25–1 Hormones and Plant Growth
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25–1 Hormones and Plant
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Patterns of Plant Growth
Patterns of Plant Growth
Biologists have discovered that plant cells send
signals to one another that indicate when to divide
and when not to divide, and when to develop into a
new kind of cell.
One difference between growth in plants and animals
is that most animals stop growing once they reach
adulthood.
In contrast, plants continue to grow new needles, add
new wood, and produce cones or new flowers.
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25–1 Hormones and Plant
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Patterns of Plant Growth
The secrets of plant growth are found in meristems,
regions of tissue that can produce cells that later
develop into specialized tissues.
Meristems can be found at places where plants grow
rapidly: tips of growing stems and roots, edges of
woody tissue, etc.
Plants grow in response to environmental factors such
as light, moisture, temperature, and gravity.
Specific chemicals direct, control, and regulate plant
growth.
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25–1 Hormones and Plant
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Plant Hormones
Plant Hormones
A hormone is a substance that is produced in one
part of an organism and affects another part of the
same individual.
Plant hormones are chemical substances that
control a plant's patterns of growth and development
and its responses to environmental conditions.
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25–1 Hormones and Plant
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Plant Hormones
Hormone
producing cells
The hormone moves
through the plant from
where it is produced to
the place where it
triggers a response. Movement of
hormone
The portion of an
organism affected by a
particular hormone is
known as its target
cell or target tissue.
Target cells
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25–1 Hormones and Plant
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Plant Hormones
To respond to a hormone, the target cell must contain
a receptor, usually a protein, to which the hormone
binds.
Cells without receptors are generally unaffected.
If the receptor is present, the hormone can influence
the target cell by:
•
changing its metabolism
•
affecting its growth rate
•
activating the transcription of certain genes
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25–1 Hormones and Plant
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Plant Hormones
Different kinds of cells may have different receptors
for the same hormone.
As a result, a single hormone may affect two different
tissues in different ways.
For example, a particular hormone may stimulate
growth in stem tissues but inhibit growth in root
tissues.
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25–1 Hormones and Plant
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Auxins
Auxins
Charles Darwin and his son Francis carried out the
experiment that led to the discovery of the first plant
hormone and published it in the book The Power of
Movement in Plants.
They described an experiment in which oat
seedlings demonstrated a response known as
phototropism, the tendency of a plant to grow
toward a source of light.
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25–1 Hormones and Plant
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Auxins
In the experiment, they
placed an opaque cap
over the tip of one of the
oat seedlings.
This plant did not bend
toward the light, even
though the rest of the
plant was uncovered.
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25–1 Hormones and Plant
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Auxins
However, if an opaque
shield was placed a few
centimeters below the tip,
the plant would bend
toward the light as if the
shield were not there.
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25–1 Hormones and Plant
Growth
Auxins
The Darwins suspected that the tip of each seedling
produced substances that regulated cell growth.
Forty years later, these substances were identified
and named auxins.
Auxins are produced in the apical meristem and are
transported downward into the rest of the plant.
They stimulate cell elongation.
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25–1 Hormones and Plant
Growth
Auxins
When light hits one side
of the stem, the shaded
part develops a higher
concentration of auxins.
This change in
concentration stimulates
cells on the dark side to
elongate.
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25–1 Hormones and Plant
Growth
Auxins
As a result, the stem
bends away from the
shaded side and
toward the light.
Recent experiments
have shown that auxins
migrate toward the
shaded side of the
stem, possibly due to
changes in membrane
permeability.
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25–1 Hormones and Plant
Growth
Auxins
Auxins and Gravitropism
Auxins are also responsible for
gravitropism; the response of a
plant to the force of gravity.
Auxins build up on the lower sides
of roots and stems. In stems,
auxins stimulate cell elongation,
helping turn the trunk upright.
In roots, auxins inhibit cell growth
and elongation, causing the roots
to grow downward.
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25–1 Hormones and Plant
Growth
Auxins
Auxins also influence how roots grow around objects
in the soil.
If a growing root is forced sideways by an obstacle,
auxins accumulate on the lower side of the root.
High concentrations of auxins inhibit the elongation of
root cells.
Uninhibited cells on the top elongate more than auxininhibited cells on the bottom and the root grows
downward.
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25–1 Hormones and Plant
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Auxins
Auxins and Branching
Auxins also regulate cell
division in meristems.
As a stem grows in length,
it produces lateral buds.
A lateral bud is a
meristematic area on the
side of a stem that gives
rise to side branches.
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25–1 Hormones and Plant
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Auxins
Most lateral buds do not start
growing right away.
The reason for this delay is
that growth at the lateral
buds is inhibited by auxins.
Because auxins move out
from the apical meristem, the
closer a bud is to the stem's
tip, the more it is inhibited.
This phenomenon is called
apical dominance.
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Auxins
Apical meristem removed
When the apical
meristem is removed,
the concentration of
auxin is reduced and
the side branches
begin to grow more
rapidly.
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25–1 Hormones and Plant
Growth
Auxins
Auxinlike Weed Killers
Chemists have produced compounds that mimic the
effects of auxins.
Since high concentrations of auxins inhibit growth,
many of these are used as herbicides—compounds
toxic to plants.
Herbicides include a chemical known as 2,4-D, which
is used to kill weeds.
A mixture containing this compound was used as
Agent Orange, a chemical defoliant sprayed during
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the Vietnam War.
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25–1 Hormones and Plant
Growth
Cytokinins
Cytokinins
Cytokinins are plant hormones produced in
growing roots and developing fruits and seeds.
In plants, cytokinins stimulate cell division and the
growth of lateral buds, and cause dormant seeds
to sprout.
Cytokinins delay the aging of leaves and play
important roles in early stages of plant growth.
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25–1 Hormones and Plant
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Cytokinins
Cytokinins and auxins often produce opposite effects.
•
Auxins stimulate cell elongation.
•
Cytokinins inhibit cell elongation and cause cells
to grow thicker.
•
Auxins inhibit the growth of lateral buds.
•
Cytokinins stimulate lateral bud growth.
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25–1 Hormones and Plant
Growth
Cytokinins
Recent experiments show that the rate of cell growth
in most plants is determined by the ratio of the
concentration of auxins to cytokinins.
In growing plants, therefore, the relative
concentrations of auxins, cytokinins and other
hormones determine how the plant grows.
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25–1 Hormones and Plant
Growth
Gibberellins
Gibberellins
A gibberellin is a growth-promoting substance in
plants.
Gibberellins produce dramatic increases in size,
particularly in stems and fruit.
Gibberellins are also produced by seed tissue and are
responsible for the rapid early growth of many plants.
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25–1 Hormones and Plant
Growth
Ethylene
Ethylene
In response to auxins, fruit tissues release small
amounts of the hormone ethylene.
Ethylene is a hormone that causes fruits to ripen.
Commercial producers of fruit sometimes use this
hormone to control the ripening process.
Many crops, like lemons and tomatoes, are picked
before they ripen so they can be handled.
Just before they are delivered to market, the fruits are
treated with synthetic ethylene to produce a ripe color
quickly, but not always a ripe flavor.
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