Paclobutrazol And Uniconazole Applications Improve The Quality Of Container-Grown Bush Morning Glory¹

Michael A. Arnold², Garry V. McDonald³, and Donita L. Bryan³

Abstract - Ipomoea caruea N. von Jacquin subsp. fistulosa (K. von Martinus ex J. Choisy) D. Austin, bush morning glories, were grown in 2.3 L black plastic nursery containers under typical outdoor container nursery conditions. Gibberellin and sterol biosynthesis inhibitors, paclobutrazol (PBZ) formulated as Bonzi® and uniconazole (UNI) formulated as Sumagic®, were applied as substrate drenches or foliar sprays. Paclobutrazol was tested at drench concentrations from 0 to 160 mg·L^-1 of active ingredient (a.i.) and as a foliar spray at concentrations from 0 to 1,600 mg·L^-1. Uniconazole was tested at drench concentrations from 0 to 24 mg·L^-1 a.i. and as a foliar spray at concentrations from 0 to 240 mg·L^-1. Genotypes with standard, dwarf, and semi-dwarf growth habits were included in the trials. After evaluation of effects of the growth regulators on height, growth index, flowering, and the number of basal shoots during production, plants were transplanted to landscape beds to evaluate residual or carry-over effects. Paclobutrazol drenches of 40 to 80 mg·L^-1 a.i. appeared consistently to produce a more compact growth form on the standard bush morning glory plants, without adverse residual effects in the landscape. Uniconazole drenches were usually effective at 1 to 6 mg·L^-1 a.i., but this varied with growth habit and experiment. Uniconazole appeared to have the potential to reduce flowering of semi-dwarf bush morning glory in the landscape at concentrations of 12 mg·L^-1 a.i. or greater. Spray applications of either compound were less consistent in their effects on bush morning glory growth than drench applications.

INTRODUCTION

Paclobutrazol and UNI are triazole plant growth regulators that inhibit gibberellin and sterol biosynthesis (Buchenauer, 1977; Fletcher et al., 2000; Hedden and Graebe, 1985). Paclobutrazol formulations, such as Bonzi, effectively reduce internode extension on various ornamental taxa (Arnold, 1998; Arnold and Davis, 1994) and fruit crops (Hunter and Proctor, 1992; Sharma and Webster,1992; Vu and Yelenosky,1992). This facilitates the production of more compact, denser canopied plants.

Tropical woody and herbaceous perennials are popular as summer annuals in hot southern climates (Arnold, 2002; Riffle, 1998; Sperry, 1991). Ipomoea caruea subsp, fistulosa, the bush morning glory, ranges in grown habit from that of a small woody tree or large shrub in subtropical climates, to a returning herbaceous perennial in USDA zones 8b through 9, and a summer annual in colder climates (Arnold, 2002; Riffle, 1998). While bush morning glory is an outstanding landscape plant, it tends to become leggy in the landscape and during conventional nursery production has a very sparse, coarse textured canopy (Arnold, 2002). Shading of the lower foliage during production can accentuate this problem. Reduced internode extension and/or promotion of lateral branch growth via inhibition of rapidly elongating terminal buds may result in a plant with greater market acceptance. Ruter (1996) and Arnold (1998) have had success with growth regulator applications to improve the appearance of another tropical woody plant, Lantana horrida H.B.K. Reports of long-term, carry-over effects on growth have been documented for some species, both of a desirable (Arnold and Davis, 1994; Arnold and McDonald, 2001; Bruner et al., 2000) and undesirable nature (Arnold and Davis, 1994). Thus, studies are needed to identify concentrations of plant growth regulators that would temporarily inhibit bush morning glory growth in the nursery, but avoid negative effects on plant growth or flowering when transplanted to landscape sites.

The objectives of this study were: 1) to determine dose response curves for I. carnea subsp. fistulosa to substrate drenches and foliar sprays of PBZ or UNI during container production, and 2) to determine if PBZ or UNI treated plants exhibit any residual effects of the production treatments following transplanting to a landscape setting.

MATERIALS & METHODS

General Nursery / Propagation Conditions.

Three genotypes were included in the trials. A standard growth habit plant for the species, a seedling from a breeding line with a dwarf growth habit (approximately half size from the standard), and an advanced semidwarf (intermediate between the dwarf and standard) selection from a breeding program tentatively named ‘Daily Beauty,. Plants were propagated from 8 to 10 cm long tip cuttings placed under intermittent mist (6 sec. every 15 min.) from stock plants over-wintered in a heated greenhouse. Cuttings were rooted in cell pack trays containing Sunshine mix #2 propagation substrate (Sun Gro Horticulture Inc., Bellevue, WA) following basal dips in 2,000 mg·L^-1 K-IBA (potassium salt of indole-3-butyric acid) solution. Once rooted, cuttings were acclimated in the greenhouse for three days prior to transplanting into a commercial 6 pine bark: 2 peat moss: 1 vermiculite: 1 hadite clay (by vol.) substrate in 2.3 L black plastic nursery containers. After potting, plants were acclimated for approximately three days under 50% light exclusion. Plants were then moved to a gravel surfaced container production area in full sun. Individual containers were top-dressed with 23N-1.7P6.6K (23-4-8 High N Southern Formula, Scotts Co., Marysville, OH) at the rate of 6.8 kg •m² Fertigation was supplied as a constant feed with N at 50 mg·L^-1 from a 24N3.5P-13K water soluble fertilizer (Scotts Co.) with Roberts Spot Spitters (#9 spot spitters, Roberts Irrigation, San Marcos, CA) placed one per container. Irrigation water was injected with sulfuric acid (93.2% H₂S0_4, Harcros Chemicals Inc., Kansas City, MO) into the irrigation stream to achieve a target pH of 6.3 to 6.5.

Data Collection.

 For the production studies, initial height and crown spread in two perpendicular directions were recorded at planting. The same measures plus the number of blooming flowers (open flowers and buds showing color) and the number of basal breaks (stems formed at the base of the plant as a result of lateral bud release or adventitious shoot bud formation) were recorded when more than half of the control plants were rated as marketable. Market ratings were determined on a 1 to 5 scale with; 1 =unacceptable plant, 2 = minimal market acceptance, but of secondary quality due to less than desired size, foliage color, or growth form, 3 = meets or exceeds minimum acceptable market standards in size and appearance, typical plant for the species; 4 = plant that exceeds minimum standards in several aspects; 5 = plant that exceeds minimum standards in nearly all aspects, a high quality plant. Plant indices were calculated as height x the first canopy diameter x the second canopy diameter, resulting in a approximation of the volume of space occupied by the canopy.

For the landscape experiments, plants were measured at planting and monthly thereafter for the remainder of the growing season. Measures recorded included height, crown spread in two perpendicular directions, and the number of blooming flowers (open flowers and buds showing color). A growth index was calculated as described for the production studies.

Preliminary Dosage Trials: Nursery.

The first set of nursery studies tested paclobutrazol [(2RS,3RS)-1-(4-chlorophenyl)-4,4-dimethyl2-1,2,4-triazol-1-yl-pentan-3-ol, formulated as Bonzi, Uniroyal Chemical Co., Middlebury, CT] substrate drenches at 0, 5, 10, 20, and 40 mg·L^-1, uniconazole [(E)-(+)-(S)-1-(4chlorophenyl) - 4,4 - dimethyl -2- (1,2,4 - triazol -1- yl) - pent - 1- ene - 3 - ol, formulated as Sumagic, Valent USA Corp., Walnut Creek, CA] drenches of active ingredients (a.i.) at 0, 1, 2, 4, and 6 mg·L^-1, PBZ foliar sprays at 0, 50, 100, 200, and 400 mg·L^{- 1} and UNI sprays at 0, 10, 20, 40, and 60 mg·L^-1 on the standard and dwarf Ipomoea carnea subsp. fistulosa. The chemicals were diluted to the desired concentrations with distilled water and applied in 30 ml aliquots. Spray applications were made to individual plants with a small mist spray bottle in a shaded location. With spray applications, container surfaces beneath the canopy were covered with paper to avoid drip or errant spray contamination of the substrate surface. Initial rates were chosen based primarily on manufacturer recommendations. The rooted cuttings were initially potted on May 13, 1999, and moved to full sun. Plants were treated with growth regulators on May 18, 1999, as new growth resumed after planting. End of production measures were recorded on May 30, 1999, when a majority of the control plants reached a marketable size. Each growth habit and chemical treatment was arranged separately in a completely randomized design in the nursery and treated as independent concurrent experiments. The different growth forms were treated as concurrent studies as it could not be predetermined whether the two growth forms would reach a marketable size at the same time, nor whether there would be shading effects of the larger growth form on the smaller growth form, either in the nursery or field portions of the trials. Completely random statistical designs were used in the nursery studies. The dosage effects of each of the four applications were determined independently for each clone. The general linear models procedures in the statistical analysis software from SAS Institute Inc. (1988) were used to perform an analysis of variance on the data. When significant dosage responses were found, first, second and third order step-wise polynomial regression equations were determined for the treatment means. Best fit regression equations, as measured by R² values, are presented in the figures.

Preliminary Dosage Trials: Landscape.

 Plants from the preliminary dosage trials were transplanted to landscape beds with 0.6 m within row and 0.9 m between row spacings on August 27, 1999. Standard and dwarf clones were established in adjacent beds. A completely randomized factorial design was utilized for each growth habit consisting of two chemicals x two application methods x five concentrations with five single plant replications per treatment combination. The 3.7 m wide by 12 m long beds were constructed from three to four layers of 10 cm diameter CCA treated landscape timbers. Beds were filled with a fine sandy loam soil. Irrigation was applied as needed from stationary risers with 4 m throws placed on 4 m centers along the sides of the beds to achieve 100% overlapping coverage. After planting, a milled pine bark mulch was applied to the entire surface at an 8 to 10 cm depth. Two weeks after planting and again on three more dates at 6 week intervals, an 18N-2.6P-1 OK formulation granular fertilizer was broadcast on the plots at the rate of 0.5 kg of N per 100 m². Height, canopy diameter, and flower number were recorded on August 27, September 28, and October 26, 1999.

Interactions between time after transplant and dosage were tested for each chemical and growth habit in the landscape experiments. If interactions were significant (P≤0.05), polynomial regression equations were determined for each rate of chemical over time. Otherwise, data from the various observation dates were pooled and regression equations were calculated as previously described for the nursery studies.

Expanded Dosage Trials: Nursery.

 In a second set of production studies, ad. rates of application were increased with substrate drenches of PBZ at 0, 40, 80, 120, 160 mg·L^-1, UNI drenches at 0, 6, 12, 18, and 24 mg·L^-1, PBZ foliar sprays of ad. at 0, 400, 800, 1200, and 1600 mg·L^-1, and UNI sprays at 0, 60, 120, 180, and 240 mg·L^-1. These rates were tested on both standard and dwarf plants. Rooted cuttings were potted in 2.3 L containers on August 5, 1999. Five single plant replicates per container per treatment were used. For each growth habit and chemical, plants of each application and concentration were arranged in a completely randomized design in the nursery on August 12, 1999. Final nursery measurements were collected on August 26, 1999.

Final Dosage Trials: Nursery.

The final set of nursery studies repeated the most promising test rates for PBZ and UNI drench applications and increased number of replication to ten plants per treatment combination and a clone with a semi-dwarf growth habit. A semi-dwarf clone was used in the final dosage trials as results from state-wide landscape trials conducted in Texas during 1998 and 1999 indicated that clones with the standard growth form grew too large for most urban landscapes, while the dwarf clones were too small on adverse sites and had reduced flowering (data not presented). This semi-dwarf clone was an advanced selection from the bush morning glory breeding program with an intermediate growth habit on a variety of sites and improved flowering characteristics compared to the standard and dwarf clones (data not presented). Test rates included a.i. drenches of PBZ at 0, 40, 80, 120, and 160 mg·L^-1 and UNI at 0, 6, 12, 18, and 24 mg·L¹. A 5 milled pine bark: 1 builders sand (by vol.) substrate was used. The substrate was amended with 16N-3.1P-l0.OK controlled release fertilizer (Southern Special, Scotts Corp., Marysville, OH) at the rate of 1.2 kg-m^-3 N, 0.89 kg-m^-3 of micromax micronutrients (Scotts Corp.), 3.6 kg-m^-3 of dolomitic lime, and 1.8 kg-m^-3 of gypsum. Cuttings were placed under mist on June 21, 2000. Ten single plant replicates were potted on July 10, 2000, in individual containers, and arranged in completely randomized designs after treatment with PBZ or UNI drenches on July 7, 2000. Final production measurements were collected on August 2, 2000.

Final Dosage Trials: Landscape.

 Semidwarf plants grown in the final dosage trial nursery study were transplanted to landscape beds on August 2, 2000. Cultural conditions were as previously described. A completely randomized factorial design was used consisting of two chemicals x five concentrations with ten single plant replications per treatment combination. Growth and flowering of individual plants were measured on August 2, September 5, and October 10, 2000, as previously described.

RESULTS & DISCUSSION Preliminary Dosage Trials: Nursery.

 No statistically significant (P≤0.05) responses were found with the standard bush morning glory during the initial nursery phase (data not presented). Likewise, PBZ drenches and UNI sprays were ineffective in reducing height, plant indices, or the number of basal breaks of the dwarf bush morning glory (data not presented). In contrast to the standard, growth indices of the dwarf bush morning glory were significantly affected by PBZ sprays and UNI drenches during the first set of production studies (Fig. 1). Paclobutrazol sprays of more than 50 mg·L^-1 reduced plant growth indices of dwarf bush morning glory, with as much as a 50% reduction in growth at 400 mg·L^-1 (Fig. 1A). Uniconazole induced a more dramatic reduction in plant index of dwarf bush morning glory with as little as 1 or 2 mg·L^-1 (Fig. 1B), but these concentrations tended also to inhibit basal breaks (Fig. 1C) that contribute a multi-stem character to the plant and enhance canopy density. Basal breaks were not affected by the UNI drenches, PBZ drenches, or PBZ sprays.

Preliminary Dosage Trials: Landscape.

 In the 1999 landscape trials, no residual effects of commercial consequence were observed with the standard bush morning glory in response to nursery applications of growth regulators (data not presented). The only statistically significant effect was a very minor reduction in height with 120 mg·L^-1 a.i. or greater UNI sprays (90.8, 91.2, 79.3, 79.5, and 80.2 cm for 0, 60,120,180, and 240 mg·L1 a.i., respectively). The dwarf bush morning glory exhibited some significant residual effects of PBZ and UNI sprays on height and plant index in the landscape (Fig. 2AC). Given the general vigor and tendency for bush morning glory to become too large for smaller home landscapes, only the highest levels of UNI sprays, 180 to 240 mg·L^-1 a.i., had a noticeable effect (Fig. 2B-C).

Expanded Dosage Trials: Nursery.

A second set of production studies were initiated with these two genotypes using the highest concentration tested in the preliminary trials as the lowest non-zero level tested. Subsequently, the higher concentrations used during the expanded dosage trials in the nursery significantly reduced vegetative growth of the standard bush morning glory (Fig. 3), but did not impact flowering during production (data not presented). Standard bush morning glory plants treated with 40 mg·L^-1 PBZ drenches or 60 mg·L^-1 UNI sprays had more compact growth forms, resulting in a more attractive plant. Plant indices of standard form plants were reduced by approximately 50% with PBZ drenches of 40 mg·L^-1 (Fig. 3B) or UNI sprays of 180 to 240 mg·L^-1 (Fig. 3C). The PBZ drenches were effective at rates similar to those reported on Plumbago auriculata Lam. (Arnold and McDonald, 2001), another tropical shrub that is used as a summer annual in cooler climates.

At the higher concentrations tested in the expanded dosage nursery trials, height or plant indices of dwarf bush morning glory were reduced by both chemicals regardless of application method (Fig. 4). Paclobutrazol drenches of 40 to 120 mg·L^-1 resulted in shorter plants (Fig. 4A) and increased flowering during production (Fig. 4B), although the increases in flowering averaged less than one flower per plant and was likely of little commercial significance. Paclobutrazol sprays produced similar vegetative responses as the drenches, but at a ten fold higher concentration (Fig. 4C, D). Uniconazole was also effective in reducing shoot elongation in dwarf bush morning glory (Fig. 4E, F, G), but at application rates six to seven fold lower than with PBZ. Uniconazole drenches of 6 to 18 mg·L^-1 stimulated slight increases in flowering, but 24 mg·L^-1 reduced flowering (Fig. 4H). However, the reductions were of such a small magnitude, approximately 0.5 flowers per plant, as to be of minimal commercial concern.

In general, PBZ and UNI sprays were less consistent in their growth and flowering effects than were drenches of the same compound. Similar inconsistencies with foliar applications of PBZ and UNI were observed with Plumbago auriculata (Arnold and McDonald, 2001). Stability of growth regulator responses across popular nursery / greenhouse substrates should be tested in light of work by Million et al. (1998a, 1998b) in which Chrysanthemum x morifolium T. de Ramatuelle and Brassica oleracea L. var. botrytis L. required applications of UNI and PBZ that were three to ten times greater with pine bark substrates compared to peat moss based substrates to achieve similar responses. Variation even occurred within pine bark and peat moss substrates based on the degree of decomposition and particle size (Million et al., 1998a). Growth regulator application responses also varied with coconut coir, vermiculite, and perlite substrates (Million et al., 1998a).

Final Dosage Trials: Nursery.

Further testing with a semi-dwarf genotype and greater replication confirmed that 40 to 80 mg·L^-1 PBZ drenches were effective for creating a more compact canopy on semi-dwarf bush morning glory (Fig. 5A). Market ratings were not adversely affected at these application concentrations (Fig. 5B). Uniconazole applied as a drench required slightly higher concentrations, 6 to 12 mg·L^-1 a.i. (Fig. 5C, D), to induce similar height control as that achieved by lower concentrations (Fig. 1B) applied to dwarf plants.

Final Dosage Trials: Landscape.

The 2000 landscape trials generally confirmed the lack of adverse residual effects of PBZ drenches below about 80 mg·L^-1 ad. (Fig. 6A). Likewise, UNI appeared to have few adverse residual effects if applied at rates of 6 mg·L^-1 a.i. or less (Fig. 6B, 2D, 2E), although if rates of 12 mg·L^-1 a.i. of UNI were employed, late season flowering was reduced.

Conclusions.

Paclobutrazol drenches of 40 to 80 mg·L^-1 a.i. produced a more compact growth form on the standard bush morning glory plants, without adverse residual effects in the landscape. Uniconazole drenches were usually effective at 1 to 6 mg·L^-1 ad., but this varied with growth habit and experiment. Uniconazole appeared to reduce flowering of the semi-dwarf forms of bush morning glory in the landscape at concentrations of 12 mg·L1 ad. or greater. Spray applications of either compound were less consistent in their effects on bush morning glory growth than drench applications. Results of this study indicated that the best treatment for effective size control of bush morning glory during nursery production without adversely affecting subsequent landscape performance was achieved using 30 ml drenches of PBZ at 40 to 80 mg·L^-1.

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¹Received for publication March 20, 2001 and in revised form February 26, 2002. Use of trade names in this publication does not imply endorsement by the authors, the Texas Agricultural Experiment Station or Texas A&M University of the products named, nor criticism of similar ones not mentioned. This study was funded in part by a grant from the Uniroyal Chemical Company, Middlebury, Conn. and the Texas Ornamental Enhancement Endowment.

² Associate Professor of Landscape Horticulture, Dept. of Horticultural Sciences, Texas A & M University, College Station, TX 77843-2133.

³Graduate Research Associates, Texas A&M University, Dept. of Horticultural Sciences, College Station, TX 77843-2133.

During greenhouse culture and when growing in wet, high humidity
situations, the bush morning glory will develop rust fungus on older
foliage. This can be easily controlled by commercial products such as
BannerMax, Terraguard, Eagle, Strike, or Systane. Home Gardeners should
use Spectricide Immunox (Not Immunox Plus necessarily) or a product
containing Neem Oil such as Greenlight Rose Defense.