Using a Potometer

Using a Potometer

• A potometer is an apparatus used to measure water uptake in plants and thereby infer the rate of transpiration.

Setting Up a Potometer

• Prior to setting up a potometer, the shoot should be freshly cut under water to prevent air entering the xylem.
• The apparatus consists of a graduated tube for measurement, connected to a water reservoir and a shoot of a plant.
• The water surface in the tube is marked at the start of the experiment.
• It’s crucial to ensure the apparatus is water-tight and air bubble free.

Procedures and Precautions

• After setting up, the plant shoot is exposed to light. This will cause the plant to transpire, and so the water level in the tube drops.
• Any changes in the water level are then recorded over a set period of time.
• Temperature, light intensity, humidity can affect the rate of transpiration so they should be kept constant to ensure a fair test.
• Care is needed with the cut shoot and the glass apparatus as both can be easily damaged.

Understanding Results

• The potometer does not directly measure the rate of transpiration. It measures water uptake by the plant. Most of the water is used for transpiration but a small amount is used for photosynthesis and other processes.
• If the rate of water uptake increases, it can be inferred that the rate of transpiration is also likely increasing.
• The data collected can be used to construct a graph to better visualise the water uptake over time.

Limitations of a Potometer

• It’s important to note a potometer only provides a rough measure of transpiration rate. It’s not accurate because it measures water uptake, which is not just for transpiration. It also assumes the conditions remain constant which is often difficult to achieve.
• It also does not truly replicate all the natural conditions plants would face in their natural environment, so it is not a wholly accurate representation of transpiration in nature.