Spring 2022


David Wm. Reed







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Charles Darwin

The Power of Movement in Plantsmarks the beginning of the modern study of plant growth”, including signals and growth stimuli, later shown to be plant hormones.

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Plant Physiology and Development, Taiz, Zeiger, Mooler and Murphy, 6th Ed, 2015 (has chapters on hormones), Sinauer Associates Inc, Sudderland, MA

Classic Text: Moore, T.C.  1979. Biochemistry and Physiology of Plant Hormones. Springer-Verlag, NY.



Hormone                  - an endogenous or naturally-occurring compound that is produced or synthesized in one part of the plant and causes a change in physiology, growth or development in another part of the plant; usually present in very small quantities.

Elicitors                    - Signaling molecules that may be involved in morphological changes, resistance to pests, and defense against herbivores, and other hormone-like activity.

Growth Substance -  all naturally-occurring or synthetically produced compounds that affect the physiology, growth and development of plants.


Plant Hormones, Elicitors and Photoreceptors

Classically, plants have been known to contain five hormones, which are auxin, cytokinin, gibberellic acid, ethylene and abscisic acid.  Recently, other endogenous compounds have been shown to elicit hormone-like reactions, which are brassinosteroids, salicylic acid, strigolactone, and jasmonic acid.  Some do not elevate these to the status of one of the five classical hormones, so often they are called elicitors.  In addition, there are photoreceptors (red/far red light, blue light) that are involved in causing plant responses.  All the classical hormones have application in horticultural practices.  The elicitors and photoreceptions have little horticultural application, but they do explain many very important development processes.


Five Classical Hormones

·         Auxin

·         Cytokinin

·         Gibberellin

·         Ethylene

·         Abscisic Acid


·         Brassinosteroid

·         Salicylic Acid

·         Strigolactone

·         Jasmonic Acid

·         Polyamine


·         Phytochrome

·     Blue-Light Responses


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Common Scheme for Hormonal Regulation

(Plant Physiology and Development, 6th Ed., Taiz, Zeigler, Moller and Murphy)




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Site of Production


·      indoleacetic acid (IAA)

·      4-chloro-IAA

·      indolebutyric acid (IBA)

·      naphthaleneacetic acid (NAA)

·      2,4-dichlorophenoxy-acetic acid (2,4-D)



·      Rapidly dividing tissue

·      shoot tips

·      young leaves

·      developing fruit

·      embryos



·         Charles Darwin – The Power of Movement in Plants 1881 – proposed existence of hormones in plants (1880s)

·         Boysen-Jensen (early 1900s) demo substance moved top down with mica sheets and diffused into gelatin

·         Frits Went – isolated auxin’s presence in plants (1920s)

·         Named auxin, from Greek auxein – to increase or to grow

·         K.V. Thimann (US) and Kogl, Haagen-Smit (Holland).isolated and identified auxin (1930s)

·         Immediately, chemists began synthesizing similar structures and testing for auxin activity.

·         Synthetic versions that have stood the test of time are listed in the above table.

o   Synthetic version almost always more effective – Why?




tryptophan  via multiple pathways to indoleacetic acid

                   requires Zn               

     tryptophan                                                           indoleacetic acid – IAA


Synthesis is in the young developing leaves of the shoot tip and just below the apical meristem.



·         3:1 basipetal transport – creates a gradient top to bottom

·         only hormone with polar transport

·         primarily in phloem parenchyma



·         NPA (naphthytphthalamic acid)

·         TIBA (2,3,5-triiodobenzoic acid)

·         Primarily used to alter auxin transport to elucidate mode of action of auxin


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Cellular Effects


1) Cell elongation - ythi is the effect Darwin studied in phototropism


2)   Cell division - stimulates


Whole Plant/Organ Effects


3)   Tropism - response of plants to environmental or physical stimuli.

Classical experiments by Darwin (1880s) Boysen-Jensen (early 1900s) began with observations on tropisms.


·         phototropism - response to light

o   auxin transport switches from basipetal to laterally to the dark side.

·         geotropism or gravitropism - response to gravity

·         thigmotropism - response to touch


Giant Leaf Pothos – combination of geotropism and thigmotropism

giant leaf
As vine grows up a
support, each leaf gets
increasingly larger.

giant leaf
                  pothos natural environment
Fill canopy once in light
Giant leaf Pothos production
Commercial production on
poles in containers.

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4)   Apical dominance –


Diagram Description automatically generated


·         determined by correlative inhibition of apical bud, high auxin produced by shoot tip.

·         dormancy of lateral buds may be a balance between high auxin/low cytokinin

·         removal of apical dominance is the bases of pruning and lateral branching

·         bud break in the spring is a period of lower apical dominance

o   This is the time when many lateral buds start growing.



5)   Branch angle

·         high auxin causes wide branch angles

·         pruning eliminates auxin coming from the apical region, thus removes apical dominance.  The lower auxin produces narrow branch angle



AJB 87:601 2000



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Re-establish central leader after topping pruning – Bald Cypress:

Natural cypress growth form

Utility company pruned top

1st bud break, narrow angle


6)   Zn Deficiency in pecan -can cause Witches Broom or Little Leaf

·         Zn required for synthesis of tryptophan

·         Tryptophan required for synthesis of auxin

·         A deficiency of Zn can result in witches broom or little leaf.

·         Common problem on pecans in Texas


Witches broom on pecan caused by Zn deficiency on alkaline soil.  Courtesy Larry Stein

Heerema Extension Guide H-659, New Mex State U.


7)   Sprout Inhibitor or Suppressants

· sprouting on Irish potatoes is a major problem

· auxin can be used to inhibit sprout growth, probably mediated through an increase in ethylene production.

· ethylene – at low concentrations; a gas

·  maleic hydrazide

·  chlorpropham (CIPC, chloroisopropyl-N phenylcarbamate) – may be most commonly
used sprout suppressant; a gas


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 Huang et al.  Carbohydrate Polymers, 107:241 2014

8)   Flower or Fruit set

·   low concentrations of auxin stimulate flower/fruit set without pollination or low rates of pollinati
Pandolfini Nutrients 1:2 2009

·      may lead to parthenocarpic (seedless) fruit or fruit with fewer seeds

      ·     no commercial formulations of which I am aware

9)   Fruit or flower thinning

· high concentrations cause flower and/or fruit abortion

· no commercial formulations

· at best chemical thinning agents are inconsistent.

·  actually, most common is hand or mechanical thinning



Cherry blossom time in DC

 Obviously, if you want cherries larger than a dime, then not every flower can make a fruit.

Hand thin to 1 peach every 6 inches

Mechanical thinning machine

You Tube videos

Hand thinning

Hand held mechanical thinner.

Tractor mechanical thinner



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10) Herbicides

· 2,4-D at high concentrations acts as a herbicide.

·  Broad-leaved plants only, e.g. dicots

·  Monocots – maize and other monocots quickly conjugate and inactivate synthetic auxin

·   2,4-D damage from drift is common and very easy to recognize.


How to recognize 2,4-D damage

2,4-D damage

Epinasty - malformation and twisting of leaves and stems due to differential growth rates.

Cupping of young leaves


11) Adventitious root formation

· stem and leaf cuttings

· tissue culture - high auxin/low cytokinin stimulates

· many commercial formulations


Stem Cuttings

AJPS vol 8 2017

Commercial rooting hormones

 Hormex series
Hormex label

DipnGrow plus liq plus label



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Site of Production



kinetin (not in plants)


benzyladenine (BA)

pyranylbenzyladenine (PBA)





root tips,


young leaves?



·   Second hormone isolated and identified.

·  Discovered investigating factors that trigger cell division.

·   1913 Haberlandt found that phloem tissue contained a diffusate that would trigger cell division in potato parenchyma.

·   1942 VanOverbeek demonstrated that extracts from coconut milk promoted growth of embryos in tissue culture.

·    1954 Skoog’s showed vascular tissue could do the same in tissue culture.  Skoog was a pioneer in tissue culture.

·    Cytokinin was named after cytokinesis.  They screened may compounds and found adenine stimulated cell division.

·    1955 Miller isolated from herring sperm DNA and adenine type compound that stimulated cell division.  It was named kinetin.  Kinetin is an adenine derivative.

·     1964 Letham identified the first natural plant cytokinin in corn endosperm, and named it zeatin.

·      Since its discovery, zeatin has been found in many plants.

·         Steward later found cytokinins effected more that cell division, including tissue differentiation, dormancy, phases of flowering/fruiting and senescence.




adenine -----> zeatin


Synthesis largely is in the root tips.  But, cytokinin is also synthesized in embryos and young developing leaves.



· xylem transported, found in root exudates

· primarily acropetally in the xylem, but not necessarily polar

· cytokinin exists in free and bound forms.


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Cellular Effects


1)      Cell division

· stimulates cell division; named after cytokinesis

2)   Protein synthesis -

· stimulates


3)   Chlorophyll breakdown

· decreases chlorophyll breakdown; promotes chloroplast development in etiolated tissue.


Whole Plant/Organ Effects


4)   Adventitious shoot formation -

leaf and root cuttings




5)   Overcome lateral bud dormancy


B. Treated with commercial product 6-BA Cytokinin (Mycorrhizal Applications, https://mycorrhizae.com/top-5-uses-of-6-ba-cytokinin-plant-growth-regulators-in-ornamental-plants/

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6)   Flower or Fruit set and pollination

·    low concentrations of cytokinin stimulate flower/fruit set without pollination or low rates of pollination
Pandolfini Nutrtient 1:2 2009

·     may lead to parthenocarpic (seedless) fruit or fruit with fewer seeds

·   Blossom set sprays– contains kinetin as active ingredient. "Label claims Blossom Set Spray, tomatoes, beans, cucumbers, melons eggplants, peppers, strawberries and grapes will often bear fruit earlier.”


                Set kinetin spray






Blossom Set label



7)    Seed germination

        -  may overcome dormancy or stimulate germination in hard to germinate seeds.


8)    Nutrient mobilization

·          -  nutrients transported towards high cytokinin concentration.

cytokinin nutrient mobilization curcurbit


9)    Leaf Aging, Senescence or Abscission

        -  may delay

        ·  decreased chlorophyll degradation

cytokinin delay senescence


10)    Root growth

         - may be inhibitory to root growth



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(Modes of action:  meristem and development)

 Wicaksono et al. Biologia Futura 72:299 2021


      auxin cytokinin

Tissue Culture – ratio of  cytokinin/auxin stimulates adventitious shoot or root formation



Apical dominance  - high cytokinin/low auxin maybe involved in overcoming apical dominance

auxin cytokinin interactions



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Site of Production


Over 130
(a “bewildering number”, named by consecutive numbers;  not all have biological activity)




(all in use are isolated from fungal cultures)




shoot tips,

root tips,





·         Existence of a stem elongation factor was known to Japanese rice farmers for many years.  Observed certain seedlings grew very tall and fall over,  called “lodging”. It was thought to be caused by a disease causing organism.   The phenomenon was called “bakanae” or “foolish seedling disease”.  Plants were shown to be infected by the fungus Gibberella fujikuroi.  A compound that caused the elongation was isolated from the filtrates of fungal culture.

·         1930s isolated crystals of the compound and termed it gibberellin, after the generic name of the fungus.

·         1950s Japanese, US and Britain identified the structure.  Termed Gibberellin A2 and A3.  They all have in common a 3 ring structure common to kaurene and sometimes termed the “gibbane backbone”.

·         1958 MacMillin identified GA in plants.

·         Many gibberellins were subsequently identified and they were named in numerical sequence.  Over 130 have been identified.  It appears as though each fungus and/or plant may synthesize its own unique GA.




·         no polarity

·         in phloem or xylem



growth retardants - chemicals that block synthesis of GA.


Commercial Growth Retardants, e.g. GA Inhibitors

Product Name

Active Ingredient

Cycocel, Citadel

chlormequat chloride

A-Rest, Abide


B-Nine, Dazide




Bonzi, Piccolo, Paczol


Sumagic, Concise




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Mode of Action of Growth Retardants

·         block ring closure between geranylgeranyl pyrophosphate and copalyl pyrophosphate

·         block ring closure between copalyl pyrophosphate and kaurene

Biosynthetic Pathway of Gibberellic Acid  (Moore Plant Phys 1979)


Rademacher Annual Rev Pt Physiol 51:501 2000



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Cellular Effects


1)   Protein synthesis - triggers de novo synthesis of some proteins, ex. a-amylase.


2)   Cell elongation - primary stimulus for cell elongation


Whole Plant/Organ Effects


3)   Rosette or dwarf plants - lack of endogenous GA often contributes to decreased height.


4)   Height control


a.       Increase height – treat with GA.

GA spray as Fresco



b.      Decrease height – apply growth retardants (see previous table).


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5)   Flowering - may cause bolting or earlier flowering in biennials

                           I can find no commercial examples


6)   Flowering – may cause or accelerate flowering or increase flower number in calla lily, spathiphyllum, and geranium.



7)   Fruit size - increases size of seedless grapes


‘Crimson’ table grapes. The cluster on the left was sprayed with gibberellic acid 



8)    Bud dormancy - may overcome and substitute for cold treatment.
                              Greenhouse Azalea, camellia?



9)   Seed germination - may increase or speed up


Penstemon seeds 24 hour GA soak; DeMello et al. HortScience 44(3) 2009


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10) Sex expression - favors staminate flower formation on monoecious plants




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Site of Production




ethephon or


(release ethylene

inside plant)




 ripening fruits,

aging flowers,

germinating seeds,

wounded tissue; thus can be made in almost all parts of the plant.


Other sources of ethylene

·         Auxin can induce ethylene synthesis

·         Natural component of natural gas

·         Incomplete combustion of any hydrocarbon; any yellow or orange flame, e.g. non-blue flame



·    Turn of the century, it was first noticed that trees in the vicinity of coal gas powered street lamps defoliated more, and that the presence of fruit may cause other fruit to ripen, and ethylene gas was involved.

·     1930s it was determined that plants produce ethylene.  At that time, ethylene was not considered a hormone.  Many thought ethylene effects might actually be auxin effects.

·     1959 Thimann’s lab demonstrated that ethylene did regulate growth.  The advent of gas chromatography to detect ethylene was critical in these studies.

·     1960s to 1980s the biosynthetic pathway was elucidated.



- diffusion as a gas throughout plant (in and out)

- because it is lipid soluble, it crosses membranes more easily than the aqueous phase-

- because it is a gas, ethylene will accumulate in flooded tissue.


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methionine --->  s-adenosylmethionine ---> 1-aminocyclopropane-1-carboxylic acid ---> ethylene

                                 (SAM)                                                 (ACC)   





ethylene inhibitors - chemicals that inhibit the synthesis or action of ethylene


Synthesis Inhibitors (block synthesis of SAM ---> ACC)

- AVG - aminoethoxyvinyl glycine

- MVG - methoxyvinyl glycine

- AOA - aminoacetic acid


Action Blockers (block ethylene ---> action)

- STS - silver thiosulfate

CO2 - carbon dioxide

- Ni - nickel

- Co – cobalt

- MCP – 1-mehtylcyclopropane

o    it is a gas that can saturate the receptor sites, and block action for several days

o    EthylBloc – commercial compound


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Cellular Effects


1)      Auxin – ethylene interactions

           - ethylene alters basipetal transport of auxin; causing abnormalities in growth

           -  auxin induces ethylene production

           -   therefore, some auxin effects may be ethylene effects, and vice versa


2)      Membrane permeability - increases


3)      Respiration - increases


4)      Cell elongation – decreases


5)      Aerenchyma formation – parenchyma that develops intercellular air spaces.


·         Aquatic plants and halophytes - naturally occurring in stems, petioles and roots


Flooded roots and stems - induces aerenchyma formation in stems and roots under anaerobic or hypoxic conditions (i.e. under low oxygen or flooded conditions)

 Sesbania javanica flooded – stem busting open with aerenchyma




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Whole Plant/Organ Effects


6) Fruit ripening in climacteric fruit


Climacteric fruit:           apple, banana, mango, papaya, pear, apricot, peach, plum, avocado, plantain, guava, nectarine, passion fruit, blueberry, cantaloupe, breadfruit, cherimoya, durian, feijoa, fig, kiwifruit, mango, muskmelon, papaya, persimmon, plantain, quince, sapodilla, sapote, soursop, nectarines, and tomato

Non-climacteric Fruit:     Citrus, berries such as blackberry, raspberry, strawberry and cherry, grapes, pineapple, melon (including watermelon), pomegranate


Ethylene stimulates climacteric fruits to ripen.


Gassing fruits in warehouses with ethylene using catalytic converters.

Catalytic converter



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Banana on store shelf - green to yellow stages of ripening

banana store shelf

 7)    Flowering - triggers flowering in some bromeliads, ex. Pineapple


·         Old time method.  Bromeliad and apple in plastic bag.  See YouTube video



·         Old time method – beat pineapple plants with sticks – cannot document


·         Ethylene applied as ethrel to rosette leaves of pineapple





·         FYI, there are many chemicals that can be used to cause pineapple to flower. 
Calcium carbide,  NAA, BOH, acetylene


8)      Flower fading

·         Pollination often triggers ethylene production, and the first sign is flower fading

·         Ethylene application also causes flower fading



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9)      Fruit color – in some non-climacteric fruit, ethylene can decrease green coloration, and increases other colors.

Thus, ethylene may appear to cause fruit ripening in these non-climacteric fruit, but it does not.  Most associate color development with ripening.  But, with non-climacteric fruit the fruit may be fully ripe, but still has coloration when the ethylene is applied.


Degreening of fruit – Example, ethylene is used to degreen citrus.  Why?  Most associate green with “not ripe”, so degreening satisfies consumer preference.

(FYI, this is the same reason why we blanch cauliflower).




10)    Flower longevity

· Ethylene triggers senescence (death) of cut flowers

· Can be stopped by ethylene inhibitors.


Synthesis inhibitors protects the flower from itself.
AVG - aminoethoxyvinyl glycine

MVG - methoxyvinyl glycine

AOA - aminoacetic acid


Block inhibitors make the plant “immune” to its own ethylene and all other/external sources of ethylene

STS - silver thiosulfate

CO2 - carbon dioxide

Ni - nickel

Co – cobalt

MCP – 1-mehtylcyclopropane


FYI, many florist and consumers use floral preservatives, but most do not contain ethylene inhibitors. 

Most contain:
- sugar (sucrose, non-diet soft drink)
- acidifier (asprin, carbonated soft drink, vinegar, citrus juice/citric acid)
- antibacterial agent (bleach, antimicrobial agents)
May help to keep the stem base from becoming clogged with bacterial or fungal growth.  Role of sugar – questionable.

Do floral preservatives really work?
Somewhat.  See Ahmad and Dole, HortTechnology 24:384 2014


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11)    Flower drop

· ethylene will trigger flower drop

·  can be causes by malfunctioning heater



12)    Seed germination

Ethylene may increase germination of dormant seeds of some species.


Plant species whose seed dormancy is broken by ethylene, ethephon (an ethylene releasing compound), or 1-aminocyclopropane-1-carboxylic acid (the direct precursor of ethylene)

Corbineau et.el. Frontiers in Plant Science 5:539 2014.


  Ethylene may help Overcomes Seed Dormancy

Type of dormancy


Primary dormancy

Amaranthus caudatus

Amaranthus retroflexus

Arabidopsis thalianap

Arachis hypogaea

Chenopodium album

Fagus sylvatica

Helianthus annuus

Pyrus malus

Rhus coriaria

Rumex crispus

Stylosanthes humilis

Trifolium subterraneum

Xanthium pennsylvanicum


Lactuca sativa

Secondary dormancy

Amaranthus caudatus
Amaranthus paniculatus
Helianthus annuus
Lactuca sativa
Rumex crispus
Xanthium pennsylvanicum



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13)    Sex expression

·         favors pistillate flower formation on monoecious plants


14)    Leaf epinasty (curling and contortion or leaves)

Ethylene causes in herbaceous plants



15)    Leaf abscission (leaf drop) - ethylene causes in some plants

· ethylene and abscisic acid are both involved in defoliation of deciduous trees in the fall.


In greenhouses, leaf epinasty and/or leaf abscission can be caused by ethylene production by a malfunctioning heater


·  incomplete combustion will produce a flame that is orange to yellow

·   properly adjusted, the flame is blue

·    must be properly vented

·   Can you heat a greenhouse with a wood burning stove?

improper venting

Yellow flame - beware



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Site of Production


abscisic acid









plastids, especially






Historically also called:

abscisin - because early investigators found caused leaf abscission

dormin - because early investigators found caused dormancy




mevalonate ---> farnesyl pyrophosphate ___> ABA




1)   Dormancy - causes bud or seed dormancy

2)   Leaf abscission (leaf drop) - may cause in some plants

3)   Stoma - causes stoma to close (a response to drought stress)


Whole Plant/Organ Effects – Practical uses?  really none.


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Jasmonic Acid



Salicylic Acid





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Light Responses and Light Signaling


Phytochrome – Red and Far Red Light Responses




·         Germination of light requiring seeds

·         Photoperiodic plants sense the time of sunrise and sunset.

o   Allows regulation of daily rhythms, called circadian rhythms

o   Nyctinasty events, e.g. nights

-  leaf rolling/folding

-  photoperiodic nighttime events/processes.


Blue-Light Responses


·         Chloroplast movement

·         Stomatal opening and guard cell functioning.

·         Regulates some gene expression.

·         Sun tracking of leaves

·         Phototropism and asymmetrical bending

·         Inhibits stem elongation – especially right after a seedling emerges from the soil.