SPECIAL ISSUE: ORIGINAL ARTICLES
Crossing and Selection of Dahlia (Dahlia Cav.) Lines with a Lower Degree of Petal Abscission
2025 Volume 94 Issue 1 Pages 15-23
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2025 Volume 94 Issue 1 Pages 15-23
Dahlia (Dahlia Cav.) is an important ornamental plant used as cut flowers. Recently, we bred the “Eternity series” cultivars with long vase life to overcome the difficulty of long-distance transport due to their short vase life. However, dahlias, including the Eternity series, are ethylene-sensitive, inducing petal abscission within a short period. Therefore, cultivars with long vase life and low degree of abscission were crossed to examine the possibility of producing lines with both traits. The flower life of offspring after treatment with 10 μL·L−1 ethylene (time to ethylene response) was investigated in open-field cultivation during the summer–autumn of 2021. The degree of abscission was classified into three types according to the time to ethylene response and the occurrence of abscission: abscission within four days (high), abscission after four days (mid), and the absence of abscission during senescence (low). We selected 17 of the 105 lines with mid or low degrees of abscission. The degree of abscission of the selected lines was re-evaluated during greenhouse cultivation in the winter–spring of 2021–2022. Three lines (F182-10, F181-14, and F182-17) that showed no petal abscission in the winter–spring were selected. The drawing resistance force of the petals from lines F182-10 and F182-17 was maintained at 1.0 N even during the senescence stage (six days after harvest), which was related to their low degree of abscission. The degree of abscission of lines F182-10, F181-14, and F182-17 was stable during the re-evaluation in summer–autumn of 2023, and their times to ethylene response were 4.8, 5.8, and 4.6 days, respectively. Moreover, the vase lives of lines F182-10 and F182-17 were 5.8 and 5.6 days, respectively, while that of ‘Port Light Pair Beauty’ was 4.4 days. These results suggested that degree of abscission was inherited from ‘Port Light Pair Beauty’, and the vase life was improved after crossing with a cultivar with a long vase life. This is the first report about breeding dahlia with a low degree of abscission, and it indicates the possibility of producing lines with both long vase life and a low degree of abscission.
Dahlia (Dahlia Cav.) is an important ornamental plant for cut flowers that belongs to the Asteraceae family and it comes in various colors, shapes, and sizes. However, despite its attractive traits, cut flowers are compromised by the short vase life (3–7 days without chemical treatment) in general household consumption. Because flowers with a short vase life have little chance of succeeding commercially, even if they show a unique color or morphology, research on postharvest treatment has been conducted to extend the vase life of dahlia cut flowers (Azuma et al., 2019; Shimizu-Yumoto and Ichimura, 2013). Furthermore, new dahlia cultivars in the “Eternity series” have been bred and were released in 2020 (Onozaki and Azuma, 2019, 2022). Among them, ‘Eternity Torch’, ‘Eternity Romance’, and ‘Eternity Rouge’ have vase lives of 6.3–12.0 days, which is 1.4–2.2 times longer than that of a commercial cultivar, ‘Kamakura’ (Onozaki and Azuma, 2022). Such crossing and selection have been repeated four times, and ‘Eternity Peach’ with a long vase life, was bred from the third generation in 2023 (Onozaki et al., 2024). Moreover, crossbreeding has improved the mean vase life of a population by 3.6 days (Onozaki and Azuma, 2019; Onozaki and Fujimoto, 2023). It is expected that cut flowers will maintain ornamental value during long-distance transport when using cultivars with long vase lives. However, it has been reported that the influence of endogenous ethylene becomes stronger in transport-related closed and stressful environments in some flower species (Müller et al., 2000; Skog et al., 2001), which is also a factor that causes petal wilting or abscission in dahlia cut flowers (Azuma et al., 2020). Additionally, owing to the high temperatures during the summer season in recent years, petal abscission has been occurred at the harvest stage (Yamagata, 2022). Therefore, both a long vase life and low degree of abscission are needed to maintain the vase life over the long term, regardless of the season, and extend the demand for dahlia cut flowers.
Ethylene sensitivity and reactions differ between plant species (van Doorn, 2001; Woltering and van Doorn, 1988). Endogenous ethylene production increases during flower senescence in ethylene-sensitive flowers such as carnation (Dianthus caryophyllus L.), rose (Rosa spp.), eustoma (Eustoma grandiflorum), and petunia (Petunia hybrida), (Ichimura et al., 1998; Mayak and Halevy, 1972; Porat et al., 1993; van Altvorst and Bovy, 1995). Moreover, treatment with ethylene action inhibitors, such as silver thiosulfate complex (STS) or 1-methylcyclopropene (1-MCP), extends flower life, while exogenous ethylene treatment shortens the flower life of carnations and eustomas (Ichimura and Niki, 2014; Ichimura et al., 1998; Wu et al., 1991a, b). However, endogenous ethylene production is low during flower senescence in ethylene-insensitive flowers such as lily (Lilium spp.), tulip (Tulipa spp.), chrysanthemum (Chrysanthemum spp.), gladiolus (Gladiolus spp.), and iris (Iris spp.) (Woltering and van Doorn, 1988). Moreover, treatment of such flower species with ethylene action inhibitors or exogenous ethylene has little or no effect on vase life (Doi et al., 2003; Elgar et al., 1999; Sexton et al., 2000; Woltering and van Doorn, 1988).
Dahlia cultivars ‘Kokucho’ and ‘Karma Thalia’ have a low sensitivity to ethylene (Dole et al., 2009; Shimizu-Yumoto and Ichimura, 2013), and pulsing treatment with STS had little effect on the flower vase life of ‘Kokucho’ (Shimizu-Yumoto and Ichimura, 2013). However, certain commercial cultivars exhibit various levels of ethylene sensitivity. For example, exogenous ethylene treatment induced petal wilting and/or abscission and shortened the vase life of ‘Carnelian’ and ‘Saisetsu’ (Azuma et al., 2020; Yang et al., 2021). Thus, cut dahlia flowers are sensitive to ethylene, whereas the response level of abscission and/or wilting vary among cultivars. Ichimura and Azuma (2022) reported different effects of ethylene inhibitors on cut dahlia flowers; pulse treatment of STS significantly improved the vase life of 7 out of 10 cultivars, but did not extend that of ‘Port Light Pair Beauty’, ‘LaLaLa’, or ‘Micchan’.
Cultivars with a low degree of abscission are needed to maintain flower value at high ethylene concentrations, such as during transportation. However, even when commercial cultivars with a low degree of abscission are used, value is lost because of their short vase life. For example, the commercial cultivar ‘Port Light Pair Beauty’ has low degree of abscission, but it cannot be used in applications requiring long transportation periods owing to its short vase life of 3–4 days (Azuma et al., 2020; Ichimura and Azuma, 2022; Onozaki and Azuma, 2019). On the other hand, in our previous study, most cultivars and lines with long vase lives showed abscission with exogenous ethylene treatment (Fujimoto and Onozaki, 2023). Thus, there are no clear relationships between vase life and degree of abscission. Among these lines, ‘Eternity Shine’ showed a relatively low degree of abscission (Fujimoto and Onozaki, 2023; Onozaki et al., 2024); however, some abscised flowers were observed in the evaluation. Nevertheless, crossbreeding to improve the degree of abscission of cut dahlia flowers has not been attempted. Since commercial cultivars that show various ethylene sensitivities are available, it is possible that lines with a low degree of abscission can be produced by selecting such cultivars and using them as breeding materials. This has been reported in begonia (Begonia spp.) (Hvoslef-Eide et al., 1995) and carnation (Onozaki, 2008). In this study, we examined the possibility of using dahlia lines with a low degree of abscission in conventional breeding. Here, we report the results of crossing, selection, and further measurement of the petal drawing resistance force during the postharvest period.
Three commercial dahlia cultivars, ‘Port Light Pair Beauty’, ‘Eternity Rouge’, and ‘Eternity Peach’ were used as initial breeding materials (Table 1). ‘Port Light Pair Beauty’ is a commercial cultivar with a short vase life (Azuma et al., 2020), while ‘Eternity Rouge’ and ‘Eternity Peach’ have long vase lives (Onozaki and Azuma, 2019, 2022; Onozaki et al., 2024). The degree of abscission of ‘Port Light Pair Beauty’ is low, while those of ‘Eternity Rouge’ and ‘Eternity Peach’ are high (Fujimoto and Onozaki, 2023). Cultivation, crossing, and selection were performed as previously described (Onozaki and Azuma, 2019, 2022; Onozaki et al., 2024). Plants were cultivated in an open field at the Institute of Vegetable and Floriculture Science, NARO, Tsukuba, Japan, during the summer–autumn season (i.e., June–November) 2020. Three crossings (F180, F181, and F182), shown in Table 1, were conducted in October 2020, and 224 seeds were collected in mid-November. Harvested seeds were sown on March 30, 2021, and 121 germinated seedlings were planted in the field on June 1, 2021, and grown as described above until November 2021. Plants that did not flower by September 29, 2021 (the last day of evaluation) were discarded, and the remaining 105 offspring were used for further evaluation. The vase life of each offspring was evaluated for the first batch of flowers during the flowering season, i.e., from July to September. Degree of abscission was evaluated using the second batch of flowers. Plants that showed a flower life of more than four days and no signs of abnormal growth, such as malformed flower buds, were selected in October 2021 and used in further investigations.
Cross combinations and result of the crosses.
After the first selection for degree of abscission, individuals were propagated vegetatively and planted in 21 cm diameter pots with standard culture soil (Royal Culture Soil; Tachikawa Heiwa Nouen Co., Ltd., Tochigi, Japan) on September 21, 2021. They were grown in a greenhouse and pinched twice to produce branches for flowering. Cultivation was carried out during the winter–spring season from September 2021 to May 2022. The greenhouse was maintained at temperatures above 10°C with heating and 14.5 h daylength with artificial lighting (K-RD110V75W/D; Panasonic Co., Ltd., Osaka, Japan). Degree of abscission and vase life were reinvestigated for the flowering season beginning in January. Subsequently, the lines that showed no petal abscission in both the summer–autumn and winter–spring seasons were considered lines with a low degree of abscission and were selected. Vegetatively propagated plantlets of the lines were grown in an open field for the summer–autumn season (June–October 2023) and used to confirm the stability of degree of abscission of the selected lines.
Additionally, the selected lines and parental cultivars were propagated vegetatively and planted in 21 cm diameter pots, grown in a greenhouse from August to December 2022, and used to investigate the drawing resistance force of the petals.
Evaluation of degree of abscission and vase lifeThe degree of abscission of the offspring and selected lines was investigated according to Azuma et al. (2020). When the outermost petals of the flower opened horizontally, the flowers were cut into 20 cm long stems, and all the leaves were removed. Then, two or three cut flowers were placed in 500 mL vessels containing approximately 400 mL of 0.5 mL·L−1 5-chloro-2-methyl-4-isothiazolin-3-one (CMIT)/2-methyl-4-isothiazolin-3-one (MIT) solution (Kathon CG; Rohm and Haas Japan K.K., Tokyo, Japan) comprising 11.3 g·L−1 CMIT and 3.9 g·L−1 MIT as an antibacterial agent. Flowers were placed in 70 L transparent acrylic boxes fitted with a septum through which ethylene was introduced at 10 μL·L−1 for exogenous ethylene treatment. The box was opened for 5 min every 24 h to release ethylene, then closed, and ethylene was injected again at the same concentration. This was repeated until petal wilting or abscission was observed, and the number of elapsed days from the start of ethylene treatment was recorded. Petal wilting was visually confirmed by observing wilting of the one-third of all the petals, while abscission was confirmed by the detachment of more than 10 petals after touching them gently by hand (Azuma et al., 2020). The degree of abscission of the lines was classified into three types according to their time to ethylene response and occurrence of abscission: abscission within 4 days (high), abscission after 4 days (mid), and absence of abscission during senescence (low). Throughout the experimental period, the box was kept at 23°C with a 12-h light period during which cool-white fluorescent lamps provided 10 μmol·m−2·s−1 irradiance.
The vase life of flowers without ethylene treatment was investigated under the same conditions described above but outside the acrylic box. Relative humidity (RH) was maintained at 70% outside the acrylic box. Additionally, the vase life of cut flowers in an acrylic box without ethylene treatment was tested under the same conditions described above from August to October 2023 to evaluate the degree of abscission.
Measurement of drawing resistance force of a petal from inflorescencesThe three selected lines (F182-10, F181-14, and F182-17) were grown in a greenhouse between August 2022 and January 2023. For measurement, the flowers were cut and prepared as described above. Throughout the experimental period, flowers were kept in 0.5 mL·L−1 CMIT/MIT solution, 23°C, 70% RH with a 12-h light period during which cool-white fluorescent lamps provided 10 μmol·m−2·s−1 irradiance, without exogenous ethylene treatment. The resistance force of the dahlia petals was investigated according to Yang et al. (2021) on the harvest day as well as three and six days after harvest. The petals in the outermost whorl of the inflorescence were clamped with a small clip and strung to a digital force gauge (HJ-DFS-5; SATOTECH, Kanagawa, Japan). The drawing resistance force was measured by slowly pulling the digital force gauge onto a flat, smooth plane. Measurements were performed for three petals per flower, and the average value was applied. Measurements were performed three times on independent inflorescences.
Using the first batch of flowers, the range of vase life was 3–12, 4–9, and 3–12 days for cross combinations F180, F181, and F182, respectively. The mean vase life of each cross combination was 6.3, 5.5 and 5.3 days for F180, F181, and F182, respectively (Table 2). In contrast, their parental cultivars, ‘Port Light Pair Beauty’, ‘Eternity Rouge’, and ‘Eternity Peach’ had vase lives of 4.2, 10.0, and 7.0 days, respectively. During harvesting, flowers were cut, leaving two nodes from the base of the shoot; however, some individuals subsequently died and could not be used for further investigation of degree of abscission. Using the second batch of flowers, the time to ethylene response of the offspring varied from 1 to 4, 1 to 4.2, and 1 to 6.2 days in cross combinations F180, F181, and F182, respectively. The mean times to ethylene response of each cross combination were 2.6, 2.9, and 3.3 days for cross combinations F180, F181, and F182, respectively (Table 2). In contrast, those of their parental cultivars ‘Port Light Pair Beauty’, ‘Eternity Rouge’, and ‘Eternity Peach’ were 3.8, 4.7, and 2.0 days, respectively. Moreover, there were significant differences in the mean time to ethylene response of each offspring between the cross combinations F180 and F182 (Table 2). Correspondingly, only cross combination F182 had individuals that showed an ethylene response longer than five days. However, there were no significant differences in the mean vase life of the offspring from each cross (Table 2). In addition, there was no relationship between the time to ethylene response and vase life or morphological traits (data not shown). Based on the degree of abscission test results, one individual with mid level abscission from cross combination F180, two and three individuals with mid or low levels from cross combination F181, three and eight individuals with mid or low levels from cross combination F182, were primary selected and used for further evaluations (Tables 2 and S1).
Vase life, degree of abscission and number of selected lines of the offspring from each cross combination evaluated in the summer–autumn season 2021.
The selected lines were re-evaluated for degree of abscission during winter–spring cultivation. As a result, most lines showed different degrees of abscission from the summer–autumn season in winter–spring season (Table S1). In particular, three lines F182-1, F182-6, and F181-18 that showed a low degree of abscission in the summer–autumn season showed high levels in the winter–spring season (Table S1). On the other hand, three lines F182-10, F181-14, and F182-17, which had times to ethylene response of 6.0, 5.6, and 7.0 days, respectively (Table S1) were subsequently selected (Fig. 1; Table 3). The line F181-14 showed mid and low degree of abscission in the summer–autumn and winter–spring seasons, respectively. On the other hand, lines F182-10 and F182-17 showed low degree of abscission both in the summer–autumn and winter–spring seasons (Table S1).
Flowers of lines F182-10, F181-14 and F182-17 selected with a low degree of abscission (A), and variations in the ethylene response of selected lines F181-14 (left) and line 003-15 (right) (B). 003-15 is a line with ultra-long vase life and a high degree of abscission (Onozaki and Fujimoto, 2023). The flowers were treated with 10 μL·L−1 ethylene that was maintained in a 0.5 mL·L−1 CMIT/MIT (isothiazolinone derivatives) solution at 23°C under a 12-h photoperiod.
Vase life, degree of abscission and number of selected lines of the first-selected lines from each cross combination evaluated in winter–spring season 2021–2022.
For the degree of abscission in the summer–autumn season of 2023, F182-10, F181-14, and F182-17 had longer times to ethylene response of 4.8, 5.8, and 4.6 days, respectively, relative to three parental cultivars without abscission (Table 4). Additionally, the vase life of the three lines outside the acrylic box was the same as the number of days inside the acrylic box, regardless of the ethylene treatment, and there were no significant differences among the conditions (Table 4). In contrast, their parental cultivars with a high degree of abscission, i.e., ‘Eternity Rouge’ and ‘Eternity Peach’ showed significantly shorter times to ethylene response than vase life due to petal abscission at 2.4 and 2.6 days, respectively (Table 4). Moreover, the vase life inside the acrylic box without ethylene treatment was significantly shorter than that outside the box, owing to petal abscission (Table 4).
Degrees of abscission of selected lines and their parental cultivars evaluated in the summer–autumn season 2023.
The drawing resistance force of petals from the three selected lines and control cultivars varied from 0.9 to 1.6 N on the day of harvest (Fig. 2). The tendency of decreased drawing resistance force three days after harvest differed among cultivars and lines; ‘Eternity Rouge’, ‘Eternity Peach’, F182-10, and F182-17 maintained relatively high values while those of ‘Port Light Pair Beauty’ and F181-14 decreased. In contrast, the drawing resistance force of parental cultivars and line F181-14 decreased to around 0.6 N six days after harvest, whereas those of lines F182-10 and F182-17 were maintained at 1.3 N and 1.0 N, respectively (Fig. 2). In particular, the drawing resistance force of petals from F182-10 increased six days after harvest. Moreover, ovaries were attached to the drawn petals in all cultivars and lines at stages with a high drawing resistance force, although they remained at the receptacle when the resistance force decreased (Fig. S1). In contrast, the ovaries of lines F182-10 and F182-17 were attached to the petals even six days after harvest.
Drawing resistance force of petals from parental cultivars and selected lines with a low degree of abscission cultivated from August 2022 to January 2023. Bars indicate SE (n = 3). Plots labeled with different letters indicate significant differences among cultivars and lines at the 0.05 level using the Tukey-Kramer’s test. E Rouge, ‘Eternity Rouge’; E Peach, ‘Eternity Peach’; PLPB, ‘Port Light Pair Beauty’.
In the present study, we carried out crossings for low degree of abscission using ‘Port Light Pair Beauty’ as a common parent of each cross combination. The time to ethylene response of the offspring varied among the cross combinations F180, F181, and F182 (Table 2). In addition, lines with a low degree of abscission were found only in cross combinations F181 and F182 (Table 2). These results suggested that genes conferring low degree of abscission in F182-10, F181-14, and F182-17 may be derived from ‘Port Light Pair Beauty’. Moreover, the possibility of producing cultivars with a low degree of abscission using conventional breeding was confirmed.
Based on the evaluation of degree of abscission in the summer–autumn season of 2021, 17 lines were selected and used for further cultivation in the greenhouse and re-evaluation of degree of abscission in the winter–spring seasons. However, the degree of abscission in most of the lines changed, and only three lines showed a low degree of abscission (Tables 3 and S1). This change may be due to the difference in temperature and environmental factors from the cultivation process between the open field in summer–autumn and the greenhouse in the winter–spring seasons. Finally, three lines, F182-10, F181-14, and F182-17, were subsequently selected as lines with a low degree of abscission (Fig. 1).
‘Eternity Rouge’ had a time to ethylene response of 4.7 days in the summer–autumn season of 2022 and 2.6 days in 2023 (Table 4). The time to ethylene response of ‘Eternity Rouge’ in the summer–autumn season of 2020 was determined to be 3.3 days (Fujimoto and Onozaki, 2023). This broad range of time to ethylene response of ‘Eternity Rouge’ may have resulted from the influence of climate and/or temperature changes during the evaluation period. Similarly, F181-14 showed abscission with the exogenous ethylene treatment for one flower out of five, and was classified as mid degree of abscission in the summer–autumn season of 2021 (Table S1). This result suggested the degree of abscission of F181-14 was basically low, but that it also exhibits a seasonal difference. In this study, the cut flowers were kept at a constant temperature after harvesting, but there was a large difference in the temperature before harvesting that may have affected the ethylene sensitivity, the amount of endogenous ethylene production or flower vase life. In cotton (Gossypium hirsutum), a high night temperature promotes the abscission of floral buds (Echer et al., 2014). In addition, in begonia, the seasonal difference in the amount of endogenous auxin controls abscission of the floral buds by ethylene response (Hänisch ten Cate et al., 1975). In the present study, the change in temperature between cultivation periods may have caused the seasonal difference in the ethylene response of ‘Eternity Rouge’ and the offspring. However, such an observation was not clarified in dahlia. On the other hand, there is a seasonal difference in the vase life of cut dahlia flowers (Naka et al., 2015). To elucidate these seasonal differences in ethylene response and vase life, further investigations are needed.
‘Eternity Rouge’ and ‘Eternity Peach’ showed a significantly shorter vase life even inside the acrylic box without ethylene treatment (Table 4). Similarly, Onozaki and Fujimoto (2023) reported that lines with a long vase life, i.e., 003-15, 012-19, 003-29, and 012-32, had vase lives of 3.0–4.8 days inside an acrylic box without ethylene treatment, which was considerably shorter than that outside the box (9.5–12.8 days). In particular, line 003-15 (Fig. 1B), which has an ultra-long vase life, had a time to ethylene response of 2.4 days (Onozaki and Fujimoto, 2023). Dahlia florets produce endogenous ethylene before and after flowering around at levels of 1.0 to 3.0 nL·h−1; however, there are differences among cultivars (Shimizu-Yumoto and Ichimura, 2013; Yang et al., 2021). Azuma et al. (2020) also reported that the RH inside an acrylic box was almost 100% when closed, and the commercial cultivars ‘Carnelian’ and ‘Namahage Cute’ showed petal abscission under conditions of 88% RH. Thus, endogenous ethylene or high RH under closed conditions may induce the abscission of dahlia petals (Azuma et al., 2020; Onozaki and Fujimoto, 2023). In contrast, there were no significant differences in the vase lives of F182-10 and F182-17 between inside and outside acrylic boxes (Table 4). Hence, the degree of abscission of lines F182-10 and F182-17 was consistently low regardless of environmental conditions, season, or cultivation method.
Petal abscission is induced by ethylene, and the subsequent reduction in structural strength is due to the formation of an abscission layer (Nakano and Ito, 2013). This can be expressed as the drawing resistance force of the petals (Craker and Abeles, 1969; Yang et al., 2021). Yang et al. (2021) investigated the formation of an abscission layer at the petal-ovary boundaries of dahlia (Fig. S1) by measuring the resistance force of the petals and clarifying the relationship between abscission layer formation and vase life. In their study, the drawing resistance force of the petals from the outermost whorl of inflorescences of the commercial cultivars ‘Kokucho,’ ‘Nesshou,’ and ‘Saisetsu’ were 1.5–1.75 N on the day of harvest. This decreased to around 0.75 N after five days (Yang et al., 2021). In the present study, the resistance force of petals from parental cultivars and line F181-14 decreased to approximately 0.6 N in the senescence stage, i.e., six days after harvest (Fig. 2). In contrast, two selected lines, i.e., F182-10 and F182-17, showed values of 1.3 N and 1.0 N even at six days after harvest (Fig. 2). This indicated that lines F182-10 and F182-17 maintained structural strength at the petal-ovary boundaries (Fig. S1), which was responsible for the absence of abscission during the evaluation of degree of abscission (Fig. 2; Table 4). This was supported by the observation that the ovaries were attached to the drawn petal even six days after harvest only in F182-10 and F182-17. Thus, the drawing resistance force of the petals is consistently associated with the low degree of abscission of lines F182-10 and F182-17. ‘Port Light Pair Beauty’ and F181-14 showed decreases in drawing resistance force three days after harvest (Fig. 2), while petal abscission did not occur when evaluating the degree of abscission (Table 4). Due to the shorter vase life of ‘Port Light Pair Beauty’ and F181-14 (4.2 or 4.4 days) than other evaluated cultivars or lines (Table 4), it was suggested that there was a decrease in physical strength due to wilting of petals in addition to the ethylene response. Further studies on the resistance force of selected lines and the Eternity series with exogenous ethylene treatment are needed to clarify whether the abscission layer is formed in each line to investigate the relationship between ethylene response and abscission layer formation in dahlia petals.
Our research group has conducted crossbreeding to improve the vase life of cut dahlia flowers, and the Eternity series with a long vase life has been bred (Onozaki and Azuma, 2019, 2022; Onozaki and Fujimoto, 2023; Onozaki et al., 2024). However, most of our cultivars and lines with long vase life show a high degree of abscission (Fujimoto and Onozaki, 2023). In the present study, the drawing resistance force of the ‘Eternity Rouge’ and ‘Eternity Peach’ with a long vase life decreased to around 0.6 N six days after harvest (Fig. 2). These results suggest that the Eternity series has ethylene sensitivity that induces petal abscission. On the other hand, a previous study reported that the senescence of dahlia petals is induced by the ethylene response and subsequent abscission layer formation, which cuts off the water supply to petals in the cultivar ‘Saisetsu’ (Yang et al., 2021). Hence, there may be some unknown factors that cause the inconsistency in the response to ethylene in the Eternity series, in which petals do not wilt in the early stage of flower senescence even when the drawing resistance force of the petals decreased (Fig. 2). Further research to confirm the abscission layer and its occurrence stage is needed to understand the relationships between vase life and the ethylene response of the dahlia Eternity series.
Although the results of the present study suggest the potential for breeding dahlia lines with a long vase life and a low degree of abscission, the mean vase life of the three selected lines ranged from 4.2 to 5.8 days (Table 4), longer than that of the parental cultivar ‘Port Light Pair Beauty’ but shorter than that of ‘Eternity Rouge’ (Table 4). The parental cultivar ‘Port Light Pair Beauty’ shows a low degree of abscission (Fujimoto and Onozaki, 2023), but time to petal wilting and detachment from inflorescences was shortened with exogenous ethylene treatment (Azuma et al., 2020). Additionally, 1-MCP treatment extended the flower vase life of ‘Port Light Pair Beauty’ (Azuma et al., 2020). Thus, ‘Port Light Pair Beauty’ has ethylene sensitivity, and subsequent responses related to petal wilting may make the vase life of offspring shorter than the Eternity series (Table 4). On the other hand, the Eternity series was bred by accumulation or duplication of genetic factors related to vase life that were originated from ‘Micchan’ (Onozaki and Azuma, 2019; Onozaki and Fujimoto, 2023). Therefore, the vase life of selected lines was longer than ‘Port Light Pair Beauty’. To improve their vase life further, crossing between the selected lines and lines with an ultra-long vase life that we previously bred (Onozaki and Fujimoto, 2023) is now in progress.
In conclusion, long vase life and low ethylene sensitivity are important for dahlia flowers to maintain their commercial value during long-term and stressful transportation. Cultivars with a long vase life and low degree of abscission were crossed to examine the possibility of producing lines with both traits. Offspring from the crossing showed various degrees of abscission, including a low level, that suggested genes conferring a low degree of abscission may have been derived from ‘Port Light Pair Beauty’. Throughout the evaluation, lines F182-10, F181-14, and F182-17 consistently showed no abscission and were selected as lines with a low degree of abscission. Moreover, the vase lives of lines F182-10 and F182-17 were longer than that of ‘Port Light Pair Beauty’. The drawing resistance force of the petals from lines F182-10 and F182-17 remained constant during the senescence stage, which is associated with the low degree of abscission of these lines. This suggests the possibility of the breeding dahlias with long vase life and a low degree of abscission. Consequently, lines F182-10, F181-14, and F182-17 are suitable crossing materials for breeding, and further breeding to improve their vase lives is now in progress.
We sincerely thank the members of the Technical Support Center of the Central Region, Tsukuba Operation Unit 6, Fujimoto-Owashi Technical Team, NARO, for their technical support in the field and greenhouse management.
