Name _______________________________________ Date _____________________ Period _______
LD 50 Lab
Background
It is suspected that salt (NaCl) applied to highways for deicing may affect the growth of vegetation
along the roadside and aquatic plants in nearby streams. Therefore, you will conduct a dose-response
experiment to determine how radish seeds will respond to various concentrations of salt. Your teacher
will provide you with a concentrated salt solution so that you can make serial dilution each of which are
half as concentrated. You will set up a wide range of concentration of salt solutions to test on radish
seed growth.
Collaborators
Emily Smith, Kat Burgert, Austin Dowler
Introduction
Especially during the winter, or when it has snowed, salt is usually applied to highways as a
method to help melt the ice found on the road that could put passing cars at risk. In this lab, we will
study the effects that salt (NaCI) could possibly have on the growth of vegetation-- in this case, radish
seeds. By applying various concentrations of salt solution to the radish seeds, overall, we will be able to
observe the effects it has on the overall germination of the seeds. We will be able to conduct this lab
through a serial dilution, in which various amounts of water will be applied in each beaker to dilute the
salt solution already present. With our results, we will then be able to create a dose-response curve-allowing us to determine when we’ve hit a threshold response from the combining of the radish seeds
with the salt water solution. Obtaining this information also allows us to identify the “LD 50”, or the
dose of the salt solution in which 50% of the radish seeds die. This refers back to the effects salt has not
only on ice found on our rods, but nearby plants as well.
Problem
How does salt impact the germination of radish seeds along with its radical length?
Hypothesis
If we apply various salt water solutions to radish seeds, then those exposed to higher salt water
solutions, will experience a decrease in the rate of seed germination and radical length.
Materials
6 ziploc bags
60 seeds
12 napkins
Water
Graduated cylinder
Concentrated Salt Water solution
Variables and Groups
Independent Variable- Salt Concentration
Dependent Variable- Percentage of seeds germinated, radical length
Controlled Variables- 2 napkins, 1 plastic bag per solution, 20 mL of solution, 4 days to germinate, 10
seeds per bag
Experimental Group- Bags 2-6; 6.25% salt concentration, 12.5% salt concentration, 25% salt
concentration, 50% salt concentration, 100% salt concentration
Control Group- 0% salt concentration, 100% water
Safety and Disposal Goggles and latex gloves should be used throughout the experiment. Completed
experimental setups can be thrown away in the trash.
Name _______________________________________ Date _____________________ Period _______
Procedure
1. Use the graduated cylinders and test tubes to prepare the various concentrations as shown below.
2. Label all six bags with your group number, the dish #, and a percent concentration of chemical:
a. Dish #1: 0%
b. Dish #2: 6.25%
c. Dish #3: 12.5%
d. Dish #4: 25%
e. Dish #5: 50%
f. Dish #6: 100%
3. Put two napkins together and cut them so that they fit into the bag.
4. Put on the safety goggles and latex gloves. Carefully pour the salt solution onto the napkins,
making sure to match the numbers and concentration percentages of the dish.
5. Count out 10 seeds. Carefully place the seeds on the moist napkins in the bag.
6. Repeat steps 3-5 for the other dishes.
7. Place the seed dishes in a stack, lying flat with the seeds up. Put the seeds in the spot designated
by your teacher.
Collecting Data and Plotting Results
You will measure the response of the radish seeds at various salt concentrations. After the seeds have
germinated, count the number of seeds that germinated and measure the length of each radical
(embryonic root). After recording your results, you will create two graphs (% seed germination and
dose-response curve) to help you analyze the data collected.
1. Remove the lid of the control dish. Count the number of seeds that germinated. Calculate the
percentage of seeds that germinated and record in Table 2. Note: if fewer than 80% of the seeds
in this control sample germinate, this indicates a problem with the experiment.
2. Measure the length of the radical for each of the germinating lettuce seeds to the nearest
millimeter (mm). Look carefully at each sprout to make sure you are measuring just the root, not
the shoot as well. In the picture below, you would measure just the part between the two arrows,
not the shoot and cotyledons to the left.
Name _______________________________________ Date _____________________ Period _______
3. Repeat steps 1-2 for each petri dish.
4. For each treatment, calculate the mean radical length for each salt solution. Add the total radical
lengths for each salt solution and divide by the total number of seeds that germinated. Do not
include data from seeds that did not germinate. Record data in column labeled, “Mean Radicle
Length (mm).”
5. Make a line graph from the data collected to show a dose-response curve. The horizontal axis
should be for the independent variable, dose (concentration of salt solutions). The vertical axis
should be for the dependent variable, response (mean radical length). Remember to give the
graph a title.
6. To help you answer “Did the radical length increase or decrease in length as compared to the
control?” subtract the mean radical length of each treatment from the mean radical length of the
control. Record your answers in the column, “Difference in Radicle Length” on the data table.
7. Make a line graph to show the percentage of seeds that germinated for each salt solution.
8. Data
Name _______________________________________ Date _____________________ Period _______
Graphs
Name _______________________________________ Date _____________________ Period _______
LD 50 of Salt Water on Seed Germination
Name _______________________________________ Date _____________________ Period _______
Dose- Response Curve
Conclusion
After completing this lab, based off of the results, the LD50 of salt for radish seeds is about 5%.
This proved my initial hypothesis to be correct; as the concentration of the salt solution increased, the
rate of seeds germinated ultimately decreased. With a salt concentration of 6.25%, the data obtained
showed that 40% of the seeds had germinated-- meaning 60% had died. By forming a line graph, it
showed that around 5% of the salt concentration would have caused 50% of the radish seeds to die.
Along with this graph, the dose-response graph proved to show similar data to support the germination
data. For the mean radicle length was indeed longer with the presence of less salt in the salt water
solution.
Although my hypothesis was correct, I expected the LD 50 of salt for the radish seeds to be
higher. The accuracy of the lab most likely would have been higher if the measurements of the slat water
solutions were more precise. With limited time, it was difficult to measure the exact amount of water
and salt that had to be combined in each solution. This caused us to have to estimate and rely on our own
idea of how much of each substance was required in each set of salt water solution. This could explain
the great amount of death of the radish seeds in sample 2 (6.25% salt concentration) and sample 3
(12.5% salt concentration). The same error applies when measuring the length of the radish seeds that
Name _______________________________________ Date _____________________ Period _______
were able to germinate, as it appeared to be more difficult than we expected to stretch out the roots from
the plant-- often resulting in it breaking apart.
Like the effects of salt on the radish seeds in this experiment, vegetation alongside highways face
similar effects. As the salt applied on these roadways run off to nearby vegetation due to the splashing of
passing cars, this has been proven to cause negative effects on plant growth. “Salt can disrupt nutrient
uptake and cause injury to seed germination, stems, leaves, and flowering ability. Salt can lead to plant
death and can also cause a colonization of salt tolerant species, such as cattails, thereby reducing species
diversity” (Environmental, Health and Economic Impacts of Road Salt). By recognizing its damaging
effects on the environment, especially vegetation, many states such as New Hampshire have demanded
to find a more effective and less damaging means of deicing the roadways.
Citation
"Water Quality Impacts - Environmental, Health and Economic Impacts of Road Salt - New Hampshire
Road Salt Reduction Initiative - Watershed Assistance Section - NH Department of
Environmental Services." Water Quality Impacts - Environmental, Health and Economic
Impacts of Road Salt - New Hampshire Road Salt Reduction Initiative - Watershed Assistance
Section - NH Department of Environmental Services. N.p., n.d. Web. 04 Feb. 2015.