3.6.3 Potometer

Setting Up a Potometer:

1. Equipment:

• Potometer.
• Plant shoot.
• Capillary tube.
• Water reservoir.

2. Steps:

• Assemble the potometer underwater to avoid introducing air bubbles.
• Cut the plant shoot underwater at an angle to maximise water uptake.
• Insert the shoot into the potometer and ensure airtight seals using petroleum jelly.
• Allow the shoot to acclimatise before starting measurements.
• Introduce an air bubble into the capillary tube.

3. Measuring Transpiration Rate:

• Record the distance moved by the air bubble over a set time interval.
• The rate of water uptake is calculated as: Rate = Distance moved by bubble ÷ Time.

Factors Affecting Transpiration Rate:

1. Number and Size of Leaves:

• More leaves or larger surface areas increase water loss.

2. Stomatal Density and Position:

• Higher stomatal density or stomata on upper surfaces increase transpiration.

3. Waxy Cuticle:

• A thicker cuticle reduces evaporation.

4. Light Intensity:

• Higher light levels open stomata, increasing transpiration.

5. Temperature:

• Higher temperatures increase evaporation and diffusion rates.

6. Humidity:

• Lower humidity increases the water vapour gradient, enhancing water loss.

7. Air Movement:

• Wind removes water vapour around the leaf, maintaining a steep gradient.

8. Water Availability:

• Limited water reduces stomatal opening and transpiration.

Using Results:

• Graphical Representation:
  • Plot rate of transpiration (e.g., bubble movement) against environmental factors such as light intensity or temperature.
• Analysing Trends:
  • Identify how factors influence the rate of transpiration.
  • Explain trends using biological principles like water potential gradients.

Limitations of a Potometer:

1. Indirect Measurement:

• The potometer measures water uptake, not actual water loss.
• Some water is used in metabolic processes or stored.

2. Air Leaks:

• Air bubbles in the apparatus can disrupt results.

3. Environmental Control:

• It is difficult to maintain constant conditions during the experiment.

Key Terms:

• Potometer: Apparatus to measure the rate of water uptake in plants.
• Transpiration: Loss of water vapour from leaves through stomata.
• Water Potential Gradient: The difference in water potential that drives water movement.