What is a potometer and how do you assemble one for measuring transpiration?

A potometer is a device used to measure water loss from leaves. Assemble it by attaching rubber tubing to a 5 mL calibrated pipette, submerging the device in water to fill it completely without bubbles, then capping the tube with a pipette bulb. Secure the potometer with clamps on a stand, keeping the capped end 1-3 inches below the bulb, and adjust the water level to zero using a transfer pipette before inserting a leaf.

How do you calculate transpiration rate from potometer measurements?

Divide the total water loss measured in milliliters by the time the experiment ran in minutes to get the rate of water loss. Then divide this value by the leaf surface area in square meters. This calculation yields transpiration rate in standardized units, allowing comparison across different leaf species and environmental conditions.

Why is it important to measure stomata density when studying transpiration?

Stomata are the primary sites of water loss in leaves. By counting stomata per unit area using microscope impressions made with nail polish, you can determine if a correlation exists between stomatal density and transpiration rate. This reveals whether plants adapted to drier environments have fewer stomata, a key adaptation for reducing water loss.

What method is used to determine leaf surface area in this experiment?

Trace each leaf outline on paper and cut it out. Weigh the tracing, then weigh a reference 4x4 centimeter paper square of the same type. Divide the tracing weight by the reference weight per square centimeter to calculate total leaf area. Convert the result from square centimeters to square meters for standardized transpiration rate calculations.

How do you prepare leaf samples to observe stomata under a microscope?

Paint the underside of each leaf with clear nail polish, covering at least one square centimeter, since most stomata are located on the leaf bottom. Once dry, press cellophane tape onto the painted area and peel it off to remove the polish impression. Mount the tape on a microscope slide and observe under low magnification first to locate stomata, then switch to higher magnification for counting.

What hypothesis would you test about transpiration rates in plants from different environments?

The experimental hypothesis is that leaves from plants adapted to hotter or arid environments will have lower transpiration rates than those from humid or wet regions. The null hypothesis states that different plant species will not differ in transpiration rates. Data analysis should reveal whether environmental adaptation correlates with both transpiration rate and stomatal density.

How does measuring water loss relate to understanding plant cellular processes?

Transpiration is a key plant process involving water movement through tissues and evaporation from leaves. Understanding this process connects to broader cellular metabolism. While transpiration itself is a physical process, it relates to cellular respiration and photosynthesis, as water availability affects metabolic rates and energy production in plant cells.

Transpiration

Concepts Instructor Prep Student Protocol

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Plants are found in almost every ecosystem in the world from deserts, to temperate forests, and down to the bottom of the sea. As a consequence of natural selection plants have evolved a stunning diversity of adaptations to deal with different environmental challenges. One of the key challenges plants face is maintaining proper hydration. Water is a critical resource that they depend upon for photosynthesis, structural support, and the transportation of nutrients and other important molecules. One way that plants can control their water balance is by regulating a process known as transpiration, which is essentially the evaporation of water from the aerial parts of a plant. This loss of water occurs primarily from pores on the leaves called stomata. But how does the water get here?

To answer this question let's look more closely underground. Here water enters the plants roots by osmosis then moves all the way up to the leaves through a vascular tissue called xylem. This channel of water is known as the transpiration stream. Because water molecules stick to each other and to the xylem walls, when it evaporates from stomata water from lower in the transpiration stream is pulled upwards to take its place…resulting in upward flow from the roots. Now let's take a closer look at a stoma. Each stomatal pore is bordered by two guard cells that can expand to open the pore and contract to close it. Plants open their stomata to take in carbon dioxide for photosynthesis and to release oxygen gas. Loss of water through transpiration is an unavoidable side effect of this process.

This trade off presents a particular challenge for plants living in arid environments and so they have evolved strategies to reduce their water loss as much as possible. One way they can do this is by growing leaves with small surface areas, presenting a small area over which transpiration can occur. This is why the leaves of desert plants such as creosote are relatively small. But going one step further plants from arid environments also have fewer stomata per unit area on their leaves allowing them to minimize water loss through transpiration.

In contrast plants that inhabit environments with plentiful water like rain forests can afford to lose a lot of water through transpiration. Such plants like this taro for example often develop leaves with large surface areas that increase their capacity to intercept light to fuel photosynthesis. These plants also have a high density of stomata compared to plants from arid habitats allowing them to maintain high rates of photosynthesis and to support large leaves and stems.

In this lab you'll measure transpiration rates and examine the frequency of leaf stomata in diverse plant species to reveal how plants from different habitats solve the problem of regulating transpiration.

Print Procedure Steps

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Comparing Transpiration Rates of Leaves
- To begin the experiment to measure the transpiration rates of leaves from different plant species, you will first create a tool called a potometer, using a piece of plastic tubing and 5 mL calibrated pipette.
- To assemble the device, simply attach the rubber tubing to the tip of the pipette.
- Then, submerge the device into a bucket of water and move the pipette around under the water to fully fill the tube, taking care not to create bubbles.
- Next, cap the open end of the tube with a pipette bulb to keep the water in.
- Now, taking care not to spill water from the potometer, use a clamp and stand to hold the capped end of the pipette around 1 - 3 inches below the pipette bulb.
- Then, use a second clamp attached lower on the same stand to secure the open end of the rubber tubing.
- Next, use a transfer pipette or dropper to remove water from the open end of the tubing until the water level in the pipette portion of the potometer rests at the zero line. HYPOTHESES: In this experiment, the experimental hypothesis might be that leaves from plant species more adapted to hotter or arid environments will have lower transpiration rates than those from humid or wet regions. The null hypothesis would be that leaves of the different plant species will not differ in their transpiration rates.
- To begin the experiment, carefully use scissors to cut the end of the petiole, or stalk, of the first leaf and then insert the leaf into the tubing.
- Then, place a layer of lubricant around the area where the plant stem and rubber tubing meet to make a watertight seal.
- Next, remove the pipette bulb and taking care not to disturb the setup, let the experiment run for the appropriate experimental period, typically 30 - 60 minutes.
- At the end of the allocated time, record the level of the water in the pipette, noting how much the level dropped in milliliters.
- Then, refill and reset the potometer water level to zero, and then repeat the procedure for each of the remaining test leaves until the transpiration rates of all four leaves have been measured.
- Next, to determine the surface area of the leaf samples, lay each leaf on a blank piece of paper and carefully trace the outlines.
- Then, cut the outlines out and record the mass of each tracing.
- Finally, cut out and weigh one four by four centimeter square of the same type of paper used to make the leaf tracings. This will act as a reference with known area and weight.
- Next, you will need to quantify the number of stomata per unit area on each leaf. To do this, first prepare a slide mount by painting at least one square centimeter of the underside of each leaf with clear nail polish. IMPORTANT: Ensure you paint the bottom of the leaf, since the majority of the stomata are located there.
- When the nail polish is completely dry, carefully press a small piece of cellophane tape directly onto each patch and then gently peel off the tape to remove the painted on polish. NOTE: Use a piece of tape that is smaller than the microscope slide.
- Then, place each impression and tape onto a separate, clean microscope slide. HYPOTHESES: In this exercise, the alternative hypothesis could be that the leaves of the plants adapted to drier environments will have fewer stomata per unit area than the leaves of plants adapted to wetter environments. The null hypothesis might be that all plants species will have an equal number of stomata per unit area.
- To observe each impression under the microscope, first use a low magnification to find areas of the leaf impression that contain stomata. These will appear as darker dots or impressions.
- Then, switch to a higher magnification, keeping the sample viewing window centered on the stomata-containing area.
- Draw and label your observations, making sure to label each sketch with the plant species.
- Then, count and record the number of stomata in the field of view. You should perform this count a total of four times in four different areas of the leaf impression, and then determine the average number of stomata per counted area on the leaf.
- To calculate the number of stomata per mm², place a transparent plastic ruler on the microscope stage under the objective and measure the diameter of the field of view.
- You can then use this number, the diameter or D, to calculate the area of the field of view in mm².
Results
- First, calculate the surface area of the 4x4 cm paper square to obtain the number of square centimeters.
- Then, divide the weight of the paper square by the number of centimeters squared it covers to obtain the weight of 1 cm² of paper.
- Finally, to calculate the surface area of each leaf, divide the weight of the leaf tracing by the weight of 1 cm² of paper. This will give you leaf area in centimeters squared.
- You should now convert this number to meters squared, the standard unit used for calculating transpiration rate.
- To calculate the transpiration rate for each leaf, divide the total water loss you measured in the potometer in mL by the time the experiment was run for in minutes.
- Then, divide this by the surface area of the leaf in m².
- Make a bar graph with the four different leaf species on the X axis, and their corresponding transpiration rates on the Y axis. Do these rates appear different?
- Next, add a second Y axis to your chart and plot the average number of stomata per millimeter squared for each leaf that was observed under the microscope. Do you see a correlation between transpiration rate and number of stomata? If so, does this seem to have any relationship to the native environment the tree would typically inhabit?