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Activities: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10
Activity Five
What's Wetland Allelopathy?
Objectives
Students will construct an experiment
of allelopathic interactions between wetland plants, including purple
loosestrife and evaluate results. Students will develop questions, hypothesis,
experiment and analyze results beginning in the field and concluding
the experiment in the classroom.
State science curriculum objectives
Constructing objectives: 13,
14, 15
Reflecting objectives: 11,
15
Using objectives: LEC
13, LEC 15
Method
Students will make plant extracts from chosen wetland plants and test
their allelopathic effects on the germination rate, radicle and hypocotyl
growth of test seeds. The experiment can take from 2- 4 or more weeks
depending on the germination time of the test seeds chosen as bio-indicators.
Terms
Allelopathy, competition, wetland,
exotic species, radicle, hypocotyl, germinate, biotic potential, environmental
resistance, succession.
Background
Plants need sunlight, nutrients, water,
air and protection from harm in order to thrive and survive. When plants
are overcrowded and do not have enough space, they cannot meet their
needs. Plants, like animals compete with other plants for the same spot
in the soil to put down their roots, take in nutrients, water and sun.
For some plants crowding, invites predation and the threat of spreading
fire. It may benefit a plant to make sure other plants of the same species
or different species do not grow too close.
Many plants produce chemical substances that
are phytotoxic (harmful to other plants). In this way, the plants keep
other plants out their space. This chemical process is called allelopathy.
There are several kinds of chemical allelopathy.
In one kind, a plant releases protective chemicals into the soil from
it's roots. Other plants growing next to the allelopathic plant or seeds
germinating near by absorb these chemicals and die. Another way allelopathic
plants keep other plants from growing too close is the leaching of chemicals
from decomposing leaves into the soil with the same results. Some plants
even release gasses through their stomata into the air "breathed"
by neighboring plants inhibiting their growth.
An example of an allelopathic plant easily observed
is the pine tree. Few plants grow where a pine tree has shed it's needles.
The decomposing needles contain acid which leaches into the soil stifling
the germination of other plants. Weedy field plants and some trees competing
in the intermediate stages of succession such as pines, tree of heaven,
some ferns and sagebrush are more likely to produce these chemicals.
Even large trees like the black walnut are allelopathic.
The exotic species, purple loosestrife, is rapidly
colonizing the wetlands of North America. It easily invades an area
that is disturbed and quickly out populates the indigenous wetland species
such as cattail, sedges and rushes. For this introduced species, there
is little environmental resistance from predators and high biotic potential
from reproduction as each plant produces up to two million seeds each
year. Could there be other factors that add to its success such as allelopathy?
Might there be other plants that are allelopathic to purple loosestrife
that might serve as a control to it's aggressive colonizing of wetlands?
The major purpose of this activity is for the
student to evaluate a wetland site and create their own experiments
using available wetland plants, including purple loosestrife, to test
for alleleopathic interactions. There is little if any research available
in this area. Findings would contribute to a growing body of knowledge
about purple loosestrife and it's control in North America.
Materials
Scissors or plant clippers, plastic
bags, access to a wetland, triple beam balance or other scale, medium
size jars with lids, distilled H2O, beaker or other liquid measuring
device, seeds, petri dishes, absorbent paper towels, small cm rulers.
Procedure
- Classroom discussion of plant competition,
succession and possible uses of chemical by plants for their protection.
How do chemicals contribute to the plants survival? What role do they
play in succession? Discuss allelopathy and the role it might play
in competition and succession. Begin journal for allelopathy and record
discussion information.
- Visit a wetland populated with some purple
loosestrife as well as other species. Identify the more dominate species.
Students ask questions about possible allelopathic interactions between
the identified plants. Choose several plants to test for possible
allelopathy. Decide which seed or seeds will be bio-indicators. Record
in journal. Collect 10 or more leaves for each chosen plant. Place
leaves in labeled plastic bags. Try to guess which plants might be
allelopathic from observing the plants in their natural setting. Record
field observations. Refrigerate leaves if next step is not immediate.
- Weigh 10 or more g of leaves Tear the leaves
into small pieces and place in a labeled jar. Add 100 ml of distilled
water and crush leaves with a spoon, stirring to mix. The amount do
not matter, just keep the procedure the same for all. The leaf liquid
should be very dense with leaves. Swirl several times a day for 3
or 4 days. If not used at this time refrigerate up to a month.
- As a class or in cooperative groups generate
a hypothesis. List the plant extracts plus a plain water control.
Students rank each plant they choose based on field observations.
Choose seed for bio-indicator. These could be seeds from one or more
wetland plants or a quick growing seed like a radish.
Label petri dishes. Place several seeds (4 or 5) in folded paper towel
in bottom of petri dish. Measure enough plant extract water onto the
paper towel to soak it. Be sure the plant extract used on the seeds
matches the label on the petri dish. Place lid on dish and place in
the dark. Check periodically, and rewater with extract. Keep the amounts
the same for all petri dishes.
- Record germination date. Record the radicle
and hypocotyl growth daily for approx. a 5 days. Record qualitative
information about the seeds. Chart qualitative results.
Calculate means, graph, chart quantitative results.
Discuss your results and draw conclusions.
Extensions
- Try other allelopathic interactions.
- Try using extract to water mature plants.
- Go on line and find the results of other
scientists investigation into allelopathic interactions.
- Create a visual representation of your findings.
Evaluation
Create a written summary of results and
their implications for wetlands and purple loosestrife. Combine allelopathic
information with other biotic potential for the purple loosestrife.
Measure that against any known environmental resistance. Suggest what
can be done?
Activities: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10
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