Longevity of previously used dip flurprimidol solutions held in darkness.

A flurprimidol solution (14 L, 30 mg·L⁻¹) was prepared using the commercial product, Topflor (SePRO, Carmel, IN) and used to dip 144 narcissus and 120 hyacinth bulbs (10 min dips) and then held in darkness at 17 °C. Three weeks later, an identical solution was prepared and used to dip a second group of bulbs and then also held in the dark at 17 °C. After another 4 weeks, a third solution was prepared before the actual start of the experiment. Thus, the experiment consisted of the 7- and 4-week-old “used” flurprimidol solutions and a freshly made (unused) solution. All solutions were held in opaque polyethylene containers.

On 1 Dec., the three solutions and water (non-treated control) were used to dip bulbs of eight hyacinth cultivars and five narcissus cultivars that had previously received 3 weeks of 9 °C treatment. All bulbs were dipped for 10 min at 20 °C. After drying at 20 °C for 2 h, bulbs were planted (one bulb per 10-cm diameter pot) in Lambert LM-111 planting mix (Lambert Peat Moss, Inc., Riviere-Ouelle, Quebec, Canada). Pots were cooled an additional 13 weeks (initially at 9 °C, gradually reducing to 1 °C, for a total cold treatment of 16 weeks) and then forced (starting 2 Mar.) in a glass greenhouse at 17 °C constant temperature under the natural light conditions prevailing in Ithaca, NY.

Hyacinth cultivars were (bulb diameter in centimeters): ‘Blue Jacket’ (17/18), ‘Carnegie’ (18/19), ‘Miss Saigon’ (18/19), ‘Pink Pearl’ (18/19), ‘Pink Surprise’ (17/18), ‘Skyline’ (17/18), ‘Top White’ (17/18), and ‘Woodstock’ (17/18). Narcissus cultivars were ‘Carlton’ (15/17), ‘Exception’ (15/17), ‘Ice Follies’ (15/17), ‘Primeur’ (15/17), and ‘Tete-a-Tete’ (12/14).

Measurements of stem height and leaf length (pot rim to tallest point) were made as each plant came into flower and again as the plant was starting to senesce. The difference of these measurements was calculated to determine growth during the flowering period. The experiment had a completely randomized design with 10 and six replicate pots per hyacinth and narcissus cultivar, respectively. JMP Version 9.0.2 (SAS Institute Inc., Cary, NC) was used for data analysis.

Photodegradation of flurprimidol.

Two experiments were conducted to determine whether prepared flurprimidol solutions lose activity when exposed to sunlight. Flurprimidol solutions of (2.5, 5, and 10 mg·L⁻¹) were prepared and exposed to full outdoor sun (Ithaca, NY) in late June in 2011 and 2012. Solar radiation (W·m⁻²) and prevailing air temperature measurements were recorded at 15-min intervals within 100 m of the research site. In general, the days were mostly sunny with some haze and no significant precipitation. In the first experiment, solutions (initially 6 L each concentration) were held outdoors in noncovered, translucent white plastic tubs. At the end of each day (sunset), the solution volume was measured and replenished to replace evaporation of water (≈400 to 500 mL·d⁻¹). After mixing, a 1-L sample was removed and subsequently held in darkness. This process continued for 6 d.

The second experiment was similar to the first with the exception that the plastic tubs containing the solutions (initially 8 L) were tightly sealed with plastic film to eliminate evaporation. One liter of solution was removed each day (for 8 d) and subsequently stored in the dark at room temperature. Over the course of the experiment, less than 20 mL liquid was lost. In both experiments, irradiated solutions were stored in glass bottles until use.

After 6 (Expt. 1) or 8 (Expt. 2) d, the solutions were used to treat (1-min bulb dip) asistic hybrid Lilium ‘Tresor’ bulbs (12/14 cm circumference). After dipping, bulbs were allowed to dry ≈1 h and then planted into 15-cm diameter plastic pots as described previously. After planting, plants were grown in a glass greenhouse under prevailing light levels at 16 °C night and 22 °C day temperature. The experiments were of completely randomized design with eight replicates (bulbs) per treatment. JMP Version 9.0.2 (SAS Institute Inc.) was used for data analysis.

Hyacinth growth.

Seven of the eight tested cultivars responded to the flurprimidol dip treatments and generally had shorter flower stems and leaves at the time of first flower and at the end of flowering (Table 1). The cultivar Woodstock was non-responsive. Of the seven responsive cultivars, plants treated with any age flurprimidol solution were significantly shorter than control plants. In six of seven cultivars, there was no significant difference in height of plants dipped into freshly prepared solution as compared with the 4- or 7-week-old solutions (Table 1). The sole exception was ‘Carnegie’, in which the height of flower stems of plants dipped into the 7-week-old solution was significantly taller than plants dipped into fresh solution. This was the only instance, and considering all other cultivars show a contrary response, is attributed to a sampling or random error. In most cultivars, post-flowering growth (stem or leaf growth from first flower to the end of flowering) was less in the flurprimidol-dipped plants compared with controls, the only exception being with stem growth in ‘Carnegie’.

Table 1.

Effect of age of flurprimidol dip solutions on growth of eight hyacinth cultivars.^z

View Table

Narcissus growth.

Four of the five cultivars were responsive to flurprimidol dips; only ‘Ice Follies’ was relatively unresponsive at the concentrations used (Table 2). This agrees with previous work showing ‘Ice Follies’ to be less responsive to flurprimidol dips than other narcissus cultivars (Miller, 2012b). In the cultivars other than ‘Ice Follies’, stem and leaf length at flowering and end of flowering were generally shorter in flurprimidol-dipped bulbs compared with untreated controls. In all cases, there were no growth differences between fresh solution and 4- or 7-week solution (Table 2).

Table 2.

Effect of age of flurprimidol dip solutions on growth of five narcissus cultivars.^z

View Table

For both genera, there were no visual effects on flower size, shape, or color.

Flurprimidol photodegradation.

The growth-retarding activity of flurprimidol solutions was significantly reduced by exposure to sunlight over a period of 1 to 8 d (Figs. 1 and 2). This effect was seen with each of the irradiated flurprimidol solutions (2.5, 5, and 10 mg·L⁻¹). The environmental conditions during the two experiments are given in Table 3. Temperature and irradiance are expressed as the average of 15-min readings and averaged from sunrise to sunset or summed over this same period (irradiance only). There was no relationship between final plant height as a percent of control and average daily temperature or average daily irradiance. When regressed against accumulated irradiance (MJ), final plant height as a percent of control gave the following equations (Expt. 2): 2.5 mg·L⁻¹: –0.0008x² + 0.2726x + 76.14 (r² = 0.841; F = 0.004); 5 mg·L⁻¹: –0.0004x² + 0.2948x + 62.42 (r² = 0.956; F < 0.0001); and 10 mg·L⁻¹: 0.0003x² + 0.2095x + 51.23 (r² = 0.947; F < 0.0001) (data not shown), clearly demonstrating a relationship of accumulated irradiance to degradation of the flurprimidol solution.

Stem length of Lilium ‘Tresor’ plants at flowering as affected by pre-plant bulb dips (1 min) into 2.5, 5, or 10 mg·L⁻¹ flurprimidol that had been exposed to full sunlight for 0 to 6 d (Expt. 1). The stem length of non-treated plants was 55.7 ± 4.3 cm. Circles: 2.5 mg·L⁻¹, y = –0.738x² + 8.929x + 27.52 (r² = 0.977, P > F 0.0005). Squares: 5 mg·L⁻¹, y = –0.167x² + 5.143x + 23.02 (r² = 0.882, P > F 0.0139). Triangles: 10 mg·L⁻¹, y = 0.274x² + 0.821x + 17.41 (r² = 0.949, P > F 0.0026).

Citation: HortScience horts 48, 1; 10.21273/HORTSCI.48.1.77

• Download Figure

View Full Size

Stem length of Lilium ‘Tresor’ plants at flowering as affected by pre-plant bulb dips (1 min) into 2.5, 5, or 10 mg·L⁻¹ flurprimidol that had been exposed to full sunlight for 0 to 6 d (Expt. 1). The stem length of non-treated plants was 55.7 ± 4.3 cm. Circles: 2.5 mg·L⁻¹, y = –0.738x² + 8.929x + 27.52 (r² = 0.977, P > F 0.0005). Squares: 5 mg·L⁻¹, y = –0.167x² + 5.143x + 23.02 (r² = 0.882, P > F 0.0139). Triangles: 10 mg·L⁻¹, y = 0.274x² + 0.821x + 17.41 (r² = 0.949, P > F 0.0026).

Citation: HortScience horts 48, 1; 10.21273/HORTSCI.48.1.77

• Download Figure

Stem length of Lilium ‘Tresor’ plants at flowering as affected by pre-plant bulb dips (1 min) into 2.5, 5, or 10 mg·L⁻¹ flurprimidol that had been exposed to full sunlight for 0 to 6 d (Expt. 1). The stem length of non-treated plants was 55.7 ± 4.3 cm. Circles: 2.5 mg·L⁻¹, y = –0.738x² + 8.929x + 27.52 (r² = 0.977, P > F 0.0005). Squares: 5 mg·L⁻¹, y = –0.167x² + 5.143x + 23.02 (r² = 0.882, P > F 0.0139). Triangles: 10 mg·L⁻¹, y = 0.274x² + 0.821x + 17.41 (r² = 0.949, P > F 0.0026).

Citation: HortScience horts 48, 1; 10.21273/HORTSCI.48.1.77

• Download Figure

Height of Lilium ‘Tresor’ plants at flowering as affected by pre-plant bulb dips (1 min) into 2.5, 5, or 10 mg·L⁻¹ flurprimidol that had been exposed to full sunlight for 0 to 8 d (Expt. 2). The height of non-treated plants was 68.4 ± 2.9 cm. Circles: 2.5 mg·L⁻¹, y = –0.212x² + 3.649x + 52.306 (r² = 0.829, P > F 0.0001). Squares: 5 mg·L⁻¹, y = –0.153x² + 4.155x + 42.742 (r² = 0.946, P > F 0.0001). Triangles: 10 mg·L⁻¹, y = 0.036x² + 2.998x + 35.062 (r² = 0.947, P > F 0.0001).

Citation: HortScience horts 48, 1; 10.21273/HORTSCI.48.1.77

• Download Figure

View Full Size

Height of Lilium ‘Tresor’ plants at flowering as affected by pre-plant bulb dips (1 min) into 2.5, 5, or 10 mg·L⁻¹ flurprimidol that had been exposed to full sunlight for 0 to 8 d (Expt. 2). The height of non-treated plants was 68.4 ± 2.9 cm. Circles: 2.5 mg·L⁻¹, y = –0.212x² + 3.649x + 52.306 (r² = 0.829, P > F 0.0001). Squares: 5 mg·L⁻¹, y = –0.153x² + 4.155x + 42.742 (r² = 0.946, P > F 0.0001). Triangles: 10 mg·L⁻¹, y = 0.036x² + 2.998x + 35.062 (r² = 0.947, P > F 0.0001).

Citation: HortScience horts 48, 1; 10.21273/HORTSCI.48.1.77

• Download Figure

Height of Lilium ‘Tresor’ plants at flowering as affected by pre-plant bulb dips (1 min) into 2.5, 5, or 10 mg·L⁻¹ flurprimidol that had been exposed to full sunlight for 0 to 8 d (Expt. 2). The height of non-treated plants was 68.4 ± 2.9 cm. Circles: 2.5 mg·L⁻¹, y = –0.212x² + 3.649x + 52.306 (r² = 0.829, P > F 0.0001). Squares: 5 mg·L⁻¹, y = –0.153x² + 4.155x + 42.742 (r² = 0.946, P > F 0.0001). Triangles: 10 mg·L⁻¹, y = 0.036x² + 2.998x + 35.062 (r² = 0.947, P > F 0.0001).

Citation: HortScience horts 48, 1; 10.21273/HORTSCI.48.1.77

• Download Figure

Table 3.

Environmental conditions during the two photodegradation experiments.

View Table

Given the different methods used, it is notable that similar results were obtained between the two photodegradation experiments. In Expt. 1, solutions were uncovered during sunlight exposure, and water evaporated lost each day was replaced before removing and saving the sample. In Expt. 2, solutions were covered with polyethylene film so that evaporation was eliminated. Given the very low vapor pressure of flurprimidol [4.85 × 10⁻² mPa (25 °C) vs. 2.3 kPa for water (20 °C) (Anonymous, 2012a, 2012b)] very little, if any, flurprimdol was lost to evaporation in the first experiment as confirmed by the very similar result of the second experiment in which essentially no evaporation occurred.

There are few data in the literature on stability of commercial floriculture PGRs. In the case of flurprimidol, it was shown to have “a half-life of 3 h in pure water under high light intensities” (Lilly Research Laboratories, 1983, cited in Chand and Lembi, 1994). Flurprimidol had a half-life of 8.4 to 9.8 d in aquatic systems (Chand and Lembi, 1994) in experiments conducted outdoors in the midwestern United States from June to approximately September, conditions that are comparable to those in the present experiments. The bioassay method used in the present work does not allow determination of a half-life per se because the result is dependent on the initial PGR concentration and sensitivity of the cultivar being used for bioassay. Even so, the significant loss of activity seen over several days of sunlight exposure supports the rapid photolysis demonstrated in the earlier report (Chand and Lembi, 1994) and suggests greenhouse and nursery producers should take steps to protect flurprimidol solutions from direct sunlight if they are to be used over an extended period.

In the present work, the 4- and 7-week-old flurprimidol solutions were stored in darkness at 17 °C before being used in the experiment, so no photolysis occurred. The results show that overall degradation in darkness must have been minimal. Assuming a half-life of 8.4 d (Chand and Lembi, 1994), the 4- and 7-week periods represent 3.3 and 5.8 half-lives, which theoretically would reduce the initial concentrations of 30 mg·L⁻¹ to 3.0 or 0.52 mg·L⁻¹ (assuming no microbial degradation or adsorption). Given that the minimal effective concentration of flurprimidol for hyacinth and daffodil height control as a pre-plant dip is in the 10- to 20-mg·L⁻¹ range (Krug et al., 2006a, 2006b; Miller, 2012b, 2012c), differences in height between bulbs dipped into the aged and freshly prepared solutions would have been apparent if significant degradation were occurring in darkness. Thus, flower bulb forcers can use flurprimidol solutions for at least 4 to 7 weeks if they are held in darkness. Tulip, hyacinth, and narcissus bulbs are free of adhering soil, and repeated dipping into a PGR solution yields relatively little debris in the solution. Additional work should be conducted to determine long-term efficacy of flurprimidol solutions used for dipping products that have a significant amount of adhering soil, peat, or other biological substrate (Lilium bulbs, young plants in plug trays, etc.).