Balancing the Water Budget of a Leaf

Discussing and viewing stomata

Ask students to open their mouths wide open for several seconds. If you really want to be dramatic you can blow a fan on them. What is happening to the water on the surface of their mouth? It is evaporating, of course. Plants have very small “mouths” called stomata (singular: stoma) that they leave open in order to move carbon dioxide and oxygen in and out of the leaf. Unfortunately, while they are doing this, they also loose water just like we loose water if we were to leave our mouths wide open. Plants are so sophisticated that when they are low on water, when there is no sunlight or when they have plenty of carbon dioxide, they close their stomata.
So how much water can they loose? According to Dr. Kim Coder at the University of Georgia, water moves at 92 feet/hr in a red oak tree in the summer, but only 3 feet/hour in a hemlock tree. The United States Geological Survey reports that an acre of corn gives off about 3,000-4,000 gallons of water each day, and a large oak tree can transpire 40,000 gallons per year (a bathtub holds about 50 gallons of water, so an oak tree can transpire 800 bathtubs worth of water in a growing season).

Either in advance or with your students, gather a few leaves of any tree, shrub or herb. A large leaf, as opposed to a grass leaf or pine needle, works best. Paint the underside of the leaf with clear nail polish. Let it dry completely (about 5 minutes). Place clear mailing tape on the nail polish. Press it down smoothly and then lift up the tape. Place the tape, sticky side down, on a microscope slide. View under a microscope to look for stomata. If this method is unavailable to you, you can download photos of stomata below. Those photos were taken of stomata “captured” with the method described here.

Which leaves transpire the most amount of water in our schoolyard?

1. Select areas, species, and branches etc. to study based upon your question, e.g., if your question "How does transpiration differ in sunny versus shaded conditions?", then find branches from the same species that have similar leaves (number, size, etc.). Or if your question is, "How do two species differ in the amount of water loss that takes place over the course of 1 week?", then find twigs from two species that are similar in all other respects (e.g., exposure to sun, size of leaves, etc.). You can have some students work on one question, and another group on a different question. You may want to have them do this outside while looking at plants in the schoolyard. Once each student or group has a question, ask them to write their question and hypothesis at the top of their data sheet.
2. Choose at least two branches approximately 15cm long (as replicates) in each of the two or more situations you've decided to focus on. All branches should be as similar as possible, and each must be able to fit into the plastic bag.
3. Mark each branch with a twist tie or tape, and label them in some way (e.g., 1 to 4). Record which number corresponds to which branch.
4. Carefully place the branch with all its leaves in the bag and seal the bag onto the branch as tightly as you can without damaging it.
5. After 4-8 days, carefully remove the plastic bag from the branch, making sure you don't spill any water.
6. Pour the water into a beaker or graduated cylinder to determine the volume of water that was lost from the twig. Record this amount.
7. Repeat this procedure for each bag.
8. Calculate the amount of water that transpired per square cm of leaf (download data sheet below)

• Clip the part of the branch that was in the bag, or just its leaves.
• Lay the clear cm grid over each leaf (or choose 2 leaves if there are many leaves in the bag) and estimate its area (you can also have student calculate area by measuring the length and width of the leaf, but this is less accurate).
• Add up the areas of all of the leaves to get a total amount of leaf area that was in each bag.
• Divide the volume of water by the total leaf area to determine transpiration as ml water per square cm of leaf.

Which leaves have the most amount of water in them?
Note: this will require a sensitive scale. If these are not available, you can simple skip this step.

1. The calculations above tell us which plants pump out the most water, but we also want to know which plants have the most water in the leaves. In order to figure this out we will dry the leaves.
2. Weigh the leaves from each bag, after wiping them with a paper towel so the surface is dry. Record this “wet weight”.
3. Dry the leaves in a dehydrator, in the sun, in a conventional oven (140F for 4-12 hours), a solar oven (see www.solarnow.org/pizzabx.htm for directions on making a pizza box solar oven) or a laboratory oven.
4. Measure the leaves dry and record the “dry weight”.
5. Subtract the dry weight from the wet weight. The difference is the amount of water that was in the leaf. One gram of water is equal to one mL of water.
6. Using the area of the leaves, divide the amount of water in the leaves by the area of the leaves to be able to compare all the bags in terms of mL of water per square cm. (see data sheet below for example calculations)

Summarize and build on systems thinking skills

1. Draw a large leaf on the board and explain the input, use and output of water (see diagram below) of the leaf “ecosystem”. These three items (input, use and output) form the basis of understanding an ecosystem, or any system.
2. Then have students use the class data to determine which leaves transpire the most and least amount of water, and which had the most amount of water in them. Depending on the branches they chose to bag, ask them if their hypothesis was correct or not.

Extension

1. Now, what if our system is not a leaf but a person? What are our inputs, outputs and internal uses of water? Develop a class list of the water inputs, outputs and uses of water in our human body system.
2. Now ask each group to choose an organism and conduct research in the library and internet to write a “water diary” for that organism. They can be artistic by making a poster and drawing that organism as a system, with input and output arrows, and a place to write water use inside the organism. As they are working in a group, you could ask them to have one or two people work on the input, one or two work on the output and one or two work on what the organism uses water for. When completed, they should present their posters to the class. This final poster can be used as an assessment of their understanding of a water budget.