Download Plant Signals

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; fruit development; apical
dominance; phototropism & gravitropism
2. Cytokinins – stim. cell division & growth; germination;
delay senescence (aging)
3. Gibberellins – promotes seed & bud germ.; stem elong. &
leaf growth; stim. flowering & fruit development
4. Abscisic Acid – inhibit growth; closes stomata during H2O
stress; encourage dormancy
5. Ethylene – promote fruit ripening; inhibit/promote growth
& dev. depending on species
The effects of gibberellin on stem elongation
and fruit growth
Ethylene gas: fruit ripening (videos)
Canister of ethylene gas to ripen
bananas in shipping container
Untreated tomatoes vs. Ethylene
treatment
Plant Movement
1.
Tropisms: growth responses  SLOW
 Phototropism – light
 Gravitropism – gravity
 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
It is time for
me to leaf you
I have taught
you well young
grasshopper
And
remember…
STUDY … if
not for you…
then for..
Captain
Pengiun
♥