1. Plant Responses to Internal and External Signals

2. 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)

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

4. Ethylene Gas: Fruit Ripening
Canister of ethylene gas to ripen bananas in shipping container
Untreated tomatoes vs. Ethylene treatment

5. 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

6. Experiments with Light
Phototropism
Experiments with Light
Conclusion: Tip of stem senses light  some signal was sent from tip to elongating region of coleoptile

7. 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
Further Experiments: Cells on darker side elongate faster than cells on brighter side AUXIN = chemical messenger that stimulates cell elongation

8. Gravitropism

9. Positive gravitropism in roots: the statolith hypothesis.

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

11. Plant Responses to Light
Plants can detect direction, intensity, & wavelenth of light
Phytochromes: light receptors, absorbs mostly red light
Regulate seed germination, shade avoidance

12. 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

13. Night length is a critical factor!
Photoperiodism: physiological response to the relative length of night & day (i.e. flowering) *more predictable than air temperature changes*
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!

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

16. Plant responses to stress

17. Flooding (O2 deprivation):
Drought (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

18. Excess 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

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