1. Ch. 39 Warm-Up
Elaborate on the methods plants use to defend themselves from pathogens and herbivores.
How do plants cope with:
Flooding
Drought
Heat stress
Salt stress
If a long day plant needs at least 9 hours of dark, which scenario(s) will prevent flowering?
16 hours light, 8 dark
14 hours light, 10 dark
14 hours light, 10 dark w/ flash of light

2. What you must know: The three steps to a signal transduction pathway.
The role of auxins in plants.
How phototropism and photoperiodism use changes in the environment to modify plant growth and behavior.
How plants respond to attacks by herbivores and pathogens.

3. Plant Responses to Internal and External Signals
Chapter 39
Plant Responses to Internal and External Signals

4. Experiments with Light and the coleoptile
Conclusion: Tip of coleoptile responsible for sensing light, but growth occurred below tip  some signal was sent from tip to elongating region of coleoptile

5. Excised tip placed
on agar block
Growth-promoting
chemical diffuses
into agar block
Agar block
with chemical
stimulates growth
Offset blocks
cause curvature
Control
(agar block
lacking
chemical)
has no
effect
Cells on darker side elongate faster than cells on brighter side AUXIN = chemical messenger that stimulates cell elongation

6. Hormones: chemical messengers that coordinate different parts of a multicellular organism
Important plant hormones:
Auxin – stimulate cell elongation  phototropism & gravitropism (high concentrations = herbicide)
Cytokinins – cell division (cytokinesis) & differentiation
Gibberellins – stem elongation, leaf growth, germination, flowering, fruit development
Abscisic Acid – slows growth; closes stomata during H2O stress; promote dormancy
Ethylene – promote fruit ripening (positive feedback!); involved in apoptosis (shed leaves, death of annuals)

7. The effects of gibberellin on stem elongation and fruit growth

8. Ethylene gas: fruit ripening
Canister of ethylene gas to ripen bananas in shipping container
Untreated tomatoes vs. Ethylene treatment

9. Plant Movement Tropisms: growth responses  SLOW
Phototropism – light (auxin)
Gravitropism – gravity (auxin)
Thigmotropism – touch
Turgor movement: allow plant to make relatively rapid & reversible responses
Venus fly trap, mimosa leaves, “sleep” movement

10. Positive gravitropism in roots: the statolith hypothesis.

11. Thigmotropism: rapid turgor movements by Mimosa plant  action potentials

12. Plant Responses to Light
Plants can detect direction, intensity, & wavelenth of light
Phytochromes: light receptors, absorbs mostly red light
Two forms: Pr (red light) and Pfr (far-red light)
Pr  Pfr: switches depending on light in greatest supply
Pfr aids in detection of sunlight
Regulate seed germination,
shade avoidance

14. Biological Clocks Circadian rhythm: biological clocks
Persist w/o environmental cues
Frequency = 24 hours
Phytochrome system + Biological clock = plant can determine time of year based on amount of light/darkness

15. Sleep movements of a bean plant
Sleep movements of a bean plant. Caused by reversible changes in turgor pressure of cells on opposing sides of the pulvini, motor organs of the leaf.

16. Night length is a critical factor!
Photoperiodism: physiological response to the relative length of night & day (i.e. flowering)
Short-day plants: flower when nights are long (mums, poinsettia)
Long-day plant: flower when nights are short (spinach, iris, veggies)
Day-neutral plant: unaffected by photoperiod (tomatoes, rice, dandelions)
Night length is a critical factor!

17. How does interrupting the dark period with a brief exposure to light affect flowering?

19. Plant Response to Stress
Causes of stress:
Drought (H2O deficit)
Flooding (O2 deprivation)
Salt excess
Heat
Cold
Herbivores
Pathogens

20. Flooding (O2 deprivation):
H2O deficit:
close stoma
release abscisic acid to keep stoma closed
Inhibit growth
roll leaves  reduce SA & transpiration
deeper roots
Flooding (O2 deprivation):
release ethylene  root cell death  air tubes formed to provide O2 to submerged roots

21. Salt: Heat: Cold: cell membrane – impede salt uptake
produce solutes to ↓ψ - retain H2O
Heat:
evap. cooling via transpiration
heat shock proteins – prevent denaturation
Cold:
alter lipid composition of membrane (↑unsat. fatty acids, ↑fluidity)
increase cytoplasmic solutes
antifreeze proteins

22. Herbivores: Pathogens: physical (thorns) chemicals (garlic, mint)
recruit predatory animals (parasitoid wasps)
Pathogens:
1st line of defense = epidermis
2nd line = pathogen recognition, host-specific

23. Match the following terms with the descriptions below:
Auxin
Cytokinins
Gibberellins
Abscisic Acid
Ethylene
Inhibits growth, closes stomata during H2O stress
Fruit ripening, promotes or inhibits growth
Affects root growth, stimulates cell division & growth, stimulates germination
Stimulates stem elongation, root growth, fruit development, photo- & gravitropism
Promotes seed & bud germination, stem elongation, flowering & development of fruit