1. Plant Responses to Internal & External Signals
Chapter 39

2. Plant Responses to Stimuli
Plants have receptors that inform the plant of specific stimuli
Just as we have learned, there are 3 steps to the response to these stimuli:
Reception – First to identify stimulus
Transduction
Second Messengers – chemical messengers that amplify the signal from the receptors to induce specific responses
Response

3. Hormone Action in Plants
Hormone-producing
cells
Plants produce hormones
Chemical messengers that travel throughout the plant causing target cells to respond
In plants, hormones control:
Plant growth and development
Plant responses to environment
Movement
of hormone
Target
cells
Cells in one blooming flower signals other blooms using hormones to open.

4. Plant Hormones Auxins Produced in embryo, apical meristems
Stimulate root growth, stem elongation

5. Plant Hormones Cytokinins Made in root, transported to other parts
Stimulates cell division, plant growth

6. Plant Hormones Gibberelins Made in meristematic tissues
Promotes bud/seed germination, stem elongation

7. Plant Hormones Brassinosteroids
Produced in seeds, fruit, shoots, leaves, floral buds
Inhibits root growth, prevent loss of leaves, promote xylem differentiation

8. Plant Hormones Abscisic acid
Produced in leaves, stems, roots, green fruit
Slows growth, promotes seed dormancy, closes stomata during water stress

9. Plant Hormones Ethylene
Gas produced in tissues of ripening fruit, nodes of stems, aging leaves/flowers
Hastens fruit ripening
“one bad apple spoils the bunch”

10. Control of Flowering Key Concepts
Phytochromes: pigments that regulate plant response to light
Critical night length: the minimum amount of night time required for regulation of plant growth

11. Short-day plants dark Critical night length
If light exceeds or interrupts critical night length, no flowering happens.
light

12. Long-day plants dark Critical night length
Night can exceed critical night length or be interrupted, flowering can still happen.
light

13. Tropism
The way a plant grows in response to stimuli in the environment
Types of tropisms:
Phototropism: growth response to light
Plants bend towards light
Geotropism: growth response to gravity
Plant roots grow down with gravity, shoots (stems) grow up against gravity and out of soil
Thigmotropism: growth response to touch
Vines grow up around trees, Venus flytrap closes when leaves are touched

14. Plant Defenses
Produce chemicals in response to predation (secondary compounds)
Antioxidants to protect against UV rays