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3.2 The Effect of UV-B on Plants

Teacher Background

The amount of potentially dangerous ultraviolet (UV) radiation reaching Earth's surface is enhanced by the austral spring depletion of stratospheric ozone, as described in Activity 3.1. Although recent research has demonstrated that increased exposure to solar UV-B radiation (between 280 and 320 nanometers) seems responsible for significant reductions in marine phytoplankton productivity, no research has addressed how enhanced exposure to UV-B affects Antarctic vascular plants, the kind which uniquely grow along the Antarctic Peninsula. In projects conducted in other regions of the world, researchers have found that increased UV-B levels can adversely affect such plants as grasses and forbs (leafy, herb-like plants) and that these plants do not have effective systems for screening UV-B. Along with enhanced UV-B levels, there is evidence that air temperatures along the Antarctic Peninsula have increased over the past 50 years and will continue to do so. It is unclear how Antarctic plants are responding to this warming.

Dr. Thomas (Tad) Day, a botanist from Arizona State University, and his research team are attempting to determine if UV-B and warming temperatures affect photosynthesis, growth and reproduction in the only two vascular plants native to Antarctica, Deschampsia antarctica (a grass), called "antarctic hair grass", and Colobanthus quitensis (a forb), called "antarctic pearlwort". At various sites around Palmer, they'll be using filters to remove UV components from sunlight over naturally growing plants, and using different filters, transparent to UV, to warm other plants as controls.

This Activity allows your students to replicate, in class, Day's experiment, and compare and contrast their results with other students across North America, observing changes across time, as the seasons change. Please note: you may wish to delay this Activity until spring in your region, since it must be done outdoors, where no window glass will filter UV and so interfere with the experiment.

Our thanks to Dr. Day for his input to this Activity, and for reviewing the science.

Objective

Students will conduct an experiment which investigates the effects of UV-B exposure and warmth on seed germination and plant growth.

Materials

  • 3 large boxes (copier paper boxes would work well)with lids
  • 10" x 48" sheet of special clear polyester, to screen/filter UV-B (labeled with red dot)
  • 10" x 42" sheet of clear cellulose acetate, transparent to UV-B: this is the control filter (labeled with green dot)
  • 30-40 pea or bean seeds (choose seeds that are relatively more cold tolerant
  • one bag potting soil
  • metric ruler
  • measuring cup
  • water
  • 36 seed pots
  • 1 dish (for soaking seeds)
  • roll of aluminum foil
  • paper towels
  • data sheet (student made)
  • Exacto Knife
  • diagram of Electromagnetic Spectrum (available in most Earth Science and Physical Science texts: see also Activities suggested in PTK's Live from the Stratosphere)

Engage

Review Electromagnetic Spectrum/light wavelengths/nanometers with class.

Place chemical equation for photosynthesis on blackboard. Ask students if they know what these symbols represent. Some students may easily recognize symbols for water and carbon dioxide.

Read Tad Day's on-line Biography and Field Journal aloud to class. Explain that students will be simulating his Antarctic research back in North America.

Explain

The equation for photosynthesis can be written as follows:

6CO2 + 6H2O + sunlight --> C6H12O6 + 6O2

Photosynthesis is the process by which plants produce their own food (C6H12O6). They use energy taken from sunlight to transform carbon dioxide, taken from the air, and water, taken from the soil, into carbohydrates. The rate of photosynthesis is governed by many factors, including temperature and light. In this experiment, students will filter the amount of ultraviolet light (UV-B) plants are exposed to, and observe the effect of this filtering on plant growth. They will also observe the effects of solar warmth on plant growth.

Explore

Review experimental procedure. Ask students to hypothesize possible effects of filtered UV-B light and solar warmth on seed germination and plant growth. Record all hypotheses on chart paper.

Also note that two variables will be manipulated in this experiment: the amount of ultraviolet radiation, and solar warmth. Students should discuss how best to control all other variables (seeds, water, environmental factors, etc.)

Procedure

  1. Fill a small dish with room temperature water, and soak seeds for 30 minutes.
  2. Line the inside of the three box lids with aluminum foil. Put 1cm depth of potting soil in each of the seed pots. Place 12 seed pots in each lid. Carefully pour sufficient water into each seed pot to saturate the soil.
  3. In the bottom of one box, carefully cut an opening that is 9 x 16 inches. Cut holes in three sides of the box so that only a 1 1/2 inch frame remains (see Figure 1). Leave fourth side of the box uncut; this side will help strengthen the frame. Cover the openings on the box with the UV-B filter (indicated with the RED dot) and tape securely. Label this box "UV-B Filtered".
    1. Repeat this procedure with the second box, covering the openings with the clear cellulose acetate (GREEN dot). Label this box "Warm".
    2. The third set of plants won't be covered by any filter/frame; label these "Control".
  4. After 30 min., carefully pour water from the small dish without spilling any seeds.
  5. Tip seeds onto paper towel. Put 3 seeds onto potting soil in each seed pot. Add a dusting of soil to lightly cover the seeds. Carefully add a little more water so that this layer is also saturated.
  6. Carefully place plants in sunny location outdoors. Place the frames over two sets of plants. Arrange plants so that uncut side of frames face NORTH. If possible, place plants in a location that is sheltered from the wind. Note: This experiment must be run outdoors; most window glass will filter UV light.
  7. Plants should remain undisturbed in warm sunny location during daylight hours. Bring plants indoors at night or during inclement weather.
  8. After two days check the seed pots daily for germination of seedlings. Keep pots well watered (water every 2-3 days). Record observations.
  9. As soon as seeds have germinated (3-4 days or sooner), students should take daily measurements of plant height, number of leaves, total leaf area (see Expand section) and make any other observations (color of plants, strength of stem, etc.). Record all data on data sheet.
  10. Continue monitoring plant growth for 2-3 week period.

Analysis

  1. Describe what happened in each box. Did the results support original hypotheses?
  2. Why was it necessary to grow plants outdoors?
  3. Which variables were difficult to control?
  4. How was this experiment similar to Tad Day's Antarctic research? How was it different?
  5. What conclusions can be drawn from the results?

drawing of box - OV-B drawing of box - OV-B warm

Expand/Adapt/Connect

math logo

Students can estimate the Total Leaf Area for each plant by:

  1. measuring the length (L) and width (W) of each leaf;
  2. applying the following formula to estimate the area of the leaf: L x W x 0.7
  3. totaling the areas

Suggested URLs

An Ozone Depletion FAQ (Frequently Asked Questions) effects of UV on various life forms.
http://www.cis.ohio-state.edu/hypertext/faq/usenet/ozone-depletion/uv/faq.html

Tells how students can take part in EPA's on-line Ozone Depletion Art Project.
http://www.epa.gov/ozone/art/enter.html

Union of Concerned Scientist's Sound Science Initiative on Ozone Depletion
http://www.ucsusa.org/textonly/global/text.ozone.html





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Last Update: 1/18/97
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