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Early 2007 Version of General
Speculations on Plant Hormones

"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

By Paul D. Pruitt
M.A. University of Pennsylvania, Biology, 1986


Taking into consideration a criticism of my previous theories on plant hormones (see here: http://www.planthormones.info/plants2003.htm), I'm willing to revise the speculations so they fit more with current findings.  The criticism that appears most salient is that GA is not made by older cells mainly but by younger cells.  I will then make this axiomatic that the two pairs of complimentary hormones Ethylene and GA/Brassinosteroid and IAA/Cytokinin are made mainly by young cells.  There are further implications to this that require some changes to my speculations.  This page will take the form of theorems or axioms about plant hormones as did the other "papers" available on the site above. 

The conclusions I make here: that Auxin is a sign of sugar and CO2 and O2 gas prosperity, Cytokinin a sign of water and mineral prosperity, Gibberellin a sign of sugar and CO2 and O2 gas deficiency and Ethylene/ACC a sign of mineral and water deficiency should perhaps be more generalized.  Perhaps Auxin is a sign of overall shoot prosperity and the existence of shoot conditions that warrant growth. Cytokinin then is a sign of overall root prosperity and the existence of root conditions that warrant growth.  Also then Gibberellin is a signal of shoot distress, which may require resources from the root be rerouted to the shoot.  Finally ACC/Ethylene may in part be a signal of root distress and result in shoot resources being rerouted to the root.

Scientist is general believe that plant hormones have many pleiotropic effects.  This means that the effects they have appear unrelated and may merely use plant hormones like money as a currency to effect unrelated types of things or at least we are far from understanding the overall commonality to all of the effects each of the hormones have.  The overall aim of this "paper" is to challenge this and although it may fall short of providing overall single definitions under which each of the hormones can fall, the attempt is made to at least cover most of the bases and hopefully move forward in explaining the reason for being of these messengers.


  • The major plant hormones Auxin, Cytokinin, Gibberellin, Brassinosteroid, Ethylene, Abscisic Acid and Salicylic Acid can be split into two groups.  The first group is Auxin, Cytokinin, Gibberellin, Brassinosteroid, and Ethylene. They are hormones made to correct nutrient imbalances.  The second group of Abscisic Acid and Salicylic Acid are made when any rapidly developing general stress occurs to the whole plant or the relief of that stress.

  • The nutrient hormones (Auxin, Cytokinin, Gibberellin, Brassinosteroid, and Ethylene) are made in the highest concentrations by dividing and young plant cells and the levels fall off precipitously, but not completely as the cells age.  The stress hormones (Abscisic Acid and Salicylic Acid) are perhaps made by all cells in equal amounts facing the same stress or release from stress conditions.

  • Auxin is made mostly by young plant cells that have more than enough shoot derived nutrients (mainly sugar, CO2 and O2) to support both them and any dependent cell, thus growth is a possibility if balanced out by an excess of water and minerals.  A root cell has no cells depending on it for sugar and gases, but a shoot cell would expect to have a similarly sized root cell depending on it for its shoot derived nutrients as well as having to fulfill its own needs. It too can make Auxin but does so at half the levels of sugars and O2 that a similar sized shoot cell.

  • In a likewise manner Cytokinin is made mostly by young plants cell that have more than enough root derived nutrients (mainly minerals and water) to support both it and any cells depending on it for root derived nutrients. Growth is thus a possibility here too if balanced out by an excesses of sugar and gases.  For a shoot cell there would be no cells depending on it for water and minerals whereas a root cell should have a counterpart shoot cell depending on it for root nutrients. Again a shoot cell would make Cytokinin at half the levels of water and minerals that root cell would synthesize the hormone.

  • Conversely now, Ethylene is made mostly by young cells when they do not have enough minerals and water to support both them and any cell depending on it for the acquisition of minerals and water.  Restating this makes similar size cells in the root producing Ethylene (really apparently ACC its precursor is made and transported upward, not Ethylene itself) when the level of minerals and water drops below 2 times that needed to maintain the cell at its present size, whereas for a similar sized shoot cell it would only have to be the amount of minerals and water dropping below what the cell itself alone needs to maintain life at its present size.  Thus a plant will need to cut back in size, if the deficit can't be made up. Ethylene, its noted by some, appears to rise precipitously whenever programmed cell senescence is needed.  I argue later, that GA needs to be also present and a burst of GA levels should also be seen whenever cell senescence is needed.  See below.

  • Also GA or Brassinosteroid (I'm lumping them for now into one hormone cascade path) is made mostly by young cells when they have less than enough sugar and gases to support both it and any cell depending on it for acquisition of these nutrients.  Thus again more plainly, if a root cell does not have enough sugar and minerals to maintain their present size, they make GA or Brassinosteroid.  A shoot cell of the same size and maturity will do a similar thing if making less than twice the needed sugar and acquiring less than twice the needed gases to maintain itself, because it is supporting a similar sized and maturity stage root cell for those nutrients.  Thus again the plant will need to cut back if the deficit can't be made up.

  • The nutrient hormones are made to correct imbalances.  They do this by affecting by at least 4 things: nutrient transport, nutrient storage, direction of growth, and new growth initiation or old growth senescence.

  • For Auxin correcting the imbalance of the perceived excess of sugar and gases is first dealt with by initiating active transport of sugar and gases away from the site of synthesis of the hormone and induction of active transport of minerals and water to the site of Auxin production.  Nutrient storage is initiated for the excess sugar and gases in vacuoles of the cell and release of store minerals and water if any from vacuoles in the cell would also initiated.  The direction of growth of the plant or plant cells is also changed by Auxin.  If the plant, plant organs, or plant cells have been broadening (because of the presence of Cytokinin or Ethylene) they are changed to lengthening strategy of growth.  Finally Auxin corrects the imbalance by initiating new roots by causing dormant root buds to grow out, thus increasing the flow of water and minerals.  Auxin is also of course known to inhibit the growth of new shoot buds with shoot apical dominance.

  • For Cytokinin correcting the imbalance of the perceived excess of water and minerals is done by initiating or increasing the transport of water and minerals away from the site of synthesis and increasing the active transport of sugar and gases towards the site of synthesis.  Likewise, Cytokinin increases the storage of water and minerals within the synthesizing cell's vacuoles and increases the release of sugar and gases from vacuoles stores if they exist within the cell.  Also Cytokinin causes or influences the growth of any cell, organ or plant toward broadening, and away from lengthening.  Finally Cytokinin induces new shoot growth and inhibits root bud growth with root apical dominance.

  • For Ethylene/ACC the correction of the deficit of water and minerals is handled by actively increasing the flow of water and minerals to the site of synthesis (and by increasing the flow of sugar and gases out of the cell too ???). Nutrient stores of water and minerals wherever they are found are encouraged to give up their storage to needy cell(s) producing Ethylene (and sugar and gases are stored in vacuoles to decrease an imbalance???).  Ethylene also is known to influences the direction of growth of a plant away from lengthening to broadening.  Finally Ethylene induces older leaves to senesce, sending the resulting freed up water and minerals to locally needy leaves and sending the sugar and gases to the root to make more roots.  Ethylene is known to induce root hairs which increases the surface area of the root and thus increases the uptake of water and minerals.  Ethylene probably inhibits new shoot growth.

  • For Gibberellin/Brassinosteroid the correction of the deficit of sugar and gases is handled by actively increasing the flow of sugar and gases to site of synthesis (and increasing the rate of transport of sugar and gases out of the cell to recreate the balance???).  Stores of at least sugars in the form of starches are known to be made available by GA during seed germination and probably made possible by the signal under all circumstances. This would be true of any stored gases too.  The direction of growth of cells is also influenced by GA toward lengthening.  Finally GA/BR inhibits root growth and probably cause the senescence of older roots. Also a change of strategy analogous to Ethylene's to root hair initiation might be GA's stimulation of bolting that moves the plant out of the shade.

  • It is widely known the Auxin and Cytokinin are needed to induce cell division.  This can be seen as the plant being reassured by these signals that it has excess amounts of all nutrients of both the shoot and root derived kind, and so cell division is warranted.  I believe it's also been shown that a cell under the influence of both Auxin and Cytokinin will draw all nutrients to itself not sending away anything even if they represent excesses.  Thus if a cell or plant organ is making Auxin and it comes under the influence of enough Cytokinin, it will change strategy and stop exporting sugar and gases and instead become a net importer of the resources even if they are a successful young shoot cell.

  • Complimentarily I'm proposing that both Ethylene and GA/BR are needed for cell senescence.  In fact when Ethylene is being released in the shoot parts that "it chooses" to senesce, may be the ones making the most GA, because these would be cells or leaves that are the least efficient at doing what the leaf should do, which is procure sugar and gases.  If a cell is synthesizing GA and comes under the influence of Ethylene, I believe it will stop actively drawing sugar and gases to it, and actually start to send them out.  GA and Ethylene acting together would then send out all nutrients, leading to the synthesis of more GA and Ethylene and an even higher rate of active transport of these nutrients out, with this active feedback loop leading to a climacteric rise in Ethylene and GA and senescence. For example when Xylem cells are fully formed they die to make hollow tubes.  Ethylene is seen to rise precipitously just before this deliberate sacrifice.  My proposal is that GA levels be examined too during this programmed cell death, along with wherever else programmed cell deaths happen, such as in fertilized flower senescence, and end of season deciduous leaf die off.

  • Just as high levels of Auxin lead to Ethylene production, high levels of Cytokinin should lead to Gibberellin/Brassinosteroid production.  What's going is that Auxin greatly increases the flow of water and minerals to young cells high in sugar and gases.  It first tries to do this by initiating new roots and root growth and changing root growth patterns from the Cytokinin induced one of broadening to lengthening, but if this doesn't succeed it begins to steal water and minerals from cells.  Particularly when it steals it from other young cells, they react by making ethylene.  The same sort of thing is true for Cytokinin and Gibberellin.  Cytokinin tries to bring sugar and gases to the young cells making it by  initiating new shoot growth and changing shoot and leaf growth from the Auxin induced pattern of lengthening to one of broadening.  When this doesn't succeed, the stealing of sugar and gases  from cells starts to occur leading to deficiencies and GA/BR synthesis.  I think it has been shown that Ethylene shuts down Auxin synthesis as would be expected to stop the causation of excess minerals and water deficiencies.  So one would also expect GA/BR to shut down Cytokinin production...

  • Auxin and Cytokinin are highest in levels during the day when the sun is out for photosynthesis and improved transpiration. (Perhaps transpiration increases mineral absorption and water intake following osmosis and the uptake of water and minerals is higher during the day than at night for most plants). Ethylene and GA/BR levels are higher at night.

  • Ethylene and GA/BR levels are higher at the beginning of the life of a plant when water, minerals, sugar and gases stored in the seed must be released.  Cytokinin and Auxin levels are highest during the middle active growth period of life of the plant.  Ethylene and GA/BR levels increase again relative to Auxin and Cytokinin at the end of the life of the plant or growing season, when the nutrients must be withdrawn from unneeded plant organs like leaves or flower petals or moved into the fruits and seeds.

  • Ethylene and GA/BR move resources more toward the center of the plant and away from the periphery.  Auxin and Cytokinin are more risk taking hormones moving resources to the active edges of the plants where "the action is".

  • As for ABA and Salicylic Acid, ABA is a "batten down the hatches" hormone that is like adrenaline and quickly potentiates a plants response to rapidly developing environmental emergencies of all kinds.  Possibly it does not do anything on its own but greatly magnifies any reaction a plant is having to a threat.  Salicylic Acid on the other hand, would be the "stand down" hormone to bring the plant back to normal operations.  I realize that ABA is famously known for closing guard cells and Salicylic Acid is found in Willow Bark, a tree more than any other in need of having open guard cells to pump out the excess water that occurs at the roots of the willow due to it's habitat of living on river banks.  However, ABA is known to be induced by heat shock, salt shock, insect damage etc.  Maybe all of these have a common denominator of water loss but my thinking is that it destroys the symmetry of the theory to say that it is actually involved per say in a nutrient issue rather than more primarily as an indicator of any kind of shock to a plant.  Also there have been failures in the past to securely tie it to all desiccation events.


These speculations do leave out some effects of some of the hormones.  For instance it appears that Ethylene is involved in shoot and leaf wound, pest and infection response.  So the question is how could this be adaptive for the root other than in maintaining long term survival of the plant.  The answer may lie in that if we look at root wounding, pest infestation and infection, we may see Gibberellin highly involved for it's abilities to inhibit and lead senescence of roots.  Additionally since shoot wounding, infestations and infections inhibit shoots from doing their job, maybe GA is involved here too and higher levels of that hormone under those conditions should be looked for too.

The commonality of GA and ACC/Ethylene is they may both be released when a plant for whatever reason needs to get smaller.  Ethylene/ACC makes the shoot system smaller and reroutes resources to the root and GA makes the root system smaller and reroutes resources to the shoot. A shoot distress may warrant and temporary caching or safekeeping of resources in the root and a plant may do this by making Ethylene/ACC which would otherwise be a signal of root distress. Environmental distresses of one sort or another, appears to require strategies of different emphasis.  Flooding prevents the root from doing its job, so resources are sent that way.  Shading on the other hand, prevents the shoot from doing its job, so GA sends resources from the root to the shoot to deal with that.  In the dark perhaps neither the root nor the shoot work as well so perhaps GA and Ethylene/ACC peak together in a daily cycle.

Thinking along the same lines the commonality of Cytokinin and Auxin is they may be released whenever a plant or a plant part needs to get bigger.  Auxin's inhibition of secondary shoot growth and the initiation of new roots seems to be the message "the shoot is doing better than needed at the moment, let's focus on root growth."  Cytokinin would be the converse.  The existence of high levels of both would signal perhaps no need for inhibition of growth of any kind and the green light for cell division.

I know the last paragraph is not completely illuminative and may appear self evident and imprecise, but plant physiologist must start explaining plant hormones more along this overall aim of each of the signals.  This will help new students understand, memorize and learn the myriad effects of hormones more clearly if they each can be understood as having an overall aim or goal or at least commonality or can be fit into some sort of scheme even if it is incomplete.  A better mind than mine and a better scholar than I am may certainly be able to better formalize the understanding of plant hormones.

Further Reading

Barrington, E. J. W. Hormone. In The New Encyclopedia Britannica, Macropaedia v. 8, pp. 1074-88. Chicago: Encyclopedia Britannica, Inc., 1975.

Brown, A. W., Reeve, D. R., and Crozier, A. The effect of light on the Gibberellin metabolism and growth of Phaesolus coccineus seedlings. Planta 126, 83-91, 1975.

Burg, S. P., and Burg, E. A. The interaction between Auxin and Ethylene and its role in plant growth. PNAS 55, 262-69, 1966.

Engelke, A. L., Hamzi, H. Q., and Skoog. F. Cytokinin-Gibberellin regulation of shoot development and leaf form in tobacco plantlets. Amer. J. of Botany 60, 491-95, 1973.

Goeschl, J. D., Pratt, H. K., and 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.

Hewett, E. W., and Wareing, P. F. Cytokinins in Populus x robusta Schneid: Light effects on endogenous levels. Planta 114, 119-129, 1973.

Jahardhan, K. V., Vasudeva, N., and Gopel, N. H. Diurnal variation of endogenous Auxin in arabica coffee leaves. J. Plant Crops 1 (Suppl), 93-95, 1973.

Lecoq, C., Koukkari, W. L., and Brenner, M. L. Rhythmic changes in abscisic acid (ABA) content of soybean leaves. Plant Physiology 72 (suppl.), 52, 1983.

McMichael, B. L., and Hanny, B. W. Endogenous levels of abscisic acid in Water stressed cotton leaves. Agron. J. 69, 979-82, 1982.

Mitsuhashi-Kato, M., Mishibaoka, H., and Shimokoriyama, M. Anatomical and physiological aspects of developmental processes of adventitious root formation. Plant and Cell Physiology 19, 393-400, 1978.

Sembdner, G., Gross, D., Liebisch, H. W., and Schneidner, G. Biosynthesis and metabolism of plant hormones. In Hormonal Regulation of Development I, ed. J. MacMillen, Heidelberg: Springer Verlag, 1980.

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.

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.

Paul D Pruitt -