The biological measurement of the relative lengths of day and night PHOTOPERIODISM The biological measurement of the relative lengths of day and night © 2016 Paul Billiet ODWS

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 approaching Days getting longer indicate summer approaching Some plants regulate their flowering this way © 2016 Paul Billiet ODWS

The control of flowering Change in day length Photoperiod mechanism in the leaves “Florigen” hormone Flower buds Flowering © 2016 Paul Billiet ODWS

Response to day length Response varies with age of the plant and varies in its intensity Broadly there are three categories. © 2016 Paul Billiet ODWS

Response to day length EXAMPLES DAY LENGTH RESPONSE Red clover > 9h Oats > 12h Winter wheat > 12h >9-16 h LONG-DAY PLANTS Chrysanthemum < 15h Tobacco < 14h < 10-16 h SHORT-DAY PLANTS (“LONG-NIGHT” PLANTS) Cucumber Holly Maize Do not respond to day length DAY-NEUTRAL PLANTS © 2016 Paul Billiet ODWS

Response to day length CRITICAL DAY LENGTH CRITICAL NIGHT LENGTH Chrysanthemum © 2016 Paul Billiet ODWS

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 © 2016 Paul Billiet ODWS

The quality of the light The wavelength of the light used is important LONG-DAY SHORT-DAY Wavelength Colour Reverses Stimulates >700nm Far red light Stimulates Inhibits 670 – 680nm Red light © 2016 Paul Billiet ODWS

The pigment This indicated that there should be a pigment that absorbs red light Therefore the pigment should be… blue-green This pigment is the mechanism capable of recognising changes in day length PHYTOCHROME © 2016 Paul Billiet ODWS

The photoperiod mechanism Phytochrome exists in two versions which are inter-convertible PR that absorbs red light PFR that absorbs far red light RED LIGHT 660nm PR PFR FAR RED LIGHT 730nm © 2016 Paul Billiet ODWS

In the short-day plant PFR PR builds up Short-day plants FLORIGEN Darkness (slow) Far red light (fast) Short-day plants FLORIGEN Activated FLOWERING © 2016 Paul Billiet ODWS

In the long-day plant PFR builds up PR Long-day plants FLORIGEN Sunlight Red light PFR builds up PR Long-day plants FLORIGEN Activated FLOWERING © 2016 Paul Billiet ODWS

Summary Sunlight Red light PR builds up Darkness (slow) Far red light (fast) PFR builds up Long-day plants FLOWERING FLORIGEN Activated PR builds up Short-day plants © 2016 Paul Billiet ODWS