Nyctinasty

[Bill Williams]

Lots of people have wondered about nyctinasty, and I've wanted to do the
definitive experiment myself for some years, but I can't figure out quite what
it is :-)

The two prevailing hypotheses (at least in the plant rhythms world, of which I
am a distantly connected part) are the "Bunning hypothesis" and the "Darwin
hypothesis."  In his book "The Power of Movement in Plants," Charles Darwin
suggested that the leaves folded up to keep them from getting too cold at
night.  Typically, this was a very advanced hypothesis for its time, requiring
quite modern concepts of leaf energy-balance to test.  Unfortunately, under
most circumstances the folding can only increase leaf temperature a few tenths
of a degree; even the most severe circumstances (windless desert nights) only
give it a very slight advantage, and of course nyctinasty isn't typically found
in desert plants.  There is some evidence that leaf folding is adaptively
warming in some Adean giant rosette plants.

Bunning thought it was to protect the phytochrome system from giving erroneous
daylengths when the moon was full!  The phytochrome system is extremely
sensitive, and the full moon could definitely supply enough photons to screw it
up (don't know about the spectrum, though -- I'll have to ask my spouse, the
photobiologist).  I'm not sure anybody has specifically addressed this
hypothesis, but it seems to me that nyctinasty is an awful lot of work to go to
for a problem that could be solved in other ways (rhythmic sensitivity of
phytochrome, for example, for which I believe there is some evidence, albeit
controversial evidence).

Unfortunately, I don't know of a good review that specifically addresses
adaptive significance of nyctinasty -- I'd like to hear of one (maybe I'll have
to write one :-)  )

For the record, we're working on adaptive significance of circadian rhythms in
stomata.