25–2 Plant Responses
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25-2 Plant Responses
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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25-2 Plant Responses
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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25-2 Plant Responses
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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25-2 Plant Responses
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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25-2 Plant Responses
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.
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25-2 Plant Responses
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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25-2 Plant Responses
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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25-2 Plant Responses
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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25-2 Plant Responses
Photoperiodism
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25-2 Plant Responses
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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25-2 Plant Responses
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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25-2 Plant Responses
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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25-2 Plant Responses
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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25-2 Plant Responses
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.
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25-2 Plant Responses
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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25-2 Plant Responses
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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