Photoperiodism

• Photoperiodism


  The response of the plant to the relative lengths of light and darkness
• Phytochromes

  • Leaf pigments used by the plant to detect periods of light and darkness
  • Exist in two forms - an active form and an inactive form
• Phytochrome conversion

  1. Inactive form (P r) converted to active form (P fr) when absorbing red light

  2. Active form (P fr) broken down to inactive form when absorbing far red light

  3. Active form gradually reverts to inactive form in darkness (darkness reversion)
• Short-day plants

  Flower when the days are short, require the night period to exceed a critical length
• Long-day plants

  Flower when the days are long, require the night period to be less than a critical length
• In short-day plants

  P fr inhibits flowering, so flowering requires low levels of P fr (resulting from long nights)
• In long-day plants

  P fr activates flowering, so flowering requires high levels of P fr (resulting from short nights)
• Methods to induce short-day plants to flower out of season

  1. Expose long-day plants to light during the night to reduce the critical night length

  2. Cover short-day plants with opaque black cloth for ~12 hours a day to increase the critical night length
• Long-day plant

  • Carnations
• Short-day plant

  • Chrysanthemums
• The purpose of flowering is to enable the plant to sexually reproduce via pollination, fertilisation and seed dispersal
• Flowers need to bloom when pollinators are most active and abundant, which is dependent on seasons