ORIGINAL ARTICLES
Effects of Bacterial Proliferation and Soluble Carbohydrate Levels on the Vase Life of Cut Dahlia (Dahlia variabilis) Flowers
2019 Volume 88 Issue 1 Pages 106-115
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2019 Volume 88 Issue 1 Pages 106-115
Cut dahlia (Dahlia variabilis) flowers have recently become popular in Japan, but have the disadvantage of only having a short vase life. Here, we sought to clarify which factors are responsible for this by investigating the effects of an antibacterial (CMIT/MIT) treatment and a combined glucose plus antibacterial (Glc + CMIT/MIT) treatment on the vase life of the cut flowers of 10 dahlia cultivars, as well as the bacterial growth kinetics in their vase solutions and the soluble carbohydrate contents of their petals. We found that the CMIT/MIT treatment extended the vase life of ‘Kamakura’, ‘Magic Pink’ and ‘Purple Stone’, all of which had relatively high numbers of bacteria in their vase solutions. By contrast, the Glc + CMIT/MIT treatment significantly extended the vase life of three cultivars and also increased the fresh weight of nine cultivars. A comparison of two cultivars with relatively long and short vase lives (‘Moon Waltz’ and ‘Port Light Pair Beauty’, respectively) showed that a longer vase life was related to a higher carbohydrate content in the petals. Together, these findings suggest that maintaining the carbohydrate level is important for extending the vase life of cut dahlia flowers.
Dahlia (Dahlia variabilis) flowers vary in terms of color, shape and size. In Japan, cut dahlia flowers have been popular in recent years. However, they only have a vase life of 3–4 days at room temperature with non-treatment, which is much shorter than the 10 or more days observed for many other species, such as carnations (Dianthus caryophyllus), chrysanthemums (Chrysanthemum spp.), lilies (Lilium spp.), and roses (Rosa spp.) (Ichimura et al., 2011).
It is known that ethylene, a water relations impairment, and carbohydrate deficiency can reduce vase life. Flowers such as carnations, sweet peas (Lathyrus odoratus), and eustoma (Eustoma spp.) are sensitive to ethylene, the production of which increases during flower senescence in these species (Ichimura and Suto, 1999; Ichimura et al., 1998; Pun et al., 2016; Wu et al., 1991). However, lilies, tulips (Tulipa spp.), and chrysanthemums have very low sensitivity to ethylene (Elgar et al., 1999; Han and Miller, 2003; Sexton et al., 2000; Woltering and van Doorn, 1988). Like chrysanthemums, dahlias belong to the family Asteraceae and do not have high sensitivity to ethylene (Shimizu-Yumoto and Ichimura, 2013). Therefore, in this study, we focused on the effects of bacterial proliferation and carbohydrate deficiency on the vase life of dahlia flowers.
It has been previously reported that bacterial proliferation reduces the vase life of some cut flowers, including roses and gerbera (van Doorn and de Witte, 1994; Zagory and Reid, 1986), due to a decrease in hydraulic conductance of the cut stems, which causes vascular occlusion (Bleeksma and van Doorn, 2003; Li et al., 2012). Consequently, the flower vase life can be extended in such flowers by adding antibacterial agents to the vase solution (Jones and Hill, 1993; van Doorn, 1997) such as silver nitrate (Liu et al., 2009; Ohkawa et al., 1999), aluminum sulfate, and 8-hydroxyquinoline sulfate (Ichimura et al., 1999). Knee (2000) also reported that the commercially prepared germicide Isocil, which also goes under the commercial names Legend MK and Kathon WT (Ichimura et al., 2006) and is a mixture of 5-chloro-2-methyl-4-isothiazolin-3-one and 2-methyl-4-isothiazolin-3-one (CMIT/MIT), is suitable for extending the vase life of cut roses, carnations, and alstromeria (Alstroemeria spp.) (Ichimura et al., 2006; Knee, 2000).
The amount of soluble carbohydrates also affects the vase life of flowers (Eason et al., 1997; Ichimura et al., 2016; Shimizu and Ichimura, 2005). Soluble carbohydrates are a necessary substrate for respiration, cell-wall components, and pigments, as well as a signal for gene expression in some cases (Lewis et al., 1995; Norikoshi et al., 2016; Yamada et al., 2009). However, they also act as osmotica, with an increase in sugar concentration largely contributing to a decrease in osmotic potential, which may facilitate the influx of water into cells to maintain the pressure potential, leading to petal cell expansion and flower opening (Norikoshi et al., 2016; Yamada et al., 2009). Furthermore, carbohydrate concentration is also related to flower coloration or decoloration (Oren-Shamir et al., 2001).
The characteristics of cut dahlia flowers during postharvest handling have previously been reported by Shimizu-Yumoto and Ichimura (2013), who demonstrated that a 6-benzylaminopurine (BA) spray treatment extended the vase life of some cultivars. Moreover, some researchers have reported that treatment with sugars or antibacterial agents also extends the vase life of cut dahlia flowers (Dole et al., 2009; Takahashi et al., 2016), although these studies were limited to only a few cultivars and did not clarify how bacterial proliferation or a deficiency of carbohydrates affect vase life. Moreover, the form of the dahlia flower is a type of bloom on the stem of a plant known as a capitulum that is common in Asteraceae plants. The capitulum is formed by many florets, and the florets wilt from the outside whorl (Shimizu-Yumoto and Ichimura, 2013). Therefore, evaluation of the vase life of florets at different whorls is useful to study their vase life in detail.
In this study, we investigated the effects of bacterial proliferation and carbohydrate deficiency on the vase life of cut dahlia flowers. To do this, we examined the effects of CMIT/MIT and a combined glucose and CMIT/MIT (Glc + CMIT/MIT) treatment on the vase life of cut flowers of 10 cultivars. We also investigated bacterial proliferation in the vase solution and soluble carbohydrate levels in the petals of these cultivars.
Ten dahlia cultivars were used in the experiments: ‘Agitate’, ‘Heavenly Peace’, ‘Kamakura’, ‘Kokucho’, ‘Magic Pink’, ‘Micchan’, ‘Moon Waltz’, ‘Namahage Cute’, ‘Port Light Pair Beauty’, and ‘Purple Stone’. Cut flowers of each cultivar were obtained from growers in Asahi, Chiba Prefecture, Japan in February and March 2017. The flowers were harvested at the stage when the first row of petals from the outside had opened, and were held in tap water and stored at approximately 10°C overnight, after which they were wet-transported to our institute, which took less than 2 h. The flowers were then cut to lengths of 40 cm and all of the leaves were removed.
2. Evaluation conditions and measurement of vase lifeWe used CMIT/MIT (Kathon CG; Rohm and Haas Japan K.K., Tokyo, Japan) solution as an antibacterial agent, which contained 11.3 g·L−1 5-chloro-2-methyl-4-isothiazolin-3-one and 3.9 g·L−1 2-methyl-4-isothiazolin-3-one as active ingredients. Two cut flowers of the same cultivar were placed in 500 mL beakers containing 450 mL distilled water (DW) and either 0.5 mL·L−1 CMIT/MIT or 20 g·L−1 glucose plus 0.5 mL·L−1 CMIT/MIT (Glc + CMIT/MIT). The cut flowers were then held at a temperature of 23°C and a relative humidity of 70% under a 12 h photoperiod at 10 μmol·m−2·s−1. The vase solution remained untouched during the experimental period and the experiment was repeated three times for each treatment.
Since dahlia flowers are made up of many florets that wilt from the outside whorl (Shimizu-Yumoto and Ichimura, 2013), we examined the vase life of florets in different whorls by examining the outer floret (in the outermost whorl), the middle floret (in the fourth whorl from the outermost whorl), and the inner floret (in the eighth whorl from the outermost whorl) (Fig. 1). The condition of the florets in each position was checked each day during the experimental period and the vase life of the florets was defined as the time from treatment application to when the florets wilted or became discolored.
The position of florets according to each whorl of the cut dahlia (Dahlia variabilis) ‘Kamakura’ flowers. The photographs show the position of the florets for each whorl. The inner florets are located between the inner whorl and the eighth whorl from the outer most whorl, the middle florets are located between the eighth whorl and the fourth whorl from outer most whorl, and the outer florets are located in the outermost whorl.
We measured the fresh weight of the cut flowers and the amount of water uptake daily during the experimental period. To determine the amount of water uptake, we first needed to determine the level of evaporation. To do this, we prepared another beaker that contained the same volume of water as the beaker holding the cut flowers and measured the level of evaporation from this beaker. This was then subtracted from the amount of water that was lost from the beaker containing cut flowers to calculate the amount of water uptake. The amount of water loss from the beakers with cut flowers was calculated by subtracting the increase in fresh weight from the amount of water uptake.
4. Bacterial proliferationWe determined the number of bacteria in the vase solution of each cultivar following Norikoshi et al. (2006). We collected vase solution on days 2 and 4, and diluted 100 μL aliquots 10-, 102-, 103-, 104-, and 105-fold with sterile DW. At each dilution, the 100 μL solution was transferred onto a nutrient agar medium (1.5% agar, pH 7.0) containing 1% Bacto Peptone and 0.5% Bacto Yeast Extract, and cultured at 23°C in the dark for 5 days, following which the number of bacterial colonies was counted.
5. Cross sectional area of stemsAt day 0, dahlia flowers were recut to 40 cm. A cross section of approximately 1 mm in thickness was prepared from the cut ends of stems with a razor blade and photographed. The cross sectional area of solid and hollow parts was calculated using Image J software (NIH, Bethesda, MD, USA). Eight flower stems were used for the measurements.
6. Soluble carbohydrate contents of the petalsWe determined the soluble carbohydrate contents of the petals following the method of Norikoshi et al. (2008). We collected petals from the middle florets of all 10 cultivars on day 0, and from ‘Port Light Pair Beauty’ and ‘Moon Waltz’ flowers that had been treated with CMIT/MIT or Glc + CMIT/MIT on days 0, 2, 4, and 6. The petals (0.2 g FW) collected from three flowers were cut into pieces (approx. 5-mm squares), immersed in 10 mL of 80% ethanol in test tubes, and heated at 75°C for 20 min. Then, 25 μL galactose solution (100 g·L−1) was gently stirred into each sample to serve as an internal standard. The extractions were transferred to filter holders (2.2-cm diameter) fitted with filter paper (No. 2; Advantec, Tokyo, Japan) on a manifold (Milipore, Medford, MA, USA) to which a vacuum pump set at 33 kPa was applied. An additional 5 mL of 80% ethanol was added to the tissue remaining in the filter holder and the filtrate was evaporated to dryness in vacuo at below 50°C and dissolved in 1 mL DW.
Carbohydrates were separated using a high-performance liquid chromatography system (PU-980; JASCO, Tokyo, Japan) equipped with a refractive index detector on a Shodex SUGAR SP0810 column (Showa Denko, Tokyo, Japan). The column was maintained at 80°C and eluted with water at 0.8 mL·min−1. The identity of each peak was confirmed using authentic carbohydrate.
7. Statistical analysisData were analyzed by three-way analysis of variance and Fisherʼs least significant difference test using SigmaPlot software (v.12.5; Systat Software, San Jose, CA, USA).
There were significant differences in vase life among the 10 dahlia cultivars or each whorl (Table 1). In cut flowers held in DW, the inner florets had a longer vase life than the outer florets in all cultivars. However, there was a relatively large difference in the vase life of the outer and middle florets among cultivars compared with the inner florets (Table 1). In most cultivars, the relative fresh weight of cut flowers held in DW increased from the first to third days, and then decreased (Fig. 2). Similarly, the water uptake of most cultivars was relatively high during the first 2 or 3 days and then decreased, although the water uptake of most cultivars slightly increased on day 6 (Fig. 3).
Effects of different vase solutions [distilled water (DW), antibacterial treatment (CMIT/MIT), and glucose plus CMIT/MIT (Glc + CMIT/MIT)] on the vase life in each whorl of cut flowers of 10 dahlia (Dahlia variabilis) cultivars.
Relative fresh weight of cut dahlia (Dahlia variabilis) flowers treated with distilled water (DW), an antibacterial agent (CMIT/MIT), or glucose plus the antibacterial agent (Glc + CMIT/MIT). Values are means ± SE of three separate experiments. Different letters indicate significant differences (Fisher’s least significant difference test, P < 0.05).
Changes in the water uptake rate of cut dahlia (Dahlia variabilis) flowers treated with distilled water (DW), an antibacterial agent (CMIT/MIT), or glucose plus the antibacterial agent (Glc + CMIT/MIT). Values are means ± SE of three separate experiments. Different letters indicate significant differences (Fisher’s least significant difference test, P < 0.05).
The CMIT/MIT treatment extended vase life for the outer and middle florets of ‘Kamakura’ and the outer and inner florets of ‘Purple Stone’. Furthermore, a delay in wilting was also observed in the outer florets of ‘Magic Pink’. All three of these cultivars also showed an increase in fresh weight (Fig. 2) and water uptake rate (Fig. 3) following CMIT/MIT treatment. Additionally, the fresh weight of ‘Port Light Pair Beauty’ slightly increased on day 2 and 3, although no difference was observed for time to wilting of petals. However, no significant delay in petal wilting or increase in fresh weight was observed in the other six cultivars (Table 1, Fig. 2).
The Glc + CMIT/MIT treatment extended the vase life of petals in all three whorls in ‘Kamakura’, the inner florets of ‘Port Light Pair Beauty’, and the outer florets of ‘Magic Pink’ (Table 1). However, in ‘Purple Stone’, Glc + CMIT/MIT treatment shortened the vase life of middle florets, although the vase life of inner florets was extended (Table 1). The Glc + CMIT/MIT treatment also significantly increased the fresh weight of nine of the cultivars, the one exception being ‘Kokucho’ (Fig. 2), with ‘Agitate’, ‘Moon Waltz’ and ‘Port Light Pair Beauty’ exhibiting particularly marked increases. In addition, this treatment promoted the coloration of the inner petals in ‘Moon Waltz’ and ‘Port Light Pair Beauty’ (Fig. 4).
Photographs of the cut flowers of the dahlia (Dahlia variabilis) cultivars ‘Port Light Pair Beauty’ and ‘Moon Waltz’ 4 days after treatment with an antibacterial agent (CMIT/MIT) or glucose plus the antibacterial agent (Glc + CMIT/MIT).
The number of bacteria in the CMIT/MIT solution was lower than in DW for all 10 cultivars on day 2 and day 4. Therefore, we investigated the number of bacteria in the vase solution (DW) after 2 and 4 days across the 10 cultivars. We found that the number of bacteria in the vase solution markedly increased to more than 107 CFU·mL−1 during the first 2 days for six cultivars (Table 2), including ‘Kamakura’, ‘Magic Pink’ and ‘Purple Stone’. Furthermore, the water uptake of ‘Kamakura’ and ‘Magic Pink’ under CMIT/MIT treatment only increased during the first 2 days. However, at 4 days in these two cultivars, the number of bacteria and the water uptake did not differ between DW and CMIT/MIT treatment. By contrast, the number of bacteria in the vase solution of ‘Moon Waltz’ remained relatively low (Table 2).
Number of bacteria (CFU·mL−1) in the untreated [distilled water (DW)] and treated (CMIT/MIT) vase solutions on day 2 and day 4 of 10 dahlia (Dahlia variabilis) cultivars.
Dahlia stems of the ten cultivars had a central hollow and there were marked variations in stem diameters (Fig. S1). There were significant differences in the cross sectional areas of the solid parts (Table 3). The cross sectional area of the solid part in ʻNamahage Cuteʼ was the largest, followed by ʻKamakuraʼ. In contrast, the cross sectional area of the solid part was much smaller in ʻMicchanʼ than in the other cultivars. The area of the hollow part varied depending on the cultivar and that of ʻKokuchoʻ was smaller than the other cultivars.
Cross-sectional area at 40 cm from the flower of 10 dahlia (Dahlia variabilis) cultivars.
We found that glucose and fructose were the major carbohydrates in the petals of all 10 cultivars, and that sucrose and myo-inositol were also present. The glucose and fructose contents were particularly high in ‘Heavenly Peace’, ‘Magic Pink’ and ‘Purple Stone’, but were relatively low in ‘Agitate’, ‘Micchan’, ‘Namahage Cute’ and ‘Port Light Pair Beauty’ (Table 4). Sucrose and myo-inositol contents were very low in all of the cultivars.
Carbohydrate contents of the petals in the middle florets of 10 dahlia (Dahlia variabilis) cultivars.
We found that the vase life of ‘Moon Waltz’ was relatively long without the Glc + CMIT/MIT treatment, whereas that of ‘Port Light Pair Beauty’ was relatively short, while the vase life of inner florets of ‘Port Light Pair Beauty’ was significantly extended by the Glc + CMIT/MIT treatment. Therefore, we performed a time course analysis of the carbohydrate contents of the petals of these two cultivars. In ‘Moon Waltz’, the glucose and fructose contents decreased gradually over time in the CMIT/MIT treatment, while the Glc + CMIT/MIT treatment suppressed these decreases, and the fructose contents slightly increased (Fig. 5). The sucrose and myo-inositol contents were very low throughout the experimental period for this cultivar irrespective of treatment. In ‘Port Light Pair Beauty’, the glucose and fructose contents decreased markedly over time in the CMIT/MIT treatment, the Glc + CMIT/MIT treatment almost completely suppressed these decreases, and the fructose contents increased. Again, the sucrose and myo-inositol contents were very low in this cultivar irrespective of treatment. In the CMIT/MIT treatment, the fructose content was higher in ‘Moon Waltz’ than in ‘Port Light Pair Beauty’ throughout the experimental period (Fig. 5).
Carbohydrate contents of the middle florets of the dahlia (Dahlia variabilis) cultivars ‘Port Light Pair Beauty’ and ‘Moon Waltz’ following treatment with an antibacterial agent (CMIT/MIT) or glucose plus the antibacterial agent (Glc + CMIT/MIT). Values are the means ± SE for three separate experiments (note: SE bars are not shown when masked by the graph symbols).
In this study, we examined the flower vase life of 10 cultivars by focusing on each of their whorls, i.e., outer florets, middle florets and inner florets, respectively, and showed clearly that the wilting of petals is caused by the outer whorls (Table 1). In previous reports, the flower vase life in dahlias was defined as the days to wilting of two thirds of all of petals (Shimizu-Yumoto and Ichimura, 2013), half of all petals (Tsujimoto et al., 2016) or the third whorl from the outer-most whorl (Takahashi et al., 2016). Thus, the definition for deciding the flower vase life in dahlias was unclear and not integrated. However, our results on the vase life for each whorl can be compared easily with the findings of previous reports (Shimizu-Yumoto and Ichimura, 2013; Takahashi et al., 2016). In this study, the results for the middle florets were thought to define the flower vase life because the results largely corresponded with the findings of a previous report. (Shimizu-Yumoto and Ichimura, 2013; Takahashi et al., 2016).
There were significant differences in the cut flower vase life of the 10 dahlia cultivars examined (Table 1). The vase life of cut flowers held in DW also varied depending on the floret’s position because the petals wilt from the outside in dahlias. Therefore, we investigated the effects of CMIT/MIT or Glc + CMIT/MIT treatment on the vase life of florets in different whorls.
The proliferation of bacteria in the vase solution is known to cause xylem blockage and a loss of hydraulic conductance (Bleeksma and van Doorn, 2003; Li et al., 2012). Therefore, we investigated the effects of the antibacterial agent CMIT/MIT on the vase life of cut dahlia flowers. We found that the effects varied among cultivars and whorls, with the CMIT/MIT treatment extending the vase life and increasing the fresh weight and water uptake rate of ‘Kamakura’, ‘Magic Pink’ and ‘Purple Stone’ (Table 1, Figs. 2 and 3), which had relatively high numbers of bacteria in the vase solution on day 2. Conversely, the CMIT/MIT treatment had little effect on these parameters in those cultivars that had a low concentration of bacteria in the vase solution, such as ‘Micchan’ and ‘Moon Waltz’ (Table 2). These findings suggest that bacterial proliferation affects the vase life of some dahlia flowers. There were differences in bacterial number in vase water among the ten cultivars (Table 2). We speculated that cultivars with thicker stems may elute larger amounts of compounds from the cut end of stems for bacterial proliferation. Thus, cross sectional area of stems was investigated. Bacterial numbers in DW and the area of the solid part were relatively small in ʻMicchanʼ, whereas they were relatively large in ʻNamahage Cuteʼ (Table 3). However, coefficients of determination (R2) between bacterial number at day 2 and 4 in the control and the area of the solid part were 0.02 and 0.05, respectively, and the correlations between them were nonsignificant, suggesting that differences in bacterial number cannot be explained by differences in stem area. Thus, we propose that there may be no correlation between stem area and amounts of eluted compounds for bacterial growth.
The vase life of cut rose and gerbera flowers is shortened by exogenous bacteria (Clerkx et al., 1989) and extended when treated with antibacterial agents (Jones and Hill, 1993), suggesting that these flowers are highly sensitive to bacteria. By contrast, treatment with antibacterial agents does not extend the cut flower vase life of other species, including tulips and lilies (Jones and Hill, 1993), suggesting that they are tolerant to bacteria. Although we did not investigate the effect of exogenous bacteria on the vase life of dahlia flowers, our finding that the vase life of some dahlia cultivars was extended by CMIT/MIT treatment suggests that these cultivars are also sensitive to bacteria. However, CMIT/MIT treatment did not greatly extend the vase life of ‘Kamakura’, ‘Magic Pink’ or ‘Purple Stone’ and had no significant effect on the vase life of other cultivars (Table 1), indicating that although the proliferation of bacteria affects the senescence of dahlia flowers, it is not a critical factor.
The concentration of soluble carbohydrates also affects the vase life of some cut flowers, such as roses, because cut flowers have a limited carbon source (Ichimura et al., 2003; Norikoshi et al., 2016) and soluble carbohydrate is important for maintaining turgor pressure in the petal cells and increasing water uptake (Oren-Shamir et al., 2001; Norikoshi et al., 2016). Therefore, we also investigated the effects of carbohydrate deficiency on the vase life of dahlia flowers. The Glc + CMIT/MIT treatment only delayed the petal wilting of outer, middle and inner florets in two, one and three cultivars, respectively (Table 1). However, we also observed that flower coloration was promoted and the flower fresh weight was increased by this treatment (Figs. 2 and 4), which indicates that carbohydrate deficiency in dahlias affects the growth of petals, as well as petal senescence.
Since the effects of the glucose treatment differed among cultivars, we also investigated the carbohydrate contents of the petals of the 10 cultivars. We found that dahlia flowers generally had low sucrose and myo-inositol contents, and relatively high fructose and glucose contents (Table 4). Furthermore, the cultivars that were not influenced by glucose treatment, i.e., ‘Heavenly Peace’, ‘Magic Pink’ and ‘Purple Stone’ had particularly high glucose and fructose contents. There were differences in carbohydrate contents in petals among the ten cultivars (Table 4). Similar results have been reported in some plants, including chrysanthemums (Ichimura et al., 2000) and roses (Ichimura et al., 2005). In general, soluble carbohydrates accumulate in vacuoles in petals (Yamada et al., 2009). For accumulation of carbohydrates in vacuoles, various factors, including transport activity (Bush, 1993) and a difference in osmotic potential between the symplast and apoplast (Yamada et al., 2009), are necessary. These factors are involved in differences in carbohydrate contents. In addition, we assume that differences in physiological stages of petals may be related to different carbohydrate contents. Carbohydrate contents in petals markedly increased during flower opening in some plants including chrysanthemums (Ichimura et al., 2000) and roses (Yamada et al., 2009). Physiological stages at harvest appear to be different depending on cultivars because carbohydrate contents of growing petals of middle whorls were determined in the present study. These findings support this assumption. We also compared the soluble carbohydrate contents during vase life in ‘Moon Waltz’, which had a relatively long vase life without glucose treatment, and ‘Port Light Pair Beauty’, which had a relatively short vase life without glucose treatment. As previously reported (Norikoshi et al., 2016; Pun et al., 2016), treatment with an antibacterial agent was used as the control in this experiment to clarify effect of glucose on carbohydrate contents. The result showed that the petals of ‘Moon Waltz’ had a higher fructose content than those of ‘Port Light Pair Beauty’ in the absence of a glucose treatment, but the sugar contents were increased and the vase life was extended in ‘Port Light Pair Beauty’ following treatment with Glc + CMIT/MIT (Fig. 5). These results suggest that the differences in vase life between these two cultivars can be attributed to the soluble carbohydrate contents of their petals, supporting the findings of Ichimura et al. (2005) for cut roses.
It has been reported that some plant tissues store a large amount of fructans. For example, chrysanthemums and many other species of the Compositae family store fructans in their stems and roots (Adachi et al., 1999; Frehner et al., 1984). In daylilies (Hemerocallis hybrid cv. Cradle Song), fructans are rapidly broken down to yield high concentrations of glucose and fructose, which is thought to contribute to the osmotic driving force involved in petal extension (Bieleski, 1993). Similarly, cut chrysanthemums accumulate sufficient reserves of fructans in their inflorescences and leaves to act as substrates for petal expansion (Adachi et al., 1999). In dahlias, fructans are stored in the tubers (Legnani and Miller, 2001). Therefore, we propose that fructans are important carbohydrate reserves in cut dahlia flowers, although further research is required to test this hypothesis.
The senescence of flowers is characterized by many phenomena, such as wilting, abscission, and decoloration of the petals, each of which is induced by a number of factors. In cut dahlia flowers, bacterial proliferation appears to be involved in petal wilting in some cultivars because treatment with antibacterial agents extended the vase life of these cultivars. In addition, the Glc + CMIT/MIT treatment promoted growth of the inner petals and increased the fresh weight in cultivars that contained low concentrations of glucose and fructose, although this depended on the cultivar. However, since treatment with antibacterial agents and glucose inhibited flower senescence only slightly, it appears that neither bacterial proliferation nor carbohydrate deficiency are the main factors that induce senescence. It has previously been reported that flower senescence is regulated by programed cell death (PCD) (Rogers, 2006). For example, PCD is involved in flower senescence in the Japanese morning glory (Ipomoea nil), where it is induced by the function of the EPH1 gene, which is a transcription factor of the NAC (NAM/ATAF1, 2/CUC2) family (Shibuya et al., 2014). Therefore, it is possible that the short vase life of dahlia flowers is also caused by PCD. We are currently attempting to investigate PCD in dahlia flowers and to isolate the gene that induces this.
In conclusion, we found significant differences in the vase life of 10 dahlia cultivars and demonstrated that treatment with the antibacterial agent CMIT/MIT delayed petal wilting in some cultivars, although its effect was not large. In addition, the Glc + CMIT/MIT treatment promoted growth of the inner petals and increased the fresh weight of most cultivars, while differences in the vase life of ‘Moon Waltz’ and ‘Port Light Pair Beauty’ were associated with differences in the carbohydrate contents of their petals. Thus, it appears that maintaining the carbohydrate content is important to increase the vase life of cut dahlia flowers.
