Introduction
Roses (Rosa hybrida L.) are valuable horticultural crops and one of the most economically important cut flowers in the world. Among flower species, cut roses are especially susceptible to water stress under the unfavorable postharvest environments and often lost their ornamental value at the retail shops or immediately after purchase by consumers (Doi et al. 2000; In et al. 2016; Van Doorn and Reid 1995). The postharvest loss in flowers is about 20 percent and a large amount of the fresh flowers are sold with low quality due to physical damages and injury by disease during the postharvest process (Mehran et al. 2008; Reid 2009). The short and unpredictable nature of the postharvest longevity has resulted in decreased in cut flower’ marketability and customer’s satisfaction (Rihn et al. 2014).
The brief vase life of cut rose flowers is related to various postharvest disorders resulted from adverse water relations, a low internal sucrose, microbial growth, ethylene production, and a loss of substrates under unfavorable postharvest conditions (Doi et al. 2000; Fanourakis et al. 2012; Ichimura et al. 2005; In et al. 2017; Van Doorn and Reid 1995). The water relation interruption mostly caused by vascular occlusion by air and microorganisms in the cut flower stems (He et al. 2006; Liu et al. 2009; van Doorn 1995). Disruption of cut flower water relation also causes physiological disorders such as bending of peduncle, inhibited flower opening, and wilting of flowers or leaves and consequently decreased the vase life of cut roses (Florack et al. 1996; Torre and Fjeld 2001; van Doorn and Perik 1990).
Therefore, improvement of the postharvest keeping quality and the vase life of cut roses is an essential requirement to provide satisfactory products for customers. The senescence of cut flowers are closely related to the reduction of the internal sucrose needed for flower opening. Although supply of exogenous sucrose retards flower senescence, sucrose cannot be treated solely as it promotes bacterial growth, which leads to s hortening of v ase life ( Ichimura e t al. 2 002 ; Kuiper e t al. 1995). Thus, substitutive methods for postharvest treatments are necessary to improve quality of cut flowers. Use of preservative solutions in combination with sucrose is effective in reducing microbial proliferation, increasing water uptake, inhibiting ethylene production, and supplying energy sources and consequently retards flower senescence and prolongs vase life of cut flowers (Ichimura et al. 1999; Son et al. 1994). Previous studies reported that silver thiosulfate, silver nitrate, aluminum sulfate, Chrysal or FloraLife effectively prolonged vase life of cut rose flowers primarily by inhibiting ethylene and preventing bacterial development in the stems, and improving water uptake and hydration of cut flowers (Ahmad et al. 2014; Ichimura et al. 2006; Liao et al. 2001; Seyf et al. 2012; Torre and Fjeld 2001). Despite the effectiveness of the products for improving postharvest longevity of fresh cut flowers, the demand for development of new postharvest treatments that are innocuous, eco-friendly, and inexpensive, has been increased in flower industry.
In this study, to develop effective pretreatment solutions that improve the vase life and postharvest quality of cut rose flowers, we determined vase life, water relations, and physiological characteristics of cut rose flowers after pretreatment of various preservative products. Effectiveness of two natural substances extracted from plants on cut roses was also investigated to develop environmentally friendly preservatives.
Materials and Methods
Plant materials
Cut roses ‘Jinny’ (Rosa hybrida L.) were obtained from a commercial rose grower in Goyang, Korea. After harvest, the cut flowers were placed immediately in buckets containing tap water and transported to the laboratory within 1 h. Flowers were selected for uniformity of size, color and development stage and then, three centimeters of the stem ends were trimmed before pretreatment.
Pretreatment
Silver thiosulfate (STS) was synthesized by adding AgNO3 solution into Na2S2O3 solution. MS-1 and MS-2 are natural antimicrobial compounds were extracted from plants of Chrysanthemum and Ranunculaceae, respectively. Lysosome (LYS) was isolated from egg white and is known to have antibacterial function. Chrysal (CHR), FloraLife (FLR), and silver nitrate (SN) were obtained from commercial companies. Based on previous studies, we selected optimal treatment concentration of the preservatives for cut roses (Table 1). Distilled water (DW) was used as control. Twelve cut flowers per each treatment were placed in a plastic jar containing 700 mL treatment solution and held for 10 h at 25°C and RH 50% in the dark. After pretreatment, the flowers were recut to a length of 50 cm and this stem contained the upper three five- leaflet leaves. Each cut flower was placed through a hole in the center of the cap on a glass jar containing 500 mL distilled and was maintained at 25°C, 50% RH, and a photoperiod of 12 supplied by fluorescent tubes at 30 μmol·m-2·s-1. Among the twelve cut flowers, nine flowers were used for vase life evaluation and the remained three flowers were used for a soluble sucrose content measurement.
Evaluation of vase life
The treatment effects were determined by measuring vase life and senescence of cut flowers. Fresh weight of cut flowers and vase water were weighed, and the water uptake and flower diameter were also measured daily. Flower diameter was determined by measuring the largest diameter of each flower and the diameter perpendicular to it using digital calipers ( CD-2 0APX, Mitutoyo C orporation, J apan). W ater balance was calculated by deducting daily transpiration from daily water uptake. The vase life of cut roses was determined as the time from the placement of cut flower vases in the environmental controlled room after pretreatment to the end of vase life. Cut flowers were considered to have reached the end of vase life when the flower showed at least one of following senescence symptoms: bluing, desiccation, abscission, and wilting of petals and bending of pedicel (bent-neck). Petal discoloration (fading) was also recorded when cut roses started to reach a senescent stage (Macnish et al. 2000). Here, the discoloration of petals was recorded when the petals showed moderate or advanced fading.
Chlorophyll (SPAD value) of the leaves was determined by a non-destructive method using a chlorophyll meter (SPAD-502Plus; Konia Minolta Sensing, Inc., Osaka, Japan). The terminal leaflets of the undermost leaves were used for the SPAD measurements every day. The internal soluble sucrose (brix) content was determined in the uppermost leaves after five days of pretreatment. Tissue samples (0.1 g) of the leaves were placed in an Eppendorf with 0.5 mL distilled water and ground using a TissueLyser (TissueLyser II; Qiagen, Hilden, Germany). The brix (%) was measured with a portable refractometer (PR-104, Atago, Tokyo, Japan).
Experimental design and data
The experiments were designed by a completely randomized block design with nine replicates for each treatment and one flower per replicate. Data were presented as means ± standard error (SE). One-way analyses of variance (ANOVA) were performed using Statgraphics Plus version 3.0 (Statistical Graphics Corp., U.S). When significant effects were detected, post-hoc pairwise comparisons of group means were executed with Duncan’s multiple range tests, with a significance level of P = 0.05.Fig. 1Fig. 2
Results
Vase life, maximum flower diameter, and senescence
The vase life of cut roses ‘Jinny’ was significantly extended by pretreatment with AS, CHR, FLR, MS-1, MS-2, SN, and STS, compared to that of control flowers (Table 2). Flowers treated with STS showed the longest vase life (19.9 d) and followed by those t reated with MS-2 (17.4 d), MS-1 (16.4 d), and SN (16.0 d). In addition, these treatments retarded occurrence of senescence compared to control (Fig. 3). While, the vase life of the cut roses treated with LYS increased only slightly (>1 d) compared to that of control flowers (Table 2). Pretreatment with FLR, MS-1, MS-2, SN, and STS also increased the maximum flower diameters (percentage of initial) compared to control flowers (Table 2).
In this study, the occurrence rate of senescence symptoms significantly varied among treatments (Fig. 1A). Petal abscission was the primary cause of vase life termination in cut rose ‘Jinny’. The percentage of vase life termination due to petal abscission was 65.5%, 50%, 87.5%, and 75% in control, FLR, MS-1, and MS-2 flowers, respectively. On the other hand, the petal abscission was significantly decreased by SN and STS and the change in petal color was also inhibited by these treatments. The primary cause of vase life termination was bluing (75%) in flowers treated with STS, whereas it was desiccation and wilting of petals in those treated with SN (62.5%) and CHR (50%), and LYS (50%) (Fig. 1A). Petal wilting was the major senescence symptoms in control, AS, CHR, FLR, LYS, and MS-2 flowers, but it was completely inhibited by STS, SN, and MS-1 treatments (Fig. 1A). Petal discoloration increased in control (65.5 %) and LYS (50.0 %) flowers, but it did not occur in STS, SN, MS-1, and MS-2 flowers ( Fig. 2 ).
One-way analyses of variance (ANOVA) were conducted for each column. Same letters (a-f) within a column represent groups that did not differ significantly at P < 0.05 based on Duncan’s multiple range test.
Water relation and relative fresh weight
Pretreatment improved water uptake rates during vase life of cut roses (Fig. 4). Among the preservative solutions, STS showed most effectively improved water uptake. Cut flowers treated with STS showed highest water uptake rate (185.6%) at d 8, whereas control flowers showed lowest water uptake (111.7%). The high water uptake of cut flowers by STS may be due to decreased stem plugging by the role of STS as antimicrobial agent.
Water balance was determined by the difference between water uptake and water loss from leaves of cut roses. The number of days that flowers retained a positive water balance was highest in flowers treated with STS (2.6 d). AS, CHR, FLR, MS1, MS2, and SN solutions also showed longer positive water balance than control (Fig. 5A). Similar to water balance, the number of days that flowers retained their initial fresh weight was also highest in flowers treated with STS (6 d), followed by SN (5.3 d), MS2 (4.9 d), and MS1 (4.6 d). Pretreatment with AS, CHR, and FLR showed relatively smaller effect on keeping fresh weight (about 2.5 d), compared to other preservatives (Fig. 5B).
Soluble sucrose and chlorophyll contents
Pretreatment with the preservative solutions significantly increased the internal soluble sucrose content (brix) in the leaves of cut roses (P < 0.05) at d 5 (Fig. 6A). The brix of control flowers was lowest (0.2%), whereas that of STS flowers was highest (0.4%) at d 5. CHR, FLR, MS1, MS2, AS and SN also significantly increased the brix level compared to control. The lowest chlorophyll content was noted in control flowers, while pretreatment with preservative solutions led to a considerable delay in degradation of chlorophyll (Fig. 6B). Flowers treated with FLR showed highest chlorophyll content (47.08 SPAD value). Other preservative solutions also showed higher SPAD values than control flowers (Fig. 6B).
Discussion
The vase life and postharvest quality of cut flowers are determined by multiple factors such as genetic factors, growth conditions, harvest time, packaging, transport, postharvest handling, and storage. In the postharvest storage stage, pretreatment with preservative solutions have various impacts on the vase life and quality of cut flowers (Halevy and Mayak 1981). The results of present study revealed that preservative solutions had significant effects not only on longevity but also on the postharvest quality of cut roses. Our results clearly showed that STS most effectively prolonged the vase life of cut rose flowers among the preservative solutions. This may be due to the inhibition of ethylene action by STS as is has been known to suppress ethylene binding to the ethylene receptors, and consequently resulting in retardation of flower senescence (Ichimura and Goto 2002; Mor et al. 1989).
The contamination of vase solution is one of major problems for cut rose flowers. An increase in microbial populations in the vase causes early wilting of flowers due to decreased water absorption caused by xylem plugging (Macnish et al. 2008). It is well known that bacterial proliferation in the vase solution shortens the vase life of cut flowers (Liao et al. 2001). In the present study, STS, SN, and MS1 were effective on inhibition of bacterial growth after 5 and 8 days of pretreatment. Since silver ion has antimicrobial properties, it reduced the degree of vascular blockage, leading to an optimum solution uptake and extending the vase life of cut roses (Liao et al. 2001; Liu et al. 2009). The results from our study also showed that MS-1 may be a promising germicide for the preservation of fresh cut roses. Pretreatment with STS increased water uptake, resulting in a relatively longer positive water balance compared to other preservative solutions. Water balance is considered as the determining factor which is changed by water uptake and transpiration rate. In this study, the initial fresh weight of cut roses was retained longest by STS, consistent with the previous observation that vase life of cut flowers was extended due to an increased in water uptake and a decreased in weight loss, by STS treatment (Al-Humaid 2004; Liao et al. 2000; Reid et al. 1989)
It is interesting to note that cut flowers with longer vase life had relatively higher brix levels in leaves of cut roses, indicating the internal sucrose may be important for providing the energy required for the sustenance of vase life. The soluble sucrose content is necessary for providing the energy sources to sustain turgor pressure in petals and facilitate flower opening (Ichimura et al. 2005; Paulin 1986). Petal discoloration is also an important factor in determining display life of cut f lowers (Macnish et al. 2000). In the current work, STS, SN, MS1, and MS2 retarded the petal discoloration rate in cut roses ‘Jinny’ and the flowers in these treatments had relatively higher sucrose content. Similarly, flower fading of cut roses was decreased by vase solution, which contained STS and sucrose (Hayat et al. 2012). These results imply that petal discoloration may be closely related to the sucrose levels of cut flowers. Water stress is strongly correlated with an imbalance in the water uptake rate and transpiration rate. This imbalance of water uptake and loss causes decrease of cell turgidity and consequent fading of cut flowers. STS solution provides an anti-bacterial activity which prevents any possible pathogenic plugging of vascular tissues and enhances water uptake of cut stems, resulting in extension of the vase life of cut flowers.
Petal abscission is associated with the leaf abscission, which is caused by ethylene production in cut flowers. The occurrence of petal abscission seems to be related to total chlorophyll and soluble sucrose contents of cut rose flowers. In our study, pretreatment with STS and SN effectively inhibited petal abscission occurred during the vase life. This result are consistent with previous study that showed that STS retarded the reduction of chlorophyll content, preserved carbohydrate contents, and inhibited ethylene production in cut roses, and thereby suppressed abscissions of petals and leaves which are induced by ethylene. These observations are consistent with the previous study on cut roses that showed that occurrence of petal abscission was remarkably reduced by STS (Hayat et al. 2012). Overall, the pretreatment with STS most effectively extended the vase life and sustained initial fresh weight longer in cut roses. The efficacy of STS on postharvest quality of cut roses could be explained by its role in inhibiting bacterial proliferation, sustaining good water uptake, and repressing ethylene action during vase life.
