Transpiration is the loss of water vapour from plant leaves through stomata. The rate of transpiration is strongly affected by temperature, which influences both the evaporation of water from mesophyll cells and the aperture of stomata. In this investigation, you will use a potometer to measure water uptake as a proxy for transpiration rate across a range of temperatures, generating a fully continuous and quantitative dataset ideal for an IB Biology IA.

This practical is suitable for IB Diploma Biology HL and SL and Edexcel IGCSE Biology.

Background Theory

Water moves from roots to leaves via the transpiration stream, driven by the evaporation of water from mesophyll cells into air spaces and then out through stomata. Temperature affects transpiration in two ways: higher temperatures increase the kinetic energy of water molecules (increasing evaporation rate) and can cause stomata to open more widely. The relationship between temperature and transpiration rate is approximately exponential at moderate temperatures, following a Q₁₀ relationship where rate roughly doubles for every 10 °C rise.

A potometer measures the rate of water uptake by a shoot, which is a reliable proxy for transpiration rate under controlled conditions. The distance moved by an air bubble in a capillary tube per unit time gives the rate of water uptake in mm min⁻¹ or cm³ min⁻¹.

Variables

• Independent variable (IV): Temperature of the surrounding air or water bath (°C) — e.g. 10, 15, 20, 25, 30, 35, 40 °C
• Dependent variable (DV): Rate of water uptake (mm min⁻¹ or cm³ min⁻¹) measured by bubble movement in the potometer capillary
• Controlled variables (CV): Same plant shoot throughout, light intensity (constant lamp at fixed distance), humidity (use a sealed chamber or consistent lab conditions), wind speed (no fan), leaf surface area

Equipment

• Potometer (bubble potometer or weighing potometer)
• Leafy shoot (e.g. Impatiens, cherry laurel, or similar with large leaf area)
• Water baths or temperature-controlled chambers at 10, 15, 20, 25, 30, 35, 40 °C
• Thermometer (±0.5 °C)
• Ruler (mm scale)
• Stopwatch
• Vaseline (to seal any air leaks)
• Clamp stand
• Constant light source (lamp at fixed distance)

Safety

⚠️ Take care with electrical equipment (lamps) near water. There are no significant chemical hazards. No waste disposal required.

Method

1. Cut the shoot under water to prevent air entering the xylem. Immediately insert into the potometer under water and seal with vaseline. Check there are no air leaks.
2. Allow the shoot to acclimatise at your starting temperature for 10 minutes.
3. Introduce an air bubble into the capillary tube by briefly removing it from water. Allow the bubble to settle.
4. Record the position of the bubble. Start the stopwatch and record the bubble position every minute for 5 minutes. Calculate the rate of movement (mm min⁻¹).
5. Reset the bubble by allowing water to flow back. Adjust the temperature to the next value and allow 10 minutes to equilibrate.
6. Repeat the bubble timing at each temperature. Take at least three readings per temperature and calculate a mean rate.
7. Record the actual temperature at each measurement using the thermometer.

Results Table

Temperature (°C)	Rate 1 (mm min⁻¹)	Rate 2 (mm min⁻¹)	Rate 3 (mm min⁻¹)	Mean Rate (mm min⁻¹)
10
15
20
25
30
35
40

Analysis

1. Plot mean transpiration rate (y-axis) against temperature (x-axis). Describe the shape of the curve.

2. Calculate Q₁₀ values between pairs of temperatures 10 °C apart using: Q₁₀ = rate at (T + 10) / rate at T. Do your values approach 2, as predicted by theory?

3. Plot ln(rate) against temperature. If the relationship is exponential, this graph should be linear. Calculate the gradient and use it to determine the activation energy of the transpiration process.

4. Note whether the rate levels off or declines above 35–40 °C. What biological explanation would account for this?

Discussion Points

• Why does transpiration rate increase with temperature? Discuss in terms of kinetic energy of water molecules and water potential gradient.
• Why might transpiration rate plateau or decrease at very high temperatures? Consider stomatal closure and protein denaturation.
• Why is water uptake by the potometer not exactly equal to transpiration rate? Where else might water be going?
• How do humidity and wind speed interact with temperature to affect transpiration? Why must these be controlled?
• What would the results look like if the leaves were coated in petroleum jelly (Vaseline) on the underside?

IA Guidance

The Q₁₀ analysis and ln(rate) vs temperature plot add significant mathematical depth to this IA. To score highly:

• Research Design: Justify your temperature range — explain why you stop at 40 °C (risk of stomatal closure and tissue damage). Explain why the same shoot is used throughout and how you will reset the bubble between readings.
• Data Analysis: Include error bars. Calculate Q₁₀ for each 10 °C interval with uncertainty. Include the ln(rate) vs temperature graph and comment on linearity. Discuss what the gradient reveals.
• Conclusion: State your Q₁₀ values and compare to the expected value of ~2. Discuss whether the relationship is truly exponential or whether there is evidence of a plateau at higher temperatures.
• Evaluation: Discuss the limitations of using a single shoot — variation in stomatal density, leaf area, and xylem diameter all affect results. Suggest using a weighing potometer for more accurate absolute measurements of water loss.

• Practical Biology