Effects of 1−MCP on Storage Quality and Enzyme Activity of Petals of Edible Rose Cultivar ‘Dianhong’ at Low Temperatures

Abstract

To explore the effect of 1-methylcyclopropene (1−MCP) on the storage quality of edible roses, flowers of the edible rose variety ‘Dianhong’ were used as materials to study the effect of post-harvest 1−MCP fumigation (concentration of 30 μL/L). The measures included respiration intensity, water loss rate, antioxidant-related substance content (soluble sugar, crude fibre, AsA, anthocyanin, total phenols and MDA), enzyme activity (SOD, CAT, APX, PAL and PPO) and petal appearance quality in different storage periods, which could provide a theoretical reference for research and development on storage and preservation technology for edible rose petals. The results showed that, compared with the control, 1−MCP treatment reduced the initial respiration intensity of petals by more than 80%, slowed the water loss rate, increased the crude fibre content and effectively maintained the contents of soluble sugars, AsA, anthocyanins and total phenols. For the petals in the treatment group at the end of storage, the rate of water loss was 3.73%, the content of soluble sugar was only 17% (62.27 μg/g)—lower than that of fresh petals—and the content of AsA (0.33 mg/g) was the same as that of pre-storage (0.34 mg/g). The contents of total phenols and anthocyanins were 30.60% and 11.63% higher than those of the control group, respectively. In addition, 1−MCP treatment increased the activity of SOD, CAT, APX and PAL and inhibited the activity of PPO. The MDA content at the end of storage was 14.36% lower than that of the control, which reduced the rate of membrane lipid peroxidation. Correlation analysis showed that sensory quality of petals in the 1−MCP treatment group was positively correlated with respiratory intensity and soluble sugar content and negatively correlated with water loss rate, MDA and crude fibre content (p < 0.05) among the four antioxidant enzymes. APX and PAL were positively correlated with anthocyanin content and total phenols content, respectively. These results confirmed that 1−MCP could effectively maintain the storage quality of edible rose petals by increasing the antioxidant capacity of petals and prolong the storage period of fresh petals to 49 days.

1. Introduction

Edible roses are a common kind of edible flower. In terms of plant classification, they belong to the perennial woody plants of the genus Rosa in Rosaceae [1]. Their petals are fragrant and rich in various amino acids, alkaloids, vitamins, trace elements and other nutrients. They have the effect of beautifying and regulating the internal secretions of the human body. They are widely used in production of various foods, cosmetics and health care drugs [2,3]. The edible parts of edible roses are petals. In addition to dry processing of the whole flower for tea and extraction of essential oil, the calyx and stamens of roses often must be removed for processing in other edible methods due to their rough texture and bitter taste [4]. Fresh petals, when separated from edible flowers, are still viable and capable of respiration and transpiration. Thus, they are subsequently prone to discolouration, wilting, dehydration and tissue browning, resulting in a rapid decline in sensory quality and nutritional value [5]. Simultaneously, the high water content of fresh petals facilitates microorganism invasion and decay, so edible flowers typically must be processed or eaten immediately after harvesting, and their extremely short shelf life limits the commercial use of edible flowers [6,7].

Low-temperature storage is the most widely used and easiest method of maintaining fresh agricultural products after harvest. As an environmental factor, a low temperature can control the shelf life of edible flowers within the temperature range of 4–6 °C [8], and the storage time of flowers after cold storage can be extended by 2–5 days [9]. The low-temperature storage effect differs for different kinds of edible flowers [10]. Compound 1-methylcyclopropene (1−MCP) is a safe and nontoxic ethylene inhibitor, which can be used as a gas preservative to protect fresh fruits and vegetables from the influence of endogenous ethylene and improve storage quality. It has become a type of preservative that is broadly used for storage of fresh agricultural products [11]. The respiration intensity and ethylene release of fresh fruits and vegetables treated with 1−MCP after harvest are inhibited, the softening speed of the outer cell wall is delayed and the decay and browning rates are greatly reduced [12]. An inhibitory effect of 1−MCP has been observed against various compounds, including enzymatic antioxidant system components (superoxide dismutase (SOD), catalase (CAT) and ascorbic acid peroxidase (APX), etc.) and nonenzymatic antioxidant system components (anthocyanins, polyphenols, flavonoids, ascorbic acid (AsA), etc.) of fruits and vegetables [13,14]. Treatment with 1−MCP for 6 h after storage can not only maintain the content of ascorbate and glutathione in leaves but also increase the activities of antioxidant enzymes, such as SOD, APX and glutathione reductase (GR), thus delaying the senescence process of Gynura bicolor after harvest [15]. Ginger rhizomes treated with 1−MCP show a reduction in reducing sugar content and an increase in starch and total phenols contents while maintaining high ascorbic acid levels, inhibiting polyphenol oxidase (PPO) activity and elevating antioxidant enzyme activity compared with the control. This method reduces the germination rate of ginger during storage and maintains a superior quality [16]. In terms of flower preservation, 1−MCP has been used to develop agents to maintain freshness of cut flower plants. However, due to its gaseous morphology, 1−MCP has not been widely used in this capacity, although it has shown good effects in maintaining the freshness of edible flowers. After 1−MCP treatment of edible carnation and snapdragons, the carnations continued to show good visual sensory performance after 14 days of storage at 5 °C, while the snapdragons exhibited brown flowers within 7 days of storage. The shelf life of the two kinds of flowers was significantly prolonged compared with that of the control [9].

After harvest, edible roses have a short shelf life. When used as raw food materials, they are usually processed into jam and then included in various foods that utilize jam as an additive [17]. Few studies have examined preservation methods for fresh rose petals, especially the effect of 1−MCP on the postharvest storage quality of edible rose petals. ‘Dianhong’ is an edible rose variety, with the largest planting area in Yunnan Province, China. The flower cake directly processed from its fresh petals is a well-known Chinese specialty food [18]. The purpose of this study was to analyse the effects of 1−MCP on enzymatic and nonenzymatic antioxidants in the petals of edible ‘Dianhong’ rose during different postharvest storage periods and to study the relationship between the storage quality of the petals, the visual quality of the appearance and the important compounds in the petals to provide a basis for long-term preservation of fresh rose petals.

2. Materials and Methods

2.1. Plant Materials

Specimens of edible rose ‘Dianhong’ (R. gallica L. cv. Dianhong) were collected from the edible rose plantation in Bajie, Anning City, Yunnan Province at an altitude of 1940.3 m. Collection was performed from 9:00 am to 10:00 am on 6 May 2021. Flowers selected as the test materials had the same openness, a uniform size, no mechanical damage and no pests or diseases after removing the receptacles and stamens according to commercial requirements.

2.2. Experimental Design

After precooling at 8 °C for 2 h, the selected petals were divided into two parts: the 1−MCP treatment group and the control group. The treatment group was fumigated with 30 μL/L 1−MCP (prepared with 1−MCP powder and distilled water) in a foam box at 8 °C for 12 h, while the control group was fumigated with the same volume of distilled water at the same temperature. After fumigation, 100 g of each bag was divided into PE bags (0.08 mm thick) and stored at 2 °C. Samples were obtained at 0, 7, 14, 21, 28, 35, 42 and 49 days to determine the indices of interest. Three bags of samples from the treatment and control groups were simultaneously randomly selected for each sampling, and three replicates were established. The samples were quickly crushed and mixed in a centrifuge tube after being frozen with liquid nitrogen, then labelled and stored in a −80 °C refrigerator for use.

The temperature of the petals and the concentration of 1−MCP fumigation were determined according to the results of previous experiments.

2.3. Sensory Evaluation

The sensory evaluation method was performed according to the quality evaluation method used in broccoli [19]. Ten people were selected to establish a scoring group. According to the sensory characteristics of rose petals, the four aspects of colour, tissue state, surface morphology and odour were set as sequential scoring items, each with a total score of 25. The evaluation score employed for each sample comprised the sum of the scores of the four sensory properties. The sensory evaluation standards are shown in

Table 1. Evaluation standard for sensory quality of petals of rose ‘Dianhong’.

Scores	Colour	Tissue State	Surface Morphology	Odour
20–25	bright rose red	The petals are fresh and full.	The petals have a smooth surface.	very strong aroma
15–20	deep rose	The petals are full.	No more than 10% of the petals lose water slightly.	strong aroma
10–15	light rose red	The petals are soft and collapsed.	10–30% of the petals were slightly dehydrated and veins were clear.	light aroma, no peculiar smell
5–10	lighter rose red	The petals are soft, thin and wilting.	30–50% of the petals have obvious water loss, and the veins were clearer.	light aroma, slight peculiar smell
0–5	pink	The petals are thin, wilting and brown	More than 50% of the petals lose water, and the veins bulged	no aroma, obvious peculiar smell

Note: the scoring criteria refer to broccoli [19].

.

2.4. Respiratory Intensity

Respiratory intensity was measured using a portable carbon dioxide analyser (LI-840, LI-COR Co., Ltd., CA, USA). The CO₂ content was determined at the beginning and end using a CO₂ metre, and the respiration intensity of the petals was calculated according to the following formula [20]: $\mathrm{X}=\frac{\left({\mathrm{W}}_1-{\mathrm{W}}_2\right)\times \mathrm{V}\times \mathrm{M}}{{\mathrm{V}}_0\times \mathrm{m}\times \mathrm{t}}\times 100$.

In the formula, X is the respiratory intensity (mg/kg·h), W₁ is the CO₂ content (%) in the closed container before the test, W₂ is the CO₂ content in the closed container after the test (%), V is the total volume of the closed container (L), M is the molar mass of CO₂ (g/mol), V₀ is the molar volume of CO₂ at the test temperature (L/mol), m is the sample mass (g) and t is the test time (h).

2.5. Determination of Water Loss

The weight of samples was measured by analytical balance, and the water loss rate was calculated by weighing method. (AUW120D, Shimadzu, Kyoto, Japan). The initial mass of petals treated at 0 days was recorded as m₀/g, and the petal mass was measured every 7 days during storage and recorded as m_x/g. The mass loss rate of each group of petals was calculated according to the following formula.

2.6. Enzyme Activity Assays

The enzyme activity was measured using a spectrophotometer (UV-2550, Kyoto, Shimadzu, Japan). Determination of SOD and CAT were performed by the method used for red-fleshed kiwifruit with slight modifications [14].

The SOD activity was determined using the nitroblue tetrazolium (NBT) method. The amount of enzyme required by the reaction system to inhibit 50% of NBT photoreduction was defined as one unit of enzyme activity (U) per minute, and the measurement wavelength was 560 nm. The reaction system (5 mL) contained 50 mM sodium phosphate buffer (pH 7.8), 13 mM methionine, 75 µM nitroblue tetrazolium (NBT), 10 µM EDTA, 2 µM riboflavin and 0.1 mL sample extract. CAT activity was measured by the hydrogen peroxide method at 240 nm, and an absorbance reduction of 0.01 per minute per gram of sample was considered 1 CAT activity unit (U). The reaction mixture consisted of 50 mM sodium phosphate buffer (pH 7.0), 40 mM H₂O₂ and 0.5 mL sample extraction solution.

APX activity was determined by the AsA colorimetric method at 290 nm, PPO activity was detected at 420 nm by catechol colorimetry and phenylalanine ammonia lyase (PAL) activity was measured at 270 nm by trans-cinnamic acid colorimetry, in accordance with methods described by Fukuoka et al. [21].

The reaction system for determination of APX included 0.2 mL extract obtained from 0.1 g of petal sample by phosphoric acid buffer, and 50 mM phosphate buffer (pH 7.2), 50 mM ascorbic acid (sodium salt), 10 mM EDTA, 10 mM H₂O₂ solution. The reaction mixture for PAL activity consisted of 1.0 mL of petal extract and of 50 mM Tris-HCl buffer (pH 8.3) containing L-phenylalanine (20 mM). PPO activity was measured using a 50 mM HEPES buffer (pH 7.5) and 1% (w/v) catechol.

2.7. Determination of AsA, Total Phenols and Anthocyanin Contents

The AsA content was measured as previously described using a spectrophotometer (UV-2550, Shimadzu, Kyoto, Japan) [22]. The trichloroacetic acid solution and the sample were homogenized and the supernatant was extracted to determine the AsA content. The total phenols content was determined using the Folin–Ciocalteu assay. After frozen petals were homogenized, phenolic compounds were extracted with ethanol:acetone (7:3, v/v) solution, and the obtained supernatant was then reacted with Folin–Ciocalteu to determine the total phenols content at 760 nm [23]. Total anthocyanins were measured using the pH difference method [24].

2.8. Analysis of Soluble Sugar, Crude Fibre and MDA Contents

The soluble sugar content was determined by the phenol-sulfuric acid method (UV-2550 spectrophotometer Shimadzu, Kyoto, Japan). The petal extract was added to a 9.0% phenol solution with the successive addition of concentrated H₂SO₄, and the absorbance value was measured at 485 nm after a sufficient reaction was achieved. Then, the soluble sugar content was calculated [25]. To determine the crude fibre content, the samples were ground and boiled with 1.25% H₂SO₄ and 12.5 g/L NaOH solution for 30 min, and the residue was washed and dried at 105 ℃. The malondialdehyde (MDA) content was measured using thiobarbituric acid colorimetry, and the absorbance of the reaction solution was measured at wavelengths of 450 nm, 532 nm and 600 nm [26].

2.9. Statistical Analysis

The data were processed using Excel 2021. The Duncan test was used to test the significance of differences among the different treatments (p < 0.05 level). One-way ANOVA, correlation analysis and PCA were performed and plotted with Origin Pro 2021. The results are expressed as the mean ± standard deviation (SD). Experimental data from each group were obtained in triplicate.

3. Results

3.1. Evaluation of Sensory Quality

The sensory evaluation scores could intuitively reflect the influence of 1−MCP treatment on the quality of edible rose. As shown in Figure 1, the sensory scores of the 1−MCP treatment and control groups showed an overall decreasing trend with prolonged storage time. There was no difference in the sensory quality of petals between the treatment and control groups over 7 days of storage. After 7 days of storage, the quality differences between the two groups of petals became increasingly high. The 1−MCP treatment significantly delayed the decline in rosette quality (p < 0.05). Compared with the control group, the comprehensive scores for the treatment group were still greater than 60 points after 49 days of storage, and the sensory quality continued to be superior. However, the control group scored only 19 points in the same period, thus largely losing its commodity value. Consequently, treatment with 1−MCP was able to prolong the storage time of edible rose petals and maintain a better appearance quality (Figure 2).

3.2. Effects of 1−MCP on Physiological Metabolism of Rose Petals

3.2.1. Respiration Intensity and Water Loss Rate

In the cold environment, the respiration intensity of rose petals showed a continuously decreasing trend in both the treatment and control groups, with the fastest decrease in respiration intensity observed within 7 days of storage (Figure 3a). After 14 days, the respiration intensity of the petals in both groups slowed, and, on the 49th day, the respiration intensity in the treatment group was only 42.90 mg CO₂/(kg·h), which was 30.21 mg CO₂/(kg·h) lower than that in the control group. Throughout the storage process, the respiration rate of petals treated with 1−MCP was significantly lower than that of control petals (p < 0.05), indicating that the respiration rate of rose petals was significantly inhibited by 1−MCP treatment.

The water content of petals could directly affect the appearance quality of petals, and the percentage of water loss gradually increased with a prolonged storage time (Figure 3b). The water loss rate increased slowly in the 1−MCP treatment, and the water loss rate was only 3.73% after 49 days of storage, while that in the control group reached 6.1%. These results indicate that 1−MCP could effectively reduce water loss in petals, which is consistent with the results of the sensory quality evaluation of rose petals.

3.2.2. Soluble Sugar and Crude Fibre Contents

Soluble sugar content is an important indicator of the edible quality and processing performance of agricultural products, which affects taste factors, such as the sweetness and flavour of products, and is related to antioxidant capacity in the ageing process of plants [27]. The soluble sugar content of rose petals in the treated and control groups exhibited a decreasing trend throughout the whole process (Figure 4a), and a longer storage time coincided with greater decreases in soluble sugar content, potentially due to respiratory metabolism of petals. During storage, soluble sugars are continuously decomposed and consumed to maintain various physiological activities of petals. Although the soluble sugar content of the petals in the 1−MCP treatment group decreased slightly in the early stage of storage, it was only 62.27 µg/g lower than that of fresh petals after 49 days of storage and 94.62 µg/g lower than that in the control group in the same period. There was a significant difference in the soluble sugar content between the two groups of petals (p < 0.05), which indicated that 1−MCP treatment could effectively delay decomposition of soluble sugar in rose petals. The tissue of fruit and vegetable products is delicate, and fibrosis and etiolation easily appear after harvest, which leads to a decline in nutritional and edible qualities. Therefore, crude fibre content can be used as the main index to evaluate freshness [28]. Within 49 days of storage, the percentage of crude fibre content in both the control and treatment groups showed an increasing trend (Figure 4b), but the percentage of crude fibre in the control group increased rapidly and was consistently higher than that in the 1−MCP treatment group. Before storage, the percentage of crude fibre content in rose petals was 2.66%. After 49 days of storage, the percentage of crude fibre content reached 2.93% in the control group, while that in the treatment group was 2.83%. Compared with that before storage, the percentage of crude fibre content in the control group increased by more than 10%, and the difference between the two groups was significant (p < 0.05), which indicated that 1−MCP could effectively inhibit the increase in the crude fibre mass fraction in postharvest edible rose and reduce the rate of fibrosis.

3.2.3. AsA Content

AsA is ubiquitous in various plant tissues. As a free radical scavenger, it directly reacts with reactive oxygen species, such as singlet oxygen, superoxide radicals, hydrogen peroxide and hydroxyl radicals, and plays an important role in plant resistance to oxidative stress [29]. As the storage period was prolonged, the AsA content of rose petals in the treated and control groups showed an initial increase followed by a decrease (Figure 5). Within 14 days of storage, the AsA content rapidly increased and reached a peak. The AsA content in the treatment group was 0.77 mg/g, which was 57.1% higher than that in the control group. Moreover, at different storage times, the AsA content in rose petals treated with 1−MCP was consistently higher than that in the control group, indicating that 1−MCP treatment had a significant effect on accumulation of AsA in rose petals.

3.2.4. Anthocyanin and Total Phenols Contents

Both anthocyanins and polyphenols are natural compounds with antioxidant activity in cells, and their content was confirmed to be significantly positively correlated with antioxidant capacity of plant tissues [5]. As shown in Figure 6a, there was a significant difference in the content of anthocyanins between the control and treatment groups, and the content of anthocyanins in the two groups first increased and then decreased over the initial 28 days. After 28 days, the anthocyanin content in the 1−MCP treatment group slowly began to decline, while that in the control group declined rapidly. When stored for 49 days, the anthocyanin content in the treatment group was still 8.54 mg/100 g, which was only 10.48% lower than that before storage, while that in the control group was 7.65 mg/100 g, which was 19.81% lower than that before storage. These findings indicate that 1−MCP treatment had a significant effect on stability of anthocyanin content under the same storage conditions. Figure 6b shows that the total phenols content in the treatment group was significantly different from that in the control group throughout the whole storage period (p < 0.05), and the contents of the two substances first increased and then decreased. The total phenols content in the 1−MCP treatment group reached a peak value of 22.75 mg/g at 14 days. After 14 days of storage, the total phenols content rapidly decreased, but, after 49 days, the total phenols content in the treatment group was 18.35 mg/g, which was still significantly higher than that in the control group. These results indicated that 1−MCP was beneficial for promoting the increase in total phenols content and enhancing the antioxidant capacity of rose petals during storage.

3.2.5. MDA Content

MDA is an important index, used to measure the degree of lipid peroxidation in cell membranes, and its content can indirectly reflect the antioxidant capacity of plant tissues. An increase in MDA content indicates accelerated destruction of cellular structures [26]. The MDA of rose petals gradually increased during storage (Figure 7). The MDA content in the control group consistently increased. On the 49th day, it rose from an initial 64.47 nmol/g to 158.65 nmol/g, while the increase in the 1−MCP treatment group was relatively gradual and remained significantly lower than that in the control group at the end of storage (p < 0.05), at only 86% of the control. These results showed that 1−MCP could significantly inhibit accumulation of MDA in rose petals, reduce damage to petal cell membranes, slow the process of membrane lipid peroxidation, maintain the integrity of cell membranes to a certain extent and improve the storage quality of petals.

3.3. Effects of 1−MCP on Enzyme Activities in Rose Petals during Different Storage Periods

The SOD activity in the 1−MCP treatment and control groups showed the same trend of change, with a fluctuating change state of first falling and then rising and then falling (Figure 8a). During the first 14 days of storage, the activity of SOD continuously decreased and rapidly increased from 14 to 28 days, until it peaked and then decreased again. During storage, the activities of CAT and APX increased first and then decreased. On the 7th day, the activities of these two enzymes reached the maximum in the 1−MCP treated group. (Figure 8b,c). After 7 days of storage, the CAT and APX activities began to slowly decline in the treatment group, as compared with a rapid decline in the control group. At 49 days of storage, the CAT activity in the treatment group was 17.15 U/mg, which was still three times that in the control group, while the APX activity in the treatment group was 2.26 U/g, which was almost four times that in the control group. PAL is a key enzyme in the metabolic pathway of phenylpropane, and its metabolites in plants include polyphenols, flavonoids, isoflavonoids, anthocyanins and other substances [30]. As shown in Figure 8d, the PAL activity in both the 1−MCP treatment and control groups first increased and then decreased, but the PAL activity in the control group reached a peak value of 237.93 U/g after 14 days of storage, in contrast to 173.51 U/g in the treatment group, indicating that 1−MCP could enhance the antioxidant enzyme activity of rose petals.

PPO can oxidize phenolic compounds into brown quinones, resulting in tissue softening and browning [26]. The PPO activity in the treatment and control groups showed an increasing trend in the first 28 days (Figure 8e). The PPO activity in the treatment group began to decline after 28 days, while that in the control group remained at a relatively high level and gradually decreased after 28 days of storage. At 49 days, the PPO activity in the 1−MCP treatment group was 28.68 U/g, which was significantly lower than that in the control group (46.54 U/g) (p < 0.05), indicating that 1−MCP could effectively inhibit the increase in PPO activity in rose petals and delay the senescence of petal tissue.

3.4. Correlation Analysis

The results showed that sensory quality of petals was significantly positively correlated with respiratory intensity and soluble sugar content and negatively correlated with water loss rate, MDA and crude fibre content (p < 0.05) (

Table 2. The correlation between different physiological parameters and sensory quality of ‘Dianhong’ rose petals treated with 1−MCP.

	Sensory Quality	Respiration Strength	Water Loss	SOD	CAT	APX	PAL	PPO	Anthocyanins	Total Phenolic	AsA	MDA	Soluble Sugar	Crude Fibre
Sensory quality	1.000
Respiration strength	0.785 *	1.000
Water loss	−0.980 *	−0.846 *	1.000
SOD	0.616	0.806 *	−0.618	1.000
CAT	0.376	−0.172	−0.287	−0.059	1.000
APX	0.204	−0.429	−0.078	−0.251	0.878 *	1.000
PAL	0.316	−0.326	−0.174	−0.249	0.759 *	0.942 *	1.000
PPO	−0.409	−0.682	0.541	−0.303	0.174	0.522	0.545	1.000
Anthocyanins	0.697	0.146	−0.635	−0.042	0.660	0.714 *	0.820 *	0.056	1.000
Total phenolic	0.686	0.100	−0.572	0.096	0.739 *	0.816 *	0.904 *	0.241	0.941 *	1.000
AsA	0.386	−0.148	−0.307	−0.300	0.419	0.687	0.845 *	0.357	0.886 *	0.830 *	1.000
MDA	−0.986 *	−0.727 *	0.942 *	−0.560	−0.401	−0.259	−0.404	0.302	−0.736 *	−0.742 *	−0.455	1.000
Soluble sugar	0.956 *	0.846 *	−0.990 *	0.612	0.301	0.053	0.132	−0.556	0.604	0.526	0.252	−0.915 *	1.000
Crude fibre	−0.987 *	−0.847 *	0.971 *	−0.726 *	−0.285	−0.106	−0.214	0.424	−0.590	−0.604	−0.277	0.962 *	−0.946 *	1.000

Note: * indicates significant correlation (p < 0.05).

). In addition, sensory quality was positively correlated with antioxidant enzymes SOD, CAT, APX and PAL, ascorbic acid, anthocyanin and total phenols content and negatively correlated with PPO, but this correlation was not significant. Among the four antioxidant enzymes, APX and PAL were positively correlated with anthocyanin and total phenols content, respectively (Figure 9).

3.5. Evaluation of Storage Quality in Rose Petals

The results of principal component analysis showed that the cumulative contribution rate of the first two components reaches 74.15%, which could represent most of the information in the original data (Figure 10). The contribution rate of the first principal component (PC1) was 63.55%, of which the sensory score load was 0.320, in which a CAT load of 0.313, APX load of 0.322, total phenols load of 0.314, MDA load of −0.311 and crude fibre load of −0.318 accounted for the highest proportion. The contribution rate of the second principal component (PC2) was 10.60%, of which the water loss rate load of 0.373, PAL load of 0.309, PPO load of 0.602, AsA load of 0.257 and soluble sugar load of −0.405 accounted for the highest proportion. In conclusion, the sensory score, water loss rate, CAT activity, APX activity, PAL activity, total phenols content, AsA content and PPO activity were extracted as the core indices for evaluating the quality of rose petals in this experiment.

4. Discussion

Petals are a component of flower organs. As they are nonphotosynthetic organs, the main cause of senescence and withering may be a result of electron transfer in the mitochondrial respiratory chain in cells [31]. Removing pistils in flowers can inhibit early withering of petals [32]. During the process of gradual browning and deterioration of petals, cells undergo a series of structural, physiological and biochemical changes. Sensory changes include lightening of flower colour and tissue collapse. Physiological factors include enhancement of or decrease in related enzyme activities and change in nonenzymatic antioxidants and MDA content [33,34].

Fresh rose petals have a high water content and could quickly rot after picking. At room temperature, petals will lose water and become brown after being placed on cement for only 2 days, and they have a short storage time [35]. In this study, the visual sensory quality of petals in the 1−MCP treatment and control groups at a low temperature showed great differences with an extension of the storage period. Fading and wilting are two typical indicators used to determine the senescence degree of rose petals [34]. The petals in the control group wilted after 28 days of storage, and the sensory quality began to significantly decline. However, the petals treated with 1−MCP still exhibited good sensory qualities after 49 days of storage, indicating that 1−MCP treatment played a positive role in maintaining the appearance quality of fresh petals. Cherry tomatoes treated with 1−MCP before storage showed enhanced fruit quality and storage, with cherry fruit freshness maintained for up to 15 days. This result further demonstrates that 1−MCP truly improves the appearance of fresh horticultural products and extends their storage period [36].

Perishability of horticultural commodities is generally proportional to their respiration rate. The stronger the respiration, the faster the decay. The process of respiration is accompanied by consumption of sugars, which leads to a reduction in the moisture content of commodities [23,37]. It has been demonstrated that moisture content of flowers has the greatest impact on the shelf life of seven edible flowers, such as Salvia discolor. Preventing moisture loss in flowers can prolong their shelf life [23]. In this study, with a prolonged storage time, the respiration intensity and soluble sugar content of petals decreased overall, and the water loss rate increased. The performance of these three indicators was significantly different between the 1−MCP treatment and control groups. After 49 days of storage, the respiratory intensity of petals in the 1−MCP treatment group decreased to 58.7% of the control group, while the soluble sugar content in the treatment group was 301.77 µg/g, which was still 32.35 µg/g higher than that in the control group. This result indicated that 1−MCP reduced the consumption of soluble sugar while minimizing the respiratory intensity of petals.

Previous studies have suggested that, with a sufficiently high sugar level in petal cells, senescence can be delayed; additionally, sugar can regulate osmotic pressure of petal cells, which plays an important role in stability of the petal shape [38,39]. Preserving apples with 1−MCP can also effectively maintain the soluble sugar content in fruit, limit hydrolysis activity of the cell wall and subsequently maintain fruit firmness [40]. Similar to the results of this study, 1−MCP treatment was shown to slow down decomposition of soluble sugar in plant cells and improve storage quality. A large amount of cellulose in plant cells is synthesized to form fibre bundles that are easily deposited in the vascular bundle network, which leads to rough plant tissue. The 1−MCP treatment could inhibit conversion of reducing sugars to cellulose in common beans, reduce synthesis and polymerization of lignin monomers and ultimately inhibit lignofibrosis of fresh common beans [28]. In this study, the crude fibre content of petals in the 1−MCP treatment group was also significantly lower than that in the control group over various storage periods, indicating that 1−MCP could reduce water loss in petals by inhibiting respiratory intensity and slowing down the increase in crude fibre content while retaining water, which was consistent with the results obtained with the 1−MCP treatment of common beans.

Total phenols, anthocyanins and AsA are nonenzymatic components in plants that not only have antioxidant functions but are also characteristic nutrients in rose petals [41]. In this study, the anthocyanin content of petals treated with 1−MCP was only 10.48% lower than that before storage for 49 days, while that in the control group was 19.81% lower than that before storage, indicating that 1−MCP had a positive effect on maintaining the stability of anthocyanin content.

Although petal colour is an independent quality indicator, the content of anthocyanins is controlled by many factors. The water content of petals can affect the stability of anthocyanins, and a moderate water shortage can enhance the synthesis of anthocyanins. In addition, metabolism of edible rose anthocyanins is regulated by AsA and PPO. During enzymatic browning, AsA can inhibit PPO activity. PPO generates quinones by oxidizing phenolic compounds in edible roses, and further oxidation of anthocyanins by quinones will cause petals to fade [42,43]. In that study, the contents of total phenols, anthocyanins and AsA in the 1−MCP treatment group were significantly higher than those in the control group, indicating that 1−MCP could improve the antioxidant capacity of rose petals. Kiwifruit treated with 1−MCP could also increase the total phenols content, accelerate accumulation of AsA and maintain high nutritional quality and antioxidant capacity [13], which is consistent with the results of the present study.

In the enzyme system, SOD is the first antioxidant enzyme to play a role in the scavenging reaction of reactive oxygen species, while other antioxidant enzymes, such as CAT and APX, subsequently convert hydrogen peroxide into water and molecular oxygen to finally protect cells from oxidative damage [44]. In this study, SOD activity in the 1−MCP treatment group and control group showed a downward trend in the early stage of storage, and SOD activity in the middle and late stages of storage first increased and then decreased, while the CAT, APX and PAL enzyme activities first increased and then decreased throughout the storage period. The petals in the 1−MCP treatment group showed significantly higher SOD, CAT, APX and PAL activities compared with the control group, while the activities of PPO and MDA were significantly higher in the control group than in the 1−MCP treatment group, indicating that 1−MCP improved the activity of antioxidant enzymes in rose petals and inhibited the activity of oxidase. As a key enzyme in the phenylpropane secondary metabolism pathway, PAL can improve the activities of CAT, APX and other antioxidant enzymes and promote synthesis and accumulation of phenylpropane secondary metabolites, such as polyphenols and anthocyanins [30]. The PAL activity in the 1−MCP treatment group was 237.93 U/g, and the contents of anthocyanins and total phenols also increased to the maximum at 14 days, which also confirmed that PAL had a significant promotional effect on anthocyanin synthesis.

5. Conclusions

Fresh petals of edible roses are light in texture and carry a great deal of heat in the field after harvest. Browning occurs quickly under normal-temperature storage, so petals need to be pre-cooled and refrigerated. Treatment with 1−MCP could decrease the respiratory intensity of petals, slow down the rate of weight loss and increase the content of crude fibre, effectively maintaining the content of soluble sugar, AsA, anthocyanin and total phenols, increasing the activities of SOD, CAT, APX and PAL and inhibiting the increase in PPO activity and MDA content. Further, 1−MCP had a good preservation effect on edible rose petals by improving the antioxidant capacity of petals and could extend the preservation time of petals to 49 days under the condition of low-temperature storage. Edible roses have a short florescence period, and preservation of petals can not only meet the consumer demand for fresh petals but also prolong the supply period of raw materials for Yunnan flower cake. The selling price of rose flower cakes made of fresh flower petal is several times higher than those made by rose paste, so the preservation technology of edible rose petal will certainly promote an increase in the output value of flower cake processing. Therefore, this study on preservation effects based on 1−MCP could provide a theoretical basis for subsequent development of preservation technology for edible roses.