Investigating Transpiration practical

• Water movement up a plant increases as the transpiration rate increases. Water evaporates from the spaces between cells in the leaf through the stomata, it moves down the water potential gradient so for water to leave through stomata there has to be less water in the atmosphere than in the leaf
• Factors that affect transpiration rate: LIGHT INTENSITY

  • Stomata open when it is light to let in CO2 for photosynthesis, when it is dark no photosynthesis is happening so the stomata close
  • Little transpiration happens at night/darkness
• Factors that affect transpiration rate: TEMPERATURE

  • In warmer conditions water particles have more kinetic energy so they are able to evaporate from the surface of the leaf faster, this increases the water potential gradient between the inside and outside of the leaf so the transpiration rate is increased
• Factors that affect transpiration rate: HUMIDITY

  • If the air around plants is dry, the water potential gradient is increased so transpiration occurs faster
  • In humid environments, the water potential gradient is reduced as there is a lot more water in the atmosphere so the rate of transpiration decreases
• Factors that affect transpiration rate: AIR MOVEMENT

  • Lots of wind blows water molecules away from the stomata increasing the water potential gradient and therefore increasing the transpiration rate
• High Transpiration rate:

  • High light intensity
  • High temperature
  • High wind
  • Low humidity
• Low Transpiration rate:

  • High humidity
  • Low light intensity
  • Low temperature
  • Low/no wind
• Using a potometer:

  • A potometer can measure water uptake by the plant and can be used to estimate how different factors affect the transpiration rate
  • eg using a fan, creating a humid environment, using different light intensities
• It is almost impossible to measure transpiration because it is extremely difficult to condense and collect all water vapour that leaves all the parts of a plant

  • However, what we can measure is the amount of water that is taken up in a given time by a part of the plant such as a leafy shoot - the rate of uptake is almost the same as the rate at which transpiration is occurring
  • Can then measure water uptake by the same shoot under different conditions
• Considerations when measuring transpiration rate:

  The shoot must be cut underwater

  • Prevent air entering the xylem (so the water column remains unbroken)
• Considerations when measuring transpiration rate:

  Shoot cut on an angle

  • Increase surface area available to uptake water
• Considerations when measuring transpiration rate:

  • End of the capillary tube must stay submerged in water
  • The diameter of the capillary tube should be known/measured
• Considerations when measuring transpiration rate:

  This is only an estimate as it assumes all water taken up is lost in transpiration but some water is used up in reactions eg photosynthesis, some produced by respiration, some used to support the plant (stored in vacuoles)
• To measure the rate of transpiration:

  1. A leafy shoot is cut underwater, using a rubber tube the leaf shoot is fitted to the potometer under water
  2. Potometer is removed from under the water to allow an air bubble to form at the end of the capillary tube
  3. Start point of the air bubble must be recorded
  4. Using a stopwatch, the time it takes for the bubble to move a certain distance can be recorded - the rate is an estimate of transpiration rate
• Measuring rate of transpiration: Restart/repeat the experiment

  • Once the air bubble nears the junction of the reservoir tube and the capillary tube, the tap on the reservoir is opened and the syringe is pushed down until the bubble is pushed back to the start of the scale on the capillary tube
  • Measurements then continue as before