1. Phototropism and Plant Hormones
AP Biology
Unit 5

2. Tropisms
Tropism = how a plant responds to a particular stimulus (light, gravity, touch, etc.)
Phototropism = growth in response to light
Gravitropism = growth in response to gravity
Thigmotropism = response to touch
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3. Auxin
The plant hormone auxin (also known as indoleacetic acid) plays a large role in many tropisms
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4. Phototropism How a plant grows in response to light
When light is placed on one side of a plant, it will bend towards the light as it grows
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5. Phototropism Why is the plant bending?
More growth
Why is the plant bending?
At a cellular level, it means that cells on one side of the plant are growing faster than the other.
Cells on the shaded side are growing faster than the lit side
Less growth
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6. Phototropism Experiments
Several scientists performed experiments to study the cause of phototropism
Darwin & Darwin: showed the plant only exhibited phototropism when the tip was exposed  the area that senses light is in the tip region
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7. Phototropism Experiments
Boysen & Jensen
Separated the tip from the rest of the plant and put mica or gelatin under it
Mica is impermeable to substances, gelatin is not
The tip with gelatin still caused the plant to bend  signal (chemical) is being passed down the plant from the tip
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8. Phototropism Experiments
Went
Removed tips and placed them on agar blocks to transfer chemical to agar
When agar block is placed on one side of the cut tip, the plant grew curving away from the side the agar was on  hormone has diffused from tips into blocks that affected growth
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9. Auxin and Phototropism
Later studies determined that the chemical at work was auxin
When light is shone on one side, it causes auxin to move AWAY from the light to the shadier side
Since the concentration of auxin is higher on the shadier side, it grows more on that side–plant bends towards the light
Note: some recent experiments indicate other molecules might also contribute to phototropism
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10. Plant Hormones
Many other plant hormones have also since been identified
Move throughout the plant
Control plant growth and development by affecting
Cell division, elongation, or differentiation
Enzyme activity
Gene expression
Properties of membranes
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11. Summary of Plant Hormones
Auxin
Growth, apical dominance (one main shoot), phototropism & gravitropism
Gibberellins
Growth (shoot elongation)
Cytokinins
Growth (cell division), promotes lateral buds
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12. Summary of Plant Hormones
Ethylene
Fruit Ripening, leaves falling off
Abscissic Acid
Stress hormone, inhibits growth, induced seed dormancy, close stomata
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