Measuring Transpiration
- There are several environmental conditions that have an impact on the rate of transpiration or water uptake
- Air movement
- Humidity
- Light intensity
- Temperature
- The table below explains how these four factors affect the rate of transpiration when they are all high; the opposite effect would be observed if they were low
Factors Affecting the Rate of Transpiration Table
Investigating the effect of environmental factors on the rate of transpiration
- We can investigate the effect of different environmental conditions (such as temperature, humidity, light intensity and wind movement) on the rate of transpiration using a piece of apparatus called a potometer
- There are 2 types of potometer:
- A mass potometer measures a change in mass of a plant as a measure of the amount of water that has evaporated from the leaves and stem
- A bubble potometer measures the uptake of water by a stem as a measure of the amount of water that is being lost by evaporation consequently pulling water up through the stem to replace it
There are 2 different types of potometer that could be used to investigate the effect of environmental conditions on transpiration
Apparatus
- Potometer (bubble or mass potometer)
- Timer
- Lamp
- Ruler
- Plant
Method
- Cut a shoot underwater
- To prevent air entering the xylem and place in tube
- Set up the apparatus as shown in the diagram and make sure it is airtight, using Vaseline to seal any gaps
- Dry the leaves of the shoot
- Wet leaves will affect the results
- Remove the capillary tube from the beaker of water to allow a single air bubble to form and place the tube back into the water
- Set up a lamp 10 cm from the leaf
- Allow the plant to adapt to the new environment for 5 minutes
- Record the starting location of the air bubble
- Leave for 30 minutes
- Record the end location of the air bubble
- Change the light intensity
- Reset the bubble by opening the tap below the reservoir
- Repeat the experiment
- Calculate the rate of transpiration by dividing the distance the bubble travelled by the time period
- The further the bubble travels in the same time period, the greater the rate of transpiration
Calculating the rate of transpiration using a bubble potometer
Investigating transpiration rates using a potometer
- Other environmental factors can be investigated in the following ways:
- Airflow: Set up a fan or hairdryer to blow air over the plant (this investigation can be extended by putting the fan at different distances from the plant or at different fan-speed settings)
- Humidity: Spray water in a plastic bag and enclose the plant within the bag
- Temperature: Change the temperature of room (e.g. cold room or warm room)
Results
- As light intensity increases, the rate of transpiration increases
- This is shown by the bubble moving a greater distance in the 30 minute time period when the lamp was placed closer to the leaf
- Transpiration rate increases with light intensity because more stomata tend to be open in bright light in order to maximise photosynthesis
- The more stomata that are open, the more water can be lost by evaporation and diffusion through the stomatal pores
Limitations
- The potometer equipment has a leak
- Solution: Ensure that all equipment fits together rightly around the rubber bungs and assemble underwater to help produce a good seal
- The plant cutting has a blockage
- Solution: Cut the stem underwater and assemble equipment underwater to minimise opportunities for air bubbles to enter the xylem
- The potometer has shown no change during the experiment
- Solution: Use the plant cuttings as soon as they have been cut, as transpiration rates may slow down when the cuttings are no longer fresh
