Transpiration and using a potometer

Laura Armstrong & Joe Wolfensohn

Teachers

Laura Armstrong Joe Wolfensohn

Recall Questions

This topic requires prior knowledge of stomatal function and transpiration. You can test your knowledge on these below.

What is transpiration?

Transpiration is the loss of water vapour from the leaves of a plant through the stomata, due to evaporation.

What environmental factors affect the rate of transpiration?

  • Temperature 

  • Humidity 

  • Wind speed 

  • Light intensity

How do stomata control transpiration?

  • Guard cells open and close the stomata to regulate water loss.

  • When turgid (full of water), guard cells open stomata to allow gas exchange.

  • When flaccid (low water content), guard cells close stomata to reduce transpiration.

Topic Explainer Video

Check out this @JoeDoesBiology video that explains transpiration and using a potometer or read the full notes below. Once you've gone through the whole note, try out the practice questions!

Factors that affect the rate of transpiration

Transpiration is influenced by several environmental factors that affect the rate at which water is lost from the leaves. The main factors include temperature, humidity, wind speed, and light intensity.

 

1. Temperature (Higher temperature = Increased transpiration rate)

  • At higher temperatures, water molecules gain more kinetic energy.

  • This increases the rate of evaporation from the mesophyll cell surfaces into the air spaces in the leaf.

  • Water vapour moves from the leaf air spaces to the external air through the stomata.

  • Higher temperatures increase the rate of diffusion of water vapour. 

2. Humidity (Higher humidity = Decreased transpiration rate)

  • Humidity refers to the amount of water vapour in the air.

  • In high humidity, the air surrounding the leaf is already moist, reducing the water potential gradient between the inside and outside of the leaf.

  • As a result, less water vapour diffuses out of the stomata, decreasing transpiration

3. Wind Speed (Air Movement) (Higher wind speed = Increased transpiration rate)

  • Around the leaf surface, there is a thin layer of still, moist air.

  • If there is little or no wind, this layer remains, reducing the water potential gradient and slowing transpiration.

  • When wind speed increases, moving air removes the still, moist layer of air and replaces it with drier air.

  • This maintains a steep water potential gradient, allowing water vapour to diffuse out of the leaf more rapidly.

4. Light Intensity (Higher light intensity = Increased transpiration rate)

  • Stomata open in the presence of light to allow carbon dioxide (CO₂) into the leaf for photosynthesis.

  • When stomata are open, water vapour can escape, increasing transpiration.

  • Increased light intensity also raises leaf temperature, which increases evaporation rates and diffusion of water vapour.

Using a Potometer to Measure Transpiration Rate

A potometer is a device used to measure the rate of water uptake by a plant.

Method

  1. Assemble the potometer underwater to prevent air bubbles forming in the xylem (air bubbles would break the continuous column of water in the xylem vessels.

  2. Cut a leafy shoot underwater at a slant to prevent air from entering and blocking the xylem.

  3. Insert the shoot into the rubber tubing of the potometer.

  4. Seal all joints with petroleum jelly to prevent leaks and make sure the apparatus is airtight.

  5. Introduce one bubble of air into the capillary tube.

  6. Measure the distance the bubble moves over a set time to calculate the rate of water uptake.

  7. Use units such as ml per minute.

  8. Repeat 3 times and calculate a mean, keeping all other variables constant.

(A reservoir or a syringe filled with water can be used to move the air bubble back to the original, starting position so the experiment can be repeated)

 

Key Assumptions

  • Water uptake is directly proportional to transpiration rate. This is not exactly true as some of the water that enters the shoot will be used by the plant for photosynthesis.

 

Factors Affecting Potometer Readings

We can alter the environmental conditions when using a potometer to investigate the effects on transpiration.

  • Temperature → Higher temperatures increase evaporation, moving the bubble faster.

  • Humidity → Higher humidity slows the rate of transpiration.

  • Wind Speed → Higher wind speed removes water vapour, increasing transpiration.

  • Light Intensity → More light opens stomata, increasing transpiration rate.

Key Terms

  • Transpiration: The loss of water vapour from leaves due to evaporation.

  • Potometer: A device used to measure the rate of water uptake, estimating transpiration rate.

  • Cohesion-Tension Theory: The mechanism explaining how water moves up the xylem due to cohesion, adhesion, and transpiration pull.

  • Stomata: Small pores on the leaf surface that regulate gas exchange and water loss.

  • Guard Cells: Cells that surround stomata and control their opening and closing.

No answer provided.

Exam Tip

When describing how a potometer works, always give reasons for the key steps:

  • Cutting the shoot underwater to prevent air bubbles from entering the xylem.

  • Sealing the setup to avoid water leaks and to keep the apparatus airtight.

No answer provided.

In one experiment, the air bubble moved 7.5 mm in 15 minutes. The diameter of the capillary tube was 1.0 mm.

Calculate the rate of water uptake by the shoot in this experiment.

Give your answer in mm3 per hour. Show your working. (The area of a circle is found using the formula, area = πr 2) (2 marks)

Correct answer 23.55 – 24 (2 marks)

(this is the correct calculation of the circular area of the capillary tube multiplied by the distance moved by the air bubble in 1 hour)

Working…

Diameter= 1.0mm, therefore radius = 0.5mm

= 0.785

0.785 x 7.5 = 5.89 (this is the volume in 15 minutes).

5.89 x 4 = 23.56

We multiply by 4 to get the volume in 1 hour.

For one mark

5.9 

(this is the correct calculation of the circular area of the capillary tube multiplied by the distance moved by the air bubble in 15 minutes)

Practice Question

Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!