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Plant Hormones

Plant Hormones. Organic compounds produced in low concentrations. Produced in one part of plant (i.e. source). Plant hormones. Transported to another part of plant (i.e. target). Cause physiological or developmental responses (stimulatory or inhibitory). Also called plant growth regulators.

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Plant Hormones

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  1. Plant Hormones • Organic compounds produced in low concentrations • Produced in one part of plant (i.e. source) • Plant hormones • Transported to another part of plant (i.e. target) • Cause physiological or developmental responses (stimulatory or inhibitory) • Also called plant growth regulators

  2. Plant Hormones • Major types of plant hormones: • Auxins • Cytokinins • Gibberellins • Ethylene • Abscisic acid

  3. Plant Hormones • Auxins • Types of auxins used in horticulture • Natural auxin is indole-3-acetic acid (IAA) • IAA is broken down in sunlight • Synthetic auxins used in horticulture: • Indole-3-butyric acid (IBA) • Naphthaleneacetic acid (NAA) • 2, 4-dichlorophenoxyacetic acid (2, 4-D) • Site of auxin production in plants: • Shoot and root tips (apical meristems) • Young, expanding leaves • Young seeds

  4. Plant Hormones • Auxins • Action of auxins in plants: • Stimulate cell elongation • Bend toward light because more auxin remains in cells on opposite side of light source • Etiolated plants have long internodes because auxin not quickly broken down • Promotes apical dominance • Pinching bud removes source of auxin and releases axillary buds from apical dominance • Promotes growth of adventitious roots • We used both IBA and NAA with geranium cuttings

  5. Plant Hormones • Auxins • Actions of auxins in plants • Promotes or inhibits abscission of leaves, flowers and fruit • NAA applied shortly after bloom is used to thin apples • NAA applied during fruit development inhibits abscission of apples • Auxin concentration determines if action is stimulatory or inhibitory • Low concentration promotes adventitious root growth, but high concentration inhibits root growth of cuttings • 2, 4-D is used as herbicide by applying at high concentrations

  6. Plant Hormones • Gibberellins • Types of gibberellins used in horticulture • Several different gibberellins (GA) produced by plants • Large, complicated molecules not synthesized • Commercial gibberellins produced by fungus • Site of gibberellin production in plants: • Shoot and root tips (apical meristems) • Young, expanding leaves • Embryos • Fruits • Tubers

  7. Plant Hormones • Gibberellins • Action of gibberellins in plants: • Stimulate cell elongation • Dwarf plants treated with gibberellins produce normal growth • Applied to grapes to elongate the peduncle (stem of flower cluster) and pedicels (stem of single flower), making looser cluster • Promotes cell division in vascular cambium • Promotes seed germination • causes production of enzymes that break down starch into energy needed for growth • Used by beer brewers to stimulate sugar production in barley malt (sugar is converted into alcohol during fermentation) • Influences flower and fruit development

  8. Plant Hormones • Cytokinins • Types of cytokinins used in horticulture • Several natural forms • Zeatin first cytokinin isolated • Synthetic cytokinins used in horticulture: • Benzyladenine (BA) • Site of cytokinin production in plants: • Embryos • Young leaves and fruit • Apical meristems of roots

  9. Plant Hormones • Cytokinins • Actions of cytokinins in plants • Promotes cell division (cytokinesis) • Contributes to cell enlargement (in leaves) • Stimulates differentiation of cells (with auxins) • High cytokinin and low auxin promotes shoot initiation in tissue culture, whereas reverse combination promotes root formation • Moderate levels of both hormones promotes callus growth • Delays senescence in leaves (maintains and promotes synthesis of chlorophyll)

  10. Plant Hormones • Ethylene • Gas at physiological temperatures • Liquid form used in horticulture (ethephon) • Site of ethylene production in plants: • Throughout plants • Actions of ethylene in plants • Inhibits root and shoot elongation by blocking transport of auxins from apical meristems • Stimulates leaf curling (epinasty)

  11. Plant Hormones • Ethylene • Actions of ethylene in plants • Induces adventitious root formation by blocking (and accumulating) auxin at tip of stem cutting • Enhances flow of latex in rubber trees • Enhances flowering in pineapples • Stimulates abscission of leaves and fruit • Used as a harvest aid for cherries • Promotes fruit ripening (apple, tomato, citrus, coffee) • Promotes senescence of flowers

  12. Plant Hormones • Abscisic Acid (ABA) • Similar structure as gibberellins • Site of abscisic acid production in plants: • All organs (e.g. roots, leaves, stems, fruits) • Actions of abscisic acid in plants • Counteracts effects of auxins and gibberellins • Maintains dormancy in seeds and buds • Stimulates guard cells to close stomates (to conserve water)

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