Plant HormonesHow do plant hormones affect plant growth and development?Why?Seeds do not usually sprout in the winter—how do they know when to germinate? Grocery stores needto have ripe, unbruised produce to sell to the public, but often this produce is packed weeks ahead oftime and shipped hundreds of miles to get to the store. How does it ripen after harvest? There are severaldifferent plant h
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Plant Hormones
How do plant hormones affect plant growth and development?
Why?
Seeds do not usually sprout in the winter—how do they know when to germinate? Grocery stores need
to have ripe, unbruised produce to sell to the public, but often this produce is packed weeks ahead of
time and shipped hundreds of miles to get to the store. How does it ripen after harvest? There are several
different plant hormones that help to orchestrate plant growth, development, ripening, and responses to
various environmental stimuli.
Germinating seeds have coleoptiles, which are the first shoots that grow from the seed. Mature plants don’t
have coleoptiles.
1. Consider the experiment illustrated in Model 1.
a. What is the stimulus in this experiment?
Amount of light
b. What is the plant’s response to the stimulus in the control?
It grows toward the light (positive phototropism).
2. The characteristic response of plants to sunlight—growing toward the light—is called
positive phototropism. Knowing that “trop” is the Greek word for “turn,” explain why this is
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an excellent term for this phenomenon.
It means that the plant is turning towards light.
3. List the three experimental groups represented in Model 1.
Covered coleoptile base, removed coleoptile tip, covered coleoptile tip.
4. Which experimental group in Model 1 has the same response as the control group?
Covered coleoptile base
5. Propose an explanation of why removing or covering the tip of the coleoptile might result in
a lack of positive phototropism.
The tip is probably the only part of the coleoptile that contains photoreceptors. If the tip is covered, it does
not recognize the presence of light, and does not grow towards it because of positive phototropism.
6. What additional evidence does the experiment with the “tip covered by cap” group tell you?
The base of the coleoptile does not have photoreceptors and is not sensitive to light.
Read This!
Other follow-up experiments showed that if the coleoptile tip is separated from the shoot by a piece of
permeable material (such as a block of gelatinous agar), the positive phototropism still occurs. If,
however, the tip is separated from the shoot by a piece of impermeable material (such as a block of
mica), then phototropism did not occur.
7. What is the major difference in the agar material versus the mica material?
The agar material is permeable, so substances can move across it, whereas the mica material
is impermeable, so nothing can pass through it between the coleoptile and the rest of the plant.
8. Hormones are chemical messengers that travel from the source of synthesis to target cells,
which they then act upon in some way. Explain how the experiment described in the Read
This! box supports the idea that a hormone is involved in phototropic response.
Hormones are synthesized in the base of the coleoptile, and then go and act in the tip of the coleoptile.
The permeable membrane allows the hormones to move into the tip and work, but the impermeable
membrane prevents the movement, and since hormones are not synthesized in the tip, the coleoptile doesn’t
grow.
9. The hormone from the coleoptile tip was later identified and named “auxin.” Auxins are a
group of plant hormones that have many actions on plant growth and development, but the
main effect is elongation of the cells in stems. If these hormones are stimulated by sunlight,
discuss with your group how the auxins might cause the coleoptile to bend towards the light.
Auxin moves to the cells that are farthest away from the light, and there is progressively less auxin as
the cells get closer to the side of the plant that is exposed to light. The auxin increases the action of proton
pumps in the plasma membranes of the cells. They pump out protons
10. Design an experiment using a seedling and a block of agar soaked with auxins that might
confirm the effect of this hormone on the elongation of the coleoptile. (Hint: Remember
that the auxins are permeable to the agar material.)
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