🌡 Temperature
Higher temperature means water evaporates faster inside the leaf and water vapour particles move faster. So the rate of transpiration increases when it is warmer.
Plant Tissues, Organs and Systems » Measuring the Rate of Transpiration
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.2.3.2
- How temperature, humidity, air movement and light intensity change the rate of transpiration
- How a potometer measures the rate of water uptake
- How to work out a rate and plot and read graphs of transpiration data
- How to find a mean and use sampling and areas for stomata data
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Unlock This CourseWhat affects the rate of transpiration?
Transpiration is not a fixed speed. A plant loses water faster in some conditions than in others. Four factors matter: temperature, humidity, air movement and light intensity.
Water vapour leaves the leaf through the stomata by diffusion. Each factor changes how quickly the vapour diffuses out, or how open the stomata are.
Key terms:
- Humidity: the amount of water vapour in the air.
- Rate of transpiration: how much water a plant loses by transpiration in a given time.
💦 Humidity
If the air is very humid, there is already a lot of water vapour outside the leaf. The concentration gradient is small, so less vapour diffuses out. Higher humidity means a lower rate of transpiration.
🌬 Air movement
Moving air blows the water vapour away from the leaf surface. This keeps the concentration gradient steep, so more vapour diffuses out. More air movement means a higher rate of transpiration.
☀ Light intensity
Guard cells open the stomata in the light, so the plant can take in carbon dioxide for photosynthesis. More open stomata let more water vapour escape. Brighter light means a higher rate of transpiration.
Measuring transpiration with a potometer
You cannot easily see water vapour leaving a leaf. Instead, we measure how fast a cut shoot takes up water. Nearly all the water a plant takes up is lost by transpiration, so the water uptake is a good measure of the rate of transpiration.
The apparatus is a potometer. It is a cut shoot joined, with no air gaps, to a tube of water with a thin capillary tube. A tiny air bubble in the capillary tube moves along as the shoot takes up water.
Method:
- Cut a shoot underwater and fit it into the potometer underwater, so no air enters the xylem.
- Make sure all joints are airtight and the leaves are dry.
- Let one air bubble into the end of the capillary tube.
- Record the starting position of the bubble.
- Measure how far the bubble moves in a set time, such as 5 minutes.
- Change one factor, such as air movement with a fan, and repeat. Keep the other factors the same.
Key terms:
- Potometer: apparatus that measures the rate of water uptake of a cut shoot.
Common mistakes
Saying the potometer measures water lost. It measures water taken up, which we use as a measure of water lost. Changing two factors at once, so you cannot tell which one caused the change. Letting air into the xylem when cutting the shoot.
Rate as a compound measure
A rate is a compound measure. It combines two measurements, an amount and a time, into one unit. The rate of transpiration is the distance the bubble moves, or the volume of water taken up, divided by the time.
Rate = distance (or volume) ÷ time
Worked example 1
The bubble in a potometer moves 36 mm in 12 minutes. Rate = 36 ÷ 12 = 3 mm per minute.
Worked example 2
Each 1 mm of the capillary tube holds 0.8 mm3 of water. The bubble moves 30 mm in 10 minutes. The volume of water taken up is 30 × 0.8 = 24 mm3. The rate = 24 ÷ 10 = 2.4 mm3 per minute.
Graphs and tables
Results from a potometer are often put in a table and then plotted as a graph. You need to move between the two.
| Temperature (°C) | Rate (mm per minute) |
|---|---|
| 10 | 1.0 |
| 20 | 2.0 |
| 30 | 3.5 |
| 40 | 4.5 |
✎ Plotting a graph
Put the factor you changed (temperature) on the x-axis and the rate on the y-axis. Choose scales that use more than half of the grid, with equal steps, for example 0 to 50 on the x-axis and 0 to 5 on the y-axis. Label both axes with units and plot each point carefully.
📈 Reading a graph
Describe the pattern first: here the rate rises as temperature rises. Then use numbers: the rate more than triples between 10 °C and 40 °C. To read a value, go up from the x-axis to the line, then across to the y-axis.
Stomata data: means, sampling and areas
You can also investigate how stomata are spread over a leaf. Take a thin layer of the leaf surface and look at it under a light microscope. Count the stomata you can see in the field of view, which is the circle of the slide you see through the eyepiece.
One count is not reliable, because stomata are not spread out evenly. So take a sample: count in several fields of view at random places, and on more than one leaf. Remember, a bigger sample gives more reliable results. Then find the mean.
Key terms:
- Mean: the total of all the values divided by the number of values.
- Field of view: the circular area you can see through the microscope.
Worked example 3
Five fields of view give counts of 9, 11, 10, 12 and 8 stomata. Mean = (9 + 11 + 10 + 12 + 8) ÷ 5 = 50 ÷ 5 = 10 stomata. The area of the field of view is 0.20 mm2. The number of stomata per mm2 = 10 ÷ 0.20 = 50.
Worked example 4
The lower surface of a rectangular leaf measures 20 mm by 15 mm, so its area is 300 mm2. With 50 stomata per mm2, there are about 50 × 300 = 15,000 stomata on that surface.
Common mistakes
Forgetting to divide by the number of values when finding a mean. Dividing the count by the wrong area. Counting in only one place on the leaf.
Exam-style question
A student set up a potometer. The air bubble moved 45 mm in 15 minutes.
(a) Calculate the rate of water uptake in mm per minute. [2 marks]
(b) The student then used a fan to blow air over the leaves. Predict what happens to the rate and explain why. [3 marks]
(c) Give two things the student should keep the same so that this is a fair test. [2 marks]
Model answer
(a) 45 ÷ 15 (1) = 3 mm per minute (1).
(b) The rate increases (1). The moving air carries water vapour away from the leaf (1). This keeps the concentration gradient steep, so more water vapour diffuses out (1).
(c) Any two from: temperature, light intensity, humidity, the same shoot (1) (1).
Exam tip
In part (a), always show the division and give the unit. In part (b), mention the concentration gradient of water vapour.