Plant Growth Regulators (PGRs)

Plant Growth Regulators (PGRs) Overview

• PGRs are synthetic or naturally occurring chemicals
• PGRs influence plant growth and development
• PGRs achieve this by altering physiological processes at low concentrations (e.g., cell division, elongation, differentiation)
• Plant hormones are naturally occurring organic compounds
• Plant hormones are produced by plants, regulating growth and development
• Examples of plant hormones include auxins, cytokinins, gibberellins, ethylene, and abscisic acid
• PGRs include synthetic and natural compounds applied externally
• Hormones are synthesized within the plant

Common Classes of PGRs

• Auxins promote cell elongation, root initiation, and apical dominance
• Gibberellins (GAs) stimulate stem elongation, seed germination, and dormancy breaking
• Cytokinins promote cell division, delay senescence, and enhance shoot growth
• Ethylene/Ethylene Releasers regulate fruit ripening, abscission, and senescence
• Growth Inhibitors/Retardants slow growth processes, promote dormancy, or reduce elongation
• "Other" PGRs include compounds like jasmonates or brassinosteroids.

Naturally Occurring Plant Hormone Groups

• Auxins
• Cytokinins
• Gibberellins
• Ethylene
• Abscisic acid (ABA)

Auxins

• Auxins promote cell elongation, apical dominance, and root initiation
• Auxins also regulate tropisms and fruit development
• 1-Naphthaleneacetic Acid (NAA) is a synthetic auxin
• NAA is used for fruit thinning, preventing fruit drop, and rooting cuttings
• 2,4-D is a synthetic auxin used as a herbicide at high concentrations and for fruit thinning
• 3-Indoleacetaldehyde (IAld) is a naturally occurring precursor to IAA
• 3-Indoleacetic Acid (IAA) is the primary natural auxin
• IAA promotes cell elongation and root formation
• 3-Indolepyruvic Acid (IPA) is naturally occurring and involved in auxin synthesis
• Indolebutanoic Acid (IBA) is a synthetic auxin, widely used for adventitious root formation in cuttings
• Naturally occurring auxins in plants include IAA, IAld, IPA, 3-indoleacetonitrile (IAN), and ethyl ester of IAA (IAE)
• Auxins reduce or increase fruit drop in apples and pears
• Auxins delay fruit maturation in citrus
• Auxins act as a blossom thinning agent in peaches
• Auxins enhance adventitious root formation for cuttings
• Auxins delay color development in grapes
• Auxins are used on apples, pears, citrus, peaches, grapes, and ornamentals
• High concentrations of synthetic auxins (e.g., 2,4-D) disrupt growth and kill weeds, acting as herbicides

Gibberellins (GAs)

• Gibberellins promote stem elongation, seed germination, and dormancy breaking
• GA3 stimulates seed germination, fruit elongation, and delays senescence
• GA3 is used in citrus for fruit set and seedless fruit production
• GA4+GA7 promotes fruit set and elongation
• GA4+GA7 is used in apples for fruit shape improvement
• Gibberellins promote cell elongation and division in grapes
• Gibberellins overcome seed and bud dormancy in cereals
• Gibberellins increase or reduce fruit set in grapes and apples
• Gibberellins affect fruit shape in apples
• Gibberellins delay senescence in citrus
• Gibberellins stimulate flowering/bolting in biennials like cabbage
• Gibberellins are used on grapes, citrus, apples, cereals, and biennials
• GA3 is a broad-spectrum gibberellin, widely used for fruit set and germination
• GA4+GA7 is more specific, used for apple fruit elongation and shape
• Only a few gibberellin structures like GA3 and GA4+GA7 are commercially used, out of over 100 that exist

Cytokinins

• Cytokinins promote cell division, shoot formation, and delay senescence
• CPPU (Forchlorfenuron) is a synthetic cytokinin that promotes fruit set and size in grapes
• Kinetin is a synthetic cytokinin that enhances cell division and delays leaf aging
• Cytokinins stimulate shoot growth in tissue culture
• Cytokinins delay senescence in leafy vegetables
• Cytokinins enhance fruit size in grapes and kiwi
• Cytokinins are used on grapes, kiwi, leafy greens, and ornamentals

Ethylene/Ethylene Releasers

• Ethylene is a gaseous hormone that regulates ripening, abscission, and senescence
• Ethylene is a natural gas that promotes fruit ripening and abscission
• Ethephon is a synthetic compound that breaks down to release ethylene
• Ethephon is used for fruit thinning and ripening
• Ethylene is used for fruit elimination, thinning, and postharvest degreening in citrus
• Ethylene increases lint strength, promotes boll opening, and aids hybrid seed production in cotton
• Ethylene induces ripening, flowering, and abscission in grain crops
• Ethylene promotes ripening in pome fruits (e.g., apples)
• Ethylene advances ripening and pigment development in tomatoes and grapes
• Ethylene enhances abscission in walnuts
• Ethylene is used on citrus, cotton, tomatoes, grapes, apples, and walnuts
• Younger leaves are less sensitive to ethylene due to higher auxin levels

Inhibitors/Retardants

• Inhibitors/Retardants slow growth processes, promote dormancy, or reduce elongation
• Abscisic Acid (ABA) is a natural inhibitor that induces dormancy and stomatal closure
• Ancymidol is a synthetic inhibitor that reduces stem elongation in ornamentals
• Carbaryl is a synthetic inhibitor used as a fruit thinner in apples
• Chlormequat is a synthetic inhibitor that shortens stems in cereals and ornamentals
• Chloro IPC is a synthetic inhibitor that inhibits growth in turf
• Daminozide is a synthetic inhibitor that reduces vegetative growth in apples
• Flurprimidol is a synthetic inhibitor that controls height in ornamentals
• Hydrogen Cyanamide (H2CN2) is a synthetic inhibitor that breaks bud dormancy in grapes
• Mefluidide is a synthetic inhibitor that suppresses turf growth
• Mepiquat Chloride is a synthetic inhibitor that reduces vegetative growth in cotton
• Paclobutrazol is a synthetic inhibitor that controls tree size in orchards
• Prohexadione Calcium is a synthetic inhibitor that inhibits gibberellin synthesis, reducing shoot growth in apples
• Succinic Acid (SADH) is a synthetic inhibitor that retards growth in ornamentals
• AVG (Aminoethoxyvinylglycine) is a synthetic inhibitor that delays fruit maturity in apples/pears and reduces flower senescence in ornamentals
• Inhibitors/Retardants reduce vegetative growth in cotton and ornamentals
• Inhibitors/Retardants induce dormancy in seeds via ABA
• Inhibitors/Retardants control plant height in cereals and turf
• Inhibitors/Retardants delay fruit maturity in apples via AVG
• Inhibitors/Retardants are used on apples, pears, cotton, cereals, and turf

Physiological Processes Regulated by PGRs

• Auxins regulate cell elongation, apical dominance, root initiation, fruit set, and tropisms
• Gibberellins regulate stem elongation, seed germination, dormancy breaking, and fruit elongation
• Cytokinins regulate cell division, shoot formation, and delayed senescence
• Ethylene regulates fruit ripening, abscission, senescence, and stress response
• Growth Inhibitors/Retardants regulate reduced elongation, dormancy induction, and stress tolerance

PGR Roles in Specific Processes

• Abscission is promoted by ethylene and ABA, and inhibited by auxins
• Dormancy is induced by ABA and broken by gibberellins and hydrogen cyanamide
• Fruit Abscission is promoted by ethylene, ethephon, and carbaryl, and inhibited by NAA and auxins
• Fruit Ripening is promoted by ethylene and ethephon
• Fruit Set is regulated by auxins (NAA, IBA), gibberellins (GA3), and cytokinins (CPPU)
• Leaf Expansion is inhibited by ethylene and promoted by cytokinins
• Plant Senescence is promoted by ethylene and delayed by cytokinins
• Root Initiation is regulated by auxins (IAA, IBA)
• Seed Germination is promoted by gibberellins and inhibited by ABA
• Stem Elongation is promoted by gibberellins and auxins, and inhibited by retardants

Effects on Plant Growth Stages

• Seed Dormancy is induced by ABA and broken by gibberellins
• Seed Growth is promoted by gibberellins and enhanced by cytokinins through cell division
• Vegetative Growth is promoted by auxins and gibberellins, and reduced by retardants
• Flower/Fruit Growth is enhanced by cytokinins, gibberellins, and auxins, with ethylene promoting ripening
• Organ Abscission is promoted by ethylene and inhibited by auxins

PGR Interactions and Effects

• PGRs are effective at low concentrations, e.g., parts per million
• Overapplication or mistiming can cause stunting, excessive growth, or abscission
• Temperature, humidity, and sunlight affect PGR uptake and efficacy
• Sensitivity to PGRs varies with developmental stage, e.g., young vs. mature tissues
• Plant condition, such as stress, fruit load, or disease, alters responses to PGRs
• Auxins/gibberellins stimulate elongation
• Retardants (e.g., paclobutrazol) inhibit elongation
• Cytokinins promote cell division and can delay senescence
• Hormones interact and auxin inhibits ethylene in young leaves, while ethylene promotes abscission

Plant Growth Concepts - Key Definitions

• Abscission: Natural shedding of leaves, fruits, or flowers
• Apical Dominance: Suppression of lateral bud growth by the shoot apex (driven by auxins)
• Apical Meristem: Tissue at shoot/root tips responsible for primary growth
• Bioassay: Test measuring biological activity (e.g., PGR effects)
• Cambium: Lateral meristem producing secondary growth (xylem/phloem)
• Cultivars: Cultivated plant varieties with distinct traits
• Dormancy: Temporary suspension of growth (e.g., seeds, buds)
• Endogenous: Originating within the plant (e.g., hormones)
• Locules: Cavities in ovaries containing ovules
• Meristem: Undifferentiated tissue capable of division
• Parthenocarpy: Fruit development without fertilization (e.g., induced by auxins)
• Phenotypic: Observable traits influenced by genetics/environment
• Phloem: Tissue transporting sugars/nutrients
• Rachis: Main axis of a compound leaf or inflorescence
• Rest Period: Dormancy phase requiring specific conditions to break
• Senescence: Aging process leading to organ death
• Xylem: Tissue transporting water/minerals

Application Technology - Key Terms

• Calibration: Adjusting equipment to deliver precise PGR doses to ensure efficacy and safety
• Parts per Million (ppm): Concentration unit for PGR solutions, critical for low-dose applications

Application Considerations

• Higher doses may not increase efficacy and can cause harm
• Volume ensures coverage without runoff
• Solution pH affects PGR stability and uptake, with acidic pH optimal for some auxins
• Surfactants enhance adhesion/spreading, needed based on leaf surface (waxy leaves require them)
• Use low-pressure nozzles, apply in calm conditions to prevent off-target damage. (Drift Avoidance)
• Some PGRs react adversely with other chemicals so you should test mixtures first. (Tank Mix Compatibility)

Factors Affecting Dosage

• High humidity enhances uptake but may increase runoff
• pH affects PGR solubility and absorption
• Sensitivity to PGRs varies with plant growth stage, e.g., flowering vs fruiting
• Stress or heavy fruit load alters response to PGRs
• Rainfall can wash off PGRs
• Sunlight influences degradation and uptake rates
• Optimal temperature ranges vary; gibberellins less effective in cold

Label Interpretation

• Labels provide dosage, timing, crops, and safety instructions
• Key terms on labels include application rates, re-entry intervals, pre-harvest intervals, and hazards

Specific Hazards

• Ethephon is corrosive and should be handled with care
• Ethephon and gibberellins are flammable and should be stored safely
• Ethephon and gibberellins can cause eye injury, use protective gear
• Hydrogen cyanamide can be a skin irritant or fatal, requiring strict safety protocols
• Carbaryl is a bee hazard, avoid application during pollination
• All PGRs can harm non-target plants, creating a drift risk