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A.P. Kamoutsis, A.G. Chronopoulou-Sereli, and E.A. Paspatis

The effects of several shading materials on the response of Gardenia jasminoides Ellis to paclobutrazol were investigated under greenhouse conditions. The three main plot treatments were shading (0%, 67%, 98%), and paclobutrazol (0.0, 0.5, 1.0, and 2 mg/pot) was applied as a soil drench in each main plot after pinching the plants. Both plant size and the number of flower buds per plant decreased as the rate of paclobutrazol increased at all levels of shading. The efficacy of paclobutrazol, however, was generally less under heavy shade, as both translocation of the growth retardant and photosynthesis were reduced. Moderate shading (67%) did not affect the size of plants receiving 0.0 or 0.5 mg of paclobutrazol per pot, but plants grown under heavy shade (98%) were 74% as large as similarly treated nonshaded plants. Medium shade reduced the size of plants receiving 1 and 2 mg paclobutrazol 4% and 6%, respectively, relative to that of similarly treated nonshaded plants, whereas heavy shade reduced plant size 11%. The number of flower buds per plant was reduced 30% by moderate shading, 90% by heavy shading. Significant quadratic relationships were observed between the rate of paclobutrazol applied and (1) plant size, and (2) the number of flower buds per plant. Chemical name used: ±-(R * ,R *)-β–[(4-chlorophenyl)methyl]-α-(1,1-dimethyl)-1H-(1,2,4-triazol)-1-ethanol (paclobutrazol).

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Shravan K. Dasoju and Brian E. Whipker

Drench applications of plant growth retardant paclobutrazol were applied at 2, 4, 8, 16, or 32 mg a.i./pot, plus an untreated control to pot sunflowers (Helianthus annuus cv. `Pacino') to determine its effect as a chemical height control. All paclobutrazol concentrations applied significantly reduced plant height by »27% when compared to the untreated control, but excessively short plants were observed at 16 and 32 mg a.i./pot. Plant diameter was also significantly decreased by »16% at 2 and 4 mg a.i./pot of paclobutrazol, when compared to the untreated control. Flower diameter decreased by »4% at 2 and 4 mg a.i./pot of paclobutrazol, but only concentrations ≥4 mg a.i./pot were significantly different from the untreated control. Paclobutrazol concentrations had no effect on days from potting to flowering. Drench concentrations of 2 and 4 mg a.i./pot of paclobutrozol produced optimum height control in relation to 16.5-cm-diameter pot size used.

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Chris A. Martin, William P. Sharp, John M. Ruter, and Richard L. Garcia

Paclobutrazol at 0 and 750 μl·liter–1 was sprayed on shoots of Feijoa sellowiana O. Berg. and Ligustrum japonicum Thunb. grown under similar production regimes in central Arizona (subtropical desert) and southern Georgia (humid temperate). Five months after application, Feijoa and Ligustrum leaves were generally smaller and thicker in Arizona than in Georgia. Arizona leaves were thicker than those in Georgia because of more layers of palisade and spongy mesophyll cells. Compared with leaves from control plants, paclobutrazol 1) increased Feijoa leaf area in Georgia, 2) decreased Ligustrum leaf area at both locations by ≈50%, and 3) decreased leaf thickness of both species in Arizona. Arizona Feijoa leaves had trichomes on adaxial and abaxial surfaces, whereas Georgia Feijoa leaves had trichomes on abaxial surfaces only. Paclobutrazol increased trichome frequency on adaxial surfaces of Arizona Feijoa leaves. Stomatal frequency of Georgia Feijoa leaves was about doubled by paclobutrazol. Reflectance of near-infrared radiation by paclobutrazol-treated Feijoa leaves was 1.4 times higher than that of nontreated leaves in Georgia and 1.9 times in Arizona. Near-infrared reflectance by Georgia Ligustrum leaves was 1.3 times higher than by Arizona Ligustrum leaves and was not affected by paclobutrazol. Leaf reflectance of photosynthetically active radiation (PAR) by Arizona Feijoa was higher than by Georgia Feijoa. Paclobutrazol increased PAR reflectance by Arizona Feijoa leaves. In contrast, Georgia Feijoa PAR reflectance was decreased by paclobutrazol. Paclobutrazol or location did not affect Ligustrum PAR reflectance. Chemical name used: (2RS,3RS)-1-(4-chlorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-1-yl)pentan-3-ol (paclobutrazol).

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George J. Wulster and Thomas M. Ombrello

Growth and flowering of Ixia hybrids as potted plants can be controlled environmentally by cool preplant storage of corms, regulation of greenhouse forcing temperatures, and application of a growth retardant. Paclobutrazol applied as a preplant corm soak, a postemergent drench, or a postemergent spray in combination with a 2- to 4-week preplant storage of corms at 7 °C, and an 18 °C day/10 °C night forcing temperature produced attractive and marketable plants. Chemical name used: β-[(4-chlorophenyl)methyl]-α-(1,1-dimethylethyl)-1H-1,2,4-triazole-1-ethanol (paclobutrazol, Bonzi®).

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George J. Wulster and Thomas M. Ombrello

Growth and flowering of Ixia hybrids as potted plants can be controlled environmentally by cool preplant storage of corms, regulation of greenhouse forcing temperatures, and application of a growth retardant. Paclobutrazol applied as a preplant corm soak, a postemergent drench, or a postemergent spray in combination with a 2- to 4-week preplant storage of corms at 7 °C, and an 18 °C day/10 °C night forcing temperature produced attractive and marketable plants. Chemical name used: β-[(4-chlorophenyl)methyl]-α-(1,1-dimethylethyl)-1H-1,2,4-triazole-1-ethanol (paclobutrazol, Bonzi®).

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J.B. Million, J.E. Barrett, T.A. Nell, and D.G. Clark

Experiments were conducted with four kinds of flowering plants to compare one-time vs. continuous application of paclobutrazol in subirrigation water. When a crop reached the stage at which it required growth regulator treatment, four concentrations of paclobutrazol were applied via subirrigation either one-time or continuously until the crop was terminated. Based upon regression equations, concentrations resulting in 30% size reduction for one-time applications of paclobutrazol were 0.01 mg·L-1 for Begonia ×semperflorens-cultorum `Cocktail Gin', 0.09 mg·L-1 for Impatiens wallerana Hook. `Super Elfin White', 0.2 mg·L-1 for Dendranthema ×grandiflorum (Ramat.) Kitamura `Tara', and 2.4 mg·L-1 for Petunia ×hybrida Vilm.-Andr. `Plum Crazy'. Respective optimal values for continuous application were 0.005, 0.02, 0.06, and 0.4 mg·L-1. Increasing the concentration for continuous application had a greater effect on paclobutrazol efficacy than did increasing the concentration for a single application. In a trial with impatiens `Super Elfin Salmon Blush', the paclobutrazol concentration was reduced 0%, 25%, 50%, 75%, or 100% (single application) for each successive subirrigation event following an initial application of 0.1 mg·L-1 of paclobutrazol. The 50%, 75%, and 100% reduction treatments provided similar levels of size control. Dilution was more important when the reduction rate was less than 50%. Chemical name used: (±)-(R*,R*)-β-[(4-chlorophenyl)methyl]-α-(1,1-dimethyl)-1H-1,2,4-triazole-1-ethanol (paclobutrazol).

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Stanislav V. Magnitskiy, Claudio C. Pasian, Mark A. Bennett, and James D. Metzger

Shoot stretching in plug production reduces quality and makes mechanized transplanting difficult. The objectives of this study were to measure seedling emergence and shoot height of plugs as affected by paclobutrazol application during seed soaking, priming, or coating on seedling emergence and height. Verbena (Verbena ×hybrida Voss. `Quartz White'), pansy (Viola wittrockiana L. `Bingo Yellow Blotch'), and celosia (Celosia cristata L. `New Look') seeds were soaked in water solutions of paclobutrazol and subsequently dried on filter paper at 20 °C for 24 h. Soaking seeds in paclobutrazol solutions before sowing reduced growth and percentage seedling emergence of verbena and pansy but had little effect on those of celosia. Verbena seeds soaked in 50, 200, or 500 mg paclobutrazol/L for 5, 45, or 180 min produced fewer and shorter seedlings than controls. Osmopriming verbena seeds with 10 to 500 mg paclobutrazol/L reduced seedling emergence. Seedling height and emergence percentage of pansy decreased with increasing paclobutrazol concentrations from 2 to 30 mg·L–1 and with soaking time from 1 to 5 min. The elongation of celosia seedlings was reduced by soaking seeds in 10, 50, 200, or 500 mg paclobutrazol/L solutions for 5, 180, or 360 min. However, these reductions were negligible and without any practical application.

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J.B. Million, J.E. Barrett, T.A. Nell, and D.G. Clark

Three experiments were conducted to evaluate media component effects on paclobutrazol activity. In Expts. 1 and 2, a broccoli (Brassica oleracea var. botrytis L.) seedling bioassay was used to compare the activity of paclobutrazol at six concentrations (0-0.32 mg·L-1). Results from Expt. 1 indicated that an average of 4-, 5-, and 10-fold higher concentrations were required in old composted pine bark, fresh pine bark, and composted pine bark samples, respectively, to achieve the same activity observed in sphagnum peatmoss (peat) samples. Activity in coir was similar to that in peat while activity in vermiculite and perlite was greater than that in peat. Activity in a fibrous peat sample was greater than in two less-fibrous peat samples. Results from Expt. 2 indicated that paclobutrazol activity was reduced more in the fine (<2 mm) fraction of fresh and composted bark samples than in medium (2-4 mm) or coarse (>4 mm) fractions. In Expt. 3, petunia {Petunia hybrida Vilm. `Madness Red') was grown in a mixture of either 60% composted pine bark: 0% peat or 0% composted bark: 60% peat. The paclobutrazol concentration required to achieve the same size control was 14 times higher in the former mixture than in the latter. Thus, media components differ greatly in their influence on paclobutrazol activity and the bioassay procedure may serve as a useful tool for predicting media-paclobutrazol interactions. Chemical name used: (±)-(R*,R*)-β-[(4-chlorophenyl)methyl]-α-(l,l-dimethyl)-lH-l,2,4-triazole-l-ethanol (paclobutrazol).

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Brian E. Whipker and Ingram McCall

Plant growth retardant (PGR) foliar sprays of daminozide at 4,000 or 8,000 mg·L-1 (ppm) and paclobutrazol drenches of 2 or 4 mg a.i. per pot were applied to `Big Smile', `Pacino', `Sundance Kid', `Sunspot', and `Teddy Bear' pot sunflowers (Helianthus annuus L.) to compare their chemical height control. Plant height varied among the cultivars due to genetic variation. The percentage reduction in plant height from the untreated control only was significant at the PGR level, indicating similar responses of all five cultivars to each PGR rate. Paclobutrazol drenches at 2 mg and daminozide foliar sprays at 4,000 or 8,000 mg·L-1 reduced plant height by about 24% when compared to the control. Paclobutrazol drenches at 4 mg produced plants that were 33% shorter than the control. Plant diameter of `Big Smile', `Pacino', or `Sundance Kid' was unaffected by daminozide, whereas `Sunspot' plants were smaller than the controls. Paclobutrazol drenches at 2 or 4 mg decreased plant diameter for all cultivars except `Teddy Bear', with the reduction being greater as paclobutrazol drench rates increased. The number of inflorescence buds increased by ≥18% with the use of daminozide sprays, while paclobutrazol drenches at 2 or 4 mg had no effect when compared to the untreated control. Paclobutrazol drenches of 2 or 4 mg offer the economic advantage to growers of increased plant density on greenhouse benches, while plants treated with daminozide had an increased bud count but would require a greater amount of bench space.

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Martin P.N. Gent

Efficacy of paclobutrazol was determined when applied to rooted cuttings before transplant. Cuttings of large-leaf Rhododendron catawbiense Michx. were treated with paclobutrazol applied as a 40-mL drench. In 1998, concentrations of 0, 1, 2, 10, or 20 mg·L-1 were applied to liners before root development was complete in February, or after cuttings were root-bound in May. The same volume of solution was applied to other plants at concentrations of 0, 5, 10, or 20 mg·L-1 in July 1998, after transplant to 1-gal pots. In 1999, a 40-mL drench of paclobutrazol at 0, 1, 2, 5, 10, or 20 mg·L-1 was only applied to liners in April. All cuttings were transplanted to 1-gal pots and set in the field. The elongation of stems was measured after each of three flushes of growth. Plants were far more responsive to paclobutrazol when it was applied before, rather than after transplant. There was a saturating response to paclobutrazol concentration and the half-maximal response occurred at 2 to 4 mg·L-1 (0.08 to 0.16 mg/plant). At low rates, later flushes of growth were affected less than earlier flushes. However if paclobutrazol was applied at 10 or 20 mg·L-1, later flushes of growth were inhibited more completely than early flushes. Flowering was enhanced by paclobutrazol. Paclobutrazol at 2 mg·L-1 applied to rooted cuttings before transplant was sufficient to inhibit growth of rhododendron, but not to the point where later flushes of growth were excessively short. Chemical name used: 2RS,3RS-1-(4-chlorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-l-yl)-pentan-3-ol (paclobutrazol).