Transcript 25-2 Plant Responses

Biology
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25–2 Plant Responses
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25-2 Plant Responses
Tropisms
Tropisms
What are plant tropisms?
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25-2 Plant Responses
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.
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25-2 Plant Responses
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.
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25-2 Plant Responses
Tropisms
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.
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25-2 Plant Responses
Rapid Responses
The secret to this movement is changes in osmotic
pressure.
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.
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25-2 Plant Responses
Photoperiodism
Photoperiodism
What is photoperiodism?
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25-2 Plant Responses
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.
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25-2 Plant Responses
Photoperiodism
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.
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25-2 Plant Responses
Winter Dormancy
How do deciduous plants prepare for
winter?
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25-2 Plant Responses
Winter Dormancy
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.
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25-2 Plant Responses
Winter Dormancy
The change in the relative amounts of auxin and
ethylene hormones starts a series of events that
gradually shut down the leaf.
First chlorophyll synthesis stops.
Light destroys the remaining green pigment. Other
pigments—including yellow and orange
carotenoids—become visible for the first time.
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25-2 Plant Responses
Winter Dormancy
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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25–2
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The rapid responses of plants such as the
Venus flytrap are caused by changes in
a. light levels.
b. osmotic pressure.
c. temperature.
d. moisture.
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25–2
The timing of seasonal activities in plants, such
as flowering and growth, depends on
a. gravitropism.
b. thigmotropism.
c. photoperiodism.
d. phototropism.
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25–2
The hormone primarily responsible for causing
plants to flower depending on the length of the
day is
a. auxin.
b. phytochrome.
c. cytokinin.
d. ethylene.
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25–2
In most flowering plants, the coming of autumn
is marked by
a. a decrease in ethylene production.
b. an increase in auxin production.
c. the production of new plant pigments.
d. a rapid growth spurt.
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25–2
In deciduous plants, the process of dormancy
involves
a. maintaining photosynthesis and storing
material in the roots.
b. turning photosynthesis off and storing
material in the roots.
c. maintaining photosynthesis and bringing
material up from the roots.
d. turning photosynthesis off and bringing
material up from the roots.
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