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Duckweed Bioassay Method
Using the method below, you will determine duckweed sensitivity to a variety of compounds, or various concentrations of a single compound. This method may be used for any compound you would like to test (see examples).

Materials

  • Fluorescent or plant grow lights
  • Duckweed plants - 5 per beaker (information on obtaining and culturing duckweed plants)
  • Beakers or clear plastic cups (3 beakers per compound or concentration being tested, plus 3 beakers for the control)
  • Miracle-Gro Liquid Houseplant Food Drops or similar fertiziler solution (N:P:K = 8:7:6)
  • Eye dropper (for fertilizer)
  • Tweezers or paper clips (for handling duckweed)
  • Clear plastic film such as Saran Wrap
  • 90 mL of each of the chemical solutions to be tested
  • 90 mL of spring water
  • 100 mL distilled water (for rinsing)

Procedure

  1. Determine which compound(s) and/or concentrations you will be using in the duckweed bioassay (information on possible compounds and dilutions)
  2. Label beakers or cups with your name, the date, and the compound (including concentration). Each compound (or each different concentration) should have 3 beakers. Label 3 additional beakers "control".
  3. Using tweezers or an unfolded paper clip, gently transfer five duckweed plants into each beaker. (Avoid using your fingers because that could introduce other chemicals into your culture solutions.) Choose only green, healthy-looking plants that have two fronds apiece and are approximately the same size.
  4. Cover the beakers with clear plastic film, and place them under 24-hour fluorescent or plant grow lights. (Artificial lighting is optimal because it provides consistent conditions from one experiment to another. Indirect natural lighting is an acceptable alternative. Avoid placing the beakers directly in a sunny window because overheating may cause the duckweeds to get scorched.)
  5. Let the beakers sit undisturbed for five days. Keep them covered with plastic, and do not add water to them during this time.
  6. At the end of the five-day growth period, count the number of fronds in each beaker. It may be difficult to decide which fronds are real, and which are too small to count. The important thing is to be consistent so that your results will be comparable across treatments.
  7. Record your data in a table like the one below, and make notes about any plants that are yellow, rootless, or sinking, or that otherwise appear unhealthy. X, Y, and Z refer to various dilutions of a particular compound.
  8. Graph the mean (average) number of fronds for each treatment (each compound, as well as the control). Then analyze your data.
Solution Concentration
# Duckweed Fronds/Beaker
Avg. # Fronds
Comments about Plant Health
Control          
Compound 1 (X %)          
Compound 1 (Y %)          
Compound 1 (Z %)          
Compound 2 (X %)          
Compound 2 (Y %)          
Compound 2 (Z %)          


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