Lab #: 8:
Plant Responses to Stimuli
(Plant Hormones), Part I
Plants & Civilization
Spring 2009
Plant Growth & Development
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Depends on many factors including:
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Light
Temperature
Gravity
Night length
Various other external factors
Hormone production
Plant Hormones
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Organic molecules that coordinate growth and
development in plants.
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Synthesized in one part of the plant & translocated
(moved) to another part.
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Active in small amounts
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Hormones cause a physiological response in the
plant.
5 Groups of Plant Hormones
1.
2.
3.
4.
5.
Auxins
Gibberellins
Cytokinins
Abscisic acid
Ethylene
The effects of these hormones can be inhibitory or
stimulatory, depending on [conc], tissue affected,
& development status of the tissue.
Auxins & Gibberellins
Today: you will set-up an experiment in which
you treat plants with an auxin or a gibberellin
to determine the effects of each on plant
growth & development.
We will not be studying the effects of other plant hormones
during the scope of this lab.
AUXINS
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Example: indole-3-acetic acid (IAA)
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Occur naturally in several different chemical forms
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Synthesized in:
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buds of young stems & leaves (e.g. apical bud)
embyros
seeds
fruits
AUXINS
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Once IAA is synthesized in a bud, it is
translocated to the base of the plant (highest
[conc] remaining in the growing tips, less in
roots).
http://www.kallus.com/otherplants/coleus/p001957.jpg
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IAA causes lengthening of cells in the elongation
region of a growing shoot/stem - just behind
apical meristem.
AUXINS
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However, if [IAA] increases beyond an optimum
level, lengthening of the stem is inhibited rather
than stimulated!!!
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IAA produced in the apical bud of a growing
shoot/stem also suppresses the development of
lateral/axillary buds.
http://koning.ecsu.ctstateu.edu/apical/decap.gif
Plants Producing Auxin
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Result: plant exhibits “apical dominance”
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Plant appears to grow upwards rather than
outwards (gets taller, but minimal lateral
branches are produced).
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Plants may appear ‘scrawny’ due to lack of
fullness in lateral branches.
What do you think will happen if the apical bud
(where IAA is produced) is removed?
Today’s Experiment #1:
(work in groups of 4)
Obtain 4 Coleus plants & treat as follows:
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1.
2.
3.
4.
Leave intact
Remove apical bud
Remove apical bud + add control lanolin paste
Remove apical bud + add 5000ppm IAA lanolin
paste
Be sure to label all plants w/ treatment type, your
names, lab section & today’s date.
Don’t forget to make your hypotheses!!!
What you will do….
You will also have a 4th plant w/ apex removed & control
lanolin paste added.
http://www.botit.botany.wisc.edu/images/130/Growth_Substances/Auxins/Apical_Dominance/Results_from_lab/Coleus.php?highres=true
Today’s Experiment #1:
(continued)
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Once you have set-up the experiment & made
your initial observations, measurements, &
hypotheses, you may continue with the next
experiment….
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You will record the results of this experiment in
lab in 2 weeks (Plant Hormones, Part 2).
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Next - testing the effect of Gibberellins on plant
growth….
Gibberellins
Example: 90+ types, gibberellic acid (GA3) is most
prevalent in flowering plants.
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Naturally occurring plant steroid hormones.
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Synthesized in meristematic regions:
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Young leaves
Shoot tips
Root tips
Seeds too!
Gibberellins
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Translocated in the xylem & promotes cell
elongation (like auxin).
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In shoots, stem elongation & rapid growth are
controlled directly by gibberellic acid.
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In seeds, GA3 causes germination to happen
quickly by enhancing cell elongation in the
embryonic root (i.e. roots grows quickly to
penetrate seed coat).
Gibberellins
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Some plants lack the ability to synthesize GA3,
but will still react to this hormone if treated with
it.
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Example: dwarf pea plants
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These plants do not produce their own GA3,
hence the name “dwarf” peas!
http://oregonstate.edu/dept/hort/faculty/Proebsting/images/isogenic.jpg
Today’s Experiment #2:
(work in pairs)
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Obtain 2 Dwarf Pea Plants: take initial
measurements & make observations.
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Then, treat as follows:
1.
2.
Leave untreated – add only control solution.
Treat by adding GA3 to the apex.
Be sure to label all plants w/ treatment type, your
names, lab section & today’s date.
Don’t forget to make your hypotheses!!!
Today’s Experiment #2:
(continued)
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Once you have set-up the experiment & made
your initial observations, measurements, &
hypotheses, you may continue with the next
experiment….
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You will record the results of this experiment in
lab in 2 weeks (Plant Hormones, Part 2).
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Next – other ‘plant responses to stimuli’
Tropisms
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Directional growth movements that occur in
response to stimuli.
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Growth TOWARD a stimulus is called a
POSITIVE response.
Growth AWAY from a stimulus is called a
NEGATIVE response.
Gravitropism
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Definition: directional plant growth that occurs in
response to gravity.
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If a plant is placed on its side…
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The stem will bend upward toward the sun (against
gravity).
The root will bend downward (with the gravitational
pull) into the soil.
Gravitropism
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Movements occur because of UNEQUAL
growth.
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Unequal growth
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Results in bending toward or away from the
stimulus.
Occurs because of auxin (or other hormone)
redistribution.
Gravitropic Response
Occurs in 2 steps:
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1.
Something in the plant must detect gravity
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2.
This “something” is starch grains called
statoliths.
Differential growth must occur
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One side of the plant grows faster than
another, causing the plant to bend.
Gravitropic Response
Plant STEMS vs. ROOTS (both placed horizontally)
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Starch grains (statoliths) fall
toward lower surface causing
an increase in [auxin] in that
region.
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Statoliths in root cap cause an
increase in a growth
inhibitor,abscisic acid, on the
lower surface.
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Lower stem surface grows
faster than upper surface,
causing upward bending.
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Inhibition of growth on lower
surface of the root causes a
downward bending of root.
http://www.greenlizzy.com/graphics/Tom.jpg
http://pmb.berkeley.edu/faculty/faculty_pages/images/FeldmanImage.jpg
Seed Viability
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Seed viability (ability of a seed to grow) is
important in the agricultural industry.
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If a seed is not capable of growth, a mature
plant cannot be produced from it.
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Several tests exist to determine whether a
seed is viable.
Testing Seed Viability
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Tetrazolium Test (TTC) – colorless chemical
when oxidized, but becomes pinkish/red
when reduced.
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Test relies on whether or not the electron
transport chain of a seed is functional.
TTC Test (continued)
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TTC added to a living cell will interact with the etransport chain by accepting e- that are
transferred from the cytochromes.
When TTC accepts these e-, it is reduced and
turns from colorless to deep pink/red.
Abnormal seed
Normal seed
http://www.bioss.ac.uk/smart/unix/mseqacc/images/i10.jpg
Note: Endosperm of normal seed is stained pink/red.
TTC Test (continued)
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TTC is a POISON,
POISON avoid contact with skin &
eyes!!
Handle only with gloves & goggles.
Wash hands & spills immediately.
You will be testing 2 groups of seeds, “A” & “B”, to
determine if the seeds are viable.
Cut seeds lengthwise & place face down in a petri
dish that contains TTC. Put in a dark place for at
least 25 minutes before examining.
Today’s Summary:
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Set-up experiments testing plant hormones –
auxin (IAA) & gibberellin (GA3). Carefully,
record ALL initial observations/measurements.
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Observe demonstrations, gravitropism of stems.
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Set-up seed gravitropism study in corn & bean
experiments.
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TTC test for seed viability.
Today’s Summary:
This lab will be relatively short in length, but you
must set-up experiments carefully. The results
will be recorded in lab two weeks from today.
The data obtained in ‘Part 2’ will be the basis of a
short write-up on this lab topic….
start thinking about your hypotheses & possible
outcomes!