Transcript 25–1 Hormones and Plant Growth
Biology
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25 –1 Hormones and Plant Growth
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25 –1 Hormones and Plant Growth 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.
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25 –1 Hormones and Plant Growth Patterns of Plant Growth
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.
The secrets of plant growth are found in meristems, regions of tissue that can produce cells that later develop into specialized tissues.
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25 –1 Hormones and Plant Growth Patterns of Plant Growth
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 Growth Plant Hormones What are plant hormones?
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25 –1 Hormones and Plant Growth 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.
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25 –1 Hormones and Plant Growth Plant Hormones 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 Growth Plant Hormones Hormone producing cells
The portion of an organism affected by a particular hormone is known as its
target cell
or target tissue.
Movement of hormone Target cells Slide 9 of 42 End Show
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25 –1 Hormones and Plant Growth Plant Hormones
To respond to a hormone, the target cell must contain a receptor to which the hormone binds.
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 Growth Plant Hormones
Cells that do not contain receptors are generally unaffected by 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 Growth Plant Hormones How do auxins, cytokinins, gibberellins, and ethylene affect plant growth?
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25 –1 Hormones and Plant Growth Auxins
Auxins
Charles Darwin and his son Francis carried out the experiment that led to the discovery of the first plant hormone.
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 Growth 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 Growth 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
.
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25 –1 Hormones and Plant Growth 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.
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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.
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25 –1 Hormones and Plant Growth Auxins
Auxins build up on the lower sides of roots and stems. In stems, auxins stimulate cell elongation, helping turn the trunk upright.
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25 –1 Hormones and Plant Growth Auxins
In roots, their effects are exactly the opposite. There, 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.
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25 –1 Hormones and Plant Growth Auxins
High concentrations of auxins inhibit the elongation of root cells.
Uninhibited cells on the top elongate more than auxin-inhibited cells on the bottom and the root grows downward.
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25 –1 Hormones and Plant Growth Auxins Auxins and Branching
Auxins also regulate cell division in meristems.
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25 –1 Hormones and Plant Growth Auxins
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 Growth 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.
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25 –1 Hormones and Plant Growth 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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25 –1 Hormones and Plant Growth 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.
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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.
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 Growth Cytokinins In plants, cytokinins stimulate cell division and the growth of lateral buds, and cause dormant seeds to sprout.
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25 –1 Hormones and Plant Growth 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
to ripen.
is a plant hormone that causes fruits Commercial producers of fruit sometimes use this hormone to control the ripening process.
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25 –1 Continue to:
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25 –1
The tendency of a plant to grow toward a source of light is a. gravitropism.
b. phototropism.
c. meristematic growth.
d. apical dominance.
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25 –1
A plant part in which hormones are produced is a. the apical meristem.
b. a target cell.
c. a hormone receptor.
d. xylem.
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25 –1
If you snip off the tip of a stem, the a. plant grows tall and narrow.
b. plant dies.
c. side branches begin to grow more quickly.
d. stem stops growing.
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25 –1
Ethylene is a plant hormone that causes a. plant cells to grow longer.
b. flowers to develop.
c. fruit to ripen.
d. roots to grow downward.
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25 –1
The herbicides produced by chemists have a structure that is similar to a. auxins.
b. gibberellins.
c. cytokinins.
d. ethylene.
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