure created by water evaporating from the surface of the leaf.

 How does water primarily evaporate from plants?

Answer- Water primarily evaporates through tiny pores on the surface of leaves called Stomata.

 What are some adaptations that plants have evolved to reduce water loss by Transpiration?

Answer- Plants that grow in dry environments have evolved adaptations such as fewer Stomata, thick cuticles, hairy leaves, reduced leaf size, water-storing tissues, and CAM photosynthesis to reduce water loss by Transpiration.

 What is the potometer method used for?

Answer- The potometer method is used to measure the rate of water intake by a plant, which is almost equal to the water lost through Transpiration.

 Why is Transpiration significant for plants?

Answer- Transpiration is significant for plants as it helps in facilitating the uptake of nutrients and water, regulating temperature, and maintaining turgor pressure. It also helps in the conduction of water and minerals to different parts of the plants, maintains water balance, and maintains osmosis and keeps the cells rigid.

What is the Ascent of Sap and what drives it?

Answer- The Ascent of Sap is the process by which water and minerals are transported from the roots of a plant to its leaves. It is mainly driven by transpiration which is the loss of water vapour from the leaves.

What are the properties of water that contribute to the Ascent of Sap?

Answer- The properties of water that contribute to the Ascent of Sap are cohesion, adhesion and surface tension. These properties allow water to be pulled up through the plant and maintain a continuous column of water in the Xylem.

What are Stomata and what is their function?

Answer- Stomata are small openings found on the surface of leaves that allow plants to exchange gases and water vapour with their environment. They play an important role in photosynthesis and respiration.

How is the opening and closing of Stomata regulated?

Answer- The opening and closing of Stomata is regulated by the turgidity or water content of the guard cells. The orientation of microfibrils within the cell walls also plays a role. When guard cells take in water Stomata open and when they lose water Stomata close.

How does the distribution of Stomata vary between dicot and monocot plants?

Answer- In dicot plants, the lower side of leaves typically has more Stomata than the upper side while in monocot plants, the number of Stomata is usually equal on both sides of the leaf.