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[APD] Re: Re: Closing up for the night - or Nyctinasty

> > Clint said:
> > > What about this hypothesis. Perhaps during the
> > > day, stem plants stretch out their leaves so
> > > that they receive as much light as possible for
> > > photosynthesis. To do this the plants may need
> > > to actively transport biochemicals to certain
> > > areas of the leaf in order to make them move
> > > (which uses energy, but the trade-off is that
> > > much more energy is gained in return via 
> > > photosynthesis). Then when either the plants
> > > have gained enough energy or when
> > > the lights go out, which ever occurs first, the
> > > plants may stop transporting the biochemicals
> > > causing the leafs to close up, which is possibly
> > > just a lower energy position. This is only
> > > speculation, but it kinda makes some sense
> > > (doesn't it?)

> Charlie replied:
> > I see where you're going, and it's possible; but,
> > <snip, usually compounds locally produced, not
> > active transport>

Stephan added:
> <snip>, but how does it explain why some processes
> which are normally stimulated by light on/off still
> occur even before the lights are switched off?
> Or that they continue even if the lights are left
> on?
> I think that Clint does have a point in that there
> is a movement of a compound concerned with 
> nyctinasty. <snip>, what about localised movements
> within the same cell, or group of cells. Since it is
> turgor pressure in cells at the base of the leaf
> that opens/closes the leaf, water must be moving in
> and out of these cells.

I think Clint is right, in that multiple environmental
factors can regulate this movement, which answers the
question about how the motion happens (sometimes) even
*before* the light changes.  For example, most
terrestrial plants accumulate water at night (stomata
is closed because no photosynthesis occurs, so the
plant doesn't lose water), but then slowly lose water
all day (it's drier during the day, and water uptake
is much slower than water loss during photosynthesis).
This results in most terrestrial plants going through
a "mid-day depression" or "diapause" where they just
"shut down" (stomata closes) and no photosynthesis
occurs.  Then, as the hottest part of the day passes
(and the plant's water uptake catches up), plants perk
up and get back to work in the early evening.  So, in
addition to all the other nutrients and hormones
available, water and becomes a simple regulator to
create the "magic" where plants are net primary
producers in the morning and the evening, where they
are most efficient.

And, Clint is right in that plants can reach
'capacity' at many other levels, and merely shut down.
It makes sense to me that these movements may appear
to "anticipate" diurnal cycles, as we're only talking
about processes that need to "guess" within a twelve
hour period, give or take a couple hours, to be 
correct most of the time.

I guess I don't have much to add, since it's possible
Clint is *entirely* right.  I tend to believe any
"ratio" or "chemical decay" explanations before any
active transport ones, though, because it can often
be done most cheaply:  Plants prove that mere ratios
of even a few compounds offer an enormous degree of
control in triggering physiologic response.  ;-)


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