Control in plants

• During the growing season, cell division takes place at the tip (apex) of the plant in the tissue called the apical meristem.


  These cells enlarge in a region below the tip called the zone of elongation; at certain times of the year the internode (the region between the nodes, or points at which the leaves develop) also elongates.
• The three plant growth hormones are: auxin cytokinin gibberellin
• Cytokinins are produced by the roots and travel upwards to stimulate cell division and delay senescence (ageing).
• Gibberellins are produced by seeds and buds and cause seed germination and flower development.
• • When P660 absorbs red light, it is converted into P730

  (Note: daylight contains mostly red light.)

  • When P730 absorbs far red light it is converted into P660.
• Long-Day plants:

  • only flower when the days are long (and the nights are short)
  • only flower when the period of darkness is less than the critical length of night
  • non-removal of P730 allows long-day plants to flower

  Note: a flash of light in the dark period will still result in flowering

  • flower in summer
• 2. Short-day plants

  • only flower when the days are short (and the nights are long)
  • only flower when the period of darkness exceeds the critical length of night
  • removal of P730 is required for short-day plants to flower

  Note: a flash of light in the dark period will result in non-flowering because P660 will be instantaneously be converted to P730.

  • flower in winter