rth Central Ftegion Extension Publication 303

IOWA

Ill IND - KANSAS uo

Plant Growth Regulators: Their Use in Crop Production

Charles L. Harms Edward S. Oplinger Department of Agronomy Department of Agronomy Purdue University University of Wisconsin-Madison

Plant growth regulators (PGRs) are organic fore, use of PGRs on field crops has not been compounds, other than nutrients, that modify as vigorously pursued by chemical companies plant physiological processes. PRGs, called and public research scientists. In fact, PGRs biostimulants or bioinhibitors, act inside plant represent the smallest market share of the cells to stimulate or inhibit specific enzymes or principal categories of chemicals applied to enzyme systems and help regulate plant me- field crops in the United States. They are pri- tabolism. They normally are active at very low marily used to control suckers in tobacco, as concentrations in plants. lodging control agents for cotton and cereals, The importance of PGRs was first recog- as harvest aids for cotton, and as ripeners for nized in the 1930s. Since that time, natural sugarcane. and synthetic compounds that alter function, shape, and size of crop plants have been discovered. Today, specific PGRs are used to modify crop growth rate and growth pattern Classes of Growth Regulators during the various stages of development, from PGRs may be naturally occurring, plant germination through harvest and post-harvest produced chemicals called hormones, or they preservation. may be synthetically produced compounds. Growth regulating chemicals that have Most PGRs, natural and synthetic, fall into positive influences on major agronomic crops one of the following classes: can be of value. The final test, however, is that Auxins primarily control growth through harvested yields must be increased or crop cell enlargement. although there are instances quality enhanced in order for PGRs to be pro- of auxin-kduced'cell division. They may act as fitable. both stimulators and inhibitors of growth, and Of the many current uses of PGRs, effects cause different plant parts (shoots, buds, and on yield are often indirect (Morgan, 1979). roots) to respond differently. For example, at Some of these uses include: (1) preventing lodg- low concentrations, the auxin-like herbicide ing in cereals, (2) preventing preharvest fruit 2,4-D stimulates cell enlargement, whereas at drop, (3) synchronizing maturity to facilitate higher concentrations. it inhibits enlargement mechanical harvest, (4) hastening maturity to or-is even toxic to cells. Auxins also stFmulate decrease turnover time, and (5) reducing labor differentiation of cells, the formation of roots requirements. Studies conducted on major on plant cuttings, and the formation of xylem grain crops, such as corn, soybean, wheat, and and phloem tissues. rice, have identified materials capable of alter- Gibberellins control cell elongation and ing individual agronomic characteristics like division in plant shoots. They have been lodging, plant height, seed number, and matu- shown to stimulate ribonucleic acid and pro- rity. Even so, these changes have not always tein synthesis in plant cells. resulted in increased yields. Cytokinins act in cell division, cell Field crops produce relatively low dollar enlargement, senescence, and transport of returns compared to horticultural crops. There- amino acids in plants.

- - Sponsored by the Cooperative Extension Services of Illinois, Indiana, Iowa, Kansas, Michigan, Minnesota, Missouri, Nebraska, North Dakota, Ohio, South Dakota, and Wisconsin. For the specific regulation of many plant carefully controlled conditions are not easy to processes and the differentiation of cells into reproduce in the field (Rappaport, 1980). specific plant parts, a variety of ratios and Effects of environment, crop management, and concentrations of these three plant hormone variety on crop responses and yields are usu- classes are required rather than a single hor- ally much more pronounced than the effects of mone acting alone. PGRs. This makes it difficult to demonstrate a Other naturally occurring regulators of yield or quality response to the application of plant growth and plant metabolic activity can a PGR. be classed as inhibitors and ethylene. Inhibitors represent a wide assortment of internally produced chemical compounds, each of which inhibits the catalytic action of a Effects of PGRs on Crop Growth specific enzyme. Since a plant cell may con- tain as many as 10,000 different enzymes, there are a wide variety of inhibitors acting Germination and Emergence inside the cell. Ethylene is internally produced Several plant hormones have been shown by plants and has a multitude of effects on cell to affect germination of seeds of some plant processes. It interacts with auxins to regulate species (Nickell, 1982). The primary event of many metabolic processes. Several chemical breaking seed dormancy is stimulated by gib- compounds that release ethylene after being berellins. Field crops, such as corn, soybean, sprayed on plants are currently commercial and small grains, have a very brief dormancy PGR products. period after seed maturity. Thus, dormancy is A wide assortment of plant growth- not a factor in stand establishment. Volunteer promoting products are being marketed with plants of field crops, which begin growing claims made for beneficial effects on crop after harvest, attest to the fact that PGRs that growth and yields. Typically, these products break dormancy are not required to aid germi- are supposed to: (1) promote germination nation. and/or emergence, (2) stimulate root growth, Germination of field crops is sometimes (3) promote mobilization and translocation of decreased by cold soil temperatures (Cole and nutrients within plants, (4) increase stress Wheeler, 1974). Cole and Wheeler's research tolerance and improve water relations in showed that cotton seeds soaked for 6 to 24 plants, (5) promote early maturity, (6) increase hours in gibberellic acid or cyclic AMP disease resistance, (7) retard senescence, or (8) increased germination percentages over a improve crop yields and/or quality. range of temperatures. To be effective, the Usually, the claims are made for plant hor- seeds must absorb the PGR into the embryo mone products or products that affect the con- cells. For absorption of PGRs to occur, seeds centrations and ratios of plant hormones inter- must be placed in a solution containing the nally. Most often, the ingredients in these PGR for 8 or more hours. More practical products are found to be: methods of seed treatment caused little seed Extracts from bacteria, yeast, fungi, response because the PGR did not penetrate marine algae, and sea kelp. Usually, low con- the seed coat (Kellerhals, 1986). centrations of auxin, gibberellin, and cytoki- While germination of several crop species nin and adenine, or adenosine monophosphate has been increased by gibberellic acid, indole (AMP) are claimed. acetic acid, succinic acid, and fusiococcin, all Adenine, AMP, and cyclic AMP. treatments required seed soaking to incor- Indole butyric acid and/or indole acetic porate the chemical into the seed. Soaking ini- acid. Both of these are auxins. tiates water imbibition, germination, and Gibberellins-a family of approximately softening of the seed, which makes planting 70 chemical compounds. with current planters difficult. Cytokinins-6 furfuryl-amino purine, 6- For most field crops, poor seedling emer- benzyl-amino purine, zeatin, dihydrozeatin, gence through soil crusts, dry soils, and cold and 20 other related chemical compounds. soils is more of a problem than is poor germi- Polyethylene glycol. nation. Large-seeded broadleaf crops emerge Dinoseb-2-sec butyl4,6dinitrophenol. by an elongating hypocotyl dragging large Proteins and/or amino acids. cotyledons through the soil. Some short- Carboxylic, phenolic, and/or humic acids. statured wheat varieties have short coleoptiles These products may provide some of the and cannot be planted at soil depths greater eight benefits listed above when applied to than 2 inches. This prevents their use in dry field crops grown in growth chambers or soils, where seeds must be planted deeper than greenhouses. However, results obtained under 2 inches to come in contact with moisture. 2 Data on the effects of PGRs on emergence translocation within plants comes from short- are very limited, although gibberellic acid has term laboratorv- studies. been shown to increase seedling height of Gibberellic acid stimulates ~hos~hate beans, wheat, and soybean. It is not known if uptake into corn root cells, potassium uptake gibberellic acid increases the force exerted by in wheat, and sulfate translocation from root seedlings against the soil to aid emergence. to shoot in pea seedlings. Indole acetic acid Seedlings emerging under stress of a soil crust both depresses and increases potassium uptake produce ethylene, which results in thickened in corn seedlings, depending upon concentra- hypocotyls and greater emergence force. tion. These studies conducted under controlled Currently, seed treatments of commercial environment conditions cannot be duplicated value are used as protectants from diseases, in field studies where foliar- or soil-applied insects, and herbicides. Commercially impor- plant hormones must exert control over long tant seed treatments include fungicides, insec- periods of time. ticides, and seed safeners. Safeners are chemi- Several of the "growth stimulant" products cals that protect seedlings so a herbicide can have been evaluated in field studies in Wiscon- be used on a susceptible crop. sin, Nebraska, Iowa, and Kansas on several field crops. Nutrient concentrations in plant Root Growth parts were slightly increased, slightly decreased, or not affected by growth stimu- Several PGRs in the auxin family of chemi- lants in the various trials. Nutrient concentra- cals will stimulate root initiation on plant cut- tions in crops were increased to a greater tings. These PGRs are commonly used for hor- extent by fertilizer additions in these trials ticultural crops (Nickell, 1982). Indole butyric than by the application of growth stimulants acid is the most frequently used PGR because (NCR-103 Committee, 1976). it is not rapidly degraded by the plant and is Nutrient uptake in field crops from the soil not translocated from the site of application. is affected by eleven factors relating to both In field crops, root growth is strongly plant and soil parameters (Barber, 1984). related to environment and soil nutrient sup- These include: (I) root length, (2) rate of root ply, not to any hormonal deficiency in plants. growth, (3) root radius, (4) maximum rate that Corn root proliferation into the soil is strongly roots can take up a nutrient, (5) rate when correlated with increasing soil temperatures nutrient uptake is half-maximal, (6) minimum during the first two weeks after planting concentrate of a nutrient in the soil solution (Barber, 1986). Another strong correlation where uptake begins to occur, (7) nutrient con- exists between root growth rate and soil mois- centration in the soil solution, (8) the soil's ture. Growth rate increases with increasing abilitv to re~lenishthe soil solution with the soil moisture up to the field capacity for the nutrient, (9)-diffusion rate of the nutrient in soil (Mackay and Barber, 1985). Seasonal root soil solution, (10) rate of water uptake by roots, density for corn varies from 4 to 15 inches of and (11) distance between competing roots, root length per cubic inch of soil caused by including their root hairs. Nutrient u~takeis a interactions of soil temperature, soil moisture, complexuprocess with many interacting factors and mineral nutrient supply. and is difficult to influence by foliar or soil Some claims for increased root growth from applications of low concentrations of PGRs. use of PGRs are made as a result of studies where plants are pulled from the soil. The Stress Tolerance and Moisture roots that remained attached are visually examined and compared with treated and non- Relations of Crops- treated plants. These roots are often broken Tolerance to soil moisture stress in croDs is off within 6 inches of the base of the stalk. related to a crop's ability to control transbira- Since any individual plant may have up to tional water loss from leaf surfaces. The open- 1.25 miles of roots in the upper 5 feet of soil, ing and closing of stomata, relative numbers root density within 6 or so inches of the stalk of stomata per unit leaf area, and thickness of may not be representative of the total amount the cuticle layer influence transpiration rates. of roots for that plant. Various attempts to decrease water losses from plants include: (I) PGRs that regulate the clo- sure of stomata during moisture stress, (2) Mobilization and Translocation chemicals that form water-barrier films over of Nutrients the upper and lower surfaces of plant leaves, Plant hormones influence mobilization of and (3) PGRs that decrease plant topgrowth inorganic plant nutrients and sugars. Most and increase the root/shoot ratio to decrease experimental evidence that indicates plant hor- water usage by the plant (Gale and Hagan, mones influence nutrient mobilization or 1966). Kellerhalls, M. 1986. Effect of plant growth regula- Nickell, L.G. 1982. Plant growth regulators, agricul- tor combinations on faba bean growth and vield tural uses. Springer-Verlag. Berlin, Heidelberg, structure. Ph.D. diss. (Diss. Abstr. Int. C. 47:597- New York. 598). Oplinger, E.S. 1983. Corn. p. 131-137. In L.G. Nick- Mackay, A.D. and S.A. Barber. 1985. Soil moisture ell (ed.) Plant growth regulating chemicals, Vol. 11. effects on root growth and phosphorus uptake by CRC Press, Inc. Boca Raton, FL. corn. Agron. J. 77:519523. Rappaport, A. 1980. Applications of gibberellins in Morgan, P.W. 1979. Agricultural uses of plant agriculture. p. 377-391. In F. Skoog (ed.) Plant growth substances: an analysis of present status growth substances, 1979. Springer-Verlag. Berlin, and future potential. Plant Growth Regulator Work- Heidelberg, New York. ing Group, Proceedings. 6:l. Stutte, C.A. and M.D. Davis. 1983. Growth regula- NCR-103 Committee. 1976. Compendium of research tors in soybean production. p. 99-112. In L.G. Nick- reports on use of non-traditional materials for crop ell (4.)Plant growth regulating chemicals, Vol. 11. production. North Central Region. American CRC Press, Inc. Boca Raton, FL. Society of Agronomy. Madison, WI.

This publication was prepared by North Central Regional Committee NCR-103, Specialized Soil Amendments, Products, and Growth Stimulants. It summarizes existing information on plant growth regulators and does not include original research results obtained by the NCR-103 com- mittee. Members of the committee include:

State Agricultural Experiment Stations Illinois Robert G. Hoeft Missouri Daryl D. Buchholz Indiana Charles L. Harms Nebraska Richard A. Wiese Iowa Regis D. Voss North Dakota William C. Dahnke Kansas David A. Whitney Ohio Paul Sutton Michigan Maurice L. Vitosh South Dakota James R. Gerwing Minnesota George W. Rehm Wisconsin Keith A. Kelling

U. S. Department of Agriculture Cooperative State Research Service Charles M. Smith Administrative Advisor Dale H. Vanderholm (Nebraska)

In cooperation with the North Central Region Educational Materials Project Programs and activities of the Cooperative Extension Service are available to all potential clientele without regard to race, color, sex, age, national origin, or handicap.

Issued in furtherance of Cooperative Extension work, Acts of Congress of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture and Cooperative Extension Services of Illinois, Iowa, Kansas, Michigan, Minnesota, Missouri, Nebraska, North Dakota, Ohio, South Dakota, and Wisconsin. H. A. Wadsworth, Director, Purdue University Cooperative Extension Service, West Lafayette, IN 47907.

NEW 12/88 (20M) Several chemicals have short-term effects, mercial use. In sugarcane, PGRs are used for lasting from several hours to several days, on uniform ripening, which improves sugar stomata1 closure. Among these chemicals are accumulation. Several PGRs and herbicides phenyl mercuric acetate, atrazine, alachlor, have been tested on cotton to promote uniform chlormequat, daminozide, indole acetic acid, boll opening, which aids in mechanical pick- and abscisic acid. Chemicals that close sto- ing and improves quality of the lint. These mata have sometimes increased crop yields PGRs, however, are not found on the list of when crops were grown under moisture stress. typical ingredients for growth stimulant pro- However, they decreased yields when crops ducts. were grown without stress. Closure of stomata limits entrv of carbon dioxide into leaves: ~ho- Disease Resistance tosynthesis is decreased as well. Thus, '{here Changing the metabolism of plants with are trade-offs when attempting to use growth PGRs to control disease has provided mixed regulators to decrease transpiration. results. Some of the auxin materials have Chemicals that form barriers to water loss given positive results, while gibberellins have have also resulted in inconsistent yield both increased and decreased disease severity responses. Oil-wax mixtures, vinyl acetate- (Nickell, 1982). acrylate copolymers, hydrocarbon films, latex, Disease resistance in crop plants is a herit- and silicon polymers coat the leaf surfaces to able characteristic of considerable economic prevent evaporation of water. Some coatings importance. Plant breeders are continually tend to plug stomata and decrease entry of car- working to incorporate new sources of disease bon dioxide, which decreases photosynthesis resistance into crop varieties. Genetic expres- and plant growth. In most cases, both upper sion of disease resistance is often a localized and lower leaf surfaces must be coated with response to invasion by the causal organism. water-bamer films in order to effectivelv The plants produce a toxic substance to kill increase yields under moisture-stressed grow- the microorganisms that cause the disease. ing conditions (Fuehring and Finkler, 1984). Disease resistance is not due to changes in Growth retardant PGRs decrease top plant hormone concentrations or ratios in growth/root mass ratios and decrease tran- infected cells. Claims for plant growth stimu- spiration. Chlormequat and other PGRs that lants improving disease resistance in crop inhibit gibberellin synthesis in plants may plants are largely unsubstantiated. increase crop yields when crops are grown under moisture stress, but decrease yields when crops are grown with adequate soil mois- Senescence ture. These PGRs decrease water consumption Senescence is the natural termination of at the expense of absolute production. Most plant processes during the development of the chemicals that have shown efficacy for seed, and is regulated by several metabolic decreasing water use are not found in the list and environmental signals. In laboratory stu- of typical ingredients for growth stimulants. dies with leaves, senescence was retarded by Only indole acetic acid as an agent that closes auxins, gibberellins, and cytokinins and stomata is on this list. accelerated by abscisic acid and ethylene. In plants growing in controlled environments, as Maturity many as 14 control signals or influences have Since crop maturity is related more to been identified that affect vegetative senes- genetic control than to environmental control, cence during reproductive development an early-maturing hybrid or a variety with a (Goldthwaite. 1987). shorter growing season requirement can be Sovbean senescence is sensitive to night grown, if desired. Seed companies have length, day and night temperature regime, and developed corn and sorghum hybrids with dif- seed fill. Maximum seed yields require com- ferent heat unit (growing-degree-day) require- ~leteleaf senescence because stored carbohv- ments from emergence to maturity. Soybean hrates and other nutrients are translocated varieties are categorized according to maturity from leaves to developing seeds. group, which is based on adaptation to north- Senescence delay in corn can be genetically south latitudes. Heading dates for wheat and manipulated. Several hybrids currently pos- other small grain varieties are normally com- sess a stav-meen characteristic for the leaves. pared to the standard variety in each area. When cornpired to hybrids of similar genetics Growth stimulants have less effect than without the stay-green characteristics, they variety or hybrid on maturity date. often have slightly lower grain yields. This is Over 50 chemicals have been tested as because the hybrids without the stay-green sugarcane ripeners, and several are in com- leaves will mobilize and translocate carbohy- drates and nutrients from the leaves and stalk come into wider usage on field crops because to the grain to a greater degree than the of inconsistencies in yield or quality improve- hybrid with stay-green leaves. Thus, retarding ments when they are used. senescence is not necessarily related to crop yield increases. Applications of plant hor- mones either in the soil or as a foliar spray on Summary the crops has less influence on senescence than environment and plant genetics. Advertising claims for benefits of applying plant hormones and PGRs often are derived from plant physiology text books. The effects Crop Yield and Quality of plant growth stimulants on plants in Commercial applications of plant hormones research are often observed on excised plant have been limited to the use of gibberellic acid parts and in controlled environment growth in fruit production. Both indole acetic acid and chambers for very brief time periods. cytokinins are broken down very rapidly by In order for a PGR to increase yields or microorganisms to inactive products when quality of a field crop, it must (1) act over a applied in the soil or on the foliage of plants. relatively long period of time, (2) over-ride the hstricted penetration of the cuticle and entry effects of environment, (3) influence many into plant cells has been another problem with varieties or hybrids of a particular crop, (4) be indole acetic and cytokinins. readily absorbed into the plant and able to Gibberellic acid is foliar applied on seedless cross cell membranes to enter cell ~roto~lasm, grapes to loosen the cluster, decrease diseases, and (5) be stable on the plant surfaces or in and increase berry size. It is also used on the the soil for a long enough time so absorption "Delicious" apple to control fruit shape, on can occur. These problems have not been lemons to delay ripening, on sugarcane to resolved; therefore, PGR use in field crop pro- increase both internode elongation and sugar duction is, at this time, strictly limited- yields, on malting barley to improve the malt- ing process, on artichokes to accelerate flower bud production, and on celery to increase petiole elongation during cool weather condi- Literature Cited tions. At present, there are no recognized com- Barber, S.A. 1986. Environmental influence on corn mercial applications of gibberellins on field root distribution. Indiana Plant Food and Agric. crops such as corn, soybean, wheat, grain Chem. Conf., Proceedings. P. 2-5. sorghum, cotton, and rice. Barber, S.A. 1984. Soil nutrient bioavailability: a The PGRs that have been studied for mechanistic approach. p. 118. John Wiley and Sons, increased yield or quality of field crops are New York. dinoseb, 2,3,5-triiodobenzoic acid (TIBA), Cole, D.F. and J.E. Wheeler. 1974. Effect of preger- ethe~hon, chlormequat, and mepiquat. mination treatments on germination and growth of Dinoseb applications on corn at low concentra- cotton seeds at suboptimal temperatures. Crop Sci. tions as a biostimulant have produced erratic 14:451-454. grain yield responses. Yields have been Fuehring, H.D. and M.D. Finkner. 1984. Effect of increased by 5 to 15% in some studies, whereas Folicote antitranspirant application on field grain no yield responses have been observed in other yield of moisture-stressed corn. Agron. J. 75:579-582. trials (oplinger, 1983). Hybrid sensitivity and Gale, J. and R.M. Hagan. 1966. Plant antitran- precise timing of applications to each h~bnd spirants. Ann. Reu. of Plant Physiol. 17:269-282. have affected results with this chemical com- Gianfagna, T.J. 1987. Natural and synthetic growth pound. regulators and their use in horticultural and agro- On "ybeans has produced nomic crops. p. 614-635. In P.J. Davis (4.) Plant sistent results across many environments hormones and their role in plant growth and (Stutte and Davis, 1983). Ethephon on small development. Martinus Nijhoff Publishers, Dor- grains provides protection from lodging and decht, Boston, Lancaster. increases yields if lodging is prevented during Goldthwaite, J.J. 1987. Hormones in plant sene+ the early part of grain-fill- In addition, it aids cence. p. 553-573. In P.J. Davis (4.)Plant hormones in mechanical harvest of small grains. Chlor- and their role in plant growth and development. mequat is another anti-lodging agent for small Martinus Nijhoff Publishers, Dordecht, Boston, grains; it is used in Europe as a crop protec- Lancaster. tant (Jung and Rademacher* 1983). Mepiquat Jung, J. and W. Rademacher. 1983. Plant growth is applied on cotton as a growth retardant to regulating chemicals-cereal grains. p. 253-271. In prevent excessive vegetative growth and L.G. Nickel1 (4.)Plant growth regulating chemi- increased boll set. However, PGRs have not cals, Vol. I. CRC Press, Inc. Boca Raton, FL. 5