Transpiration with a Potometer
Transpiration is an important concept in both biology and environmental science, especially in terms of role it plays in the water cycle. As water evaporates from the stoma of leaves water is pulled up (due to hydrogen bonding) through the xylem from the roots which have drawn the water from the surrounding soil.
Because transpiration is essentially an invisible process, a potometer is used to measure the rate of water lost to the air. The advantages that sensor technology makes in many investigations in biology and environmental science are that it allows students to see the data in real time while great improving the accuracy and significantly decreasing the time needed to capture data.
Setting up a classic potometer with a Wireless Pressure Sensor is one example of how integrating sensors can improve the data collection process. With the included Leur connectors and tubing, all you need is a plant sample and optional stand with clamps to complete the lab. Students can choose from any plants available, but there are three general guidelines which help ensure success. Students should choose a plant with
- a woody stem/branch that will fit snugly into the tubing, making it less prone to crushing and easier to setup.
- relatively soft cuticle leaves because they generally have higher rates of transpiration and good stomatal density.
- high leaf surface area (either large leaves or lots of leaflets) per stem/branch.
Insert the plant stem into the tubing as shown, making sure there are no bubbles in the tubing and that you have a few centimeters of air between the sensor and water. This can take a few tries to get right, and having a sink or tub to submerse the tubing in will help. The cohesion and adhesion of the water along with a slight positive pressure created when connecting the sensor will keep water out of the sensor even if a stand is not available.
Figure 1. Potometer Setup with Wireless Pressure Sensor
Data collection usually takes 5-10 minutes depending on the plant. For the control run (taken at room temp with ambient light) wait for a change of at least 5.0 kPa before stopping data collection. After the control run is complete, find the rate of transpiration in kPa/min using the curve fit tool and save this into a data table. Save the plants from each trial so the surface area can be calculated and the trial data normalized for comparison.
Figure 2. Sample data from control run at room temperature with ambient lighting
Calculating surface area (SA) can be done using the tried and true method with graph paper, but if you have cameras and computers available students can also use ImageJ— a free image analysis tool from the National Institute of Health. This is a powerful software and the basics are pretty easy to master. The steps for conducing area and size calculations in ImageJ can be found in this blog article or on this video. Although not part of the PASCO software suite, this is another tool that eliminates some repetitive work from the procedure and let students really focus on the data and analysis that support learning.
Figure 3. ImageJ program analyzing leaf SA from control trial.
When the SA is determined, add it to the data table in SPARKvue. A simple calculation provides the adjusted rate in kPa/Min/cm2. In subsequent trials students can investigate the impact of environmental variables such as light intensity, humidity, temperature, and wind— or they can compare different species of plants.
Figure 4. Data analysis table with control and windy trial data
You can download the sample data with the table formatted and calculations created. After students go through the procedure once they can easily iterate this setup to conduct their own inquiry— where the true learning transpires!