1. Growth Regulators Original by Libby Astrachan Modified by Georgia Agriculture Education Curriculum Office- July Modified by Lindsey Cottrell Nov. 2009

2. Plant Hormones Two types: Also called PGRs (plant growth regulators)
Natural (made by plants)
Synthetic (man made)
Also called PGRs (plant growth regulators)
Purposes: start growth, stop growth, modify growth & development

3. 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

4. Auxins Stem elongation Produced in tips of stems (“B” in photo)
Migrate from cell to cell in stems

5. 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
Phototropism Video

6. 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

7. Apical dominance
Auxins – move down the stem from the terminal bud and inhibit growth of side shoots

8. 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

9. 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

10. 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

11. 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.

12. 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 $$$.
B-Nine is a growth regulator that inhibits gibberellin and controls plant height in bedding plants.

13. 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 researched

14. 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.

15. Ethylene Gas Colorless gas
Produced in nodes of stems, ripening fruits, dying leaves

16. Ethylene exposure Thickens stems Breaks down chlorophyll
Weakens cell membranes
Softens cell walls

17. The Holly and the Ethylene

18. Abscisic Acid – The Plant Stress Hormone
Growth inhibiting hormone
Responsible for seed dormancy
Responsible for closing stomata during drought

19. Synthetic Growth Regulators
Rooting Compounds –
increase rooting %
speed rooting
increase number and quantity of roots
increase uniformity of roots

20. Rooting compounds
Liquid or mixed with talc

21. Growth Retardants Widely used in the greenhouse industry
Inhibit action of gibberellins on stem elongation

22. Goal of Synthetic Growth Retardants:
Compact plants
More attractive
Greener
Easier to transport
Flowering NOT affected

23. Application Guidelines
Read label
Wear safety gear
Apply correct amount at correct time
Plant should be well-established
Apply uniformly as foliar spray or drench
Split into two half-strength applications