Abscission

• Why do plants lose their leaves?

  • Winter survival (as rate of photosynthesis drops)
  • Reduces water loss
  • Avoids frost damage and fungal infection
  • Rate of photosynthesis affected by temperature and light- not enough glucose produced to balance keeping the leaves
  • plant less affected/damaged by extreme temperatures/wind without leaves than with leaves
• What is leaf abscission?

  Plants losing their leaves
• How are hormones involved in leaf abscission

  Auxins

  Ethene

  Abscissic acid

  Cytokinnins

  • Auxins inhibit leaf abscission
  • Ethene produced by maturing or aging tissues (promotes ripening)
  • Abscisscic acid produced when plant under stress - eg like cold temperatures, lack of water
  • Cytokinnins stop the leaves of deciduous plants (which have leaves all year round) from senescing. If cytokine production drops, less nutrients are available so senescence begins
• What is senescence?

  condition or process of deterioration with age- loss of a cells power of division or growth
• Describe the process of shedding leaves(abscission)

  1. falling light levels decrease auxin conc
  2. Drop in auxin production triggers the production of more ethene
  3. cells in abscission zone at the base of a leaf stalk become more sensitive to ethene
  4. ethene initiates gene switching in these cells, results in the production of new enzymes
  5. Enzymes digest cell walls in outer layer of abscission zone (separation layer)
  6. vascular bundles sealed off
  7. fatty material deposited in separation layer to prevent path entry and form waterproof scar
  8. abiotic factors strain the leaf and cause it to detach

  
• describe what the abscission zone is made of

  • Forms where the petiole meets the stem
  • Two layers- separation layer on leaf side (small cells within thin cell walls) protective layer on stem side (cell walls contain suberin- waxy and waterproof)