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Factors affecting the rate of transpiration

Transport in plants · Transpiration · note 2 of 2

Spec B8.3.2
Supplement (what this means)Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels mean
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Factors affecting the rate of transpirationSpec B8.3.2

In short

Transpiration is faster at higher temperature and higher wind speed and slower in humid air; a potometer measures water uptake by a cut shoot.

The rate of transpiration changes with the conditions around the plant. You must be able to describe the effects of temperature and wind speed.

Effect of environmental factors on the rate of transpiration
FactorEffect on rate
Temperaturehigher temperature gives a faster rate
Wind speedhigher wind speed gives a faster rate

Measuring the rate: the potometer

A potometer measures how much water a cut shoot takes up. Most of the water taken up is lost by transpiration, so water uptake is used as an estimate of the rate of transpiration. As the shoot takes up water, an air bubble in the capillary tube moves along a scale.

  1. Cut a leafy shoot under water and fit it into the potometer, keeping the apparatus full of water with no air bubbles.
  2. Introduce an air bubble into the capillary tube and note its starting position.
  3. Change one factor, for example place a fan at different distances to vary wind speed, or use rooms or water baths at different temperatures.
  4. Measure the distance moved by the bubble in a set time.
  5. Repeat and calculate a mean. Keep everything else the same.
A potometer: a leafy shoot sealed in a bung, a water reservoir with a tap to refill, and a capillary tube with an air bubble beside a scale in mm; an arrow shows the bubble moving as water is taken up. (opens full size in a new tab)
As the shoot takes up water, the air bubble moves along the scale.
rate of water uptake = distance moved by bubble ÷ time

Calculating a rate

In a potometer, the air bubble moved 45 mm in 15 minutes. Calculate the rate of water uptake in mm / min.

  1. rate = distance ÷ time
  2. rate = 45 ÷ 15

Answer: 3 mm / min

Practical skill:

Name the independent variable (for example wind speed), the dependent variable (distance moved by the bubble in a set time) and at least two controlled variables (such as the same shoot, temperature and light). Repeat the experiment for reliability.

Quick check

  1. Define transpiration.

    Show answer

    The loss of water vapour from leaves.

  2. Supplement Name two factors that increase the rate of transpiration when they increase.

    Show answer

    Temperature and wind speed.

Written and checked against the Cambridge IGCSE Combined Science (0653) specification · Updated October 2026

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