The following classroom activities were developed over two summers by Donna Schmidt as part of a teacher summer research experience funded by the National Science Foundation. They have been designed as an ongoing investigative experience that to some extent mimics the actual process of science as it occurs in a research facility. Donna Schmidt is a high school biology teacher at Pattonville High School in St. Louis, MO.

Zea mays, Responding to the Environment
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Name_____________________________

Zea mays, Responding to the Environment

A drive through Missouri, particularly through the major river valleys, gives a visual demonstration of the importance of corn and other crops to the local economy. In both St. Louis and St. Charles Counties there are many farms growing corn. We are very fortunate to have a fairly regular rainfall, although, some years, or even months can be very dry compared to others. Farmers in our area solve the problem of irregular rainfall with irrigation. Because of the Missouri and Mississippi rivers, they can be assured that water will always be available to them for this purpose. Not all farmers are so lucky. For instance, many countries in Africa have great difficulty feeding their populations because of drought. They do not have dependable rainfall or a source of fresh water for irrigating crops. In these poor nations most people cannot afford food that has been imported from far away and without the ability to grow their own food they may starve. Scientists all over the world, including those at Monsanto and the Donald Danforth Plant Science Center in St. Louis, are conducting research to find a solution to this problem. While the basic mechanism of water movement through plants is known it isn’t well understood how some plants are better able to survive under drought conditions. Read in your textbook to remind yourself how water moves from the roots to the leaves of a plant.

In order to photosynthesize, plants must take in CO₂ and release O₂. These gases are exchanged with the environment through openings in the leaves of plants called stomata. Unfortunately, water vapor is also lost through these openings and water is another necessary ingredient for photosynthesis. So, for a plant, it is important to maximize the gain of CO₂ while minimizing the loss of H₂0. For example, during the daylight hours the plant needs lots of CO₂ to perform photosynthesis and so the loss of H₂0 is justified. But, what if it is a cloudy day? How much photosynthesis is the plant likely to perform? Would opening the stomata to take in C0₂ be worth the loss of H₂0? Under environmental conditions that don’t allow a high rate of photosynthesis or that increase the evaporative loss of water, a plant would benefit most from having the stomata closed. Like all living things, plants must be able to respond to the environment in such ways. For each environmental condition listed below, circle open or closed to indicate the stomatal position that would most benefit the plant. Remember that you are weighing the cost of excess water loss with the benefit of photosynthesis.

1. Sunny day with lots of water available.	Open/closed
2. Sunny day but very dry soil.	Open/closed
3. Very high temperature	Open/closed
4. Moderate temperature	Open/closed
5. Strong winds	Open/closed
6. Mild winds	Open/closed
7. Total cloud cover	Open/closed

As you just read in the book, this process where water is transported from the roots, through the xylem cells of the plant to the stomata in the leaves where it evaporates into the atmosphere is called transpiration. How might you measure whether or not a corn plant was losing water by transpiration through the stomata in its leaves? Water has mass; therefore, when it evaporates from the plant or soil, the pot holding the soil and plant would lose weight. By ensuring that no water is lost due to evaporation from the soil, then any weight loss must come from water being lost through the stomata. The following experiment is designed to measure the rate of water being lost through the stomata, or, transpiration rate of the plant.

Purpose

To conduct an assay to measure the amount of transpiration occurring in a plant.
To develop and conduct an experiment using this assay that compares transpiration rates of groups of plants in varying environmental conditions.

Materials

Ruler	Hole punch	Fan
Electronic balance for weighing	Soil to fill three pots	Light stand
Dark cabinet	Plastic wrap	Tape
Three identical pots	Corn seed (3)

Procedure

Part 1: Planting seed

Fill each pot with soil, and then add enough water to thoroughly soak the soil in each pot. This should have caused the soil to settle.
In one pot plant a seed at least 2 centimeters deep in the soil. You may need to create a hole in the soil using your finger so that when the pot is refilled to the top with soil the seed will be at the correct depth. Use a ruler to measure. The other pot is the control.
Add a small amount of water to wet the soil you added
Label your pots and place them on the plant stand until the seedling emerges in approximately 5-6 days. Water daily.
Make a data table to record the initial weight of each pot, (12 controls and 12 with plants) and space to record a daily weight loss for 7 days.

Part 2: Bioassay

When the seedling emerges you will wrap the top and bottom of the pot with Saran wrap to prevent evaporation of water from the soil.
Before wrapping, add 30 ml of water to each pot.
Tear off a section of wrap approximately 15 cm long.
Place a 2-3 cm piece of masking tape in the center of the wrap and use the hole-punch to make a hole in the center of the tape.
Place the wrap over the top of the pot so that the seedling sticks up through the hole (see figure 1). Gently place a rubber band around the wrap and pot to secure it in place.
Tear off a second piece of wrap the same size and place it over the bottom of the pot and secure it in place with a rubber band.
Follow the same procedure for the control pot, but place a plastic straw through the opening.

figure 1
Weigh pots on the electronic balance and record the initial weight in a data table. Continue weighing pots daily for seven days. Record data from other groups each day.
When pots have lost a total of 30 grams, add 30 ml of water by gently lifting up the edge of the top Saran covering and adding the water. (30ml of water weighs 30 grams)
After adding water, remember to take a new initial weight and use the new initial weight to determine weight loss for the next day.
At the end of seven days, make a graph in your lab spiral to show the average weight loss of each group of pots over the seven day time period.

Answer the following questions in your lab spiral:

What was the total average weight lost by each group?
How can you explain the difference in total weight lost by each group?
From what part of the plant is water being lost?
The weight loss from the plant can be considered evidence that the stomata are open/closed.

Part 3: Experiment

Now that you have a method for measuring transpiration rate, you will design an experiment to test the effects of different environmental conditions on transpiration rate. Go back to the introduction and review the various conditions that might cause the stomata to open or close. This experiment will be conducted in table lab groups.

All of the following questions should be answered in your lab spiral.

Dependent Variable

What did you measure in the assay to determine if the plants were transpiring, meaning their stomata were open?____________________

This is your dependent variable.

Independent Variable

What environmental condition will you test? Or, what do you think will cause the stomata to close, thereby slowing the rate of transpiration?_______________

This is your independent variable.

Hypothesis

Now put your independent (cause) and dependent (effect) variables into a statement that predicts what you expect to see in the results of this experiment.

Constants

What are all of the other variables that could affect the dependent variable? You must make sure to keep these variables the same for all groups in the experiment to make it a "fair test" of your independent variable. Make a list of variables you will keep constant.

Procedure

Write a detailed description in your spiral of how you will carry out the experiment. This is basically the procedure we used in the assay, but you will add the experimental conditions that you are testing. You will also want to use plants as the control, so, what will be the conditions you put the control in vs. the experimental group? Go back to the materials list to help you decide how you will mimic the environmental conditions you are testing. Ask the teacher if there is something you need that is not on the list.

Data Table

Make a table in your lab spiral to record your measurements.

Results

Use Microsoft Excel to analyze the data and make a graph. Explain the graph in this section.

Conclusion

What did you find out? Was your hypothesis supported by this experiment? Did the plant respond to the changed environmental condition by closing its stomata? How do you know? Use specific data collected to support your conclusion. Discuss experimental design errors that may have affected your results.

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