New Ideas on the Central Role of Ethylene and Epinasty to Moving the Transpiration Stream - With a Note on Salicylic Acid
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or Philosophy are (at least sometimes) I think call apologists. So
what follows is a little bit of an attempt to reconcile part of the
previously posted theory that may seem to be inconsistent or not
fitting with experimental findings.
The main finding I want to deal with is the increase in Ethylene
caused by Auxin. If the speculations are correct, how does Auxin lead
to an increase in a signal that indicates low minerals and water
uptake when Auxin induces new roots in plants?
The answer may be that Ethylene is released by active meristematic
cells in the root or shoot that have less than enough minerals and
waters than is required for their location in the plants (twice as
much in root cells as in shoot cells). If you thus bathed the root
with essential minerals and water, exogenous Auxin should have little
or no effect on Ethylene production. Ethylene is an indication of
active meristematic tissue failing to get enough minerals and water.
Before exogenous Auxin activates the quiescent secondary meristems in
the roots, they do not produce much Auxin or Ethylene, but once
active, they are "required" to take in "profitable" or growth
conducive amounts of minerals and water. This would mean more than
enough minerals and water to support both it and a similar sized
meristematic cell in the shoot. In the shoot on the other hand as
mentioned in other places in this group, Ethylene is only made when
the active meristematic cell has less than enough minerals and water
to support growth period, just for itself, since it is not responsible
for gathering minerals and water for any cells but itself.
My contention is that Ethylene is a meristematic cells attempt to find
an alternate way of getting minerals and water. Auxin production by
the plant is an indication of growth conducive environmental
conditions for dividing cells via shoot gathered resources. Cytokinin
would be an indication of growth conducive environmental conditions or
resources gathered via the root (I realize conditions and resources
are two separate things but let me blur the distinction for now). In
the absence of Cytokinin or with low levels of it, Auxin is given the
"green light" to try to compliment the growth conducive levels of
sugar and gases with minerals and water, by lengthening the roots and
inducing new ones.
Only when this doesn't succeed, does a plant make Ethylene, which
tries alternative methods. Ethylene broadens roots instead of
lengthening. It causes root hairs to grow, increasing root surface
area, rather than induce new roots. It causes Epinasty of the leaves,
increasing the rate of flow of the Xylem and thus water and mineral
flow through the physical pumping action induced by leaves acting as
handles of a water pump and as sails as such, being blown up and down
by the wind. Thus Ethylene probably engages a "valve" in the Xylem
for this pump, either by inducing it's growth or reactivating it
(Ethylene levels probably peak in the plant every night anyway, so
"valves" that engage nightly are a distinct possibility in my mind).
Three other effects of Ethylene could also increase the minerals and
water available to meristematic cells. First Ethylene causes older
leaves to senesce, particularly leaves not making Auxin and thus no
longer gathering "profitable" or growth conducive amounts of sugar and
gases (also an absence of Cytokinin being made by the older leaves
would be an indication that the leaf is a mineral water sink and is
not supporting itself - although maybe older leaves don't make
Cytokinin period, nor older roots Auxin).
This senescence of older leaves of course, significantly decreases the
water and mineral load requirements.
Secondly though I never saw much discussion about this, just like
there are starch granules stored in vacuoles in the cells probably
under the direction of Auxin that are released by Gibberellin, there
must be water and minerals also stored in vacuoles under the direction
of Cytokinin and released by Ethylene.
Finally the last way Ethylene could help "feed the babies", i.e. get
the meristematic cells the water and minerals they need to grow, is by
the release of the sugars and gases that occur with the cannibalizing
of the no longer efficient older leaves (probably only occurring with
the added environmental condition of high levels of Gibberellin).
Thus just like Cytokinin and Auxin are needed for cell division,
Gibberellin and Ethylene are always needed "to pronounce judgment" on
a cell and lead it on a path of senescence. The sugar and gases can be
used to support the new root hairs or maybe a further retry at
lengthening the roots again.
Getting into Gibberellin and Cytokinin is the other part of the theory
which I'm not concentrating on here, so that is an aside more or less.
However I include it to hint at interdependence of all of four major
hormone pathways.
It is quite possible that Brassinosteroid, Abscisic Acid, Salicylic
Acid and an unknown eighth hormone are really involved in this system
instead of four hormones. In this case, each hormone would either be
an indication of abundance or scarcity of one nutrient group. Perhaps
roughly, Auxin is an indication of sugar abundance, Gibberellin -
sugar deficiency, Cytokinin - mineral abundance, Ethylene (or Hormone
X) - mineral deficiency, Brassinosteroid - gas abundance, Hormone X
(or Ethylene) gas deficiency, Salicylic Acid - water abundance (thus
found in high amounts in Willow Trees growing on river banks),
Abscisic Acid - water deficiency.
If you feel I'm cheating by sneaking in a big scheme at the end, its
probably because I just made it up and am excited enough to include
it :-))). I hope you don't find this disenguous. It is true I have
been kicking these schemes around in my head for 22+ years, but the
idea that Ethylene induces Epinasty as another way to move the
transpiration stream other than Van Der Waal forces of surface
tension, is a new one. As is seeing Salicylic Acid as a water
abundance signal.
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regular basis, say nightly, to continue transpiration at night even
when there is less evaporation. Thus the pumping action of the leaves
at night in the wind supplements the lower transpiration that occurs
at night due to evaporation, Van Der Waals Forces and surface tension
of water.
This might definitely explain why certain plant leaves (Nightshade
Genus plants?) droop at night and perk at day. There would be a
physiological profit to this if a continued flow of root exudate were
needed by these plants at night and this need could not be met by
transpiration alone.