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Transpiration in Plants – Overview, Types, Factors and Significance

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  • Last Updated : 17 Aug, 2022
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Plants absorb mineral nutrients and water from the soil but most of the water absorbed is not utilized by the plants. Like all living beings plants too need to release the excess water. The plants release the excess water through evaporation through different plant parts such as stems or the stomata present on the surface of the leaves by the process of transpiration. The evaporation of water from leaves creates a suction pull which can pull water to great heights in the plants. Transpiration helps in cooling the plant in hot weather.

Transpiration in Plants

 

Types of Transpiration

There are three types of transpiration. They are:

Stomatal Transpiration 

The process of loss of water through the stomata of leaves is known as Stomatal Transpiration.

Mechanism of Stomatal Transpiration

Open and Closed stomata

 

Stomata are minute pores on the epidermis of the leaves which are surrounded by two bean-shaped guard cells that surround it. The inner wall of the guard cell is thicker than the outer wall. So, what basically happens in stomatal transpiration? In stomatal transpiration, the water near the surface of the leaves vaporizes through the stomata when the stomata are open. 

As we know, from the roots the plants absorb water from the soil which is transferred to the xylem present in the plant stem. From here the water is distributed to different branches and leaves of the plant. Now inside the leaves, from the upper epidermis, the water enters the palisade mesophyll cells from which it enters the spongy mesophyll cells. When the spongy mesophyll cells become excess with water, they form a thin layer of water around them. When the spongy mesophyll cells become filled with water, the water moves to the intercellular space which lies just next to the mesophyll cells. When the intercellular space gets saturated with water vapors they transfer the extra water vapor to the substomatal space which is in direct contact with the stomata present on the lower epidermis. And thus the excess water moves out from the stomata. This is the whole mechanism of Stomatal Transpiration.

Cuticular Transpiration

The process of loss of water through cuticles of leaves is known as Cuticular Transpiration. Cuticular transpiration holds 5 to 7% of the total transpiration that occurs inside the plant. Cuticular transpiration mostly takes place at night or when stomata are closed during dry conditions, more water is transpired through the cuticles. Cuticles are present on the upper and lower epidermis of the leaf. It is not completely impermeable for water. Plants containing thin cuticles allow more cuticular transpiration. 

Lenticular Transpiration 

The process of loss of water through lenticels of the stem is known as Lenticular Transpiration. Lenticular transpiration holds 1 to 2% of the total transpiration that occurs inside the plant. Lenticels are microscopic openings present on the bark of woody stem plants. It is also involved in gaseous exchange.  Lenticel pores consist of loosely packed cells and thin-walled cells. Their cell shape is oval and spherical. 

Factors Affecting Transpiration in Plants

The factors affecting the rate of transpiration are as follows

  • Relative Humidity – Humidity refers to the amount of water vapor present in the atmosphere at a particular time and temperature. If the atmosphere is filled with water vapors the transpiration rate decreases and vice versa. Therefore, the rate of transpiration is inversely proportional to relative humidity.
  • Air Movement – If the air movement is fast, saturated air in contact with the leaf surface containing the water vapor is removed, thus increasing the transpiration. But, if the air is still, the rate of transpiration is low. Therefore, the rate of transpiration is directly proportional to air movement.
  • Temperature – High temperature helps in lowering the atmospheric humidity and thus increasing the rate of transpiration. So, the rate of transpiration is directly proportional to temperature.
  • Light – During bright daylight, the stomata remain open, and thus the transpiration rate increases. The increase in light in light intensity also increases the temperature which in turn increases the rate at which the transpiration occurs. Thus, the rate of transpiration is directly proportional to light intensity.
  • Surface Area of Leaf – The rate of transpiration is higher if the leaf has more surface area. On the other hand, if we consider the rate of transpiration per unit surface area, then, smaller leaves have a higher rate because the number of stomata in smaller leaves is higher.
  • Water Availability – The rate of transpiration also depends upon the rate of absorption of water from the ground by roots. With a lack of water availability, less water uptake by the root causes partial dehydration of the leaf cells resulting in wilting of the plant.
  • Opening and Closing of Stomata – Stomatal pores present in the leaf allow gas exchange where water vapor leaves and carbon dioxide enters the plant. So, when stomata are open the rate of transpiration increases, and when they are closed the rate decreases.
  • Ascent of Sap – Ascent of sap means the upward movement of water and minerals from the roots to the aerial parts of the plant through the stem. Evaporation of water vapors from the leaf cells creates a suction that pulls water from the xylem cells of roots and thus contributing to transpiration. 

Significance of Transpiration

  • Cooling Effect – Transpiration is responsible for cooling the surface of the leaf. When the temperature is extremely high even the plant starts facing burns, so, in order to cool down the plant gives out more amount of water that gives transpiration to have a cooling effect on the plant.
  • Suction Force – Suction force is the force that is required to pull the water from the stem towards the branches and the leaves of the plant against gravity. When water is lost continuously from the leaves the roots absorb more and more water from the soil and transfer it to the leaves.  So, the suction force is experienced more by the plants if there is more amount of transpiration.
  • Distribution – Due to transpiration plants get a proper distribution of minerals. During transpiration, when water is going out from the leaves as vapors the roots in turn pulls the water in the soil towards itself. So, when it is pulling water it is obvious that all the minerals dissolved in the water also get absorbed by the roots. Thus, the branches, stems and the whole plant gets a proper amount of mineral content. There is a perfect balance of water and minerals maintained in the plant.
  • Climate – The amount of water lost by the plant influences a lot in the climate of a region. An area containing more plants has more amount of rainfall.
  • Turgor Pressure – Transpiration helps in maintaining the turgidity of the cells and thus helps in cell division. A proper plant form and structure is maintained because with the lack of water the plants become floppy and wilt.

Disadvantages of Transpiration

Even though transpiration is a very important process, it has a few drawbacks.

  • Wilting of leaves – Due to excessive transpiration during high temperature wilting and loss of turgidity is quite common which in turn reduces photosynthesis and other metabolic processes of the plant.
  • Loss of energy – A lot of energy is lost during transpiration for absorption and conduction of water as 98-99% of absorbed water is lost through transpiration.
  • Reduced Growth – Due to transpiration the water availability inside the plant gets reduced thus giving the plant a stunted growth.

Conclusion

Transpiration in plants is a very vital process. Without transpiration, the plants will not get enough nutrients and due to the lack of loss of water, the plant cells will eventually burst. Transpiration is also responsible for maintaining the water balance of the world. 

Frequently Asked Questions 

Question 1: What is transpiration?

Answer: 

Transpiration is the process of evaporation of excess water from the aerial parts of the plant such as leaves and stems.

Question 2: Explain the different types of transpiration.

Answer:

Transpiration in plants occurs in the following three processes: 

  1. Stomatal Transpiration: The process of loss of water through stomata of leaves.
  2. Lenticular Transpiration: The process of loss of water through lenticels of stem
  3. Cuticular Transpiration: The process of loss of water through cuticles of leaves

Question 3: Differentiate between stomata and lenticel.

Answer: 

Stomata

Lenticel

Stomata are found on the surface of the leaves and on the green stems of the plant. Lenticels are present on the woody stem.
Stomata are open only during the daytime and close during the night. Lenticels are open all the time.
Stomata have special bean-shaped guard cells. Lenticels have no such special cells.

Question 4: How is transpiration important to plants?

Answer: 

Transpiration helps in cooling the surface of the leaves, helps in maintaining the turgidity of the cells, and helps in the proper distribution of water and minerals throughout the plant.

Question 5: How is transpiration important to the surroundings?

Answer: 

Transpiration helps in cooling the environment. Water vapors are released from the plant which in turn contributes to the rainfall of that region and thus lowering the temperature of the surroundings.

Question 6: What are the disadvantages of transpiration?

Answer: 

Excessive transpiration results in the wilting of leaves and loss of turgidity and also stunted growth of the plants. It can result in the desiccation of plants due to scarcity of water. 


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