Stomatal Transpiration is a critical process for Plant survival and growth. It refers to the loss of water vapour from the Stomata, which are small pores located on the surface of leaves and stems. Stomatal Transpiration plays a vital role in regulating Plant water balance, temperature and nutrient uptake. 

The loss of water in the form of vapor from the Aerial parts of the plant is known as Transpiration.

What are Stomata?

Definition 

Stomata  are tiny pores found on the surface of leaves, stems and other Plant organs. They are surrounded by two specialised epidermal cells, known as guard cells. The guard cells are responsible for opening and closing the Stomata, allowing for gas exchange and Transpiration.

What is Stomatal Transpiration?

Stomatal Transpiration is the process by which water vapour is lost from the leaves of Plants through the Stomata. It is an essential process for Plant survival, as it facilitates the uptake of nutrients and water from the soil as well as the exchange of gases necessary for Photosynthesis and respiration.

Most of the foliar Transpiration (up to 97%) takes place through Stomata. The Stomata are minute elliptical pores in the epidermis of the Plant leaf. The pore is bounded by two kidney shaper cells called guard cells. 

Also Check – Lenticular Transpiration

How does Stomatal Transpiration occur?

Stomatal Transpiration occurs as a result of a diffusion gradient of water vapour between the leaf surface and the surrounding air. Water vapour moves from areas of high concentration (inside the leaf) to areas of low concentration (the outside air) through the Stomata.

Stomatal Transpiration Diagram

3 main stages of Stomatal Transpiration

1. Diffusion of water vapour from the mesophyll cells to the Stomata-

• Water vapour is produced in the mesophyll cells of a leaf through the process of Transpiration, which is the loss of water vapour from the Plant through evapouration. 
• As water evapourates from the surface of the leaf, water molecules are pulled up through the Plant’s xylem from the roots and into the leaf. 
• Once inside the leaf, the water molecules diffuse into the air spaces within the leaf’s tissue. These air spaces are interconnected and allow for the movement of gases, including water vapour.
• The water vapour then diffuses through the air spaces and reaches the Stomata, which are small pores located on the underside of the leaf. 
• The movement of water vapour from the mesophyll cells to the Stomata is driven by a gradient of water vapour concentration. As the water vapour diffuses into the air spaces within the leaf, the concentration of water vapour in the air spaces increases, while the concentration of water vapour in the mesophyll cells decreases. This concentration gradient drives the diffusion of water vapour towards the Stomata.

2. Diffusion of water vapour through the Stomata

• The Stomata are small openings on the underside of the leaf that allow for the exchange of gases between the Plant and the environment. 
• Each Stoma is surrounded by two guard cells, which can change shape to open or close the Stomatal pore. When the guard cells are turgid (swollen with water) the turgor pressure makes the  Stomatal pore open allowing for the diffusion of gases and water vapour.
• Once the Stomatal pore is open, the water vapour diffuses out of the leaf and into the surrounding air. 
• The rate of diffusion is influenced by several factors, including the size of the Stomatal pore, the concentration of water vapour in the air surrounding the leaf and the concentration of water vapour inside the leaf.

3. Removal of water vapour from the atmosphere-

After the water vapour diffuses out of the leaf through the Stomata, it enters the surrounding air. 

The water vapour can be removed from the atmosphere through the process of evaporation, where it is converted back into liquid water, or it can contribute to the formation of clouds or precipitation. 

Also Check – How do the Guard cells Regulate Opening and Closing of Stomatal Pores

Transpiration pull can draw up water to about 50 meters or more in tall trees.

Stomatal Transpiration mechanism

Stomatal Transpiration is a complex mechanism that involves the movement of water vapour through the Stomata in a Plant’s leaves. Following is the detailed mechanism of Stomatal Transpiration – 

1. Water uptake- The process of Stomatal Transpiration begins with the uptake of water by the roots of a Plant. Water is drawn up through the Plant’s xylem vessels and transported to the leaves.
2. Water movement- Once water reaches the leaves, it moves into the Plant’s mesophyll cells through osmosis. Within the mesophyll cells, water is used for a variety of purposes, including Photosynthesis and the production of other compounds.
3. Evapouration- As water is used by the Plant, some of it is converted into water vapour through a process called Evapouration. Water vapour is produced within the intercellular spaces of the leaf and diffuses through the air spaces.
4. Stomatal openings- The Stomata are small pores located on the surface of a Plant’s leaves that allow gases to enter and exit the leaf. The size of the Stomatal opening is regulated by the Plant in response to a variety of environmental cues, such as light, temperature and humidity.
5. Water vapour diffusion- Water vapour that is produced within the leaf diffuses through the intercellular spaces and reaches the Stomatal opening. Once it reaches the opening, water vapour can diffuse out of the leaf and into the surrounding air.
6. Transpiration- The final stage of Stomatal Transpiration occurs when water vapour is released into the atmosphere through the Stomata. This process is called Transpiration and it is an important way for Plants to regulate their internal water balance and maintain proper hydration.

Also Check – What is Transpirational Pull

Factors that affect Stomatal Transpiration

There are several factors that can affect Stomatal Transpiration in Plants-

1. Light- Stomata tend to open in response to light. When light levels are high, the guard cells of the Stomata become turgid and the Stomatal pore opens, allowing for the exchange of gases. Conversely, when light levels are low, the Stomata tend to close to conserve water.
2. Temperature- Temperature can also affect Stomatal Transpiration. High temperatures can cause the Stomata to open wider, allowing for increased Transpiration. However, if the temperature is too high, the Stomata may close to prevent excessive water loss.
3. Humidity- The concentration of water vapour in the air can affect the rate of Transpiration. If the air is very dry, water vapour will diffuse more quickly out of the leaf through the Stomata. Conversely, if the air is very humid, the rate of Transpiration will be lower.
4. Wind- Wind can increase the rate of Transpiration by removing water vapour from the air around the leaf, creating a concentration gradient that drives the diffusion of water vapour out of the Stomata.
5. Soil moisture- The availability of water in the soil can also affect Stomatal Transpiration. If the soil is dry, the Plant may close its Stomata to conserve water. Conversely, if the soil is moist, the Plant may open its Stomata wider to allow for increased Transpiration.
6. Carbon dioxide concentration- Stomata also plays a role in the exchange of gases necessary for Photosynthesis. The concentration of carbon dioxide in the air can affect the rate of Stomatal Transpiration. If the concentration of carbon dioxide is low, the Stomata may open wider to allow for increased uptake of carbon dioxide.
7. Plant species and genetics- Different Plant species may have different responses to environmental factors that affect Stomatal Transpiration. Additionally, some Plants may have genetic variations that affect the size or density of their Stomata, which can in turn affect their Transpiration rates.

Also Check – Excessive Transpiration

Importance of Stomatal Transpiration

1. Water transport- Stomatal Transpiration is an essential part of the water transport system in Plants. Water is absorbed by the roots and transported through the Plant’s vascular system to the leaves, where it is released into the air through Stomata. This process helps to maintain a continuous flow of water through the Plant and is critical for Plant survival.
2. Temperature regulation- Stomatal Transpiration also helps to regulate the temperature of the Plant. When a Plant transpires, water is evapourated from the surface of the leaves, which cools the Plant down. This process is similar to sweating in animals and helps prevent the Plant from overheating in hot environments.
3. Gas exchange- In addition to water, Stomatal Transpiration also allows for the exchange of gases between the Plant and the atmosphere. During Photosynthesis, Plants take in carbon dioxide and release oxygen. Stomata facilitates this gas exchange by allowing for the diffusion of gases into and out of the Plant.
4. Nutrient uptake– Stomatal Transpiration can also help Plants take up nutrients from the soil. As water is transpired from the leaves, it creates a negative pressure gradient that pulls water and nutrients up from the soil through the Plant’s roots.
5. Growth and development- Stomatal Transpiration is also important for Plant growth and development. Without enough Transpiration, Plants can become water-stressed and may not be able to grow properly. Conversely, if Transpiration rates are too high, Plants may lose too much water and become dehydrated, which can also stunt growth.

Also Check – Why is Transpiration Important for Plants

How do Plants regulate Stomatal Transpiration?

Plants have evolved complex mechanisms for regulating Stomatal Transpiration, which allow them to balance their need for gas exchange and water conservation. Here’s an overview of how Plants regulate Stomatal Transpiration-

1. Stomatal Aperture– Stomatal aperture refers to the opening and closing of the Stomata on a Plant’s leaves. When Stomata are open, water vapour and gases can be exchanged between the leaf and the atmosphere. When they are closed, Transpiration is reduced, which conserves water.
2. Control of Stomatal opening and closing- Plants control Stomatal opening and closing in response to a variety of internal and external signals. For example, changes in light intensity, humidity and temperature can all trigger changes in Stomatal aperture. Additionally, Plants respond to signals from their roots, such as changes in soil moisture levels, by adjusting Stomatal aperture.
3. Role of Plant hormones- Plant hormones play a key role in regulating Stomatal aperture. Two hormones in particular, abscisic acid (ABA) and auxin, are involved in Stomatal regulation. ABA is produced in response to water stress and triggers the closure of Stomata to conserve water. Auxin, on the other hand, promotes Stomatal opening in response to light.
4. Mechanisms of Stomatal regulation- The mechanisms by which Plants regulate Stomatal aperture are complex and not fully understood. However, researchers have identified several key players in the process. For example, guard cells, which surround the Stomata, can change shape in response to changes in water pressure, which can cause the Stomata to open or close. Additionally, ion channels in the guard cells can control the movement of water and ions in and out of the cells, which can also affect Stomatal aperture.

Also Check – 15 Important Difference between Transpiration and Guttation

Impacts of environmental factors on Stomatal Transpiration

Environmental factors can have a significant impact on Stomatal Transpiration. Here’s an overview of how drought, excess water and pollutants can affect Stomatal Transpiration-

1. Drought- Drought can significantly reduce Stomatal Transpiration in Plants. When water is scarce, Plants may close their Stomata to conserve water, which can lead to reduced rates of Photosynthesis and growth. In response to drought stress, Plants also produce the hormone abscisic acid (ABA), which promotes Stomatal closure.
2. Excess water- Excess water can also affect Stomatal Transpiration. When soil is saturated with water, oxygen availability can become limited, which can lead to reduced rates of Photosynthesis and growth. In some cases, excess water can also lead to flooding, which can result in the death of Plant tissues.
3. Pollutants- Pollutants in the air or soil can also have an impact on Stomatal Transpiration. For example, high levels of ozone can cause damage to the Stomata, which can reduce their ability to exchange gases and water vapour. Similarly, heavy metals and other toxins in the soil can also affect Stomatal function and reduce Transpiration rates.

Also Check – 8 Important Functions of Stomata

Frequently asked questions on Stomatal Transpiration

Question –  What is Stomatal Transpiration?

Answer-  Stomatal Transpiration is the process by which water vapour is lost from the leaves of Plants through the Stomata. It is essential for Plant survival as it facilitates the uptake of nutrients and water from the soil as well as the exchange of gases necessary for photosynthesis and respiration.

Question –  How does Stomatal Transpiration occur?

Answer-  Stomatal Transpiration occurs as a result of a diffusion gradient of water vapour between the leaf surface and the surrounding air. Water vapour moves from areas of high concentration (inside the leaf) to areas of low concentration (the outside air) through the Stomata.

Question –  What are the three main stages of Stomatal Transpiration?

Answer-  The three main stages of Stomatal Transpiration are: 1) diffusion of water vapour from the mesophyll cells to the Stomata, 2) diffusion of water vapour through the Stomata, and 3) removal of water vapour from the atmosphere.

Question –  What are the guard cells?

Answer-   The guard cells are cells that surround the Stomatal pore on the underside of the leaf. They can change shape to open or close the Stomatal pore, regulating the exchange of gases and water vapour between the Plant and the environment.

Question –  What are some factors that affect Stomatal Transpiration?

Answer-  Some factors that affect Stomatal Transpiration include light, temperature, humidity, wind, and atmospheric CO2 concentration.

Question –  Why is Stomatal Transpiration important for Plants?

Answer-  Stomatal Transpiration is important for Plants because it helps them take up water and nutrients from the soil, facilitates the exchange of gases necessary for photosynthesis and respiration, and helps regulate their internal water balance and temperature.

Question –  What is the mechanism of Stomatal Transpiration?

Answer- The mechanism of Stomatal Transpiration involves water uptake by the roots, movement of water to the leaves, evaporation of water from the intercellular spaces of the leaf, diffusion of water vapour through the Stomata, and release of water vapour into the atmosphere through Transpiration.

Question –  How do Plants regulate Stomatal Transpiration?

Answer-  Plants regulate Stomatal Transpiration by adjusting the size of the Stomatal pore through changes in the turgor pressure of the guard cells. They do this in response to environmental cues such as light, temperature, humidity, and atmospheric CO2 concentration.

e turgidity. In the evening or during the night, the stomata are constricted and the temperature is not high, therefore, there is no loss of water through transpiration and the turgidity of the leaves is re-acquired and they stand out erect.