Topic 8.3 · Core + Supplement
Factors Affecting Transpiration
Temperature and wind speed both increase transpiration; humidity decreases it. All three sit on the syllabus, but not in the same way — Cambridge asks Core candidates to investigate and describe the effects of temperature and wind speed, and asks Supplement candidates to actually explain all three mechanistically. Getting the direction right for each factor matters, but the explanation is where the real marks are.
Temperature
Increasing temperature generally increases the rate of transpiration, other conditions being equal. At a higher temperature, water molecules have more kinetic energy, so evaporation from the mesophyll cell surfaces happens faster, and the resulting water vapour also moves and diffuses out more quickly. The chain worth stating in full is: higher temperature → faster evaporation → faster diffusion of water vapour → higher transpiration rate. A fall in temperature produces the reverse effect.
Wind speed
Increasing wind speed generally increases the rate of transpiration too, but the mechanism is different from temperature’s, and it’s worth keeping the two separate. Water vapour leaving a still leaf tends to accumulate in the layer of air immediately around it, making that boundary layer more humid and reducing the concentration difference driving further diffusion. Wind removes this humid layer and replaces it with drier air, which keeps the concentration gradient between the inside of the leaf and the air outside steeper for longer, so diffusion out through the stomata continues at a faster rate. The chain: higher wind speed → humid air around the leaf is removed → a steeper water-vapour concentration gradient is maintained → faster diffusion → higher transpiration rate.
Humidity
Humidity works in the opposite direction to temperature and wind speed, which is exactly why it’s worth learning as its own case rather than assuming every environmental factor pushes transpiration the same way. When the air outside the leaf is already humid, it already contains a relatively large amount of water vapour, so the difference in water-vapour concentration between the leaf’s internal air spaces and the air outside is smaller. A smaller concentration gradient means diffusion out through the stomata slows down, so higher humidity lowers the transpiration rate. Lower humidity has the reverse effect: a larger gradient and a faster rate.
| Factor increases | Effect on transpiration | Main reason |
|---|---|---|
| Temperature | Increases | Faster evaporation and faster molecular movement |
| Wind speed | Increases | Humid boundary layer removed; concentration gradient maintained |
| Humidity | Decreases | Smaller water-vapour concentration gradient |
Investigating the rate of transpiration
Cambridge’s current Core practical scope for this topic is specifically temperature and wind speed — humidity has to be explained, above, but investigating it is not a Core requirement, and light intensity doesn’t appear in this chapter’s required factor set at all. A potometer is the standard piece of apparatus for this investigation, but there’s an important distinction to hold onto: a potometer measures the rate of water uptake by a cut shoot, not water vapour leaving the leaf directly. Uptake is used as an estimate of the transpiration rate because most of the water a shoot takes up is replacing water already lost by transpiration — it is an estimate, not a direct measurement, and that limitation is worth stating explicitly if a question asks you to evaluate the method.
A typical bubble-potometer investigation follows a consistent structure: connect a leafy shoot to a water-filled potometer, make sure every joint is airtight so water can only leave through the shoot, introduce a single air bubble into the capillary tube, let the shoot acclimatise to the test condition, then measure how far the bubble moves in a fixed time. That distance-over-time figure gives you a rate. Change one environmental factor at a time — wind speed using a fan, or temperature by testing at different temperatures — while keeping everything else, such as light conditions and humidity, as constant as possible, then repeat the measurement and compare rates across conditions. Repeating each condition and taking a mean improves the reliability of the result.
This is a directly examinable practical skill on Cambridge’s Paper 6, so be ready to interpret unfamiliar apparatus, identify the independent, dependent and controlled variables in a described investigation, spot what would make a comparison unfair, and explain why the bubble’s movement is being used as an estimate of transpiration rather than a direct measurement of it.
Cross-links
The evaporation-and-diffusion mechanism these factors act on is explained in full on the transpirationpage — read that first if the “why” behind any of the explanations above feels unclear.