# Practical: Investigating the Rate of Transpiration ## Using a potometer - 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

A potometer is a device used to measure the rate of water uptake by a plant shoot, which estimates the rate of transpiration

### Transpiration Rate Factors | Factor | Condition | Effect on the rate of transpiration (more/less) | |--------|-----------|--------------------------------------------------| | **Air movement** | High | More – good airflow removes water vapour from the air surrounding the leaf which sets up a concentration gradient between the leaf and the air, increasing water loss | | **Humidity** | High | Less – humidity is a measure of moisture (water vapour) in the air; when the air is saturated with water vapour the concentration gradient is weaker so less water is lost | | **Light intensity** | High | More – guard cells are responsive to light intensity; when it is high they are turgid and the stomata open allowing water to be lost | | **Temperature** | High | More – at higher temperatures, particles have more kinetic energy so transpiration occurs as a faster rate as water molecules evaporate from the mesophyll and diffuse away faster than at lower temperatures | ## 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** - Place the shoot in the tube - Set up the apparatus as shown in the diagram - Make sure it is **airtight**, using vaseline to seal any gaps - If air enters the apparatus the readings will be affected - Dry the leaves of the shoot - Any moisture present on the leaves will affect the rate of transpiration - 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 the environmental factor you are investigating - 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 - Record the **end location** of the air bubble - Change the light intensity or wind speed or level of humidity or temperature (only one - whichever factor is being investigated) - Reset the bubble by opening the tap below the reservoir - Repeat the experiment

The further the bubble travels in the same time period, the faster transpiration is occurring and vice versa

## Investigating different environmental factors Environmental factors can be investigated in the following ways: - **Airflow**: Set up a fan or hairdryer - Position at different distances or set to different speeds to vary air movement - A desk fan on low speed simulates gentle breeze conditions - Higher speeds simulate windier environments - **Humidity**: Spray water in a plastic bag and wrap around the plant - This creates a humid microenvironment around the leaves - The water vapour concentration in the air immediately surrounding the plant increases - More water spray = higher humidity levels - **Light intensity**: Change the distance of a light source from the plant - A lamp placed closer to the plant increases light intensity - Moving the lamp further away decreases light intensity - Ensure the lamp doesn't heat the plant significantly (use LED bulbs if possible) - **Temperature**: Temperature of room (cold room or warm room) - Use water baths at different temperatures around the base - Or conduct experiments in rooms with different ambient temperatures - Measure the actual temperature with a thermometer to ensure accuracy

When designing an investigation to ensure a fair test you must keep all factors the same other than the one you are investigating

## Understanding the results - Calculate the distance travelled by the air bubble in mm - Divide by the time taken to calculate rate of water uptake (mm/min or mm/hour) - The rate of water uptake approximates the rate of transpiration - Repeat readings should be taken and a mean calculated to improve reliability - If investigating one variable: - Plot a graph of rate against the environmental factor being tested - For example, temperature (°C) on x-axis versus rate of transpiration (mm/min) on y-axis - This allows you to identify relationships and patterns ### Example Results: Temperature Investigation Below is an example of what results might look like when investigating the effect of temperature on transpiration rate:

- The positive correlation shows that as temperature increases, the rate of transpiration increases - This is because water molecules have more kinetic energy at higher temperatures - They evaporate and diffuse away from the leaf surface more rapidly ### Example Results: Light Intensity Investigation Light intensity can be varied by changing the distance of a lamp from the plant. Example results below:

- As the lamp moves closer (distance decreases), light intensity increases - Higher light intensity causes stomata to open wider, increasing transpiration rate - Note: Distance is inversely related to light intensity (closer = brighter) ### Example Results: Comparing Different Humidity Levels Humidity can be compared using different experimental setups:

- Higher humidity reduces the concentration gradient between the leaf and the air - This decreases the rate of water vapour diffusion from the leaf - In very humid conditions, transpiration is significantly slower ## Key considerations - The potometer measures water uptake, not water loss directly - We assume most water taken up is lost through transpiration - A small amount is used in photosynthesis and to maintain turgidity - The shoot must be cut underwater to prevent air bubbles entering the xylem - Air bubbles would block water transport through the xylem vessels - This would prevent water reaching the leaves and give inaccurate results - The apparatus must be airtight - Any leaks would allow water to escape from the apparatus - This would make it impossible to accurately measure water uptake by the plant - Allowing the plant to adapt to new conditions for 5 minutes ensures: - Stomata have time to respond to the new environment - The plant reaches a steady rate of transpiration - More reliable and representative measurements can be taken ## Safety and practical tips - Take care when cutting the shoot with sharp instruments (scalpel/scissors) - Ensure the plant shoot is healthy and has several leaves - A wilted or damaged shoot will give unreliable results - Keep all other variables constant: - If investigating temperature, keep light intensity, humidity and airflow constant - If investigating light intensity, keep temperature, humidity and airflow constant - The same plant species should be used for all repeats - Different species may have different transpiration rates - Use plants like Busy Lizzie (Impatiens) or Pelargonium which transpire readily