Investigating the Effect of Environmental Factors on the Rate of Transpiration

Aim

To investigate the effect of an environmental factor, such as wind, light, or humidity, on the rate of transpiration in a leafy shoot, using a potometer.

Variables

• Manipulated variable: the environmental factor being tested, for example the presence of moving air from a fan, bright light from a lamp, or high humidity created by covering the shoot with a plastic bag.
• Responding variable: the rate of water uptake by the shoot, measured as the distance an air bubble moves along the capillary tube of the potometer per unit time.
• Controlled variables: the same shoot and leaf area used throughout a run, the size of the air bubble, all other environmental factors kept constant, and the time interval used for each reading.

Materials and apparatus

• A potometer, consisting of a capillary tube, a reservoir, and rubber tubing
• A leafy shoot
• Water
• Petroleum jelly, to seal all joints
• A stopwatch
• A ruler
• An electric fan
• A lamp
• A large plastic bag
• Scissors

Procedure

1. Cut a leafy shoot under water to prevent air from entering the xylem vessels.
2. Assemble the shoot into the potometer while it is still under water, sealing every joint with petroleum jelly to make the apparatus airtight.
3. Allow a single air bubble to enter the capillary tube.
4. Measure the distance the air bubble moves in a fixed time under normal room conditions to obtain a baseline rate.
5. Introduce one environmental factor at a time, for example switching on the fan, shining a bright lamp on the shoot, or covering the shoot with a plastic bag.
6. Measure the distance moved by the air bubble in the same fixed time under each condition.
7. Return to baseline conditions between tests and refill the bubble as needed before testing the next factor.

Expected results

The rate of transpiration, shown by the distance the air bubble moves per unit time, increases when wind is introduced and when light intensity or temperature is increased. The rate decreases when the shoot is enclosed in a plastic bag, which raises the humidity around the leaves.

Conclusion

The rate of transpiration is affected by environmental factors. It increases with wind speed, light intensity, and temperature, and decreases with higher humidity.

These factors change the diffusion gradient of water vapour between the air spaces inside the leaf and the surrounding air, which speeds up or slows down the diffusion of water vapour out of the stomata.

Paper 3-style questions

Context. Using a potometer, a student measured the distance an air bubble moved in 5 minutes under different conditions: 20 mm in normal room conditions, 38 mm with a fan, 34 mm under a bright lamp, and 9 mm inside a plastic bag.

Question 1 (hypothesis). State a suitable hypothesis for the effect of wind on the rate of transpiration.

Model answer. Moving air (wind) around the leaves increases the rate of transpiration, so the air bubble moves a greater distance in a fixed time when a fan is switched on.

Question 2 (variables). State the responding variable and explain why it is taken as a measure of the rate of transpiration.

Model answer. The responding variable is the distance the air bubble moves per unit time, which measures the rate of water uptake by the shoot. Because most of the water taken up is lost as water vapour through the stomata, the rate of water uptake is used as an indirect measure of the rate of transpiration.

Question 3 (tabulation and graph). Describe how the results should be recorded and displayed.

Model answer. Record each condition and the distance moved by the bubble in a table with units. Since the conditions are separate categories, display the results as a bar chart with the condition on the horizontal axis and the distance moved (or the rate) on the vertical axis, using bars of equal width with gaps between them.

Question 4 (inference). Using the readings, compare the effect of the fan and the plastic bag and explain the difference.

Model answer. The fan increased the distance from 20 mm to 38 mm, while the plastic bag reduced it to 9 mm. The fan removes water vapour from around the leaf and keeps the diffusion gradient steep, so water vapour diffuses out faster. The plastic bag traps water vapour and raises the humidity, which reduces the diffusion gradient, so transpiration slows down.

Safety

• Cut the leafy shoot with sharp scissors or a blade, cutting away from your fingers and on a firm surface.
• Keep the electric fan and the lamp, with their plugs and cables, away from the water to reduce the risk of electric shock; dry your hands before using the switches.
• Handle the glass capillary tube of the potometer carefully, as it is thin and breaks easily.
• Wipe up any spilled water at once so the floor and bench do not become slippery.

Common mistakes

Source:SRC-DSKP-EN

Frequently asked questions