growth stimulants, retardants, and rooting hormones · 2019. 12. 9. · explain why chemical...

Growth Stimulants, Retardants, andRooting Hormones

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OBJECTIVETolistfourtypesofgrowth

regulatorsusedinthe

horticultureindustry.

KEY TERMSallelopathy

apicaldominance

auxins

blossomset

cytokinins

gibberellins

greenmanure

hormones

inhibitors

node

PGR

photorespiration

retard

retardants

U N I T

5

COMPETENCIES TO DEVELOPAfterstudyingthisunit,youshouldbeableto:

listoneexampleofasubstanceusedtostimulateplant

growth.

explainwhychemicalretardantsareappliedtofloralcrops

andnametwocommonlyusedretardants.

explaintheuseofrootinghormonesoncuttingsandlist

severalrootinghormones.

describeorallythesourceofoneplantgrowthstimulant.

demonstratetheproperapplicationofarootinghormoneto

cuttings.

explainhowrootstockisusedindwarfingfruittrees.

definebiostimulants.

MATERIALS■ chrysanthemumandpoinsettiaplants

■ rootinghormones(Hormodin#3and#1)

■ gibberellicacid

■ B-Nine

■ AmericanandEnglishhollycuttings(semihardcuttingsfromdiffer-

entshrubsmaybeused)

■ mistsystemorplasticenclosedpotsorflats

■ knifeformakingcuttings

80392_05_Unit05_p062-071.indd 6280392_05_Unit05_p062-071.indd 62 2/26/10 10:54:02 PM2/26/10 10:54:02 PM

Copyright 2011 Cengage Learning, Inc. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part.

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GROWTH STIMULANTS, RETARDANTS, AND ROOTING HORMONES UNIT 5

GROWTH-REGULATING substances in plants

are called hormones. Hormones are organic

chemicals that act and interact to affect growth rate.

An example is seen in Figure 5–1. The gardenia plant

on the right was treated with the growth retardant

B-Nine. A more compact, lower-growing plant is

produced. Prominent hormones are

❉ auxins (Greek word meaning to increase): accelerate growth by stimulating cell enlargement.

❉ gibberellins: stimulate growth in stem and leaf by cell elongation. Also, they stimulate premature flowering, growth of young fruits, and breaking of dormancy.

❉ cytokinins: stimulate cell division. They work along with auxins (will not work without auxins present).

❉ inhibitors (abscisic acid): inhibit seed germination and stem elongation, and hasten ripening of fruit (ethylene gas).

The auxins and gibberellins promote cell enlarge-ment, and the cytokinins stimulate cell division.

Growth hormones are organic chemicals principally produced by actively growing plant tissue such as shoot tips and young leaves. They move throughout the plant and may be found in many plant parts.

These chemicals react with one another in a very complex system in the plant. In some cases one concentration or amount of a hormone stimulates growth and a different concentration or amount restricts growth.

Apical DominanceA good example of regulating growth in plants is seen in the dominance of the terminal bud or shoot. Where apical dominance exists, the terminal bud secretes chemicals that inhibit or prevent the growth of axillary buds on the same shoot. Axillary buds are found in the axil or angle between a leaf and the stem. This causes the plant to grow tall and not send out side branches. Once the plant reaches flowering age and the ter-minal bud becomes a flower bud, the chemicals are no longer secreted. At this time, the plant starts sending out side branches. It appears that this is a genetic program directing the plant to grow above competing plants in its early years. Once height and access to sunlight are secured, the plant spreads over its competitors. Pinching out the terminal leaf bud of the shoot nullifies the effect on axillary buds, and branching occurs sooner than the plant would have branched if left alone. This method is commonly used by nurser-ies to encourage early branching of shrubs and the production of more compact plants.

For years, humans have been develop-ing methods other than pruning to control the growth rate, size, and shape of plants. Some major achievements include the discovery of

❉ chemical and natural stimulants that cause plants to grow taller or faster.

❉ chemical retardants that cause plants to grow slower or cease growing at a certain point.

❉ hormones that cause cuttings to root faster.

❉ dwarfing rootstock for tree fruits.

FIGURE 5–1 The growth retardant B-Nine used on the gardenia plant on the right results in a compact, low-growing plant.

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G R E E N T I PGoGreenbyusingthenaturallyoccurring

chemicalsintheplantbyprevention,monitor-

ingandcontrollingplantgrowth.

StimulantsApplications of certain chemicals enable plants to grow taller. The most common chemical of this type is gibberellic acid. An example of the



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SECTION 2 PLANT SCIENCE

A natural growth stimulant has been found in alfalfa, and although this stimulant has been known by nurserymen for more than 30 years, interest in it has recently been renewed. The chemical that causes the stimulation of growth is the alcohol triacontanol.

Plant stimulation is brought about by mulch-ing with alfalfa hay or watering plants with a tea made by soaking alfalfa feed pellets in water and using the tea to water the plants. The amount of triacontanol needed is very small (4 milligrams per acre of crop). Too much seems to reduce the stimulating effect. This material is also sold commercially.

Results with rhododendron seedlings were

1. 55.1 percent increase in height.

2. 27.6 percent increase in diameter.

3. 27.6 percent increase in number of leaves.

4. 53.5 percent increase in leaf length.

5. 19.3 percent increase in leaf width.*

This increased plant growth resulted with no added fertilizer. See Figure 5–3.

If you wish to experiment with this material see the Student Activities section at the end of this unit for detail.

effect this chemical has on plants is shown in Figure 5–2. The chrysanthemum shown here was treated with gibberellic acid. Notice how much taller than normal it is.

Gibberellic acid causes the stems of plants to stretch out. The nodes, the joints at which buds, leaves, and branches grow out from the stem, are farther apart. Notice how much longer the stems that hold the blossom buds are on the treated plant. Gibberellins also occur naturally in plants.

FIGURE 5–2 A plant treated with gibberellic acid shows elongated stems. The space between nodes on the stem is lengthened, causing the plant to grow taller than normal. Gibberellic acid is naturally present in plants.

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*Research done by Dr. Hinderman of Ann Arbor, Michigan

G R E E N T I PByusingnaturalEpsomsalts,magnesiumsul-

phate,9.8%magnesiumand6%sulfur,youfor-

tifythecrownofaplantandstimulatebudsto

sproutnewwoodfromthebase.Epsomsalts

helproseplantsorotherfloweringshrubsto

increasethenumberofblossoms.

FIGURE 5–3 Growth increases on rhododendron using alfalfa as a stimulant. Examples A and B are untreated controls; C and D have been treated with alfalfa.

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Gibberellic acid causes stems to elongate or lengthen. This chemical is taken up by the leaves, stem, and roots and moves through the plant to the growing point where it inhibits the production of gibberellic acid.

Plant retardants are also being used in the landscape to control growth on shrubbery and hedges. One application of the PGR Atrimmic after pruning or some regrowth is claimed to last the entire season on plants that have only one flush of growth, thus reducing or eliminating the need to prune. Plants that have a continuous growth habit would need a treatment again in eight weeks.

Atrimmic is a systemic (a material that pene-trates the plant, enters the plant sap, and moves through the plant) that works by blocking the plant hormones that stimulate growth. Applied as a foliar spray, it is absorbed by the leaves and moves to the shoot tips where it temporarily stops shoot growth. Figure 5–6 shows an exam-ple of the use of Atrimmic on a hedge.

Rooting HormonesWhen plants are propagated from cuttings, it is important that a large percentage of the plants root and that they root as quickly as possible. Some plants root very easily from cuttings with-out any chemical treatment. Geranium, azalea, and many soft, succulent houseplants require only a moist, well-aerated medium and high

Chemical RetardantsAt times, plants may grow too tall and open to be pleasing to customers. In these cases, chemi-cals are used to retard the growth of the plants, causing them to be shorter and more compact. Not only are these plants more attractive, but also the shorter stems are better able to sup-port the flowers, thereby reducing the need for staking or tying. Today, chemical retardants ( chemicals that retard or slow down growth) are used commercially as a normal part of the grow-ing process of many plants. Figure 5–4 illustrates the effects of B-Nine used on azaleas.

One of the newest uses of plant growth retar-dants (PGR,s) is the application of a chemical to retard lawn grass growth. If applied at first gree-nup, after the first mowing of grass in the spring, a chemical called Limit is absorbed by the roots and restricts grass growth for six to eight weeks. Another material called Embark is leaf-absorbed and also restricts grass growth.

Figure 5–5 shows two strips of turf grass. Grass on the left side was treated with Embark, a PGR that is absorbed through the leaves and moves to the growing point where it interrupts cell division, stem elongation, and seed head formation. One big advantage of this material is that roots continue to grow. Embark should be applied after the first or second mowing in spring.

A new PGR called Sumagic reduces the height of plants by inhibiting the natural pro-duction of the plant hormone gibberellic acid.

FIGURE 5–5 Two strips of turf grass. The strip on the left was treated with Embark. There is a tremendous reduction in growth rate compared with the untreated grass on the right.

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FIGURE 5–4 The plant growth retardant (PGR) B-Nine was applied to the two azaleas on the right. Notice the compact growth and heavier flowering compared with the untreated plant on the left.

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humidity to root. Many plants must have some assistance if any or very many of the cuttings are to root successfully.

Why is there a difference in ease of rooting? Early researchers discovered that some plants have more natural root-promoting chemicals than others. Indoleacetic acid is a natural plant hormone that causes roots to form on plant stems. It is found in various plants in differing amounts.

Plants also root more easily or with more difficulty at different stages of maturity, which affects the hardness of the wood. Some root best when the wood is soft, others when it is almost hard.

The development of chemical rooting hor-mones made it possible to root certain plant cuttings that were considered impossible to root before. These chemicals also shortened the length of time required to root cuttings.

The chemical most commonly used for root-ing cuttings is indolebutyric acid. Naphthale-neacetic acid and alpha-naphthaleneacetic acid are also widely used. Indoleacetic acid, the first naturally occurring plant hormone to be used, is rarely, if ever, used today. It is generally not as effective as indolebutyric acid or naphthale-neacetic acid.

Rooting hormones are either mixed with talc and used as powders or dissolved in liquid and used as a wet dip. Some of the new liquid dips give better rooting percentages than do the

powders. The strength of the active chemical varies from 0.1 percent to 0.8 percent, although concentrations as great as 2 percent of indolebu-tyric acid have been used on certain evergreens such as camellias and yews. Whatever their con-centration, all rooting hormones should contain a fungicide (a chemical that kills fungi) such as captan to prevent rotting of the cutting. Captan also seems to help promote faster rooting.

The holly cutting on the left in Figure 5–7 was treated with indolebutyric acid; the one on the right was untreated. After 21 days, the treated cutting was rooted. The untreated cutting had developed no roots.

Dwarfing RootstocksAs the cost of labor increased, orchardists began looking for smaller fruit trees that could be picked without the use of ladders. Research was started to discover a way to prevent apple trees from grow-ing very tall. Pruning had been used to reduce growth, but the researchers realized that it was a time-consuming, expensive, and only partially effective method. It was then discovered that trees growing from certain types of roots did not grow as large as other trees. These trees also bore fruit at an earlier age. Using this principle, a complete series of rootstock known as the malling rootstock was developed in England. (Rootstock is a root or piece of root that is used for grafting.) It became possible to control the size and rate of growth of apple trees by selecting the proper rooting stock.

FIGURE 5–7 The holly cutting on left treated with 1.5% indolebutyric acid developed a good root system, whereas the untreated cutting did not.

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FIGURE 5–6 The shrubbery shown above was treated with the PGR atrimmec. The treated area on the left has made very little growth compared with the untreated area on the right.

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Some varieties rooted from the malling rootstock grow close to normal size, whereas others are only one-fourth to one-half the normal size.

Since the original development of the mall-ing stock, many other types have originated, including stock for apple, peach, and pear trees. The dwarf trees themselves can be purchased commercially. Check with your local agricultural experiment station to determine which root-stock is best for grafting or budding fruit trees in your area. Consult nursery catalogs for listings of dwarf fruit trees.

The Chemical Blossom SetOne type of growth hormone is used on tomato blossoms early in the season to cause earlier development of fruit. Normally, the first blos-soms on tomato plants often do not set (develop) fruit because of cool weather, lack of pollination, or for other reasons. The chemical blossom set

causes seedless fruit to set on the blossoms and results in tomatoes ready to eat as much as 10 days earlier than normal. Experiment with some of the commercially used plant growth regula-tors in Figure 5–8.

Plant BiostimulantsBiostimulants are natural products. They are organic, meaning from living organisms, and work to both stimulate soil microbial activity and improve soil cation exchange capacity, stim-ulate plant growth, and promote disease resis-tance. Humic acid, a product of humus or rotting organic matter, is an example of a soil microbial stimulant.

Root growth biostimulants improve water and nutrient uptake by the roots and also increase the number of fibrous roots. High levels of these growth stimulants are found in kelp sea-weed. Fresh kelp is most effective and has been found to greatly increase growth and yields of plants. Processing such as drying, freezing, boil-ing, and chemical treatment decreases the stim-ulating effect. There is a product on the market that is processed to retain the stimulating effect of kelp.

Use of biostimulants can greatly reduce the need for fertilizers, especially nitrogen. This not only saves the grower money, but also

FIGURE 5–8 Commercially used plant growth regulators.

TRADENAME EFFECT USED ON

A-Rest shortens stem chrysanthemum length lily poinsettia

B-Nine shortens stem chrysanthemum length hydrangea petunia azalea poinsettia

Cycocel shortens stem on poinsettia poinsettia induces azalea heavier flower bud geranium set on azalea

Phosfon shortens stem lily length chrysanthemum

Florel induces basal rose breaks on roses poinsettia retards growth carnation on poinsettia tomato increases number cuttings of branches on carnation speeds ripening of tomatoes assists in faster rooting of cuttings

Bonzi shortens stem poinsettia length geranium petunia chrysanthemum impatiens coleus pansy snapdragon celosia

Sumagic shortens stem poinsettia length chrysanthemum celosia coleus marigold periwinkle petunia ageratum dahlia

Growth retardants work best when the humid-ity is high, but plant foliage is dry. Always follow exactly the directions on the label.

(Note: wear protective clothing when called for on the label.)

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reduces water pollution from excess nitrogen run-off or percolation into ground or stream water.

Methanol (a form of alcohol) used in small amounts appears to speed up plant growth. In tests by Dr. Andrew A. Benson (University of California), methanol (methyl alcohol) increased plant yield by 36–100 percent. A 10–30 percent solution of methanol was used.

The alcohol seems to work by blocking pho-

torespiration. Plants use the water for growth rather than transpiring it into the air. This mate-rial works best on plants grown in full sum-mer sun. Water use was reduced by as much as 50 percent from some plants.

C a u t i o nStayonthelowsideuntilplants’responseor

possibledamageisobserved.

This product is already on the market. Other plant stimulants such as triacontanol and DCPTA are being studied as possible growth regulators.

An aspirin to stimulate plants? Scientists have discovered that salicylic acid is produced by plants when attacked by a disease organism. Spraying the plants with salicylic acid stimulates them to put their defenses into action. Applying the acid before plants are attacked may prepare them to resist diseases even before they are present.

Plants’ Biological ClocksPlants have clocks that operate on a 24-hour cycle and go off each morning just as your alarm clock does.

According to Autar Mattoo, a plant physiolo-gist in Vegetable Laboratory of the Agricultural Research Service in Beltsville, Maryland, the clock is located in the nuclei of plant cells. The clock controls an enzyme that modifies a pro-tein which is necessary for photosynthesis as day starts and light energy is available.

The clock also regulates plants’ adjustment to the brightest, hottest part of the day by stimu-lating them to put on sunscreen. Too much sun destroys leaf protein, which is essential in photo-synthesis. Plants use natural sun blockers called flavonoids for protection during mid-day in sum-mertime.

AllelopathyAllelopathy is the production of a chemical com-pound by one plant that slows down or stops the growth of another plant. This is a natural growth regulator some plants use to stop competition from other plants. Some green manure or cover crops grown for weed control, such as millet, have this effect on most weeds and prevent weed growth. The effect of these growth regulators may last for months or in some cases, such as the black walnut, years.

Some plants do not transplant well into beds where older plants of the same type are or recently were growing. Members of the rose fam-ily are good examples (hawthorn, apple, rose, peach, pear, etc.).

SummaryPlant growth is the result of many naturally occurring chemicals in the plant. The materials work

together to promote natural growth rates, flowering, rooting, and other activities. Humans have learned

to add greater amounts of the materials than naturally found in the plant to change growth rate (faster or

slower) or to more easily root cuttings. Some manmade chemicals slow down or speed up production of

growth-regulating chemicals in plants.



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Student Activities

1. Apply gibberellic acid to an actively growing plant. Keep an identical plant that has not been treated as a check on the results.

2. Apply B-Nine to five poinsettias and five mum plants. Keep five untreated plants of each type as check plants. Record the differences noted in the plants’ growing habits. (Be sure to follow exactly directions on the label.)

3. Start 20 holly cuttings. (If necessary, refer to Unit 7, Semihardwood Cuttings, for procedural suggestions.) Use a commercial rooting compound on 10 of the cuttings. Keep 10 untreated plants as checks. Be sure to follow the directions when using the rooting hormone. Record the results.

4. Contact a local orchardist or a state experiment station for details on dwarf fruit trees.

5. Alfalfa growth stimulus experiment. Take some potted plants just starting their growth cycle (about a week after transplanting) and treat as follows with alfalfa.a) Finely chop some good-quality alfalfa hay.b) Mix 1/2 cup of the chopped hay with some hardwood mulch and place on top of the soil in a 6- to

10-inch pot; water it well.c) Compare with control plants with no alfalfa.

An alternate method is to soak alfalfa pellets (cattle feed) in water (1 1/2 cups of pellets per 5 gallons of water) for at least a day. Water the plants with 1/2 cup of this tea every 7 to 14 days.

6. Search the Internet using the following terms: plant hormones; growth stimulants; U.S.D.A.

7. Explore the Internet web site: www.usda.gov.

Self-Evaluation

Multiple ChoiceSelect the best answer from the choices offered to complete the statement or answer the question.

1. Plants may be made to grow taller by applying the chemicala) naphthaleneacetic acid. c) phosphon.b) gibberellic acid. d) indoleacetic acid.

2. A naturally produced plant growth stimulant has been found ina) corn. c) alfalfa.b) the soil. d) hardwood mulch.

3. Plants are often treated with chemicals that dwarf or shorten them. This is done toa) make them more attractive and to eliminate the need for staking.b) make it easier to spray them.c) force them to bloom earlier.d) all of the above

4. B-Nine is a commercially used chemical thata) makes plants taller. c) causes plants to bloom sooner.b) helps cuttings root faster. d) shortens or dwarfs plants.

5. Gibberellic acid causesa) plants to grow taller.b) plant stems to stretch out.c) the development of a greater distance between nodes.d) all of the above



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6. One rooting hormone occurring naturally in plants isa) indoleacetic acid. c) naphthaleneacetic acid.b) indolebutyric acid. d) talc.

7. Holly cuttings treated with indolebutyric acid should be well rooted in abouta) 10 days. c) 14 days.b) 21 days. d) 31 days.

8. The chemical most often used commercially for rooting cuttings isa) indoleacetic acid. c) naphthaleneacetic acid.b) indolebutyric acid. d) talc.

9. The strength of the active chemical in the majority of rooting hormones on the market ranges froma) 0.1 percent to 0.8 percent. c) 5.0 percent to 15.0 percent.b) 1.0 percent to 8.0 percent. d) 0.01 percent to 0.08 percent.

10. A fungicide such as Captan added to a rooting hormonea) helps prevent cuttings from rotting.b) makes the hormone last longer.c) causes the hormone to break down chemically.d) none of the above

11. Fruit trees may be dwarfed bya) not fertilizing. c) budding or grafting using a special rootstock.b) chemical sprays. d) none of the above

12. Blossom set is used toa) increase flower production. c) extend the life of flowers.b) stimulate early fruit set on tomatoes. d) none of the above

13. Biostimulants affect plants bya) improving water uptake. c) increasing the number of fibrous roots.b) stimulating growth. d) all of the above

14. An aspirin may help your plants toa) prepare for a disease attack. c) grow thicker leaves.b) appear shiny to the eye. d) all of the above

15. Plants are in continuous slow motion, which is stimulated bya) air temperature.b) growth regulators formed in the root and shoot tips.c) air movement.d) soil fertility.



growth stimulants, retardants, and rooting hormones · 2019. 12. 9. · explain why chemical...

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