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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
- 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.
- Add one solution to the deep base of each system.
Do NOT use fertilizer solution in this activity.
- Plant an approximately
equal number of seeds in each system (15-20).
The study period starts at this point.
- 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.
- Once plants have sprouted, thin them
to an equal number in each system (e.g., 10 –15 plants).
- Using your “Salt
Of The Earth” Data Sheet, track the
growth of plants in each system over time.
- 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.
- Continue pollinating for 4 – 5
days until about day 18. After that, pinch any new flowers off at
their bases.
- 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.
- Remove water
and allow the plants to dry until crispy brown — about
one week.
- 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.
- 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.
- Collect the seeds from
these pods, keeping the seeds from each population separate.
Selective
breeding
- Use the same set up for selective breeding.
- Sow the seeds collected
from plants grown in each specific solution in a system containing
that same
solution.
- Observe through the life cycle to see how survival
and reproduction is affected in the next
generation.
Activity Questions
Before the study period begins
- What are some indicators of superior survival
and reproduction of an individual plant among others in the same population?
- How might superior survival and reproduction
become more common in subsequent generations of plants?
- How do you think the concentration
of salt in a plant’s water
supply might affect it?
- If you wanted to develop a population of
salt-tolerant plants, how might you do this?
- How would you know
if you were successful?
After the study period ends
- What occurred during the study period
to the plants in distilled water? In the 2% salt solution? In the 4%
salt solution?
- In each solution, what was the extent of variation
with regard to plant survival and reproduction?
- 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.
- If you collected
and sowed seeds from superior survivors, what do you expect to happen
in the next generation? Explain your reasoning.
- 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?
- 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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