Plant Hormones

• Hormones are naturally occurring, organic substances that, at low concentration, exert a profound influence on physiological processes.
• There are numerous chemical substances - natural and synthetic - that profoundly influence the growth and differentiation of plant cells and organs.
• Auxins are synthesized in the stem and root apices.
• Auxins have the capacity to stimulate cell elongation
• Auxins are involved in root initiation.
• Auxins are involved in vascular differentiation
• Auxins are also responsible for tropic responses.
• Auxins are involved in the development axillary buds, flowers, and fruits.
• Physiological Action of Auxin
• Physiological Action of Auxin

  • Cell growth and differentiation
  • Shoot and root development
  • Leaf abscission
  • Root elongation and development
  • Flower and fruit development
• Gibberellins are produced by both fungi and higher plants
• Exogenous application of gibberellins causes hyper elongation of intact stems.
• Gibberellins are involved in seed germination, mobilization of endosperm reserves during early embryo growth, as well as flower and fruit development.
• Physiological Actions of Gibberellins

  • Control of shoot elongation
  • Seed germination
  • Flowering
• Kinetin was the first cytokinin to be isolated.
• Zeatin was the most widespread naturally occurring cytokinin in higher plants.
• Cytokinins stimulate cell division
• Cytokinins influence shoot and root differentiation in tissue culture, the growth of lateral buds and leaf expansion, chloroplast development, and leaf senescence.
• Physiological role of Cytokinins

  • Cell division and morphogenesis
  • Nutrient mobilization and senescence
• Abscisic acid is a terpenoid involved primarily in regulating seed germination, inducing storage protein synthesis, and modulating water stress.
• Physiological role of Abscisic Acid

  • Seed maturation
  • Stomatal function
• Ethylene is a simple gaseous hydrocarbon
• Ethylene can have a significant impact on the development of roots and shoots
• Ethylene is synthesized primarily in response to stress
• Physiological Role of Ethylene

  • Vegetative development
  • Fruit development
  • Flowering