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Plant Hormones

Plant Hormones

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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)