To determine what these compounds might be, and in what concentrations, a number of different experiments are possible. You might want to try adding a mineral compound that is commonly found in tap water, such as MgCO 3 or CaCO 3 . Another possibility would be to test the effects of supplying a nutrient such as NH 4 NO 3 or K 2 HPO 4 . See if you can come up with a recipe for a solution that promotes better lettuce seed growth than you get in deionized or distilled water. If you want to be able to compare results from one experiment to another, it is important to keep everything the same except for the test variable.
Temperature is one of the factors that usually is kept as constant as possible.
You might want to experiment with this, though. If you make temperature your test variable, you will be able to find out how sensitive the lettuce seed bioassay is to heat or cold. In this case, you will want to use the same solution in all of your petri dishes but incubate them at different temperatures. You could make a simple incubator using a light or heating pad enclosed in a box. For cold temperatures, you could try a refrigerator or chilly windowsill.