1. Chapter 39: Plant Responses to Internal and External Signals
39.2
Admit Slip
3. List 3 words you think of when you look at the picture/diagram
2. Write 2 ideas you have based on the picture and your words. If possible, use your words as you write your ideas.
1. Write 1 question you have.

2. Hormones
Plant hormones help coordinate growth, development, and responses to stimuli
Hormones: chemical messengers that coordinate the different parts of a multicellular organism (produced in one location and transported to another in the body)

3. Tropism
A plant growth response from hormones that results in the plant growing either toward or away from a stimulus
Phototropism: growth of a shoot in a certain direction in response to light
Positive phototropism: growth toward light
Negative phototropism: growth away from light

5. Greening response to light in potatoes

6. Types of hormones
Auxin
Cytokinins
Gibberellins
Abscisic acid
ethylene

7. AUXINS Natural auxin in plants is idoleacetic acid (IAA)
Plays a key role in phototropisms and gravitropisms
Stimulate elongation of cells in young shoots
Produced in apical meristems and activate proton pumps in plasma membrane (lowers pH)
Cell wall is weakened, turgor pressure expands cell wall=cell elongation
Apical dominace: apical bud (left) apical bud removed (right)

8. How Auxins Cause Cell Elongation
Increase in H+ activates expansins
Weakened cell wall allows more water to enter
H+ pump

9. Synthetic Auxins Used as herbicides
Monocots (grasses) can quickly inactivate synthetic auxins but dicots cannot
High concentrations therefore kill broadleaf (dicot) weeds while grasses (like corn and turf grass) are not harmed

10. Other Hormones
Cytokinins: stimulate cytokinesis or cell division (aide in growth). Work with auxins to stimulate cell division and differentiation
Gibberellins: work with auxins to stimulate cell elongation by loosening cell walls, allowing for expansion of cells and stems. Effects: fruit growth, stem elongation, seed germination

11. Other Hormones
Abscisic Acid: slows growth , promotes seed dormancy, drought tolerance (water stress=close stomata to save water)
Ethylene: gas, role in programmed cell death (apoptosis) such as shedding of leaves/death of annual after flowering. Promotes ripening of fruits which triggers more ethylene (positive feedback loop)-1 rotten apple can spoil the bunch…

13. Went Experiments

14. Chapter 39: Plant Responses to Internal and External Signals
39.3

15. Plants and Light
Plants detect presence, direction, intensity, and wavelength of light
Red and blue light are the most important to plants
Blue-light receptors initiate plant response such as phototropisms and opening of stomata
Red-light receptors=Phytochromes

17. Phytochrome photoreceptors
Molecular switch reaction to red light
conversion of Pr  Pfr in sunlight stimulates germination, flowering, branching…
conversion of Pfr  Pr in dark inhibits response, & stimulates other responses: growth in height
Chromophore
Photorecptor
Light induced
Kinase activity
Phytochrome
Phytochrome
Response:
Vertical growth

18. Practical Application
Why do you plant lettuce seed by scattering them on the ground instead of burying seed?
What is the evolutionary advantage to lettuce seeds?

19. Phytochromes and Circadian Rhythyms
Circadian Rhythms: physiological clocks that usually run on a 24 hour cycle (not paced on environmental cues)
Surge of Pfr at dawn resets clock
Combination of phytochrome system and biological clock allow plant to asses the amount of daylight/darkness and time of year

20. Photoperiodism Photoperiod: relative length of night and day
A physiological response (such as flowering) to a photoperiod=photoperiodism
Controls when plants flower
Length of night is more critical than length of day
Old terminology remains!! We refer to plants by day-length!

21. Day-length Plants
Short-day plants: require a period of continuous darkness longer than a critical period to flower.
Flower in early spring or fall
Actually ‘Long-night” plants
Long-day plants: require a period of continuous darkness sorter than critical period to flower
Flower in late spring/early summer
Actually “short-night” plants
Day-neutral plants: flower in days of any length

22. Figure 39.22 Photoperiodic control of flowering

23. Figure 39.23 Reversible effects of red and far-red light on photoperiodic response