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Phototropism and Plant Hormones

Phototropism and Plant Hormones. AP Biology Unit 5. 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. Slide 2 of 13.

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Phototropism and Plant Hormones

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  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 Slide 2 of 13

  3. Auxin • The plant hormone auxin (also known as indoleacetic acid) plays a large role in many tropisms Slide 3 of 13

  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 Slide 4 of 13

  5. More growth Less growth Phototropism • 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 Slide 5 of 13

  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 Slide 6 of 13

  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 Slide 7 of 13

  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 Slide 8 of 13

  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 Slide 9 of 13 Image taken without permission from http://bcs.whfreeman.com/thelifewire/default.asp

  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 Slide 10 of 13

  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 Slide 11 of 13

  12. Summary of Plant Hormones • Ethylene • Fruit Ripening, leaves falling off • Abscissic Acid • Stress hormone, inhibits growth, induced seed dormancy, close stomata Slide 12 of 13

  13. Slide 13 of 13

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