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Plant Hormones: Fun Facts and Applications
Target audience: High School Biology students
Time allotted for this activity: Two 45-minute class periods (or one class period
and finish for homework); or one 90-minute block.
Materials needed: Computer, handouts (optional)
National Science Standards:
12CLS1.4 Cell functions are regulated. Regulation occurs both through changes in the
activity of the functions performed by proteins and through the selective expression of
individual genes. This regulation allows cells to respond to their environment and to
control and coordinate cell growth and division.
12CLS6.2 Organisms have behavioral responses to internal changes and to external
stimuli. Responses to external stimuli can result from interactions with the organism’s
own species and others, as well as environmental changes; these responses either can
be innate or learned. The broad patterns of behavior exhibited by animals have evolved
to ensure reproductive success. Animals often live in unpredictable environments, and
so their behavior must be flexible enough to deal with uncertainty and change. Plants
also respond to stimuli.
Objectives for this activity:
1. The student will be able to identify five major plant hormones.
2. The student will be able to list at least three effects that each hormone has on
plant growth and development.
3. The student will be able to list a commercial application for each plant hormone.
To begin the trackstar, go to:
http://trackstar.4teachers.org/trackstar/ts/viewTrack.do?number=405358
Click on “View in Frames” at the bottom of the page.
Teachers can use the guided questions that correlate with each website in the following
ways:
1. Have students copy the questions from the top of each new screen and write out
their answers.
2. Teachers can give students a handout with the questions and directions (see student
copy).
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3. Students can copy and paste the questions into a Word document and type their
responses. This is helpful for electronic submission of the assignment.
Abscisic Acid - http://www.plant-hormones.info/abscisicacid.htm
1. Discuss how abscisic acid (ABA) got its name.
Scientists were studying compounds that were responsible for abscission of fruits.
The compounds that were isolated were named abscisin I and abscisin II.
2. Where is ABA produced in plants? List both the plant organ and organelles.
Construct a flow chart showing the synthesis of this compound from the source
to the activated compound..
ABA is produced in chloroplasts and other plastids which are primarily in leaves.
The pathway of synthesis is: carotenoids (violaxanthin) →isomerase reaction →
oxidation reaction → xanthonin →ABA aldehyde →oxidation →ABA →glucose or
oxidation produces the active form.
3. Read the functions of ABA. Based on the original notion that this substance is
involved in abscission, why do the listed functions make sense?
All of the functions involve inhibition or slowing down the growth processes of
plants. Some of the functions also involve dormancy or preparing for dormancy.
Auxins- http://plantphys.info/plant_physiology/auxin.shtml
1. Look at the diagrams showing the historical experiments that enabled scientists
to learn about the function of auxin (IAA). Briefly describe what each scientist
did.
Charles Darwin cut the tops off of coleoptiles to see the response of the plant to
light. He determined that the tip of the plant responded to light. The BoysenJensen experiments cut the top of the coleoptile off but they inserted a piece of
mica between the cut tip and the tip itself. They believed there was a chemical
response to light was occurring further down the tip. The Went experiments used
agar blocks between the coleoptile tips and the rest of the coleoptile. The blocks
were placed off-center and the coleoptile growth responded accordingly. This
proved that a chemical was involved the growth pattern of the coleoptiles.
2. What is the optimal concentration for IAA for elongation?
10-5M
3. When a plant produces auxin in excess of the optimal concentration, how does
the plant compensate? Look at the diagram showing the sources and sinks of
auxin.
The auxin is compartmentalized, transported somewhere else in the plant,
degraded, or conjugated and then possibly degraded.
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4. Discuss how auxin moves through a plant. Look at the diagrams showing
basipetal movement. What does this word mean? Google this term to find a
definition.
The auxin always moves from the section of the stem that is towards the top of the
plant to the bottom of the plant. The diagrams show that no matter which way the
stem sections were turned, the auxin would move from the area closest to the top
of the stem towards the bottom part of the stem. Basipetal means to proceed from
the apex to the base.
Auxins continued - http://www.plant-hormones.info/auxins.htm
5. Scroll down to the “Functions of auxin.” Contrast the main function of this
hormone with that of ABA.
Auxin promotes growth of cells and cell division. It also promotes growth of flower
parts and delays cell death or dormancy of the plant. It also suppresses lateral buds
so apical growth is encouraged. ABA promotes cell death or dormancy. In a sense,
it has the opposite effects on the plant from auxin.
Cytokinins - http://www.plant-hormones.info/cytokinins.htm
1. Before reading the information about cytokinin, speculate what the job of this
hormone is based on its name.
Students may have learned the term cytokinesis and associate this word with cell
division. The role of cytokinins is promotion of cell division.
2. Summarize the history of how cytokinin was discovered.
Scientists discovered that substances which promoted cell division were in phloem
and in endosperm. The first kinetin was isolated from herring sperm in 1955. The
first cytokinin isolated from a plant was zeatin – named for the plant it was taken
from (Zea mays - corn).
3. Where is cytokinin concentrated in a plant?
The meristematic tissue.
4. Read the functions of this hormone. Contrast one function of cytokinin with that
of auxin.
Auxin does not promote lateral buds but cytokinin does stimulate lateral growth.
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Cytokinin continued - http://www.ars.usda.gov/is/pr/2010/100310.htm
Read the article on how cytokinin is used for growing cotton. Discuss why the
application of this hormone is beneficial to cotton farmers.
Farmers that grow cotton in dry areas have found that cytokinin “tricks” the young
plants into growing a deeper root system and also the plant produces a waxy
coating that prevents water loss. The cytokinin is applied at a low concentration
and did not have any deleterious effect on plants grown in wet conditions. The
hormone can be applied at the same time that weed killers are applied.
Gibberelins - http://4e.plantphys.net/article.php?ch=5&id=372
1. Read the paragraph on commercial uses for gibberellins. What are the various
uses for this hormone in the grape, cherry, apple, and citrus crop industries?
This hormone is used to promote growth of fruit crops and increase stem length. In
the grape industry, the gibberellins substitute for seeds and fruits will form without
the seed. It also elongates the rachis of the grape clusters which allows for larger
fruit and less fungal growth since the fruits are farther apart and air flow is better.
Cherry production is enhanced because the hormone produces bigger fruit. In
apples the hormone is used to promote fruiting in the “off years” since apples tend
to be biennial producers of fruit. One type of gibberellin is used for prevention of
russetting on Golden Delicious apples. For citrus crops, the gibberellic acid can
prevent rind-aging in oranges and tangerines and in lemons and limes it helps
ripening occur when the fruit is at an optimal size.
Ethylene – http://plantphys.info/plant_physiology/ethylene.shtml
1. Scroll half-way down the web page until you reach the section on Ethylene and
Fruit Ripening. What process accelerates after an increase in ethylene release?
What does the graph show that verifies this conclusion? What is the term that
describes this effect?
Respiration increases after ethylene is released in plants. The graph shows that
carbon dioxide release increases right after ethylene release increases. The term
for an increase in respiration shortly after ethylene release is climacteric.
2. Ethylene accelerates cell death and ripening of fruits. Read and summarize how
the shelf-life of cut flowers can be extended if ethylene is blocked.
Ethylene requires a copper cofactor in the cell-membrane receptor protein that can
be eliminated if silver is present instead. By adding silver thiosulfate, the ethylene
does not bind to the cells and flowers do not die as quickly.
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Extra credit – http://www.plant-hormones.info/index.htm
Go to the website and click on the links for each plant hormone from this activity.
List the compound that is the hormone is synthesized from in the plant. For
example, ABA →violaxanthin (a carotenoid).
ABA →violaxanthin (a carotenoid)
Auxin→tryptophan
Cytokinin→adenine
Ethylene→methionine
Gibberellin→acetyl Co-A