Gerry Deitzer gd3 at UMAIL.UMD.EDU
Mon Nov 17 12:02:31 EST 1997

Carl Pike wrote:
> In our class discussion of the photoperiodic control of flowering, after I
> presented the experiments showing (by means of night interruption) that
> plants are timing the length of the night, rather than the length of the
> day, a student asked **why** (in an evolutionary sense) it is the night
> that is timed.
> Any suggestions out there?
> Carl S. Pike                             (717) 291-3958
> Department of Biology                    FAX (717) 399-4548
> Franklin and Marshall College            Internet  C_PIKE at ACAD.FANDM.EDU
> P.O. Box 3003
> Lancaster, PA  17604-3003  USA

Hi Carl,

I guess that I should respond to this excellent question.  

First of all, the simple answer is that all organisms (plant, animal,
protist, whatever) evolved an ability to detect the relative amounts of
light and darkness in a 24 hour cycle.  The seasonal change in this
parameter is extremely stable with little or no variation from year to
year.  The amount of light is quite variable, while the amount of
darkness is not.  Dark is essentially dark and does not show much
variation in the degree of darkness.  It is; therefore, a much more
stable environmental cue than the amount of light.  

However, the timing of only the night length is a bit of an
oversimplification.  It is true for all short-day plants, but is not so
clear for long-day plants, which flower optimally in continuous light
and seldom respond to a brief night break.  However, the basis for the
photoperiodic induction of flowering is an endogenous circadian rhythm
(biological clock), the phase of which is set by either a light-on or a
light-off signal.  It is generally considered that the light-on signal
gets the clock going, while the light-off signal sets the phase.  A
night break re-sets the phase (new light-off signal) and so inhibits
flowering in short-day plants.  Long-day plants are inhibited by a long
dark period and generally require a long daylength extension rather than
a night break to promote flowering.  Apparently a night break is
insufficient to re-set the phase.  I realise that this is a bit
confusing and involves a great deal of hand-waving, but we simply do not
understand how light regulates the phase of a circadian rhythm that is
the basis for the photoperiodic response.

Gerry Deitzer
Department of Natural Resource Sciences
University of Maryland
College Park, MD 20742-4452

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