1. Plant Hormones and Their Effect
24.3

2. 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.

3. How do hormones work?
Specific target cell that have a receptor for the hormone
Response depends on the receptors

4. Auxin Auxin Apical Dominance Also play a role in phototropism
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

5. 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

6. More Hormones Gibberellins Abscisic Acid
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

7. 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

8. Tropisms Tropism – a response to the environment Phototropism
Gravitropism
Thigmotoropism

9. Phototropism Tendency to grow towards light Auxin in responsible
Migrate towards shaded area
Seedlings can quickly change to grow in opposite directions

11. 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

12. Thigmotoropism Response to touch
Plant will wrap around an object to help support its growth
Vines and climbing plants
Tomato plants and wind

13. 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

14. 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

15. Photoperiodism
Page: 713

16. 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

17. 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

18. 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