Rate of Transpiration


Our objective is to compare the rate of transpiration between the upper and lower surfaces of a leaf.


What is Transpiration?

Transpiration is the process of water movement through a plant and its evaporation into the atmosphere from its aerial parts. In leaves and in young shoots the epidermal layer contains minute microscopic pore like structures called stomata. Transpiration occurs chiefly through the stomata of the leaves.  The stomata are mainly concerned with exchange of gases during the process of photosynthesis and respiration. Each stomata has a slit like opening called the stomatal pore, which is surrounded by two special cells called the guard cells. These special cells help to regulate the rate of transpiration by opening and closing the stomata.


Importance of Transpiration

  1. Transpiration helps in the absorption of water from the soil.
  2. The absorbed water is transported from the roots to the leaves through the xylem vessels that are greatly influenced by transpiration pull.
  3. Transpiration helps to cool down the plant surface during evaporation.

Environmental Factors that Affect the Rate of Transpiration

  1. Light:Stomata are triggered to open in light so plants transpire more rapidly in the presence of light than in the dark.
  2. Temperature: Plants transpire more rapidly at higher temperatures because water evaporates more rapidly as the temperature rises.
  3. Humidity:Humidity is expressed as the percentage of water vapour present in the atmosphere. The higher the relative humidity of the outside atmosphere, the lower the rate of transpiration.
  4. Wind:When there is no breeze, the air surrounding a leaf surface becomes increasingly humid, thus decreasing the rate of transpiration. The increase in the wind velocity increases the rate of transpiration by removing the humidity from the leaf surface.

In different plants, distribution, number, size and type of stomata vary. Even within a plant, the upper and lower surfaces of the leaf may have different distributions. In some plants a greater number of stomata is present on the on the lower surface than on the upper surface of the leaf. Therefore, the loss of water from the lower surface is greater than from the upper surface. 

We can study the rate of transpiration from the two surfaces of a leaf by comparing the loss of water vapour from the two surfaces of the leaf.

Rate of transpiration can be easily demonstrated by cobalt chloride paper test.  Dry cobalt chloride paper that is blue in colour turns pink when it comes in contact with water. Using this property of cobalt chloride paper we can demonstrate water loss during transpiration.

We can measure the rate of transpiration by using the time taken for the paper to change its colour from blue to pink.

Learning Outcomes

  • Students understand the concept of transpiration.
  • Students understand the importance of transpiration.
  • Students understand the factors that affect the rate of transpiration.
  • Students will be able to do the experiment more accurately in the real lab once they understand the steps through the animation and simulation.