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Powerpoint Presentation: Photoperiodism
Powerpoint Presentation: Phototropism

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.

The control of flowering

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


The pigment

This indicated that there should be a pigment that absorbs red light (in other words this pigment should be blue-green).

This pigment 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
PFR that absorbs far red light

In the plant

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© Paul Billiet 2016