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A Sketch of an 8 Part
Plant Hormone Theory
"Fools have no interest in
understanding; they only want to air their own opinions." Proverbs 18:2
NLT
"Whatever exists has already been named..."Ecclesiastes
6:10 NIV
Informally since 1986 and on the Web since 1996, I have
written several fairly different versions of comprehensive speculations on the
functions and behavior of plant hormones. In early 2008 I was reading the
Wikipedia articles on Jasmonates, and the article made me question the role I
had made for Auxin as the indicator of excess sugar. This was because by
Jasmonic Acid is involved in tuber formation and de-chlorophylling leaves
actions we might expect from Auxin. Presumably both these events occur because
of an excess of sugar. So if this is true, my new eight hormone scheme from
2007 needs rearranging again.
Furthermore on 08/12/2008 I read an article on
Brassinosteroid, causing me to reclassify again it as working with GA as a sugar
deficiency indicator and not as a mineral deficiency one. I now leave the
mineral deficiency signal up for question although the newly discovered
strigolactones may be the signal. Strigolactones help mediate the
interaction with symbiotic fungi that help the plant absorb minerals and inhibit
branching of the shoot. We might expect that from a mineral deficiency
signal.
That is mineral deficiency might cause a suppression of
growth and branching of the shoot and attempt to increase the uptake of minerals
through an increase in the hosting of symbiotic mineral absorbing fungi.
The table below is almost complete, however I removed
Salicylic Acid as a sign of Water abundance. In 1986 it was found to
reverse
ABA mediated closing of stomata which is why
I originally placed it there in the scheme of things. More recently it has
been found that when working alone, it closes stomata. This may be due to
it's role in pathogen defense. Stomata are open avenues to the interior of
the plant. It is known that Salicylic Acid is released.
So here's the break down:
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Deficiency
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Abundance
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Sugar
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Gibberellin/Brassinosteroid
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Jasmonic Acid
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Oxygen and Carbon Dioxide
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?
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Auxin
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Minerals
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Strigolactones
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Cytokinin
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Water
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Abscisic Acid
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Ethylene?
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One of the problems with this scheme is there appears to be
some question about whether Brassinosteroid increases root growth or inhibits
it. If it inhibits it short term and increase it long term, than this
understandable as just the behavior we would expect from a sugar shortage
message. Roots don't make sugar, and should be the first place to
experience sugar deficiency. The hormone may be an attempt to restart root
growth on the long term. However, on the short run it might want to change
the behavior of the shoot to bring down more sugar through the stem to the
roots. It might want also minimize any increase in deficiency the root
might be experiencing, through the means of inhibiting it's root growth.
Another problem, is seen when I explain below in the
big table below that going against common belief, I believe Jasmonic and
Salicylic Acid are needed to be present in addition to Auxin and Cytokinin for
cell division. However from my now rather ancient researches into the matter I
never found such a reference or indication...Additionally those two hormones do
not seem to be involved in crown gall formation as we would anticipate if these
hormones were additional crucial indicator "green lights" for cell division.
Keeping these in mind we might postulate two other ways to
organize the overall roles of the hormones. One is to think for each of
the four major nutrients hormones are released when there is not enough of the
nutrient, a different one is released when there are growable amounts and
finally a still different hormone is released when there is too much of any
nutrient. You then might end up with the following table:
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Nutrient
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Deficiency Hormone
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Growable Amount Hormone
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Excess Hormone
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Sugar
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Gibberellin/Brassinostreroid
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Auxin
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Jasmonic Acid
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Gases
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?
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Auxin
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Ethylene?
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Water
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Abscisic Acid
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?
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Ethylene
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Minerals
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Strigolactones
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Cytokinin
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Abscisic Acid
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A third possible scenario is to return to a very simple
system I postulated some time ago. Auxin would be released when a root or
shoot meristematic cell finds that it contains more than enough shoot derived
nutrients mainly sugar, and all other environmental conditions are favorable for
growth. Cytokinin would be made when meristematic cells are bathed in more
than enough nutrients of the sort normally provided by the root, mainly water
and minerals and all other conditions are favorable for growth. Conversely
Gibberellin/Brassinostreroid would be made when mature cells have less than
enough shoot nutrients, i.e. sugar and Oxygen to survive especially if
environmental conditions are poor. Finally Ethylene might be released when
mature cells are receiving less than enough nutrients normally received from the
roots, mainly minerals and water, to support life at all, thus senescence of the
cell is warranted. Again this effect may be accentuated by poor
environmental conditions.
In this scheme Abscisic Acid might fulfill the role akin to
adrenaline or cortisol in animals, signaling a need emergency action under most
kinds of rapidly developing environmental stress, not just water shortages.
Complimentarily, Salicylic Acid may be the hormone released when things are
running normally and no special rapid response is needed from the plant.
It might be the "feel good" hormone.
The problem with this scheme has been pointed out that GA is
made by meristematic cells not mature ones. This is not fatal to the
speculations, but does kind of make them a little less symmetrical and
compelling.
A third table emerges from this speculation:
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Root Derived Nutrient Abundance + Good
Environmental Conditions
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Root Derived Nutrient Deficiency + Bad
Environmental Conditions
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Shoot Derived
Nutrient Abundance + Good Environmental Conditions
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Auxin & Cytokinin - produces cell division
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1st Cytokinin then 2nd Gibberellin/BA - produces 1st
root broadening then older root cell senescence
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Shoot Derived
Nutrient Deficiency + Bad Environmental Conditions
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1st Auxin then 2nd Ethylene - produces 1st stem
lengthening then older stem cell senescence
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GA/BA & Ethylene - produces cell senescence
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One thing not discussed so far is that root oxygen is
probably mostly obtained from the soil surrounding the roots, not from the
leaves. This resolves the perplexing property of Ethylene causing the
senescence of leaves because the shoot and leaves aren't the providers of O2
for the root. So the plant wouldn't be shooting itself in the foot if it were to
trim older inefficient leaves and stems and the resources freed could be used
for making oxygen harvesting adventitious roots under anoxia and flooding
conditions.
Hormone Table - Under Construction
Note the information in the table is
constructed to defend the speculations of the first table and hasn't yet been
modified to provide the equal treatment to all the three ways of understanding hormones.
| Chemical Structure |
Synthesis and Transport |
Proven Effects |
Insight Provided by
Genetic Manipulation |
Speculative
Explanation |
Synthesis; Exogenous
Treatment; Inhibition and Stimulation; Storage; Nutrient and Hormone
Attraction and Repulsion; Apical Dominance; Hormone Transport; Cell
Division; Senescence; Specific to Overall Role |
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Abscisic Acid
(ABA)

ABA's main role is clearly resistance to drought conditions -
A Wikimedia Commons Image
Speculative Overall Role: Signal of Water Deficiency

ABA May Figure Prominently in Desert Plants Like Cactus -
A Wikimedia Picture
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- Under consistent levels of desiccation, ABA levels
normally peak at night.1
- Closes stomata via ABA synthesized in the root.2
- Induce by drought.3
- ABA coming up from the root, synergizes with auxin
coming down from the apex to produce apical dominance.12
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- Closes stomata via ABA synthesized in the root.2
- At high concentrations, inhibits root growth, but
after removal. stimulates greater root lengthening and branching
than controls.4
- Mediate adaptation to salt.5
- Mediate adaptation to heat.6
- Mediate adaptation to cold.7
- Inhibits Kinetin nucleotide formation8
- Down regulates enzymes needed for photosynthesis9
- Induces bud dormancy. Lower levels of ABA is
associated with dormancy termination in winterized plants.10
- ABA Promotes tuberization.11
- ABA coming up from the root, synergizes with auxin
coming down from the apex to produce apical dominance.12
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Actions of ABA attempt to mitigate drought
effects.
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Water loss is slowed by closing of the stomata.
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Shoot growth does not rectify water deficiency,
however, new root growth may.
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High salt levels may water stress plants.
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Heat stress may induce water stress.
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Cold may make water less available to plants.
Transpiration cools plants, so this might be avoided by the ABA
induced stomata closing.
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The photosynthesis process uses twice as much
water as it makes and Kinetin stimulates this.
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-
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Synthesis: Dry plants should have
high levels of ABA, well watered plants, low levels. Like abundance
signals ABA may be mostly made in meristematic cells and much less so as
cells mature. (Or for real theoretical beauty, deficiency hormones
should be made mostly in mature cells and much less so in meristematic
cells). ABA should be made when a cell has less than enough water
to support both it any cell dependent on it for water acquisition.
Thus ABA is an indication that water exists in less than enough amounts
to continue the plant at its current size, thus the plant must use
emergency stores of water, find new sources of the liquid and cut down
on water sinks.
Exogenous Treatment: High levels of
exogenously applied ABA should induce SA synthesis, because many of
ABA's effects may be to increase water levels within the plant, if only
temporarily. This may include making dormant reactions that
are normally dependent on water.
Inhibition and Stimulation: ABA
should encourage root and new root growth, but inhibit shoot growth and
even encourage shoot and leaf senescence.
Storage: ABA should cause the
emptying of stored water reserves found in vacuoles or tubers.
Nutrient and Hormone Attraction and Repulsion:
ABA should generally push all nutrients and abundance signals/hormones
out of cells. ABA should attract the deficiency signals/hormones,
GA, ET and BA, leading to positive feedback and cell senescence.
Apical Dominance: Should break root
apical dominance because low water levels are an indication of poor
performance by the currently dominant apical root. ABA may
strengthen the currently dominant shoot apex in order not to encourage
any new shoot growth which would be a further sink on water levels.
Hormone Transport: Water deficiency,
on average should be detected in the leaves first, the point furthest
from the source of water. Water may be repelled from tissues high
in ABA, thus ABA may be built up in the roots, in order to force water
toward the shoots.
Cell Division: Although it may
encourage it in the roots, if it is inducing new ones, ABA should
generally inhibit cell division, as a water deficient plant is in no
condition to expand.
Senescence: Just as I am
hypothesizing that SA, JA, IAA and CK all need to be present to induce
cell division, ABA, GA, ET and BA may all need to be present for cell
senescence to proceed. ABA should encourage senescence,
particularly of shoot tissue whose nutrients can be cannibalized and
used to make more water absorbing root tissue.
Specific to a Water Deficiency Signal:
Since GA causes lengthening, ET broadening, BA lengthening, what's left
is ABA should cause cell and tissue broadening when it induces growth if
it does. (I believe I may have seen such a finding but I have to find
the reference again).
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Jasmonates
(JA)

Jasmonates are somewhat similar chemically to prostaglandins, which are
chemical messengers found in animals. both are made from fatty
acids. However prostaglandins always contain 20 carbons and JA
only contains 12.
-
A Wikimedia Commons Image
Speculative Overall Role: Signal of
Sugar Abundance

JA prompts plants to store excess sugar in tubers like the Potato -
A Wikimedia Commons Image
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- JA exist at high levels in flowers and developing
pericarps13
- JA exist at high levels in the chloroplasts of
illuminated plants13
- JA increases in response to mechanical stress and
produce tendril coiling14
- Jasmonate is made in response to wounding.15
- Jasmonates levels increase along with those of ABA
under desiccation conditions.19
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- JA increases in response to mechanical stress and
produce tendril coiling14
- JA is involved in the tuber storage proteins
system and a derivative may stimulate tuber formation.16
17
- JA induces
chlorosis
inhibits photosynthesis gene transcription13
- What JA does in flower and fruits is unknown, but
it may be involved in the converting green leaf cells contents into
seed storage proteins, carotenoid and the sugars.13
- JA and ethylene appear to act in tandem to enact
plant defense response.18
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- Flowers have high amounts of sugar available in
order to produce nectar.
- Chloroplast have high amounts of sugar due to
photosynthesis.
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-
Bruising and wounding ruptures cells and releases
vacuole sequestered sugars. Also neighboring plant cell
reaction to wounding may be to release amylases (by inducing GA?)
which increases sugar levels in the inter cell spaces.
-
Storage proteins and tubers are induced by JA in
order to store excess sugar.
-
Chlorosis induced by JA, is a negative feedback
loop to cannibalize excess photosynthesis machinery. JA is an
indication of excess photosynthesis capacity.
-
Fruits and developing seeds may use JA to store
necessary sugar reserves in seeds and fruit.
-
-
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Synthesis: Well lit plants should
have high JA levels, darkened plants should have low levels. JA should
be mostly made in meristematic cells and much less so as cells mature.
JA should be made when a cell has more than enough sugar to support both
it any cell dependent on it for sugar acquisition. Thus JA is
always an indication that growth amounts of sugar exist and if
conditions warrant, that the plant has enough sugar to grow at least in
the specific cell where the JA is. (Shoot cells are responsible for
acquiring sugar for both it and similar size cells in the root whereas a
root cell is only responsible to itself for it own sugar level).
Exogenous Treatment: Should induce
GA, because JA up regulates various processes limited by sugar levels.
Exogenously applying JA leads the plant to falsely believe that it has
high levels of sugar, thus engaging all sorts of reactions that use
sugar, thus further depleting what may simply be a homeostatic level of
existing sugar and moving this level into the deficiency range.
Inhibition and Stimulation: Should
Induces new root growth, just like Auxin. If Auxin is also present, JA
should inhibit shoot growth because high JA and IAA levels are an
indication of at least a short term lack of need to expand the shoots.
Storage: should cause sugar to be
stored in proteins and tubers for less propitious times.
Nutrient and Hormone Attraction and Repulsion:
Should attract all nutrients and abundance signals to a cell and repulse
deficiency signals.
Apical Dominance: Should induce shoot
apical dominance along with Auxin, however the possibility exists for
two dominant apices if one is particularly good at sugar production (in
the light) and one good at oxygen harvesting (in the wind). May
break root apical dominances under conditions of low CK and SA.
Hormone Transport: May be expected to
travel in the direction of the roots, away from the shoots and
particularly the shoot meristems. Regions of a cell or tissue or
plant part that contains high JA, may particularly attract sugar and
transport of sugar may follow active JA transport down a plant.
Cell Division: Is actually necessary
for cell division along with Auxin, Cytokinin and Salicylic acid.
If there are some plant callus lines that will divide with only Auxin
and Cytokinin present it is because these cell lines are mutants that
produce SA and JA natively, or these other hormones are unknowingly
being included with the "other" nutrients/vitamins that are also added
to calluses to get them to divide.
Senescence: Should protect plant
tissue from senescence, particularly root tissue.
Specific to a Sugar Abundance Signal:
Because ET and IAA show complimentary growth patterns with ET broadening
and IAA lengthening and the same is true for CK (broadening) and BA
(lengthening), we might expect that JA should show a complimentary
growth pattern to GA's cell lengthening, thus JA should broaden cells
and plant tissue.
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Gibberellin
(GA)

GA1 - there are many different gibberellin molecules -
A
WIkimedia Commons Image
Speculative Overall Role: Signal of
Sugar Deficiency

The so called "Foolish Seedling" a disease first characterized in Japan
which is due to too much GA - Image from
here
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- GA levels go up in the dark when Sugar cannot be
manufactured and down in the light.20
- "The highest content of GA was characteristic of
leaves in the period of growth cessation."31
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- Promotes shoot and flower stem lengthening
especially in the dark. 21
22 23
- Greatly promotes bud growth.24
-
GA reverses ABA effects on growth inhibition and
dormancy.
- Dissolves stored starch.26
- At low concentrations GA (Gibberellin A3) and
other Gibberellins promote lateral root growth but high
concentrations markedly inhibit it.27
28
- GA stimulates flowering some plants.29
- GA inhibits tuberization.30 11
- GA in concert with Auxin induce phloem
differentiation.32
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GA increases stem length in the dark, to move
shoots out of the shade or the ground, into the light and into sugar
production. GA induces shoot growth to increase sugar levels.
Breaks bud dormancy to increase sugar levels.
GA temporarily increase sugar levels by dissolving
stored starch.
GA inhibits root growth which is counterproductive
to increased sugar levels.
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Synthesis: Darkened plants should
have high levels of GA, well lighted plants, low levels. Like abundance
signals GA may be mostly made in meristematic cells and much less so as
cells mature. (Or for real theoretical beauty, deficiency hormones
should be made mostly in mature cells and much less so in meristematic
cells). GA should be made when a cell has less than enough sugar
to support both it any cell dependent on it for sugar acquisition.
Thus GA is an indication that sugar exists in less than enough amounts
to continue the plant at its current size, thus the plant must use
emergency stores of starch, find new sources of the molecule and cut
down on its sinks.
Exogenous Treatment: High levels of
exogenously applied GA should induce JA synthesis, because many of GA's
effects may be to increase sugar levels within the plant, if only
temporarily. This may include making dormant reactions that
normally depend on sugar.
Inhibition and Stimulation: GA should
encourage shoot and new shoot growth, but inhibit root growth and even
encourage root senescence. This may be a particularly apparent
when ethylene levels are high and ABA and BA levels low as this is an
indication that resources need to rerouted from the root to the shoot.
Storage: GA should cause the emptying
of stored sugar reserves found in vacuoles or tubers.
Nutrient and Hormone Attraction and Repulsion:
GA should generally push all nutrients and abundance signals/hormones
out of cells. GA should attract the deficiency signals/hormones,
ABA, ET and BA, leading to positive feedback and cell senescence.
Apical Dominance: GA should break
shoot apical dominance because low sugar levels are an indication of
poor performance by the currently dominant apical shoot. GA may
strengthen the currently dominant root apices in order not to encourage
any new root growth which would be a further sink on sugar levels.
Hormone Transport: Sugar deficiency,
on average should be detected in the lroots first, the point furthest
from the source of wsugar. Sugar may be repelled from tissues high
in GA, thus GA may be built up in the shoots, in order to force sugar
toward the roots.
Cell Division: Although it may
encourage it in the shoots, if it is inducing new ones, GA should
generally inhibit cell division, as a sugar deficient plant is in no
condition to expand.
Senescence: Just as I am
hypothesizing that SA, JA, IAA and CK all need to be present to induce
cell division, ABA, GA, ET and BA may all need to be present for cell
senescence to proceed. GA should encourage senescence,
particularly of root tissue whose nutrients can be cannibalized and used
to make more sugar producing shoot and leaf tissue.
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Auxin
(IAA and others)

Indol-3-ylacetic Acid , the most common Auxin -
A Wikimedia Commons Image
Speculative Overall Role: Signal of
Oxygen Abundance

Crown galls are caused by Agrobacterium tumefaciens bacteria; they
produce and excrete Auxin and Cytokinin and I argue Salicylic acid and
jasmonic acid, which interfere with normal cell division and cause
largely undifferentiated calluses of cells -
A
Wikimedia Image (Caption is also partially from Wikipedia - see
here)
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- Made mostly in meristematic cells of the shoot and root
decreasing as cells mature and age.
34 35
- More is made in the shoot meristem than the root.34
35
- Overall levels of auxin peak during the day.36
- An internal gradient within the ovary effects the
development of the of the embryo.
44
46
- The developing seed releases auxin, stimulating
fruit growth.44
47
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- Auxins induces new adventitious root development
and growth.33
-
Involved in shoot and root phototropism.
(The Cholodny-Went theory).37
-
May mediate positive root and negative
shoot gravitropism.38
50
-
Induce xylem differentiation.39
-
Auxin in concert with GA induce phloem
differentiation.32
-
Inhibits secondary buds below site of synthesis
producing apical dominance.40
41
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High levels of auxins induce ethylene synthesis
especially in the roots.42
-
Induce cell lengthening.43
-
An internal gradient within the ovary effects the
development of the of the embryo.
44
46
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The developing seed releases auxin, stimulating
fruit growth.44
47
-
Young leaves strongly attract auxin preventing
new leaves from growing out of the meristem too soon.44
48
-
When Auxins are no longer produced by leaf, this
initiates leaf senescence and abscission.49
-
The Shade-Avoidance Effect.44
51
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- Auxins is integral to flower formation.
Knockout auxin mutants do not flower. 44
45
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-
Auxin induces new root growth to compliment
oxygen abundance. Since oxygen is mostly taken in by the
leaves, abundance of it shifts growth away from the leaves to roots.
- All the abundance signals are indications that certain meristems
are worth sending nutrients to, e.g. investing in, and the strongest
candidate in many species wins out to the exclusion of all others.
Although possibly there is one dominant shoot apex for all four
nutrient groups, water, minerals, sugar and gases making a total of
four apices. Perhaps most of the time, the mineral and water apex
and the sugar and gas apices are the same making two dominant
apices, one for the root and one for the shoot.
-
-
-
-
-
Opposite of what is expected, unless secondary
xylem remains a hollow tube for the transport of oxygen, not the up
flow of water. However, maybe xylem differentiation may be to
bring water and minerals to compliment the oxygen indicated by Auxin
and the sugar indicated by JA.
-
-
Cuts down on excess growth of the shoot.
Excess oxygen already exists, so the mission is that it needs to be
complimented by excess sugar, minerals and water. Perhaps
producing these complimentary nutrients is not accomplished by
letting secondary shoot buds grow in an uncontrolled way, but by a
second shoot apex establishing itself as the best possible site of
sugar producing and putting all other resources into establishing
productive roots (basically your guess is as good as mine :-).
-
Opposite what is expected if speculative overall
role is true. It is known the Auxin is made in the highest
levels in the shoot apex and this is often the most light exposed
part of the plant. Perhaps light induces an increase in Auxin
biosynthesis, due to the photosynthesis reaction producing oxygen,
but that light induces a greater active transport away of Auxin than
darkness such that real levels appear to fall in illuminated plant
parts.
-
Cell lengthening requires processes that require a
lot of oxygen? Excess oxygen is sequestered in vacuoles, blowing a
cell up like a balloon?
-
Perhaps flower initiation normally takes a lot of
respiration, so point of highest concentration of Auxin, the
dominant oxygen apex, is the best place to initiate a flower.
-
-
-
-
A leaf no longer producing Auxin, may no longer be
"pulling its weight" in terms of oxygen harvesting and thus needs to
be excised.
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Synthesis: Well aerated plants should
have high IAA levels, anoxia treated plants should have low levels. IAA
should be mostly made in meristematic cells and much less so as cells
mature. IAA should be made when a cell has more than enough oxygen
to support both it any cell dependent on it for oxygen acquisition.
Thus IAA is always an indication that growth amounts of oxygen are being
procured by the plant and if conditions warrant, that the plant has
enough oxygen to grow at least in the specific cell where the IAA is.
(Shoot cells are responsible for acquiring oxygen for both it and some
of the oxygen for a similar size cell in the root. Whereas a root
cell is only responsible to itself for it own oxygen level and may even
obtain some oxygen from spaces between soil particles).
Exogenous Treatment: Should induce
ET, because IAA up regulates various processes limited by oxygen.
Exogenously applying IAA leads the plant to falsely believe that it has
high levels of oxygen, thus engaging all sorts of reactions that use O2,
thus further depleting what may simply be a homeostatic level of
existing O2 and moving this level into the deficiency range.
Inhibition and Stimulation: Should
Induces new root growth, just like JA. Especially if JA is also present,
IAA should inhibit shoot growth because high JA and IAA levels are an
indication of at least a short term lack of need to expand the shoots.
Storage: should cause O2 to be stored
in proteins and tubers for less propitious times.
Nutrient and Hormone Attraction and Repulsion:
Should attract all nutrients and abundance hormones/signals to a cell
and repel deficiency hormones/signals.
Apical Dominance: Should induce shoot
apical dominance along with JA, however the possibility exists for two
dominant apices if one is particularly good at sugar production (in the
light) and one good at oxygen harvesting (in the wind). May break
root apical dominances under conditions of low CK and SA.
Hormone Transport: May be expected to
travel in the direction of the roots, away from the shoots and
particularly the shoot meristems. Regions of a cell or tissue or
plant part that contains high IAA, may particularly attract O2 and
transport of O2 may follow active IAA transport down a plant.
Cell Division: Along with
Cytokinin and JA and Salicylic acid, IAA should be necessary for cell
division. If there are some plant callus lines that will divide
with only Auxin and Cytokinin present it is because these cell lines are
mutants that produce SA and JA natively. Alternatively these
latter two hormones are unknowingly being included with "other"
nutrients/vitamins that are also added to calluses to get them to
divide.
Senescence: Should protect plant
tissue from senescence, particularly root tissue.
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Ethylene
(ET)

Ethylene is an extremely simple gas -
A
Wikimedia Commons Image
Speculative Overall Role: ?

The earliest discovery of Ethylene occurred
when when lamp post near a green house
caused wilting and leaf abscission. A Wikimedia Commons Image
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- Induced by high levels of Auxin, especially in
the roots but this can be moderated by red light which
characteristic of shading.55
- Ethylene levels increase during flooding,
probably due to entrapment rather anoxia. Most plant appear to
have a net inhibition of Ethylene production under anoxic or carbon
dioxide deficient conditions.56
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-
Promotes the ripening of fruit with climacteric
respiration releasing additional ethylene.52
-
Broadens/thickens plant parts.53
54
-
Inhibits leaf expansion.54
-
Inhibits geotropism.55
-
Inhibits Auxin transport55
and production?
-
Induces leaf, fruits, and flower petal
abscission.52
-
Stimulates seed germination.52
57
-
Flooding produces the epinasty reaction through
Ethylene, where leaf surfaces deliberately grow from a position
perpendicular to the stem to one which is more horizontal.
52 58
-
Induces air spaces called Aerenchyma used for gas
diffusion in roots during flooding of non-water based plants.59
-
Carbon Dioxide inhibits Ethylene action.58
-
Inhibits embryogenesis of cell cultures.60
-
Induces root hair growth.61
-
Ethylene upregulates auxin biosynthesis at least
in the roots.61
-
Flood induce ethylene sensitizes plants to the
existing steady Auxin levels, inducing adventitious roots formation.62
-
Induces flower formation in some species.52
63
-
Etephon (ethylene precursor) has a dual role in
tuberization. It promotes already formed tubers by inhibiting
stolon growth. Differently though it inhibits the formation of new
tubers.64
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-
-
-
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-
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Epinastic leaves act like the handles of water
pumps to actively pump up flooded water from roots so that it can be
evaporated from the leaves and the roots move away more quickly from
being water logged. Also or may be instead, leaves parallel to
the stem have a greater amount of transpiration than leaves
perpendicular to the stem because they flap up and down in the wind,
which increases evaporation rates.
-
-
-
-
-
-
Prop roots are like snorkels. They are
hollow and let the water logged roots more easily get oxygen.
-
The flooding induced air spaces in the roots
also serve to increase aeration of the roots and remove excess
Ethylene.
|
Synthesis:
Exogenous Treatment:
Inhibition and Stimulation:
Storage:
Nutrient and Hormone Attraction and Repulsion:
Apical Dominance:
Hormone Transport:
Cell Division:
Senescence: Just as I am
hypothesizing that SA, JA, IAA and CK all need to be present to induce
cell division, ABA, GA, ET and BA may all need to be present for cell
senescence to proceed. ET should encourage senescence,
particularly of root tissue whose nutrients can be cannibalized and used
to make more oxygen harvesting shoot and leaf tissue.
|
|
Cytokinin
((CK)

Zeatin - the most common Cytokinin -
A Wikimedia
Commons Image
Speculative Overall Role: Signal of Mineral Abundance e.g. Nitrogen,
Phosphate, Potassium etc.

Cytokinin is synthesized in the greatest amounts in root tips -
A Wikimedia Commons Image
|
- Made in high amounts in dividing shoot
meristematic cells.
- Made in the highest concentrations in the root meristematic cells.
|
-
Exogenous CK inhibits senescence of leaves.65
66
67
-
Induces new shoots in undifferentiated calluses in
stumps whose stems have been cut down or off.
-
Is integral to differentiation of the shoot
meristem.
-
Stimulates the development of lateral buds and
branching
-
Induces cell broadening.
-
Integral to root differentiation
-
Integral to leaf formation
-
Along with Auxin, necessary to be present to
induce cell division.
-
Integral to chloroplast development
|
|
-
Preserves shoots in order to compliment minerals
with sugar and gases. May actually inhibit root growth and
induce root senescence, just as Jasmonic Acid induces chlorosis.
-
Attempts to compliment root derived mineral
abundance with shoot derived nutrients of sugars and gases.
-
-
-
- CK is a signal that certain meristems cells are
good bets for sending resources too. CK is in effect an
indication that these cells are producing the amounts of minerals
they supposed to and enough to warrant growth.
-
-
-
-
|
Synthesis: Well fertilized plants
should have high CK levels, plants living in poor soils should have low
levels. CK should be mostly made in meristematic cells and much less so
as cells mature. CK should be made when a cell has more than
enough essential minerals to support both it any cell dependent on it
for mineral acquisition. Thus CK is always an indication that
growth amounts of minerals are being procured by the plant and if
conditions warrant, that the plant has enough minerals to grow at least
in the specific cell where the CK is. (Root cells are responsible for
acquiring minerals for both it and similar size cells in the root.
Conversely a shoot cell is only responsible to itself for it own mineral
nutrition levels).
Exogenous Treatment: Should induce
BA, because CK up regulates various processes limited by minerals.
Exogenously applying CK leads the plant to falsely believe that it has
high levels of minerals, thus engaging all sorts of reactions that use
or are normally limited by mineral levels, thus further depleting what
may simply be a homeostatic level of the existing fertilizers and moving
this level into the deficiency range.
Inhibition and Stimulation: Should
induces new shoot growth, just like SA. Especially if SA is also
present, CK should inhibit root growth because high SA and CK levels are
an indication of at least a short term lack of need to expand the roots.
Storage: CK should cause excess
minerals to be stored in vacuoles, storage proteins and tubers for less
propitious times.
Nutrient and Hormone Attraction and Repulsion:
Should attract all nutrients and abundance hormones/signals to a cell
and repel deficiency hormones/signals.
Apical Dominance: Should induce root
apical dominance along with SA, however the possibility exists for two
dominant apices if one is particularly good at fertilizer absorption (in
good soil) and one good at water harvesting (in the moist part of the
soil). CK may break shoot apical dominances under conditions of
low JA and IAA.
Hormone Transport: CK may be expected
to travel in the direction of the shoots, away from the roots and
particularly away from root meristems. Regions of a cell or tissue
or plant part that contains high CK, may particularly attract fertilizer
type minerals and transport of important minerals may follow active or
passive CK transport up a plant in the xylem or other tissue.
Cell Division: Along with IAA
and JA and Salicylic acid, CK should be necessary for cell division.
If there are some plant callus lines that will divide with only Auxin
and Cytokinin present it is because these cell lines are mutants that
produce SA and JA natively. Alternatively these latter two
hormones are unknowingly being included with "other" nutrients/vitamins
that are also added to calluses to get them to divide.
Senescence: Should protect plant
tissue from senescence, particularly shoot tissue.
|
|
Brassinolides
(BA)

Brassinolide was the first Brassinosteroid discovered -
A
Wikimedia Commons Image
Speculative Overall Role:
Signal of Mineral Deficiency e.g. Nitrogen, Phosphate, Potassium
etc.

Brassinosteroids were first discovered in Brassinus napus, Rapeseed,
from which comes Canola Oil (from a variety). Brassins are from the
Mustard Family -
A
Wikimedia Commons Image
|
- "One well-supported hypothesis is that all tissues
produce BRs, since BR biosynthetic and signal transduction genes are
expressed in a wide range of plant organs, and short-distance
activity of the hormones also supports this."
- BA is transported acropetally (upward).
|
- Promotes epinasty through synthesis of Ethylene.63
-
Inhibit leaf abscission.
- Plants found deficient in brassinolides suffer
from
dwarfism.
- Promotes cell expansion and cell elongation; works
with
Auxin
to do so
-
Promote leaf senescence and accelerates senescence
in dying tissue cultured cells; delayed senescence in BR mutants
supports that this action may be biologically relevant
-
Provide protection to plants during chilling and
drought stress
- Low levels of BA promotes root lengthening
independent of Auxin and Ethylene. 51
- Higher BA levels inhibits root growth.
51
- Promote apical dominance
-
Enhance seed germination
-
Enhance gravitropism
- Increases the production of ethylene
- Prevents premature abscission of fruit
- Increases the yield of Wheat and Rice
|
- Plants found deficient in brassinolides suffer
from
dwarfism.
|
- BA does this to maximize the level of sugar coming from the
leaves. Leaf abscission would cut down on some sugar production.
- Brassinolide encourages stem lengthening just like Gibberellin,
because the signal could be an indication the plant's leaves are in
the shade and thus the stem needs to lengthened to move it back into
the sun.
- All the deficiency hormones should speed senescence in those
plant parts that are sinks and net users of their respective
nutrients, but should preserve those plant parts that are harvesters
or net producers of the nutrients they represent.
- All the deficiency hormones may do this by causing plant cells
to either senesce (if they are in bad shape) of go into hibernation.
- Most sugar deficiencies should occur in the roots, so this make
sense.
|
Note BA actually appears to be involved in the GA
pathway whether as the principal and GA being a secondary messenger or
vice versa. From the evidence BA is like GA, actually appears be a
sugar deficiency hormone.
Synthesis: If BA is a sugar deficiency signal,
plants grown in the shade or with a lot of darkness, should have high
levels of BA. Alternatively well illuminated plants should show
low levels. BA should be made when a cell has less than enough sugar to
support both it any cell dependent on it for sugar acquisition.
Thus BA is an indication that sugar levels exists in less than enough
amounts to continue the plant at its current size, thus the plant must
use emergency stores of sugar (starch), find new sources of light and
cut down on the sugar sinks, by for instance inhibiting root growth.
Exogenous Treatment: If BA is truly a sugar
deficiency hormone, high levels of exogenously applied BA should induce
JA synthesis, because many of BA's effects may be to raise sugar levels
within the plant, if only temporarily.
Inhibition and Stimulation: BA should inhibit root
and new root growth and even encourage root senescence. It should
encourage shoot growth, preserve leaves from senescing and even
encourage new shoot growth initiation. This may be a particularly
apparent when ABA levels are low and GA and ET levels high as this is an
indication that resources need to rerouted from the shoot to the root.
Storage: If BA is really a sugar deficiency signal,
it should cause the emptying of stored starch reserves found in vacuoles
or tubers if such things exist.
Nutrient and Hormone Attraction and Repulsion: BA
should generally push all nutrients and abundance signals/hormones out
of cells. BA should attract the deficiency signals/hormones, ABA,
GA and ET, leading to positive feedback and cell senescence.
Apical Dominance: BA should break shoot apical
dominance because low sugar levels are an indication of poor performance
by the currently dominant apical shoot. BA may strengthen the
currently dominant root apice in order not to encourage any new root
growth which would be a further sink on essential sugar levels.
Hormone Transport: sugar deficiency, on average
should be detected in the roots first, the point furthest from the
source of mineral harvesting, the shoot.
Cell Division: Although it may encourage it in the
shoot, if it is inducing new ones, BA should generally inhibit cell
division, as a sugar deficient plant is in no condition to expand.
Senescence: Just as I am hypothesizing that SA, JA,
IAA and CK all need to be present to induce cell division, ABA, ET , BA
and GA (with BA and GA working together) may all need to be present for
cell senescence to proceed. BA should encourage senescence,
particularly of root tissue whose nutrients can be cannibalized and used
to make more shoots.
Specific to a Sugar Deficiency Signal: Should induce
C4 photosynthesis which is a more efficient but more risky form of
photosynthesis/sugar making. It's more risky because it causes the
buildup of poisons in the plant.
GA and BA may be part of the same chemical hormone cascade pathway.
|
|
Salicylates (SAs)

Salicylic Acid
Speculative Overall Role:
?

|
- Decreases with increasing salinity
- Found in high amounts in Willow bark
|
-
Reverses ABA induced opening of stomata.
-
Closes stomata
-
Induces future resistance to pathogen infection
when released during infection
-
Inhibits Ethylene Synthesis
-
Inhibits seed germination
-
Salicylic Acid causes
"global repression of
auxin-related genes...
and inhibition
of auxin responses"
-
"Phenolic compounds, viz., trans-cinnamic
acid, chlorogenic acid, ferulic acid, salicylic acid, tannic acid
and quercetin when applied with ABA, antagonize ABA action and
restore normal seedling growth."
|
|
-
-
-
-
-
-
-
Inhibits seed germination – by inhibiting ABA
synthesis
-
-
May also block the wound response and act
antagonistically to ABA – preventing the wound response from
spreading further than necessary
|
Synthesis:
Cells returning from
Exogenous Treatment:
Inhibition and Stimulation:
Storage:
Nutrient and Hormone Attraction and Repulsion:
Apical Dominance:
Hormone Transport:
Cell Division:
Senescence:
|
|

Jasmonic Acid
|
- Increase in desiccated plants.
|
-
Effect of elevated ABA levels
-
JA-induced proteins are lacking in the roots, in
bleached leaves, and in leaves of chlorophyll-deficient
|
|
-
Growth inhibition
-
Senescence promotion
-
Stimulates wound responses
-
Germination inhibition
-
Tuber formation promotion
-
Fruit ripening and fruit abscission promotion
-
Pigment formation promotion
-
May have a role in plant defense
|
Synthesis:
Exogenous Treatment:
Inhibition and Stimulation:
Storage:
Nutrient and Hormone Attraction and Repulsion:
Apical Dominance:
Hormone Transport:
Cell Division:
Senescence:
Specific to a Sugar Abundance Signal:
|
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Khan, Wajahat; Salicyl Acid And Related Phenolic Compounds For
Increasing Photosynthesis In Plants, World Intellectual Property
Organization. 19.04.2001, (WO/2001/026464).
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S. D. Ray, GA, ABA, phenol interaction in
the control of growth: Phenolic compounds as effective modulators of
GA-ABA interaction in radish seedlings. Biologia Plantarum
Publisher, Volume 28, Number 5 / September, 1986, Pages361-369
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K. Grossmann and T. Schmülling, The effects of the herbicide
quinclorac on shoot growth in tomato is alleviated by inhibitors of
ethylene biosynthesis and by the presence of an antisense construct
to the 1-aminocyclopropane-1-carboxylic acid (ACC) synthase gene in
transgenic plants. March, 1995, Volume 16, Number 2: Pages 183-188
Further Reading
References pulled from my own library research started in the mid 1980's (and
some say limited to this period and before). Nevertheless I believe the
information is relevant to today as the earliest findings are crucial to a
general theory here, not the more detailed ones of later years.
Abeles, F. B., Holm, R. E., & Gahagan, H. E. Abscission: the role of
aging. Plant Physiology 42, 1251-56, 1967.
Adedipe, N. O., Hunt, L. A., & Fletcher, R. A. Effects of Benzyladenine
on Photosynthesis growth and senescence of the bean plant. Phys. Plant.
25, 151-53, 1979.
Addicott, F. T., Carns, H. R., Lyon, J. L., Smith, O. E., & McMeans, J. L. On
the physiology of Abscisins. Recrulateurs Naturels de la Croissance Vegetale,
pp. 687-703. Paris: C.N.R.S., 1964.
Barrington, E. J. W. Hormones. The New Encyclopaedia Britannica. Macropaedia
v. 8, pp. 1074-88. Chicago: Encyclopaedia Britannica, Inc., 1975.
Beevers, L., Loveys, B., Pearson, J. A., & Wareing, P. F. Phytochrome and
hormonal expansion and greening of etiolated wheat leaves. Planta 90, 286-94,
1970.
Black, M. Abscisic Acid in seed germination and dormancy. Abscisic Acid, ed.
F. T. Addicott, pp. 331-364. New York: Praeger, 1983.
Booth ? Nature London 194-204
Brown, A. W., Reeve, D. R., & Crozier, A. The effect of light on the
Gibberellin metabolism and growth of Phaesolus coccineus seedlings. Planta 126,
83-91, 1975.
Burg, S. P., & Burg, E. A. The interaction between Auxin and Ethylene and its
role in plant growth. PKAS 55, 262-69, 1966.
Davis & Wareing ? Planta 65 p. 129
Engelke, A. L., Hamzi, H. Q., & Skoog. F. Cytokinin-Gibberellin regulation of
shoot development and leaf form in tobacco plantlets. Amer. J. of Botany 60,
491-95, 1973.
Esashi, Y., & Leopold, A. C. Plant Physiology 44, 1470, 1970.
Goeschl, J. D., Pratt, H. K., & Bonner, B. An effect of light on the
production of Ethylene and the growth of the plumula portion of the etiolated
pea seedling. Plant Physiology 42, 1077-80, 1967.
Goldthwaite, J. J. Further studies of hormone regulated senescence in Rumex
leaf tissue. Plant Growth Substances, 1970, ed. D. J. Carr, pp. 581-88. Berlin:
Springer, 1972.
Hayes, P. M., & Patrick J. W. Photosynthate transport in stems of Phaesolus
vulgaris treated with Gibberellic Acid, Indole 3-Acetic Acid or Kinetin. Effects
at the14site of hormone application. Planta 166: 371-79, 1985.
Hewett, E. W., & Wareing, P. F. Cytokinins in Populus x robusta Schneid:
Light effects on endogenous levels. Planta 114, 119-129, 1973.
Houck, D. H., & Lamotte, C. E. Primary phloem regeneration without
concomitant xylem regeneration--its hormone control in Coleus. Amer. J. Botany
64, 799-809, 1977.
Imaseki, H. Hormonal control of wound-induced responses. Encyclopedia of
Plant Physiology. v. 11, ed. R. P. Pharis & D. M. Reid, p. 504, Heidelberg:
Springer Verlag, 1985.
Jahardhan, K. V., Vasudeva, N., & Gopel, N. H. Diurnal variation of
endogenous Auxin in arabica coffee leaves. J. Plant Crops 1 (Suppl), 93-95,
1973.
Kawase, M. Effects of flooding on Ethylene concentration in horticultural
plants. J. Am. Soc. Hortic. Sci. 97, 584-88, 1972.
Lecoq, C., Koukkari, W. L., & Brenner, M. L. Rhythmic changes in Abscisic
Acid (Abscisic Acid) content of soybean leaves. Plant Physiology 72 (suppl.),
52, 1983.
Manos, P.J., & Goldthwaite, J. A kinetic analysis of the effects of
Gibberellic acid, Zeatin, and Abscisic Acid on leaf tissue senescence in Rumex.
Plant Physiology 55, 192-98, 1975.
Marre, E. Effects of fusiccocin and hormones on plant cellmembrane
activities, observations and hypothesis. Regulation of Cell Membrane Activities
in Plants. eds. Marre, E. & Caffer, O. Amsterdam: Elsevier/North Holland
Biomedical Press, pp. 175-202, 1977.
McMichael, B. L., & Hanny, B. W. Endogenous levels of Abscisic Acid in Water
stressed cotton leaves. Agron. J. 69, 979-82, 1982.
Mitsuhashi-Kato, M., Mishibaoka, H., & Shimokoriyama, M. Anatomical and
physiological aspects of developmental processes of adventitious root formation.
Plant and Cell Physiology 19, 393-400, 1978.
Nooden, L. D. Senescence in the whole plant. Senescence in Plants, ed. K. V.
Thimann, Boca Raton, FL: CRC Press, 1980.
Palmer, J. H., & Phillips, I. D. J. The effect of the terminal bud indole
acetic acid and nitrogen supply on the growth and orientation of the petiole of
the Helianthus. Annus. Physiol. Plant 16, 572-84, 1963.
Pooviah, B. W., and Leopold, A. C. Deferral of leaf senescence with calcium.
Plant Physiology 52, 236-39, 1973.
??, Phys. Plant v. 51 375-79
Reid, D. M., and Wample, R. L. Water relations and plant hormones. Chapter 14
in Volume 11, Hormonal Regulation of Development III, eds. A. Pirson and M. H.
Zimmerman. Heidelberg: Springer Verlag, 1985.
Reinhold, L. Phytohormones and the orientation of growth. Phytohormones and
Related Compounds a Comprehensive Treatise, v. II, ed. by D. S. Letham, P. B.
Goodwin, and T.J. V. Higgins. Amsterdam: Elsevier/North Holland Biomedical
Press, 1978.
Ross, E. L. Growth regulators and conifers: their physiology and potential
uses in forestry. Plant Growth Regulating Chemicals, v. II, ed. by L. G.
Nickell. Boca Raton, FL: CRC.
Sachs, T., and Thimann, K. V. The Role of Auxin and Cytokinin in the Release
of Buds from Dominance. Amer. J. Bot. 54, 126-44, 1967.
Sembdner, G., Gross, D., Liebisch, H. W., and Schneidner, G. Bio-synthesis
and metabolism of plant hormones. Hormonal Regulation of Development 1, ed. J.
MacMillen, Heidelberg: Springer Verlag, 1980.
Skoog, F., and Miller, W. Chemical regulation of growth and organ formation
in plant tissues cultured in vitro. Symp. Soc. Exp. Biol. 11, 118, 1957.
Snow, R. Plagiotropism and correlative inhibition. New Phytologist 44,
110-117, 1945.
Sutcliffe, J. F. Regulation in the whole plant. In Transport in Plants v. 2.
Part B: Tissues and Organs. Encyclopedia of Plant Physiology, ed. U. Luttge and
M. G. Pitman. Heidelberg: Springer Verlag, 1976.
Thimann, K. V. Cell Enlargement and Growth. Hormone Action in the Whole Life
of the Plant Amherst: Univ. of Mass. Press, 1977.
Thompson, M. J., Meudt, W. J., Mardava, N. B., Dutky, S. R.1Lusby, W. R., and
Spaulding, D. W. Synthesis of Brassinosteroid and relationship of structure to
plant growth promoting effect. Steroid 39 #1, 89-105, 1982.
Torrey, J. G. Auxin control of vascular pattern formation in regenerating pea
root meristems grown in vitro. Amer. J. Bot. 44, 859-870, 1957.
Van Staden, J., and Smith, A. R. The synthesis of Cytokinin in excised roots
of maize and tomato under aseptic conditions. Annals Bot. 42, 751-753, 1978.
Varner, J. E. GA-controlled synthesis of alpha-amylase in barley endosperm.
Plant Physiology 39, 412-415, 1964.
Wain, R. L. Some development in research on plant growth inhibitors. Proc.
Roy, Soc. B. 191, 335-352, 1975.
Wareing, P. F., and Phillips, I. D. J. Growth and Differentiation in Plants.
Great Britain: Pergamon Press, 1981.
Webb, D. P., Van Staden, J., and Wareing, T. F. Seed dormancy in Acer. In J.
Exp. Bot. 24, 105-106, 1973.
Wright, S. T. C. The effect of plant growth regulator treatments on the
levels of ethylene emanating from excised turgid and wilted wheat leaves. Planta
148, 381-88, 1980.
ZeevABArt, J. A. D. Giberellin and flowering. The Biochemistry and Physiology
of Giberellin, v. 2, ed. A. Crozier, New York: Praeger, 1983.
Zimmerman, P. W., and Hitchcock, A. E. Initiation and stimulation of
adventitious roots caused by unsaturated hydrocarbon Gases. Contributions to the
Boyce Thompson Institute 5, 351-369, 1933.
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