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Sensing daylength

> Several of the enzymes that control functions within the plant's life
> are light sensitive. Although the plant pretty much generates them
> continually, their concentrations can only build up during the hours of
> darkness. ...
> - -Y-
> David A. Youngker

To follow up on this (and not to be pedantic, particularly with someone who
has forgotten more about aquaria than I will probably ever know), it's
primarily not actually enzymes building up, but rather a family of light
receptors called phytochromes that shift form upon exposure to light
(specifically red and far red light), and potentially some blue light
receptors as well, that sense light and the duration of daylight.  A good
flash of light in the middle of the night can indeed reset the clock on
plants that are sensing daylength (or rather nightlength).  Most of these
processes are most dramatic in temperate plants, although some tropical ones
seem to be able to sense small variations in daylength on the order of an
hour or two.  In tropical dry forests, some trees can tell the difference
between an 11 hour day and a 13 hour day and use those cues to determine (in
part) when to leaf out, bloom, etc.

Probably more stuff than you cared to know.