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Plant flowering and the first day of Summer

Ross Koning koning at ECSUC.CTSTATEU.EDU
Tue Jun 8 10:05:03 EST 1999

At 6:37 AM -0400 6/8/99, I don't like spam! wrote:
>Out of curiosity, more than anything else, this question has mulled
>around in the back of my mind for some time now. Forgive me for the
>lack of more specific, scientific terms:
>As the longest day of the year approaches, the first day of Summer,
>the day to day change in daylight hours gets very small. Lacking a cue
>from a changing photoperiod, do fewer plant species in an area begin
>their flowering period around the first day of summer than at other
>times of the year? On a similar thread, are there "peak" times of the
>year for the number of plant species blooming based upon rapidly
>changing photoperiods, around the first day of Spring and Fall? I
>realize that the Spring bloomers are taking advantage of additional
>available light before the deciduous trees leaf out. But in areas
>where there are no deciduous seasons, are there peak blooming periods
>around these days of rapidly changing photoperiods?
>(No, this is not a test or essay question, simply a curiosity.)

The progression of photoperiods (and noctoperiods) is slow
and steady, not sudden.  The plants in the environment are
also now subjected to a range of disturbances to their
photoperiods (and noctoperiods) thanks to supplemental
lighting by humans.  So, the longest day is not particularly
special or vastly different from the previous or subsequent
days.  And, depending on what we do, the longest day might
be some other day in certain microenvironments.

Many plants have a critical photoperiod (or noctoperiod) that
initiates flowering.  Some plants only need one day of the
critical length of day/night to initiate flowering.  Others
need several days meeting the criteria before a flowering
response is initiated.  Yet others flower, but do so more
abundantly as the days meeting minimum criteria are increased.
So the type of response varies among plants.

Moreover, the critical day/night length is different for
various species.  Thus, the critical minimum might be exceeded
well before or well after the summer solstice.  It is also
known that various ecotypes of a single species vary in their
critical values.

These factors, taken together, mean that flowering time is
not a sudden phenomenon and that for many species it is a
much more gradual, if admittedly seasonal, process.

Finally, there are day-neutral species that flower when they
reach critical size rather than in response to day-length
changes.  Others respond only to water availability, etc.

So it is really a complex story and one needs to focus on
a single species, and must select certain model species if
you wish to find "sharply focused" flowering times.


Ross Koning                 | koning at ecsu.ctstateu.edu
Biology Department          | http://koning.ecsu.ctstateu.edu/
Eastern CT State University | phone: 860-465-5327
Willimantic, CT 06226 USA   | fax: 860-465-4479

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