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PHOTOPERIODISM The biological measurement of the relative lengths of day and night
Photoperiodism Photoperiodism the response by an organism 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 © 2010 Paul Billiet ODWSODWS
The control of flowering Flowering “Florigen” hormone Flower buds Photoperiod mechanism in the leaves Change in day length © 2010 Paul Billiet ODWSODWS
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 © 2010 Paul Billiet ODWSODWS
Response to day length Cucumber Holly Maize Do not respond to day length DAY-NEUTRAL PLANTS Chrysanthemum < 15h Tobacco < 14h < hSHORT-DAY PLANTS (“LONG-NIGHT” PLANTS) Red clover > 9h Oats > 12h Winter wheat > 12h >9-16 hLONG-DAY PLANTS EXAMPLESDAY LENGTHRESPONSE © 2010 Paul Billiet ODWSODWS
Response to day length Some plants flower only after a CRITICAL DAY LENGTH Some plants only flower after a CRITICAL NIGHT LENGTH Image Credit: Chrysanthemum Chrysanthemum © 2010 Paul Billiet ODWSODWS
The night break phenomenon 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 © 2010 Paul Billiet ODWSODWS
The quality of the light The wavelength of the light used is important StimulatesInhibits670 – 680nmRed light ReversesStimulates>700nmFar red light LONG-DAYSHORT-DAYWavelengthColour © 2010 Paul Billiet ODWSODWS
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 PHYTOCHROME © 2010 Paul Billiet ODWSODWS
The photoperiod mechanism Phytochrome exists in two versions which are inter-convertible P R that absorbs red light P FR that absorbs far red light PRPR RED LIGHT FAR RED LIGHT P FR © 2010 Paul Billiet ODWSODWS
In the short-day plant P FR P R builds up Darkness (slow) Far red light (fast) Short-day plants FLORIGEN Activated FLOWERING © 2010 Paul Billiet ODWSODWS
In the long-day plant P FR builds up PRPR Sunlight Red light Long-day plants FLOWERING FLORIGEN Activated © 2010 Paul Billiet ODWSODWS
Summary Sunlight Red light Darkness (slow) Far red light (fast) P FR builds up Long-day plants FLOWERING FLORIGEN Activated P R builds up Short-day plants FLORIGEN Activated FLOWERING © 2010 Paul Billiet ODWSODWS
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