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Life Science: Field Population System

Salt of the Earth

How does salt affect the growth of plants? In a population of plants, are some plants more resistant to the effects of salt than others? If so, how might such a population be selectively bred to tolerate higher concentrations of salt?

Salt sources in real-world situations include road salts, such as those that are applied during the winter to melt snow, and seawater. The salt in question may be common table salt — sodium chloride — or other salts such as calcium chloride (CaCl2) used in road salts.

“Salt Of The Earth” is an activity that is designed to find answers to the above questions. Using a population of Fast Plants, you’ll be able to do your own investigation and perhaps do some selective breeding — just like Paul Williams did to create Fast Plants! This is an advanced activity that is meant to help you apply the principles of selective breeding. However, you may find it appropriate for students in later elementary grades. Kathy Vandiver’s sixth-grade class — featured in Session 5 — was very successful doing an activity like this.

While this activity uses salt as a variable to affect plant growth, this same design can be used to vary any number of environmental conditions:

  • Nutrient levels
  • Temperature
  • Light
  • Water
  • Soil type
  • Soil moisture
  • Sound
  • Atmospheric conditions (e.g., smoke)
  • Pollutants (e.g., detergents, pesticides, oil, etc.)

Materials Needed

You can use seeds of any type for this activity. We suggest Fast Plants because of their rapid life cycle and predictable growth habits under the controlled conditions in a Light House. You can also substitute the Bee Stick with a cotton swab or fine paintbrush.

Instructions

Effects of salt

  1. Prepare three different salt-water solutions: bottled water, 2% salt (2.0 grams table salt in 100 ml water — about 1/8 teaspoon) and 4% salt (4.0 grams salt in 100 ml water — about 1/4 teaspoon). The bottled water will act as a control, and the two concentrations of salt water will allow you to compare how different levels of salt affect plant reproduction. Keep the extra solution for maintaining water levels.
  2. Add one solution to the deep base of each system. Do NOT use fertilizer solution in this activity.
  3. Plant an approximately equal number of seeds in each system (15-20). The study period starts at this point.
  4. Place each system in a Light House so that the seeds sprout about 10 cm from the light. Adjust this distance to the top of the plants as they grow.
  5. Once plants have sprouted, thin them to an equal number in each system (e.g., 10 –15 plants).
  6. Using your “Salt Of The Earth” Data Sheet, track the growth of plants in each system over time.
  7. Once the plants have begun flowering (about day 14 in the control population), use your Bee Stick to pollinate them. Gather pollen from the flowers of one plant and transfer it to the flowers of another. You can also use a fine paint brush or cotton swab.
  8. Continue pollinating for 4 – 5 days until about day 18. After that, pinch any new flowers off at their bases.
  9. Continue to supply the appropriate salt-water solution to each Bottle Growing System for another 20 – 22 days to allow pods to develop and seeds to set.
  10. Remove water and allow the plants to dry until crispy brown — about one week.
  11. On the day you remove the water, note three individuals in each population that seem to have survived and reproduced better than the others and mark them in some way (e.g., a toothpick at the base). These will be the plants from which you will collect seeds for selective breeding.
  12. After about one week, the plants will be dry and the seeds will be ready for collection. Carefully remove the pods from the plants of the individuals you selected.
  13. Collect the seeds from these pods, keeping the seeds from each population separate.

Selective breeding

  1. Use the same set up for selective breeding.
  2. Sow the seeds collected from plants grown in each specific solution in a system containing that same solution.
  3. Observe through the life cycle to see how survival and reproduction is affected in the next generation.

Activity Questions

Before the study period begins

  1. What are some indicators of superior survival and reproduction of an individual plant among others in the same population?
  2. How might superior survival and reproduction become more common in subsequent generations of plants?
  3. How do you think the concentration of salt in a plant’s water supply might affect it?
  4. If you wanted to develop a population of salt-tolerant plants, how might you do this?
  5. How would you know if you were successful?

After the study period ends

  1. What occurred during the study period to the plants in distilled water? In the 2% salt solution? In the 4% salt solution?
  2. In each solution, what was the extent of variation with regard to plant survival and reproduction?
  3. If the plants in each population were left to reproduce undisturbed (i.e., without selecting superior survivors) what would the next generation of plants be like? Explain your reasoning.
  4. If you collected and sowed seeds from superior survivors, what do you expect to happen in the next generation? Explain your reasoning.
  5. Applying what you understand about heredity, account for variability in the plants’ salt tolerance. How does selective breeding for salt-tolerance impact the genome of a plant population?
  6. Could a salt-tolerant plant population develop in the natural world? Explain how this might occur.

SHARE YOUR RESULTS: Salt of the Earth

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