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Plant Responses to Internal & External Signals
Chapter 39
Tropism
A plant growth response from hormones that results in the plant growing either toward or away from a stimulus
Hormones – chemical messengers that coordinate the different parts of a multicellular organism
Types of tropisms: Phototropism – growth toward or away from light Gravitropism – growth toward or away from a
gravitational source Thigmotropism – growth toward or away from a touch
Vines grow toward a support and then grow toward (around) that support
Phototropism video
Gravitropism Video
Thigmotropism Video
Positive v. Negative Tropisms Positive is growth toward a stimulus
Positive Phototropism – growth toward light Positive Gravitropism – growth toward a
gravitational source Positive Thigmotropism – growth toward a
tactile stimulus
Negative is growth away from a stimulus
Replace the above terms’ “Positive” with “Negative” and then replace “toward” with “away”
Hormones
Auxins Stimulate cell elongation (remember that
zone?) Cause proton pumps to activate Lower pH
weakening of cell wall turgor pressure expands & elongates cell wall
High concentrations of synthetic auxins KILL certain plants, usually weeds
Synthetically produced auxins in high concentration = Herbicides
How do you make a …?
Cytokinins Stimulate cell division or cytokinesis Proper ratio of auxins & cytokinins cell
division & differentiation Gibberellins
Work with auxins to stimulate stem elongation Loosen cell walls allowing cellular expansion
stem expansion Signal the seed to cease dormancy and
germinate Many dwarf plant varieties have non-functional
gibberellins
Hormones (Page 3)
Abscisic acid SLOWS Growth Antagonistic to previously mentioned hormones
Promotes seed dormancy, but gibberellins cease it
Causes stomata to close to conserve water Ethylene
Gas Plays crucial role in programmed cell death
(apoptosis) Promotes ripening of fruit Ripening in one fruit ripening in other fruits
Positive feedback mechanism = rapid ripening of fruit
One bad apple does spoil the lot.
Plants respond to Light
Plants can detect presence, direction, intensity, and wavelength of light
Red & Blue wavelengths are most important
Red light is sensed by phytoreceptors Blue light is most important for
phototropisms and light-induced opening of stomata
Sensed by photoreceptors
Phytochromes
Photoreceptors for red light (mainly) Exist in 2 isomer forms which can switch
forms depending on wavelength available
Phytochrome form triggers plant’s developmental responses to light
Responsible for Circadian Rhythms 24 hour cycle & not paced by environmental
variables Chickens & Summer in Nome, Alaska
Photoperiodism
Physiological response to a photoperiod (relative lengths of day and night)
Night is always the more important of the 2!! But we (humans) focus on the daylight, we’re
dumb. Controls Flowering Short-day plants
Require a period of continuous darkness exceeding a critical measure in order to flower
Early Spring or Fall flowering AP Biology students call them “Long-Night”
plants
Besides Short-Day plants,
Long-Day Plants Flower only if a period of continuous darkness
is less than a critical value Flower in Late Spring or early Summer Considered “short-night” plants
Day-Neutral Plants No length of continual darkness is needed for
flowering
Plants respond to things other than light
Gravitropism – growth toward or away from light
Auxin plays key role If root is plased horizontally, then gravity
causes an accumulation of auxins in root’s (radicle’s) lower side
Remember HIGH [auxin] = inhibition of growth, so
lower side = no growth Upper side = growth Allows the root to grow down into the ground
Auxin Accumulation
Plants respond to other things… Drought
Stomata close Leaves will cease growth Leaves roll into shape that reduces
transpiration Deep roots (where H2O is) will speed their
growth, but shallow roots will stop growing Predators
Thorns, chemicals, distasteful compounds Some plants even attract parasites First layer of defense - Epidermis
Coordinated Plant-Parasite Defense