How do plants know when
to flower?
The Phytochrome
System
Flowering of plants is
determined in part by the
length of the day.
long days &
short nights
short days &
long nights
Plants are of three general
types:
Example
short-day
strawberry
long-day
spinach
day-neutral
sunflower
Short-day plants flower in
spring or autumn when the
day lengths are shorter than
the critical day length..
For example, the common
cocklebur (Xanthium strumarium)
is induced to
flower only when
exposed to 16
hours or less
of light.
Examples of short-day plants :
poinsettias
strawberries
primroses.
Long day plants flower
chiefly in the summer and
will flower only when exposed
to light for longer than a
critical length.
Spinach has a
critical daylength
of 13 hours.
Examples of long-day plants
spinach
lettuce
henbane
What determines if plants
are long or short-day is not
so much the length of the
exposure time but whether it
is longer or shorter than
some critical interval.
Day-neutral plants flower
without respect to daylength.
Examples of day-neutral plants
cucumber
corn
sunflower
cucumber
Plants contain a photoreceptor
called phytochrome (P)
which comes in two forms
which are interconvertible,
and
Pfr.
Pr
Pfr
Pr
Pr absorbs red
light and Pfr
absorbs far-red
light.
Visible light lies between
400 and 700 nm.
Red light Far Red 730
nm
660 nm light
The Electromagnetic spectrum
Pr
Dark
reversion
Slowly
reverts
during the
night.
Red
light
Far-red
light
Straight
away
Pfr
Straight
away
Physiological response