Photoperiodism

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Photoperiodism

  • Plants do not have a nervous system, their growth is determined by hormones; one response to light is photoperiodism; where flowering is influenced by day length.
  • Phytochromes are responsible for absorbing light and are found in minute-quantities, they exist in two forms which are inter-convertible:
    -red light  <----------------------------------------------------------------> far-red light
     P660 (Pr)                                                                      P30 (Pfr)
  • Sunlight contains more 660nm than 730nm, therefore Pr is converted to Pfr in the daytime.
  • However Pfr is unstable and breaks down at night back in to Pr.
  • Flowering of plants is thought to be initiated via florigen.
  • Day neutral - flowering does not seem to be affected by the day length
  • Long-day - flowering is induced by exposure to dark periods shorter than a critical length; so the length of the day/light is long and the darkness short (ie. cabbage)
  • Short-day - flowering is induced by exposure to dark periods longer than critical length; so the length of the day/light is short and the darkness longer (ie. poinsetta)
  • Any disruptance of the darkness means that short-day plants do not flower.
  • Horticulturists use knowledge of photoperiodism to:
    -Breed plants that don't normally flower at the same time.
    -To delay flowering until Christmas/other occasions. 
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