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Plant Hormones and Their Effect
24.3
Hormones
• A hormone is a chemical signal that affects growth, activity or development in a living organism
• A plant hormones control development, but also coordinate responses to environment.
How do hormones work?
• Specific target cell that have a receptor for the hormone– Response depends on
the receptors
Auxin
• Auxin– Stimulate cell elongation– Promote growth of roots– Regulate division in
meristem
• Apical Dominance– Prevent growth of buds
closer to the stem’s tip
• Also play a role in phototropism– Bending towards a light
source
Cytokinins
• Produced in growing roots and developing seeds and fruits
• Stimulate cell division• Regeneration of
damaged tissue• Delay aging of leaves
• Usually have the opposite affect of Auxin– Auxin: made in shoots,
sent to roots– Cytokinins: made in
roots, sent to shoots– Helps to restore lost
organs
More Hormones
• Gibberellins– Stimulate growth
• Dramatic increase is size, specially in stems and fruits
• Abscisic Acid– Controls seed dormancy until a shift in hormones
brought on by environmental conditions– Inhibits the cellular division
• There opposite affect helps to control homeostasis
Ethylene
• In the form of a gas
• Stimulates fruits to ripen
• Helps to “seal off” and drop organs that are no longer useful for the plant
Tropisms
• Tropism – a response to the environment– Phototropism– Gravitropism– Thigmotoropism
Phototropism
• Tendency to grow towards light
• Auxin in responsible– Migrate towards shaded
area
• Seedlings can quickly change to grow in opposite directions
Gravitropism
• Shoots grow upward, roots grow downward
• Auxin is also responsible for this
• Still not understood why auxin migrates to the areas that cause this
Thigmotoropism
• Response to touch
• Plant will wrap around an object to help support its growth
• Vines and climbing plants– Tomato plants and wind
Rapid Response
• Some responses are extremely quick– Ex: Mimosa pudica• “sensitive plant” – fold together in seconds when
touched
– Ex: Venus flytrap• Close when an organism lands inside• Triggers sensory to close the leaves
Response to Seasons
• Plants flower according to photoperiod– Length of daylight– Photoperiodism
• Major factors in the timing of seasonal activities– Flowering and growth
• Phytochrome – absorb red light and different signaling pathways within the plant– Not completely undestood
Photoperiodism
Page: 713
Winter Dormancy
• Deciduous trees stop photosynthesis, transport important material to roots and seal leaves off from rest of plant– This is why leaves drop
• Phytochrome absorbs less due to day length less auxin produced ethylene production increases
Winter Dormancy
• Because of the change in hormone levels– Chlorophyll breaks down and pigments that were
there but were not displayed become more visible• Yellow and red pigments
• This is why leaves change color. What hormone helps them fall off?– Ethylene
Winter Dormancy
• More effects:– Meristems produce protecting scale over buds• Waxy coating
– Xylem and Phloem are occupied by many ions and organic compounds• Anti-freeze• Prevents sap from freezing
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