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Horticulture Unit 5. GROWTH STIMULANTS, RETARDANTS AND ROOTING HORMONES. Original Power Point Created by Rebecca Johnson Modified by Georgia Agricultural Education Curriculum Office June 2002. Growth Regulating Substances.

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horticulture unit 5

HorticultureUnit 5


Original Power Point Created by Rebecca Johnson

Modified by Georgia Agricultural Education Curriculum Office

June 2002

growth regulating substances
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.
growth hormones
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.
apical dominance
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.
major discoveries
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
chemical stimulants
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
natural stimulants
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
chemical retardants
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)
rooting hormones
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.
rooting hormones1
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.
dwarfing rootstock
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.
student activities
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.