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[APD] Re: Re: Closing up for the night - or Nyctinasty
> Clint Brearley 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 Bay replied:
> I see where you're going, and it's possible; but,
> because of circulatory system effiencies, plants tend
> to do things differently than animals.
> 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.
> 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.
Charley, I agree with all you wrote, 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. OK, true,
it does not usually happen from one organ to another in
plants. But 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. To do that
the osmotic potential must have been changed, which means
that solutes must have been pumped in/out of the
vacuole/cytoplasm of these cells. That is still movement.
Lets hypothesise that these solutes are a product of
photosynthesis then Clint's hypothesis would be partially
There's more to this hypothesis, accumulation and critical
levels, but I am feeling sleepy, and its time for my
nocturnal nyctinastic movement.
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