PGR

• Plant growth

  • Plants retain the capacity for unlimited growth throughout their life due to the presence of meristems
  • New cells are always being added to the plant body by the activity of the meristem (open form of growth)
• Meristems in plants

  • Root apical meristem
  • Shoot apical meristem
  • Vascular cambium
  • Cork cambium
• Phases of growth

  1. Meristematic

  2. Elongation

  3. Maturation
• Arithmetic growth


  Only one daughter cell continues to divide while the other differentiates and matures
• Geometric growth

  Both the progeny cells following mitotic cell division retain the ability to divide and continue to do so
• Geometric growth


  Follows a sigmoid or S-curve
• Absolute growth rate


  Measurement and comparison of total growth per unit time
• Relative growth rate


  Growth of the given system per unit time expressed on a common basis, e.g., per unit initial parameter
• Oxygen and nutrients


  • Very essential elements for plant growth
• Plant cell growth

  1. Cell enlargement

  2. Requires water

  3. Turgidity of cells helps in extension growth
• Plant growth and development

  Intimately linked to the water status of the plant
• Water


  Provides the medium for enzymatic activities needed for growth
• Oxygen


  Helps in releasing metabolic energy essential for growth activities
• Nutrients (macro and micro essential elements)


  Required by plants for the synthesis of protoplasm and act as source of energy
• Optimum temperature range

  Best suited for plant growth
• Deviation from optimum temperature range


  Could be detrimental to plant survival
• Environmental signals

  • Light and gravity affect certain phases/stages of plant growth
• Absolute and relative growth rates

  • Leaves A and B have increased their area by 5 cm2 in a given time to produce A1, B1 leaves
• Differentiation

  Act leading to maturation of cells derived from root apical and shoot-apical meristems and cambium
• Differentiation

  • Cells undergo structural changes in cell walls and protoplasm, e.g. tracheary elements lose protoplasm and develop strong, elastic, lignocellulosic secondary cell walls
• Dedifferentiation

  Living differentiated cells regain capacity for division under certain conditions
• Dedifferentiation

  • Formation of meristems - interfascicular cambium and cork cambium from fully differentiated parenchyma cells
• Redifferentiation

  Cells produced by dividing meristems/tissues lose capacity to divide but mature to perform specific functions
• Differentiation in plants is open, as cells/tissues arising from the same meristem have different structures at maturity</b>
• Final structure of a cell/tissue is determined by its location within the plant
• Development

  All changes an organism goes through during its life cycle from germination to senescence
• Plants follow different pathways in response to environment or phases of life to form different kinds of structures
• Plasticity


  Ability of plants to form different structures, e.g. heterophylly
• Growth, differentiation and development are closely related events in the life of a plant
• Development in plants is under the control of intrinsic (genetic, chemical) and extrinsic (light, temperature, water, oxygen, nutrition) factors
• Plant growth regulators (PGRs)


  Small, simple molecules of diverse chemical composition that regulate plant growth and development
• Types of PGRs

  • Auxins
  • Gibberellins
  • Cytokinins
  • Abscisic acid
  • Ethylene
• Growth promoting PGRs

  Auxins, gibberellins, cytokinins
• Growth inhibiting PGRs

  Abscisic acid, ethylene
• Discovery of major PGRs was accidental
• Discovery of PGRs

  • Auxin from coleoptile tips
  • Gibberellins from 'bakanae' disease
  • Cytokinins from tobacco callus
  • Abscisic acid from inhibitors
  • Ethylene from ripened oranges
• Auxins

  Promote rooting in stem cuttings, flowering, prevent fruit and leaf drop, induce parthenocarpy, control xylem differentiation, cell division
• Gibberellins

  Increase length of grape stalks, cause fruits like apple to elongate and improve shape, delay senescence
• 2, 4-D


  Widely used herbicide that kills dicotyledonous weeds but does not affect mature monocotyledonous plants
• 2, 4-D


  • Used to prepare weed-free lawns by gardeners
  • Controls xylem differentiation and helps in cell division