Allelopathy
Chapter 1: Introduction
History
Theophrastus (ca. 300 B.C.E.), a
student and successor to Aristotle, wrote about allelopathic reactions in his
botanical works. He has been called the
"father of Botany", and wrote of how chickpea "exhausts" the
soil and destroys weeds.
In 1 C.E., Gaius Plinius Secundus, also
known as Pliny the Elder, a roman scholar and naturalist, wrote about how chick
pea and barley "scorch up" cornland.
He also mentioned that Walnut trees are toxic to other plants.
Augustin Pyramus De Candolle, a
botanist and naturalist, in 1832, suggested that soil sickness was caused by
chemicals released by the crop.
And, in 1907-1909, two researchers,
Schreiner and Reed investigated the isolation of a number of phytotoxic
chemicals from plants and soils.
What is Allelopathy?
The word allelopathy derives from two
separate words. They are allelon
which means "of each other", and pathos which means "to
suffer". Allelopathy refers to the
chemical inhibition of one species by another.
The "inhibitory" chemical is released into the environment
where it affects the development and growth of neighboring plants.
Allelopathic chemicals can be present
in any part of the plant. They can be
found in leaves, flowers, roots, fruits, or stems. They can also be found in the surrounding soil. Target species are affected by these toxins
in many different ways. The toxic
chemicals may inhibit shoot/root growth, they may inhibit nutrient uptake, or
they may attack a naturally occurring symbiotic relationship thereby destroying
the plant's usable source of a nutrient.
Are all plants Allelopathic?
Not all plants have allelopathic
tendencies. Some, though they exhibit
these tendencies, may actually be displaying aggressive competition of a non-chemical
form. Much of the controversy
surrounding allelopathy is in trying to distinguish the type of competition
being displayed. In general, if it is
of a chemical nature, then the plant is considered allelopathic. There have been some recent links to plant
allelotoxins directed at animals, but data is scarce.
Environmental Impact
Allelopathy is a form of chemical
competition. The allelopathic plant is
competing through "interference" chemicals. Competition, by definition, takes one of two forms--exploitation
or interference.
Competition is used by both plants and
animals to assure a place in nature.
Plants will compete for sunlight, water and nutrients and, like animals,
for territory. Competition, like
parasitism, disease, and predation, influences distribution and amount of
organisms in an ecosystem. The
interactions of ecosystems define an environment.
When organisms compete with one
another, they create the potential for resource limitations and possible
extinctions. Allelopathic plants prevent
other plants from using the available resources and thus influence the
evolution and distribution of other species.
One might say that allelopathic plants control the environments in which
they live.
For Discussion
How can one tell whether a plant is
exhibiting allelopathy as opposed to non chemical competition?
Can allelopathic chemicals affect
animals, including humans?
How did people first become aware of
allelopathy?
Have you noticed allelopathy or
allelopathic-like influences in your neighborhood? Elsewhere?
Why is studying allelopathy important?
What are some beneficial implications
of allelopathy? How can these be used for the betterment of the planet?
How would you go about testing for
allelopathy?
Can you name or identify some allelopathic
plants?
Focus: Inquiry based learning
This unit on Allelopathy is inquiry
based. The labs and field trips are
hands-on and all of the activities require constant student interactions. Because of this, the unit can be modified
and adjusted without negative affect.
Chapter 2: Chemical Warfare in the
Plant Kingdom
Black Walnut
One of the most famous allelopathic
plants is Black Walnut (Juglans nigra).
The chemical responsible for the toxicity in Black Walnut is Juglone (5
hydroxy-1,4 napthoquinone) and is a respiration inhibitor. Solanaceous plants, such as tomato, pepper,
and eggplant, are especially susceptible to Juglone. These plants, when exposed to the allelotoxin, exhibit symptoms
such as wilting, chlorosis (foliar yellowing), and eventually death. Other plants may also exhibit varying
degrees of susceptibility and some have no noticeable effects at all. Some plants that have been observed to be
tolerant of Juglone include lima bean, beets, carrot, corn, cherry, black
raspberry, catalpa, Virginia creeper, violets, and many others.
Juglone is present in all parts of the
Black Walnut, but especially concentrated in the buds, nut hulls, and
roots. It is not very soluble in water
and thus, does not move very rapidly in the soil. Toxicity has been observed in all soil with Black Walnut roots
growing in it (roots can grow 3 times the spread of the canopy), but is
especially concentrated closest to the tree, under the drip line. This is mainly due to greater root density
and the accumulation of decaying leaves and hulls.
Tree-Of-Heaven
The Tree-Of-Heaven, or Ailanthus
(Ailanthus altissima) is a recent addition to the list of allelopathic
trees. Ailanthone, an allelotoxin
extracted from the root bark of Ailanthus, is known for its "potent
post-emergence herbicidal activity".
Ailanthus poses a serious weed problem in urban areas.
Sorghum
The major constituent of sorghum that
causes allelopathic activity is sorgolene
(2-hydroxy-5-methoxy-3-{(8'z,11'z)-8',11',14'-pentadecatriene}-p-benzoiquinone). Sorgolene is found in the root exudates of
most sorghum species and has been shown to be a very potent allelotoxin that
disrupts mitochondrial functions and inhibits photosynthesis. It is being researched extensively as a weed
suppressant.
Others
There are many other known allelopathic
species, and many that are highly suspected of being allelopathic including
various wetland species, grasses, and other woody plants such as Fragrant Sumac
(Rhus aromaticus). Tobacco (Nicotiana
rustica), Rice (Oryza sativa), Pea (Pisum sativum), and many others, are known
to have root allelotoxins.
For Discussion:
Have the students research and discuss
other allelopathic plants.
Chapter 3: Procedures and Protocol
Protocol 1. Learning To Identify Signs
Of Allelopathy
The best way to study allelopathy is to
find signs of it occurring in nature.
It is impossible to "see" the toxins at work, but it is
possible to see the signs and symptoms caused by the chemicals on surrounding
plants. For example, very few plants
grow under a Black Walnut and those that do often times look sickly and
chlorotic. This is a sign of the
allelotoxin, Juglone, at work.
Along with recognizing the signs of
allelopathy, one must also be able to identify the plants. Some allelopathic plants, such as Black
Walnut, grow in our backyards and on our streets and are easy to identify. Others, like sorghum or chick pea, may be
easier to find in rural areas where they are grown as crops or alongside farm
land. Some allelopathic plants, especially
many of the wetland species, may require special field trips and extra time to
find them first and then identify them.
Protocol 2. Harvesting Plants and Plant
Parts
Many of the known allelotoxins are very
expensive and not easy to come by. Some
companies such as Sigma Chemical and Caroline Biological may carry the
chemicals, but in solid form that will require extra time and effort to bring
to a soluble form that can be used in the lab.
However, not every class will have the funds or the access to these
chemicals. Thus, it may be that the
only way to run the experiments is to have the class harvest their own
allelotoxins.
Some research will be required to
investigate what plant parts have the highest concentrations of allelopathic
species. For instance, the Juglone
found in Black Walnut can be found throughout the plant but particularly within
the nut hulls, leaves, and roots.
Therefore, a class project may be to break into groups and harvest each
part and test them accordingly.
It is important when harvesting plants
or plant parts to be sure that the plant is not endangered and to be sure that
the procedure is carried out in such a way as to bring no harm to the plant or
the surrounding area. Of course, in the
case of harvesting the entire plant, accommodations must be made.
This field exercise can be done when
the class is identifying the allelopathic plants as described in Protocol 1, or
can be done as a separate exercise.
Protocol 3. Testing for Allelopathy in
the Lab
The effects of allelopathic toxins on
sensitive plants can easily be tested in the lab or greenhouse setting. Seeds are the easiest and least expensive to
test. Seeds that do not germinate in the
presence of allelotoxins are probably displaying toxicity effects. Plants that become chlorotic and eventually
die in the presence of allelotoxins are also showing signs of toxicity to the
chemical.
Solanaceous crops, such as tomatoes and
peppers, are most susceptible to juglone (the allelotoxin found in Black Walnut
trees). The laboratory setting is the
perfect place to test the susceptibility of certain plants to various
alleltoxins.
Other scientific or research based
concepts, such as graphing, dilutions, and general lab protocol will also be
covered when certain allelopathy activities are conducted in the lab or
classroom setting.
Procedures
1. Familiarize yourself with the
allelopathic species in your area. In
particular, focus on mature species that are established. These tend to have higher concentrations of
the allelotoxin and thus will display better signs and symptoms on any
susceptible surrounding plants.
2. If possible, contact a local
conservation organization or extension agency, that might have some insight
about allelopathy. Your research may be
of interest to them and they may offer professional advice or important
information.
3. Decide which species and areas
should be the focus of your survey.
4. Decide on a survey method. For instance, you may want to conduct the
identification field trip one day and then follow up with the harvest field
trip another, or you may want to conduct both on the same day.
5. Learn how to identify the species
that you will be studying. There are
many good Field Guides available, as well as many excellent web sites.
6. Decide how to divide up the area you
will be working in.
7. Record what allelopathic signs and
symptoms were found, and the species they were found by.
8. Discuss ways to study allelopathy in
the laboratory.
9. Gather needed materials. See the materials list at the end.
Lab & Classroom Exercises
A} Identifying Allelopathic Plant
Parts: Black Walnut
Research Focus
Have the students discuss why they
believe certain plant parts may be more allelopathic than others. Have them conduct some research into this. They can scan the web, be involved in
personal communications with professionals in the field, or visit a library.
For Discussion:
*Why are some parts more allelopathic than others?
*How
does this affect the organism's ability to out-compete other plants?
*Does
this change from season to season? Day to night?
*How
can we test these parts and their properties?
Field Harvesting
Conduct a field trip to harvest parts
of the Black Walnut tree. These parts
will be brought back to the classroom for further investigation and
observation.
Students should collect the following:
1. Leaves
*5-10
leaves per group
*Place them
in paper bags with the group's name or number on them and where on the tree
canopy the leaves were collected from.
2. Nut Hulls
*2-3
nut hulls per group
*Note:
Black Walnut nuts stain skin and clothes.
3. Roots
*One
bunch of roots for the entire class is sufficient
*Dig
a clean hole, CUT a small amount of roots by the drip line
*Replace
the soil
*Removal of
roots is invasive and should ONLY be done if the teacher and students are
confident in their techniques and methods.
4. Soil
*Have
students remove soil from a range of areas starting at the trunk of the tree
and moving out beyond the drip line.
*One
bucket-full per group will be sufficient
5. Other
Laboratory Procedures
Leaves:
1. Prepare jars with tomato or pepper
seeds
*Place
tissue in bottom jar
*Make
moist
*Place
seeds on moist towels (usually ten seeds per treatment)
2. Crumble leaves and place in
cheesecloth
3. Rubber band or tie the cheesecloth
closed and place into mouth of jar so that the leaf bundle dangles over, but
does not touch, the seeds.
4. Place jars in windowsill or under
grow light.
5. Observe
Nut Hulls:
1. Using a blender, food processor, or
hammer, pulverize the nut
2. Decide on whether to add water or
leave as is (a comparison of both ways may be best)
3. Place the nut juice in a petri dish
that has been lined with paper towel.
4. Place ten seeds into each prepared
petri dish
5. Observe.
Roots:
See Nut Hulls Procedure Above
Soil:
1. Place some potentially allelotoxic
soil into containers.
2. Place a known un-contaminated soil
into other containers (control)
3. Plant seeds into each
4. Observe
Note: Tomato plants can be used instead
of seeds where appropriate.
B} Allelopathy: Tomato Seed
Dose/Response Bioassay
Research Focus:
Have students discuss the idea of only
certain amounts of allelotoxins being effective against other organisms. Have them conduct some research into
this. As with the afore mentioned
activity, they can scan the web, visit a library, or be involved in personal
communications with a professional in the field.
For Discussion:
*Why would certain amounts of allelotoxins be more effective than
others?
*How
would this affect susceptible organism's defenses?
*What
other factors may be involved?
*How
can we test for this?
Doing Dilutions to Test a Chemical’s
Toxicity Thresholds
Conduct a laboratory experiment where
certain allelotoxins are tested at varying dilutions or concentrations. These dilutions, once observed and analyzed,
should give some insight into what amounts of that chemical are most effective
against other organisms. This
particular experiment uses tomato seeds as a bioassay because tomatoes are a known
susceptible species to Juglone.
However, the class can decide on other seeds or plants to use (in the
case of plants, the dilutions would be poured into the containers or into the
solution in the case of hydroponics).
Allelopathy: Tomato seed Dose/Response Bioassay Data Sheet
Name
Date
Chemical tested
100% Concentration
Length of Experiment
Table 1: Seed Germination Data
Concentration (%) |
Concentration (mg/L) |
# Seeds Germinated/Dish |
Average # Seeds Germinated |
Control |
|
|
|
0.001% |
|
|
|
0.01% |
|
|
|
0.1% |
|
|
|
1% |
|
|
|
10% |
|
|
|
100% |
|
|
|
Table 2: Radicle Length Data
Concentration (%) |
Radical Length (mm) |
Average Length (mm) |
Control |
|
|
0.001% |
|
|
0.01% |
|
|
0.1% |
|
|
1% |
|
|
10% |
|
|
100% |
|
|
What are students looking for?
1. Whether or not seeds germinate.
2. Whether or not plants become
affected.
3. For changes in reactions according
to treatments.
4. Other: Let them decide what to look
for and why.
General Notes on above procedures:
Solanaceous crops, such as tomatoes, peppers,
eggplants, etc., are most susceptible to juglone and are thus best to use.
Tomato, eggplant, or pepper plants can
be used instead of seeds where appropriate.
Juglone is generally non-toxic to humans but, as with any substance, individuals may be sensitive and might react. It is recommended that pregnant women not come in direct contact with crystallized juglone.