1. Chapter 39- The Response in Plants to Internal and External Stimuli

2. Background
The goal of all plants in a response is to enhance survival and reproductive success
Signal Transduction Pathways- Reception of stimulus and then a response

3. Background Example- Potatoes
- Etiolation- Potato is left in the dark. All resources are being used by the potato to lengthen the stem, and there is no energy being used to produce leaves.
- De-etiolation- Plant exposed to light. Resources are shifted into the production of leaves.

5. Signal Transduction Pathways
How it works
1. Reception of Stimulus
- Signals detected by receptors, which are often proteins embedded in the membrane

7. Signal Transduction Pathways
- Receptors produce secondary messenger chemicals which amplify the signal inside of the plant

9. Signal Transduction Pathways
3. Response
- Signal is received somewhere inside of the plant, and usually increases an enzymatic activity (might be the transcription of new mRNA or the activation of proteins already present)

11. Plant Hormones - Chemical signals
- Produced in one part of the plant and then is spread throughout the entire plant
- Only needed in small quantities in order to work
- Interact with each other to either inhibit or to promote the hormones effect

12. Plant Hormones
- Discovered by looking at tropisms (a reaction to a stimulus)
- The Darwin family discovered phototropism- a plant will grow towards light, and it is controlled strictly by hormones in the plant

14. Effects of Light on Plants
- Photomorphogenesis- A plant changing because of light. Two important spectrums of light:
- Blue- Absorbed by blue light photoreceptors
- Red- Absorbed by phytochromes
- Circadian Rhythms- Changes based on a 24 hour day, but not influenced by external stimuli
- Biological Clock- Can be desynchronized (ie- “jet lag”) - Controlled by light
- Photoperiodism- Length of daylight and/or darkness affects the flowering of most plants

16. Effects of Gravity on Plants
- Gravitropism
- Positive gravitropism- growth with gravity (roots)
- Negative gravitropism- growth away from gravity (the shoots)
- Controlled by statoliths- specialized plastids inside of each cell that tells position of the cell

18. Effects of Mechanical Stimuli on Plants
- Thigmomorphisms- Reacting to being disturbed (wind, touch…)
- Thigmotropism- Attraction to mechanical supports (many vines)