>At 1:31 PM -0500 11/15/97, 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.
Oh, heck. That is going waaaay back in plant physiology. If I remember
correctly, it has to do with a light-sensitive chemical (phytochrome?) that
is switched from one form (red) to another (far-red) in the light and which
slowly changes back in the dark. To trigger flowering, the right amount has
got to change back. The change from dark form to light form, though, can be
very rapid. This is why a flash of light at night will inhibit flowering in
"short-day" (actually, long-night) plants like poinsettias.
I guess this is useful to the plant because while your day might be
interrupted, once it's night, it's dark until morning.
M. Reed
(trying frantically now to remember how plants deal with moonlight....)
