Short -day & Long-day plants

• Photo/critical period = minimum amount of light needed for flowering
• short day plants

  • Pfr inhibits flowering
  • Low levels of Pfr removes this inhibition
  • Need long uninterrupted nights
  • Where Pfr is converted to Pr
  • E.g. chrysanthemums
• long day plants

  • Pfr promotes flowering
  • High levels of Pfr stimulates flowering
  • Need long days/short nights
  • Where more Pr is converted to Pfr
  • E.g. spinach & lettuce