PURPOSE: To investigate photosynthesis in winter evergreen leaves.
MATERIALS: For the class -- leaf samples of several winter evergreen species, one large geranium plant, black paper, paper clips. For each group -- hot plate, 600 ml beaker, 400 ml beaker, 250 ml 95% ethyl alcohol, tongs, forceps, 1 petri dish, water, iodine solution.
INTRODUCTION: Northern plants have many adaptations for survival during winter. Some retain their leaves and remain green throughout the year; others lose their leaves in autumn and grow new ones every spring. The function of leaves in both plant types is the manufacture of carbohydrates by the process of photosynthesis. Glucose, the main product of photosynthesis, is converted into starch for storage. Photosynthetic activity in leaves is easy to detect by using iodine to test for the presence of starch.
Many studies of photosynthesis have been done on broad-leaved plants during the summer or indoors. Less is known about photosynthetic activity in winter evergreen leaves. Do plants with evergreen leaves carry on photosynthesis in winter?
In this investigation, you will perform two controlled experiments to test for photosynthesis in winter evergreen leaves outdoors and in a common broad-leaved house plant indoors. You will be able to draw conclusions about the effect of light on photosynthesis in these two kinds of plants.
A. Prepare evergreen plant(s) growing outdoors as follows: Locate one or more evergreen plants that are in sunlight during school hours. Clip a folded piece of black paper over half a leaf or half the needles on a twig as shown in the diagram below. Leave the paper masks in place for at least 24 hours.
B. Prepare a large geranium plant as follows: Clip a folded piece of black paper over half a leaf as shown in the diagram below. Place the plant on the window sill for at least 24 hours at room temperature.
C. At the end of the day prior to the experiments, pick the experimental leaves from both plants and submerge them in alcohol in a beaker overnight.
PREDICTIONS: Use what you know about the effect of light on photosynthesis and the effect of temperature on enzymes to make some predictions:
1. Which leaf samples do you think will carry on photosynthesis in these two experiments? Record your predictions by writing YES or NO in the "Predict" spaces on the diagram below.
PROCEDURE: Now test your predictions experimentally using iodine to indicate the presence (positive test) or absence (negative test) of starch. If starch is present, photosynthesis must have occurred. Each group should test one geranium leaf and one winter evergreen sample.
D. Put about 200 ml water into the 600 ml beaker and heat until boiling on the hot plate.
E. Pick an experimental leaf from one of the geranium plants and one of the evergreen samples. Remove the black paper. Record the species of your evergreen sample:
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F. Submerge the geranium leaf and the winter evergreen sample into the boiling water for 2-3 minutes. Using forceps, transfer the leaf samples to the 400 ml beaker
G. Boil the leaves in alcohol as follows:
(1) Pour 250 ml of alcohol over the leaves in the 400 ml beaker.
(2) Using tongs, set the 400 ml beaker inside the 600 ml beaker to make a boiling water bath. This will dissolve the waxy cuticle on the leaves and extract the chlorophyll.
(3) Boil the leaves in alcohol for 20 minutes. The geranium leaf will turn pale green; the evergreen leaf will remain green.
(4) Remove the leaves from the alcohol using forceps. Rinse the leaves in a petri dish containing water. Drain off the water.
H. Add iodine solution to the leaves in the petri dish. Note the color of the leaves. A blue-black color indicates a positive test for starch. No color change or a reddish stain indicates a negative test for starch. Record whether your tests were positive (YES) or negative (NO) for photosynthesis in the "Results" spaces on the diagram above.
CONCLUSIONS
2. How did your predictions compare to the results of the two experiments?
3. According to the results of the geranium experiment, what abiotic (physical) condition is necessary for photosynthesis to occur?
4. According to the results of the evergreen experiment, do evergreens carry on photosynthesis in winter?
DISCUSSION:
5. What was the experimental variable in both experiments?
6.In the evergreen experiment, which leaves (masked or unmasked) were the experimental group Which leaves were the control group? Explain your ideas.
7. Why is it impossible to determine the effect of temperature alone on photosynthesis by comparing the geranium results to the evergreen results?
8. Do you think that evergreens could carry on photosynthesis during a midwinter thaw? Design an experiment to test your ideas.
9. Evergreens are often found growing where the winter weather is cold and the soils are low in minerals. Suggest a possible advantage of having leaves that are evergreen to the survival of northern plants under these conditions.
GOING FURTHER: Use the following information to answer questions 10-12. Microscopic examination of winter evergreen leaves suggests that their stomata are closed. Other evidence indicates that the fluid within the vascular tissue is often frozen during the winter. Photosynthesis is a biochemical process that requires specific enzyme catalysts. The rate of photosynthesis is influenced by the effect of temperature on enzyme action.
10. What is the function of the stomata? How would closed stomata affect the ability of an evergreen plant to carry on photosynthesis in the winter?
11. What is the function of the vascular tissue? How would frozen fluid affect the ability of an evergreen plant to carry on photosynthesis in the winter?
12. How would low temperatures affect the ability of an evergreen plant to carry on photosynthesis in the winter?