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Plant Hormones

Growth Regulators Original by Libby Astrachan Modified by Georgia Agriculture Education Curriculum Office- July 2004. Plant Hormones. Natural (made by plants) – also called hormones Synthetic (man made) Also called PGRs (plant growth regulators)

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Plant Hormones

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  1. Growth RegulatorsOriginal by Libby AstrachanModified by Georgia Agriculture Education Curriculum Office- July 2004

  2. Plant Hormones • Natural (made by plants) – also called hormones • 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

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

  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

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