Growth Regulators Original by Libby Astrachan Modified by Georgia Agriculture Education Curriculum O

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Growth RegulatorsOriginal by Libby
AstrachanModified by Georgia Agriculture
Education Curriculum Office- July 2004Modified
by Georgia Organics 2008
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Plant Hormones

• Natural (made by plants) also called hormones
• Synthetic (man-made)
• Also called PGRs (plant growth regulators)
• Purposes start growth, stop growth, modify
  growth development

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5 Known Plant Hormones

• Auxins (ox ins)
• Gibberellins (jib ber ill ins)
• Cytokinins (site oh kine ins)
• Ethylene (eth el een)
• Abscisic acid (ab sis ick)
• Hormones may act individually or together

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Auxins

• Stem elongation
• Produced in tips of stems (B in photo)
• Migrate from cell to cell in stems

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Phototropism ability to bend towards light

• Auxins - responsible for plants bending towards
  light.
• Auxins - move down shaded side of the stem and
  cause cells to elongate

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Gravitropism (geotropism) plant response to
gravity

• Auxins responsible for plant response to
  gravity
• Auxins move to lowest side and cause stem
  tissue to elongate stem curves upwards

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Apical dominance

• Auxins move down the stem from the terminal bud
  and inhibit growth of side shoots

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Pinching

• Pinching - removing the terminal bud
• Pinching - stops flow of auxins down the stem and
  allows side shoots to develop
• Produces bushy, well-branched crops

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Root development

• Auxins encourage root development in cuttings
• Some plants produce plenty of auxins to make
  rooting cuttings easy
• Other plants need synthetic auxins such as IBA

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Gibberellins

• Cell elongation and cell division
• Stimulate development of flowers (as in gibbing
  camelias)
• Cause internodes to stretch
• Produced in stem and root apical meristems, seed
  embryos, young leaves

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Internode Elongation

• Gibberellins cause internodes to stretch in
  relation to light intensity.
• High light intensity no stretch
• Low light intensity long internodes. Leaves
  are raised to capture light

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Problems with Internode Elongation

• Greenhouse problem plants spaced too closely to
  one another
• Plants shade one another results in stretching,
  less compact plants, weaker stems, loss in value
  

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Cytokinins

• Cell division (used in tissue culture)
• Cell differentiation (used in tissue culture for
  plant organ formation)
• Formation of callus tissue
• Delay aging process in plants
• Produced in roots
• Transported through xylem
• Still being researched

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Cytokinins vs. Auxins

• In stems auxins inhibit lateral shoots,
  cytokinins promote lateral shoots
• In roots auxins promote root branching,
  cytokinins inhibit root branching
• Work together to control cell differentiation and
  cell division

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Ethylene Gas

• Colorless gas
• Produced in nodes of stems, ripening fruits,
  dying leaves

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Ethylene exposure

• Thickens stems
• Breaks down chlorophyll
• Weakens cell membranes
• Softens cell walls

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The Holly and the Ethylene
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Abscisic Acid The Plant Stress Hormone

• Growth inhibiting hormone
• Responsible for seed dormancy
• Responsible for closing stomata during drought

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Synthetic Growth Regulators

• Rooting Compounds
• increase rooting
• speed rooting
• increase number, quantity of
  roots
• increase uniformity of roots

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Rooting compounds

• Liquid or mixed with talc