1. Hormones and Plant Growth

2. IB Assessment Statement
Explain the role of auxin in phototropism as an example of the control of plant growth.
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3. Patterns of Plant Growth
Biologists have discovered that plant cells send signals to one another that indicate the following:
when to divide and when not to divide,
and when to develop into a new kind of cell.

4. How do plants know they need to grow? Plant Hormones
A hormone is a substance that is produced in one part of an organism and affects another part of the same individual.

5. 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
Plant hormones are chemical substances that control patterns of development as well as plant responses to the environment. Hormones are produced in apical meristems, in young leaves, in roots, and in growing flowers and fruits. From their place of origin, hormones move to other parts of the plant, where target cells respond in a way that is specific to the hormone.

6. changing its metabolism affecting its growth rate
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

7. 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.

8. Auxins
Darwin’s Experiment
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.
The effect of light on growing plants was shown in experiments by Francis and Charles Darwin and other scientists. They demonstrated that chemical substances are produced in the growing tip of a plant. Auxins stimulate cell elongation. A higher concentration of auxins accumulate in the shaded part of a stem and cause the plant to bend toward a light source.

9. 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.
The effect of light on growing plants was shown in experiments by Francis and Charles Darwin and other scientists. They demonstrated that chemical substances are produced in the growing tip of a plant. Auxins stimulate cell elongation. A higher concentration of auxins accumulate in the shaded part of a stem and cause the plant to bend toward a light source.

10. Forty years later, these substances were identified and named 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.

11. 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 (get longer).
The effect of light on growing plants was shown in experiments by Francis and Charles Darwin and other scientists. They demonstrated that chemical substances are produced in the growing tip of a plant. Auxins stimulate cell elongation. A higher concentration of auxins accumulate in the shaded part of a stem and cause the plant to bend toward a light source.

12. 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.
The effect of light on growing plants was shown in experiments by Francis and Charles Darwin and other scientists. They demonstrated that chemical substances are produced in the growing tip of a plant. Auxins stimulate cell elongation. A higher concentration of auxins accumulate in the shaded part of a stem and cause the plant to bend toward a light source.

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Location of Auxins in Plant….auxins are found in
the embryo of seeds
meristems of apical buds
And young leaves
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What cells do auxin affect?:
Auxins will only affect cells that have auxin receptors
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15. How does auxins affect cells?:
Cells with auxin will elongate
In stems auxin concentrate on the shaded side.
Auxin causes transport of hydrogen ions from cytoplasm to cell wall
This decrease in pH / H+ pumping breaks bonds between cell wall fibres;
This makes cell walls flexible/extensible/plastic/softens cell walls and makes cells enlarge/grow;
This elongation of the stem will cause the plant to bend towards the light.
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