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

[Charley Bay <charleyb123 at yahoo_com>]

> Charley Bay wrote:
> > As far as the biochemical change in plants driving
> > nictinasty, yes, several bioactive substances have
> > been identified that regulate leaf movement, and
> > some <snip>...

Clint Brearley responded:
> 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?)

I see where you're going, and it's possible; but,
because of circulatory system effiencies, plants tend
to do things differently than animals.

Remember that critters with a circulatory system and
internal organs *specifically in support of*
circulation (like a 'heart') tend to produce chemicals
and proteins that are transported *elsewhere* in the
organism to do a (chemistry) job.  Even large insects
have a 'heart' organ for this.  But, plants don't
usually do that since they must rely on diffusion
or some piggy-back to water circulation.  But, even
that internal water circulation only applies to
*vascular* plants.

It's true that some nutrients/chemicals in plants are
*mobile*:  They move from one spot in the plant to 
another where they are needed.  Nitrogen, phosphorus,
and potassium are always plant mobile nutrients, so
a plant lacking them always shows defficiency signs in
*older* growth *first* (since the nutrients are moved
to *new* growth, which is *always* more efficienct).
However, other nutrients like calcium and boron are
*always immobile*.  There is no chemistry that the
plant can use to get them to move.  Others like
sulphur, chloride, copper, zinc, manganese, iron,
and molybdenum are 'intermediate', which means they
are only mobile under certain circumstances (linked
to breakdown under low nigtrogen conditions of 
amino acids and proteins in older parts of the plant
for movement to younger parts of the plant).

In general, "big" molecules and compounds are *not*
mobile, or it's a lot of work/infrastructure to move
them (which is why vascular plants might have a phloem
system for transport of glucose, a very big molecule).

Unlike critters, plants don't tend to 'produce' a
compound in one spot to perform an effect in another
part of the organism, like a hormone.  While there 
are many dozens of hormones identified for critters, 
there have been only four, maybe five, hormones
identified for plants.  Rather, plants tend to rely
upon *ratios* of hormones and other chemicals locally
produced, and which are usually very environmentally
sensitive.  Some break down faster or slower in light,
in temperature, or in the presence of other stimuli.
It's the *ratios* of these chemicals in the local
part of the plant that causes dramatic physiologic
response (like growth form, or even motion).

And, remember chemistry can be *fast*.  Feel free
to review the chemistry of an ephedrine 'cascade'
to show how chemistry can run through your entire
body in a fraction of a second.

For example, levels for abscisic acid (one plant
hormone) tend to rise in the Autumn as plants shift
towards synthesizing 'storage' proteins to prepare for
winter (seasonal influence on synthesis, usually due
to daily photoperiods and sometimes temperature
variations).  As another example, indole-3-acetic acid
(another hormone) tends to be produced at the
terminal meristem (the 'tip' or growth bud of many
stem plants), and that inhibits lateral growth.  Since
the compound breaks down the farther it is from the
growing tip, its relatively high ratio at the growing
top gives things like conifers the 'cone-like' growth
form.

BTW, there's some cool plant hormone info at:
<http://www.plant-hormones.info/Index.htm>

So, my guess is that things like sunflowers that 
move dramatically to 'follow the sun' actually produce
compounds right where it bends (somewhere around the
receptacle or pedicel), and the compound either breaks
down very fast in light causing cells to shrink, or
is produced very fast in light causing the cells to
expand (usually through establishing an osmotic
gradient), and that causes the flower head to 'move'.
The shadow on one side of the flower head's 'base' is
the gradient that causes the chemistry to kick in.

Sorry for the long post.

--charley


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