Plant Responses to Internal and External Signals
Experiments with Light and the coleoptile
Conclusion: Tip of coleoptile senses light  some signal
was sent from tip to elongating region of coleoptile
Excised tip placed
on agar block
Growth-promoting
chemical diffuses
into agar block
Cells on darker side
elongate faster than
cells on brighter side
AUXIN = chemical
messenger that
stimulates cell
elongation
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
– 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
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 responses to stress
1. Drought (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. Excess 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