Re: Daylength


>Subject: day light length
>I just read in Aquarium Pflanzen, that Echinodorus sp. can be induced to
>flower by providing them with either a long day or a short day. Some species
>are day length neutral and flower all the time.
>Nowhere, however, can I find the definition of "day". Is a short day,
>a period with less than 12 hours of illumination - and then total black-out?
>Or does the night light in the corner put out enough photons to make the plant
>"think" it still lit?  What about the car lights that strike the window
>blinds, opposite from the tank? Does the mometary bright flash have an
>If no, how long is too long?
>Also, over what period do the plants need a short day before the flower?
>One night? One week? Several weeks?
>Sorry for all these questions, but I would really like to see my swords to
>flower again and I think this is an area that hasn't received a lot of
>press. :-)

Daylength control of flowering in plants is complicated, In general, the
mechanism that plants use to measure daylength really measures the length
of the night.   Long day plants should, more correctly, be called short
night plants, where the night has to be shorter than X hours.  Short day
(really long night) plants have to have a night longer than X hours.  Just
what X is, varies with the plant species, and even variety.  There is a
plant pigment, phytochrome, that exists in two forms, Pr and Pfr.  Pr has
an absorbtion maximum at around 600 nanometers (red) and Pfr has an
absorbtion maximum at 720 nm (far red). Red light is absorbed by Pr and
converts it to Pfr.  Far red light is absorbed by Pfr and converts it to
Pr.  Pfr in the dark slowly converts to Pr, and it is this slow conversion
that is the main 'clock' by which the plant measures night length.   (Told
you it was complicated!)

Flowering in short night plants is stimulated _only_ if the level of Pfr
does not drop below a certain level.  This would happen in a night shorter
than X hours.  Flowering in long night plants is stimulated if the level of
Pfr drops below a certain level.  This would happen in a night longer than
X hours.

Sunlight has a balance of red and far red light that leaves plants with a
mix of about 60% Pfr, 40% Pr at sunset.  The phytochromes are relatively
sensitive to dim light, however, and street lights, which are higher in red
than far red light can mess up the local weeds growing nearby, and cause
them to stay vegetative too long into the fall season, resulting in their
being killed by frosts before they can set seed.  A brief one minute
exposure of red light in the middle of the night can convert a bunch of Pr
to Pfr and 'reset' the plant's clock to zero.  It basically breaks up a
long night into two short nights, and would cause a short night plant to
bloom and a long night plant not to bloom.  A brief one minute pulse of far
red light can convert a bunch of Pfr to Pr, and make the plant think it had
a long night.  With some plants, all it takes is one night of the critical
length to stimulate flowering.  With others it takes more than one, up to a
week of nights.

I have always wanted to do some experimenting with pulses of red and far
red light on my tank plants, but havn't gotten around to doing it yet.  It
is a lot easier to give a tank a short night than a long one, because room
lights left on after the tank light go off can contribute to the conversion
of Pr to Pfr.  A breif flash of car lights, however, probably wouldn't have
any appreciable effect.  If one is serious about giving a long night, one
should keep the tank pretty much in darkness after the tank lights go off.
That proviso makes giving the tank a long day followed by a minute or two
of far red light just after the lights go off an attractive alternative,
because the shot of far red converts what would have been a short night to
a long one by setting the plant's clock ahead.  Does anyone out there know
where I can get a relatively cheap filter that lets mostly far red light
(730) through?


Paul Krombholz                  Tougaloo College, Tougaloo, MS  39174
In rainy Jackson, Mississippi.