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Chapter 39. Plant Responses to Internal & External Signals. 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:

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chapter 39
Chapter 39

Plant Responses to Internal & External Signals

tropism
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
positive v negative tropisms
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
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
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
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 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
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
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
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
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
plants respond to other things
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