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

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Auxin

• The plant hormone auxin (also known as indoleacetic acid) plays a large role in many tropisms

N

CH2COOH

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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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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

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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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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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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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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

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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

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