Background:

Transpiration is the evaporation of water from a plant surface. If water lost from leaves is not replaced by water transported up from the plant roots, the plant will wilt and die.

Transpiration involves a linked chain of processes:

• the concentration of water molecules in the air surrounding the leaf is lower than the concentration of water molecules in the moist air surrounding the leaf's mesophyll cells, resulting in the movement of water vapor through the leaf's stomatal openings into the surrounding air;
• the concentration of water molecules in the moist air spaces surrounding the mesophyll cells is now decreased, resulting in evaporation of water from the outside of the mesophyll cells;
• evaporation of water from the outside of the mesophyll cells causes water to be drawn from the inside of the mesophyll cell to its outer surface;
• the mesophyll cells now have a lower water potential than the xylem, resulting in water moving from the xylem to the mesophyll cells;
• the water potential in the xylem at the top of the plant becomes less than the water potential in the xylem in the lower part of the plant;
• through the combined forces of the differences in water potential, cohesion of water molecules, adhesion of the water molecules to the walls of the xylem cells, and root pressure, the upward force on the water molecules becomes greater than the force of gravity, and the water moves upward in the plant;
• as water moves up the xylem, the water potential in the root xylem tissue decreases;
• when water potential in the roots becomes less than that of the surrounding soil, water moves into the roots, allowing the transpiration process to continue.

In this activity, you will use a low pressure sensor to measure the change in pressure at the end of the plant stem as a result of transpiration. You will then use a fan to blow air across the plant's leaves and use the sensor to measure the change in pressure.

How does the rate of transpiration under normal conditions compare to the rate of transpiration on a windy day?

Variables:

Identify the type of data you will collect to support your hypothesis and state the manipulated, responding and controlled variables in this investigation.

SAFETY REMINDER: Be careful when cutting with sharp objects.

• Placing your thumb over the free end of the tubing, put the tubing under water and insert the cut plant stem into the tubing. Avoid creating any air bubbles in the tubing; pull the tubing away from the stem if air bubbles form.
• Spread the petroleum jelly around the end of the tube to create an airtight seal between the top edge of the plastic tubing and the plant stem.
• Secure the plant in an upright position with the clamp to the lab stand.
• Mount the Pressure Sensor to the lab stand support rod using the other clamp (note that a blue USB pressure sensor will be oriented downward when clamped to the stand, unlike the photo below to the right). The pressure port should be 5-7 cm above the cut end of the plant stem to prevent water from entering the Pressure Sensor.

• Align the quick-release coupling on the end of the plastic tubing with the pressure port of the Pressure Sensor. Push the coupling onto the port, and then turn the coupling clockwise until it clicks (about 1/8 of a turn). Make sure no water enters the sensor.

Data collection:

Step 4:

• Click the Start button ( ) to begin collecting data.
• Collect data for approximately 10 minutes, then click the Stop ( ) button. Note: If the pressure does not change or starts to increase, it is likely that your system has developed a leak. Try reseating the plant in the tube and applying more petroleum jelly around the end of the tube to create a seal.
• Set up the electric fan at least one meter away from the plant. Turn the fan on to a low setting so that it blows a light breeze over the plant.
• Record another data run, again for approximately 10 minutes.

Analyzing and Interpreting:

Step 5:

• Use the graph built-in analysis tools to find the rate of change of pressure for each run of data - click the 'Smart Tool' button in the graph, then move the Smart Tool to the point on Run #1 where the pressure begins to change. Move the cursor to a corner of the Smart Tool. Notice that the cursor changes to a delta shape:

• Click-and-drag the delta cursor to the end of Run #1. The Smart Tool shows the change in time and the change in pressure between the starting and ending points:

• Record the change of pressure and the change of time between the beginning and end points for the first run of data.
• Repeat the process to find the change of pressure and time for the second run of data.

Step 6:

Convert the amount of time to minutes. Calculate the 'Rate of change of pressure' by dividing the change in pressure by the amount of time:

Forming Conclusions:

Based on the data you have collected, write a summary statement for the following questions:

1.	How does the rate of transpiration under normal conditions compare to the rate of transpiration on a windy day?
2.	Does a decrease in pressure in the tubing correspond to an increase or a decrease in water loss through the seedling's stomata? Explain.
3.	Did the fan affect the rate of pressure change? Explain how the fan affects transpiration.
4.	Describe some adaptations that enable plants to minimize water loss from their leaves.

Extension:

• Try repeating this experiment with a data run where you cut off leaves from the plant at regular intervals (for example, every 3 minutes remove a leaf). Explain the resulting pressure change graph in terms of transpiration rate of the plant.