Plant material.

On 3 Mar. 2005, rooted vegetative cuttings of argyranthemum ‘Sunlight’, calibrachoa ‘Callie Dark Blue’, petunia ‘Cascadias Vivid Red’, scaevola ‘Jacob's White’, and verbena ‘Rapunzel Red’ grown in liners [50-cell size (3.8 × 2.5 cm)] and in a peat-based medium were received in East Lansing, Mich., from a commercial greenhouse (Raker's Acres, Litchfield, Mich.). Plants were subsequently grown at 20 °C under a 9-h photoperiod maintained by pulling opaque black cloth from 0800 to 1700 hr. All plants were pinched to three or four nodes 7 d before the chemical application.

Chemical applications.

Plants were irrigated on the day preceding the application. On 14 Mar., each liner tray was cut into sections containing 10 plants. Each 10-cell section was then placed into a tray containing 2.5 cm paclobutrazol (Piccolo; Fine Americas, Walnut Creek, Calif.) at 4, 8, or 16 mg·L⁻¹ or uniconazole (Sumagic; Valent USA Corp., Marysville, Ohio) at 2, 4, or 8 mg·L⁻¹ for 30 s. Ten plants of each species that did not receive a chemical application were designated as the control. The chemical applications were made between 1100 and 1200 hr and were performed in a glass-glazed greenhouse. The average air temperature during the application was 20.6 °C and the maximum photosynthetic photon flux (PPF) was 700 μmol·m⁻²·s⁻¹. After the chemical application, liners were placed on a greenhouse bench for 3 h and then plants were subsequently transplanted into 4.5-inch-diameter round plastic pots (volume, 591 mL) containing a commercial soilless medium (Suremix; Michigan Grower Products, Galesburg, Mich.).

Greenhouse environment.

Plants were grown at a constant temperature set point of 20 °C in a glass-glazed greenhouse with exterior whitewash. The actual average daily temperature during the experiment was 20.1 ± 2.6 °C, as measured by aspirated thermocouples and recorded by a CR10 data logger (Campbell Scientific, Logan, Utah). Natural photoperiods were extended to a 16-h photoperiod (0600 to 2200 hr) by high-pressure sodium lamps that delivered a PPF of 25 to 50 μmol·m⁻²·s⁻¹ at plant height [as measured with a line quantum sensor (Apogee Instruments, Logan, Utah)]. Plants were irrigated as necessary with reverse osmosis water supplemented with a water-soluble fertilizer at each irrigation providing 125 mg·L⁻¹ N, 12 mg·L⁻¹ P, 100 mg·L⁻¹ K, 65 mg·L⁻¹ Ca, 12 mg·L⁻¹ Mg, 1.0 mg·L⁻¹ Fe and Cu, 0.5 mg·L⁻¹ Mn and Zn, 0.3 mg·L⁻¹ B, and 0.1 mg·L⁻¹ Mo (GreenCare Fertilizers, Kankakee, Ill.).

Data collection and analysis.

Total plant height (from the medium surface to the tip of the longest extended stem) was measured on the day of application and at 28 d after application. Height increase and height inhibition percentage compared with that of control plants were calculated. Date of first flowering (open flower) was recorded for each plant, and time to flower was calculated. On the day that each plant flowered, the total number of flower buds was counted and the average was calculated for each treatment.

A completely randomized design was used. Data were analyzed using an SAS (SAS Institute, Cary, N.C.) mixed model procedure (PROC MIXED), and pairwise comparisons between treatments were performed using Tukey's honestly significant difference (hsd) test at P ≤ 0.05. An exponential decay model was fitted to data for height increase as a function of chemical rate using SigmaPlot v. 8.02 (SPSS, Chicago).

Argyranthemum.

Plant height increase of ‘Sunlight’ displayed an exponential decay response (r² = 0.65 and 0.73) as the rate of paclobutrazol or uniconazole liner dip increased (Fig. 1A, B). At 28 d after application, stems were 2.9 to 4.4 cm shorter in plants treated with paclobutrazol at 8 or 16 mg·L⁻¹ or with uniconazole at all rates tested. Stem elongation in plants treated with paclobutrazol at 4 mg·L⁻¹ was not statistically different from that of control plants. Paclobutrazol at 16 mg·L⁻¹ and uniconazole at 8 mg·L⁻¹ suppressed final plant height by 36% and 42% respectively. The effectiveness of uniconazole on controlling stem elongation of argyranthemum is in agreement with data from Gibson et al. (2003), who reported that a foliar spray of 40 mg·L⁻¹ inhibited plant height by 25% at 22 d after application.

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Plants treated with paclobutrazol at 16 mg·L⁻¹ or uniconazole at all rates flowered 9 or 10 d later than untreated plants (Table 1). Time to flower in plants treated with paclobutrazol at 4 or 8 mg·L⁻¹ was not significantly different from that of control plants and ranged from 37 to 43 d. The total number of flower buds ranged from 34 to 46 and was not significantly different among treatments.

Table 1.The effects of paclobutrazol and uniconazole liner dips on days to flower and number of flower buds in argyranthemum ‘Sunlight’, calibrachoa ‘Callie Dark Blue’, petunia ‘Cascadias Vivid Red’, scaevola ‘Jacob's White’, and verbena ‘Rapunzel Red’.

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Calibrachoa.

The stem elongation response of ‘Callie Dark Blue’ to paclobutrazol or uniconazole was fit to an exponential decay model with an r² value of 0.82 or 0.78 respectively (Fig. 1C, D). Total stem elongation after 28 d was increasingly inhibited as the rate of either chemical increased. Paclobutrazol at 8 and 16 mg·L⁻¹ or uniconazole at 8 mg·L⁻¹ suppressed plant height by 56%, 64%, and 67% respectively. The highest application rate of both chemicals continued to inhibit stem elongation 49 d after application (data not presented).

Flowering time was not significantly different among any of the treatments and ranged from 29 to 33 d (Table 1). However, plants treated with paclobutrazol or uniconazole at the highest rates (16 mg·L⁻¹ or 8 mg·L⁻¹ respectively) had, on average, 20 fewer flower buds compared with untreated plants, but we believe plants were still of acceptable commercial quality.

Petunia.

Stem length of ‘Cascadias Vivid Red’ at 28 d after application decreased as the concentration of paclobutrazol and uniconazole increased, and data were fit to an exponential decay model with an r² value of 0.61 or 0.60 respectively (Fig. 1E, F). The subsequent height increase of liners dipped in paclobutrazol or uniconazole was 3.2 to 6.0 cm (27% to 51%) less than that of untreated plants.

Plants dipped in paclobutrazol at 16 mg·L⁻¹ flowered, on average, 6 d later than untreated plants (Table 1). Time to flower for plants dipped in uniconazole was similar to that of untreated plants and averaged 24 d. Plants treated with paclobutrazol at 8 or 16 mg·L⁻¹ or uniconazole at all rates had, on average, 5 to 10 fewer flower buds compared with control plants. However, we believe that plants were still of acceptable commercial quality.

Scaevola.

The stem elongation response of ‘Jacob's White’ to increasing concentrations of paclobutrazol or uniconazole was fit to an exponential decay model with an r² value of 0.73 or 0.55 respectively (Fig. 1G, H). At 28 d after application, the dip in paclobutrazol or uniconazole suppressed subsequent stem elongation by 4.2 to 10.2 cm. Paclobutrazol or uniconazole applied at the highest rates (16 and 8 mg·L⁻¹ respectively) were judged to have caused excessive inhibition of stem elongation. For example, paclobutrazol at 16 mg·L⁻¹ inhibited plant height increase by 66% compared with that of untreated plants. At 49 d after application, plants treated at these high rates remained excessively compact, and we believe that they were not marketable (data not presented). In ‘Blue Wonder’ scaevola, a medium drench of paclobutrazol at 4 mg·L⁻¹ or uniconazole at 1 mg·L⁻¹ (120 mL/10-cm-diameter pot) suppressed final plant width by 42% or 29% respectively (Starman and Williams, 2000).

Paclobutrazol at 8 or 16 mg·L⁻¹ or uniconazole at 8 mg·L⁻¹ delayed flowering by an average of 12 d compared with that of untreated plants (Table 1). Liners dipped in uniconazole at 8 mg·L⁻¹ had 33 fewer flower buds than control plants. Therefore, rates of uniconazole greater than 4 mg·L⁻¹ can compromise plant quality and are not recommended under the growing conditions provided in this experiment.

Verbena.

Stem length of ‘Rapunzel Red’ was increasingly inhibited (by 25% to 42%) as the concentration of the paclobutrazol or uniconazole dip increased (Fig. 1I, J). The stem elongation response to increasing concentrations of paclobutrazol or uniconazole was fit to an exponential decay model with an r² value of 0.39 or 0.52 respectively. The inconsistent stem elongation response of ‘Rapunzel Red’ to 2 mg·L⁻¹ uniconazole compared with 0 and 4 mg·L⁻¹ uniconazole is unclear. Therefore, data for height increase in plants treated with 2 mg·L⁻¹ uniconazole were judged to be outliers and were not included in the exponential decay model. The average time to flower ranged from 20 to 23 d and was not significantly different among treatments (Table 1). Uniconazole at 8 mg·L⁻¹ was the only chemical and rate that reduced flower bud number. Magnitskiy et al. (2006) reported that soaking seeds of ‘Quartz White’ verbena in paclobutrazol at 50 mg·L⁻¹ for 5 min inhibited seedling height by 1.6 cm when measured 39 d after sowing. However, radical emergence from the media 20 d after seed sow was also reduced by 50%.

Paclobutrazol and uniconazole were effective at inhibiting stem elongation in all species investigated, but not surprisingly, the magnitude of the height suppression depended on the chemical, rate, and plant species. For example, in argyranthemum and calibrachoa, application of uniconazole at 2 mg·L⁻¹ suppressed stem elongation more than paclobutrazol at 4 mg·L⁻¹ (statistical analysis not presented). In petunia, scaevola, and verbena, the lowest rates of paclobutrazol and uniconazole effectively controlled stem extension. Stem elongation responses in all species were statistically similar among plants treated with paclobutrazol at 8 mg·L⁻¹ and uniconazole at 4 mg·L⁻¹. Similarly, plants treated with paclobutrazol at 16 mg·L⁻¹ and uniconazole at 8 mg·L⁻¹ were also of similar height for each species investigated (statistical analysis not presented).

The chemical applications also delayed flowering time and decreased flower number in some species treated at the higher chemical rates. In all species except argyranthemum, average time to flower was similar for plants treated with paclobutrazol at 4 mg·L⁻¹ and uniconazole at 2 mg·L⁻¹, paclobutrazol at 8 mg·L⁻¹ and uniconazole at 4 mg·L⁻¹, and paclobutrazol at 16 mg·L⁻¹ and uniconazole at 8 mg·L⁻¹. For scaevola and verbena, liners dipped in uniconazole at 8 mg·L⁻¹ had, on average, 29 and 9 fewer flowers compared with plants treated with paclobutrazol at 16 mg·L⁻¹ respectively. In calibrachoa, plants treated with uniconazole at 2 mg·L⁻¹ had 22 fewer flowers than plants treated with paclobutrazol at 4 mg·L⁻¹.

The effectiveness of a liner dip at controlling stem elongation could be influenced by several environmental and cultural factors, such as duration of the dip treatment, moisture level of the media, irradiance during application, and size of plant material (Schnelle and Barrett, 2006; Schnelle et al., 2005). In our study, the medium was moderately dry at application and was allowed to absorb the PGR solution for 30 s. Longer application durations or a lower medium moisture content could increase the volume of PGR solution absorbed and thus elicit a stronger treatment effect. Bearce and Singha (1992) reported that poinsettia cuttings rooted in foam and irrigated 36 h before a uniconazole dip absorbed 9.3 mL more solution and were ≤9.9 cm shorter than those irrigated immediately before treatment. Therefore, to reduce variability when a PGR liner dip is used for height control, a repeatable protocol is required to maintain consistency among applications. Further studies are needed to determine effective liner dip rates of paclobutrazol and uniconazole for controlling stem elongation in other bedding plant species.