Plant Response to Stimuli Stimuli and a Stationary Life

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Plant Response to Stimuli
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Stimuli and a Stationary Life

• Plants, being rooted to the ground
• Must respond to whatever environmental change
  comes their way

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• For example, the bending of a grass seedling
  toward light
• Begins with the plant sensing the direction,
  quantity, and color of the light

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• A potato left growing in darkness
• Will produce shoots that do not appear healthy,
  and will lack elongated roots
• These are morphological adaptations for growing
  in darkness
• Collectively referred to as etiolation

(a) Before exposure to light. Adark-grown potato
has tall,spindly stems and nonexpandedleavesmor
phologicaladaptations that enable theshoots to
penetrate the soil. Theroots are short, but
there is littleneed for water absorptionbecause
little water is lost by theshoots.
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• After the potato is exposed to light
• The plant undergoes profound changes called
  de-etiolation, in which shoots and roots grow
  normally

(b) After a weeks exposure tonatural daylight.
The potatoplant begins to resemble a typical
plant with broad greenleaves, short sturdy
stems, andlong roots. This transformationbegins
with the reception oflight by a specific
pigment,phytochrome.
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The Discovery of Plant Hormones

• Any growth response
• That results in curvatures of whole plant organs
  toward or away from a stimulus is called a
  tropism
• Is often caused by hormones

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A Survey of Plant Hormones
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• In general, hormones control plant growth and
  development
• By affecting the division, elongation, and
  differentiation of cells
• Plant hormones are produced in very low
  concentrations
• But a minute amount can have a profound effect on
  the growth and development of a plant organ

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Light Response

• Responses to light are critical for plant success
• Light cues many key events in plant growth and
  development
• Photomorphogenesis
• Is the effects of light on plant morphology
  (structure)

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• Plants not only detect the presence of light
• But also its direction, intensity, and wavelength
  (color)
• A graph called an action spectrum
• Depicts the relative response of a process to
  different wavelengths of light

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• Action spectra
• Are useful in the study of any process that
  depends on light

Researchers exposed maize (Zea mays) coleoptiles
to violet, blue, green, yellow, orange, and red
light to test which wavelengths stimulate the
phototropic bending toward light.
EXPERIMENT
The graph below shows phototropic effectiveness
(curvature per photon) relative to effectiveness
of light with a wavelength of 436 nm. The photo
collages show coleoptiles before and after
90-minute exposure to side lighting of the
indicated colors. Pronounced curvature occurred
only with wavelengths below 500 nm and was
greatest with blue light.
RESULTS
CONCLUSION
The phototropic bending toward light is caused
by a photoreceptor that is sensitive to blue and
violet light, particularly blue light.
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• Research on action spectra and absorption spectra
  of pigments
• Led to the identification of two major classes of
  light receptors blue-light photoreceptors and
  phytochromes

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Blue-Light Photoreceptors

• Various blue-light photoreceptors
• Control hypocotyl elongation, stomatal opening,
  and phototropism

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Phytochromes as Photoreceptors

• Phytochromes
• Regulate many of a plants responses to light
  throughout its life

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The Effect of Light on the Biological Clock

• Phytochrome conversion marks sunrise and sunset
• Providing the biological clock with environmental
  cues

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Photoperiodism and Responses to Seasons

• Photoperiod, the relative lengths of night and
  day
• Is the environmental stimulus plants use most
  often to detect the time of year
• Photoperiodism
• Is a physiological response to photoperiod

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Photoperiodism and Control of Flowering

• Some developmental processes, including flowering
  in many species
• Requires a certain photoperiod

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Critical Night Length
During the 1940s, researchers conducted
experiments in which periods of darkness were
interrupted with brief exposure to light to test
how the light and dark portions of a photoperiod
affected flowering in short-day and long-day
plants.
EXPERIMENT
RESULTS
Darkness
Flash oflight
24 hours
Criticaldarkperiod
Light
(b) Long-day plantsflowered only if aperiod
of continuousdarkness was shorterthan a
critical darkperiod for thatparticular species
(13hours in this example).
(a) Short-day plantsflowered only if a period
ofcontinuous darkness waslonger than a critical
darkperiod for that particularspecies (13 hours
in thisexample).
CONCLUSION
The experiments indicated that flowering of each
species was determined by a critical period of
darkness (critical night length) for that
species, not by a specific period of light.
Therefore, short-day plants are more properly
called long-night plants, and long-day plants
are really short-night plants.
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Gravity

• Response to gravity
• Is known as gravitropism
• Roots show positive gravitropism
• Grow with gravity
• Stems show negative gravitropism
• Grow against gravity

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• Plants may detect gravity by the settling of
  statoliths
• Specialized plastids containing dense starch
  grains

GRAVITY
Statoliths
20 ?m
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Environmental Stresses

• Environmental stresses
• Have a potentially adverse effect on a plants
  survival, growth, and reproduction
• Can have a devastating impact on crop yields in
  agriculture

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Drought

• During drought
• Plants respond to water deficit by reducing
  transpiration
• Deeper roots continue to grow

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Flooding

• Enzymatic destruction of cells creates air tubes
  that help plants survive oxygen deprivation
  during flooding

Vascularcylinder
Air tubes
Epidermis
100 ?m
100 ?m
(b) Experimental root (nonaerated)
(a) Control root (aerated)
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Salt Stress

• Plants respond to salt stress by producing
  solutes tolerated at high concentrations
• Keeping the water potential of cells more
  negative than that of the soil solution