Affecting Water Uptake by a Plant (OCR Gateway GCSE Biology)

• Air movement, humidity, temperature and light intensity all have an effect on the rate at which transpiration occurs
• 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

Transpiration Rate Factors 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