In this article, we will discuss the factors that affect the rate of transpiration in detail. But before proceeding to discuss the factors, first, let us recall what is transpiration and how the process occurs in plants.

What is transpiration?

Transpiration is the movement of water through the xylem in plants mainly due to the evaporation of water vapors from the leaves and the cohesion between water molecules. The gradient in water potential drives a force that allows the movement of water from the soil (higher water concentration) to the atmosphere (lower water concentration) through plant cells.

Plants constantly take in the water at their roots and lose water through tiny pores in the leaves known as stomata. Plants lose approximately 99% of absorbed water through evaporation from their stems and leaves through a process known as transpiration. Hence, we can define transpiration as:

The loss of water vapor from the plant to its environment by diffusion is referred to as transpiration

On the other hand, we can define transpiration stream as:

The movement of water molecules from the roots to the leaves of the plants is known as the transpiration stream

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How is Transpiration Steam Created?

  • There is an attraction between the water molecules due to cohesive forces between them. These forces create a continuous column of water up the plant.
  • Water travels through the xylem vessels in a continuous transpiration stream from roots to leaves through stem
  • Transpiration creates a “pull” or “tension” on the water in the xylem vessels by the leaves.
  • Because the cohesive forces hold up the water molecules, hence the water is pulled up through the plant.
  • The water molecules are pulled up the xylem vessels faster as the rate of transpiration from the leaves increases.

In the next section of the article, we will discuss the advantages of transpiration.

Advantages of Transpiration

The advantages of transpiration are listed below:

  • Due to the process of transpiration, the plants remain cool because the evaporation of water vapors utilize heat energy
  • The transpiration stream is beneficial in taking up the mineral ions from the soil
  • Transpiration keeps the plant cells turgid. The presence of water as it moves up the plant creates the turgor pressure of the cells that provides support to the leaves and stems of non-woody plants.

The Role of Xylem in Transpiration

In this section, we will discuss what role does xylem plays in transpiration.

Water moves up the xylem from the roots into the plant's leaves to replace the water that is lost because of the process of transpiration.

Xylem in plant cells have certain adaptations which are listed below:

  • A substance known as lignin is deposited in the cell walls which kills the xylem cells
  • These cells become hollow as they lose their cytoplasm and organelles. They join end to end to create a continuous tube for water and mineral ions to move through the roots.
  • Lignin provides strength to the plant and helps it to bear the pressure of water movement.
  • Water movement in the xylem is unidirectional. It means that this movement takes place in a single direction only, i.e. from roots to leaves.

 

The Role of Stomata in Transpiration

In this section, we will discuss the role of stomata in transpiration in detail.

  • A pair of guard cells that surround stomata usually control the process of transpiration in plants.
  • When the guard cells are turgid, they open the stomata and when they lose water, they close the stomata.
  • The opening of stomata increases the rate of transpiration and gaseous exchange.
  • The closing of stomata decreases the rate of transpiration and exchange of gases.
  • Stomata generally remain open during the day to allow the exchange of gases, i.e. letting in carbon dioxide and letting out oxygen.

 

In the next section of the article, we will discuss the factors that affect the rate of transpiration in detail.

Factors affecting transpiration

The following list explains the factors that affect the transpiration rate. Factors affecting the transpiration rate are the same as the factors affecting the evaporation rate.

Temperature

The rise in the temperature of the air heats the water insider the leaves more quickly, causing it to evaporate faster. It also enhances the capacity of the air to absorb more water. In short, higher temperature increases the molecular movement of water molecules so that more water can evaporate from the surfaces of the cell. As the rate of diffusion of water molecules also increases, hence we can conclude that the rate of transpiration increases with the temperature rise.

Sunlight

Direct sunlight and temperature have the same effect on transpiration. Because the direct sunlight heats the leaves quicker, therefore it increases the transpiration rate. Sunlight also speeds up the process of photosynthesis. More sunlight causes stomata to open, resulting in a quicker rate of transpiration during the day. We can also conclude from these factors that the rate of transpiration is more on a sunny day as compared to cold dull days.

Wind

Transpiration depends on diffusion. Winds cause air molecules to be blown away from the leaves. Because the wind removes water vapours from the surfaces of the leaves, there is a greater diffusion of water from the leaves. Hence, we can conclude that wind increases the rate of transpiration.

Humidity

Humidity refers to the concentration of water vapours in the air. There is a lower concentration of water molecules in the air surrounding the leaves at low humidity. This concentration gradient supports the transport of water molecules from the leaves through the process known as diffusion. When humidity is higher, the air surrounding the leaves becomes already saturated and contains a higher concentration of water molecules inside the leaves. Hence, we can say that when humidity is higher, the rate of transpiration increases.

Investigating the rate of transpiration

The evaporation of water from the leaf drives the water movement through the plant. A piece of apparatus, referred to as a potometer, is employed to investigate the loss of water from a plant in various environmental conditions. Through a potometer, the effect of temperature, wind speed, humidity, and light intensity can be investigated.

 

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Rafia Shabbir