Role of Ethylene and 1-MCP in Flower Development and Petal Abscission in Zonal Geraniums

Geraniums are sensitive to ethylene during shipping and respond by abscising their petals. Treatment of stock plants with ethylene (ethephon) in order to increase cutting yield resulted in earlier flowering in Pelargonium × hortorum ‘Kim’ and ‘Veronica’, but did not result in increased susceptibility to petal abscission following exposure to 1.0 μL·L–1 ethylene. Treatment of ‘Kim’, ‘Veronica’, ‘Fox’, and ‘Cotton Candy’ with 1.0 μL·L–1 ethylene resulted in increased petal abscission within one hour, with ‘Fox’ being the most sensitive and ‘Kim’ the least. Pretreatment of florets with 1-MCP for 3, 6, 12, or 24 hours at concentrations of 0.1 or 1.0 μL·L–1 decreased petal abscission in all cultivars following exposure to 1.0 μL·L–1 ethylene. Treatment with 0.1 μL·L–1 1-MCP for 1 hour reduced petal abscission rates in ethylene treated florets to that of non-ethylene treated controls in all cultivars except Fox. ‘Fox’ florets, which are more sensitive to ethylene, required 12 to 24 hours of exposure to 1-MCP to reduce petal abscission rates to that of control flowers. Pretreatment of geranium plants with 1-MCP can be used to reduce petal shattering during shipping. Chemical names used: 2-chloroethanephosphonic acid (ethephon); 1-methylcyclopropene (1-MCP). sensitivity of four zonal geranium cultivars to exogenous ethylene; and 3) to investigate the effectiveness of 1-MCP at preventing petal shattering in these cultivars. Materials and Methods Plant materials. Rooted cuttings of zonal geraniums were obtained from a commercial propagator (Busch Greenhouses, Denver). One-half of the rooted cuttings were taken from stock plants that had been treated with ethylene in the form of ethephon, to prevent flower formation and ensure maximum yield of cuttings. Ethephon applications were initiated 28 d after establishment of geranium stock plants and continued at 21 d intervals for a total of 3 applications at 400 mg·L a.i. and 32 L·m. The other half of the cuttings came from stock plants that were not treated with ethephon, but were manually pinched. All stock plants were grown in a single layer polycarbonate covered greenhouse and maintained at 17 °C night and 24 °C day temperatures. Cuttings were harvested from both types of stock plants and rooted into a sphagnum peat-based medium. Plants grown from these rooted cuttings were used in the described experiments. These plants will be referred to as ethephon-treated and untreated plants. Cultivars used in all experiments included ‘Cotton Candy’, ‘Fox’, ‘Kim’, and ‘Veronica’. All rooted cuttings were transplanted into 15-cm azalea pots filled with a commercial growing medium (Pro-Mix HP; Premier Horticulture, Red Hill, Pa.) on 23 Dec. 1998. Plants were grown in a fiberglass reinforced plastic greenhouse under natural photoperiod at 25 °C day/16 °C night, and were fertilized at each irrigation (about twice per week) with N at 250 mg·L from 15N–5P–15K CalMag (The Scotts Co., Marysville, Ohio). To the surface of the media in each pot, four g of slow release fertilizer (17N–7P–12K, Osmocote, The Scotts Co., Maryville, Ohio) was applied one month after transplant. Geranium plants from each cultivar (‘Cotton Candy’, ‘Fox’, ‘Kim’, and ‘Veronica’) and stock plant treatment (ethephon-treated and untreated) were placed on four greenhouse benches in a randomized complete-block design. Two treatment combinations were randomly assigned to each block of 50 plants each. From the 50 plants, peduncle length, number of florets per inflorescence, and number of days to floret formation and visible bud color for the first six florets were determined from 10 randomly selected plants (n = 10). Data collection was terminated 11 weeks after transplanting. General linear models analysis of the data was conducted using the SAS statistical software package version 6 (SAS Institute, Cary, N.C.). Exposure to exogenous ethylene. To identify cultivar differences in sensitivity to ethylene, excised inflorescences were exposed to exogenous ethylene. Inflorescences were harvested and immediately placed into beakers with deionized water. Fully opened florets (with the stigmatic lobes fully reflexed) and flower buds were removed in order to obtain Received for publication 31 Oct. 2000. Accepted for publication 28 Feb. 2001. We would like to thank Dan Busch of Busch Greenhouses in Denver, Colo., for supplying us with rooted geranium cuttings. This research was funded in part by the Colorado Floriculture Foundation and the Colorado Agricultural Experiment Station (project #738). Assistant Professor. Graduate Student. Associate Professor. Geraniums (Pelargonium × hortorum) are popular landscape plants because they flower prolifically, bloom continually, and have a wide variety of flower colors. The major postharvest problem with geraniums is petal shattering during shipping (Oglevee, 1998). Zonal geraniums are one of the most ethylene-sensitive plant species, with some cultivars abscising all of their petals after a 1 h exposure to 1.0 μL·L ethylene (Clark et al., 1997; Evensen, 1991; Evensen et al., 1993). Ethylene sensitivity has been shown to vary between cultivars of both regal (P. × domesticum) and zonal (P. × hortorum) geraniums, with the severity of the response to ethylene increasing with the age of the floret (Clark et al., 2001; Deneke et al., 1990). The postharvest quality of many flowering plants is reduced by ethylene. Ethylene causes premature wilting, color fading and abscission of flower petals. Ethylene induced senescence and abscission can result from exposure to exogenous ethylene during shipping or increased endogenous ethylene production by the flower resulting from stress, wounding, pollination, or infection (Abeles et al., 1992). Cameron and Reid (1983) demonstrated that spraying seedling geraniums with the ethylene action inhibitor, silver thiosulfate (STS), effectively prevented petal shattering during shipping. While the application of STS reduced petal abscission in geraniums, it also predisposed them to Phythium root rot (Hausbeck et al., 1987). An additional drawback of spraying potted flowering plants with STS is that it has a narrow range of concentrations at which it is effective as an ethylene action inhibitor without being phytotoxic (Nell, 1993). A new ethylene action inhibitor, 1-MCP has been developed as an alternative to STS. 1MCP has been shown to improve the postharvest quality of a number of flowering potted plants (Serek et al., 1994). It is as effective an ethylene action inhibitor as STS, but it is nontoxic to plant tissues at concentrations much higher than those needed for maximum effectiveness (Serek et al., 1994,1995; Sisler et al., 1996). While ethylene exposure is generally detrimental to the postharvest quality of flowering plants, the ethylene treatment of stock plants is utilized to eliminate flowers in order to produce more compact plants and maintain them vegetatively. Geranium stock plants treated with ethylene in the form of the commercial growth regulator ethephon can yield as many as 20% more cuttings than non-ethephontreated plants (O’Donovan, 1993). Although this is a benefit to producers, growers in Colorado have expressed concern as to whether ethylene treatment of stock plants might predispose the flowers to increased rates of petal abscission by increasing the plants’ sensitivity to ethylene. The objectives of the research presented in this paper were 3-fold: 1) to investigate whether the treatment of stock plants with ethylene has any residual effect on time to flowering or petal abscission in plants produced from cuttings of these stock plants; 2) to determine the 6674, p. 1305-1309 1/3/02, 11:15 AM 1305 HORTSCIENCE, VOL. 36(7), DECEMBER 2001 1306 POSTHARVEST BIOLOGY & TECHNOLOGY only young morphologically similar florets from each cultivar. Florets were staged by visualizing the stigma and anthers rather than the petals as described by Deneke et al. (1990). Three open florets in which the stigmatic lobes had not begun to separate and the anthers had not begun to discolor were left on each inflorescence (Deneke et al., 1990). Florets were then placed inside a sealed 24-L treatment chamber. Ethylene was injected into the chamber to a final concentration of 1.0 μL·L. The ethylene concentration in the chambers was verified by testing a sample with a gas chromatograph equipped with a flame ionization detector and Haysep R packed column (model 3800; Varian, Walnut Creek, Calif.). Florets from all cultivars were exposed to ethylene for 1, 3, 10, and 24 h in order to determine differences in ethylene sensitivity. Control florets were harvested the same as the treated florets, but were held in ethylene-free chambers. After ethylene treatment, petal abscission was evaluated by holding the florets under a light stream of air for 90 s. The number of abscised petals was recorded. Treatments consisted of a factorial arrangement of four cultivars, two stock plant treatments (ethephontreated and untreated), two ethylene concentrations (0.0 and 1.0 μL·L) and four ethylene treatment durations (1, 3, 10, and 24 h). Four inflorescences with three florets each were evaluated per treatment (n = 4), and the experiment was conducted twice. Statistical analysis of the data was conducted as described above. 1-MCP treatment. To determine if 1-MCP could effectively prevent petal shattering with subsequent exposure to exogenous ethylene, various 1-MCP treatment concentrations and times were evaluated. Inflorescences were harvested as described above and placed in treatment chambers. 1-MCP (Ethylbloc, BioTechnologies for Horticulture, Walterboro, S.C.) was added to the chambers to three final concentrations of 0, 0.1 μL·L, or 1.0 μL·L for 1, 3, 6, 12, or 24 h. Following 1-MCP treatment, florets were treated with 0 or 1.0 μL·L ethylene for 24 h and petal abscission was evaluated as described above. In summary, 1-MCP experiments included five exposure durations of 1-MCP pretreatment (1, 3, 6, 12, or 24 h) with three 1-MCP concentrations (0, 0.1, and 1.0 μL·L), two ethylene treatments (0 or 1.0 μL·L), and four cultivars in a factorial arrangement. Only flowers from the ethephon-treated plants were used for the 1-MCP experiments because no differences in ethylene sensitivity were detected between ethephon-treated and untreated plants (see Results). Four inflorescences with three florets each were evaluated per treatment (n = 4), and the experiment was conducted twice. Statistical analysis of the data was conducted as described above. Results and Discussion Of all of the characteristics evaluated between the ethephon-treated and untreated plants, only the timing of the first floret was affected (Table 1). Ethephon treatment had no effect on the number of days to floret formation and visible color of florets two through six, and had no effect on peduncle length or number of florets per inflorescence (data not shown). Among ‘Kim’ plants, buds were visible on the ethephon-treated plants 8 d earlier than on untreated plants. Ethephon treatment also resulted in a decrease in the number of days until the observation of visible color from the flower buds of ‘Kim’ plants. Untreated plants showed visible color in 83 d while treated plants showed color at 74 d. Ethephon treatment also influenced the time to bud formation, but not color in ‘Veronica’. In ‘Veronica’, treatment with ethephon resulted in a 4 d decrease in the time to the observation of the first flower bud. Among ‘Cotton Candy’ and ‘Fox’ plants, no differences in time to first bud or flower color were observed. Ethylene’s effects on flowering vary between species, but it has been well established that ethylene induces flowering in geophytes (Abeles et al., 1992). The mechanism by which ethylene stimulates flower induction in these plants is largely unknown. It has been proposed that the induction of flowering may be the result of changes in the source and sink activities, which lead to increases in the carbohydrate status of the apex Fig. 1. Ethylene sensitivity of ‘Cotton Candy’, ‘Fox’, ‘Kim’, and ‘Veronica’ geraniums. Florets were treated with 1.0 μL·L ethylene in a 24-L chamber. Controls were held in a chamber with ethylene free air. The percentage of petal abscission was calculated after various exposure times. Each time point represents the average percentage of abscission value for four inflorescences ± SE. Table 1. The influence of ethephon on four cultivars of zonal geranium when applied to stock polants on days to first floret formation and days to first color after transplant of rooted cuttings.