Growth Stimulants, Retardants

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Unit 5

• Growth Stimulants, Retardants Growth Hormones

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Growth Regulating Substances

• Hormones (organic chemicals that act and interact
  to affect growth rate).

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Prominent Hormones

• Auxins accelerate growth by stimulating cell
  enlargement.
• Gibberellins stimulate growth in stems and
  leaves by cell elongation. Also stimulate
  premature flowering, growth of young fruit and
  breaking of dormancy.

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Prominent Hormones cont.

• Cytokinins Stimulate cell division, work with
  auxins.
• Inhibitors Inhibit seed germination, stem
  elongation hasten the ripening of fruit.

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Apical Dominance

• The terminal bud secretes chemicals that inhibit
  or prevent the growth of auxiliary buds on the
  same shoot.
• At flowering age the terminal bud becomes a
  flower bud the chemicals are no longer
  secreted. This may induce side branching.

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Apical Dominance
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Pinching Back

• Pinching out (removing) the terminal leaf bud of
  the shoot nullifies the affect on auxiliary buds
  and branching (thickening) occurs sooner.

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Pinching Back
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Stimulants

• Gibberellic acid causes the stems of plants to
  stretch out. The node are further apart.
• May be found in alfalfa tea.

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Chemical Retardants

• Causes short compact plants, making them more
  attractive and stronger.
• Commercial examples
• B-Nine
• Limit
• Sumagic

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Chemical Retardants
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Rooting Hormones

• Soft succulent houseplants typically only require
  moist conditions to root. They contain large
  amount of the chemical indoleacetic acid. This is
  a naturally occurring hormone that causes roots
  to form on stems.

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Commercial Rooting Hormones

• IBA (indolebutyric acid)
• NAA (naphthaleneacetic acid)
• Most are mixed with talc (powder) or dissolved in
  a liquid.
• Many contain a fungicide.

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Rooting Hormones
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Dwarfing Root Stocks

• Trees growing from certain types of roots did not
  grow as tall and had a dwarfing effect.
• Root stocks were developed to allow fruit tree
  growers to control the height of trees.

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Plant Biostimulants

• Natural products
• Stimulate soil microbial activity
• Improve water nutrient uptake
• High levels are found in kelp seaweed
• Can reduce the need for nitrogen fertilizers
• Methanol is being tested

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Plant Biostimulants
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Allelopathy

• When the presence of a chemical compound by one
  plant slows down or stops the growth of another.
• May be the same plant.
• Millet is used as a cover crop.