9.1.6 Skills: Experiments Investigating the Rate of Transpiration

• The effect of environmental factors on the rate of transpiration in plants can be measured using a piece of equipment called a potometer
  • Note that while potometers are used to measure transpiration rates, they technically measure the rate of water uptake rather than the rate of transpiration, as a small amount of the water taken up by a plant will be used in photosynthesis
    
    • Because the amount of water used in photosynthesis is so small in relation to the total amount of water that passes through a plant, the rate of water uptake can reasonably be used to represent the rate of transpiration
• Different types of potometer exist
  • Bubble potometers measure the movement of an air bubble along a water-filled tube connected to a plant shoot as water is drawn up by the shoot
    • The position of the air bubble is recorded at the start of an experiment, and then a researcher can either measure how far the bubble moves in a set amount of time, or time how long it takes for the bubble to move a certain distance
  • Mass potometers measure the change in mass of a water-filled test tube connected to a plant shoot as it loses water over a set amount of time
• The effect of various environmental factors on transpiration can be measured by placing the potometer in different conditions e.g.
  • Wind speed
  • Humidity
  • Light intensity
  • Temperature

Investigating the effect of light intensity on the rate of transpiration

Apparatus

• Plant shoot
• Cutting board
• Scalpel/scissors
• Paper towels
• Potometer
• Volume scale
• Beaker
• Capillary tube
• Stopwatch
• Vaseline

Method

• Cut a shoot underwater
  • This is done to prevent air from entering the xylem; this could block the movement of water through the plant
• Set up the apparatus as shown in the diagram, ensuring that it is airtight, and using vaseline to seal any possible gaps
• Dry the leaves of the shoot
  • Any water present on the leaves might affect the rate of transpiration as it could block the stomata
• Remove the capillary tube from the beaker of water to allow a single air bubble to form and then place the tube back into the water
• Set up a light source from which the light intensity can be varied
  • This could be achieved by varying the light bulbs used or by varying the distance between the light source and the plant shoot
• Allow the plant to adapt to the new environment for 5 minutes
• Record the starting location of the air bubble, leave for a set period of time, and then record the end location of the air bubble
• Change the light intensity by a measurable amount e.g. moving the lamp 10cm further away from the plant shoot
• Reset the bubble by opening the tap below the reservoir
• Repeat the experiment at the new light intensity, and again at a range of different intensities

• A potometer can be used to test hypotheses about the effect of various environmental factors, including temperature or humidity, on transpiration rates
• Environmental factors can be investigated in the following ways
  • Air movement
    • A fan on different settings could be used to vary the flow of air around a plant shoot
  • Humidity
    • Enclosing the plant shoot in a plastic bag can increase the humidity
    • A humidifier or dehumidifier could be used to give a measurable variation in humidiy
  • Light intensity
    • A lamp at different distances or with different types of light bulb can be used to vary light intensity
  • Temperature
    • A thermometer or temperature probe can be used to find surroundings with different air temperatures
    • A heater or air conditioner can be used to give a measurable variation in temperature
• A researcher would need to be aware of the importance of controlling any variables other than the variable of interest to ensure that any results were valid e.g. placing a plant shoot in different rooms could be a way of varying temperature, but might bring the risk of also varying light levels and humidity; these variables would need to be controlled

Factors affecting the rate of transpiration

• Air movement
  • More air movement leads to increased rates of transpiration 
    • The air outside a leaf usually contains a lower concentration of water vapour than the air spaces inside a leaf, causing water vapour to diffuse out of the leaf
    • When the air is relatively still, water molecules can accumulate just outside the stomata, creating a local area of high humidity
    • Less water vapour will diffuse out into the air due to the reduced concentration gradient
    • Air currents, or wind, can carry water molecules away from the leaf surface, increasing the concentration gradient and causing more water vapour to diffuse out
• Temperature
  • Higher temperatures lead to higher rates of transpiration, up to a point at which transpiration rates will slow 
    • An increase in temperature results in an increase in the kinetic energy of molecules
    • This increases the rate of transpiration as water molecules evaporate out of the leaf at a faster rate
    • If the temperature gets too high the stomata close to prevent excess water loss
    • This dramatically reduces the rate of transpiration
• Light intensity
  • Higher light intensities will increase the rate of transpiration up to a point at which transpiration rates will level off 
    • Stomata close in the dark and their closure greatly reduces the rate of transpiration
    • Stomata open when it is light to enable gas exchange for photosynthesis; this increases the rate of transpiration
    • Once the stomata are all open any increase in light intensity has no effect on the rate of transpiration
• Humidity
  • Higher humidity levels reduce the rate of transpiration 
    • If the humidity is high that means the air surrounding the leaf surface is saturated with water vapour
    • This causes the rate of transpiration to decrease as there is no concentration gradient between the inside of the leaf and the outside
      • At a certain level of humidity, an equilibrium is reached; water vapour levels inside and outside the leaf are the same, so there is no net loss of water vapour from the leaves