Toxicity Tests with Lettuce Seeds
This is called a dose/response experiment. You vary the dose of a selected compound, then measure the response of the bioassay organism.
First, make a 0.2M NaCl solution by mixing 11.69 g NaCl with enough deionized water to make 1 liter.
Second, label a series of beakers with the following concentrations:
0.2M, 0.1M, 0.075M, 0.05M, and 0.025M. Make up these concentrations from the
0.02M solution using the proportions listed in the following table:
To determine the sensitivity of an organism to a chemical compound, scientists carry out reference toxicity tests. To do this, you measure the response of the organism to a wide range of concentrations of the selected chemical. What concentrations should you use? That of course depends on both the bioassay organism and the chemical being tested. You might want to start by searching through published student reports included on this web site to see whether anyone else has already generated data that would be of use to you.
Before scientists begin an experiment, usually they search through published scientific literature for papers that relate to the procedure they have in mind. If you have access to scientific journals, it would be a good idea to look for papers that report bioassays using the organism and compound you are interested in (see References for example papers). This is a good way to get an idea about an appropriate range of concentrations.
If you can't find any appropriate data, that's ok -- you'll just have to start with a broader range of concentrations to make sure you hit the range that your test organism responds to. (With too high a concentration, the test organisms will all die, or in the case of seeds, none will sprout. With too low a concentration, you will not be able to detect any difference between your samples and your control.) Ideally, you want to test concentrations that cover both of these endpoints plus a range of concentrations in between. Then you will be able to conclude whether your test organism responds in a predictable way to the compound you are testing.
Serial dilutions are one way to set up a broad range of concentrations. For example, suppose you suspect that in a 100 mg/L solution of a selected compound, no lettuce seeds will sprout, and you are interested in narrowing this down to find out the range of concentrations in which germination will occur. You might decide to start with a 10-fold dilution series, testing solutions of 100, 10, 1, 0.1 and 0.01 mg/L. Another possibilitity would be a dilution series in which each solution is half the strength of the previous solution in the series: 100, 50, 25, 12.5, and 6.25 mg/L.
Once you have collected data using an initial set of concentrations, you may find that it would be useful to carry out a follow-up experiment using a more narrow set of concentrations. For example, if none of the seeds sprout at one concentration in your series, and all of them sprout at the next level of dilution, it would make sense to carry out a dilution series between these two concentrations in order to further define the sensitivity of lettuce seeds to your selected compound.