Transpiration – Definition, Types, Factors Affecting Transpiration

Transpiration: In Botany, transpiration is defined as the loss of water from a plant, mainly through the stomata of leaves. However, since stomatal openings are necessary for carbon dioxide and oxygen exchange during photosynthesis, transpiration is generally considered to be a necessary evil. Transpiration was first measured by English botanist Stephen Hales. He also postulated that transpiration encourages a continuous upward flow of water and dissolved nutrients from the roots.

Nevertheless, excessive transpiration can be harmful to a plant. When water loss is more than the plant’s water intake, it can retard the growth of the plant and may lead to the plant’s death by dehydration. As per the latest research, as much as 99 per cent of the water taken in by the roots of a plant is released into the air as water vapour through transpiration. Read on to learn more about this phenomenon.

Transpiration Definition

Transpiration can be described as the loss of water from the living tissues of aerial parts of the plant in the form of water vapour. In simple terms, it is the removal of water vapour from the surface of the plant, mostly leaves.

Transpiration in Plants

Just like any other living organism, plants also require an excretory system to let the excess water from their body. The process of eliminating the excess water from the plant body is known as transpiration. It is generally done through evaporating of water from the surfaces of the leaves.

Fig. Transpiration

Most of the water which is absorbed by roots is lost into the surrounding through the aerial parts of the plants by the transpiration process. Less than \(2\% \) is used in various metabolic activities in the plant body.

Types of Transpiration

Based on the plant surface, transpiration is classified into three types:
1. Stomatal
2. Cuticular 
3. Lenticular

1. Stomatal Transpiration

Stomatal transpiration is the most important type of transpiration as compared to all the other types, as more than 95% of a total loss of water takes place through stomata. Stomata are small openings that are usually confined to leaves but may also be present in herbaceous stems.
During this type of transpiration, the water molecules present near the surface of leaves evaporate in the form of vapour through open stomata.

2. Cuticular Transpiration

When transpiration takes place from the cuticles of the plant, it is called cuticular transpiration. Cuticles are waxy covering on the leaf surface and are made of cutin. The rate of transpiration depends upon the thickness of the cuticle. The thicker the cuticle, the lower is the rate of transpiration.
Most of the transpiration takes place forms stomata; only about \(5 – 8\% \) of water is lost through cuticular transpiration. When stomata are closed due to hot weather the rate of cuticular transpiration is comparatively high.

3. Lenticular Transpiration

The type of transpiration that occurs through the lenticels is called lenticular transpiration. Lenticels are small regions on the bark or twigs that bear small, loosely arranged cells called the complementary cells.
Only a negligible amount of water is lost through this transpiration as lenticels are not found in most plants.

Evaporation and Transpiration

Fig. Evaporation and Transpiration

Evaporation and transpiration are two natural processes often confused as both involve loss of water from the surface. But, there are many differences between these two processes.

Some of These Differences Are Mentioned Below:

Factors Affecting Transpiration

Numerous factors affect the process of transpiration. Some of them are as follows:
1. Environmental Factors
2. Cellular Factors

Environmental Factors

Certain environmental factors have a direct or indirect impact on the rate of transpiration. These can also be called external factors. To know more about these factors, check the table below:

Cellular Factors

Cellular factors are the internal factors that affect the rate of transpiration. Examples of cellular factors are:

Ascent of Sap

Ascent of sap is a biological process of upward movement of water from the tip of the root to the aerial parts of the plant body. Now, you might think, how is it related to transpiration?

Fig. Ascent of Sap and Transpiration Pull

During evaporation of water through the stomata of leaves, a pull is formed inside the xylem tissue that helps in the upward movement of water into the xylem vessels. This pressure is called a transpiration pull.
The ascent of sap that is induced by transpiration depends on the following features of water:
1. Force of Cohesion: This is the mutual attraction between molecules of water that helps to maintain continuity of the water column.
2. Force of Adhesion: This is the attraction between the water column and walls of the xylem. It prevents acclimation of air blocks in the xylem as well as breaks in the water column.  
3. Suction Force (Transpiration Pull): It is the force responsible for pulling the water column in the upward direction (ascent of sap) even in tall plants.

Opening and Closing of Stomata

Stomata are tiny openings or pores that enable gaseous exchange and transpiration in plants. In general, stomata open during the daytime and close at night. This opening and closing of stomata occur due to turgor changes in guard cells caused by the osmotic flow of water. Stomata consist of a pair of guard cells with a gap in between.
1. Due to endosmosis, an increase in the turgor of guard cells takes place which results in bulging and stretching out of their outer thin walls resulting in an opening in the guard cells of stomata.
2. When turgor pressure decreases, inner walls sink due to loss of water, leading to the closure of space between them. Thus, stomata are closed.

Fig. Opening and Closing of Stomata

Significance of Transpiration

The significance of transpiration is explained below:
1. Transpiration conducts water and minerals to different parts of the plants.
2. Transpiration maintains osmosis and keeps the cells rigid.
3. There is a balance of water maintained in the plant by the continuous elimination of water from the plant body.
4. The upward movement of water in plants is due to the suction force created by the transpiration pull.
5. The leaves remain moist as some hydrophilic salts are accumulated on the surface of the leaves.
6. It helps in cell division and maintains the turgidity.
7. Proper growth of plants is maintained due to optimum transpiration.
8. Due to the evaporation of water from its leaves, the tree has a cooling effect.

Summary

Roots absorb a large amount of water from the soil, which is not required for plants. Only a small quantity of water is used for metabolic and other activities. The rest is removed by transpiration. In absence of transpiration, cells of the plant accumulate excess water, become inactive, and may eventually die.
Transpiration is also an important process of the water cycle. It is one of the major sources of water in the atmosphere.

Frequently Asked Questions (FAQs) on Transpiration

Some of the most commonly asked questions about transpiration are answered here:

Q1. What is transpiration?
Ans: Transpiration can be defined as the loss of water from the living tissues of aerial parts of the plant in the form of water vapour. In simple terms, it is the removal of water vapour from the surface of the plant, mostly leaves.

Q2. How does humidity affect transpiration?
Ans: It is easier for water to evaporate into dry air than into humid air. Hence, the rate of transpiration is reduced when the atmosphere is humid, whereas the rate of transpiration increases when the air is dry.

Q3. What is the significance of transpiration?
Ans: Transpiration is the removal of water vapour from the surface of the plant. Some significance of transpiration are:
1. It conducts water and minerals to different parts of the plants, helps in cell division, and maintains the turgidity and proper growth of the plant.
2. Transpiration maintains osmosis and keeps the cells rigid.
3. There is a balance of water maintained in the plant by the continuous elimination of water from the plant body.
4. The upward movement of water in plants is due to the suction force created by the transpiration pull.
5. The leaves remain moist as there are some hydrophilic salts that are accumulated on the surface of the leaves. Due to the removal of water from its leaves, the tree has a cooling effect.

Q4. What is transpiration pull?
Ans: During evaporation of water through the stomata of leaves, a pull is formed inside the xylem tissue that helps in the upward movement of water into the xylem vessels. This pressure is called a transpiration pull. It helps in the ascent of sap and is responsible for the absorption of a major quantity of water by roots.

Q5. How does still air affect transpiration?
Ans: The rate of transpiration is low when the air is still. This is because the transpiring organs are accumulated by water vapours and reduce the diffusion pressure deficit of the air.

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