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Re: Darkness period and plant flowering

>From: "ALEX PASTOR" <alexp at idirect_com>

>When I studied botany at university (many moons ago) we were taught that
>there are "short-day" and "long-day" plants.  Since that time this has all
>been overturned.  Quite recently, there was an article in the newspaper
>saying that there was no such thing, and that in fact it was the duration of
>the night period only that determines whether a particular plant will flower
>or not.  Apparently that is that.
>Dr. Momfish

The distinction between describing a plant as a short day plant  or a long
night plant is not important as long as the plant is on a 24 hour cycle.
If it gets short days, it will automatically get long nights.  The
distinction was made because it was found that plants measure the night
length, not the day length.  There is a pigment in plants called
phytochrome that exists in two forms, phytochrome red (P660) and
phytochorme far red (P700).  Plants begin their nights with most of the
pigment in the P700 form, which slowly converts to P660 during the night.
The amount converted is the measure of the night length.

P660 absorbs red light, with a peak absorbance at a wavelength of 660
micrometers. When P660 absorbs red light, it converts to P700.  P700
absorbs far red light, with a peak absorbance at 700 micrometers.  When
P700 absorbs far red light, it converts back to P660.  Daylight has a lot
more red light than far red light, and that is why the plant starts off its
night with mostly P700, the form that slowly reverts to P660.  A short day
(long night) plant needs a long night to accumulate enough P660 to trigger
the hormonal sequence that leads to blooming.  If the night is too short,
P660 does not build up to high enough levels to trigger blooming.  The two
phytochromes are quite sensitive to light, and even room lighting has
enough red light to keep the 'clock' from running, i.e., keep any P660 from
building up.  Even the relatively dim light from street lights has enough
red light to slow down the clock and give plants the "misinformation" that
the night is a lot shorter than it really is.  Every November I see weeds
growing near street lights that delayed blooming and got killed by the
frosts while still in the vegetative state.  Further away from the lights,
the weeds have gone to seed in plenty of time.

Steve Pushiak mentioned on Jan. 22 that his Ocelot sword bloomed after he
had been on vacation for a week.  While the house was unoccupied, there
were no room lights on after dark to prevent the clock from running, and
the plant got the long nights required for blooming.  Room light strong
enough to keep the clock from running is not strong enough for any
meaningful photosynthesis.

"Normal" light that plants are likely to encounter has much more red than
far red light, and so the effect is always to reset the clock to the point
where nearly all the phytochrome is in the P700 form.  With just a brief
flash of red light in the middle of a long night, the clock will be reset,
and the plant starts counting from the beginning.  With a special filter
that only allows far red light through, it is possible, with a flash of far
red light, to run the clock to the end and create the effect of a long

Paul Krombholz, in cloudy, chilly, central Mississippi