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· It has been known for a long time that some tiny seeds
such as lettuce and mint require light to germinate.
· With thin seed coats and minimal food reserves, they will
only germinate in optimum conditions i.e. close to the soil
· Experiments into the effect of different wavelengths of
light showed that:
­ Red light (wavelength 580-660nm) is most effective in
stimulating germination in these seeds.
­ Far red light (wavelength 700-730nm) could actually inhibit
­ Very brief exposures produced these effects and the colour of the
final flash of light determined whether seeds germinated…read more

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· Scientists suggested that these effects involved a
pigment that reacts with different types of light.
· In the 1960s this pigment was isolated from plants and
called phytochrome
· It is a blue-green pigment which exists in two
interconvertible forms, or isomers:
­ PR (P660), red absorbing
­ PFR (P730), far-red absorbing
· Plants synthesise phytochromes as PR; absorption of
red light converts this to the PFR form.
· Conversely, when PFR absorbs far red light, it is
converted to PR…read more

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red light
far-red light
slow conversion in
Sunlight contains more red than far-red light so
during the day phytochrome is converted to PFR
In the dark, PFR slowly converts to PR
This works like a clock ­ the plant can "measure"
the length of a dark period because the longer
the dark, the more PR is present…read more

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Absorption spectra of the
two forms of phytochrome…read more

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How does it work?
· When lettuce seeds are exposed to red light
(sunlight), the PR phytochrome is quickly
converted to PFR
· This is the biologically active form and
stimulates the responses which lead to
· If the seeds are exposed to far red light, the
PFR is converted back into PR and germination
is inhibited…read more

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