Lecture Notes: Measuring Transpiration with Potometers

Introduction

• Purpose: Understanding how potometers measure water uptake as an estimate for the rate of transpiration.
• Transpiration Overview:
  • Evaporation of water vapor from open stomata mainly found on leaves.
  • Continuous water column flow through xylem from soil due to transpiration.

Factors Affecting Transpiration Rate

1. Temperature
2. Humidity
3. Air movement (wind)
4. Light intensity

Measuring Transpiration

• Potometer: Tool to measure water uptake by a plant section.
  • Measures water uptake, not direct transpiration, as water uptake correlates with transpiration.
  • Useful for comparing transpiration rates among different plant species or under varying conditions (light, air movement, humidity, temperature).

Setting Up a Potometer

1. Cutting the Plant:
   • Cut a small section of the plant underwater to prevent air from entering the xylem, which could break the water column.
2. Filling the Potometer:
   • Ensure all parts are filled with water, submerged to remove air bubbles.
3. Sealing:
   • Insert the plant into the potometer through a rubber seal.
   • Use petroleum jelly on joints to ensure no air entry or water leakage.
4. Introducing an Air Bubble:
   • Create a single air bubble for measurement by briefly exposing the capillary tube to air.
   • Use the bubble's movement to measure water uptake.

Using the Potometer

• Measurement:
  • Start the stop clock when the air bubble reaches zero on the scale.
  • Measure the distance traveled by the bubble to calculate water volume taken up.
  • Calculate water uptake rate by measuring the volume of water divided by time.

Experimental Repeats

• Use the reservoir and tap to reset the setup for multiple trials.

Common Exam Questions

1. Setup:
   • Importance of cutting the plant underwater due to cohesion-tension theory.
2. Sealing:
   • Role of petroleum jelly in sealing joints to prevent air entry and water leakage.
3. Calculating Water Uptake:
   • Calculate volume using the formula: πr² × length.
   • Example: Air bubble movement = 15.28 mm/min, radius = 0.5 mm -> Volume = 12 mm³/min.
   • Possible unit conversions may be required.
4. Controlling Variables:
   • For comparing different species, control leaf surface area (number and size of leaves).

Conclusion

• Potometers provide an effective way to estimate the rate of transpiration indirectly by measuring water uptake.
• Importance of controlling variables and understanding measurement techniques for accuracy.

• Remember: Practical setup and proper sealing are crucial for reliable data in potometer experiments.