1. Growth Stimulants, Retardants, and Rooting Hormones

2. Growth regulators Hormones
Organic chemicals that act and interact to affect growth rate
Auxins – accelerate growth by stimulating cell enlargement

3. Gibberellins Stimulate growth in stem and leaf by cell elongation
Stimulates premature flowering, growth of young fruits and breaking of dormancy

4. Cytokinins Stimulate cell division Work along with auxins
Will not work without auxins present

5. Inhibitors Abscisic Acid and Ethylene Gas inhibits seed germination
Inhibits stem elongation
Hasten ripening of fruit – ethylene gas

6. Auxins and Gibberellins promote cell enlargement
Cytokinins stimulate cell division

7. Growth Hormones
Organic chemicals produced by actively growing plant tissue
Shoot tips and young leaves
Move throughout the plant and can be found in most tissues

8. These chemicals react with one another in a very complex system in the plant
In some cases a concentration of one hormone stimulates growth and a different concentration restricts growth

9. Apical Dominance Dominance of the terminal bud
Apical dominance exists there
Terminal bud secretes chemicals that inhibit or prevent the growth of axillary buds on the same shoot

10. Apical Dominance
Axillary buds are found in the axil – angle between a leaf and the stem.
Causes the plant to grow tall and not branch
Once the plant reaches flowering age and the terminal bud becomes a flower, the chemicals are no longer secreted

11. Apical Dominance The plant then starts to send out side branches
This allows the plant to grow above competing plants
Once height and access to sunlight are secured, the plant spreads out over its competitors

12. Apical Dominance
Pinching off terminal buds removes the hormone and causes branching to happen faster
Common practice in greenhouses and nurseries to produce bushier plants with many flower buds

13. Stimulants Allow plants to grow taller
Most common is Gibberellic Acid (GA)
Causes stems of plants to stretch out
Nodes are farther apart

14. Growth Stimulants Natural growth stimulants have been found in alfalfa
It causes stimulation of growth
It is the alcohol called Triacontanol
Stimulation is brought about by mulching plants with alfalfa hay

15. Or watering plants with a “tea” made from soaking alfalfa feed pellets in water
Amount needed is very small
Also sold commercially

16. Chemical Retardants
Chemicals are used to retard growth of plants causing them to be shorter and more compact
Plants are more attractive and the plants themselves are stronger
Used commercially
B-Nine is commonly used on Azaleas

17. Newest uses of Plant Growth Retardants (PGR) is the application to lawns
After first mowing in the spring “Limit” is applied and absorbed by the roots
Restricts growth for 6-8 weeks

18. “Embark” is absorbed by the leaves and also restricts growth
PGR’s absorbed by the leaves move to the growing point where it interrupts cell division, stem elongation and seed head formation
Roots continue to grow
Applied after first or second mowing

19. Sumagic New growth retardant
Reduces height of plants by inhibiting production of GA
Chemical is taken up by leaves and moves through the plant to the terminal bud

20. Used to control growth of shrubbery and hedges
PGR Atrimmic applied after pruning can last an entire season
Reduces or eliminates the need to prune

21. Atrimmic
A systemic
Penetrates the plant, enters the plant sap, and moves through the plant
Blocks plant hormones that stimulate growth
Applied as a foliar spray

22. Rooting Hormones Important when propagating plants by cutting
Helps cuttings to develop more roots faster than without a rooting hormone
Indoleacetic Acid (IAA) naturally occurring, causes roots to form on plant stems

23. IBA – Indolebutyric Acid
Most widely used rooting hormone
Most effective rooting hormone

24. Rooting Hormones
Either mixed with talc and used as a powder or dissolved in water and used as a wet dip
Liquid can be slightly more effective
All rooting hormones should contain a fungicide

25. Fungicide
Helps prevent cuttings from rotting

26. Dwarfing Rootstock Used in fruit trees
Shorter trees enable you to pick fruit without using a ladder
First research done to prevent apple trees from growing very tall
Trees growing from certain types of roots didn’t grow as tall

27. These trees also bore fruit at an earlier age
Complete series of rootstock known as malling rootstock developed in England
Controls size and rate of growth or apple trees

28. Stock has also been developed for peach and pear trees as well
Dwarf trees can be purchased commercially

29. Chemical Blossom Set Used on tomato blossoms early in the season
Causes earlier development of fruit
Causes seedless tomatoes to set on first blossoms and results in tomatoes ready to eat as much as 10 days earlier than normal

30. Plant Biostimulants Natural products Organic
Work to stimulate soil microbial activity and improve soil cation exchange capacity, stimulate plant growth and promote disease resistance

31. Humic Acid
Product of rotting organic matte is an example of a soil microbial stimulant

32. Root growth biostimulants
Improve water and nutrient uptake
Increase number of fibrous roots
High levels found in kelp plants

33. Biostimulants Greatly reduce the need for fertilizers, especially N
Saves money, reduces pollution

34. Methanol A form of alcohol Speeds plant growth
Research in California showed an increase in plant yield of %
Works by blocking photrespiration
Plants use water for growth rather than transpiring it into the air

35. Methanol Works best on plants frown in full summer sun
Water use reduced by as much as 50% in some plants

36. Allelopathy
Production of a chemical compound in one plant that slows or stops the growth of another plant
Natural herbicide
Black Walnut, Millet