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

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

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

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 Slide 5 of 13

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

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

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

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

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

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

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

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