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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