Plant Responses to Internal and External Signals

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Plant Responses to Internal and External Signals

• Plant Hormones
• Plant Movements
• Control of Daily and Seasonal Responses
• Phytochromes

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Plant Hormones

• Hormone-compound produced by one part of an
  organism that is transported to other parts where
  it triggers a response in target cells
• Sends signals between different plant parts
• Tracks time of day and year
• Sensing and responding to gravity, direction of
  light, etc
• Adjusts growth patterns and development
• Phototropism-responsible for many of the
  discoveries about plant hormones

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Experiments

• Darwin experiment-removed coleoptiles result-no
  phototropism conclusion- tip responsible for
  sensing light
• Peter Boysen-Jensen experiment-separated tip w/
  block of gelatin result-normal behavior
  conclusion-signal was a mobile substance
• F.W.Went experiment-removed tip, placed it on
  agar block, placed block back on plant
  result-normal behaviorconclusion-chemical in
  block from tip was responsible. AUXIN

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Functions of Plant Hormones

• Coordinate growth and development by affecting
  division, elongation and differentiation of cells
• Regulate responses to environmental stimuli
• Difficult to identify-varying concentrations
  affect target cells differently and different
  target cells are affected differently

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Classes of Plant Hormones

• Auxins (IAA)
• Cytokinins
• Gibberellins (GA)
• Abscisic Acid (ABA)
• Ethylene

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Auxins

• Produced by apical meristem
• Stimulate cell growth
• Induces vascular cambium cell division and
  differentiation of secondary xylem
• Promotes formation of adventitious root
• Promotes fruit growth
• 2,4 D-selective herbicide-dicots

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Cytokinins

• Modified adenine
• Stimulates cytokinesis
• Controls cell division and differentiation (in
  conjunction with auxins) by stimulating RNA and
  protein synthesis
• Controls apical dominance
• Anti-aging hormone

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Gibberellins

• More than 80 gibberellins have been identified
• Primarily produced in roots and young leaves
• Stimulate growth of leave and stems, but not
  roots
• Work with auxins to stimulate cell elongation
• Control fruit development along with auxins
• Causes seeds to break dormancy, stimulated by
  imbibing water

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Abscisic Acid

• Produced by terminal bud-prepares plant for
  winter (suspends primary and secondary growth)
• Stress hormone-closes stomata as a result of
  excess transpiration

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Ethylene

• Growth inhibitor
• Produced by high auxin concentrations
• Gas
• Promotes sinescence (aging) examples xylem, leaf
  fall, withering of flowers, death of annuals
  after flowering
• Fruit ripening
• Leaf abscission

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Plant Movements

• Tropisms-growth responses ( or -), one time
  events, not repeatable
• Phototropism-light, differential distribution of
  auxins, blue light
• Gravitropism-gravity, roots positive, stems
  negative process involve statoliths (starch
  grains), calcium and auxin concentration changes
• Thigmotropism-touch, twining of a tendril

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Turgor Movements (Nastic Movements)

• Reversible movements caused by changes in turgor
  pressure
• Rapid leaf movements (Mimosa)
• Sleep movements
• K movements changes osmotic conditions and leads
  to turgor changes

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Control of Daily and Seasonal Responses

• Circadian rhythm-physiological cycle with a
  frequency of about 24 hours
• Photoperiodism-physiological response to day
  lengthflowering,
• Phytochromes play a critical role in seasonal
  cycles.
• Two photoreversible forms Pfr and Pr