Transcript Warm-Up

Ch. 39 Warm-Up
1.
Elaborate on the methods plants use to defend themselves
from pathogens and herbivores.
2.
How do plants cope with:
a. Flooding
b. Drought
c. Heat stress
d. Salt stress
3.
If a long day plant needs at least 9 hours of dark, which
scenario(s) will prevent flowering?
a. 16 hours light, 8 dark
b. 14 hours light, 10 dark
c. 14 hours light, 10 dark w/ flash of light
Ch. 39 Warm-Up
Match the following terms with the descriptions below:
A. Auxin
B. Cytokinins
C. Gibberellins
D. Abscisic Acid
E. Ethylene
1.
2.
3.
4.
5.
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
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.

Chapter 39
Plant Responses to Internal and External Signals
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
Excised tip placed
on agar block
Cells on darker side
elongate faster than
cells on brighter side
AUXIN = chemical
messenger that
stimulates cell
elongation
Growth-promoting
chemical diffuses
into agar block
Control
(agar block
lacking
chemical)
has no
effect
Control
Agar block
with chemical
stimulates growth
Offset blocks
cause curvature
Hormones: chemical messengers that coordinate
different parts of a multicellular organism
Important plant hormones:
1. Auxin – stimulate cell elongation  phototropism &
gravitropism (high concentrations = herbicide)
2. Cytokinins – cell division (cytokinesis) & differentiation
3. Gibberellins – stem elongation, leaf growth, germination,
flowering, fruit development
4. Abscisic Acid – slows growth; closes stomata during H2O
stress; promote dormancy
5. Ethylene – promote fruit ripening (positive feedback!);
involved in apoptosis (shed leaves, death of annuals)
The effects of gibberellin on stem elongation
and fruit growth
Ethylene gas: fruit ripening
Canister of ethylene gas to ripen
bananas in shipping container
Untreated tomatoes vs. Ethylene
treatment
Plant Movement
1.
Tropisms: growth responses  SLOW
 Phototropism – light (auxin)
 Gravitropism – gravity (auxin)
 Thigmotropism – touch
2.
Turgor movement: allow plant to make
relatively rapid & reversible responses
 Venus fly trap, mimosa leaves, “sleep”
movement
Positive gravitropism in roots: the statolith
hypothesis.
Thigmotropism: rapid turgor movements by
Mimosa plant  action potentials
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
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
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.
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!
How does
interrupting the
dark period with
a brief exposure
to light affect
flowering?
Plant Response to Stress
Causes of stress:
1.
2.
3.
4.
5.
6.
7.
Drought (H2O deficit)
Flooding (O2 deprivation)
Salt excess
Heat
Cold
Herbivores
Pathogens
1.
H2O deficit:
 close stoma
 release abscisic acid to keep stoma closed
 Inhibit growth
 roll leaves  reduce SA & transpiration
 deeper roots
2.
Flooding (O2 deprivation):
 release ethylene  root cell death  air tubes
formed to provide O2 to submerged roots
3.
Salt:
 cell membrane – impede salt uptake
 produce solutes to ↓ψ - retain H2O
4.
Heat:
 evap. cooling via transpiration
 heat shock proteins – prevent denaturation
5.
Cold:
 alter lipid composition of membrane (↑unsat.
fatty acids, ↑fluidity)
 increase cytoplasmic solutes
 antifreeze proteins
6.
Herbivores:
 physical (thorns)
 chemicals (garlic, mint)
 recruit predatory animals (parasitoid wasps)
7.
Pathogens:
 1st line of defense = epidermis
 2nd line = pathogen recognition, host-specific