15
Plant Growth Regulators
LOUISEFERGUSON ANDJAMESE. LESSENGER
Key words: plant hormones, auxins, gibberellins, cytokinins, ethylene, abscisic acid, phenolics alkaloids
Plant growth regulators (PGRs) were first discovered in plants at the begin- ning of the 20th century. An endogenous PGR is a plant hormone, in aca- demic terms “an organic compound synthesized in one part of a plant and translocated to another part of a plant where, in very low concentrations, it causes a physiological response” in the plant. As knowledge and commercial use of PGRs grew, compounds formerly extracted could be synthesized. Thus PGRs are now both natural (extracted) and synthetic (synthesized) in origin.
Even if a synthesized PGR is identical in structure to a hormone extracted from a plant, it is not considered a plant hormone (1).
Use of Plant Growth Regulators in Agriculture
In agricultural application a PGR is defined as “a substance used for con- trolling or modifying plant growth processes without appreciable phytotoxic effect at the dosage applied.” In order for a PGR to be registered by the U.S.
Environmental Protection Agency, its use, as recommended on the label, must be safe for the plant, its applicator, and the environment as far as can be feasibly determined. Thus, neither plant nor human injury is to be expected from most properly applied PGRs (2).
The PGRs are divided into five general groups of compounds based on their chemical structures and effects on plants. The groups are auxins, gibberellins, cytokinins, ethylene, and a group called inhibitors, which includes abscisic acid, phenolics, and alkaloids. Some new PGRs do not fit neatly into these classifica- tions but are described as having effects that resemble those for known PGRs.
For example, cytokinin-like is a term used to describe new products extracted from seaweed products. A new PGR may be developed to counter the effects of a known plant hormone by interfering with natural plant hormone production.
A general description of each growth regulator’s effect on plant growth follows.
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Plant Growth Regulators 157
Auxins
Auxins stimulate cell division and elongation, stimulating a plant stem to grow taller, or in a specific direction.
Gibberellins
Gibberellins stimulate extension growth of plants and delay aging. This gen- erally produces a plant that grows and produces faster, or fruit peels that remain green longer than an untreated plant.
Cytokinins
Cytokinins affect cytokinesis, or cell division, cell enlargement, dormancy, flowering and fruiting, and senescence. In agricultural application this trans- lates into propagating new plants in the nursery, breaking seed dormancy, or delaying senescence.
Ethylene
Ethylene affects growth, ripening, and senescence in plants. In agricultural applications it is used primarily to promote ripening.
Abscisic Acid
Abscisic acid generally inhibits growth and germination and promotes dormancy.
Phenolics
Phenolics affect the metabolic system of plants. In agricultural application, phenolics are used primarily to delay abscission of fruits.
Toxic Effects of Plant Growth Regulators
Details of the effects of some representative PGRs on various species, includ- ing humans, are given in Table 15.1. If properly used, PGRs have an excellent safety record. However, if the wrong concentration is used, if safety equip- ment is not properly used, or if the application times are not correct, poison- ing can occur in plants, animals, and humans.
A good example is hydrogen cyanamide. This PGR is considered very effective and economical in assuring uniform bud break in crops including peaches, kiwis, and grapes. Uniform maturation of buds results in the uni- form ripening of the fruit so that all the fruit on one tree can be harvested at
TABLE15.1.Plant growth regulators. Compound Chemical name Common name Trade nameTarget cropAction on plantSentinel human effect Chemical name:A-Rest® controls the height A-Rest® is a plant growth ●Acute toxicity (ancymidol) (α-cyclopropyl-ofcontainer-grown lilies,regulator that reduces Mouse:LD505000 mg/kg α-(4-methoxyphenyl)-poinsettias,chrysanthemums,internode elongationRat,adult:LD504500 mg/kg 5-pyrimidine methanol)dahlias,tulips,and foliage Rat,newborn:LD50350 mg/kg and bedding plants;Dog:LD50> 500 mg/kg (emesis) Common name(s):effectively reduces internode Monkey:LD50> 500 mg/kg (emesis) ancymidolelongation,resulting in more Chicken:LD50> 500 mg/kg (registered with EPA)compact plantsFor A-REST®,the oral LD50in rates is > 5 mL/kg Trade name(s) and ●Subacute toxicity:all rats survived without manufacturer(s):significant toxicity when fed 8000 ppm ofancymidol (A-Rest®) for 3 months.Dogs given daily oral 200 mg/kg doses Dow Elanco & Co.ofancymidol for 3 months survived without significant toxicity Dermal toxicity and inhalation dangers:when ancymidol and A-REST® at a concentration of 5 mg/L ofair were applied to the back ofrabbits, subjects were unaffected Chemical name:Increases the number ofSynthetic cytokinin,which ●General toxicity to wildlife and fish:TLM n-(phenylmethyl)-9-lateral branches developed stimulates growth ofdormant – 96 hr – 52 mg/L (blue gill) (tetrahydro-2h-pyran-2-yl)-on chrysanthemums,lateral buds in several TLM – 24 hr – 3 mg/L (trout) 9h-purine-6-aminecarnations,and rosesplant species●Acute toxicity—LD50926 mg/kg (mice), 1640 mg/kg (rats) Common name(s):●Subacute toxicity—NEL > SD8339 (registered 10,000 ppm (mallard) with EPA)NEL > 4,640 ppm (bobwhite quail)
Trade name(s) and ●Chronic toxicity—no teratogenic effect at manufacturer(s):320 mg/kg in rabbits ACCEL® ●Dermal toxicity and inhalation dangers—no Plant Growth Regulator,identified hazards at use concentrations Abbott Laboratories Chemical name:Chemical control ofaxillary There is no information on ●Acute toxicity to mammals (2-chloro-n-[2,6-dinitro-4-bud (sucker) growth after the mechanism ofaction ofCGA-40065 (trifluoromethyl)-topping the floral portion CGA-41065 in controlling the TechnicalPrime +1.2E phenyl]-n-ethyl-6-oftobaccogrowth and development ofAcute oral LD50>5000 mg/kg4400 mg/kg fluorbenzenemethanaminetobacco suckers (rat) Common name(s):Interferes with cell division by Acute dermal LD50>2000 mg/kg2010 mg/kg CGA-41065M,PRIME+binding to tubulin,preventing (rabbit) tubulin from forming Eye irritation ModeratelyCorrosive on Trade name(s) and microtubules which are required (rabbit)Irrigatingcontact manufacturer(s):for proper spindle development Primary skin Mildly Moderately PRIME+1.2Eand mitotic divisionirrigationirritatingirritating Chemical name:Improving fruit set ofGA3is a growth promoter;it ●Acute toxicity—oral administration ofmassive Gibberellic acidblueberries with insufficient accelerates vegetative growth single doses of1 g kg−1produced no toxic pollination;used for better ofshoot producing larger symptoms in rats;mice – 15 g kg−1no toxic Common name(s):color,firmer fruit,larger fruit,plants;mostly due to cell symptoms Gibberellic acid,GA3and to prolong the harvest elongation but sometimes cell ●Subacute toxicity—dogs and rats have no ill effects (registered with EPA)period ofcherries;used to multiplication may be involved;from daily doses of1 g kg−16 days per week for reduce cherry flowering and induces flowering by breaking 90 days;one group ofrats has been fed for Trade name(s) andfruiting to minimize the dormancy 14 months,no toxic symptoms observed manufacturer(s):competitive effect ofearly●Chronic toxicity—no ill effect on rats fed 5 to Berelex (ICI),observedfruiting on tree development;8 weeks with 5% GA3in diet;no changes found in Gib-Tabs (Microbial for cluster loosening and organs tissue;guinea pigs – no toxic signs fed with Resources),Gib-Sol elongation and berry size kale for 3 months,treated with 250 ppm. (Microbial Resources),increase ofgrapes Pro-Gibb (Abbott),Pro-Gibb Plus (Abbott) continued
TABLE15.1.Plant growth regulators.(continued) Compound Chemical name Common name Trade nameTarget cropAction on plantSentinel human effect Chemical name:Systemic plant growth regulator Decreases plant height,increases ●Acute oral toxicity—oral LD50(rats) = 1420 mg/kg; 1.1-dimethyl-piperidinium for limiting undesired boll set and yield;inhibits all no eye irritation chloridevegetative growth ofthe elongation and node formation ●Dermal toxicity and inhalation dangers—acute Common name(s).cotton plant;reduces growth,and increases leafmesophyll dermal LD50(rats) is greater than 5000 mg/kg; Mepiquat-chloride increases boll set;affects structure;possibly inhibits inhalation toxicity (rats) no reaction after (registered with EPA)maturity,yield,boll rotbiosynthesis ofgibberellin acid4-hour exposure Trade name(s) and manufacturer(s): PIX-BAS 08300E by BASF Wyandotte Corp. Chemical name:Regulates the growth ofInhibition ofgrowth and ●Acute toxicity n-[2,4-dimethyl-5-various species ofturfdevelopment ofthe Mice oral LD501920 mg/kg [[(trifluoromethyl)-sulfonyl] grasses and broadleafmeristematic regions ofRats oral LD50> 4000 mg/kg amino]phenyl]acetamidevegetation by suppressing responsive plantsRabbits > 4000 mg/kg seed-head formationdermal LD50 Common name(s):●Subacute toxicity Mefluidide (WSSA,ANSI) Dogs 90-day feeding “no effect”at 1000 ppm in diet (registered with EPA)Rats 90-day feeding “no effect”at 6000 ppm in diet ●Chronic toxicity—in lifetime feeding studies with Trade name(s) and rats and mice no adverse effects related to ingestion manufacturer(s):ofmefluidide at a dose level of600 ppm in the diet EMBARK®were observed.Reproduction and pup survival were 2-s PBI/Gordon Col.not affected by mefluidide up to 6000 ppm in the diet during a 18-month 3-generation study with rats
●Dermal toxicity and inhalation dangers—rabbits primary skin irritation study:no irritation to abraded and nonabraded skin;rats 4-hour acute aerosol inhalation LD50:> 8.5 mg/L air Chemical name:Thin apples,olive and pear Auxin activity●Acute toxicity (rats)—LD50approximately 1000 mg/kg 1-naphthaleneacetic acidblossoms;control apple body weight and pear preharvest drop;●Chronic toxicity—8-day dietary LC50(bobwhite Common name(s):stimulate root formationquail):acute LC50estimated to be greater than NAA (registered 10,000 ppm;8-day dietary LC50(mallard duck): with EPA)acute LC50estimated to be greater than 10,000 ppm ●Dermal toxicity and inhalation dangers— Trade name(s) and acute dermal toxicity (rabbits)—LD50greater than manufacturer(s):5.0 mg/kg body weight;not considered to have NAA-800;Fruitone®;potential as a primary skin irritant;primary In Rootone® Rhone-eye irritation (rabbits)—considered an eye irritant; Poulenc Ag Company acute inhalation toxicity (rats)—not considered toxic Products Co.,Inc.by the route ofadministration Chemical name:Spray gynoecious cucumbers Close similarity in growth ●Acute toxicity—the acute oral LD50ofgibberellin A4, ga4-(1α,2β,4aα,4bβ,10β)-to develop male flowersregulating activity ofGA3but A7is >500 mg/kg (for mice) 2,4a-dihydroxy-differs in its capability of●Dermal toxicity and inhalation dangers—avoid 1-methyl-8-methylene influencing growth in some use in poorly ventilated areas gibbane-1,10-dicarboxylic species in ways not achievable acid,1,4a-lactonewith gibberellic acid ga7-(1α,2β,4aα,4bβ,10β)-2, 4a-dihydroxy-1-methyl-8- methylene gibb-3-ene-1, 10-dicarboxylic acid,1, 4a-lactone continued
TABLE15.1.Plant growth regulators.(continued) Compound Chemical name Common name Trade nameTarget cropAction on plantSentinel human effect Common name(s): Mixture ofGA4+GA7 (registered with EPA) Trade name(s) and manufacturer(s): Pro-Gibb 47,Abbott Labs Regulex,ICI Plant Protection Chemical name(s):Increases red delicious apple Gibberellin activity●Acute toxicity:the acute oral LD50ofN-(phenyl ba-6-benzylamino purinesize,weightmethyl)-1H-purine on mice is 1690 mg/kg,while for gibberellin A4,A7it is greater than 500 mg/kg Ga4-(1α,2β,4aα,4bβ,10β)-2,●Dermal toxicity and inhalation dangers:promalin 4a-dihydroxy-1-methyl-8-is nonirritating in dermal toxicity testing but does methylene gibbane-1,cause injury to the eye 10-dicarboxylic acid,1, 4a-lactone ga7-(1α,2β,4aα,4bβ,10β)-2, 4a-dihydroxy-1-methyl-8- methylene gibb-3-ene-1, 10-dicarboxylic acid,1, 4a-lactone Common name(s): Mixture ofBA +GA4+GA7 (registered with EPA)
Trade name(s) and manufacturer(s): Promalin,Abbott Labs Chemical name:Apples:promoting fruit Releases ethylene in plant tissues●Acute toxicity:rats—LD504229 mg/kg body weight (2-chloroethylphosphoric maturity and loosening fruit;●Subacute toxicity:8-day dietary LC50(mallard acid)promoting uniform ripening ducks) 196 LC50greater than 10,000 ppm;static Common name(s):and coloring ofred varieties 96-hour toxicity (blue-gill sunfish)—TL50estimated Ethephon (ANSI) without loosening fruit;to be about 311 ppm Static 96-hour toxicity (registered with EPA)increasing flower bud (rainbow trout)—TL50estimated to be about development on young trees357 ppm Trade name(s) and Cantaloupes:promoting ●Chronic toxicity:considered noncarcinogenic, manufacturer(s):fruit abscissionnonmutagenic,nonteratogenic CERONE®,Cherries:uniform ripening ●Dermal toxicity and inhalation dangers:acute ETHREL®,Plant and loosening fruit,increasing dermal irritation (rabbits)—5730 mg/kg—a single Regulator,ETHREL® dormant fruit bud hardiness,instillation of0.1 ml ofEthrel into the eye showed Pineapple Growth and delaying spring bloom corneal damage in the nonirrigated group at day Regulator,PREP,ofsweet cherries in the 14 whereas the irrigated group had recovered;acute CHIPCOR®,FLOREL™ Pacific Northwest.inhalation toxicity (rats)—inhalation of2 mg/L air Plant Growth Regulator – Cotton:accelerating uniform caused signs ofirritation during 1 hour exposure; Rhone-Poulenc Ag Companyboll opening all animals appeared normal during 14 days Cucumbers andsquash:modifying following exposure sex expression for seed production Daffodils:use a drench for shortening stems offorced bulbs. Geraniums:to increase branching Greenhouse roses:use for basal bud stimulation Peppers:early,uniform ripening and coloring fruit continued
TABLE15.1.Plant growth regulators.(continued) Compound Chemical name Common name Trade nameTarget cropAction on plantSentinel human effect Tobacco:hastening “yellowing” ofmature tobacco and reducing curing time Tomatoes:accelerating and concentrating fruit ripening Walnuts:loosening walnuts and improving full removal Chemical name:Used on fruit-bearing trees,Plant growth regulant of●Acute toxicity:acute oral LD50(rat):8,400 mg/kg Butanedioic acid mono (2,2-peanuts,ornamentals,grapes,unknown mechanism●Subacute toxicity:in a 90-day feeding study with dimethylhydrazide) (formerlyand tomatoes;daminozide rats,technical-grade daminozide had no adverse succinic acid-2,2-has a wide range ofeffects,effect when fed at 43,200 ppm in a daily diet dimethylhydrazide)from retarding vegetative ●Chronic toxicity:a 2-year feeding test with rats growth,to controlling and dogs indicated that technical-grade daminozide Common name(s):harvest quality factors,had no adverse effect at 3000 ppm in the daily diet Daminozide (ANSO,ISO,increase red color in apples,(highest rate tested);a three-generation reproduction BSI);Former literature to concentrating maturityand lactation study with rats fed 300 ppm references include SADH,technical-grade daminozide in the diet showed no B995,B9,and aminozide.significant effect on either fertility or reproductive Note:Food uses will be capacity canceled.Ornamental uses ●Dermal toxicity and inhalation dangers: remain in effect.Acute dermal LD50(rabbit):> 5000 mg/kg Acute inhalation LC50(rat):> 147 mg/L Trade name(s):and ●Symptoms ofpoisoning:no known symptoms manufacturer(s): ALAR-85,KYLAR-85, B-NINE-SP by Uniroyal Chemical
Chemical name:Pears and apples:promotion ●Acute toxicity:LD50oral:male 2-chloroethyltrimethyl offruit bud formation,albino rats 0.48–0.94 g/kg ammonium chlorideand increasing yields;also ●Subacute toxicity:repeated feeding to rats for allows young trees to 29 days at 500,1000,and 2000 mg/kg;all animals Common name(s):bear soonerhad normal behavior and food intake and weight Chlormequat chloride(BSI);Azaleas:to produce earlygain ofthe test rodents remained within other names are CCC,budded,compact symmetrical normal limits chlorocholine chloride,plants for use in commercial ●Chronic toxicity:12 months offeeding ofrats at El 38,555 (Registered forcing ofearly500 and 1000 ppm gave no symptoms of with EPA)blooming azaleaspoisoning,differences in mean body weight, appearance or behavior,no differences in various Trade name(s) and hematological and clinical chemical determinations, manufacturer(s):and no untimely deaths were observed;in 12-month CYCOCEL® feeding studies with dogs on a diet containing Plant Growth Regulant,300 ppm (calculated on dry weight),regular American Cyanamid examination ofanimals showed no gross or Company;HORMOCEL,microscopic pathology referable to the compound; All India Medical Corporationdeterminations ofacetyl-cholinesterase in erythrocytes and plasma made after 4,8,and 12 months on test indicated that the compound had no effect on red blood cells or plasma Chemical name:Tree growth retardantInhibitor ofgibberellin ●General toxicity to wildlife and fish:contains (±)-r*,r*)-β-[(4-chlorophenyl)biosynthesismethanol and cannot be made nonpoisonous methyl]-α-(1,1-dimethylethyl)-●Symptoms ofpoisoning:same as methanol 1h-1,2,4-triazole-1-ethanol Common name(s): Paclobutrazol (registered with EPA) Trade name(s) and manufacturer(s): Clipper® Tree Growth Regulator Source:Data from Cutler and Schneider (2).
the same time. For many years hydrogen cyanamide was used as a medication for the treatment of alcoholism in humans (albeit in much smaller concen- trations than is used on plants). The chemical causes an Antabuse reaction in people who consume alcohol, including nausea, vertigo, hypotension, and, in extreme cases, circulatory collapse and death. Therefore, people who mix and apply the chemical must wear full protective ensembles, used closed air sup- ply cabs for the tractors, and never consume any alcohol in liquid form or as a perfume or after-shave. Care must be exercised to isolate the field until the reentry period has passed. Treatment is supportive (see Chapter 13) (3,4).
References
1. Salisbury FB, Ross CW. Plant Physiology, 2nd ed. Belmont, CA: Wadsworth, 1978.
2. Cutler HG, Schneider BA. Plant Growth Regulator Handbook. LaGrange, GA:
Plant Growth Regulator Society of America, 1990.
3. Lessenger JE. Case study: hypotension, nausea and vertigo linked to hydrogen cyanamide exposure. J Agromed 1998;5:5–11.
4. Formoli TA. Estimation of exposure in persons in California to pesticide products that contain hydrogen cyanamide. Sacramento: California Environmental Protec- tion Agency, 1993.
166 L. Ferguson and J.E. Lessenger
