Horticulture Unit 5

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HorticultureUnit 5

• GROWTH STIMULANTS, RETARDANTS AND ROOTING HORMONES

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Growth Regulating Substances

• Called hormones organic chemicals which act and
  interact to affect growth rate
• Auxins accelerate growth by stimulating cell
  enlargement
• Gibberellins stimulate growth in stem and leaf
  by cell elongation
• Cytokinins stimulate cell division (only works
  with auxins present)
• Inhibitors inhibit seed germination, stem
  elongation and hasten ripening of fruit.

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Growth Hormones

• Organic chemicals principally produced by
  actively growing plant tissue such as short tips
  and young leaves.
• They move throughout the plant and can be found
  in many parts.
• They react with one another.

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Apical Dominance

• The terminal bud secrets chemicals which inhibit
  or prevent growth of lateral buds.
• This causes the plant to grow tall and not send
  out side branches.
• Seems to be a genetic program directing the plant
  to grow above competing plants.
• Once the plant reaches flowering age, the
  terminal bud becomes a flower bud and the
  chemicals are no longer secreted.
• Pinching the terminal bud has the same effect and
  side branching occurs sooner.

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Major Discoveries

• Chemical and natural stimulants that cause plants
  to grow taller or faster
• Chemical retardants that cause plants to grow
  slower
• Hormones that cause plants to root faster
• Dwarfing rootstock for fruit trees

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Chemical Stimulants

• Enables plants to grow taller.
• Most common is gibberellic acid
• Gibberellic acid causes the stems of plants to
  stretch out
• The nodes (the joints at which bulbs, leaves and
  branches) are further apart

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Natural Stimulants

• Chemical known as alcohol Triacontanol
• Found in alfalfa and has been known to nursery
  men for 30 years.
• Stimulation is brought about by mulching with
  alfalfa hay or by watering plants with an alfalfa
  tea

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Chemical Retardants

• Used to retard the growth of plants making them
  shorter and more compact and therefore more
  attractive.
• One of the newest applications for growth
  retardants is on lawns, a chemical called Limit
  is used new and it restricts grass growth for 6
  to 8 weeks.
• A new plant regulator called Sumagic reduces
  the height of plants by inhibiting production of
  the hormone Gibberellic acid. (which cause stems
  to elongate)

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Rooting Hormones

• When propagating plants from cuttings, it is very
  important that they root as quickly as possible
• Indoleacetic acid (IAA) is a natural plant
  hormone that causes roots to form
• Chemical hormones, indolebutyric acid (IBA) or
  naphaleneacetic acid (NAA) are the most common
  used today.

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Rooting Hormones

• Rooting hormones are either mixed with talc and
  used as powders or mixed with liquid and used as
  a wet dip
• All rooting hormones should contain a fungicide.

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Dwarfing Rootstock

• As the cost of labor has increased, growers are
  looking for smaller trees.
• Discovered trees from certain types of roots, do
  not grow as large and bear fruit earlier.
• A complete series of rootstock known as malling
  rootstock was developed.

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Commercially Used Plant Growth Regulators
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Student Activities

• Complete self-evaluation at the end of this unit.
• Using nursery catalogs and books, make a list of
  available dwarf fruit trees and details.