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Plant Physiology Index
The biological measurement of the relative lengths of day and night.
Photoperiodism is a response by a plant or an animal to synchronise its body with changes in day length. At high latitudes this is important because the change in length of the day indicates the season. Days getting shorter indicate winter is approaching, days getting longer indicate summer is approaching.
Some plants regulate their flowering this way
Response to day length
Response varies according to the age of the plant and varies in its intensity.
Broadly they can be grouped into three categories:
Some plants flower only after a CRITICAL DAY LENGTH
Some plants only flower after a CRITICAL NIGHT LENGTH
The night break phenomenon
Plant biologists observed that for plants with a critical night length, a short flash of light in the middle of the night would make the plant behave as if it had been exposed to a long day.
The quality of the light
The wavelength of the light used is important
This indicated that there should be a pigment that absorbs red light (in other words this pigment should be blue-green).
This pigment, phytochrome, is the mechanism capable of recognising changes in day length
The photoperiod mechanism
Phytochrome exists in two versions which are inter-convertible
PR that absorbs red light
In the plant
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