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Plant Growth Regulators A hormone is a naturally produced chemical synthesized in one part of the plant and then travels to another part where it effects growth and development. There are five main groups: Auxin or Indole Acetic Acid (IAA) Gibberellin or Gibberellic Acid (GA) Cytokinin (Ck) Ethylene (C2H4) Abscisic Acid (AbA) Hormones discovered more recently include salicylic acid, jasmonates, brassinolides, etc. Plant growth regulators are man-made chemicals applied to plants to produce a desired effect (some are chemically identical to hormones). https://en.wikipedia.org/wiki/Plant_hormone Light source Phototropism: shoots bend towards light due to auxin migrating to darker side of stem. Plant Growth Regulators © KPU.ca/Hort Gibberellic acid and low light levels cause stretching due to the elongation of internode cells. Plant Growth Regulators © KPU.ca/Hort Effects of gibberellic acid and low light on bean seedlings and petunia. Plant Growth Regulators © KPU.ca/Hort Auxin regulates apical dominance: pinching/ shearing plants will result in more branches. Plant Growth Regulators © KPU.ca/Hort Control Senescence (aging) and dormancy: Ethephon causes plants to release ethylene and senesce faster. Plant Growth Regulators © KPU.ca/Hort Control Senescent parts due to Ethephon spray abscise faster due to more ethylene gas being released. Plant Growth Regulators © KPU.ca/Hort A separation layer forms in the abscission zone where the petiole joins the stem. Leaf abscission: occurs when auxin levels decrease and ethylene levels increase. Plant Growth Regulators © KPU.ca/Hort Control Fruit and flower part drop: ethylene released from apple cause separation layer formation. Plant Growth Regulators © KPU.ca/Hort Ethephon can also be used to de-green citrus fruits, and to ripen bananas or tomatoes. Plant Growth Regulators © KPU.ca/Hort Shoot proliferation in tissue culture: high cytokinin and auxin level promotes shoot growth. Plant Growth Regulators © KPU.ca/Hort BA >> IAA BA << IAA Effect of cytokinin (e.g. Benzyl Adenine) to Auxin (IAA) levels on African violet in culture. Synthetic auxin (e.g. 0.1-0.8% IBA) can be used to stimulate adventitious rooting on cuttings. Plant Growth Regulators © KPU.ca/Hort Underwatered Underwatered Underwatered Under-watered Under watered plants are stressed and have stomata closed more often. Why? Plant Growth Regulators © KPU.ca/Hort Summary: Main Effects of Plant Hormones (and related PGR’s) Auxins Cell enlargement, role in apical dominance Gibberellins Cell (internode) elongation, seed germination Cytokinins Cell division, delays senescence Abscisic Acid Promotes dormancy, stomatal regulation Ethylene Fruit ripening, senescence, organ abscission
