Design and Maintenance of Green House 3(2+1)

Lesson 27 Chemical Growth regulation

27.2 CLASSIFICATION

Chemicals used to control growth are either naturally occurring plant hormones or synthetically produced compounds.

27.2.1 Hormones

Hormones are compounds  produced in the plant at one site and then transported to a different part of the plant at one site and then transported to a different part of the plant where they affect growth.

27.2.1.1Auxins

Auxins promote growth primarily through cell enlargement. The major auxin produced in plants is indole-2-acetic acid (IAA). Synthetic auxins include indolebutyric acid (IBA), indolepropionic acid (IPA), and naphthalene acetic acid (NAA). Auxins play an important role in plant propogation.

Auxins are involved in tropistic growth movements. Such movements include the downward growth of roots, the upward growth of shoots, and the growth of shoots and leaves toward the light. It is believed that shoots grow toward the light source because auxin is inactivated by light. This occurs more on the bright side of the stem; thus, there is greater promotion of growth on the darker side.

Auxins also inhibit lateral shoot development. When the top of the main shoot of a plant is removed, the source of auxin is lost from that shoot, and lateral shoots are free to develop. This is why pinching (the removal of shoot tips) is practiced on some floral crops; multiple lateral shoots are promoted. A plant is said to display apical dominance when only one shoot predominates. When apical dominance is lost, several lateral shoots usually develop simultaneously.

Auxins are effectively used for promoting root formation on cuttings. Many types of cuttings benefit from the use of rooting substances. Root formation occurs faster, and in the end the root system is usually more extensive. The benefit is least on plant species that normally root quickly, and there are a few species where no benefit is seen.

Rooting compounds are very concentrated and so are always diluted. Talc powder is a customary diluent. Active-ingredient concentrations of 0.1 to 1.0 percent are used—the lower concentrations for easy-to-root soft cuttings and the higher concentrations for slower-to-root woody cuttings. The base of the cutting is dipped into the powder and then tapped to remove all but a thin film of powder. To reduce the possibility of disease transfer, a duster is often used.

Rooting compounds can be diluted by another method for use on woody cuttings where penetration is difficult. A concentrated stock solution of the rooting compound is made by dissolving it in alcohol. The stock solution is further diluted with water to a final concentration in the range of 500 to 5,000 ppm (0.05 to 0.5 percent). The cut end of cuttings is dipped in this solution for a short time and then “stuck” into propagation substrate in a propagation bed. The concentration of the solution and the length of dipping time (five seconds to a few minutes) are determined by the ease of rooting and the penetrability into the woody stem of the cutting.

Rooting compounds are a very common and valuable aid to the propagators of greenhouse crops, since so many crops are propagated by cuttings. African violet, azalea, begonia, carnation, chrysanthemum, geranium, hydrangea, kalanchoe, poinsettia, and many green plants are examples of plants that benefit from rooting compounds.

27.2.1.2 Gibberellic acid (GA)

GA promotes growth through cell enlargement. Various gibberellins have been isolated from species of the fungus Gibberella.  It promotes growth , but unlike that of Auxins, the promotion is uniform throught the plant tissue.

GA inhibits root formation on leaves and stems; thus, it is not found in root-promoting products. It is used by gardeners for enlarging the size of camellia blooms. Also, GA sprayed on geranium flowers at the time of first colour appearance (at a concentration of 5 ppm) stimulates a 25 to 50 percent increase in flower size. The number of petals remains constant, but each petal is larger. When greater concentrations are applied, however, increased responses carry an adverse effect. Stems and flower stalks elongate and become thinner. Stems may become adversely weak; flowers that are normally flat may become undesirably spikelike.

Flowering of cyclamen can be accelerated by four to five weeks with a single spray of 50 ppm GA 60 to 75 days prior to the anticipated flower date (Widmer, Stephens, and Angell, 1974). Higher concentrations result in adversely tall and weak flower stems. Lyons and Widmer (1983) suggest applying 0.25 ounce (8 mL) of 15 ppm GA₃ solution to the crown of the plant below the leaves 150 days after seed is sown.

Researchers have used gibberellins to replace the cold treatment of azalea. In the cold treatment, when the plant has reached sufficient size, it is pinched for the last time. New shoots are allowed to develop for about six weeks, and then flower-bud initiation is induced by about six weeks of long-night treatment. Once flower buds are established, a period of six weeks at a temperature of 45°F (7°C) or lower is required for development of flower buds. After this treatment, the plants are moved to the greenhouse and forced into bloom in four to six weeks.

The cold treatment is expensive, requiring costly moving of plants and also cooler facilities.  Considerable efforts have been made to reduce or eliminate the cold treatment (Boodley and Mastalerz, 1959). Five weekly sprays of a combination of gibberellins 4 and 7 (GA_{4+ 7}) or gibberlin 3 (GA₃) at a concentration of 1,000 ppm have proven effective (Figure 13-2) (Larson and Sydnor, 1971; Nell and Larson, 1974). The five consecutive weekly sprays begin when flower buds are well developed after the short day treatment. Plants treated in this manner usually flower earlier and have larger blossoms than plants given the cold treatment. Most cultivars respond well; however, there can be some variation. For instance, flower pedicels (flower stems) may become too long, causing flowers to droop.

There have also been studies on partial replacement of the cold treatment. In one such study, after three weeks of cold treatment, plants were moved to the greenhouse for forcing, and three weekly sprays of GA₃ at 250 ppm were made. Half of the cold treatment was eliminated, thereby permitting twice the volume of plants to be moved through the cooler facilities.

Hydrangea is also subjected to a period of cold storage. On occasion, the plants are removed prematurely, and slow development, small flowers, and short stems ensue. Research studies show promise of eliminating this situation by a spray of GA at a concentration of 5 to 50 ppm.

A 250-ppm GA spray applied to fuchsia four times at weekly intervals temporarily prevents flowering and stimulates rapid growth (Heins, Widmer, and Wilkins, 1979). This could lend itself well to production of tree-type fuchsia. Tree-type geraniums can likewise be produced (Carlson, 1982) by applying GA₃ as a spray to plants two weeks after potting. A total of five weekly applications of 250 ppm must be applied. A tolerable delay in flowering occurs. Excessive GA application results in distorted growth and poor plant quality.

27.2.1.3 Ethylene

It is naturally produced in fruits, seeds, flowers, stems, leaves and roots and controls a multitude of processes. Ethylene lends itself to numerous commercial applications.

27.2.1.4 Abscisic acid (ABA)

ABA promotes abscission of leaves and petals as well as a number of other processes. It is not a major hormone in the vegetative stages of growth but comes into play in the later stages of maturity and sensescence. ABA is not commercial important in greenhouse crop production.

27.3 SYNTHETIC COMPOUNDS

A number of synthetic compounds also exist for control of greenhouse plant growth. These include the height retarding chemicals A-rest^® , B-Nine^®, and Cycocel^®, the chemical pinching agents Atrinal^® and Off-Shoot-O^® and ethylene producer ethephon.      

27.3.1 Florel^®

Ethephon is the common name for the commercial product Florel (produced by Rhone-Poulenc Ag. Co., Research Triangle Park, NC 27709). It is a 3.9 percent liquid concentrate of the chemical [(2-chloroethyl) phophonic acid]. Ethephon undergoes a chemical conversion that releases ethylene to the plant. Depending on the plant type and stage of growth, one or more of the following desirable responses can be induced: flowering, increased branching, height retardation, and leaf drop.

The commercial appeal of bromeliads is enhanced by the presence of a flower stalk. Flowering can be induced in two months’ time by pouring 1/3 ounce (10 mL) of a diluted solution of ethephon (1.54 oz ethephon/gal water; 12 mL/L) into the vase of plants at least 18 to 24 months old (Heins, Widmer, and Wilkins, 1979). Flowering of Dutch iris bulbs is likewise affected by ethephon. A number of bulbs, particularly smaller bulbs, fail to bloom when they are greenhouse forced. A spray of 156 ppm (4 mL ethephon/L water) applied to green plants in the Dutch bulb-production fields just prior to bulb harvest led to earlier flowering, less bud abortion, and fewer leaves during greenhouse forcing (Kamerbeek, Durieux, and Schipper, 1980). British work showed that the reduction in leaf number permitted increased plant density from 14 to 30 bulbs/ft² (150 to 320 bulbs/m²) (Krause, 1984). Many Dutch iris bulbs are now treated by the producer in the storage area after harvest with 500 ppm ethylene gas for 24 hours to promote earlier and more extensive flowering on higher-quality plants.

As might be expected, ethylene plays a role in fruit maturation. Ethylene gas, or the ethylene-producing compound ethephon, is used to ripen apples, bananas, coffee, grapefruit, oranges, peppers, tobacco, and other fruit. Ethephon spray is applied to processing tomatoes in the field to hasten maturity. Fresh-use field tomatoes are treated after harvest in the mature-green stage with 200 ppm ethylene gas. This enhances color formation and hastens ripening by about two days (Lutz and Hardenburg, 1968).

Leaf abscission, like flower and fruit formation, is part of the maturation process. It is likewise enhanced by ethephon. This has a commercial advantage in hydrangea production, where it is desirable to remove the leaves for the six-week cold treatment that occurs after flower-bud initiation and just prior to greenhouse forcing. An application of 1,000 to 5,000 ppm ethephon two weeks prior to the start of cold treatment has been shown to result in the defoliation of the cultivars “Merville” and “Rose Supreme” (Tjia and Buxton, 1976).

Light intensity at the crown of rose bushes diminishes as the canopy grows larger over the years. This discourages the development of new canes from the base of the plant. Renewal of the plant is dependent in great part on the large, floriferous shoots that come from these basal breaks. The best hope for encouraging such breaks has come from scoring lower canes with a saw blade dipped in 7,500 ppm ethephon (Zeislin et al., 1972). Ethephon sprays have, in fact, been used commercially in Israel to stimulate basal branching of “Baccara” rose.

Surprisingly, ethephon is used as a height retardant as well. Drenches or sprays serve well to control the height of narcissus. Ethephon sprays prior to floret color result in shorter hyacinths and prevent stem topple, which is a problem with some cultivars (DeHertogh, 1989).

27.3.2 Cycocel^®

Potted plants must be grown to a height compatible with the environment in which they will be used. Many plants grow too tall if not checked. In past years, water and nutrients were withheld to reduce height, resulting in bad side effects in the appearance of the foliage and size of the bloom. Poinsettia stems were sometimes folded to reduce height, which was an effective but time-consuming process. Cycocel is used today.

Height retardants, in general, result in shorter stem internodes but do not affect the number of leaves formed. Stems are thicker, and leaves are deeper green because chlorophyll is more dense in the smaller cells. As a result, plants have a very pleasing appearance.

Cycocel [(2-chloroethyl) trimethylammonium chloride] is available in a liquid formulation containing 11.8 percent active ingredient. The common name of the chemical is chlormequat. (Cycocel is produced by American Cyanamid Company, P.O. Box 400, Princeton, NJ 08540.) Since Cycocel is considerably cheaper to apply as a spray than a drench, it is generally sprayed on crops.

Cycocel is applied as a spray to azaleas when about 1 inch (2.5 cm) of new growth occurs after the plants have been pinched for the last time. This checks growth and prompts early flower-bud initiation. Quite often, a larger number of flower buds develop. The retardant helps further by reducing the formation of vegetative shoots at the time of flower-bud development. These undesirable side shoots give the plant an unbalanced appearance.

Cycocel is recommended as a drench or a spray for poinsettia height retardation. Sprays can result in blotchy yellowing of foliage about 24 hours after application. This is a temporary situation and is not noticed at flowering time. Combinations of Cycocel plus B-Nine have proven more effective than either alone on poinsettia. Cycocel is also labeled for height control of geranium, hibiscus, Jerusalem cherry, and many bedding plants and green plants.

27.3.3 B-Nine WSG

B-Nine WSG (N-dimethyl amino succinamic acid) is known under the common name daminozide. (B-Nine WSG is produced by Uniroyal Chemical Company, Inc., Crop Protection Div., Specialty Products Group, World Headquarters, Middlebury, CT 06749 <www.uniroyalchemical.com>.) It is an effective height retardant labelled for use on azalea, begonia, browallia, caladium, pot chrysanthemum, cut chrysanthemum, crossandra, exacum, gardenia, gerbera, gloxinia, hydrangea, hibiscus, kalanchoe, liatris, poinsettia, and many plug seedlings, bedding plants, perennials, and green plants. B-Nine WSG is sold as a water-soluble granule containing 85 percent active ingredient plus a wetting agent. It is not used as a drench but rather as a foliar spray to the upper leaf surfaces. It is slowly absorbed, thus foliage should not be wetted for 24 hours after application.

Azalea is treated with B-Nine WSG for the same reason that Cycocel is used—to promote early and more extensive flower-bud set and to retard vegetative shoot development. Some cultivars of standard chrysanthemum develop a long pedicel, which is unattractive. A compact flower with a short pedicel can be produced by spraying the upper third of the foliage to the point of runoff two days after disbudding with a 0.25 percent concentration of B-Nine WSG. Pot mums are most often sprayed when new shoots are 1.5 inches (4 cm) long, about two weeks after the pinch. No delay in flowering occurs. B-Nine WSG is particularly useful in producing compact bedding plants but is not effective on coleus, French-type marigold, pansy, or snapdragon.

27.3.4 A-Rest

The chemical A-Rest [a-cylopropyl-a-(p-methoxyphenyl)-5-pyrimidinemethanol] (produced by SePRO, 11550 N. Meridian St., Suite 200, Carmel, IN 46032) takes the common name ancymidol. A-Rest effectively controls the height of and is labeled for azalea, pot chrysanthemum, gardenia, geranium, gerbera, hydrangea, liatris, lilies (including Easter lily), poinsettia, tulip, several green (foliage) plants, and numerous annual and perennial plants. It is purchased as a solution containing 250 mg of active ingredients per quart (264 ppm). A-Rest can be applied as a spray or as a drench, depending on the crop.

A-Rest loses activity at low pH levels. Consequently, the effectiveness of
A-Rest drenches in pine-bark substrates has been found to be poor (Larson, Love, and Bonaminio, 1974; Tschabold et al., 1975). A solution to the problem was found by Simmonds and Cumming (1977) by dipping hybrid lily bulbs in A-Rest solution. This procedure worked well for Easter lily as a dip prior to cold-storage treatment (Lewis and Lewis, 1982). Larson (1985) refined the procedure, calling for a dip in 24-ppm A-Rest for 30 minutes after cold-storage treatment.

Another interesting function of A-Rest has been seen in research, where it was used to induce flowering of Clerodendron (Koranski, Struckmeyer, and Beck, 1978). Retardation of vegetative growth prompts flowering in this plant. Cycocel has a similar effect in Clerodendron (Hildrum, 1973).

27.3.5 Bonzi

Bonzi is a more recent height retardant. This product contains 0.4 percent of the active ingredient [(±)-(R*,R*)-b ((4-chlorophenyl)methyl)-a-(1,1,-dimethylethyl) -1 H-1,2,4-triazole-1-ethanol]. This ingredient is a member of the triazine group of compounds; its common name is paclobutrazol. (Bonzi is distributed by Uniroyal Chemical Co., Inc., Crop Protection Div., Specialty Products Group, World Headquarters, Middlebury, CT 06749 <www.uniroyalchemical.com>.)

Bonzi can be absorbed by the roots from a soil drench or through the shoots from a spray. In either event, it is translocated to the upper portion of each shoot, where it reduces internode elongation by inhibiting gibberellin biosynthesis. If a spray is used, it is very important that the spray coat all the stems of the plant; otherwise, some shoots will grow longer than others. Bonzi is primarily absorbed by stems as opposed to leaf blades and moves upward in the plant via the xylem. As in the case of A-Rest, this material is less effective in pine bark–based root substrates. Higher rates must be used if it is applied as a drench to these root substrates. Higher rates are also needed when it is applied to very vigorous-growing varieties compared to those that are naturally shorter, and when it is applied during high-temperature periods.

Bonzi is labeled for application on ornamental crops including plug seedlings, bedding plants, flowering pot plants, green plants, and bulb crops. Instructions for specific crops are presented on the label. Other crops should be trialed first to determine safety, effectiveness, and required rates.

27.3.6 Sumagic

Sumagic is the most recent height retardant to be introduced. The common name for this chemical is uniconazole. Uniconazole is produced by Sumitomo Chemical Co., Ltd.; in the United States, it is distributed as Sumagic for greenhouse use by Valent USA Corp., P.O. Box 8025, Walnut Creek, CA 94596-8025 <www.valentpro.com>.

It has chemical properties closer to Bonzi than to the other height retardants and is also a member of the triazine chemical group. The active ingredient is (E)-(+)-(S)-1-(4-chlorophenyl)4, 4-dimethyl-2-(1,2,4-triazol-1-yl)-pent-1-ene-3-ol. Like Bonzi, it is effective at very low rates.

Sumagic can be applied as foliar spray, drench, bulb or cutting dip or to the surface of substrate prior to planting. It can be used on the whole range of ornamental plants. However, when used on crops for which specific instructions are not given on the label it should first be tried for safety, effectiveness, and required rate. As in the cases of A-Rest, and Bonzi, Sumagic drenches are less effective in root substrates containing pine bark, thus higher rates are required in these substrates.

27.3.7 Off-Shoot-O

Off-Shoot-O is composed of methyl esters of C₆ to C₁₂ fatty acids, primarily methyl octanoate and methyl decanoate, in combination with an emulsifying agent. It is a product of the Cochran Corporation, P.O. Box 14603, Memphis, TN 38114. Off-Shoot-O is termed a chemical pinching agent because it causes death of the terminal bud on shoots, which in turn results in the development of side shoots (Figure 13-5). Often, more side shoots are produced from a chemical pinch than from a manual pinch. Off-Shoot-O is applied in a very fine spray to wet the shoot tips. The remainder of the plant need not be treated, and spraying is stopped before the point of runoff. Runoff increases the possibility of injury to lateral buds and leaves.

Azaleas are effectively pinched with this chemical. Considerable labor is saved, since azaleas must be pinched many times in order to produce a large plant with numerous shoots. Concentrations of 2 to 5 ounces of product per quart (63 to 155 mL/L) are used, depending upon the cultivar. A concentration of 3.2 ounces per quart (100 mL/L) is common. Several species of woody ornamentals can also be chemically pinched, including Cotoneaster, Juniperus, Ligustrum, Rhamnus, and Taxus.

27.3.8 Atrimmec

Atrimmec [sodium salt of 2,3,:4,6-bis-O-(1-methylethylidene)-a-L-xylo-2-hexulofuranosonic acid] is known by the common name dikegulac. (Atrimmec is produced by PBI/Gordon Corp., P.O. Box 014090, Kansas City, MO 64101-0090.) The predecessor to this product was Atrinal.

Atrimmec temporarily stops shoot elongation, thereby promoting lateral branching. It is thus a pinching agent for greenhouse crops, including azalea, Elatior begonia, bougainvillea, clerodendron, fuchsia, gardenia, grape ivy, ivy geranium, kalanchoe, lantana, lipstick vine, Pachystachys lutea (shrimp plant), Schefflera arboricola, and verbena. Branching can also be enhanced in 41 species of landscape ornamentals. Atrimmec is used to prevent flowering, and ultimately fruiting, in glossy privet, Japanese holly, multiflora rose, and ornamental olive.

27.4 COST OF MATERIALS

It is difficult to make cost comparisons for height retardants and pinching agents. Not all are effective on each crop. Some are applied as a drench, while others are applied as a spray. The number of pots that can be sprayed with a gallon depends upon the density of pots in the bench and the spray equipment; a high-pressure, fine-droplet spray covers more area. The concentration of growth regulator required varies according to the crop, its stage of growth, and the weather conditions.

REFERENCE:

1. Paul V.Nelson (1985).Greenhouse Operation and Management, Reston publishing company, Inc. A Prentice Hall Company reston, Verginia PP.378-399.