Plant Responses to Internal

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Chapter 39

• Plant Responses to Internal External Signals

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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

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Phototropism video
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Gravitropism Video
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Thigmotropism Video
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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

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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

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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

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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.

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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

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Phytochromes

• Photoreceptors for red light (mainly)
• Exist in 2 isomer forms which can switch forms
  depending on wavelength available
• Phytochrome form triggers plants developmental
  responses to light
• Responsible for Circadian Rhythms
• 24 hour cycle not paced by environmental
  variables
• Chickens Summer in Nome, Alaska

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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, were
  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

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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

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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 roots
  (radicles) 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

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Auxin Accumulation
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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

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Coordinated Plant-Parasite Defense