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25–2 Plant Responses
Photo Credit: ©Dr. Paul A. Zahl/Photo Researchers, Inc.
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Tropisms
Tropisms
Plants change their patterns and directions of growth in response to a multitude of cues.
The responses of plants to external stimuli are called tropisms.
Plant tropisms include gravitropism, phototropism, and thigmotropism.
Each of these responses demonstrates the ability of plants to respond effectively to external stimuli, such as gravity, light, and touch.
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Tropisms
Gravitropism
Gravitropism, the response of a plant to gravity, is controlled by auxins.
Gravitropism causes the shoot of a germinating seed to grow out of the soil—against the force of gravity.
It also causes the roots of a plant to grow with the force of gravity and into the soil.
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Tropisms
Phototropism
Phototropism, the response of a plant to light, is also controlled by auxins.
This response can be so quick that young seedlings reorient themselves in a matter of hours.
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Tropisms
Thigmotropism
Thigmotropism is the response of plants to touch.
An example of thigmotropism is the growth of vines and climbing plants.
The stems of these plants do not grow straight up. The growing tip of each stem points sideways and twists in circles as the shoot grows.
When the tip encounters an object, it quickly wraps around it.
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Tropisms
Some climbing plants have long, twisting leaf tips or petioles that wrap tightly around small objects.
Other plants, such as grapes, have extra growths called tendrils that emerge near the base of the leaf and wrap tightly around any object they encounter.
One effect of thigmotropism is that plants curl and twist around objects, as shown by the stems of this grapevine. Photo Credit: ©Charles D. Winters/Photo Researchers, Inc.
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Rapid Responses
Rapid Responses
Not all plant responses involve growth.
One example is the rapid closing of leaflets that occurs in the Mimosa pudica.
If you touch the leaves of a mimosa plant, within seconds, the leaves snap shut.
The secret to this movement is changes in osmotic pressure.
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Rapid Responses
The leaves are held apart due to osmotic pressure where the two leaflets join.
When the leaf is touched, cells near the center of the leaflet pump out ions and lose water due to osmosis.
Pressure from cells on the underside of the leaf, which do not lose water, forces the leaflets together.
A venus fly trap works in a similar manner, using osmotic pressure and cell wall expansion to snap the leaf shut around a prey item.
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Photoperiodism
Photoperiodism
Plants such as chrysanthemums and poinsettias flower when days are short and are therefore called short-day plants.
Spinach and irises flower when days are long and are therefore known as long-day plants.
Photoperiodism is the response to periods of light and darkness.
Photoperiodism in plants is responsible for the timing of seasonal activities such as flowering and growth.
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Photoperiodism
Photoperiodism controls the timing of flowering and seasonal growth. The response of flowering, shown here, is controlled by the amount of darkness plants receive. Short-day plants, such as chrysanthemums, flower only when exposed to an extended period of darkness every night—and thus a short period of light during the day. Long-day plants, such as irises, flower when exposed to a short period of darkness or to a long period of darkness interrupted by a brief period of light.
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Photoperiodism
It was later discovered that a plant pigment called phytochrome is responsible for photoperiodism.
Phytochrome absorbs red light and activates a number of signaling pathways within plant cells.
Plants respond to regular changes in these pathways and these changes determine the patterns of a variety of plant responses.
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Winter Dormancy
Winter Dormancy
Phytochrome also regulates the changes in activity that prepares many plants for dormancy as winter approaches.
Dormancy is the period during which an organism's growth and activity decreases or stops.
As cold weather approaches, deciduous plants turn off photosynthetic pathways, transport materials from leaves to roots, and seal leaves off from the rest of the plant.
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Winter Dormancy
Leaf Abscission
At summer’s end, the phytochrome in leaves absorbs less light as days shorten and nights become longer.
Auxin production drops, but the production of ethylene increases.
The change in the relative amounts of auxin and ethylene hormones starts a series of events that gradually shut down the leaf.
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Winter Dormancy
First chlorophyll synthesis stops.
Light destroys the remaining green pigment.
Other pigments—including yellow and orange carotenoids—become visible for the first time.
Production of new plant pigments—the reddish anthocyanins—begins in the autumn.
Every available carbohydrate is transported out of the leaf, and much of the leaf’s water is extracted.
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Winter Dormancy
Finally, an abscission layer of cells at the petiole seals the leaf off from the plant’s vascular system.
Before long, the leaf falls to the ground, a sign that the tree is fully prepared for winter.
Deciduous plants undergo changes in preparation for winter dormancy. An abscission layer of cells forms at the petiole to seal the leaf off from the rest of the plant . Eventually, the leaf falls off.
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Winter Dormancy
Overwintering of Meristems
Hormones also produce important changes in apical meristems.
Instead of continuing to produce leaves, meristems produce thick, waxy scales that form a protective layer around new leaf buds.
Enclosed in its coat of scales, a terminal bud can survive the coldest winter days.
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Winter Dormancy
At the onset of winter, xylem and phloem tissues pump themselves full of ions and organic compounds.
These molecules act like antifreeze in a car, preventing the tree’s sap from freezing, thus making it possible to survive the bitter cold.
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