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Phototropism and Plant HormonesPhototropism and Plant Hormones
AP BiologyUnit 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
N
CH2COOH
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?– 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
More 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
Image taken without permission from http://bcs.whfreeman.com/thelifewire/default.aspSlide 9 of 13
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 budsSlide 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
Slide 13 of 13