Bioassays: Testing Environmental Samples with Lettuce Seeds

1. Soak lettuce seeds for 20 minutes in a 10% bleach solution (add 1 part household bleach to 9 part deionized or distilled water). Then rinse five times. This kills fungal spores that can interfere with seed germination. Note: Tap water can be used if you do not have access to deionized or distilled water, but it will introduce more variability into your experiment because of the varied minerals and other compounds it contains.

2. For water samples, place a 7.5-cm paper filter in each 9-cm petri dish. Add 2 ml of water sample to each dish

3. For sediment or soil samples, place 3 grams of sample in the bottom of each petri dish and cover with filter paper. If the sample does not contain enough moisture to saturate the filter paper, add up to 2 ml deionized water as needed.

4. Prepare a control by setting up dishes using 2 ml deionized or distilled water as your test solution.

5. To each dish, add 5 lettuce seeds, spaced evenly on the filter paper so that they do not touch each other or the sides of the dish.

6. Place the dishes in a plastic bag, and seal it to retain moisture. Incubate in the dark at constant temperature (preferably 24.5 degrees C) for 5 days (120 hours).

7. At the end of this time, count how many seeds in each dish have germinated, and measure the root length of each to the nearest mm. Look carefully at the plants to make sure you are measuring just the root, not the shoot as well.