Transcript Chapter 15 Plant Responses to Stimuli

Plant Responses to Stimuli
Plants versus Animals
► Plants as living organisms have the ability to:
► 1) use energy to obtain materials from the
external environment, and use energy to
rearrange those materials into new plant
substances.
► 2) With very few exceptions, plants acquire all the
matter and energy they need: without changing
location (as must most animals, protists, and
bacteria), without preying on other living
organisms (as animals do), and without relying on
matter assembled by other organisms (as fungi
do).
How are new individuals formed?
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Some plants, such as the
Bryophyllum grow new
individuals from a leaf.
Some trees, such as sumac
and poplar, grow new
individuals, called suckers,
from the roots.
Spider plants and
strawberries grow stems
from which new individuals
can become established at
some distance from the
original plant.
Many grasses grow from
nodes in their roots.
Attainment of Nutrients
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Most plants obtain their nutrients
from the air and the soil.
Some plant species however, are
able to “prey”, or eat and gain
nutrients from animals.
The pitcher plant (the provincial
flower of Newfoundland and
Labrador) and the Venus flytrap
are consumers.
They trap and digest insects in
order to obtain nutrients that are
not available in the nutrient-poor
soil they grow in.
These plants are also producers
because they photosynthesize, as
well.
P.Plant
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Newfoundland)
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Development in the Meristem
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What gives plants their amazing
ability to grow throughout their
lives?
Although mitosis and cell division
occurs throughout a plant as it
grows, eventually new growth is
restricted to small regions of
unspecialized tissue collectively
called the meristems
Growth there results from the
accumulation of rapidly dividing
cells.
When a cell in the meristem
divides, one of the two resulting
cells remains in the meristem.
The other cell becomes part of
the plant body.
Initially, all meristem cells are
identical in structure, and they
have no specialized function.
Development in the Meristem
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As they divide repeatedly,
they begin to differ in:
 Shape
 relative proportions of their
various organelles
 functions they can perform.
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Changes result in the cells
becoming specialized for
particular functions, such
as photosynthesis,
storage, and support.
Types of Meristem Tissue
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Two main types of meristem
tissue.
1) apical meristem tissue:
located in the root and shoot tips
of plants. Division of apical
meristem cells results in growth
of roots, leaves, and flowers.
Protected by a root cap in the
root.
Protected by a terminal bud in
the stem.
In colder climates, the terminal
buds stop growing in the winter
and resume growing in the
spring.
These buds are protected by bud
scales, which fall off when
growth begins in the spring.
Types of Meristem Tissue
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Lateral meristem
results in the growth of
tissue beneath the bark
of tree stems
► division of cells results
in the thickening of
cylinders of tissue.
► Most woody plants
have two kinds of
lateral meristems: a
vascular cambium
and a cork cambium.
Meristems
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Meristem tissue enables
plants to grow from
cuttings.
Growing plants from
cuttings is the basis of
plant cloning: growing
genetically identical
copies of an organism
from a single cell or part
of an organism.
For some species,
growing plants from
cuttings can be much
faster than growing them
from seed.
Internal Regulation of Plant
Growth and Development
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Plants can grow to their maximum height
when environmental conditions are
optimal.
► Fertilizer
Optimal conditions include adequate
moisture, warmth, light, and
nutrients.
Fertilizers promote plant growth and
development by providing additional
nutrition.
Pesticides and fungicides promote
plant growth by controlling numbers of
insects and fungi that feed on plants.
Plant growth and development are also
controlled by the plant’s own hormones.
Hormone: a chemical compound
manufactured by specialized tissue in one
body part of an organism but that
►Pesticide
governs or regulates the activity of
another body part or parts.
Even though a hormone may circulate
throughout an organism, it will act only
on specific “target” tissues or
organs.
Alice…R.I.P
Hormonal Control of Plant
Growth: Auxin
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In the early 1800s, experiments undertaken by
Charles Darwin and his son Francis described
the effects of a mysterious “influence” that
affected the growth of grass seedlings.
The seedlings normally grew toward a light
source, however this behaviour was not seen if
the tips of the grass seedlings were covered
with an opaque capsule that did not let light
through.
The remainder of the plant, where the growth
actually occurs, was still exposed to light.
If the tip of the seedling were covered with a
gelatin capsule, which allowed light to pass
through, then the seedling would grow towards
the light as expected.
For many years, researchers conducted
experiments to try to explain the nature of this http://www.tutorvista.com/content/biolo
observation.
gy/biology-iv/plant-growthIn 1926, Frits Went performed a series of
movements/growth-regulators.php
experiments that showed there was a chemical
messenger in the grass seedlings. This
chemical could enhance plant growth. He
named the substance auxin, from the Greek
work auxein, which means “to increase”.
Hormonal Control of Plant
Growth PROMOTOR HORMONES
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Other discoveries about plant hormones came as a result
of people noticing unusual growth in plants.
For example, observers noticed that a rice plant infected
with the fungus Gibberella fujikoroi grew abnormally tall.
In 1935, researchers were finally able to isolate the
chemical compound that caused the accelerated growth,
and named it gibberellic acid. They discovered that
applying artificially-manufactured gibberellic acid to a plant
not infected by the fungus caused the plant to grow
abnormally tall.
Scientists and researchers continued to search for other
plant hormones that might affect growth and development.
Two types of plant growth hormones: promoter
hormones, which are hormones that cause growth, and
inhibitor hormones, which are hormones that block
growth.
The Classes of
Hormones
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Auxins are a class of hormone that
is produced in the apical meristem
of shoots.
There are both natural and
synthetic auxins that promote
cell elongation, the development of
vascular tissue, and trigger the
development of above-ground
stems, which help support the
plant.
These hormones also cause leaves
to drop after they are no longer
needed.
The stimulation of cell elongation
occurs because auxin increases the
plasticity of the plant cell wall.
The more plastic the cell wall is, the
more it can stretch during active
cell growth.
The Classes of
Hormones
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Gibberellins are also produced in the apical meristem and act
to increase stem length.
Increase the uptake of starch in the embryo of germinating
seeds and stimulate the development of vascular tissue.
The effects of gibberellins include taller and stronger plants,
plants that flower early, or genetically dwarf plants that grow to
normal heights.
Used in commercial crops to increase fruit size, and cluster size
in grapes.
They can delay the ripening of citrus fruits and speed up the
flowering of strawberries.
The Classes of Hormones
Cytokinins promote cell division and cell differentiation.
► Cell differentiation occurs when specialized cells are
needed to perform certain functions.
► They also delay the aging of leaves and fruit.
► Work by influencing the synthesis and activation of
proteins that are required for mitosis.
► Oligosaccharins are a recently discovered class of
growth promoters. They stimulate plants to manufacture
antibiotics in response to attack by fungi or bacteria.
► This allows the plant to grow to its full potential because
the negative influences of pests are diminished.
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The Classes of Hormones:
Inhibitors
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are two classes of hormone growth
inhibitors.
► 1) abscisic acid (ABA): synthesized mainly in
mature green leaves, fruits, and root caps.
► Inhibits the germination of seeds, inhibits the
growth of buds in plant stems, and blocks the
intake of carbon dioxide by controlling the opening
and closing of leaf stomata.
► Abscisic acid also blocks the action of growth
promoting hormones.
The Classes of Hormones:
Inhibitors
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2) Ethylene: a gaseous hydrocarbon. It occurs as a
natural plant hormone.
Stimulates the aging of plant tissues, the ripening and
sweetening of fruit, and can also speed up the dropping of
leaves from trees.
The production of ethylene gas by plants can stimulate
other plants to ripen.
Initially noted when bananas ripened quickly if they were
left near oranges.
The ripening action of ethylene has led to its use in
agriculture. For example, tomatoes may be picked while
green and then ripened artificially by the application of
ethylene.
Plant Tropisms
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Plants exhibit the ability to orient
themselves in response to external
stimuli such as light.
A directional growth response to
unequal stimulation from the external
environment is called a tropism, and
it controls the growth pattern of the
plant.
Various external stimuli affect the
production of plant hormones - results
in the directional growth of a plant.
In tropism, the plant may grow either
toward or away from the stimulus.
Growth toward the stimulus is a
positive tropism. Growth away from
the stimulus is a negative tropism.
There are three major kinds of
plant tropisms that are affected by
light, gravity, and touch.
Plant Tropisms
1) Phototropism occurs when the
growth of a plant is affected by light.
► In general, plants are positively
phototropic, that is, they grow toward
light.
► Roots are negatively phototropic and
grow away from light.
► The growth is caused by differing
amounts of auxin produced on the
light and dark sides of the stem.
► Auxin accumulates on the shaded side
of the stem, which causes the cells
there to elongate.
► This causes the stem bend toward the
light.
► Turning the plant around will cause the
stem to bend in the other direction,
however, it will not change the original
curve in the stem because it is the
result of growth.
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Plant Tropisms
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2) Gravitropism is a plant’s response to gravity.
Causes roots to grow downwards (positive gravitropism)
and shoots and stems to grow upward (negative
gravitropism).
Benefits the plant, because shoots that grow upward will
receive light and roots that grow downward will receive
nutrients from the soil.
3) Thigmotropism is the response of plants to touch.
This behaviour is a caused by specialized cells in the
epidermis of the plant. Vining plants demonstrate a strong
positive thigmatropism
The vines grow toward the object touching them causing
them to coil around the object. Other plants demonstrate a
negative thigmotropism.
Nastic Responses in Plants
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Another type of response, called nastic
movements, are caused by a stimulus that
is not directional.
For example, the leaves on a mimosa plant
fold up when the plant is touched
This response might seem to illustrate a
negative thigmotropism, however, it is
neither directional nor permanent.
The leaflets fold downward in the same way
regardless of the direction of the stimulus.
These movements are not a result of
growth, but rather a change in turgor
pressure in the cells at the base of each
leaflet.
A sudden drop in pressure causes the cells
to become limp and the leaflets fold down.
Once the stimulus has ceased, the turgor
pressure in the cells rises once again and
the leaflets open.
Another example of a nastic response is the
hinged leaf of a Venus’s flytrap.
The movement of an insect on the leaf
triggers the hinged leaf to close, trapping
the insect between the leaves.
Commercial Use of
Growth Regulators
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Over the past century, scientists have
learned much about plant growth
hormones.
Horticulturists and other agricultural
scientists use this knowledge of plant
growth regulators to influence the
growth and development of crops and
ornamental plants.
Most growth regulating hormones used
for commercial purposes are
synthetically produced rather than
extracted from plants.
For example, there appears to be only
one naturally occuring auxin, but many
more synthetic auxin-like growth
regulators exist.
Although these synthetically produced
hormones are not identical to natural
auxins, their chemical action is similar,
and the plants respond as they would
to naturally occurring auxin.
Commercial Use of
Growth Regulators
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A large industry is based on the
manufacture of artificial plant
growth regulating hormones.
Some of the growth regulating
hormones that are produced can
be very specialized
For example, a chemical treatment
can be applied to ornamental trees
to prevent them from growing too
tall and interfering with utility
lines. From the ground, the trees
look normal, but the tops look as if
they have been pruned flat.
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