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3-4 Plants

The Effect of Increased UV on Plants


 A healthy flower. (Photo by John Pickle.)

Photosynthesis is the process that plants use to produce food for themselves. Studies in Sweden and in the United States have shown that ultraviolet energy destroys the chlorophyll molecules that plants need to capture light energy for photosynthesis. That’s how UV damages photosynthesis. Other studies suggest that UV energy reduces the insect resistance of many plants. Some plants have evolved defenses against damage by ultraviolet energy. Some can produce large amounts of pigments that absorb the radiation; others block out sunlight with waxy leaves or fine, fuzzy hairs. There are plants that develop enzymes that undo the DNA damage. However, these defenses are not present in all vegetation. Many of the plants sensitive to ultraviolet energy are ones we depend on for food.

The purpose of this investigation is to compare the growth of plants under normal outdoor conditions, under a grow light, and under light with the same wattage, but increased ultraviolet energy.


4 potted plants of the same variety (pea, bean, clover, alfalfa, cucumber, or soy bean)

1 suntanning lamp or bulb (which is not a “growlight”)

2 growlight lamps or bulbs of the same wattage as the suntanning lamp

1 suntanning lamp or bulb with wattage as low as possible, preferably 30% or less of the wattage of the growlight


graph paper and pencil

Strategies for Investigation

1. Measure the area of a few sample leaves on each of the plants and calculate the average leaf area. Also record the total number of leaves on each plant. (Leaf area can be measured by putting a piece of graph paper under each leaf and tracing its outline.)

2. Place one plant in a spot where it can get direct sunlight and where it will be away from the lamps. It will serve as the “control” plant.

3. Place the other three plants indoors, one illuminated solely by the growlight, the second solely by the tanning lamp of equal wattage as the growlight, and the third illuminated by the growlight and the low-wattage suntanning light. The lamps for all three plants should be an equal distance above the soil in each pot. Turn off the lamps each day at about sundown and then turn them on again the next morning. (If this is not convenient put the control plant in the same light conditions as the experimental plants when their lamps are turned off.)

4. Make a prediction. Do you think the number and size of leaves will be affected by the kind of light? If so, how? If not, why not?

5. Leave the plants for several weeks, watering as needed. Make sure each plant has equal watering.

6. After several weeks, measure the average leaf area once again.


 Will our crops be affected as the amount of UV radiation changes?
(Photo by John Pickle.)

Make a bar graph to show the effects of the different lighting conditions on the average leaf area for all four plants. What conclusions can you draw from your experiment? What did you find out? Discuss whether or not your experiment indicates that increased UV energy could affect agriculture. If you anticipate problems, how do you think their effects could be avoided?