Photoperiod - the relative length of day and night, varies with time of year
Photoperiodism - the response of a plant to changes in the photoperiod (flowering, fruit and seed production)
Long day plants - flower when light periods are long, late spring, early summer, e.g. lettuce, clover
Short day plants - flower when light periods short, late summer/early spring, tobacco, strawberries
Phytochrome - the photoreceptor resposnible for absorbing light. Blue green pigment that absorbs in the red part of the visible spectrum. Exists in 2 interconvertible forms:
Pr absorbs red light Pfr absorbs far red light
When Pr absorbs red light it is rapidly converted to Pfr and vice versa.
As sunlight contains more red light, during the day Pr is converted to Pfr
At night, the unstable Pfr slowly reverts back to Pr
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