Theory & Procedure, Rate of Transpiration | Additional Study Material for NEET PDF Download

Objective

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

Theory

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. 

Materials Required

Real Lab Procedure

• Take 3 % cobalt chloride solution from beaker and pour into the Petri dish.
• Take some filter paper strips and dip them in the cobalt chloride solution.
• Keep the strips in the solution for 3-5 minutes. They become pink in colour when wet.
• Remove the strips from the solution using forceps.
• Place the strips on the wire gause to allow them to dry.
• The filter paper becomes blue in colour on drying.
• Select one healthy leaf and clean the leaf to remove the water droplets using a filter paper.
• Take the dry pieces of cobalt chloride paper from the wire gause.
• Place the dried strips of cobalt chloride paper: one on the upper and the other on the lower surface of a leaf of the potted plant.
• Take two glass slides and place one over the upper and the other  over the lower side of the leaf.
• Clip the slides together using binder clips.
• Note the time taken by the cobalt chloride paper to change its blue colour to pink.

Simulator Procedure (as performed through the Online Labs)

• To select a particular plant, click on the corresponding icon.
• Click on the proceed button to view the leaves more closely.
• We can observe dried cobalt chloride paper strips which are blue in colour on upper and lower surfaces of the leaf.
• Click on the start button to start the experiment.
• A timer is shown and you need to wait till the timer stops.
• We can observe cobalt chloride paper strips on both surfaces of the leaf slowly change their colour from blue to pink.
• Small icons on both surfaces of the leaf shows the time taken to change from blue cobalt chloride paper to pink in colour.
• The expected colour and the original colour of the cobalt chloride paper shows in the left pane of the simulator.
• Click on the information icon to see the inference.
• You can redo the experiment by clicking on the ‘Reset’ button.

Note: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 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.

Observation

The time taken to change colour of the cobalt chloride paper from blue to pink on the lower leaf surface is less than the upper surface.

Conclusion

The quick change in the colour of cobalt chloride paper on the lower surfaces indicates higher rate of loss of water vapour from this surface than the upper one.

Precautions

• Always use a well watered potted plant for the experiment.
• Always handle the dried cobalt paper with dry hands or forceps.
• The leaf surface should not be wet while applying the cobalt chloride strips.