4.9 Factors Affecting Transpiration

A free GCSE Biology lesson for AQA (8461) from OBA School: temperature, humidity, air movement and light intensity; using a potometer; reducing water loss. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Explain how temperature, humidity, air movement and light intensity affect the rate of transpiration
• Describe how to use a potometer and its limitations
• Calculate the rate of water uptake from potometer readings, including volume (Higher tier)
• Describe how some plants are adapted to reduce water loss

Key words

• Rate of transpiration: How much water vapour is lost from the leaves in a given time.
• Humidity: The amount of water vapour in the air.
• Potometer: Apparatus that measures how fast a leafy shoot takes up water.
• Concentration gradient: The difference in concentration between two areas. Water vapour diffuses faster down a steeper gradient.
• Wilting: Drooping of a plant when its cells lose turgor because it is losing water faster than it can take it in.

Four factors

Anything that speeds up the evaporation of water inside the leaf, or the diffusion of water vapour out through the stomata, increases the rate of transpiration.

• Temperature ↑; Effect on rate increases; Explanation water molecules have more kinetic energy, so evaporation and diffusion are faster
• Humidity ↑; Effect on rate decreases; Explanation more water vapour in the air, so the concentration gradient between the inside of the leaf and the air is less steep
• Air movement (wind) ↑; Effect on rate increases; Explanation water vapour is blown away from the leaf surface, keeping the concentration gradient steep
• Light intensity ↑; Effect on rate increases; Explanation more stomata open (to take in carbon dioxide for photosynthesis), so more water vapour can diffuse out

Exam tip: Always explain using the concentration gradient of water vapour (humidity, wind) or the stomata (light) or kinetic energy (temperature). "It is hotter so it transpires more" will not score.

Measuring transpiration with a potometer

• Cut the shoot under water, so no air gets into the xylem, and fit it into the potometer under water.
• Make every joint airtight, and dry the leaves.
• Let one air bubble into the capillary tube. Record how far it moves in a set time (for example 5 minutes).
• Reset the bubble using the reservoir, then repeat to calculate a mean. Change one factor (for example a fan on different settings) and keep the others the same.

A potometer really measures water uptake. This is close to the rate of transpiration, but a little of the water is used in photosynthesis or kept by cells to stay turgid.

Worked example: rate of water uptake (Higher tier)

In 6 minutes the bubble moves 45 mm. The capillary tube has a radius of 0.40 mm. Calculate (a) the rate in mm/min and (b) the volume of water taken up per minute in mm³.
(a) rate = 45 ÷ 6 = 7.5 mm/min
(b) the water fills a cylinder: volume = πr² × length = π × 0.40² × 7.5 = 3.77 mm³ per minute

Answer: (a) 7.5 mm/min (b) 3.8 mm³/min (2 s.f.)

Reducing water loss

If a plant loses water faster than its roots can take it in, its cells lose turgor and it wilts. Wilting reduces the surface area exposed to the sun, and stomata close, so less water is lost. Plants in hot, dry or windy places have extra adaptations.

• thick waxy cuticle; How it reduces water loss waterproof layer reduces evaporation from the leaf surface
• fewer stomata, or stomata sunken in pits; How it reduces water loss less water vapour diffuses out; still, humid air is trapped over the stomata
• hairs on the leaf, or rolled-up leaves; How it reduces water loss trap a layer of moist air, so the concentration gradient is less steep
• leaves reduced to spines (e.g. cacti); How it reduces water loss much smaller surface area for water loss

Key fact: Most of these adaptations work by making the water vapour concentration gradient less steep or by reducing the surface area.

Common mistakes

• Saying high humidity increases transpiration. More water vapour in the air makes the gradient less steep, so the rate goes down.
• Saying light increases transpiration because "the plant gets hotter". The main reason is that stomata open in the light.
• Forgetting that a potometer measures water uptake, not exactly the water lost.
• Using the diameter instead of the radius in πr² × length.

Revision questions

Try these, then check your answers below each one.

Question 1. Which change would decrease the rate of transpiration? (Choose from: An increase in temperature, An increase in humidity, An increase in wind speed, An increase in light intensity.)

Answer: An increase in humidity

Working: Higher humidity means more water vapour in the air, so the concentration gradient is less steep and less water vapour diffuses out.

Question 2. True or false? A potometer measures the rate of water uptake, which is not exactly the same as the rate of water loss.

Answer: True

Working: True. A little water is used in photosynthesis or kept by cells, so uptake is close to, but not exactly, the transpiration rate.

Question 3. What apparatus is used to measure the rate of water uptake by a leafy shoot?

Answer: potometer

Working: A potometer.

Related lessons

• Previous lesson: 4.8 Evaporation and Transpiration
• Next lesson: 5.1 Health and Disease
• In this chapter: 4.1 The Blood
• In this chapter: 4.2 The Blood Vessels
• In this chapter: 4.3 The Heart

Continue in the full course

This lesson is section 4.9 (AGCBH0409) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.