Principles of Seed Propagation

• germination: the activation of a seed's metabolic machinery leading to the emergence of a new seedling plant
• three conditions to initiate germination:

  • the seed must be viable
  • primary dormancy must be overcome
  • seed must receive appropriate environmental conditions
• three phases of germination:

  1. imbibition
  2. lag phase
  3. radicle protrustion
• imbibition: rapid increase in water uptake
• lag phase: active metabolic activity
• radicle protrusion: additional water uptake
• Dry seeds have low water potential, water moves from high to low water potential (outside to inside seed)
• Due to metabolic processes, osmotically active solutes increase in a cell, OP becomes negative and as a result, water moves into the cell
• Imbibition:

  • water uptake
  • rapid uptake followed by slow uptake
  • volume of seed increases
  • seed parts may wet differentially depending on their contents
  • leakage
• lag phase:

  • reduced or no water uptake, highly physiologically active period
  • mitochondria maturation
  • protein synthesis
  • metabolization of storage reserves
  • enzymes loosen cell walls
• radicle protrusion:

  • cell enlargement
  • food reserves continue to be used
  • enzymes loosen cell wall to allow emergence of radicle
  • GA promotes enzymatic cell wall hydrolysis and radicle emergence
  • ABA inhibits enzymatic cell wall hydrolysis
  • growth potential
• growth potential: relative force generated by the radicle during germination
• Seed germinates when the radicle force is sufficient to penetrate the seed covering
• hypocotyl: portion of seedling axis between the root and the cotyledons
• cotyledons: storage structure; will shrivel and drop off
• epicotyl: includes all seedling structures above the cotyledons
• terminal bud: at the tip of the epicotyl at the apical meristem
• coleoptile: protective sheath enclosing the terminal bud and developing leaves
• coleorhiza: covers the root apex
• dicot seedling structure:

  • hypocotyl
  • cotyledons
  • epicotyl
  • terminal bud
• monocot seedling structure:

  • coleoptile
  • coleorhiza
• epigeal: hypocotyl hook raises the cotyledons above the soil
• hypogeal: cotyledons remain below ground and the epicotyl and shoot emerge from the soil
• emergence patterns in dicot seedlings:

  1. cryptocotylar hypogeal reserve cotyledons
  2. cryptocotylar epigeal reserve cotyledons
  3. phanerocotylar epigeal foliaceous cotyledons
  4. phanerocotylar epigeal storage cotyledons
  5. phanerocotylar hypogeal storage cotyledons
• major energy sources converted to amino acids/sugars to fuel early embryo growth:

  • proteins
  • carbohydrates (starch)
  • lipids (oils)
• steps in starch mobilization in cereal grains:

  1. gibberellin is released from the embryo axis and scutellum into the aleurone layer
  2. gibberellin induces the production of enzymes that degrade storage carbohydrates
  3. enzymes are secreted from the aleurone cells into the nonliving endosperm
  4. starch is converted to sugars, while enzymes digest the cell walls in the endosperm
  5. sugars are absorbed by the scutellum for transport to the growing embryo axis
• measures of germination:

  • germination percentage
  • germination rate
  • germination uniformity
• germination percentage: number of seedlings produced from a seed population
• germination rate: measure of how rapid a seed lot germinates
• germination uniformity: how close in time seeds germinate or seedlings emerge
• environmental factors affecting germination:

  • temperature
  • gases
  • light
  • water
• temperature affects both germination percentage and germination rate
• cool temperature tolerant: native to temperate zones, grow over wider range of temp 4-30C
• cool temperature requiring: cool season species adapted to a Mediterranean climate; no germination if >25C
• warm temperature requiring: native to subtropical and tropical regions, fail to germinate below 10-15C, susceptible to chilling injury
• alternating temperature requiring: day/night temperature fluxes are better than constant temperatures; 10C difference often used
• Oxygen is essential for respiratory processes in germinating seeds
• light quality: wavelength
• photoperiod: light duration
• Light sensitive seeds are generally small in size or are epiphytes