Re: inducing flowering

>From: nfrank at nando_net (Neil Frank)
>Date: Sun, 17 Mar 96 07:49:11 EST
>Subject: Re: adjusting daylength (to flower horemanii)
>For anyone who has experimented with artificial light to change daylength in
>order to flower plants (as per discussion for E. horemanii), how important
>is the annual cycle?
>- - When a short day (e.g. 8-hours) is needed for flowering, is a
>complementary long day followed by transition periods needed?
>- - Or, can a short day immediately follow a 12-hour day?
>- - In either case, for how many weeks is the short day needed to trigger the
>- - And what about concomittant changes in temperature, light intensity, etc.

I have not messed with daylength, but I hope to find the time do do so
sometime.  From what I have been able to read, it works something like

flowering plants fall into three categories---long day, short day, and
day-neutral.  It is actually the NIGHT length that is critical.  Long day
plants need a short night (shorter than X hours), short day plants need a
long night (longer than X hours), and day-neutral plants don't pay
attention to day or night length, but bloom when they reach a certain size.

There is a pigment, phytochrome, in flowering plants that exists in two
forms, P660, (also called Pr) and P730 (also called Pfr).  Pr has an
absorbtion peak at 660nm (red light) and Pfr has an absorbtion peak at
730nm (far red light).  Here is where it gets tricky.  Red light is not
only absorbed by Pr, red light also converts Pr to Pfr.  The same is true
for far red light and Pfr.  Far red light converts Pfr to Pr. The pigment
that causes biological responses in plants is Pfr.

 Because of the mixture of red and far red light in sunlight, plants end
their day with about 60 percent Pfr and 40 percent Pr.  During the night
some of the Pfr slowly converts to Pr and some of the Pfr just breaks down.
Both these processes result in a declining percentage of Pfr during the
night.  A high level of Pfr promotes blooming in the long day (short night)
plants.  These must end their night with a relativly HIGH level in order to
bloom.    A high level of Pfr inhibits blooming in the short day (long
night) plants.  These must end their night with a relativly LOW level in
order to bloom.

Now, here is where it gets interesting.  If you have a plant that needs a
long night to bloom (needs a low level of Pfr), and, let us say you are
giving it a long night.  If you come in in the middle of the night and give
it a short one minute pulse of red light, you will convert most of the Pr
in the plant to Pfr, in effect, starting the night all over again.  The Pfr
won't get low enough by morning to trigger blooming.  The plant won't
bloom.  If you are giving the same plant a short night, and, sometime
during that night, you come in and give it a shot of far red light, you
will convert most of the Pfr to Pr, and the plant will bloom.  I will leave
it an exercise for the reader, as they pedantically say in textbooks, to
work out what shots of red and far red light do during the night to short
night (long day plants).

I bring up all this stuff about shots of red and far red light in the night
because it is hard to give your indoor plants a long night.  The
phytochrome system is quite sensitive to light, and the room light after it
gets dark outside or the daylight coming in the window in the morning is
enough to affect the plant.  You can have the lights on your tank on an 8
hour day-16 hour night cycle, but the tank had better be covered over with
some kind of shroud so that it doesn't get any stray light when the tank
lights are off.  Just the light from street lights is enough to make
various annual short day (long night) weeds growing nearby continue to grow
vegetatively and not flower and go to seed during the fall.  They are
supposed to go to seed and die back in late August and September, but,
instead, they are still growing vegitatively when they are killed by the

So, if you want to give your E. hormanii a long night, you will have to
protect the tank from stray light with a shroud.  On the other hand, you
could give it a one minute shot of far red light when you turned out the
room lights and went to bed, and then, even if the sun came up 5 or 6 hours
later, the plant would "think" it had a long night.  The problem with the
latter method is finding a filter that gives far red light, but cuts out
red light.  Carolina Biological supply sells one, but it is rather
expensive.  They sell two sizes, and I got a little one, but havn't had
time to do any experiments with it yet.  The cocklebur has been found to
need only a single short night to trigger flowering.  Other species,
however may  require weeks of the proper photoperiod.  It is anybody's
guess how long sword plants will need to be exposed to the proper

I hope that all this will stimulate somebody to do some experiments and let
us know what happens.

Paul Krombholz                  Tougaloo College, Tougaloo, MS  39174

I should be grading lab reports and exams!