Ch 38: Regulation of Plant Growth

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Ch 38 Regulation of Plant Growth
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Figure 38.4 Embryos Mobilize Their Reserves
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Plant hormones

• Hormone chemical signals that coordinate parts
  of an organism produced in one part of the body
  and then transported to other parts of the body
  low concentrations
• Tropism movement toward or away from a stimulus
• Went experiments (phototropism)
• Hormone auxin
• Others gravitropism, thigmotropism

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Auxin

• IAA (indoleacetic acid)
• Location seed embryo meristems of apical buds
  and young leaves
• Function stem elongation root growth,
  differentiation, branching fruit development
  apical dominance tropisms

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Auxin Affects Plant Growth and Form

• Phototropism is the tendency for plants to grow
  toward light sources.
• In the 1800s, Charles Darwin and his son Francis
  experimented with canary grass seedlings grown in
  the dark.
• They found that when the top millimeter of the
  coleoptile of a grass plant is covered, the plant
  cannot respond to the direction of light.
• The photoreceptors are in the coleoptile tip.
  However, the bending takes place in the growing
  region below the tip. A signal must pass from the
  tip to the growing region.

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Figure 38.7 The Darwins Phototropism Experiment
(Part 1)
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Figure 38.7 The Darwins Phototropism Experiment
(Part 2)
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Figure 38.8 Wents Experiment
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Auxin Affects Plant Growth and Form

• The movement of auxin is polarit travels in just
  one direction along a line from apex to base.
• This movement is not due to gravity. The apex to
  base direction is not reversed by inverting
  plants.
• Polar transport depends on auxin anion efflux
  carriers, membrane proteins found only at the
  basal ends of cells.
• At pH 7 in the cytoplasm, auxin exists as an
  anion. Auxin anions can leave the cell only by
  way of the protein carriers.

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Auxin Affects Plant Growth and Form

• The lateral redistribution of auxin is involved
  in both phototropism and gravitropism.
• Redistribution occurs when the carrier proteins
  move to one side of the cell and allow exit of
  auxin only on that side.
• When light strikes a coleoptile from one side,
  the auxin moves to the shaded side, growth on
  that side is increased, and the seedling bends
  towards the light.
• If a shoot is tipped over, auxin moved to the
  lower side and causes more rapid growth there.
  The seedling bends upward.

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Figure 38.10 Plants Respond to Light and Gravity
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Auxin Affects Plant Growth and Form

• Auxin affects plant growth in many ways
• Initiating root growth
• Inhibiting leaf abscission
• Maintaining apical dominance
• Promoting stem elongation and inhibiting root
  elongation
• Controlling fruit development

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Auxin Affects Plant Growth and Form

• Shoot cuttings can produce roots if
  undifferentiated cells in the shoot can be
  induced to differentiate into the meristem of a
  new root.
• Shoot cuttings of many plant species develop
  profuse roots when the cut surfaces are dipped
  into an auxin solution.
• This observation suggests a role for auxin in the
  initiation of lateral roots.
• Commercial rooting powders usually contain
  synthetic auxin.

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Auxin Affects Plant Growth and Form

• Apical dominance is the tendency for lateral buds
  to remain dormant. Apical buds inhibit the growth
  of lateral buds.
• Removing apical buds stimulates lateral bud
  growth.
• If auxin is applied to the cut surface in place
  of the apical bud, the lateral buds are inhibited.

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Figure 38.12 Auxin and Apical Dominance
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Auxin Affects Plant Growth and Form

• Auxin stimulates stem elongation but inhibits
  root elongation. Why different organs respond
  differently to the same hormone is a subject of
  current research.
• In many species, treatment of unfertilized
  ovaries with auxin or gibberellins causes fruit
  formation.
• This process is called parthenocarpy and is
  useful in the production of seedless fruits.

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Auxin Affects Plant Growth and Form

• Synthetic auxins have been produced and studied.
• One of them, called 2,4-D, is lethal to eudicots
  at concentrations that are harmless to monocots.
• This auxin has been used as a selective herbicide
  on lawnsgrasses are monocots, and most of the
  weeds in lawns are eudicots.
• 2,4-D takes a long time to break down, however,
  so it pollutes the environment.

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Cytokinins

• Zeatin
• Location roots (and actively growing tissues)
• Function root growth and differentiation cell
  division and growth germination delay
  senescence (aging) apical dominance (w/ auxin)

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Gibberellins

• GA3
• Location meristems of apical buds and roots,
  young leaves, embryo
• Function germination of seed and bud stem
  elongation leaf growth flowering (bolting)
  fruit development root growth and differentiation

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Figure 38.5 The Effect of Gibberellins on Dwarf
Plants
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Abscisic acid

• ABA
• Location leaves, stems, roots, green fruit
• Function inhibits growth closes stomata during
  stress counteracts breaking of dormancy

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Ethylene

• Gaseous hormone
• Location ripening fruit tissue stem nodes
  aging leaves and flowers
• Function fruit ripening oppositional to auxin
  (leaf abscission) promotes/inhibits
  growth/development of roots, leaves, and flowers
  senescence

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Daily and Seasonal Responses

• Circadian rhythm (24 hour periodicity)
• Photoperiodism (phytochromes)
• Short-day plant light period shorter than a
  critical length to flower (flower in late summer,
  fall, or winter poinsettias, chrysanthemums)
• Long-day plant light period longer than a
  critical length to flower (flower in late spring
  or early summer spinach, radish, lettuce, iris)
• Day-neutral plant unaffected by photoperiod
  (tomatoes, rice, dandelions)
• Critical night length controls flowering

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Phytochromes

• Plant pigment that measures length of darkness in
  a photoperiod (red light)
• Pr (red absorbing) 660nm
• Pfr (far-red absorbing) 730nm