Melatonin Treatment Delays the Senescence of Cut Flowers of “Diguan” Tree Peony by Affecting Water Balance and Physiological Properties

Abstract

Tree peony (Paeonia suffruticosa Andr.), which is a traditional flower cultivated in China, is rapidly becoming an important species in the cut flower industry. Thus, extending the vase life of tree peony cut flowers is a major goal in the cut flower industry. Melatonin, which is a new type of antioxidant, plays an important regulatory role in the preservation of cut flowers. Therefore, this study employed the cut flower of tree peony “Diguan” as the test material to investigate the preservative effects of the antioxidant melatonin on the cut flower of tree peony “Diguan”. We examined tree peony cut flowers in terms of their morphology, lifespan, relative fresh weight, relative diameter, and water balance value after treatments with different melatonin concentrations (0.2, 0.3, 0.4, and 0.5 mg·L⁻¹) to select the optimal treatment concentration. Considered together, these analyses clarified the effects of melatonin on the preservation of “Diguan” tree peony cut flowers. Specifically, the exogenous application of melatonin positively affected the preservation of tree peony cut flowers by improving the water balance value and increasing the soluble protein content and antioxidant enzyme activities, thereby prolonging the ornamental period of tree peony cut flowers. The fresh weight of flower branches is significantly positively correlated with soluble protein, and cut flower lifespan increases with the values of soluble protein and the fresh weight of flower branches, with a large correlation coefficient. It can be used as an important indicator to measure cut flower lifespan in subsequent research. The 0.4 mg L⁻¹ melatonin treatment was optimal for preserving tree peony cut flowers because of its positive effects on the duration of the ornamental period and ornamental quality.

1. Introduction

Tree peony (Paeonia suffruticosa Andr.), which is a perennial deciduous shrub in the family Paeoniaceae, is a traditional flower known as the “king of flowers” in China, with high ornamental and medicinal value [1,2,3]. The plant exhibits a rich diversity, with large, vibrant blooms that are both elegant and regal. It is widely celebrated and regarded as the national flower of our country. They are widely regarded as the national flower of China. The thriving development of the domestic and international cut flower markets has resulted in the gradual increase in the tree peony cut flower market [4]. Thus, tree peony has become one of the high-end floral materials in the domestic and international cut flower markets, with substantial demand and economic potential [5]. However, as a woody perennial flower species, it has a short natural flowering period (i.e., seasonality). Furthermore, the rapid deterioration of cut flowers and an imperfect preservation system contribute to the short vase life of the tree peony. These issues have significantly hindered the development of the tree peony cut flower industry [3,6,7].

There have recently been numerous studies on tree peony cut flowers, with a particular focus on the evaluation and selection of cut flower varieties, development of efficient cultivation and production techniques, screening of preservatives, and analysis of energy metabolism and stress physiology during the bottle insertion period [3,8,9,10,11]. Screening and optimizing preservatives can effectively reduce the number of bacteria at the stem end of cut flowers and in the vase solution, decrease the extent of vessel blockage, and maintain the water balance value, thereby extending the flowering period [12,13]. Accordingly, there is considerable scientific value to conducting research aimed at optimizing the methods and technology used to preserve tree peony cut flowers. Recent research on preservatives for cut flower vase solutions has mainly examined antioxidants, fungicides, and respiratory inhibitors [9,11,14]. Many studies have shown that treating cut flowers with antioxidants can effectively prolong the full flowering period. In addition, nanochitosan-encapsulated melatonin is also an eco-friendly strategy to delay petal senescence in cut gerbera flowers [15]. Another antioxidant, hydroquinone, was observed to significantly influence the preservation of cut rose flowers [16], while glutathione, ascorbic acid, and β-carotene reportedly delay cut rose flower processes and significantly improve water stress tolerance [17,18]. Salicylic acid treatment alleviates post-harvest chilling injuries of anthurium cut flowers [19]. These findings indicate that including antioxidants in the preservation solution can delay the senescence of cut flowers and enhance the quality of cut flowers in vases.

Melatonin (N-acetyl-5-methoxytryptamine) is a multifunctional signaling molecule and natural phytohormone that has received increasing attention in recent years. It has extremely strong antioxidant properties. More specifically, it can effectively inhibit the accumulation of reactive oxygen species, scavenge free radicals, and protect cell structure and function. There has recently been considerable interest in melatonin because of its potential utility for the storage and transport of vegetables and the post-harvest preservation of fresh cut flowers [20,21,22]. Cut flower wilting is a key limiting event, frequently terminating vase life. This symptom is elicited by adverse water relations, arising when the transpirational water loss exceeds the vase solution uptake. In this perspective, down-regulating transpiration or up-regulating solution uptake is a direct modus of postponing wilting symptoms and, thus, prolonging vase life. Incorporating melatonin in the holding solution favored the balance between water loss and uptake, as well as increased both the cut flower fresh weight and the duration of increased fresh weight [23,24].

Earlier research indicated that exogenous melatonin can enhance the resistance of apples to Botrytis cinerea, thereby inhibiting the development of post-harvest diseases [25]. Melatonin can enhance antioxidant enzyme activities, prevent the accumulation of superoxide anions and hydrogen peroxide, and delay the senescence of grape berries [26]. Applying melatonin during the lignification of bamboo shoots reportedly decreases the lignification rate, with beneficial effects on the preservation of bamboo shoots [27]. The exogenous application of melatonin can significantly improve the stress resistance of soybean and coffee [28,29]. Some studies have demonstrated that melatonin can prolong the flowering time of rose cut flowers [30,31] and significantly extend the post-harvest life of freshly cut cauliflower and the quality of Eustoma grandiflorum inserted in vases [32,33]. Moreover, there are a few reports describing research on the applicability of melatonin for the post-harvest preservation of tree peony cut flowers, applied to post-harvest treatments to extend the freshness period. The application of melatonin during the harvesting, transportation, and storage of cut flowers can effectively extend their freshness and shelf life. By incorporating melatonin into these critical stages, the natural aging process is slowed down, ensuring that the flowers remain vibrant and durable for a longer period. This approach not only enhances the quality of the cut flowers but also supports more efficient logistics and storage solutions in the floral industry. Therefore, this study used “Diguan” tree peony to investigate the effects of exogenous melatonin at different concentrations on the post-harvest vase life and quality, as well as the physiological and biochemical properties, of tree peony cut flowers, with the aim of providing the theoretical basis for further analyses of the aging mechanisms of tree peony cut flowers treated with melatonin during their vase life.

2. Materials and Methods

2.1. Plant Materials and Treatments

The tree peony cut flower variety “Diguan” with pink double flowers was selected as the test material and obtained from the planting base of Shenzhou Tree Peony Garden in Luoyang, Henan, China (34.70683° N, 112.40026° E). The region benefits from ample sunlight, with an average annual relative humidity of 45%. Natural light exposure averages between 10 to 12 h daily, and the mean annual temperature ranges from 13 to 14.5 degrees Celsius. For autumn cultivation of this cut flower, it is recommended to plant in loose, fertile, well-drained neutral loam soil, maintaining a planting distance of 50 × 50 cm. Five-year-old tree peony branches at the initial blooming stage (III) with robust and uniform flowers under similar growth conditions were collected. The flower development stage was chosen as previously described [34]. Following the morning harvest, the tree peony cut flowers were immediately transported to the laboratory and rehydrated in distilled water for 2 h. The tree peony cut flowers with a consistent growth status had their lower stems diagonally trimmed in distilled water for an approximate length of 25 cm, with 2–3 compound leaves retained.

In the control group, the tree peony cut flowers were treated with either distilled water (CK₀) or a base bottle solution containing 2% sucrose (Tianjin Damao Chemical Reagent Factory, Tianjin, China), 200 mg·L⁻¹ citric acid (Coolaber, Beijing, China), 100 mg·L⁻¹ 8-hydroxyquinoline (Coolaber, Beijing, China), and 10 mg·L⁻¹ EDTA-Na₂ (Coolaber, Beijing, China) (CK₁). The following melatonin (Coolaber, Beijing, China) concentrations were added to the base bottle solution for the other treatments: 0.2 mg·L⁻¹ (T₁), 0.3 mg·L⁻¹ (T₂), 0.4 mg·L⁻¹ (T₃), and 0.5 mg·L⁻¹ (T₄). Each treatment involved three flowering branches and was replicated six times for a total of eighteen flowering branches per group. The treated tree peony cut flowers were placed in glass bottles filled with 450 mL of liquid to ensure each flowering branch was absorbing liquid appropriately before being sealed with plastic wrap. The bottles were then placed in a laboratory with natural diffused light (300–500 Lux), with a relative air humidity of 50% to 60%, a light period of 12 h, a dark period of 12 h, and a temperature of 22 ± 2 °C. The bottle solution was replaced every 3 days.

2.2. Analyzed Variables

2.2.1. Morphological Observation and Vase Life Determination

During the vase period, three cut flowers per treatment group were randomly selected daily at 9 a.m. They were photographed, and their morphological characteristics were examined. It was observed whether the petals maintain an upright stance, exhibit increased brittleness, or become thinner. It was inspected whether the stem incisions show signs of blackening or decay and whether the leaves retain their green color or undergo yellowing or browning. Additionally, the vase life was measured as a comprehensive morphological parameter [35].

2.2.2. Relative Flower Diameter

Each day at a consistent time, the maximum flower diameter of three randomly selected tree peony cut flowers was measured using a digital vernier caliper with a precision of 0.01 mm. The data should include fields such as sample number, treatment/control group, measurement time, and flower diameter. Touching the petals was avoided to minimize mechanical damage. This procedure was repeated three times, and the average value was calculated. Calculate according to the following formula: D1 and D2 are the flower diameters at times t1 and t2, respectively [36].

$\mathrm{R}={\displaystyle \frac{\mathrm{I}\mathrm{n}\left({\mathrm{D}}_2\right)-\mathrm{I}\mathrm{n}\left({\mathrm{D}}_1\right)}{{\mathrm{t}}_2-{\mathrm{t}}_1}}$

2.2.3. Fresh Weight of Flowering Branches

With a precision electronic balance (accuracy ≥ 0.01 g), after blotting the flower stems dry with absorbent paper to remove surface moisture, weigh the same stem rapidly 3 times and take the average value to reduce measurement errors. Repeat the weighing every 24 h until the end of vase life, then return the stems to their original vase solution for continued observation [37]. FW₀: initial fresh weight (measured before treatment). FW_t: fresh weight after 24 h of treatment (measured post-treatment).

$\mathrm{R}\mathrm{e}\mathrm{l}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{e}\mathrm{f}\mathrm{r}\mathrm{e}\mathrm{s}\mathrm{h}\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{o}\mathrm{f}\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}\mathrm{e}\mathrm{r}\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{b}\mathrm{r}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{h}\mathrm{e}\mathrm{s}={\displaystyle \frac{{\mathrm{F}\mathrm{W}}_{\mathrm{t}}-{\mathrm{F}\mathrm{W}}_0}{{\mathrm{F}\mathrm{W}}_0}}$

2.2.4. Water Balance Value

The vase solution volume changes were measured using an electronic balance (0.01 g accuracy). The vase openings were sealed with parafilm to reduce the evaporative losses. The water uptake, transpiration rate, and WB values were recorded daily. All experiments were conducted under controlled environmental conditions (25 °C, 60% RH) to ensure consistency.

Water absorption of a single cut flower = the next day’s bottle solution and bottle weight − the previous day’s bottle solution and bottle weight

Water loss of a single cut flower = the next day’s bottle solution and bottle weight and the total weight of cut flowers in the bottle − the previous day’s bottle solution and bottle weight and the total weight of cut flowers in the bottle

Water balance value = water absorption of a single cut flower − water loss of a single cut flower

2.2.5. Determination of the Soluble Protein Content and Antioxidant Enzyme Activities During the Vase Period

Petals (0.5 g) were collected from three randomly selected tree peony cut flowers without senescing the petals. This process was repeated six times. Liquid nitrogen was used to grind the sample, and PBS lysate (Beijing Solarbio Science & Technology Co., Ltd., Beijing, China) was added to extract the enzyme solution. The activity of superoxide dismutase (SOD) was determined based on its ability to inhibit the reduction in nitroblue tetrazolium (NBT) under light conditions. For peroxidase (POD) activity, hydrogen peroxide catalyzed by POD oxidizes guaiacol to produce a brown product. Catalase (CAT) activity was assessed by measuring the breakdown of hydrogen peroxide, which terminates the reaction. The activity of catalase was determined by monitoring the rate of change in absorbance as the reaction proceeded. After that, the formula is substituted to calculate the enzyme activity. The reactions were carried out on a microplate reader (Infinite 200 PRO, Tecan, Männedorf, Switzerland) after the reagents were added. The peroxidase (POD) activity, superoxide dismutase (SOD) activity, and catalase (CAT) activity were measured as previously described [38].

2.3. Data Processing and Analysis

The data were processed and analyzed using the SPSS 22.0 software (http://www.downcc.com/soft/103627.html, accessed on 3 February 2025) and Microsoft Office 2016 (http://www.downcc.com/soft/78568.html, accessed on 3 February 2025). To assess the differences between the treatment groups, a one-way ANOVA was conducted. Subsequently, Duncan’s method was utilized for multiple comparisons, with a significance level set at p < 0.05. To ensure the reliability of the results, all data were subjected to three repetitions, and the mean standard error (SE) was calculated.

3. Results

3.1. Melatonin Treatments at Different Concentrations Significantly Enhance the Vase Life of Cut Flowers of “Diguan” and Effectively Delay Petal Abscission

The effects of different melatonin concentrations on tree peony cut flower morphology are presented in Figure 1 and Figure 2. The CK₀ and T₄ treatments caused the flowers to open relatively slowly (Figure 1 and Figure 2, CK₀, T₄). The cut flowers that underwent the other treatments opened significantly faster than the CK₀-treated cut flowers, reflecting the effects of melatonin on the preservation of the cut flowers (Figure 1 and Figure 2, CK₀, T₄). All flowers were in bloom on day 4 and were at their peak ornamental value. The CK₀ treatment resulted in obvious withering on day 6, whereas the T₁ and T₄ treatments caused the cut flowers to enter the decline period (Figure 1 and Figure 2, CK₀, T₁, T₄). In contrast, the cut flowers treated with T₃ or CK₁ had not fully entered the decline period (Figure 1 and Figure 2, T₃, CK₁). More than 50% of the cut flowers had entered the decline period, implying that the best viewing period of the tree peony cut flowers had passed. However, the T₃ treatment was able to delay entry into the decline period better than the other treatments (Figure 1 and Figure 2, T₃). Hence, the T₃ treatment resulted in the longest blooming period (Figure 1 and Figure 2, T₃). These observations indicate that a melatonin treatment can significantly extend the flowering time of tree peony cut flowers and improve their ornamental value. More specifically, T₃ was the best treatment (Figure 1 and Figure 2, T₃).

3.2. Melatonin Treatments at Varying Concentrations Promote a Significant Relative Increase in the Diameter of Cut Flowers

The relative cut flower diameter is an important index for assessing the ornamental quality of cut flowers. An increase in the diameter is associated with an increase in the cut flower quality and ornamental value. The effects of different melatonin concentrations on the relative cut flower diameter are presented in Figure 3. The relative cut flower diameter gradually increased as the duration of the treatment period increased. For the CK₀ treatment, the relative cut flower diameter peaked on day 5 and tended to be stable (Figure 3, CK₀). For the other treatments, the relative cut flower diameter gradually increased and reached the maximum value on day 6. Among all treatments, the increase in the relative cut flower diameter was the highest for T₃ (10.1 cm), followed by T₁ (9.8 cm) (Figure 3, T₃, T₁). The increase in the relative cut flower diameter was similar for the T₂ and T₄ treatments (Figure 3, T₂, T₄). Moreover, the smallest increases in the relative cut flower diameter were observed for the CK₀ and CK₁ treatments (Figure 3, CK₀, CK₁). Although the T₁ and T₃ treatments both increased the relative cut flower diameter, T₃ had a slightly stronger effect, implying it is the optimal treatment for increasing the relative cut flower diameter among the treatments included in this study (Figure 3, T₁, T₃).

3.3. Melatonin Treatments at Varying Concentrations Exert a Significant Influence on the Peak Relative Fresh Weight of Flowering Branches

The relative fresh weight of flowering branches is an important factor for evaluating the preservation of cut flowers. The effects of different melatonin concentrations on the relative fresh weight of flowering branches are indicated in Figure 4. The relative fresh weight of flowering branches initially increased and then decreased during the treatment period. For the CK₀ treatment, the relative fresh weight of the flowering branches was highest on day 2, after which it rapidly decreased (Figure 4, CK₀). In contrast, for all other treatments, the relative fresh weight of flowering branches gradually increased, peaked on day 3, and then slowly decreased. The increase in the relative fresh weight of cut flowers was highest for the T₃ treatment (10.22 g) (Figure 4, T₃). Although the peak increase for the CK₁ treatment (9.19 g) did not differ much from that of the T₃ treatment, the subsequent decrease in the relative fresh weight of cut flowers was lower for the T₃ treatment than for the CK₁ treatment (Figure 4, CK₁, T₃). Although the decrease in the relative cut flower fresh weight was higher for T₃ than for T₄, the T₃ treatment resulted in the highest relative cut flower fresh weight (Figure 4, T₃, T₄). Thus, T₃ was the best treatment for increasing the relative fresh weight of flowering branches (Figure 4, T₃).

3.4. Melatonin at Different Concentrations Affects the Water Balance and Water Retention Capacity of Cut Flowers

The water balance value of cut flowers during the growth period is typically greater than 0, whereas it tends to be 0 in the blooming stage, after which it gradually decreases to less than 0 as cut flowers age and wilt. The effects of different melatonin concentrations on the water balance value are presented in Figure 5. For each treatment, the water balance value of the cut flowers gradually decreased over the treatment period. The water balance value remained at 0 the longest for the T₃ treatment, indicating that this treatment was better for maintaining the water holding capacity of flowering branches and delaying the aging process of cut flowers compared to the other treatments (Figure 5, T₃). The initial water balance value was the highest for the T₃ treatment, implying the cut flowers in this treatment group were in a better state than the other cut flowers as early as day 1 (Figure 5, T₃). In addition, the water balance value was only slightly lower for the T3 treatment compared to the T₄ treatment on the last day (Figure 5, T₃, T₄). The CK₀ treatment had the worst effect on the water balance. Although the CK₁ treatment positively affected the water balance in the early part of the treatment period, its effect was similar to that of the CK₀ treatment on the last day, indicating that melatonin had a certain effect on the preservation of the tree peony cut flowers (Figure 5, CK₁, CK₀). These findings suggest that T₃ was the best treatment for maintaining the water balance (Figure 5, T₃). On the basis of the examined morphological and physiological indices, T3 was considered to be the ideal treatment (Figure 5, T₃).

3.5. The Addition of 0.4 mg·L⁻¹ Melatonin Increases the Soluble Protein Content in Cut Tree Peony Flowers

The soluble protein contents of cut flowers following the T₃ and CK₁ treatments are provided in Figure 6. The soluble protein contents in these two treatment groups increased at first and then decreased during the treatment period. Notably, the soluble protein content was higher for the T₃ treatment than for the CK₁ treatment (Figure 6, T₃, CK₁). In the first 3 days of the treatment period, the soluble protein content of both groups increased, but the increase was greater for T₃ than for CK₁ (Figure 6, CK₁, T₃). The peak soluble protein content was higher for the T₃ treatment (0.29 mg·g⁻¹ FW) compared to the CK₁ treatment. From day 4 to day 6, the soluble protein content was higher for the samples that underwent the T₃ treatment than for the CK₁-treated samples, indicating that T₃ may be the most appropriate treatment for increasing the soluble protein content of tree peony cut flowers (Figure 6, CK₁, T₃).

3.6. The Addition of 0.4 mg·L⁻¹ Melatonin Resulted in the Optimal Peroxidase (POD) Activity in Cut Flowers

The effects of the T₃ and CK₁ treatments on the POD activity were determined (Figure 7). The POD activity in the cut flowers in both treatment groups initially increased and then decreased as the duration of the treatment period increased. Overall, the POD activity was higher for the T₃ treatment compared to the CK₁ treatment. In the first cycle (i.e., change in the bottle solution), the peak POD activity was higher for the T₃ treatment (98.67 U·g⁻¹ (FW)·min⁻¹) compared to the CK₁ treatment (72.89 U·g⁻¹ (FW)·min⁻¹) (Figure 7, CK₁, T₃). The rate of the increase in POD activity was also higher for the T₃ treatment compared to the CK₁ treatment, whereas the lowest POD activity was similar for the two treatments (Figure 7, CK₁, T₃). In the second cycle, the peak POD activity was much higher for the T₃ treatment (47.33 U·g⁻¹ (FW)·min⁻¹) compared to the CK₁ treatment (18.22 U·g⁻¹ (FW)·min⁻¹) (Figure 7, CK₁). Moreover, the rate of the increase in POD activity was greater for the T₃ treatment compared to the CK₁ treatment, but the lowest POD activity was similar for the two treatments (Figure 7, CK₁, T₃). Hence, the T₃ treatment had a greater positive effect on the POD activity compared to the CK1 treatment (Figure 7, T₃).

3.7. The Addition of 0.4 mg·L⁻¹ Melatonin Enhances the Superoxide Dismutase (SOD) Activity in Cut Flowers of the “Diguan” Tree Peony Variety

The differences in the effects of the T₃ and CK₁ treatments on the SOD activity are presented in Figure 8. For the cut flowers in both treatment groups, the SOD activity increased at first and then decreased as the duration of the treatment period increased. The overall SOD activity was higher for the T₃ treatment compared to the CK₁ treatment (Figure 8, CK₁, T₃). The peak SOD activity was higher after the T₃ treatment (267.61 U·g⁻¹ (FW)·min⁻¹) than after the CK₁ treatment (236.78 U·g⁻¹ (FW)·min⁻¹) (Figure 8, CK₁, T₃). The rate of the increase in SOD activity was higher for T₃ compared to CK₁ (Figure 8, CK₁, T₃). Although the SOD activity was lower for the T₃ treatment compared to the CK_1. treatment on days 4 and 5, the SOD activity was higher for T₃ compared to CK₁ on day 6 (Figure 8, CK₁, T₃). Considered together, these results indicate that the T₃ treatment was better than the CK₁ treatment for enhancing SOD activity (Figure 8, CK₁, T₃).

3.8. The Addition of 0.4 mg·L⁻¹ Melatonin Enhances the Catalase (CAT) Activity in Cut Flowers of the “Diguan” Tree Peony Variety, and This Activity Is Higher than That of the CK₁

According to the analyses of CAT activity (Figure 9), the T₃ and CK₁ treatments caused the CAT activity in cut flowers to increase, decrease, and then increase again over the treatment period (Figure 9, CK₁, T₃). Because the bottle solution was changed on day 3, the CAT activity in both treatment groups increased again after it decreased. Compared with the effects of the CK₁ treatment, the T₃ treatment delayed the initial occurrence of the highest and lowest CAT activities by 1 day (Figure 9, CK₁, T₃). In addition, the first CAT activity peak was higher for the T₃ treatment (885 U·g⁻¹ (FW)·min⁻¹) compared to the CK₁ treatment (780 U·g⁻¹ (FW)·min⁻¹), reflecting the positive effects of melatonin on the antioxidant capacity. After changing the bottle solution, the CAT activity was higher for the T₃-treated cut flowers than for the cut flowers that underwent the CK₁ treatment (Figure 9, CK₁, T₃). The peak CAT activity was higher for the T₃ treatment (915 U·g⁻¹ (FW)·min⁻¹) compared to the CK₁ treatment (855 U·g⁻¹ (FW)·min⁻¹) (Figure 9, CK₁, T₃). Therefore, the T₃ treatment induced the CAT activity in tree peony cut flowers better compared to the CK₁ treatment (Figure 9, T₃).

3.9. Analysis of Correlations Among the Physiological Indices

Seven indices were included in a correlation analysis. As shown in Figure 9, there were some correlations among the indices. For example, the fresh weight of flowering branches was significantly positively correlated with the soluble protein content (correlation coefficient of 0.92) (Figure 10). Melatonin can maintain the water-holding capacity of the fresh weight of flower branches and increase the content of soluble protein (Figure 10). The fresh weight of flower branches is positively correlated with the POD and negatively correlated with the water balance (correlation coefficients of 0.54 and −0.37) (Figure 10). The floral diameter was significantly positively correlated with SOD (a correlation coefficient of 0.88) (Figure 10). The correlation between SOD and the floral diameter, soluble protein, and the fresh weight of flower branches with SOD was weak (Figure 10). Soluble proteins were positively correlated with CAT and POD (Figure 10). The water balance was also negatively correlated with the soluble protein, CAT, and SOD (Figure 10). When evaluating the lifespan of cut flowers, the fresh weight of flower branches, soluble protein, floral diameter, and SOD can be used as indicators for measurements (Figure 10).

4. Discussion

The tree peony cut flower industry is constrained by the inherent characteristics of its extremely dense and short-lived blooming period, coupled with a post-harvest explosive accumulation of ROS. These factors lead to rapid quality deterioration, including petal wilting and stem bending, which severely limit commercial value and marketability [38,39]. Previous studies have demonstrated that exogenous antioxidant treatments can effectively improve cut flower quality and extend vase life by mitigating oxidative damage [9,14,40]. As a novel antioxidant, melatonin has shown remarkable efficacy in delaying ripening and senescence in post-harvest fruits and vegetables. Notably, its application in cut flowers, such as roses, chrysanthemums, and lisianthus, significantly enhances carbohydrate metabolism, water balance, and overall post-harvest performance, resulting in prolonged ornamental longevity and improved visual quality [30,31,33,41,42]. These findings highlight melatonin’s potential as a strategic solution to address the dual challenges of concentrated flowering and ROS-driven deterioration in tree peony cut flowers.

In this study, melatonin at varying concentrations was supplemented into the vase solution of tree peony cut flowers (Paeonia suffruticosa “Diguan”). The results demonstrated that the addition of melatonin at appropriate concentrations effectively maintained the flower diameter and relative fresh weight of the cut peonies. Notably, it significantly prolonged the time required for the water balance value to approach zero, thereby sustaining the water balance status of the flower stems and extending the period of water balance homeostasis. Among all treatments, 0.4 mg·L⁻¹ melatonin exhibited the most favorable overall performance. This treatment not only markedly extended the vase life of the tree peony cut flowers “Diguan” but also increased the flower diameter, demonstrating a significant improvement in the post-harvest quality. Specifically, on day 6, the cut flowers treated with T₃ still had ornamental value, whereas the cut flowers that underwent the other treatments were sagging or their outer layer of petals had dropped.

Through the establishment of treatment systems featuring diverse concentration gradients, it was discovered that the treatment involving 0.4 mg·L⁻¹ of melatonin demonstrated the most optimal comprehensive effect. This treatment not only extended the vase life of the flowers but also augmented the maximum flower diameter by 12.4%. More significantly, it re-engineered the water metabolism equilibrium system. Specifically, it notably postponed the time at which the water balance value (WBA) shifted from positive to negative. By modulating the frequency of stomatal opening and closing, a dynamic equilibrium between the transpiration rate and the water absorption rate was accomplished. This finding aligns with the reports regarding the regulation of water transport by MT in cut flowers, as cited in references [30,33].

Nevertheless, considering that the tree peony, as a woody flower species, exhibits a relatively high degree of vessel lignification. MT might alleviate the blockage at the stem base by regulating the expression of genes associated with lignin synthesis, such as PAL and CAD. However, this proposed mechanism awaits further verification. After cut flowers are detached from the plant, soluble proteins serve as both osmotic adjustment substances and crucial carriers for energy storage [43,44]. Under the treatment of 0.4 mg·L⁻¹ melatonin, the peak value of the soluble protein content (0.29 mg·g⁻¹ FW) increased by 7.4% compared to that of CK₁ (0.27 mg·g⁻¹ FW), and the degradation rate decreased. It is speculated that melatonin may maintain protein stability by activating the proline synthesis pathway (such as the P5CS gene) and the expression of a heat-shock protein (HSP70) [45]. As protective enzymes in plants, antioxidant enzymes, such as superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), are key enzymes for scavenging reactive oxygen species (ROS). Their activities directly influence the growth and development of plants. These antioxidant enzymes can work in synergy to maintain the dynamic balance of ROS metabolism within plants [46,47].

Floral diameter was significantly negatively correlated with water balance (a correlation coefficient of −0.84). The relative flower diameter also exhibits a negative correlation with POD (with a correlation coefficient of −0.29). It is speculated that this is due to vessel blockage and water transport obstacles. Specifically, the blockage of the stem vessels may prevent the effective transportation of absorbed water to the petals. Another possibility is that the increased activity of peroxidase (POD) promotes stem lignification, which either hinders the upward movement of water or fosters the growth of microorganisms. Additionally, an imbalance in cell turgor regulation might occur, where the accumulation of reactive oxygen species (ROS) leads to membrane lipid peroxidation. This increases the permeability of the cell membrane, causing the leakage of intracellular solutes and a subsequent decline in turgor pressure [48,49]. Recent work also indicates that the application of melatonin in the holding solution has successfully prolonged cut carnation vase life by both improving water balance and boosting antioxidant activity. This natural and environmentally friendly substance exhibited promising results, suggesting that it could serve as a viable alternative to harmful chemical compounds, posing risks to both the environment and human health.

The fresh weight of flower branches is significantly positively correlated with soluble proteins (with a correlation coefficient of 0.92). It is hypothesized that soluble proteins are an essential component of the cytoskeleton. When their content is high, the structures of the cell wall and cell membrane are more stable, reducing water loss and maintaining the fresh weight of flower branches. Soluble proteins have an osmotic adjustment function, participating in the regulation of cell osmotic potential, maintaining cell turgor pressure, and delaying petal wilting [50]. Soluble proteins can reduce membrane lipid peroxidation (such as the accumulation of MDA) by binding to membrane lipids or antioxidant molecules, thus maintaining membrane integrity and preventing water leakage [51]. However, this mechanism awaits further verification.

Treatment with melatonin can elevate the vase-keeping quality of cut peonies to commercial-grade standards. The effect is significantly superior to the previously reported treatment schemes using SA (salicylic acid) or 1-MCP, as documented in references [3,14]. Future research can further verify this hypothesis through molecular docking techniques. The research findings not only provide an industrially viable preservation solution for the cut tree peony industry chain, reducing costs by approximately 40%, but also establish a theoretical model for the post-harvest physiological regulation of other woody flowers, such as camellias and herbaceous peonies. Thus, these findings hold significant value for industrial transformation.

5. Conclusions

The addition of melatonin at a suitable concentration to the vase solution has beneficial effects on the preservation of tree peony cut flowers. Specifically, melatonin can effectively maintain the water balance value of tree peony cut flower stems, increase the soluble protein content, and improve antioxidant enzyme activities, thereby extending the vase life and ornamental period and significantly improving the vase life quality of tree peony cut flowers. A treatment with 0.4 mg·L⁻¹ melatonin in the base bottle solution was revealed to have the best preservative effect on tree peony cut flowers. The study results provide theoretical and technical evidence of the utility of melatonin for improving the post-harvest preservation and quality of tree peony cut flowers and other flowers and vegetables. Furthermore, the study findings may be applied to benefit the cut flower industry.

Author Contributions

Study conception and design, Y.S. (Yinglong Song), Z.W. and S.H.; data collection, M.W. and P.Z.; analysis and interpretation of the results, Y.S. (Yuke Sun) and W.S., draft manuscript preparation, M.W., Y.S. (Yinglong Song), L.S. and S.Y. (Shuiyan Yu). All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures

Figure 1. Melatonin treatments at different concentrations significantly enhance the vase life of cut flowers of “Diguan” and effectively delay petal abscission. (T1, 0.2 mg·L−1 melatonin + Base bottle insertion; T2, 0.3 mg·L−1 melatonin + Base bottle insertion; T3, 0.4 mg·L−1 melatonin + Base bottle insertion; T4, 0.5 mg·L−1 melatonin + Base bottle insertion; CK0, Distilled water; CK1, Base bottle insertion (2% Canesugar + 200 mg·L−1 Citric acid + 100 mg·L−1 8-hydroxyquinoline + 10 mg·L−1 EDTA-Na2). 1d is the first day of insertion in the cut flower insertion bottle, and so on).

Figure 2. The vase life of cut flowers treated with different concentrations of melatonin. Within a column, the different letters indicate significant differences (p [less than] 0.05). Values are presented as means ± SD (n =10).

Figure 3. Melatonin treatments at varying concentrations promote a significant relative increase in the diameter of cut flowers. Within a column, the different letters indicate significant differences (p [less than] 0.05). Values are presented as means ± SD (n = 3).

Figure 4. Melatonin treatments at varying concentrations exert a significant influence on the peak relative fresh weight of flowering branches. Within a column, different letters indicate significant differences (p [less than] 0.05). Values are presented as means ± SD (n = 3).

Figure 5. Melatonin at different concentrations affects the water balance and water retention capacity of cut flowers. Within a column, the different letters indicate significant differences (p [less than] 0.05). Values are presented as means ± SD (n = 3).

Figure 6. The addition of 0.4 mg·L−1 melatonin increases the soluble protein content in cut tree peony flowers (Note: different lowercase letters indicate significant differences among treatments (p [less than] 0.05)). Within a column, the different letters indicate significant differences (p [less than] 0.05). Values are presented as means ± SD (n = 3).

Figure 7. The addition of 0.4 mg·L−1 melatonin resulted in the optimal peroxidase (POD) activity in cut flowers. Within a column, the different letters indicate significant differences (p [less than] 0.05). Values are presented as means ± SD (n = 3).

Figure 8. The addition of 0.4 mg·L−1 melatonin enhances the superoxide dismutase (SOD) activity in cut flowers of the “Diguan” tree peony variety. Within a column, different letters indicate significant differences (p [less than] 0.05). Values are presented as means ± SD (n = 3).

Figure 9. The addition of 0.4 mg·L−1 melatonin enhances the catalase (CAT) activity in cut flowers of the “Diguan” tree peony variety, and this activity is higher than that of the CK1. Within a column, different letters indicate significant differences (p [less than] 0.05). Values are presented as means ± SD (n = 3).

Figure 10. Correlation analysis of physiological indices.

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