Seed Selection

• Stages of seed development:

  1. histodifferentiation
  2. cell expansion
  3. maturation drying
• Stage I: histodifferentiation (embryo differentiation)
• histodifferentiation

  • zygote to proembryo
  • embryo differentiation
  • cell division
  • beginning of cotyledon stage
  • rapid increases in fresh and dry weight
  • differs in monocots, dicots, and gymnosperms
• embryo differentiation in dicots: proembryo, globular, cotyledon, mature stage
• Suspensor function is to push embryo into the embryo sac cavity and to absorb and transmit nutrients to the proembryo
• embryo differentiation in monocots: proembryo, globular, scutellar, coleoptilar
• Stage II: cell expansion
• funiculus: conductive tissue between fruit and ovule (seed)
• cell expansion

  • endosperm enlargement
  • rapid increase in food reserve storage
  • embryo enlargement
• hilum: the scar left on the seed coat after the funiculus abscises
• Stage III: maturation drying
• maturation drying

  • physiological changes
  • physiological maturity
  • maximum embryo dry weight
  • removed embryos can germinate
  • dessication
  • rapid water loss via hilum
  • LEA (late embryogenesis abundant) proteins develop
• Auxin and gibberellins signal fruit development and are used during early seedling growth
• Cytokinins control cell division in developing seed
• Ethylene is high in some developing seeds and its role is unclear
• Not all seeds undergo maturation drying
• recalcitrant seeds

  • seeds are unable to withstand maturation drying
  • seeds will lose viability rapidly after drying
  • seed germination must occur quickly after maturity
  • must be stored at high humidity
  • relatively short storage life
  • typically found in fleshy fruits, many tropical/subtropical
• Orthodox (normal) desiccated seed stores for extended periods
• quiescent: will germinate under proper conditions
• dormant: will not germinate even under proper conditions
• viviparous: seed prematurely germinates in the fruit; inability of the embryo to produce or perceive abscisic acid
• apomixis: asexual development of seeds; production of an embryo without meiosis and fertilization
• facultative apomictic: both apomictic and sexual embryos are produced on the same plant
• obligate apomictic: all embryos produced are apomictic
• gametophyte apomixis: the megaspore mother cell degenerates or bypasses meiosis and unreduced cells divide to produce reproductive cells
• diplospory: megaspore mother cell does not undergo or complete meiosis
• apospory: megaspore mother cell undergoes normal meiosis, but the resulting cells usually degrade before they are fertilized
• sporophytic apomixis: adventitious embryony; more important from horticulture production standpoint
• During sporophytic apomixis, megaspore mother cell undergoes normal meiosis and forms a normal sexual embryo sac. At the same time, nucleus cells begin to abnormal cell division and form multiple asexual embryos
• polyembryony: development of multiple embryos within the same seed
• adventitious embryony: nucleus
• polyembryogenesis: embryo splitting; conifers
• false polyembryogenesis: fertilization of many egg nuclei within a single embryo sac
• parthenocarpy: fruit development without seeds
• vegetative parthenocarpy: fruit development without pollination
• stimulative parthenocarpy: fruit development after pollination but not fertilization
• objective of breeding program: use the observed variability within genus or species to create new, table populations with improved plant characteristics
• breeding: selection of parents, specific breeding procedures, genotype stabilization
• fixing the genotype: the process of stabilizing the genotype of a seedling population to make it homozygous so that it will "breed true"
• homogeneous: seedlings that are phenotypically very similar in appearance to each other