5 - Leaf Senescence and Programmed Cell Death

• Leaf Development:

  • Leaf primordia begin from apical meristem
  • Periclinal divisions start the process
  • Anticlinal divisions form the leaf primordium buttress
  • Auxin and gibberellin stimulate primordium formation
  • Protein expansin leads to the formation of primordium-like outgrowths
  • Three phases: leaf initiation, primary morphogenesis, expansion and secondary morphogenesis
• Leaf initiation:

  • Lateral expansion of leaf blade
  • Formation of a primordial leaf axis called phyllopodium with dorsiventral symmetry
  • Phyllopodium becomes the petiole and midrib
  • Leaf lamina forms as outgrowths on either side from cytokinesis in marginal meristems
  • Compound leaves have subdivided marginal meristems
  • Basal meristem in phyllopodium for petiolate leaves
  • Provacular tissue differentiates to reflect the mature system of veins
• Expansion and secondary morphogenesis:

  • Young leaf continues growth and differentiation
  • Increase in surface area and volume
  • Marginal meristems are short-lived
  • Parenchyma differentiates as mesophyll
  • Lobing development occurs for the final form of leaves
• Morphology:

  • Dicotyledon leaf has reticulate venation
  • Monocotyledon leaf has parallel venation
  • Differentiation of veins in leaves occurs in a hierarchical manner
  • Major veins develop first and act as conduits for nutrient transport
  • Last-order veins enclose small regions of parenchyma called areoles
• Leaf Senescence:

  • Genetically determined process
  • Triggered by environmental cues
  • Active developmental process that requires energy
  • Involves reabsorption of nutrients
  • Different from necrosis, which is caused by external injury
• Programmed Cell Death:

  • Specialized type of senescence
  • Involves individual cells activating an intrinsic senescence program
  • Accompanied by morphological and biochemical changes called apoptosis
  • Plays a role in development and protection against pathogens