Responses in plants

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Definitions

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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Flowering

Pfr is the critical form: Low levels of Pfr induce flowering in short day plants by stimulating florigen production. High levels of Pfr induce flowering in long day plants by stimulating florigen production

The unbroken dark period is also crictical, as a brief light period will bring about the rapid conversion of Pr to Pfr. 

Key points to know about photoperiodism experiments:

- only one leaf needs to be exposed to the appropriate photoperiod for flowering to occur

- the photoperiod is detected by the leaf (phytochrome present in leaves)

- a chemical substance such as Florigen must be transported from leaf to bud for flowering

- the chemical substance is transported in the phloem

- the chemical substance has the same effect in long day and short day plants

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