PHOTOSYNTHESIS IN WINTER

PURPOSE:  To determine if evergreens have the ability to carry on photosynthesis in winter.

MATERIALS: For the class - duplicate leaf samples of several winter evergreen species, 2 large geranium plants, black paper, paper clips, 2 or 4 lamps, 1 or 2 extension cords, refrigerator.  For each group - hot plate, 600 ml beaker, 400 ml beaker, 250 ml 95% ethyl alcohol, tongs, forceps, 2 petri dishes, 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 have the ability to carry on photosynthesis in winter? 

In this investigation, you will test for photosynthesis in winter evergreen leaves and in a common broad-leaved house plant.  You will be able to draw conclusions about the effect of temperature and light on photosynthesis in these two types of plants.

ADVANCE PREPARATION:

A.  Collect two samples of several different species of plants having evergreen leaves in winter.  Your teacher will help with their identification.  Put the cut ends in water and place them in the refrigerator for several hours to acclimate the leaves and minimize wilting.

B.  Prepare 2 large geranium plants as follows:  Clip a piece of black paper over half of the top and bottom of a leaf on one of the plants, as shown in the diagram below.  Place both plants in continuous illumination for about 24 hours, one at room temperature and the other in the refrigerator. 

C.  Prepare two sets of evergreen samples as follows:  Use black paper to cover half a leaf or half the needles on a twig on an evergreen sample.  Expose the duplicate sets of evergreen samples to continuous light for about 24 hr, one set at room temperature and the other set in the refrigerator.

PREDICTIONS:

1.  Which leaf samples do you think will carry on photosynthesis under these conditions?  Record your predictions by writing YES or NO in the "predict" spaces on the diagram below.

   plant leaves hot and cold

  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.  Circle the temperature condition of each leaf and record the species of your evergreen sample:

Geranium: WARM COLD Evergreen _______________________: WARM COLD

F.  Submerge the geranium leaf and the winter evergreen sample in 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 for 20 min. The geranium 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.  Share your results with others in your class.

CONCLUSIONS:

2. How did your predictions compare to the results of the experiment? 

3. According to the results using geranium leaves, what abiotic (physical) conditions are necessary for photosynthesis to occur?

4. According to the class data, does temperature affect the ability of evergreens to carry on photosynthesis? 

DISCUSSION: 

5. Do you think that evergreens carry on photosynthesis during a midwinter thaw?  Use class data to support your ideas.

Use the following information to answer questions 6-8.  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.

6. 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?

7. 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?

8. How would low temperatures affect the ability of an evergreen plant to carry on photosynthesis in the winter?

9. Evergreens are often found growing where the winter weather is cold and the soils are low in minerals.  Suggest a possible advantage of evergreen leaves to the survival of northern plants under these conditions.