Paclobutrazol for height control of two Lilium L . A . hybrids grown in pots

Height control is often required in order to obtain bulb plants that are proportional to their pot size and to reduce post-harvest stem elongation. Plant growth regulators afford an effective means of controlling plant height. In this study, paclobutrazol (Pbz) was studied for use in the chemical dwarf ing of potted Lilium plants —the hybrids ‘Ercolano’ and ‘Royal Respect’— for marketing. Their bulbs were dipped in the growth regulator (0, 50, 100, and 150 ppm) for 10 min before planting. Plant height (measured at 7 day intervals), the duration of the different stages of the growth cycle, the number of flower buds formed, and the number of open and aborted flowers were recorded. The average day and night air temperatures were 25 and 13oC, respectively. Increasing Pbz dose progressively reduced the height of the ‘Ercolano’ hybrid by between 29% (46.2 cm; 50 ppm) and 45% (35.3 cm; 150 ppm) compared to control plants (64.4 cm), however, neither the duration of flowering nor the number of flowers was modified. The ‘Royal Respect’ hybrid responded with a marked shortening – by 45% (20.8 cm; 50 ppm) to 59% (28.2 cm; 150 ppm) in comparison with the average control height (51.1 cm), but in this case both the duration of flowering and the number of open flowers was reduced at the highest concentration. The feasibility of cultivating these Lilium hybrids for marketing in pots is demonstrated. However, the concentration of paclobutrazol should be adjusted according to the hybrid grown. Additional key words: bulb dips, chemical dwarfing, growth regulator, Lilium ‘Ercolano’ hybrid, Lilium ‘Royal Respect’ hybrid.

Interest in producing potted Lilium spp.plants has recently emerged among cut flower producers due to the increase in the popularity -and hence the profitability-of these species as potted plants.Apart from their relative ease of cultivation, the value of potted Lilium spp.plants per unit greenhouse surface area is high (Miller et al., 1998).
When producing bulb crops, height control is often required in order to obtain plants proportional to their pot size, to meet shipping requirements, and to reduce post-harvest stem elongation.An effective means of controlling plant height is to use plant growth regulators.
ethanol} is commonly used with bulb crops in today's floriculture industry (Krug, 2004).The pre-planting soaking of bulbs offers advantages over other application methods, such as time and labour savings, accuracy in dosage application, and its reasonable cost (Ranwala et al., 2002).Many of the most vigorous lily cultivars traditionally grown for cut flowers can be raised in pots if growth regulators are used to obtain plants of a marketable size (Whipker and Hammer, 1993).Some authors suggest an acceptable height for Lilium in pots to be 30-40 cm (Krug, 2004), while others suggest 20-50 cm (Beattie and White, 1993).However, many cultivars can grow to 50 or even 60 cm in height (DeHertogh, 1996).
The use of Pbz to reduce plant height has already been tested in oriental hybrids by pre-planting bulb dipping for 30 min at concentrations of 100-200 ppm (Miller et al., 1998;Dole and Wilkins, 1999).Ball and Miller (1996) demonstrated the effectiveness of this method with Pbz and other growth regulators (ancymidol and uniconazole) for the control of the height of Lilium L.A. hybrids.However, evidence has been found of a cultivar-dependent response, which emphasizes the need for appropriate dose adjustments (White, 1990;Ranwala et al., 2002).
The aim of the present work was to determine the effectiveness of Pbz on the control of the growth of two popular L.A. hybrids of Lilium ('Ercolano' and 'Royal Respect'), by dipping their bulbs in solutions of this growth regulator before planting.The results show that by the use of appropriate doses of Pbz plants of marketable size can be obtained.
This experiment was performed at the INTA-San Pedro Agricultural Experimental Station (lat.33º4'S, long.59º4' W), Buenos Aires Province, Argentina, in the spring of 2005.The hybrids used were 'Ercolano' and 'Royal Respect'.Size 12/14 bulbs were planted (one per pot) in 1L black polyethylene pots (diameter 12 cm), and grown in a metallic greenhouse with a polyethylene cover (25% ventilation) from September 6 th .The bulb bases were placed at a soil depth of 8 cm.The substrate was a handmade mixture of 25% peatmoss (from the Epuyen region, Chubut, Argentina) + 75% soil (apparent density 0.8 kg m -3 , porosity 2.2%, water retention 58.2%).The density of the pots on the greenhouse bench was 35 plants m -2 .
Water was provided by individual drip irrigation according to necessity (equal conditions for all treatments).The electrical conductivity of the irrigation water was 0.9 mS cm -1 .Once the shoots were 5-7 cm tall, a liquid fertilizer was applied at the rate of 200 ppm nitrogen and potassium (Smith, 2003).
To mimic the cultivation conditions of most local flower producers, the experiment was undertaken without greenhouse temperature control.
Paclobutrazol (commercial product Kultar SC TM , 25% active ingredient) was applied by dipping the clean bulbs in 1 L of 0 (water control), 50, 100 or 150 ppm solutions -concentrations chosen according to the Bonzi TM and Sumagic TM pre-plant dip guidelines for a number of hybrid lily cultivars (Miller, 2003).The bulbs were placed in net bags (30 per treatment) and dipped in the treatment solutions for 10 min.These bags were then hung to drain, and the dry bulbs planted.
The experiment had a randomised complete block design with three replicates.Each experimental unit contained 10 plants.The analysis for each hybrid is presented separately.
Growth and development data are shown in Table 1.
Plants were inspected daily to check for damage and/or disease.The canopy air and substrate temperatures (5 cm depth) were averaged hourly; these data were stored using an ETG Multirecorder-P data logger.
The results were treated using the GLM and Mean procedures of the SAS statistical software package version 6 (SAS Institute, 1989).The Tukey test for nonadditivity was used to confirm the normal distribution of the data, which were subjected to ANOVA (α = 0.05).The variables that did not show a normal distribution (days to emergence, height at all dates, and the number of open flowers) were transformed using the (x) square root procedure.For the variable «number of aborted flowers» the (x + 1) square root transformation procedure was used.Treatments were compared using the Tukey test (α = 0.05), and the best linear, quadratic, or cubic f itting of the variables and the Pbz concentrations determined (α = 0.05).
At 28 days after planting, dose showed a significant quadratic relationship with height in both hybrids (Table 2).The maximum height reached before the opening of the first flower bud decreased with increasing paclobutrazol doses.'Ercolano' responded to concen-trations of 50, 100, and 150 ppm with a shortening of 29%, 33% and 45% respectively compared to the control treatment (64.4 cm).'Royal Respect' responded to 50, 100 and 150 ppm with a shortening of 45%, 53% and 59% compared to the control (51.1 cm).For both hybrids, the maximum height attained after dipping in the assayed Pbz doses was in the range of 20-50 cm (Table 2), an appropriate height for marketing plants well proportioned to their pots (Beattie and White, 1993).However, the varietal response was different; 'Royal Respect' plants were restricted to a height lower than 30 cm even with the lowest dose (50 ppm) of Pbz (Fig. 1).Ball and Miller (1996) reported a stronger effect of Pbz on the Lilium L.A. hybrid 'Royal Fantasy'; 10 min dips reduced the height at flowering by 53% at 100 ppm and 66% at 150 ppm.These authors reported a day temperature similar to that of the present study (24°C), and a higher night temperature (17°C) (Table 3).Moore (1979) suggested a strong interaction between the temperature difference between day and night (DIF) and the endogenous levels of gibberellic acid to explain why the effect of another plant growth regulator, ancymidol, on stem elongation diminished as DIF increased.Temperate night temperatures (16ºC) reduced stem length in comparison with colder night tempera-tures (10ºC) in the cultivars 'Enchantment', 'Sterling Star', 'White American' and 'Uchida' (Marlogio et al., 1987).In the present study, the night temperatures were below 13.1ºC from planting until the first flower bud with colour appeared (sale time), and the DIF varied between 12 and 13ºC depending on the plant cultivation stage.Future assays should determine whether variations in DIF affect the responses of 'Ercolano' and 'Royal Respect' to Pbz.

Growth stages after planting
In 'Ercolano', effects of treatments on different stages of cultivation and even on the characteristics of flowering were not observed, except a slightly earlier visible appearance of flower buds (less than 0.3 day) between anyone of the Pbz doses and the control (Table 4).
In 'Ercolano', Pbz (at any dose) appeared to have no effect on the time to reach the different growth stages or on the characteristics of flowering [except for a slightly earlier appearance of the flower buds (difference < 0.3 days)] (Table 4).In 'Royal Respect', however, slight treatment effects were observed on emergence   4).Although significant, these effects cannot be considered of commercial importance.Working with 'Royal Fantasy', Ball and Miller (1996) observed a lag time of 2 and 3 days in flowering after 10 min bulb dips in Pbz at 100 and 200 ppm respectively.In 'Royal Respect', the duration of flowering was slightly reduced in plants thus treated with doses of 50 and 100 ppm, but by more than 1 day in those treated with a 150 ppm dose.In this hybrid a detrimental effect on the number of open flowers was also observed with the 150 ppm dose (Table 4).
For the Asiatic cultivars 'Enchantment' and 'Lemonglow', ancymidol has been reported to have similar effects on flowering variables; following a brief bulb dip treatment (5 s) with 16.5 ppm ancymidol, a minimum lag time in flower opening was seen although the number of flower buds was not affected (Adzima, 1986).
Under the spring greenhouse climatic conditions of the north of the Buenos Aires Province, Argentina (average night/day air temperatures 13/25ºC), potted 'Royal Respect' and 'Ercolano' Lilium hybrids of commercially acceptable height can be obtained by immersing their bulbs in Pbz for 10 min before planting.However, the concentration of paclobutrazol should be adjusted according to the hybrid grown.
appearance of the flower sprout Tips of flower bud visible to the naked eye Flower bud with separated tip petals End of flowering defined as when the last open flower shows areas of minimum wilting at the petal edges Time elapsed between the first open flower and the last wilted flower Maximum plant height reached by either the leaves or flowers without modifying their natural position Number of flower buds completely formed Length of the second flower bud when completely formed but still closed Number of flowers showing maximum petal aperture Difference between the number of flower buds formed and the number of open flowers

Figure 1 .
Figure 1.Differential responses by Lilium hybrids 'Ercolano' (A) and 'Royal Respect' (B) to paclobutrazol.From left to right: appearance of plants at the stage of full flowering after bulb soaking in 150, 100, 50 and 0 ppm paclobutrazol respectively for 10 min.

Table 1 .
Definitions of Lilium hybrid growth and flowering variables

Table 2 .
Change in height (cm) of plants whose bulbs were treated with different paclobutrazol doses (0, 50, 100, 150 ppm) until the time prior to the first open flower

Table 3 .
Average air and substrate temperatures recorded at different culvitation stages

Table 4 .
Comparison of variables related to the duration of the different Lilium growth stages and characteristics of flowering in plants raised from bulbs treated with the different concentrations of paclobutrazol(0, 50, 100, 150 ppm)