Experimental Design Lab
(Modified and Abbreviated)
ISCI 2001

The Effects of Ionizing Radiation on Plant Growth and Development

I. Introduction

The original investigation took three weeks. You will be using a modification which allows you to do the experiment in one week. You will be following the method used by scientists. The process or method is called scientific methodology and is a combination of common sense and logic. Each time that you try to find the answer to a question or the solution to a problem you are usually following this scientific methodology to some extent. You may not use the same terminology as a scientist to describe the sequence of actions you are following, but you are using the same framework to find your answer or solve your problem.

There is no one scientific method carved in stone on some giant rock that says this is the only way to carry out this process. There are a number of common elements to all forms of scientific methodology.  The first step is always the same. In the most concise and precise way, you must define your problem or clearly frame the question you seek to answer. In this case, the question has already been posed for you. What are the effects of ionizing radiation on the growth and development of a seed of a specific type of plant?  We will use radish seeds because they are easy to grow, easy to care for, and will show distinctive differences as the result of different doses of radiation.   

The next step in the process is to develop some statements that describe what you believe will happen to the seeds as the result of their exposure to the different doses of radiation. This is known as developing a hypothesis. In the nonscientific world you would normally say that this is what you think will answer the question or solve the problem. At this point you might begin to search various scientific or agricultural databases or scientific journals for information about this problem. For example: what is ionizing radiation and what happens when seeds are exposed to it? Are seeds different than fully formed plants in terms of radiation effects? There are lots of questions that can be posed and a good search engine such as Google can provide lots of answers. Just remember that the answers you get depend on the questions you ask. The more precise the question, the more likely a more focused response.

 Eventually you will need to set up a controlled experiment to test your hypothesis. A controlled experiment is one where comparisons are being made between what would be considered the normal condition (the control)

and one or more situations where a single condition or variable has been changed (the experimental groups). Your experiment is designed to determine if you can accept your hypothesis or if you must reject it. Once again, nonscientists would simply say that it worked or it didn’t work. The actual procedure you will follow is outlined for you later in this exercise. This controlled experiment should be carried out many times rather than just once to confirm your results.  Don’t worry! You only have to do it once but if other lab sections were also doing it there would be a large database with which to work.

Once you have been assigned to a treatment group, you and your group members will need to develop two hypotheses. Write these hypotheses in the space provided at the end of this section of the lab. Your first hypothesis should predict the effects of different radiation doses on the germination of the seeds. Your second hypothesis should predict the effects of different radiation doses on the time of germination.

The seeds contained in these packets were treated with gamma radiation. Gamma rays are of short wavelength, high energy, and readily penetrate most matter. They are one of several forms of ionizing radiation (other forms include alpha, beta, and X‑rays). Ionizing radiation can cause mutations in DNA molecules. Since DNA molecules contain the coded information that guides the growth and development of an organism, any change in that information may cause changes for that organism. While certain doses of radiation may have a stimulatory or beneficial impact, most radiation tends to have an inhibitory or negative effect on the organism’s growth and development. Humans are exposed to a variety of different types and dosages of radiation as a result of their activities and technologies.

In order to study the biological effects of ionizing radiation, a person should be able to determine how much radiation has been delivered to or absorbed by a living organism. The rad or radiation absorbed dose is the amount of radiation absorbed by an organism. This unit is applicable to all types of radiation and all biological  materials. Our seeds were irradiated with the rad amounts indicated on the seed packets. One packet contains the non‑irradiated control seeds.

Procedure:

 1. The class will be divided into 5 groups. Each group will be responsible

     for one radiation category of seeds.

 2. Each group will procure two Petri dishes (9 cm in diameter, 1.5 cm

     deep). Using a sharpie marker, label the bottoms and the lids with the

     groups initials, start date, and radiation dosage.

 3. Place 25 seeds in each of the two dishes and cover with 25 ml of   

     deionized water.

 4. All Petri dishes will be placed under a fluorescent light source. The

     lights are on timers thus exposing the plants to a photoperiod of 12

     hours of light and 12 hours of darkness.

 5. Students will monitor the progress of the seeds daily since things

     happen rapidly with radish seeds.

 6. Record the date of first germination of the seeds in your data table.

 7. To prevent the water from stagnating, students should agitate the

     water daily using a transfer pipette. Draw water from the Petri dish

     into the pipette and gently expel the water back into the Petri dish.

     Do this several times. Replace the water on a regular basis.

 8. On day 7, count the total number of seeds that germinated in each

     group (germinated seeds must have some measurable length growth –

     seeds with cracked hulls are not considered germinated). Divide by 25

     to get the percent germination. Add the numbers of germinated seeds

     in the two dishes and divide the total by 50.

 9. There are several metrics which could be measured. These include

     overall plant length, root length and stem length. You are only looking

     at the date of first germination and the percent germination for each

     radiation dosage.

Protocol revision by:

Dr. Scott Mateer 2008

Dr. Don Emmeluth 2009