The Seeds Tell the Story

Radish Seed Bioassay Student Worksheet

(adapted from Rathbun, 1996)

Wear protective gloves and safety glasses when handling sample solutions!

  1. Soak radish seeds for 20 minutes in about 20 ml of a 10% solution of household bleach in distilled water, then rinse 5 times with distilled water. The solution kills fungi, which could interfere with seed germination.

  2. Bioassays are carried out in zip-lock plastic bags containing a paper towel saturated with the test solution. Set up three replicate bags for each sample to be tested, plus three bags for each control solution. Label each bag with the sample number, plus a letter to identify the individual replicates (so for sample number 1, there would be three bags labeled “1-A,” “1-B,” and “1-C”).

  3. Pour in enough sample solution to saturate the filter paper, as directed by your teacher. Use the same volume in all tests.

  4. Using forceps, place 10 seeds on the paper, spaced evenly. Seal the top of the bag.

  5. Incubate the bags at room temperature for 5 days. Do not place the bags in direct sunlight! Each day, record the number of seeds that germinated in each plate, and measure (to the nearest mm) the length of the root that has emerged from each germinated seed.

  6. For each sample (including the controls), calculate the mean and standard deviation of root lengths. Comparisons can be made by using the Student’s t-test. If you are not familiar with these calculations, there are many examples on the internet, such as http://www.sonoma.edu/users/h/hanesda/B324/stats.html or http://www.physics.csbsju.edu/stats/t-test.html.

A more approximate method is to compare the mean ±1 standard deviation of each sample to the control. If a sample’s mean plus 1 standard deviation is less that the mean of the control minus 1 standard deviation, there’s a strong likelihood that the sample is significantly more toxic than the control.

 

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