An Explanation of

Plant Hormones

Paul Pruitt, M.A. Biology, University of Pennsylvania 1984

This is the third version, Version III, of my Plant Hormones ideas first written and posted on the Web in 1999. The most recent version is available written in 2003. The first version of the paper was written in 1986 and had not been previously published anywhere or posted on the Web until 06/06/2003. The 1995 version is also available. When this and 1995 the versions were posted on the Internet, they received considerable comment, both positive and negative.

In this article I will show, that if we make 8 groups of assumptions about plant hormones, many of the most important questions of plant physiology can be answered. Auxin is seen here as mainly made when there are good shoot growing conditions, more particularly when any cell is receiving a good supply of shoot derived nutrients: Sugar, CO2, and O2. Conversely Gibberellin (GA) is seen as made mainly under poor shoot conditions, more particularly when any cell is facing a scarcity of Sugar, CO2, and O2. Cytokinin is seen as made in the most part under good root conditions, but more specifically when any cell has a good supply of root derived Water and Minerals. Conversely Abscisic acid (ABA) is seen as made mostly under bad root conditions or more specifically when any cell is up against a dearth of Water and Minerals. Ethylene, as is well accepted, is seen as made under overall stress conditions. Conversely, I add that a yet to be determined hormone (possibly NO, Nitric Oxide) is made under good overall non-stressful conditions. I believe positive feedback loops are induced in the shoot and root meristems by Auxin, Cytokinin and the yet to be determined hormone under good growing conditions. This is because each of the "positive" (Auxin, Cytokinin and the yet to be determined hormone) hormones draw all nutrients (not just the nutrients that induce their synthesis) to the cells where these "positive" hormones exist. What happens, is the better the nutrient conditions, the more of these hormones are made, this causes more nutrients to be attracted to the immature cells, and this in turn causes more hormones to be made, etc. This effect is responsible for apical dominance in the shoot and root and works because mature cells make far less of these hormones than the immature cells. Nutrients are drawn away from the mature cells that produce them, to immature cells that need them to grow. This drainage does not complete to the point of senescence for 3 reasons. First because Auxin is transported down and Cytokinin up, and the nutrients follow these hormones' journey away from the nutrient concentrating meristems. Secondly if the mature cells are still efficiently making or taking in nutrients, the mature cells continue to make a small amount of Auxin, Cytokinin and/or a yet to be determined hormone. This small amount of hormone has been shown to be protective of mature plant parts. Thirdly there is a possibility that there are negative feedback loops where the "positive" hormones, when they drain the surrounding tissue of nutrients, cause the surrounding tissue to make GA, ABA, and/or Ethylene and these hormones when they reach the cells where the positive hormones are made directly inhibits the enzymes producing the positive hormones. Explaining senescence, if a mature cell is not "pulling its own weight" nutrient-wise, that cell will start making GA, ABA, and/or Ethylene. This will induce a positive feedback loop in the opposite direction as to those causing the apical dominances, because these hormones push nutrients out of mature cells (toward immature cells), and the more nutrients that are pushed out, the more of these negative hormones will be made. A vicious cycle is born, leading to senescence of inefficient mature cells and plant parts. Also in contrast to the "positive" hormones, the "negative" hormones are only made in small quantities in immature cells. This quantity is only enough to cause hibernation not senescence, so secondary buds, while not "profitable" nutrient-wise at a given time, are protected for possible future use.

This theory is designed to explain, in a simple way, the conditions under which hormones are made, how they are vital to nutrient transportation, how they induce apical dominance and senescence, the Auxin-Ethylene effect, and the hereto lack of totipotency found in many cultured calluses of plant species. Additionally, in response to criticism by Dr. Michael Jackson, some attempt is made to look at how plant hormones affect tissues, not just the conditions under which production occurs. Finally, I give a brief alternative theory which differs from the body of this work in some key ways.

Any theory of Plant Hormones needs to recognize the work of K. V. Thimann, F. Went, F. Abeles, F. Skoog, G. Avery, P. F. Wareing, P. Davies, P. W. Morgan, W. P. Jacobs, A. C. Leopold, A. W. Galston, R. Cleland, and F. Addicott. Forgive me for leaving out countless names of others who have made major contributions to the field.  Special thanks goes to Mark Jacobs for getting me so interested in plants in the first place.

Disclaimer

I'm not a professional scientist, and this "paper" is considered by most plant scientists to be pure speculation. Nevertheless I stand by what I write here because I believe it summarizes and draws valid conclusion from a large body of findings, producing a theory which is simple, cohesive and powerful. This "paper" suggests bold new directions for experiments and may have no other value than this. The use of "positive" and "negative" to describe the hormones, is not meant to put a value judgment on the hormones, but is instead meant to reflect the conditions of production and the effect of the hormone. In other words "positive," Plant Hormones are made under good growing conditions and produce further growth, whereas "negative" hormones are produced under bad growing conditions, and produce a cutting back on the size of the plant. They are simply names, however unfortunate some may consider them to be, that I currently use to describe the two sets of contrasting and complimentary Plant Hormones. At a later date the names can be changed, but they certainly are vivid.

Theory

• I believe Plant hormone production can be tied to abundance or scarcity of nutrients or good or bad conditions as follows:
   

  • Auxin is made by all cells, to varying degrees, in response to a good supply of shoot-derived nutrients: Sugar, CO2, and O2.

  • Cytokinin is made by all cells, to varying degrees, in response to a good supply of root-derived nutrients: Water and Minerals.

  • A yet to be determined hormone (perhaps NO, Nitric Oxide) made by all cells, to varying degrees, under good overall, non-stressful growing conditions.

  • Gibberellin (GA) is made by cells to varying degrees, facing a scarcity of shoot-derived nutrients: Sugar, CO2, and O2.

  • Abscisic Acid (ABA) is made by all cells, to varying degrees, facing a scarcity of root-derived nutrients: Water and Minerals.

  • Ethylene is made by all cells, to varying degrees, faced with overall stress such as weather, pest or disease, as is generally thought.

From here on I will use the term positive hormones for those made under positive growing conditions: Auxin, Cytokinin, and the yet to be determined hormone. I use the term negative hormones, as those made under negative growing conditions: GA, ABA, and Ethylene. Positive and negative hormones are assumed to have largely opposite effects.

• Research has shown that Auxin is mainly made by young cells and drops as cells mature (Sembdner, et al., 1980). I speculate that all the positive hormones are made in large amounts in immature cells and drop off precipitously as cells mature. That is, faced with the same positive growing conditions, immature cells will make far greater amounts of positive hormones than mature cells. I also speculate, that the negative hormones are made in small amounts in immature cells, and rise precipitously as the cells mature. That is, faced with the same negative conditions, a mature cell will make far more negative hormones than an immature cell.
   

• Other research has shown that both Auxin and Cytokinin induce the uptake of all nutrients and hormones to their site of application (missing reference). I postulate that a yet to be determined hormone also has this effect. Coupled with the group of assumptions in point 1, this produces a run-away positive feedback loop. That is let's say, the shoot apical meristem is experiencing good growing conditions. It will then produce much Auxin, because the shoot apex is immature tissue (see assumption 2). The cells' attraction of Sugar, CO2,  O2, Minerals and Water from surrounding tissue will induce even more Auxin, Cytokinin, and a yet to be determined hormone's production, and this will lead to an even greater uptake of nutrients and thus a positive feedback loop is created. By analogy I also predict that the negative hormones push nutrients out of cells. This also induces a positive feedback loop in the opposite direction as the positive hormones, because a deprivation of nutrients particularly in mature cells leads to negative hormone production, which pushes out nutrients which in turn leads to a greater production of negative hormones. This should lead to senescence in mature cells but not immature ones, see below. Partial evidence is shown by the observation that Ethylene leads the senescence of older leaves (Wareing and Phillips, 1981) as does ABA (reference missing).
   

• I suggest that plant hormones affect the plant cells in 2 reversible stages according to their amounts. The first step involves activity in the cell. At low levels the positive hormones increase cell activity, whereas the negative hormones decrease, or induce suspension of activities. Secondly, at intermediate levels, plant hormones affect cell dimensions. It has been documented that Auxin, Cytokinin, GA, and Ethylene. My guess is that positive hormones increase average cell size in the plant overall, but tend to increase growth in the peripheral parts (leaves and outlying roots) faster than core parts of the plant (the stem and root core).  Two of the negative hormones, GA (Engelke, et al., 1973) and Ethylene (Burg and Burg, 1966) have been shown to cause increased cell size in some cells. However, I predict all three negative hormones cause a net shrinkage of cell size if averaged over the whole plant. GA for instance is a hormone concerned with shoot-derived nutrient deficiencies, thus GA may cause a shrinkage of less needed root cells. Certainly GA has been shown to stop root growth (Mitsuhashi-Kato, 1978). Along the same lines ABA is a hormone concerned with root-derived nutrient deficiencies. Perhaps then ABA causes shrinkage of less needed shoot cells. I also predict the negative hormones induce some growth of the core plant parts at the expense of the peripheral parts, making the plant smaller but stronger.
   

• It has been shown that Auxin and Cytokinin are needed for cell division. I suggest that the yet to be determined positive hormone is also actually needed for cell division, and this hasn't been seen yet because the unknown positive hormone has been natively made by the cell lines where scientists have had success in producing cell division. By analogy again, I also postulate that Ethylene, ABA and GA are all three necessary for complete cell senescence.
   

• Production of a small amount of a positive hormone in a mature cell can protect that cell from senescence. This is already well known. The production of a small amount of Auxin for instance can prevent a leaf treated with ABA from going into senescence (reference missing). Conversely, it is possible that the production of a small amount of negative hormones made in immature cells, perhaps in some cases, can negate treatment with positive hormones. If the cell is still an efficient producer of nutrients, I suggest that mature cells will make a small amount of life-saving positive hormones. For example: if the shoot cell is taking more than enough shoot derived materials to support both it and a sister root cell with their Sugar, CO2, and O2 needs, than the cell is "profitable" and will make a small amount of Auxin. If it is not making a "profit" of Sugar, CO2, and O2, it starts making GA and also the other negative hormones. A similar schema, I would suggest, exists for root cells, Cytokinin, and ABA, where Cytokinin is made if enough Minerals and Water are taken in to support both the root cell and a cell of similar size or maturity in the shoot. If the root cell doesn't take in enough Minerals and Water, it makes ABA, and is eventually excised.
   

• I predict that positive hormones have the direct effect of inhibiting negative hormones and the indirect effect of promoting negative hormones and vice versa. For example the direct effect of Auxin might be to inhibit ABA and Ethylene production within the shoot apical meristem, but the indirect effect is to draw nutrients from surrounding tissue inducing nutrient deprivation, particularly Water and mineral deprivation (as this is the shoot where Water and Minerals are in short supply). This Water and mineral deprivation lead to the production of ABA and perhaps Ethylene as nutrient deprivation is stressful to cells. When the ABA and Ethylene reach the shoot apical meristems they directly induce a moderation of Auxin production.
   

• I predict that the reaction of cells to negative hormones is context sensitive.  For example if there is an excess of Water (enough for growth) but a deficiency of Minerals, the plant will still make ABA, but the cells will not react to this ABA in the fashion typically thought of.  That is ABA is thought by others to be a Water deficiency hormone and leaf cells will react to it by closing the guard cells.  However in line with my theory, I predict that ABA is still made in the face of good amounts of Water, but in the face of deficiencies of Minerals.  The guard cells closing may be inappropriate under these conditions instead the plant may want to concentrate its Minerals by transpiring off some Water. Therefore the guard cells may remain open under high Water and low mineral conditions.  The reaction of cells to negative hormones may reflect the conditions within those cells rather than always exhibiting the same response to the hormones.

Predictions

• The major question that has been asked about plant hormones, is, what is their function or why are they needed? I will go into detail about this below. However to sum up, I would say they allow the plant to respond in a balanced way to good or bad situations. For example let us say there are good shoot conditions and poor root conditions (e.g. plenty of light, but little Water). This will produce Auxin in greater overall amounts than Cytokinin. As has been shown, this will lead to the induction of new roots (Torrey, 1957). I suspect the good shoot and poor root conditions also leads to an increase in ABA, which inhibits shoot growth (ABA's inhibition of shoot growth probably has been shown but I don't have the reference) and probably shifts energy towards the roots. This then leads to new supplies of root nutrients.
   

• Apical dominance looks to me like a simple case of the rich getting richer and the poor staying poor. The successful shoot apical meristem, by means of positive feedback multiplication eventually wins out in a war for nutrients. The secondary buds, who lose out in this war, are only immature tissue, they do not make anywhere as near as much negative hormones to induce senescence, only enough to induce dormancy. Assumptions 1 & 3 would also explain the finding that both Cytokinin and a mineral solution can break secondary bud inhibition. That is, the application of Cytokinin to a secondary bud begins a new process of positive feedback for the bud where it attracts all of the nutrients and hormones it needs to allow it to grow (see assumption 3) and it induces the production of additional amounts of Cytokinin in immature secondary bud cells once the Minerals and Water arrive (see assumption 1). The flood of nutrients eventually starts a production of Auxin, which will only be sustained if the former secondary bud is in a good position to receive the Sugar making light.
   

• Senescence is explained by the positive feedback loops for negative hormones mentioned in assumption 3 and the efficiency issues mentioned in assumption 6. That is, a newly shaded shoot cell, for example, that can no longer make enough Sugar, and take in enough CO2, and O2, will start making GA (see assumption 6). The cell will first go into hibernation and the GA will cause the stem to lengthen perhaps bringing the leaf into better sunlight. If this allows the leaf to start making enough Sugar, CO2, and O2, then the cell will start making Auxin again and come out of hibernation.
   

• If the stem lengthening induced by GA does not work, the GA will eventually start pushing nutrients out of the cell, inducing even more production of GA and some production of ABA as well. This will cause stress to the cell inducing the production of Ethylene. Now we have all three negative hormones pushing nutrients out of the cell, a real positive feedback loop, culminating in senescence. This is perhaps a simplistic model of what goes on, but I believe the general principle stands.
   

• Plant hormones have a big effect on nutrient transport. The positive hormones attract nutrients from the sites of harvesting or production in the mature cells, to the growing immature cells where the nutrients are needed. Auxin is transported downward from the apical meristem in the phloem. This suggests that Auxin may be responsible for drawing Sugar, CO2, and O2, from the leaves into the phloem for downward transport to the roots. The negative hormones also have the overall effect of pushing nutrients from inefficient mature cells toward  efficient immature cells.
   

• The positive feedback loops produced by the positive hormones do not get carried away to the point of draining all the nutrients away from adjacent areas because of possibly 3 different mechanisms.  First as mentioned above, Auxin is transported down the stem so shoot nutrients don't just get attracted to the apical meristem, but to the entire phloem as the Auxin is transported down in it.  Cytokinin is transported in the xylem, and I would predict that Water and Minerals follow it up out of the roots and then up the stem, so root nutrients don't stay concentrated in the root apex. Secondly as mentioned, a small amount of Auxin produced by efficient mature plant parts protects it from the production of negative hormones. Thus efficient mature parts are protected from complete draining because they never go into a negative hormone positive feedback loop. Thirdly it has been shown (reference missing) that some of the negative plant hormones may directly curtail the production of the positive hormones. This would be a negative feedback loop where the positive hormones induce nutrient deprivation in negative hormone producing tissue (non-efficient tissue), but the positive hormone levels are dampened, once the negative hormones reach a high enough level and travel back to the site of production of the positive hormones. Thus for instance the shoot apical meristem might from the power of its positive hormones, start draining nearby leaves of needed Water and Minerals. This might cause a large production of ABA. When this ABA reached the positive hormone producing enzymes in the shoot meristem (as this author assumes it would) it might directly slow these enzymes, and the flow of Water and Minerals to the leaf may resume.
   

• The reason why secondary buds do not grow out, may not just be the simple reason that they only make a small amount of negative hormones, but may be a very dynamic process.  For example the Auxin production by the shoot apex induces the draining of nutrients from the secondary buds, inducing GA, ABA and Ethylene.  Eventually these travel up to the shoot apex and directly inhibit the production of Auxin.  With a decrease in Auxin there becomes a favorable Cytokinin-Auxin balance.  Cytokinin is known to stimulate secondary bud growth.  With the influx of nutrients to the secondary buds the negative hormones decrease precipitously, and the Auxin production by the shoot apical meristem can start again. Thus the secondary buds may be poised between losing all their nutrients and dying off, or gaining nutrients and growing out and may be go through a periodic draining and refilling of nutrients to at least some extent.
   

• It has been shown that Auxin is made in greater amounts in the shoot than in the root (Sembdner, et al., 1980). I would suggest this is because there is more Sugar, CO2, and O2, in their point of origin, the shoot, than in the root. It has also been shown that more Cytokinin is made in the root than in the shoot (Van Staden and Smith, 1978). I believe this is because there are more root-derived nutrients in the root than in the shoot. GA has been found more in the root than in the shoot (Barringtion, 1975) as one would expect, because there are less shoot derived nutrients there. Finally I would suggest that ABA is found in greater amounts in the shoot than the roots, because of the greater scarcity of Water and Minerals there.
   

• I predict that the positive hormones control the day life of the plant, since we can expect that they are made in greater concentrations in the day than the night. Auxin has been shown to peak during the day (Jahardhan, et al). Although Hewett and Wareing (1973) found Cytokinin to peak once during the day and once at night not enough research has been done to show this conclusively for this effect to be thought to exist in an "average" plant. Conversely the negative hormones rule the night, because we can expect with the lack of light and the decrease in temperature (slowing down nutrient harvesting machinery), less nutrients are brought in or created. Ethylene emanation from plants has been shown to decrease in the presence of light (Goeschl, et al., 1967). GA production has also been shown to go up in the dark and down in the light (Brown, et al., 1975). ABA has also been shown to peak at night (Lecoq, et al., 1983; McMichael and Hanny, 1977), although the latter only occurred under Water stress. Perhaps we may go so far as to predict that ABA and GA reverse the flow of nutrients at night. Possibly stores of Sugar, CO2, and O2, found in the roots are hydrolyzed by GA and dumped into the xylem for shipment upward.  
  
  

• Because of the direct and indirect influences hormones have on each other, we can expect that negative and positive hormones rise and fall in contrasting waves.  That is when positive hormones are high, then negative hormones are low, and when negative hormones are high positive hormones are low.  Thus a plots of the amounts of negative and positive hormones should be two sinusoidal curves staggered by 180°.  Although the biggest difference between the levels of positive and negative hormones should occur at the peak of the night and of the day, there should be a rise and fall of all hormones periodically during the whole night and day.  Conceivably waves of hormones sweep through the plant as a kind of breathing many times a day.
   

• I suggest the quest for totipotency has been hindered because of the failure to recognize of the role of the yet to be determined hormone. Possibly the success that has been had, is because some cell lines have a mutation allowing unprovoked native synthesis of the yet to be determined hormone.
   

• Plants always respond to positive hormones by increasing activity or growing. They respond to negative hormones by becoming less active or smaller but stronger.  Negative hormones cause downsizing.
   

• Auxin has been known to cause an increase in Ethylene production upon application to tissues in high enough doses. My explanation of this comes from assumption 3. That is, Auxin draws to it all kinds of nutrients from surrounding cells. This induces stress in surrounding tissue, thus causing Ethylene production. I would guess that ABA and GA are also produced in these surrounding tissue. I would suspect the other two positive hormones produce the same production of all the negative hormones. Conversely the application of negative hormones should eventually cause an increase in positive hormones as measured in parts of the plant away from the site of application.  This is because the negative hormones free up nutrients for use in other parts of the plant, which then stirs up a fresh wave of positive hormone production.
   

• An interesting question is whether a cell can make positive and negative hormones at the same time. An answer might be yes, because a cell might be experiencing, for example, a plethora of nutrients from the shoot, and a dearth of nutrients from the root. In that case it would make Auxin, but also make ABA.
   

• It is possible that the effects of plant hormones are different according to which tissue they are in.  For instance negative hormones may affect the peripheral parts differently than the core parts. I believe that under the effects of negative hormones, the peripheral parts (the leaves and peripheral roots and tubers) first undergo hibernation, then cell shrinkage.  Finally after the cell has undergone enough nutrient deprivation and stress and all the three hormones are present, senescence. On the other hand, I believe the core parts (the stem and root core) undergo first increased activity, then increased size and then cell division. In other words, in the presence of negative hormones the stem and the root core become stronger whereas the peripheral parts decrease in biomass. Again the plant becomes smaller but physically stronger under these environmental conditions.  Also we can postulate that under the effect of negative hormones nutrients are stored in the core parts where they are less vulnerable. For example, under the influence of ABA, Water is stored in a stem of increased girth so it will face less surface area and thereby evaporation. Positive hormones may be the reverse of negative hormones in this respect.  That resources may switched from less vulnerable core parts, outward to peripheral parts when the secure growing conditions signaled by the positive hormones are present.

Alternative Theory

An alternative to the theory above, would accept ABA as a Water deficit signal alone (Wain, 1975), with nothing to do with Minerals. Rearranging the above theory, Cytokinin would be a Water-abundance signal. Auxin and GA roles would then take roles of Sugar abundance and Sugar deficiency signal respectively. A yet to be determined hormone (perhaps NO, Nitric Oxide) would then be a signal of the abundance of all nutrients, perhaps even including CO2, and O2, but excepting Sugar and Water.  The part of this alternative theory that would be hard to swallow would be that Ethylene would have to take the role of a signal of all nutrient (excepting Sugar and Water) deficiency, not the widely held notion that it is a stress hormone. However, clever experimentation could tease out whether these traits are true.
 

Conclusion

Many possible experiments could be done examining these ideas. The main theory may have possible weaknesses, for example, I am not aware that ABA has been tied to mineral deficiencies. Yet, the lack of supporting experiments may simply reflect the fact that scientists have not been looking at hormones in the light of way outlined here. I have come to believe that most Plant Physiologists are frustrated and do not believe an encompassing theory can be found.  Thus they have not been looking for a theory.  As ever though, "Seek and you shall find." (Matthew 7:7, Luke 11:9)
 

Qualifications, Contact Information and Guestbook

My name is Paul Pruitt. I received a BA from Swarthmore College in 1984 where I studied under Mark Jacobs. My Bachelor's thesis was an examination of all aspects of Plant Senescence, including the role of hormones. I also received an MA from the University of Pennsylvania in 1986, where I studied plants under Scott Poethig among others. I have been studying the Plant Physiological Hormone Literature and thinking about Plant Hormones for 20 years. I'm currently an unemployed but experienced IT Support Analyst who has his own small file recovery and virtual Helpdesk business.  The Website can be seen here.  If you have any questions or comments send them to socrtwo@s2services.com.

References

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

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