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Article

Effect of Exogenous Melatonin Application on Maintaining Physicochemical Properties, Phytochemicals, and Enzymatic Activities of Mango Fruits During Cold Storage

by
Narin Charoenphun
1,
Somwang Lekjing
2 and
Karthikeyan Venkatachalam
2,*
1
Faculty of Science and Arts, Burapha University, Chanthaburi Campus, Chanthaburi 22170, Thailand
2
Faculty of Innovative Agriculture, Fisheries and Food, Prince of Songkla University, Surat Thani Campus, Makham Tia, Mueang, Surat Thani 84000, Thailand
*
Author to whom correspondence should be addressed.
Horticulturae 2025, 11(2), 222; https://doi.org/10.3390/horticulturae11020222
Submission received: 3 February 2025 / Revised: 14 February 2025 / Accepted: 18 February 2025 / Published: 19 February 2025
(This article belongs to the Section Postharvest Biology, Quality, Safety, and Technology)
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Abstract

:
Mango fruits are susceptible to cold stress under prolonged storage. Melatonin (MT) is a phytohormone well known for enhancing the tolerance and overall quality of various tropical and subtropical fruits during cold storage. This study investigated the effects of MT treatment on the postharvest quality of mango fruits during prolonged cold storage. Mangoes were treated with different concentrations of MT (1.0 mM (T1), 1.5 mM (T2), 2.0 mM (T3), and 2.5 mM (T4)) and stored for 45 days under cold conditions (15 °C and 90% relative humidity). Control fruits had no MT treatments. Various physicochemical, phytochemical, antioxidant, and enzymatic activities were monitored every 5 days throughout the storage period. MT treatment significantly reduced the weight loss and decay rates compared to control samples, with T3 and T4 treatments showing superior effectiveness. Due to severe decay in the control samples, the storage period was terminated on day 25, whereas the MT treatment protected the mango fruits and allowed for the completion of all 45 days of storage. The MT treatments effectively maintained color characteristics, reduced respiration rates, and suppressed ethylene production in mango fruits compared to the control samples. Higher MT concentrations preserved firmness and controlled malondialdehyde accumulation (p < 0.05). Chemical properties, including the starch content, total soluble solids, and titratable acidity, were better maintained in MT-treated fruits. The treatments also enhanced the retention of phytochemicals (ascorbic acid, total phenolic, and total flavonoid contents) and improved antioxidant activities against DPPH and ABTS radicals. Furthermore, MT treatment effectively regulated the activities of browning-related enzymes (polyphenol oxidase (PPO) and peroxidase (POD)), cell wall-degrading enzymes (polygalacturonase (PG), pectin methylesterase (PME), and lipoxygenase (LOX)), and antioxidant enzymes (superoxide dismutase (SOD) and ascorbate peroxidase (APX)). The results demonstrate that MT treatment, particularly at higher concentrations (T3 and T4), effectively extends the storage life and maintains the quality of mango fruits during prolonged cold storage.

1. Introduction

Mango (Mangifera indica L.) is one of the most widely grown and consumed tropical fruits, with global production exceeding 55 million metric tons annually [1,2]. Mango is considered an essential fruit crop due to its distinct flavor, high nutritional value, and significant economic impact. It plays a vital role in the agricultural economies of major producing countries such as India, Thailand, and Mexico, which lead the global export market [3]. However, mangoes face substantial postharvest challenges, with softening and spoilage accounting for significant losses during storage and transportation. Postharvest losses can range from 25% to 40% depending on storage conditions, leading to considerable economic impacts on producers and exporters [4,5]. Postharvest softening is a significant factor limiting mango shelf life. The ripening process involves the breakdown of starch into soluble sugars and the degradation of cell wall components such as pectin, hemicellulose, and cellulose, which lead to changes in the texture, firmness, and overall fruit quality [6,7]. Ethylene, a gaseous hormone, plays a central role in regulating the ripening process in climacteric fruits including mangoes. It accelerates starch breakdown and cell wall degradation by activating enzymes such as amylase and polygalacturonase, contributing to fruit softening [8]. While these physiological changes enhance fruit flavor and aroma, uncontrolled softening can lead to over-ripening, spoilage, and declining market value. Furthermore, postharvest mango losses, particularly under low-temperature storage, pose a major challenge for the fruit industry. Cold storage is commonly employed to extend the shelf life and reduce metabolic rates; however, chilling-sensitive fruits such as mangoes are prone to chilling injury, which leads to physiological and biochemical changes that adversely affect fruit quality. Chilling conditions can induce symptoms such as browning, pitting, uneven ripening, increased decay, and loss of firmness, all of which significantly diminish the market value and consumer acceptability of mangoes [9]. These chilling-induced losses highlight the need for effective postharvest treatments to mitigate stress responses, preserve quality, and extend shelf life in mangoes stored under low temperatures.
In recent years, melatonin (MT) has emerged as a promising postharvest treatment for extending the shelf life of mangoes by slowing down ripening and reducing oxidative damage. MT, an indoleamine with well-documented antioxidative properties, enhances cellular defense mechanisms and mitigates oxidative stress, thereby delaying senescence and softening [7,10]. MT’s primary mechanism involves increasing the activities of antioxidant enzymes such as superoxide dismutase (SOD) and ascorbate peroxidase (APX), which play critical roles in scavenging reactive oxygen species (ROS) and preventing lipid peroxidation—a process that would otherwise result in cellular damage and the loss of fruit quality [4,11]. In addition to strengthening the antioxidant system, MT regulates other biochemical pathways that help delay ripening and senescence. One such mechanism is the modulation of cell wall metabolism. By reducing the activity of cell wall-degrading enzymes such as polygalacturonase (PG) and pectin methyl esterase (PME), MT maintains cell wall integrity, preserving fruit firmness over extended storage periods [9,12]. Moreover, MT influences gene expression related to cell wall remodeling, suggesting that its effects on softening result from direct enzymatic inhibition and transcriptional regulation [7]. MT’s impact on membrane stability is also essential for preserving mango quality during cold storage. MT stabilizes cell membranes by regulating membrane lipid composition, reducing electrolyte leakage, and lowering susceptibility to chilling injury—an issue that mangoes are particularly prone to under low-temperature conditions [11,13]. Studies on other fruits, such as loquat and peaches, further support the ability of MT to improve membrane stability, reduce chilling injury, and maintain structural integrity [14,15]. Additionally, MT enhances phenolic metabolism, which is linked to its ability to reduce browning and extend the fruit’s shelf life. An increase in the phenolic content provides antioxidant benefits and reduces enzymatic browning, a key factor in consumer acceptance and marketability. This MT-induced enhancement in phenolic levels has been observed in other fruits, including peach and mulberry, indicating MT’s potential for improving mango storage life through multiple pathways [10,16]. Furthermore, the effectiveness of MT and the optimal concentration required for protecting fruits under chilling conditions are commodity-dependent. Several studies have shown that MT treatment in the range of 1.0–2.5 mM has a positive effect on preventing fruit deterioration and spoilage, making it a suitable concentration range for the postharvest preservation of tropical and subtropical fruits [17,18].
This study explores MT’s efficacy in delaying softening and extending the shelf life of postharvest mangoes. By examining changes in enzymatic activities, membrane stability, and cell wall integrity, this research seeks to establish MT as a reliable treatment for improving mango shelf life, with implications for the broader horticultural industry.

2. Materials and Methods

2.1. Raw Materials, Chemicals, and Reagents

Fully mature ’Nam Dok Mai’ mangoes (Mangifera indica L.) were harvested at the physiological maturity stage, approximately 120 days after flowering, from a commercial orchard in Chachoengsao Province, Thailand. Fruits were carefully selected for uniformity in size (400–450 g), color (greenish-yellow), and ripeness, ensuring that only fruits free from physical defects or damage were included in this study. All chemicals and reagents used in the experiment were of analytical grade. MT powder, with a purity of ≥98%, was obtained from Sigma-Aldrich Inc. (Missouri, MO, USA). Distilled water was used to prepare all solutions, and analytical-grade reagents were purchased from recognized suppliers.

2.2. Melatonin Treatment and Storage

The selected mangoes were divided into five treatment groups, each consisting of 50 fruits. The treatment groups included a control, which was dipped in water, and four groups treated with varying concentrations of MT. The treatment groups were as follows: 1.0 mM (T1), 1.5 mM (T2), 2.0 mM (T3), and 2.5 mM (T4) MT solutions. MT solutions were freshly prepared by first dissolving MT powder in a small volume of ethanol (99%) to enhance solubility. Specifically, MT was dissolved in ethanol at 1% v/v of the final solution volume. The ethanol–MT mixture was stirred continuously at room temperature (25 °C) for 5–10 min to ensure complete dissolution. The resulting solution was then gradually diluted with distilled water to achieve the desired concentrations, ensuring uniform mixing. The final solutions contained minimal ethanol content, which did not affect the mango fruit quality. Fresh solutions were prepared before each treatment. The fruits were immersed in the treatment solutions for 15 min at room temperature, ensuring that the entire surface of the fruit was exposed to the solution. During the immersion process, gentle agitation was applied to ensure uniform coverage of the solution on the fruit surface. After the treatment, the mangoes were air-dried at ambient temperature for 30 min to remove excess solution. Following drying, the treated fruits were placed in storage at 15 °C with 90% relative humidity for 45 days. Quality assessments were conducted at 5-day intervals throughout the storage period, beginning on day 0 and continuing until day 45. At each sampling interval, fruits from each treatment group were randomly selected for analysis. Throughout the storage period, the temperature and humidity conditions were carefully monitored to ensure consistency, and the fruits were stored in open plastic crates to allow adequate air circulation and prevent condensation. Figure 1 provides an infographic representation of the MT treatment applied to mangoes at varying concentrations.

2.3. Quality Analysis

2.3.1. Determination of Weight Loss

The weight of the mango fruit was measured at the beginning of storage and at each sampling interval point to calculate the weight loss (WL) in the fruit, and WL was expressed as a percentage and calculated using the following formula:
W L % = ( W e i g h t   a t   t h e   b e g i n n i n g − W e i g h t   a t   e a c h   s a m p l e   t i m e ) W e i g h t   a t   t h e   b e g i n n i n g × 100

2.3.2. Determination of Decay Rate

The decay rate (DR) was measured based on the method of Cao et al. [16]. The decay rate in the samples was defined as the number of fruits showing signs of decay, expressed as a percentage relative to the total number of fruits in each treatment. The total decay rate was calculated using the following formula:
D R % = T o t a l   n u m b e r   o f   d e c a y e d   f r u i t s T o t a l   n u m b e r   o f   e x a m i n e d   f r u i t s × 100

2.3.3. Determination of Color Characteristics

To analyze the color characteristics of mangoes, a Hunter Lab colorimeter (model WR18, Jedto, Lam Luk Ka, Pathum Thani, Thailand) was used to record the L* (lightness: 100 = white, 0 = black), a* (negative = greenness, positive = redness), and b* (negative = blueness, positive = yellowness) values. Additionally, the total color difference (ΔE) during storage was calculated using the L*, a*, and b* values. Each fruit was measured at three equidistant points along its equatorial circumference to ensure uniform color assessment.

2.3.4. Determination of Respiration Rate

The respiration rate (RR) of mango fruit was measured following the method described by Caleb et al. [19]. Mangoes from each replication were placed in an air-tight glass chamber (3 L) and incubated for 3 h at ambient conditions. After incubation, a 1 mL gas sample was extracted from the chamber and analyzed using a gas chromatograph (GC; Auto system XL, Perkin Elmer, Waltham, MA, USA). The GC was equipped with a Porapack Q column (80/100 mesh) and a thermal conductivity detector. The results are expressed as mL CO2 kg−1 h−1.

2.3.5. Determination of Ethylene Production

The ethylene production in the mango fruit was measured in accordance with the method of Bhardwaj et al. [2] with some modifications. Samples were placed in an air-tight glass container (3 L) for 2 h at ambient conditions, and after that, the probe was used to pierce the glass container to reach the headspace and quantify the ethylene production using an ethylene analyzer (Bioconservacion, Barcelona, Spain). The measurements of the apparatus range from 0 to 100 ppm through the septum fixed to the container lid. Ethylene production was measured and was expressed as μL kg−1 h−1 by using the following formula:
E t h y l e n e   p r o d u c t i o n = E t h y l e n e   p r o d u c e d × h e a d   s p e a c e   v o l u m e f r u i t   w e i g h t k g × t i m e h

2.3.6. Determination of Firmness

Mango fruit firmness was assessed using a texture analyzer (LFRA 4500, Brookfield Engineering, Middleborough, UK) fitted with a 2 mm diameter probe. The device was configured with a pretest speed of 1.5 mm s−1, a test speed of 0.5 mm s−1, and a post-test speed of 10 mm s−1. Each measurement was conducted to a penetration depth of 5 mm at two equidistant points around the equatorial region of the fruit, with three measurements taken per replicate and each replicate consisting of three fruits. The results were recorded in Newtons (N).

2.3.7. Determination of Malondialdehyde (MDA) Content

The MDA content was measured according to the method of Kakaei et al. [10]. A 2 g fruit sample was homogenized in 5 mL of trichloroacetic acid and then centrifuged at 8000× g for 10 min. Next, 5 mL of supernatant was collected and mixed with 5 mL of a chemical reagent containing 0.5% thiobarbituric acid (w/v) and 20% trichloroacetic acid (w/v). The resulting mixture was heated in water at 95 °C for 30 min, then quickly cooled in an ice bath, and centrifuged at 3000× g for 10 min. The absorbance of the mixture was recorded at 532 nm and 600 nm using a spectrophotometer. MDA accumulation was calculated and expressed as nmol g−1 fresh weight (FW) using the following formula:
M D A   c o n t e n t = ( A 532 − A 600 ) C u v e t t e   p a t h l e n g t h × 155 × d i l u t i o n   f a c t o r

2.3.8. Determination of Starch Content

The starch content was measured following the method of Mesa et al. [20] with some modifications. Mango samples were juiced, and a 10 mL sample of mango juice was autoclaved for 20 min at 120 °C. The starch was then enzymatically digested by incubating at 45–50 °C in a water bath for 1 h with a solution comprising 1 mL of 2 M Na-acetate buffer, 1 mL of 200 mM Na-acetate buffer (both at pH 4.5), and 1 mg of amyloglucosidase. The samples were then left to rest at room temperature for 12–24 h. After this digestion period, the samples were centrifuged at 5000× g for 10 min, and the supernatants were collected in a 10 mL flask. The residue was then centrifuged with a solution of 1 mL of Na-acetate buffer and 1 mL of distilled water. After that, centrifugation was performed on the residue with a solution of 2 mL of distilled water. Then, the supernatants were combined and brought to a final volume of 10 mL with distilled water. Subsequently, 1 mL aliquots were taken from each of the 10 mL samples, and 10 mL of anthrone reagent was added to a glass tube with a glass cap. The samples were shaken and placed in boiling water for 15 min in the dark and then cooled to room temperature for 30 min. Finally, the samples were quantified by measuring the absorbance with a UV-Vis spectrophotometer (F-15001, Shimadzu, Kyoto, Japan) at 620 nm. The concentrations were obtained by comparison with a standard glucose curve (0–1000 μg mL−1).

2.3.9. Determination of Total Soluble Solids

The total soluble solids (TSS) level in the mango fruit was measured using a digital refractometer (PR-32a, ATAGO Co., Ltd., Tokyo, Japan), and the results were expressed in Brix (°).

2.3.10. Determination of Titratable Acidity

The titratable acidity (TA) level in mango fruits was measured in accordance with the method of Rastegar et al. [4]. Mango fruits were extracted to obtain the juice using a fruit processor, and then 5 mL of juice was extracted with NaOH (0.1 N) and phenolphthalein. Then, the titratable acidity results were measured and expressed as % citric acid.

2.3.11. Determination of Phytochemicals and Antioxidant Activities

Determination of Ascorbic Acid Content (AsA)

Mango fruit samples were processed into juice using a food processor. Then, 2 mL of the juice was mixed with 2 mL of 5% trichloroacetic acid as a stabilizer. The mixture was titrated with 2,6-dichlorophenol indophenol until it turned a constant purple. The results were calculated and expressed as mg 100 g−1 FW.

Determination of Phenolic Contents and Antioxidant Activities

Prior to determining the phenolic content and antioxidant activities, mango fruits were extracted. For this, 5 g of fruit sample was homogenized with 25 mL of 80% ethanol. The homogenate was then centrifuged at 10,000× g for 20 min under refrigerated conditions. After centrifugation, the supernatant was collected and used for measuring the total phenolic content (TPC), total flavonoid content (TFC), and antioxidant activities as described below.
The TPC and TFC in the mango fruit were measured in accordance with the method of Bharadwaj et al. [2] with some modifications. For TPC, the reaction mixture consisted of 0.1 mL of supernatant and 1 mL of 10% (v/v) Folin–Ciocalteu reagent, to which 1 mL of 15% (w/v) sodium carbonate was added and thoroughly mixed. The reaction mixture was then incubated for 90 min at room temperature under dark conditions, and the absorbance was recorded at 765 nm using a UV-Vis spectrophotometer (F-15001, Shimadzu, Kyoto, Japan). The TPC was calculated based on a standard curve of gallic acid (GA), and the results were expressed as mg GA equivalents per g FW. For TFC, the reaction mixture consisted of 0.1 mL of supernatant and 0.1 mL of 1 M potassium acetate and 10% (w/v) aluminum chloride. The volume of the reaction mixture was made up to 3 mL with distilled water. Then, the reaction mixture was incubated at room temperature under dark conditions for 45 min. After incubation, the reaction mixture was mixed well, and the absorbance was measured against a blank at 510 nm using a UV-Vis spectrophotometer. The TFC level in the samples was calculated based on a standard curve prepared using quercetin (Q), and the results were expressed as mg QE per g FW.
The DPPH and ABTS radical scavenging activities in mango were measured in accordance with the method of Keawpeng et al. [21] with some modifications. For the DPPH (2,2-diphenyl-1-picrylhydrazyl) radical scavenging assay, 0.1 mL of supernatant was mixed with 3.9 mL of 60 µmol/L DPPH in a test tube, and after that, the reaction mixture was thoroughly mixed and incubated in dark conditions for 30 min. Then, the absorbance of the reaction mixture was measured at 515 nm using a UV-Vis spectrophotometer. The results were expressed as a percentage of DPPH radical scavenging ability. For the ABTS (2,2′-azino-di-3-ethylbenzthiazoline sulfonic acid) radical cation scavenging assay, 1 mL of supernatant was mixed with 1 mL of ABTS reagent in a test tube, and then, the reaction mixture was mixed well and incubated at room temperature for 30 min. After incubation, the absorbance of the sample was measured at 734 nm using a UV-Vis spectrophotometer. The results were expressed as a percentage of ABTS radical scavenging ability.

2.3.12. Determination of Browning-Related Enzymes

Determination of Polyphenol Oxidase Activity

Polyphenol oxidase (PPO) activity in the mango fruits was measured by following the method of Gao et al. [11] with some modifications. A total of 5 g of mango flesh was homogenized in 25 mL of 50 mM potassium phosphate buffer (pH 6.8) and centrifuged at 12,000× g for 15 min at 4 °C. The supernatant was collected as the enzyme extract. For the assay, the reaction mixture was prepared with 2.8 mL of 100 mM catechol and 0.2 mL of the enzyme extract. The reaction was monitored by measuring absorbance changes at 420 nm using a UV-Vis spectrophotometer. PPO activity was expressed as units g−1 FW.

Determination of Peroxidase Activity

The peroxidase (POD) activity in mango fruits was determined by following the method of Hassan et al. [22] with some modifications. A total of 5 g of mango flesh was homogenized in 25 mL of phosphate buffer (pH 7). Then, the homogenate was centrifuged at 12,000× g for 15 min at 4 °C, and the supernatant was collected as the enzyme extract. For the assay, the reaction mixture consisted of 0.5 mL of extraction buffer, 0.3 mL of pyrogallol, 1 mL of H2O2 and 5 mL of distilled water. The reaction mixture was thoroughly mixed, and the enzyme activity was measured at 425 nm using a UV-Vis spectrophotometer. PAL activity was expressed as units g−1 FW.

2.3.13. Enzyme Activities

Determination of Cell Wall-Degrading Enzymes

  • Determination of polygalacturonase activity
Polygalacturonase (PG) activity in mango fruit was determined based on the method of Cao et al. [16]. A total of 10 g of mango sample was ground with 35 mL of sodium acetate buffer (40 mmol/L, pH 5.2) containing 5 g/100 mL polyvinyl pyrrolidone, 2 mL/100 mL mercaptoethanol, and 100 mmol/L NaCl. After that, the homogenate was centrifuged at 15,000× g for 20 min at 4 °C. Then, 0.5 mL of the supernatant was collected and mixed with 0.5 mL of polygalacturonic acid (10 g/L). The reaction mixture was then thoroughly mixed, and the absorption was measured at 540 nm using a UV-Vis spectrophotometer. The activity of the PG enzyme was expressed as units per gram fresh weight (units g−1 FW).
2.
Determination of pectin methylesterase activity
Pectin methylesterase (PME) activity was measured in accordance with the method of Cao et al. [16]. A total of 50 g of mango sample was homogenized in 100 mL of sodium phosphate buffer (pH 7.0) for 10 min, and then, the homogenate was adjusted to pH 7 with 0.50 N NaOH. After that, 0.5 M NaCl (approximately 50 mL) was added to obtain a final concentration of 0.5 M to aid in enzyme solubilization. Then, the mixture was centrifuged at 3000× g for 30 min at 4 °C, and the resulting supernatant was collected and adjusted to pH 4.2 using 0.5 N HCl to stabilize PME activity and filtered through a syringe filter (0.22 µm) to obtain the crude enzyme extract. Next, a reaction mixture containing 2 mL of 0.5% pectin solution, 0.15 mL of 0.01% bromothymol blue, 0.85 mL of distilled water, and 0.4 mL of enzyme extract was thoroughly mixed and the changes in absorbance were observed at 620 nm using a UV-Vis spectrophotometer. PME activity was expressed as units g−1 FW.
3.
Determination of lipoxygenase activity
Lipoxygenase (LOX) activity was measured by following the method of Gao et al. [11] with slight modifications. A total of 5 g of mango sample was homogenized in 25 mL of 50 mM potassium phosphate buffer (pH 6.8) and centrifuged at 12,000× g for 15 min at 4 °C. The supernatant was collected as the enzyme extract. The reaction mixture consisted of 2.775 mL of 100 mM sodium acetate buffer (pH 5.5), 25 µL of linoleic acid, and 0.2 mL of enzyme extract. Absorbance changes were measured at 234 nm using a UV-Vis spectrophotometer, with the changes in the absorbance indicating conjugated diene formation, and LOX activity was expressed as units g−1 FW.

2.3.14. Determination of Antioxidant Enzyme Activities

Determination of Superoxide Dismutase Activity

Superoxide dismutase (SOD) activity in the mango fruits was determined by following the method of Zhang et al. [23]. First, 2 g of fruit sample was homogenized in 15 mL of precooled 50 mM phosphate buffer (pH 7.8) containing 0.1 mM EDTA to stabilize the enzyme, and the homogenate was centrifuged at 12,000× g for 15 min at 4 °C. Then, the supernatant was collected and used in the SOD assay. For the assay, the reaction mixture was composed of 1.5 mL of 50 mM phosphate buffe (pH 7.8), 0.3 mL of methionine, 0.3 mL of 75 µM nitroblue tetrazolium, 0.3 mL of 0.1 mM EDTA, 0.3 mL of 2 µM riboflavin, and 0.3 mL of enzyme extract. The reaction was incubated under fluorescent light (approximately 4000 lux) for 15 min at room temperature, while a control reaction was kept in the dark. The absorbance was measured at 560 nm using a UV-Vis spectrophotometer. SOD activity was expressed as units g−1 FW.

Determination of Ascorbate Peroxidase Activity

Ascorbate peroxidase (APX) activity in the mango fruits was determined in accordance with the method of Wang et al. [24]. In total, 5 g of fruit sample was homogenized in 25 mL of 50 mM phosphate buffer (pH 7.0) that contained 1 mM EDTA and 1 mM ascorbic acid. After that, the homogenate was centrifuged at 12,000× g for 15 min at 4 °C, and the supernatant was collected to measure the APX activity. For the assay, the reaction mixture (3 mL) was prepared by combining 2.6 mL of extraction buffer, 0.1 mL of enzyme extract, and 0.3 mL of 2 mM H2O2. The reaction was initiated by adding H2O2, and the decrease in absorbance was measured at 290 nm every 30 s for 3 min using a UV-Vis spectrophotometer. The APX activity was expressed as units g−1 FW.

2.4. Statistical Analysis

All experiments were conducted in triplicate, and the data are expressed as the mean ± standard deviation (SD). Statistical analysis was performed using one-way analysis of variance (ANOVA), followed by Tukey’s post-hoc test to determine significant differences between treatment groups. Statistical significance was set at p < 0.05. The statistical analysis was conducted using SPSS software (version 6) for Windows, supplied by SPSS Inc., based in Chicago, IL, USA. The heatmap of Pearson’s correlation was generated using TBtools Version 2.142.

3. Results and Discussion

3.1. WL and DR

WL and DR levels in the MT-treated and untreated mango fruits are shown in Figure 2A,B. The WL and DR patterns observed during the 45-day storage period revealed significant differences between treated and control samples. Generally, the WL in mango fruit results from moisture loss through continuous respiration and transpiration, even at low temperatures [25]. The present results showed that the control group exhibited a rapid increase in WL, reaching a value of 15.8% by day 25, at which point further measurements were discontinued due to severe decay, rendering the samples unsuitable for continued analysis (Figure 2A). In contrast, the treated samples (T1-T4) demonstrated markedly improved stability throughout the storage period. Among the treatments, T3 and T4 showed superior performance, maintaining the lowest WL of approximately 10.8–12.8% by day 45, while T1 and T2 exhibited intermediate values of 14–14.5%. These results demonstrate that MT treatment, particularly at higher concentrations, effectively reduced moisture loss, contributing to prolonged fruit freshness. While there is no direct mechanism explaining how MT controls WL in mango fruit, several studies suggest that its effectiveness is mainly attributed to its enhancement of antioxidant systems, maintenance of cell wall integrity, and reduction in metabolic activity [2,26]. Additionally, MT reduces the respiration rate and ethylene production, which lowers metabolic activity and slows down processes, leading to controlled WL and quality deterioration [27]. Furthermore, the DR analysis emphasized the effectiveness of the treatments in preserving sample quality. The control group showed an aggressive decay progression, escalating dramatically from day 15 onwards and reaching approximately 80% decay by day 25, necessitating the termination of the control sample analysis (Figure 2B). This rapid deterioration in control samples underscores the critical need for protective treatments to extend shelf life. The treated samples, however, maintained significantly lower DRs throughout the storage period. T4 emerged as the most effective treatment, with a DR remaining below 30% even after 45 days, followed by T3 showing similar protective effects. T1 and T2, while still considerably more effective than the control, demonstrated moderate protection, with final DRs of approximately 45% and 37%, respectively. These findings collectively indicate that all treatments effectively extended the shelf life beyond the 25-day limit observed in control samples, with T3 and T4 showing particular promise in maintaining product quality through reduced WL and DR. The reduced DR in MT-treated fruits can be attributed to its ability to maintain cell wall integrity by inhibiting cell wall-degrading enzymes, including PG and PME, which are instrumental in the softening and decay of fruit tissue [27]. This is in accordance with the present study. Furthermore, MT-treated fruit could effectively suppress ethylene production, enhancing stress-inducing mechanisms that positively maintain cellular homeostasis and, thus, consequently maintain the lower DR in the mango fruits [28]. Additionally, MT possesses antimicrobial properties, which can inhibit the growth of decay-causing microorganisms on the fruit surface.

3.2. Color Characteristics

The color characteristics of MT-treated and untreated mango fruits during storage are illustrated in Figure 3A–D. Overall, the L* values, which indicate lightness, continuously declined across all samples throughout the storage period. The control group exhibited the most dramatic decrease in L* values by day 25, while the treated samples maintained relatively higher values compared to the control by day 45. Among the treated groups, fruits treated with higher melatonin concentrations (T4) showed better control against L* reduction, followed by the other concentrations (p < 0.05). T3 and T4 treatments particularly demonstrated superior preservation of L* values. On the other hand, the a* values, representing redness, showed an increasing pattern throughout the storage period. The control group displayed a sharp increase in a* values, reaching a peak at the end of day 25. In contrast, the treated samples exhibited a more gradual increase in redness, ultimately reaching values between 7 and 8 units by day 45. T3 and T4 samples effectively controlled the increase in a* values, especially towards the end of the storage period. Furthermore, the b* values, which indicate yellowness, progressively increased during storage. The control group showed the most pronounced increase until the end of storage (25 days), whereas the treated samples displayed a more moderate and controlled increment. These findings suggest that the treatments effectively moderate the color changes during storage, with T3 and T4 showing promising results in maintaining color stability compared to the control group. The total color difference (ΔE) increased progressively in all samples during storage, with the control group exhibiting the highest ΔE values, compared to the MT-treated samples (Figure 3D). In contrast, MT-treated mangoes showed significantly lower ΔE values (p < 0.05), indicating better color retention. Among the treatments, T3 and T4 were the most effective, maintaining ΔE below 30, while T1 and T2 displayed intermediate values. In general, the higher color changes, particularly the loss of L* and increase in a* and ∆E, indicate browning in mango fruits. This browning is mainly linked to enzymatic and chemical changes during storage, leading to color degradation [13]. MT’s role in mitigating browning is attributed to its modulation of key biochemical pathways and enhancement of antioxidant defenses. MT reduces browning by inhibiting enzymes such as PPO, which catalyzes phenolic oxidation, and by maintaining cellular integrity through its antioxidant effects [11,23,29]. Similar results were observed in mulberry fruits, where MT treatment reduced browning by controlling MDA accumulation and enhancing antioxidant capacity during storage [10]. Additionally, MT has been shown to upregulate PpPAL and suppress PpPPO and PpPOD in peach fruits, leading to increased phenolic accumulation and reduced browning [15]. Overall, the present results showed that MT treatments, especially at higher concentrations, effectively preserve the color characteristics of mango fruits during storage by reducing browning and controlling color changes.

3.3. Respiration Rate, Ethylene Production, Firmness, and MDA Content

The changes in the respiration rate, ethylene production, firmness, and MDA content in the MT-treated and untreated mango fruits during storage are illustrated in Figure 4A–D. Overall, the results exhibited distinct patterns between control and treated samples throughout storage. Respiratory activity is a key indicator of metabolic processes and increases in cold-stored fruit during extended storage, which is observed as a response to chilling stress [30]. The control samples displayed a pronounced increase in the respiration rate, escalating from an initial rate of 15 mL CO2 kg−1 h−1 to approximately 88 mL CO2 kg−1 h−1 by day 25 (Figure 4A). In contrast, MT-treated samples exhibited better control against the increment in the respiration rate. Among the MT treatments, T3 and T4 samples maintained significantly lower respiration rates, reaching approximately 70–75 mL CO2 kg−1 h−1 by day 45, indicating a moderated metabolic response and, consequently, a slowing of the respiration process. Cold storage generally increases the activity of respiratory metabolism enzymes, particularly phosphohexose isomerase, succinate dehydrogenase, and cytochrome C oxidase [31]. The ethylene production pattern is generally closely correlated with respiratory behavior [32]. Lal et al. [33] reported that MT can reduce ethylene production during cold storage by inhibiting ethylene synthesis, which is critical for accelerating fruit ripening and respiration rates. This is in accordance with the present study, in which MT effectively suppressed the production of ethylene in the mango fruit during storage (Figure 4B). Notably, treatments T3 and T4 effectively suppressed ethylene production, maintaining levels below 2.1 μL kg−1 h−1 throughout the storage period, whereas the control samples exhibited an accelerated increase in ethylene production, reaching 4.8 μL kg−1 h−1 by day 25. Overall, the results indicate that the reduced ethylene production by MT treatment suggests a successful delay in ripening and senescence processes, contributing to an extended shelf life. The texture analysis results revealed significant differences in firmness retention among treatments (Figure 4C). Generally, the degradation of cell wall components and the increase in the sugar content are key factors that lead to the softening of mango fruit during storage [3]. The control samples demonstrated rapid softening, with firmness values declining dramatically from 37 N to 13 N within 25 days. Treatment T4 proved most effective in maintaining structural integrity, with firmness values remaining above 24 N until day 40. This enhanced firmness retention correlates with the reduced respiratory and ethylene production rates, suggesting a comprehensive delay in ripening-related processes. MT treatment has been found to suppress the activities of enzymes such as PG, PME, and β-glucosidase, which are responsible for cell wall degradation. This inhibition helps maintain higher levels of protopectin, cellulose, and hemicellulose, thereby preserving mango fruit firmness [9]. This is in accordance with the present study on inhibiting cell wall-degrading enzymes. Changes in membrane degradative byproducts, such as MDA, in treated and untreated mango fruit during storage are shown in Figure 4D. The malondialdehyde (MDA) content is a marker for lipid peroxidation and oxidative stress in fruits during the deterioration process, significantly impacting their storage quality and shelf life. Monitoring and managing MDA levels is crucial for preserving the freshness of fruits by reducing oxidative damage, as its accumulation damages cellular organelles and plasma membranes [16,34]. Control samples exhibited rapid membrane deterioration, with MDA levels increasing to 3.8 nmol g−1 by day 25, indicating severe oxidative stress. Compared with control samples, all MT treatments significantly controlled the MDA level in mango fruits. MT treatments such as T3 and T4 significantly mitigated lipid peroxidation, maintaining MDA levels below 1.5 nmol g−1 throughout storage. Cao et al. [16] showed that MT application significantly controlled MDA accumulation in kiwifruit during low-temperature storage. MT reduces the levels of harmful compounds such as superoxide anions, hydrogen peroxide, and MDA, indicators of oxidative stress, by enhancing endogenous antioxidant enzymes [7,24]. Wan et al. [14] reported that MT-treated loquat fruits significantly lowered MDA accumulation due to MT’s inhibitory activity on reactive oxygen species, thus reducing membrane peroxidation.

3.4. SC, TSS, TA, and TSS/TA Ratio

The chemical properties such as the SC, TSS, TA, and TSS/TA ratio of the MT-treated and untreated mango fruits during storage are shown in Figure 5A–D. The SC, which is a primary indicator of product maturity and senescence, showed distinctive degradation patterns across treatments. In general, the SC in mango fruit decreases during storage due to the conversion of starch into sugars, which increases sweetness [35]. Cold storage is often considered as a safer method for storing mango fruits as it slows down starch degradation and sucrose accumulation; however, when mangoes undergo cold stress, it activates the amylolytic enzymes and induces starch degradation [36]. The control samples demonstrated rapid starch degradation, declining sharply from an initial content of approximately 80 g kg−1 to 40 g kg−1 by day 25. In contrast, the MT treatments, particularly T3 and T4 samples, effectively delayed starch breakdown, maintaining higher levels throughout storage and reaching approximately 48–55 g kg−1 by day 45 (Figure 5A). The reduced rate of starch degradation observed with MT treatment indicates a successful modulation of amylolytic enzyme activity and a delay in senescence processes. Rastegar et al. [4] reported that MT modulates the activity of amylase and slows the conversion of starch into sugars, thereby maintaining a higher SC in the fruits. TSS is a key indicator of fruit sweetness and ripeness [37]. The TSS content in mango fruits exhibited a reciprocal relationship with starch degradation, as expected due to the conversion of starch into simple sugars during ripening [38]. Control samples showed the most rapid increase in TSS, reaching 18.5 °Brix by day 25, indicating accelerated ripening. MT treatments T3 and T4 demonstrated a more gradual increase in TSS content, achieving final values of approximately 18–18.5 °Brix by day 45, while T1 and T2 showed intermediate effects with values around 17 °Brix. This moderated increase in TSS suggests the effective regulation of ripening-related metabolic processes. The application of MT effectively postpones mango ripening by inhibiting the starch-to-sugar conversion while preserving cell wall integrity, modulating antioxidant metabolism, and regulating ethylene and abscisic acid synthesis [39]. Priyadarsani and Kar [1] reported that mangoes of the ’Arka Aruna’ variety had increased TSS with a corresponding decrease in TA and firmness. This is in accordance with the present study. TA is an important factor in flavor development and product quality [40]. The results revealed that the control samples exhibited a rapid decline in acidity from an initial value of 2.0% to 0.5% by day 25, indicating accelerated metabolic processes, whereas the MT treatments such as T3 and T4 maintained higher acidity levels throughout storage, declining more gradually to approximately 0.7–0.85% by day 45. This preserved acidity profile suggests better maintenance of the organic acid content and cellular compartmentalization. MT has been shown to inhibit the reduction of TA by reducing metabolic activity, specifically the respiration rate and ethylene production, which are key factors in fruit ripening and acidity loss [2]. The TSS/TA ratio is a critical indicator of taste and ripening progression [41]. Control samples showed an exponential increase in the TSS/TA ratio, reaching approximately 37 by day 25, indicating over-ripening. In contrast, all treatments effectively moderated the increase in the TSS/TA ratio compared to the control, with T4 showing the most balanced progression, reaching about 22 by day 45. This moderate ratio suggests a better preservation of the organoleptic properties and delayed ripening processes. The lower TSS/TA ratio in MT-treated mango fruits could be due to the slower rate of increase in TSS and the decrease in TA levels as a result of MT treatment. This is consistent with the study by Medina-Santamarina et al. [42], which found that MT-treated pomegranate exhibited delayed postharvest ripening and acid regulation, contributing to a lower TSS/TA ratio over time.

3.5. Phytochemicals and Antioxidant Activities

The changes in phytochemicals such as AsA, TPC, and TFC in the MT-treated and untreated mango fruits during storage are shown in Figure 6A–C. The AsA level in the mango fruit represents a crucial indicator of nutritional quality and antioxidant capacity. The study results showed a distinct degradation in AsA levels in all the samples. At the beginning of the storage, all the samples exhibited an AsA level of approximately 24 mg g−1, and a slight increase was observed during the first 5 days of storage, possibly due to metabolic adjustments [43]. However, after 5 days of storage, the AsA level in the control samples demonstrated a rapid degradation, and it declined to approximately 12.5 mg g−1 by day 25. In contrast, the MT-treated samples proved to be the most effective compared to the control and maintained a higher level of AsA even at 45 days of storage. Among the treated samples, T4 proved most effective in preserving AsA, maintaining the AsA level in mango above 17 mg g−1 even after 45 days of storage. T3 showed a similar protective effect, while T1 and T2 exhibited intermediate preservation capacities, reaching final values of approximately 13.8 and 14.2 mg g−1, respectively. Overall, the results showed that MT-treated mango fruit significantly controlled the loss of AsA by enhancing antioxidant activity by reducing oxidative stress and metabolic activity, which helped maintain a higher AsA content, delay fruit softening, and improve storage quality during cold storage [2,9]. The activities of APX and SOD play a crucial role in maintaining AsA levels by neutralizing ROS, which would otherwise contribute to oxidative stress and deplete AsA [44]. Kul et al. [45] reported that MT enhances the activities of APX and SOD, which are critical in maintaining ROS homeostasis under abiotic stress conditions. Similarly, throughout the storage period, all treatments exhibited a continuous decrease in TPC levels (Figure 6B). Control samples exhibited the most rapid decline in the phenolic content, decreasing from an initial value of 72 mg GAE/g to approximately 48 mg GAE/g by day 25. On the other hand, the MT-treated mango fruits demonstrated superior protection from the extensive loss of phenolic compounds during storage. Among the treated samples, the T4 sample retained a higher level of TPC, with the levels remaining above 60 mg GAE/g throughout the storage, and at the end of storage (45 days), the TPC levels in the T4 samples reached approximately 61 mg GAE/g. This represents a significant improvement in phenolic retention compared to the other MT treatments, where T3 maintained intermediate levels (56 mg GAE/g), while T1 and T2 showed moderate preservation effects (45–50 mg GAE/g) by the end of storage. Similarly, TFC levels in mango fruits also exhibited a gradual degradation across all treatments (Figure 6C). In the beginning, the TFC levels in mango fruits were approximately 45 mg QE/g despite the sample variations. However, it decreased steadily throughout the storage. Control samples decreased to 30 mg QE/g on day 25. On the other hand, MT-treated mango fruits retained higher TFC levels. However, it was strongly dose-dependent. Higher MT treatment, particularly T4 samples, demonstrated the most effective preservation of TFC, maintaining the levels above 27 mg QE/g at 45 days of storage, followed closely by T3 samples (25 mg QE/g). The enhanced retention of flavonoids in treated samples suggests effective protection of these important secondary metabolites during storage. This preservation may be attributed to reduced metabolic activity and enhanced cellular integrity, as evidenced by the lower respiration rates and better firmness retention observed earlier. During cold storage, the TPC and TFC levels in mangoes decline due to oxidative degradation from ROS, residual activity of enzymes such as PPO and POD [46], and cold-induced metabolic shifts that metabolize phenolics as part of the fruit’s stress response [8]. Polymerization and binding to cellular components also reduce their measurable levels [47]. However, MT treatment preserves TPC and TFC in mangoes during cold storage by scavenging ROS, boosting antioxidant enzymes, inhibiting oxidative enzymes (PPO and POD), and stabilizing stress responses, which collectively reduce phenolic and flavonoid degradation [2,39]. The antioxidant properties, such as the DPPH and ABTS radical scavenging activities of MT-treated and untreated mango fruits during storage, are shown in Figure 7A,B. In general, TPC and TFC are significant contributors to antioxidant activities in mango fruit [48,49]. The DPPH radical scavenging activity initially showed high values of approximately 80% across all samples, indicating strong antioxidant potential at the beginning of storage. Control samples exhibited a rapid decline in DPPH scavenging activity, decreasing to about 60% by day 25. This rapid decline suggests the accelerated degradation of antioxidant compounds and reduced protective capacity against free radicals. In contrast, the treatments showed varying degrees of effectiveness in maintaining antioxidant capacity. MT treatments, particularly T4, demonstrated a superior preservation of DPPH radical scavenging activity, maintaining levels above 65% even after 45 days of storage. This represents a significant improvement in antioxidant retention compared to the other treatments. T3 showed similar protective effects, with scavenging activity remaining above 62% throughout the storage period. Treatments T1 and T2 exhibited moderate preservation, with final DPPH scavenging activities of approximately 56% and 59%, respectively. Similarly, the ABTS radical scavenging activity of mango samples exhibited a decline comparable to that of the DPPH assay, with initial values being slightly lower, at approximately 70%. Among the variables, the control samples showed the most rapid decline, reaching approximately 48% on day 25. On the other hand, the MT-treated fruits maintained the highest ABTS scavenging activity throughout the storage; T4 samples had the highest scavenging activity, with levels remaining above 60% on day 45, while T3 demonstrated similar protective effects, and T1 and T2 showed intermediate preservation capacity, reaching final values of approximately 50–51%. This study found that the preservation patterns of both DPPH and ABTS radical scavenging activities strongly correlate with the retention of bioactive compounds observed earlier, particularly the maintenance of phenolic compounds, flavonoids, and ascorbic acid. This study’s results suggest that MT treatment enhanced the antioxidant defense systems of fruits, with higher concentrations showing a better preservation of antioxidant activity. MT strengthens antioxidant defenses against free radicals by enhancing antioxidant enzymes, reducing ROS production, and activating redox pathways such as Nrf2, which together bolster cellular resistance to oxidative stress [2,50,51].

3.6. Enzyme Activities

3.6.1. Browning-Related Enzyme Activities

Browning-related enzymes such as PPO and POD are mainly associated with the browning of mango fruit, and their changes in the MT-treated and untreated samples during storage are shown in Figure 8A,B. PPO activity is closely associated with browning reactions in mango fruit, and the results showed an initial value of approximately 15 units g−1 across all samples. The control group demonstrated a rapid increase in PPO activity, reaching about 42 units g−1 by day 25, indicating accelerated enzymatic browning processes. Treatments showed varying degrees of effectiveness in suppressing PPO activity, with T4 demonstrating the most significant inhibition, maintaining the lowest activity levels throughout storage (reaching approximately 35 units g−1 by day 45). T3 showed similar protective effects, while T1 and T2 exhibited intermediate inhibition, with final PPO activities of approximately 45 units g−1. Similarly, the POD activities in mango fruits also continuously increased during storage. Among the tested samples, control samples exhibited a dramatic increase in POD activity between days 5 and 10, reaching approximately 42 units g−1 by day 15 and maintaining elevated levels until measurements were discontinued at day 25. This rapid increase suggests enhanced oxidative processes and potential stress responses in untreated samples [52]. In contrast, T4 treatment demonstrated superior regulation of POD activity, gradually increasing throughout storage and maintaining the lowest activity levels (approximately 38 units g−1 by day 45). T3 showed similar effectiveness in POD inhibition, while T1 and T2 displayed moderate control over enzyme activity, reaching final values of approximately 45 units g−1. The reduction in POD activity in MT-treated fruits suggests that MT influences phenolic metabolism, contributing to reduced browning and enhanced antioxidant capacity [23]. Rastegar et al. [4] reported that MT treatment regulates PPO and POD activities by enhancing antioxidant activity. Similarly, Kebbeh et al. [53] found that MT modulates PPO and POD activities in mango by influencing proline metabolism, protecting the fruit from chilling injury under stress conditions. Dong et al. [12] also reported that MT treatment regulates PPO and POD activities in mangoes, leading to delayed ripening and improved membrane integrity by reducing cell wall degradation during postharvest storage.

3.6.2. Cell Wall-Degrading Enzyme Activities

The activities of cell wall-degrading enzymes, including PG, PME, and LOX, in the mango fruits that were MT-treated and untreated and stored under prolonged cold storage are illustrated in Figure 9A–C. These enzymes are key contributors to fruit softening by breaking down cell wall components, which are closely linked to physiological and biochemical changes during ripening and storage [6]. Among these enzymes, PG activity plays a fundamental role in cell wall degradation and fruit softening [54]. The results indicated an initial value of approximately 1.5 units g−1 across all mango samples. However, the control fruits exhibited rapidly increased PG activity during extended storage, reaching approximately 12 units g−1 by day 25. This indicates an accelerated breakdown of pectin substances in the cell wall. The MT treatments demonstrated varying degrees of effectiveness in suppressing PG activity, with T4 showing the most significant inhibition, reaching approximately 13 units g−1 by day 45 and maintaining the lowest activity levels throughout storage. T3 displayed similar protective effects, while T1 and T2 showed intermediate inhibition, with final PG activities of approximately 17 units g−1. Furthermore, PME activity, which catalyzes the de-esterification of pectin and contributes to cell wall modification [55], exhibited similar trends in all the mango samples. The initial PME activity was approximately 0.15 units g−1 across all treatments. The control samples showed a steady increase in PME activity, reaching about 0.66 units g−1 by day 25, suggesting the progressive demethylation of cell wall pectins. Among the MT treatments, T4 samples demonstrated superior regulation of PME activity, maintaining the lowest levels throughout storage (approximately 0.75 units g−1 by day 45). T3 showed comparable effectiveness in PME inhibition, while T1 and T2 exhibited moderate control over enzyme activity, reaching final values of approximately 0.89 units g−1. These findings suggest that MT treatment, particularly at higher concentrations, effectively suppresses the activities of the PG and PME enzymes, contributing to delayed softening and better firmness retention during storage. MT’s effect may involve enzyme suppression, the regulation of gene expression related to cell wall metabolism [23], the enhancement of antioxidant defenses [56], and the maintenance of cell wall polysaccharides [27], collectively contributing to improved fruit firmness during storage. LOX activity, associated with membrane lipid deterioration and textural changes [57], displayed a consistent upward trend across all samples. The initial LOX activity was approximately 2.8 units g−1, with the control group showing the most rapid increase, reaching about 8 units g−1 by day 25. T4 treatment proved most effective in controlling LOX activity, maintaining the lowest levels throughout storage (approximately 10 units g−1 by day 45). T3 demonstrated similar effectiveness, while T1 and T2 showed intermediate levels of enzyme inhibition, with final LOX activities of approximately 11.5 and 10.9 units g−1, respectively. These results suggest that the T4 and T3 treatments most effectively maintained cell wall integrity and reduced textural degradation during storage. This reduction in LOX activity indicates that MT treatment mitigates lipid oxidation and off-flavor development, improving fruit quality during storage. Qu et al. [27] reported that blueberries treated with MT treatment also displayed inhibited LOX activity and reduced membrane lipid peroxidation with preserved fruit quality. Li et al. [58] examined the effect of MT treatment on long green peppers and found that MT application effectively controlled LOX activity, delayed chilling injury, and consequently enhanced firmness retention.

3.6.3. Antioxidant Activities

The activities of antioxidant enzymes, specifically SOD and APX, in mango fruits that were MT treated and untreated and stored under prolonged cold storage are shown in Figure 10A,B. SOD serves as the first line of defense against reactive oxygen species by catalyzing the dismutation of superoxide radicals in cold-stress fruits [59]. The SOD results displayed an initial activity of approximately 10 units g−1 across all samples. The control group exhibited a rapid increase in SOD activity, reaching about 39 units g−1 by day 20, followed by a slight decline, indicating an enhanced oxidative stress response. Interestingly, T4 treatment showed a more gradual but sustained increase in SOD activity, reaching the highest level of approximately 50 units g−1 by day 45, suggesting a more controlled and prolonged antioxidant response. T3 demonstrated similar patterns to T4, while T1 and T2 maintained intermediate SOD activity levels, reaching final values of approximately 42 and 45 units g−1, respectively. The enhanced SOD activity in MT-treated fruits indicates that the treatment improved the antioxidant defense system, contributing to reduced oxidative damage and delayed senescence. Luo et al. [43] reported that MT treatment significantly enhanced SOD activity in papaya, improving antioxidant defenses and reducing oxidative damage during storage. Similarly, Bhardwaj et al. [2] found that MT-treated mangoes exhibited higher SOD activity than controls, contributing to delayed senescence and better fruit quality during cold storage. Similarly, the APX activities, which work in conjunction with SOD by detoxifying hydrogen peroxide, showed more pronounced differences among treatments. The initial APX activity was approximately 15 units g−1 across all samples. The control group demonstrated a sharp increase in APX activity between days 10 and 15, reaching about 55 units g−1 by day 15, indicating an acute oxidative stress response. Furthermore, this rapid increase in control samples suggests a sudden activation of the antioxidant defense system, possibly due to accumulated oxidative damage. In contrast, T4 treatment exhibited a more gradual increase in APX activity throughout storage, ultimately reaching the highest level of approximately 73 units g−1 by day 45. This pattern suggests that T4 treatment promoted a more sustained and effective antioxidant response. T3 showed similar trends but with slightly lower final activity, while T1 and T2 maintained lower APX activities throughout storage, reaching final values of approximately 65 and 68 units g−1, respectively. The higher APX activity in MT-treated fruits helped reduce oxidative stress by removing H2O2. This is consistent with Rastegar et al. [4], who found that MT treatment significantly enhanced APX activity in mango fruits during storage, contributing to improved antioxidant defenses and reduced oxidative damage. Bhardwaj et al. [2] also observed higher APX activity in MT-treated mangoes than controls, which was associated with delayed senescence and better maintenance of fruit quality during cold storage. These studies suggest that MT treatment strengthens the antioxidant defense system in mango fruits, thereby mitigating oxidative damage and slowing the onset of senescence.

3.7. Correlation Coefficient and Hierarchical Clustering

The Pearson correlation heatmap illustrates complex interactions among physiological, biochemical, and quality-related attributes in mangoes during storage (Figure 11), highlighting the impact of MT treatments on mango quality retention. The analysis reveals strong interdependencies between antioxidant activity, structural integrity, ripening progression, and oxidative stress, emphasizing the complex dynamics governing fruit quality during storage. Phytochemicals such as TPC, TFC, and AsA form a distinct network with strong positive correlations (TFC-AsA: r = 0.94; TFC-TPC: r = 0.92; AsA-TPC: r = 0.94) and with antioxidant activity assays (ABTS-TPC: r = 0.97; DPPH-TFC: r = 0.93). This tight clustering indicates a coordinated antioxidant response critical for mitigating oxidative stress during storage. MT-treated samples maintained higher antioxidant activity than the control group, as evidenced by the strong correlations with oxidative stress markers (MDA-TPC: r = −0.92; LOX-TFC: r = −0.98; MDA-AsA: r = −0.90). These results suggest that MT enhances the fruit’s natural antioxidant defense, reducing lipid peroxidation and delaying quality loss. The antioxidant system’s effects extend to ripening and structural parameters, with firmness emerging as a central component of the structural integrity network, displaying strong positive correlations with TA (r = 0.96) and SC (r = 0.95). These relationships underline the importance of acidity and starch reserves in maintaining fruit texture during storage. Conversely, firmness showed strong negative correlations with ripening-related variables such as ethylene production (r = −0.90), WL (r = −0.92), and PME (r = −0.87). Color parameters exhibited significant correlations, with L* showing strong relationships with ethylene production (r = −0.91) and firmness (r = 0.93), while a* and b* demonstrated strong correlations with ripening indicators (a*-WL: r = 0.95; b*-WL: r = 0.97). The total color difference (ΔE) showed a strong positive correlation with WL (r = 0.92) and negative correlations with firmness (r = −0.88) and L (r = −0.91), suggesting that as fruit softening and weight loss increased, color changes became more pronounced. Additionally, ΔE was strongly correlated with the enzymatic browning markers PPO (r = 0.94) and POD (r = 0.91), indicating that oxidation-related color degradation contributed to the overall color deviation observed during storage. The enzymatic network revealed complex interactions, with PPO showing strong correlations with oxidative stress markers (PPO-MDA: r = 0.89) and other degradative enzymes (PPO-PME: r = 0.96; PPO-POD: r = 0.94). The antioxidant enzymes SOD and APX displayed strong positive correlations with each other (r = 0.97) and with TPC (SOD-TPC: r = −0.70; APX-TPC: r = −0.68), suggesting their coordinated role in the antioxidant defense network. Hierarchical clustering analysis revealed three distinct functional groups: (1) antioxidant parameters (TFC, DPPH, AsA, SC, TA) forming the tightest cluster, (2) structural and enzymatic parameters (firmness, TPC, ABTS) forming an intermediate cluster, and (3) ripening-related parameters (RR, a*, PME) grouping together. MT treatments enhanced antioxidant activity and reduced oxidative stress, resulting in the delayed ethylene-induced degradation of firmness, starch, and color. The analysis indicates that interventions targeting oxidative stress or ethylene sensitivity create cascading effects across multiple quality attributes, improving storage outcomes. The strong interconnections between these parameters demonstrate the complex nature of fruit quality maintenance and MT treatment effectiveness in modulating these relationships to extend storage life.

4. Conclusions

This study comprehensively demonstrates the significant potential of MT treatment in extending the postharvest life and maintaining the quality of mango fruits during prolonged cold storage. The results revealed that MT treatment effectively controls various physiological and biochemical processes contributing to fruit deterioration. Higher concentrations of MT (T3 and T4) proved particularly effective in reducing weight loss and decay rates, maintaining firmness, and preserving color characteristics. The effectiveness of MT treatment can be attributed to multiple mechanisms, including regulating cell wall-degrading enzymes, enhancing antioxidant defense systems, and modulating ripening-related processes. MT treatment effectively preserved important quality parameters such as the starch content, total soluble solids, and titratable acidity while maintaining higher levels of bioactive compounds and antioxidant activities. The regulation of various enzyme activities, including browning-related enzymes (PPO and POD), cell wall-degrading enzymes (PG, PME, and LOX), and antioxidant enzymes (SOD and APX), suggest that MT’s protective effects operate through multiple pathways. Based on the findings, T4 treatment is effective and recommended, providing the most effective preservation across multiple quality parameters. This concentration is suggested for future studies and potential commercial applications to enhance postharvest quality retention in mangoes during extended storage.

Author Contributions

Conceptualization, K.V. and N.C.; methodology, K.V., S.L. and N.C; software, K.V. and N.C.; validation, K.V., N.C. and S.L.; formal analysis, K.V. and S.L.; investigation, K.V. and S.L.; resources, K.V., N.C. and S.L.; data curation, K.V. and N.C.; writing—original draft preparation, K.V., N.C. and S.L.; writing—review and editing, K.V., N.C. and S.L.; visualization, K.V. and N.C.; supervision, K.V. and N.C.; project administration, K.V.; funding acquisition, K.V. All authors have read and agreed to the published version of the manuscript.

Funding

This research was financially supported by Prince of Songkla University, Surat Thani Campus, 2024.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors sincerely thank Prince of Songkla University, Surat Thani Campus, and Burapha University Chanthaburi Campus for providing the resources and facilities to complete this research. Furthermore, the authors gratefully acknowledge the Center for Food Innovation and Research Laboratory for providing equipment and laboratory support to conduct this research. In addition, the graphical abstract and infographic of the treatment process were created with BioRender (https://biorender.com/).

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Infographic presentation of MT treatment of mango fruit and storage.
Figure 1. Infographic presentation of MT treatment of mango fruit and storage.
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Figure 2. Changes in weight loss (A) and decay rate (B) in MT-treated and untreated mango fruits stored under cold storage.
Figure 2. Changes in weight loss (A) and decay rate (B) in MT-treated and untreated mango fruits stored under cold storage.
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Figure 3. Changes in lightness (A), redness (B), yellowness (C), and total color (D) in MT-treated and untreated mango fruits stored under cold storage.
Figure 3. Changes in lightness (A), redness (B), yellowness (C), and total color (D) in MT-treated and untreated mango fruits stored under cold storage.
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Figure 4. Changes in respiration rate (A), ethylene rate (B), firmness (C), and MDA (D) levels in MT-treated and untreated mango fruits stored under cold storage.
Figure 4. Changes in respiration rate (A), ethylene rate (B), firmness (C), and MDA (D) levels in MT-treated and untreated mango fruits stored under cold storage.
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Figure 5. Changes in starch content (A), total soluble solids (B), titratable acidity (C), and TSS/TA ratio (D) of MT-treated and untreated mango fruits stored under cold storage.
Figure 5. Changes in starch content (A), total soluble solids (B), titratable acidity (C), and TSS/TA ratio (D) of MT-treated and untreated mango fruits stored under cold storage.
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Figure 6. Changes in ascorbic acid (A), total phenolic content (B), and total flavonoid content (C) in MT-treated and untreated mango fruit stored under cold storage.
Figure 6. Changes in ascorbic acid (A), total phenolic content (B), and total flavonoid content (C) in MT-treated and untreated mango fruit stored under cold storage.
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Figure 7. Changes in DPPH (A) and ABTS (B) radical scavenging activity of MT-treated and untreated mango fruits stored under cold storage.
Figure 7. Changes in DPPH (A) and ABTS (B) radical scavenging activity of MT-treated and untreated mango fruits stored under cold storage.
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Figure 8. Changes in PPO (A) and POD (B) activities of MT-treated and untreated mango fruits stored under cold storage.
Figure 8. Changes in PPO (A) and POD (B) activities of MT-treated and untreated mango fruits stored under cold storage.
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Figure 9. Changes in PG (A), PME (B) and LOX (C) activities of MT-treated and untreated mango fruits under cold storage.
Figure 9. Changes in PG (A), PME (B) and LOX (C) activities of MT-treated and untreated mango fruits under cold storage.
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Figure 10. Changes in SOD (A) and APX (B) activities in MT-treated and untreated mango fruits stored under cold storage.
Figure 10. Changes in SOD (A) and APX (B) activities in MT-treated and untreated mango fruits stored under cold storage.
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Figure 11. Pearson’s correlation coefficient heatmap matrix among the different attributes of mango fruit. The purple and yellow colors represent the positive and negative correlations, respectively. TFC = total flavonoid content, TA = titratable acidity, AsA = ascorbic acid, TPC = total phenolic content, SC = starch content, ABTS = ABTS radical scavenging activity, L* = lightness, DPPH = radical scavenging activity, ER = ethylene rate, TSS = total soluble solids, POD = peroxidase, DR = decay rate, MDA = malondialdehyde, LOX = lipoxygenase, WL = weight loss, b* = yellowness, PG = polygalacturonase, RR = respiration rate, PME = pectin methyl esterase, a* = redness, ΔE = total color difference, SOD = superoxide dismutase, PPO = polyphenol oxidase, APX = ascorbate peroxidase.
Figure 11. Pearson’s correlation coefficient heatmap matrix among the different attributes of mango fruit. The purple and yellow colors represent the positive and negative correlations, respectively. TFC = total flavonoid content, TA = titratable acidity, AsA = ascorbic acid, TPC = total phenolic content, SC = starch content, ABTS = ABTS radical scavenging activity, L* = lightness, DPPH = radical scavenging activity, ER = ethylene rate, TSS = total soluble solids, POD = peroxidase, DR = decay rate, MDA = malondialdehyde, LOX = lipoxygenase, WL = weight loss, b* = yellowness, PG = polygalacturonase, RR = respiration rate, PME = pectin methyl esterase, a* = redness, ΔE = total color difference, SOD = superoxide dismutase, PPO = polyphenol oxidase, APX = ascorbate peroxidase.
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Charoenphun, N.; Lekjing, S.; Venkatachalam, K. Effect of Exogenous Melatonin Application on Maintaining Physicochemical Properties, Phytochemicals, and Enzymatic Activities of Mango Fruits During Cold Storage. Horticulturae 2025, 11, 222. https://doi.org/10.3390/horticulturae11020222

AMA Style

Charoenphun N, Lekjing S, Venkatachalam K. Effect of Exogenous Melatonin Application on Maintaining Physicochemical Properties, Phytochemicals, and Enzymatic Activities of Mango Fruits During Cold Storage. Horticulturae. 2025; 11(2):222. https://doi.org/10.3390/horticulturae11020222

Chicago/Turabian Style

Charoenphun, Narin, Somwang Lekjing, and Karthikeyan Venkatachalam. 2025. "Effect of Exogenous Melatonin Application on Maintaining Physicochemical Properties, Phytochemicals, and Enzymatic Activities of Mango Fruits During Cold Storage" Horticulturae 11, no. 2: 222. https://doi.org/10.3390/horticulturae11020222

APA Style

Charoenphun, N., Lekjing, S., & Venkatachalam, K. (2025). Effect of Exogenous Melatonin Application on Maintaining Physicochemical Properties, Phytochemicals, and Enzymatic Activities of Mango Fruits During Cold Storage. Horticulturae, 11(2), 222. https://doi.org/10.3390/horticulturae11020222

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