Exogenous application of melatonin maintains storage quality of jujubes by enhancing anti-oxidative ability and suppressing the activity of cell wall-degrading enzymes

Phytomelatonin: From Intracellular Signaling to Global Horticulture Market

Melatonin (N-acetyl-5-methoxytryptamine), a well-known mammalian hormone, has been having a great relevance in the Plant World in recent years. Many of its physiological actions in plants are leading to possible features of agronomic interest, especially those related to improvements in tolerance to stressors and in the postharvest life of fruits and vegetables. Thus, through the exogenous application of melatonin or by modifying the endogenous biosynthesis of phytomelatonin, some change can be made in the functional levels of melatonin in tissues and their responses. Also, acting in the respective phytomelatonin biosynthesis enzymes, regulating the expression of tryptophan decarboxylase (TDC), tryptamine 5-hydroxylase (T5H), serotonin N-acetyltransferase (SNAT), N-acetylserotonin O-methyltransferase (ASMT), and caffeic acid O-methyltransferase (COMT), and recently the possible action of deacetylases on some intermediates offers promising opportunities for improving fruits and vegetables in postharvest and its marketability. Other regulators/effectors such as different transcription factors, protein kinases, phosphatases, miRNAs, protein-protein interactions, and some gasotransmitters such as nitric oxide or hydrogen sulfide were also considered in an exhaustive vision. Other interesting aspects such as the role of phytomelatonin in autophagic responses, the posttranslational reprogramming by protein-phosphorylation, ubiquitylation, SUMOylation, PARylation, persulfidation, and nitrosylation described in the phytomelatonin-mediated responses were also discussed, including the relationship of phytomelatonin and several plant hormones, for chilling injury and fungal decay alleviating. The current data about the phytomelatonin receptor in plants (CAND2/PMTR1), the effect of UV-B light and cold storage on the postharvest damage are presented and discussed. All this on the focus of a possible new action in the preservation of the quality of fruits and vegetables.

Melatonin extends shelf life in postharvest okra via delaying fruit softening and reducing weight loss

BACKGROUND

Melatonin, a hormone present in animals and some plants, has garnered attention for its potential in preserving harvested produce. Softening due to changes in cell wall composition and wilting caused by weight loss are the major reasons for the loss of commercial value in postharvest okra. This study aimed to evaluate the impact of melatonin on the softening and weight loss of postharvest okra.

RESULTS

The results revealed that the application of melatonin had a significant influence on the maintenance of fruit firmness by inhibiting the breakdown and dissolution of cell wall polysaccharides by suppressing the expression of specific genes responsible for cell wall degradation in okra. Conversely, melatonin treatment positively influenced the expression of genes involved in the synthesis of cell wall components. Furthermore, the treatment exhibited notable benefits in reducing weight loss in okra, which was accomplished by promoting the closure of stomata - the tiny pores on the surface of the fruit.

CONCLUSION

Melatonin could serve as a novel approach to reduce water loss, delay fruit softening and extend the shelf life of okra. © 2024 Society of Chemical Industry.

Melatonin improves the storage quality of rabbiteye blueberry (Vaccinium ashei) by affecting cuticular wax profile

Cuticular wax is the first line of structural defense for plants against external stresses. This study investigated the effects of melatonin (MT) on chemical composition and accumulation profile of wax, as well as fruit quality of rabbiteye blueberry during storage. The results indicated a significant reduction in the overall wax content during storage. Nevertheless, MT effectively delayed the decline, with a higher amount of 9.8% and 15.17% in the treated 'Baldwin' and 'Garden Blue' compared to their respective controls at 21st day of storage. The wax composition significantly varied depending on storage time, MT treatment, and cultivars. Additionally, MT markedly improved the fruit quality of rabbiteye blueberries. Correlation analysis revealed water loss and decay rates were negatively correlated with triterpenoids and fatty acids. Taken together, this study highlights the positive effects of post-harvest MT application on shelf life and fruit quality of blueberry by modifying the wax profile during storage.

Effect of exogenous melatonin on antioxidant properties and fruit softening of 'Fengtang' plum fruit (Prunus salicina Lindl.) during storage at room temperature

'Fengtang' plums soften quickly and lose flavor after harvest. This study comprehensively evaluated the effect of exogenous melatonin on the fruit quality of 'Fengtang' plums. According to our findings, exogenous melatonin prevented plum fruit from losing water, delayed the decline in firmness, and preserved a high TSS/TA level. Additionally, exogenous melatonin also enhanced the activity of antioxidant enzymes and increased the non-enzymatic antioxidants, thereby further increasing the antioxidant capacity of plum fruit. Notably, exogenous melatonin delayed the degradation of covalent soluble pectin (CSP), cellulose, and hemicellulose, as well as the rise in water-soluble pectin (WSP) concentration and the activity of cell wall degrading enzymes. Further investigation using atomic force microscopy (AFM) revealed that the chain-like structure of ionic-soluble pectin (ISP) and the self-assembly network structures of CSP were depolymerized, and melatonin treatment retarded the depolymerization of pectin structures. Our results showed that exogenous melatonin preserved the postharvest quality of plum fruits by controlling fruit softness and antioxidant capacity during storage.

Sucrose, cell wall, and polyamine metabolisms involve in preserving postharvest quality of 'Zaosu' pear fruit by L-glutamate treatment

'Zaosu' pear fruit is prone to yellowing of the surface and softening of the flesh after harvest. This work was performed to assess the influences of L-glutamate treatment on the quality of 'Zaosu' pears and elucidate the underlying mechanisms involved. Results demonstrated that L-glutamate immersion reduced ethylene release, respiratory intensity, weight loss, brightness (L*), redness (a*), yellowness (b*), and total coloration difference (ΔE); enhanced ascorbic acid, soluble solids, and soluble sugar contents; maintained chlorophyll content and flesh firmness of pears. L-glutamate also restrained the activities of neutral invertase and acid invertase, while enhancing sucrose phosphate synthetase and sucrose synthase activities to facilitate sucrose accumulation. The transcriptions of PbSGR1, PbSGR2, PbCHL, PbPPH, PbRCCR, and PbNYC were suppressed by L-glutamate, resulting in a deceleration of chlorophyll degradation. L-glutamate concurrently suppressed the transcription levels and enzymatic activities of polygalacturonases, pectin methylesterases, cellulase, and β-glucosidase. It restrained polygalacturonic acid trans-eliminase and pectin methyl-trans-eliminase activities as well as inhibited the transcription levels of PbPL and Pbβ-gal. Moreover, the gene transcriptions and enzymatic activities of arginine decarboxylase, ornithine decarboxylase, S-adenosine methionine decarboxylase, glutamate decarboxylase, γ-aminobutyric acid transaminase, glutamine synthetase along with the PbSPDS transcription was promoted by L-glutamate. L-glutamate also resulted in the down-regulation of PbPAO, PbDAO, PbSSADH, PbGDH, and PbGOGAT transcription levels, while enhancing γ-aminobutyric acid, glutamate, and pyruvate acid contents in pears. These findings suggest that L-glutamate immersion can effectively maintain the storage quality of 'Zaosu' pears via modulating key enzyme activities and gene transcriptions involved in sucrose, chlorophyll, cell wall, and polyamine metabolism.

Carbon monoxide is involved in melatonin-enhanced drought resistance in tomato seedlings by enhancing chlorophyll synthesis pathway

BACKGROUND

Drought is thought to be a major abiotic stress that dramatically limits tomato growth and production. As signal molecule, melatonin (MT) and carbon monoxide (CO) can enhance plant stress resistance. However, the effect and underlying mechanism of CO involving MT-mediated drought resistance in seedling growth remains unknown. In this study, tomato (Solanum lycopersicum L. 'Micro-Tom') seedlings were used to investigate the interaction and mechanism of MT and CO in response to drought stress.

RESULTS

The growth of tomato seedlings was inhibited significantly under drought stress. Exogenous MT or CO mitigated the drought-induced impairment in a dose-dependent manner, with the greatest efficiency provided by 100 and 500 µM, respectively. But application of hemoglobin (Hb, a CO scavenger) restrained the positive effects of MT on the growth of tomato seedlings under drought stress. MT and CO treatment promoted chlorophyll a (Chl a) and chlorophyll a (Chl b) accumulations. Under drought stress, the intermediate products of chlorophyll biosynthesis such as protoporphyrin IX (Proto IX), Mg-protoporphyrin IX (Mg-Proto IX), potochlorophyllide (Pchlide) and heme were increased by MT or CO, but uroporphyrinogen III (Uro III) content decreased in MT-treated or CO-treated tomato seedlings. Meanwhile, MT or CO up-regulated the expression of chlorophyll and heme synthetic-related genes SlUROD, SlPPOX, SlMGMT, SlFECH, SlPOR, SlChlS, and SlCAO. However, the effects of MT on chlorophyll biosynthesis were almost reversed by Hb.

CONCLUSION

The results suggested that MT and CO can alleviate drought stress and facilitate the synthesis of Chl and heme in tomato seedlings. CO played an essential role in MT-enhanced drought resistance via facilitating chlorophyll biosynthesis pathway.

Nanovehicles for melatonin: a new journey for agriculture

The important role of melatonin in plant growth and metabolism together with recent advances in the potential use of nanomaterials have opened up interesting applications in agriculture. Various nanovehicles have been explored as melatonin carriers in animals, and it is now important to explore their application in plants. Recent findings have substantiated the use of silicon and chitosan nanoparticles (NPs) in targeting melatonin to plant tissues. Although melatonin is an amphipathic molecule, nanocarriers can accelerate its uptake and transport to various plant organs, thereby relieving stress and improving plant shelf-life in the post-harvest stages. We review the scope and biosafety concerns of various nanomaterials to devise novel methods for melatonin application in crops and post-harvest products.

Exogenous Melatonin Promotes Cold Tolerance in Grape Seedlings: Physiological, Transcriptomic, and Functional Evidence

Melatonin (MEL) is an antioxidant molecule that enhances plant tolerance to environmental stress. However, the mechanisms by which MEL regulates cold signaling pathways in grapes under cold stress remain elusive. Here, we investigated the physiological and transcriptomic changes in grape seedlings treated with exogenous MEL to determine their protective role under cold stress. Results showed that 150 μM MEL effectively attenuated cold-induced cell damage by reducing reactive oxygen species (ROS) and preserving the chloroplast structure and function. MEL also inhibited tannin degradation, which contributed to its protective effect. Exogenous MEL promoted the synthesis of endogenous MEL, abscisic acid, auxin, and cytokinin while inhibiting gibberellin. Transcriptomic profiling revealed 776 differentially expressed transcripts in MEL-treated samples compared to controls. Functional analysis of a candidate hub gene, VvHSFA6b, showed that its overexpression in grape calli enhances cold tolerance by activating jasmonic acid synthesis pathway genes, promoting JA accumulation, and inhibiting JAZ-repressed transcription factors.

Melatonin treatment delays the softening of blueberry fruit by modulating cuticular wax metabolism and reducing cell wall degradation

The effects of postharvest melatonin (MT) treatment on cuticular wax and cell wall metabolism in blueberry fruit (Vaccinium spp.) were evaluated. The results revealed that MT treatment maintained the cuticular wax rod-like structure and delayed wax degradation. The gas chromatography-mass spectrometry analysis results revealed that MT application changed the cuticular wax composition in blueberries, and 25 metabolic components were screened. The metabolic regulation of wax quality in blueberry fruit may therefore be influenced by MT. Additionally, MT slowed down pectin and cellulose degradation by reducing the activities of cell wall degrading enzymes like pectin methyl esterase polygalacturonase, β-galactosidase, and cellulose in the later stages of storage. It also downregulated the transcriptional expression of related genes like VcPE, VcPG, VcBG6, and VcGAL1. Thus, MT prevented softening and senescence by postponing the degradation of the cell wall in postharvest blueberry fruit.

Prolong the postharvest shelf life of spinach through the antioxidative ability of melatonin

Spinach is also known as Persian cuisine, it is rich in nutrients such as protein, vitamin C and minerals, and has high nutritional value. In this study, Spinach was treated with melatonin in order to prolong its shelf life. Melatonin has strong antioxidant effects as an endogenous free radical scavenger. The spinach was sprayed with 0.10, 0.20 and 0.30 mg/mL melatonin solution after harvesting, and distilled water was used as control for low temperature storage at 4 °C. The results showed that melatonin spraying Spinach delayed the degradation of chlorophyll, especially the treatment of 0.20 mg/mL melatonin was the most effective. The content of soluble sugar and soluble protein in spinach tissue was kept high, the accumulation of malondialdehyde (MDA) was reduced, and the activities of superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) were increased. These findings suggested that melatonin treatment may be a useful technique to prolong the postharvest life of spinach and improve its quality.

Exogenous melatonin activates the antioxidant system and maintains postharvest organoleptic quality in Hami melon (Cucumis. melo var. inodorus Jacq.)

Hami melon is prone to postharvest perishing. Melatonin is a signaling molecule involved in a variety of physiological processes in fruit, and it improves fruit quality. We hypothesized that melatonin treatment would improve the storage quality of Hami melon by altering its respiration and reactive oxygen species (Graphical abstract). Our results indicated that optimal melatonin treatment (0.5 mmol L-1) effectively slowed the softening, weight loss, and respiratory rate of the Hami melon fruit. Furthermore, melatonin markedly improved the antioxidant capacity of the fruit and protected it from oxidative damage by decreasing its contents of superoxide anions, hydrogen peroxide, and malondialdehyde. Melatonin significantly enhanced the activities of superoxide dismutase, catalase, ascorbate peroxidase, and peroxidase. The total phenol, total flavonoids, and ascorbic acid contents were maintained by melatonin treatment. This treatment also repressed the activities of lipase, lipoxygenase, and phospholipase D, which are related to lipid metabolism. Thus, exogenous melatonin can maintain postharvest organoleptic quality of Hami melon fruit by increasing its antioxidant activity and inhibiting reactive oxygen species production.

Biochemistry behind firmness retention of jujube fruit by combined treatment of acidic electrolyzed water and high-voltage electrostatic field

Harvested jujube (Zizyphus jujuba Mill) is prone to softening due to active metabolism. This study investigated the effects of acidic electrolyzed water (AEW), high-voltage electrostatic field (HVEF) and their combination (AEW + HVEF) on softening and associated cell wall degrading enzymes (CWDEs), cell membrane integrity and antioxidant system of 'Huping' jujube during storage at 0 ± 1 °C. The results indicated that fruit subjected to AEW + HVEF, AEW or HVEF treatments maintained firmness 15.7%, 10.7%, and 5.3% higher than that of untreated control fruit at the end of 90 days cool storage. Fruit treated with AEW + HVEF could better maintain cell membrane integrity and exhibit lower activities of CWDEs and higher antioxidant capacity than that treated with either AEW or HVEF. Correlation analysis suggested that inhibition of softening was associated with reduction of CWDEs activities, and maintenance of membrane integrity and antioxidant system.

Melatonin Treatment Maintains the Quality of Fresh-Cut Gastrodia elata under Low-Temperature Conditions by Regulating Reactive Oxygen Species Metabolism and Phenylpropanoid Pathway

The application of melatonin (MT) has been shown to improve the quality during the storage of fruits and vegetables. The primary objective of this study is to investigate the effects of MT on the quality of fresh-cut Gastrodia elata during low-temperature (4 °C) storage. The results indicated that MT treatment not only suppressed the respiratory rate and malondialdehyde content but also slowed down the decline in total acidity and total soluble solids, effectively inhibiting microbial growth and enhancing the product safety of fresh-cut G. elata. The treatment with MT reduced the superoxide anions and hydrogen peroxide production, as well as inhibiting the activity and expression of peroxidase and polyphenol oxidase. Additionally, it led to increased activity and the expression of antioxidant-related enzymes, including superoxide dismutase, catalase, ascorbate peroxidase, glutathione reductase, monodehydroascorbate reductase, and dehydroascorbate reductase, while also resulting in elevated levels of ascorbic acid and glutathione. Furthermore, the treatment with MT induced an increase in the total phenolic and flavonoid content of fresh-cut G. elata and enhanced the activity and expression of key enzymes involved in the phenylpropanoid pathway (phenylalanine ammonia-lyase, cinnamate-4-hydroxylase, 4-coumarate: CoA ligase). In summary, MT enhances the antioxidant capacity by activating both the ROS metabolism and phenylpropanoid pathway, thus maintaining the quality of fresh-cut G. elata.

Glycine betaine inhibits postharvest softening and quality decline of winter jujube fruit by regulating energy and antioxidant metabolism

Winter jujube fruit easily softens after harvest. To investigate the effects of glycine betaine (N,N,N-trimethylglycine; GB) treatment on the quality of postharvest jujubes, fresh winter jujubes (Zizyphus jujuba Mill. cv. Dongzao) were immersed in 20 mmol·L^-1 GB for 20 min. The results showed that GB application can effectively maintain cell wall component content by restraining gene expression and enzyme activities, including PG, CX, PME and β-Glu. Meanwhile, the activities of antioxidant enzymes (APX, CAT, SOD, POD) and the contents of nonenzymatic antioxidants (MDA, H₂O₂, ASA, GSH) were enhanced in treated jujubes, thereby reducing the content of ROS. In addition, energy metabolism enzyme activities (H⁺-ATPase, Ca²⁺-ATPase, SDH and CCO) and gene expression were also significantly increased, thus maintaining higher energy levels (ATP, ADP, AMP and EC). In summary, GB enhances ATP biosynthesis by increasing energy metabolism. It offers essential energy for the antioxidant metabolism, thus retarding the softening of postharvest jujubes.

CaCl2 treatment effectively delays postharvest senescence of passion fruit

This study aimed to systematically investigate the changes in peel color, physicochemical characteristics, textural properties, and peel ultrastructure between CaCl₂-treated and water-soaked passion fruit under short-term storage at room temperature (20 °C) for eight days. The fruit peel was further analyzed and compared for the differences in calmodulin (CaM) gene expression between the two groups. The data were analyzed using principal component analysis. The results confirmed that CaCl₂ treatment effectively maintained the appearance and color of passion fruit, inhibited peel browning, and improved fruit quality. The treatment had an effect on maintaining the physiological properties of passion fruit parenchyma, effectively delayed the passion fruit senescence, and kept the structural integrity of the fruit peel. The relative expression of PeCaM gene in the CaCl₂-treated fruit peels was higher than that of the control peels. The Ca²⁺ stimulated the relative expression of the PeCaM gene, which delayed the senescence of passion fruit.

Exogenous Melatonin Spray Enhances Salinity Tolerance in Zizyphus Germplasm: A Brief Theory

Fruit orchards are frequently irrigated with brackish water. Irrigation with poor quality water is also a major cause of salt accumulation in soil. An excess of salts results in stunted growth, poor yield, inferior quality and low nutritional properties. Melatonin is a low molecular weight protein that shows multifunctional, regulatory and pleiotropic behavior in the plant kingdom. Recently, its discovery brought a great revolution in sustainable fruit production under salinity-induced environments. Melatonin contributed to enhanced tolerance in Zizyphus fruit species by improving the plant defense system's potential to cope with the adverse effects of salinity. The supplemental application of melatonin has improved the generation of antioxidant assays and osmolytes involved in the scavenging of toxic ROS. The tolerance level of the germplasm is chiefly based on the activation of the defense system against the adverse effects of salinity. The current study explored the contribution of melatonin against salinity stress and provides information regarding which biochemical mechanism can be effective and utilized for the development of salt-tolerant germplasm in Zizyphus.

Functions of Melatonin during Postharvest of Horticultural Crops

Melatonin, a tryptophan-derived molecule, is endogenously generated in animal, plant, fungal and prokaryotic cells. Given its antioxidant properties, it is involved in a myriad of signaling functions associated with various aspects of plant growth and development. In higher plants, melatonin (Mel) interacts with plant regulators such as phytohormones, as well as reactive oxygen and nitrogen species including hydrogen peroxide (H2O2), nitric oxide (NO) and hydrogen sulfide (H2S). It shows great potential as a biotechnological tool to alleviate biotic and abiotic stress, to delay senescence and to conserve the sensory and nutritional quality of postharvest horticultural products which are of considerable economic importance worldwide. This review provides a comprehensive overview of the biochemistry of Mel, whose endogenous induction and exogenous application can play an important biotechnological role in enhancing the marketability and hence earnings from postharvest horticultural crops.

UV-C delays senescence in 'Lingwu long' jujube fruit by regulating ROS and phenylpropanoid metabolism

Ultraviolet (UV-C), a no residual environmentally friendly physical treatment, plays an important role in delaying the senescence in fruit. In this study, 'Lingwu long' jujubes were treated with UV-C (5 kJ m^-2) to investigate the impacts of cell wall degrading enzymes (CWDEs) activities, reactive oxygen species (ROS) metabolism, and phenylpropanoid metabolism under storage at 4 ± 1 °C for 30 d. UV-C treatment reduced respiration rate and decay index. Treated fruit exhibited lower polygalacturonase (PG), pectinate lyases (PL), cellulase (Cel), and β-galactosidase (β-gal) activities which ultimately delayed the reduction of firmness. UV-C treatment increased hydrogen peroxide (H₂O₂), free radical scavenging ability, and superoxide dismutase (SOD) and catalase (CAT) activities, reduced superoxide anion (O₂^-) and malondialdehyde (MDA) content. In addition, ascorbate peroxidase (APX), glutathione reductase (GR), dehydroascorbate reductase (DHAR), and monodehydroascorbate reductase (MDHAR) activities were activated by UV-C treatment, leading to glutathione (GSH) and ascorbic acid (AsA) increased. Besides, phenolic compounds of jujube fruit treated with UV-C were also increased, which might be due to the enhanced phenylalanine ammonia-lyase (PAL), cinnamate-4-hydroxylase (C4H), and 4-coumarate-CoA ligase (4CL) activities. In conclusion, UV-C was recommended for improving overall quality and alleviating senescence in jujube fruit.

Melatonin Treatment Affects Wax Composition and Maintains Storage Quality in 'Kongxin' Plum (Prunus salicina L. cv) during Postharvest

Cuticular wax is an essential barrier against biological and abiotic stress and is also an important factor affecting fruit storage quality. This paper investigated the effect of melatonin treatment on cuticular wax and the storage quality of plum fruit at low temperature storage of 4 ± 1 °C. 'Kongxin' plum was treated with 150 μmol·L^-1 melatonin, dried overnight at room temperature 25 ± 1 °C, and then stored at 4 ± 1 °C for 40 d. The microstructure of the fruit epidermis was examined after 0, 20, and 40 d of storage, and the wax composition and fruit storage quality were measured at 10 d intervals. The results demonstrated that melatonin promoted the disintegration and thickening of rod-shaped waxy crystals of 'Kongxin' plum fruit and inhibited the combination of disintegrated wax and inner wax. Melatonin maintained fruit firmness and decreased the correlation between fruit firmness and other storage quality parameters. The correlation between firmness and wax composition was enhanced. Melatonin promoted long-chain alkanes that were positively correlated with firmness and water retention and strengthened the correlation between the length of the alkane chain and storage quality parameters but reduced the difference between alkane isomers and storage quality parameters.

Exogenous melatonin treatment on post-harvest jujube fruits maintains physicochemical qualities during extended cold storage

This study was conducted to investigate the visual appearance and physicochemical changes of postharvest jujube fruits (Ziziphus jujuba Mill. cv. Shucuizao) stored under 0 °C for 15 days. The fruits were dipped in 0 (control), 50, 100, 200 and 400 µM melatonin solutions after harvest. The results showed that treatment with a suitable melatonin concentration improved the rate of crisp fine fruits, delayed weight loss and firmness decline, and suppressed changes in total soluble solids (TSS) and titratable acidity (TA) contents of jujube fruits compared with the control. In addition, jujube fruits soaked with melatonin showed improved antioxidant capacity through increased ascorbic acid (AsA) content, enhanced superoxide dismutase (SOD) activity, and decreased malonaldehyde (MDA) content. As a result, 50 µM melatonin showed the greatest improvement of visual appearance and quality maintenance, and could be used as an effective treatment to preserve postharvest jujube fruit.

Interactions of melatonin, reactive oxygen species, and nitric oxide during fruit ripening: an update and prospective view

Fruit ripening is a physiological process that involves a complex network of signaling molecules that act as switches to activate or deactivate certain metabolic pathways at different levels, not only by regulating gene and protein expression but also through post-translational modifications of the involved proteins. Ethylene is the distinctive molecule that regulates the ripening of fruits, which can be classified as climacteric or non-climacteric according to whether or not, respectively, they are dependent on this phytohormone. However, in recent years it has been found that other molecules with signaling potential also exert regulatory roles, not only individually but also as a result of interactions among them. These observations imply the existence of mutual and hierarchical regulations that sometimes make it difficult to identify the initial triggering event. Among these 'new' molecules, hydrogen peroxide, nitric oxide, and melatonin have been highlighted as prominent. This review provides a comprehensive outline of the relevance of these molecules in the fruit ripening process and the complex network of the known interactions among them.

Functions and prospects of melatonin in plant growth, yield, and quality

Melatonin (N-acetyl-5-methoxytryptamine) is an indole molecule widely found in animals and plants. It is well known that melatonin improves plant resistance to various biotic and abiotic stresses due to its potent free radical scavenging ability while being able to modulate plant signaling and response pathways through mostly unknown mechanisms. In recent years, an increasing number of studies have shown that melatonin plays a crucial role in improving crop quality and yield by participating in the regulation of various aspects of plant growth and development. Here, we review the effects of melatonin on plant vegetative growth and reproductive development, and systematically summarize its molecular regulatory network. Moreover, the effective concentrations of exogenously applied melatonin in different crops or at different growth stages of the same crop are analysed. In addition, we compare endogenous phytomelatonin concentrations in various crops and different organs, and evaluate a potential function of phytomelatonin in plant circadian rhythms. The prospects of different approaches in regulating crop yield and quality through exogenous application of appropriate concentrations of melatonin, endogenous modification of phytomelatonin metabolism-related genes, and the use of nanomaterials and other technologies to improve melatonin utilization efficiency are also discussed.

Melatonin treatment reduces qualitative decay and improves antioxidant system in highbush blueberry fruit during cold storage

BACKGROUND

Blueberry is considered as a 'functional food' because it contains bioactive compounds such as flavonoids, phenolic acids, tannins and anthocyanins. The blueberry is one of the most consumed berries in the world and is highly appreciated by consumers because of its unique taste and sensory properties. Fresh blueberries decay rapidly because of mould and water loss. To preserve the qualitative and nutraceutical traits of fresh highbush blueberries during storage, the efficacy of 1 mm melatonin treatment was investigated at 5 °C for 3 weeks.

RESULTS

The results demonstrated that melatonin treatment reduced weight loss and delayed postharvest ripening. Compared to the control, melatonin treatment induced an overproduction of polyphenols, flavonoids, anthocyanins and ascorbic acid, consequently increasing antioxidant activity. The enzymatic antioxidant system was also affected by the treatment. An increase in the activity of catalase, superoxide dismutase and ascorbate peroxidase was observed in treated fruit compared to that in control fruit. Enzymatic browning, controlled by assaying the content of malondialdehyde and hydrogen peroxide, polyphenol oxidase, guaiacol peroxidase and lipoxygenase activities, appeared to slow down under melatonin treatment.

CONCLUSION

Melatonin coating is a valid tool for delaying the perishability and qualitative decay of highbush blueberry fruit during cold storage. Furthermore, this treatment increases the production of secondary metabolites such as polyphenols, flavonoids, anthocyanins and ascorbic acid, improving the nutraceutical traits of this fruit during storage. Melatonin treatment can be considered as an environmentally sustainable, non-harmful-to-human-health alternative for the postharvest preservation of highbush blueberry fruit. © 2022 Society of Chemical Industry.

Melatonin Alleviates Chilling Injury Symptom Development in Mango Fruit by Maintaining Intracellular Energy and Cell Wall and Membrane Stability

The efficacy of the signaling molecule melatonin for alleviating chilling injury (CI) in mango (Mangifera indica L.) fruit was studied to investigate the potential role of membrane integrity, energy charge, and ripening-related changes in the development of CI, and its management by melatonin. ‘Langra’ and ‘Gulab Jamun’ cultivar mango fruit was immersed in 100 μM of melatonin before storage for 28 days at 5°C with weekly transfers to shelf life at 25°C. CI symptom development was associated with compositional and enzymatic aspects of textural changes, cell membrane deterioration, and chemical energy status. Melatonin-treated ‘Langra’ fruit exhibited very low CI (5 vs. 21%) while ‘Gulab Jamun’ fruit exhibited higher CI (36 vs. 38%) during 28 days of storage at 5 ± 1°C. Higher chilling tolerance in melatonin-treated ‘Langra’ was associated with lower softening, ascribed to lower cell wall degrading exo- and endo-polygalacturonase, pectinesterase, and endo-1,4-β-D-glucanase. In addition, lower membrane deteriorating-phospholipase D and lipoxygenase activity in melatonin-treated ‘Langra’ corresponded to lower palmitic and stearic acids and higher oleic, linoleic, and linolenic acids accumulation, thus, higher unsaturated/saturated fatty acids ratio. Additionally, there was a higher intracellular energy supply with melatonin, represented by a higher adenylate energy charge (AEC) arising from higher ATP and ADP and lower AMP accumulation, related to higher H⁺-ATPase, Ca²⁺-ATPase, succinate dehydrogenase, and cytochrome c oxidase activities. This study for the first time provides evidence, suggesting that melatonin alleviation of CI is related to the preservation of membrane integrity, thereby protecting the intracellular energy supply, and preserving cell wall integrity via impeding cell wall degrading enzyme activities.

Melatonin treatment maintains the quality of cherry tomato by regulating endogenous melatonin and ascorbate-glutathione cycle during room temperature

Changes in quality attributes, ascorbate-glutathione (AsA-GSH) cycle, and melatonin (MLT) synthesis were evaluated in cherry tomato fruit treated with MLT solution at 0.1 mM during storage at room temperature for 16 days. According to the results, the MLT treatment was beneficial to maintaining fruit quality as indicated by the declines in weight loss, fruit decay, and titratable acid (TA), accompanied by the maintenance of fruit firmness, total soluble solids (TSS) as well as TSS/TA ratio. Also, the MLT treatment not only effectively inhibited oxidative damage via reducing relative electrolyte leakage and malondialdehyde content, but also improved antioxidant capacity via stimulating AsA-GSH cycle. Moreover, the MLT treatment promoted endogenous MLT synthesis by upregulating the expressions of biosynthetic genes consisting of SlTDC, SlT5H, SlSNAT, and SlASMLT. Thus, our results suggested that the MLT treatment might be involved in maintaining quality in cherry tomato fruit during room temperature by promoting antioxidant capacity and enhancing endogenous MLT. PRACTICAL APPLICATIONS: As a typical climacteric fruit, cherry tomato fruit ripen rapidly and are easily infected by various pathogenic fungi during storage under ambient conditions, which leads to short storage life and a decrease in economic value. The results showed that the application of MLT maintained cherry tomato quality via improving antioxidant capacities and enhancing endogenous MLT. Therefore, MLT treatment could become a promising postharvest strategy for quality maintenance in cherry tomatoes during room storage.

Co-Application of 1-MCP and Laser Microporous Plastic Bag Packaging Maintains Postharvest Quality and Extends the Shelf-Life of Honey Peach Fruit

Honey peach (Prunus persica L.) is highly nutritious; it is an excellent source of sugars, proteins, amino acids, vitamins, and mineral elements. However, it is a perishable climacteric fruit that is difficult to preserve. In this study, “Feicheng” honey peach fruit was used as a test material to investigate the synergistic preservation effect of 1-methylcyclopropene (1-MCP) and laser microporous film (LMF). The peach fruits were fumigated for 24 h with 2 μL L⁻¹ 1-MCP, then packed in LMF. In comparison with the control treatment, 1-MCP + LMF treatment markedly decreased the respiration rate, weight loss, and rot rate of peach fruits. Moreover, the combination of 1-MCP and LMF suppressed the increase in soluble solids (SS) and reducing sugars (RS), as well as the decrease in titratable acid (TA) and ascorbic acid (AsA). The combined application also maintained a high protopectin content and low soluble pectin content; it reduced the accumulation of superoxide anions (O₂⁻) and hydrogen peroxide (H₂O₂). Except in a few samples, the catalase (CAT) and ascorbate peroxidase (APX) activities were higher when treated by 1-MCP + LMF. Conversely, the phenylalanine deaminase (PAL), peroxidase (POD), lipase, lipoxygenase (LOX), polygalacturonase (PG), β-glucosidase, and cellulase (Cx) activities were lower than in the control. Furthermore, 1-MCP + LMF treatment reduced the relative abundances of dominant pathogenic fungi (e.g., Streptomyces, Stachybotrys, and Issa sp.). The combined treatment improved the relative abundances of antagonistic fungi (e.g., Aureobasidium and Holtermanniella). The results indicated that the co-application of 1-MCP and LMF markedly reduced weight loss and spoilage, delayed the decline of nutritional quality, and inhibited the physiological and biochemical metabolic activities of peach during storage. These changes extended its shelf-life to 28 days at 5 °C. The results provide a reference for the commercial application of this technology.

Introducing melatonin to the horticultural industry: physiological roles, potential applications, and challenges

Melatonin (N-acetyl-5-methoxytryptamine) is an emerging biomolecule that influences horticultural crop growth, flowering, fruit ripening, postharvest preservation, and stress protection. It functions as a plant growth regulator, preservative and antimicrobial agent to promote seed germination, regulate root system architecture, influence flowering and pollen germination, promote fruit production, ensure postharvest preservation, and increase resistance to abiotic and biotic stresses. Here, we highlight the potential applications of melatonin in multiple aspects of horticulture, including molecular breeding, vegetative reproduction, production of virus-free plants, food safety, and horticultural crop processing. We also discuss its effects on parthenocarpy, autophagy, and arbuscular mycorrhizal symbiosis. Together, these many features contribute to the promise of melatonin for improving horticultural crop production and food safety. Effective translation of melatonin to the horticultural industry requires an understanding of the challenges associated with its uses, including the development of economically viable sources.

Influence of sequential exogenous pretreatment and contact ultrasound-assisted air drying on the metabolic pathway of glucoraphanin in broccoli florets

In this investigation, the combinations of exogenous pretreatment (melatonin or vitamin C) and contact ultrasound-assisted air drying were utilized to dry broccoli florets. To understand the influences of the studied dehydration methods on the conversion of glucoraphanin to bioactive sulforaphane in broccoli, various components (like glucoraphanin, sulforaphane, myrosinase, etc.) and factors (temperature and moisture) involved in the metabolism pathway were analyzed. The results showed that compared with direct air drying, the sequential exogenous pretreatment and contact ultrasound drying shortened the drying time by 19.0-22.7%. Meanwhile, contact sonication could promote the degradation of glucoraphanin. Both melatonin pretreatment and vitamin C pretreatment showed protective effects on the sulforaphane content and myrosinase activity during the subsequent drying process. At the end of drying, the sulforaphane content in samples dehydrated by the sequential melatonin (or vitamin C) pretreatment and ultrasound-intensified drying was 14.4% (or 26.5%) higher than only air-dried samples. The correlation analysis revealed that the exogenous pretreatment or ultrasound could affect the enzymatic degradation of glucoraphanin and the generation of sulforaphane through weakening the connections of sulforaphane-myrosinase, sulforaphane-VC, and VC-myrosinase. Overall, the reported results can enrich the biochemistry knowledge about the transformation of glucoraphanin to sulforaphane in cruciferous vegetables during drying, and the combined VC/melatonin pretreatment and ultrasound drying is conducive to protect bioactive sulforaphane in dehydrated broccoli.

Induced sulfur metabolism by sulfur dioxide maintains postharvest quality of 'Thompson Seedless' grape through increasing sulfite content

BACKGROUND

The commercial preservation of table grapes largely depends on the application of sulfur dioxide (SO₂ ). However, little is known about whether SO₂ participates in sulfur metabolism to improve the postharvest quality of table grapes. In this study, the contents of sulfur-containing compounds, activities of enzymes, and expression of genes involved in sulfur metabolism in table grapes (Vitis vinifera cv. Thompson Seedless) were evaluated.

RESULTS

The results indicated that SO₂ treatment maintained the postharvest quality of table grapes. The sulfite content in rachises and berries, but not the sulfate content, increased in response to SO₂ treatment. SO₂ caused high activities of sulfite reductase, O-acetylserine (thiol)-lyase, and γ-glutamylcysteine synthetase, thereby increasing the contents of cysteine, hydrogen sulfide, and glutathione in the rachises and berries. The expression of VvSURTL, VvATPS1, VvATPS2, and VvAPR3 decreased in response to SO₂ treatment; however, the transcript levels of VvSiR1 and VvOASTL exhibited the opposite tendency.

CONCLUSION

These findings indicated that the sulfite converted from SO₂ participated in sulfur metabolism and maintained the postharvest quality of table grapes by modulating the contents of metabolites, activities of enzymes, and expression of genes related to sulfur metabolism. © 2021 Society of Chemical Industry.

Melatonin treatment improves postharvest quality and regulates reactive oxygen species metabolism in "Feizixiao" litchi based on principal component analysis

Postharvest quality of litchi reduces rapidly during storage at room temperature. This study aimed to investigate the effect of melatonin treatment on postharvest quality and oxidative stress markers of litchi fruit during cold storage. The "Feizixiao" litchi was treated with melatonin solution concentrations of 0.2 and 0.6 mmol·L^-1 and then stored at 4°C for 12 days. The results confirmed that the melatonin treatment effectively maintained the appearance and color of the litchi fruit, suppressed the peel browning, and improved the litchi quality. The treatment also significantly enhanced the levels of endogenous melatonin, antioxidant components (total phenolics, flavonoids, and anthocyanin), and antioxidant enzyme activities of the fruit. It also inhibited the other oxidative stress markers, such as O 2 - , H₂O₂, MDA, and protein carbonyl content, and upregulated the expressions of antioxidant and Msr-related genes. Correlation and principal component analyses further confirmed that the melatonin treatment effectively delayed the fruit senescence by enhancing the antioxidant enzyme activities and modulating peel browning and reactive oxygen species metabolism of the litchi fruit via regulating gene expression of the related enzymes (SOD and PPO). These findings suggested that the exogenous application of melatonin to litchi during the postharvest is an ideal way to preserve the fruit quality and delay fruit senescence.

Attenuation of Postharvest Browning in Rambutan Fruit by Melatonin Is Associated With Inhibition of Phenolics Oxidation and Reinforcement of Antioxidative Process

Rambutan is a famous tropical fruit with a unique flavor and considerable economic value. However, the high vulnerability to postharvest browning leads to a short shelf life of rambutan fruit. Melatonin (MT) is an excellent bioactive molecule that possesses the potential to improve the storability of the harvested crops. In this study, the physiological mechanism of exogenous MT in affecting pericarp browning and senescence of postharvest rambutan fruit was investigated. Experimental results showed that the application of MT at 0.125 mmol L^-1 appreciably retarded the advancement of pericarp browning and color parameters (L*, a*, and b*). MT treatment inhibited the increase in membrane relative electrolytes leakage (REL) while lowering the accumulation of reactive oxygen species (ROS) (_■O₂ ^- and H₂O₂) and malonaldehyde (MDA). Reduced phenolics oxidation, as indicated by higher contents of total phenolics, flavonoids, and anthocyanins along with fewer activities of peroxidase (POD) and polyphenol oxidase (PPO), was detected in MT fruit compared with control fruit. MT treatment maintained the cellular redox state by inducing antioxidant enzyme activity and reinforcing the ascorbate-glutathione (AsA-GSH) cycle. Furthermore, the ultrastructural observation revealed that the spoilage of cellular and subcellular structures was milder in MT fruit than that in control fruit. The results suggest that MT could ameliorate the browning and senescence of rambutan fruit by inhibiting phenolic oxidation and enhancing the antioxidative process.

Exogenous Application of Melatonin and Methyl Jasmonate as a Pre-Harvest Treatment Enhances Growth of Barhi Date Palm Trees, Prolongs Storability, and Maintains Quality of Their Fruits under Storage Conditions

Fresh date palm fruits (cv. Barhi) have received much attention due to their sweet taste and popularity in marketing. There is a critical need to prolong their storability, as well as maintain their quality during the postharvest and marketing periods. In this study, the effects of spraying date palm trees with melatonin (Mt) and/or methyl jasmonate (Mj) at 10, 20, and 50 ppm, on their growth and yield were investigated. In addition, impacts on quality and storability of the fruits were also studied. In general, application of Mt was mostly more effective than that of Mj, even at 50 ppm, with regard to all evaluated parameters. However, the dual treatment at 50 ppm recorded the highest relative chlorophyll and nutrient content in date palm leaves, as well as the yield and its components. Regarding the date palm fruits stored at 4 °C for 28 days, this dual treatment recorded the lowest weight loss and fruit decay values (0.14 and 2%, respectively), the highest firmness (6 g·cm⁻²), total soluble solids content (36 °Brix), total sugar content (32.5 g/100 g fresh weight), and the lowest total acidity (0.16 g citric acid/100 mL juice). Moreover, the highest total phenolic content and activity of peroxidase and polyphenol oxidase enzymes in the stored fruits were also recorded for the dual treatment. In contrast to the untreated fruits, scanning electron microscopy observations showed that the sprayed fruits had a very good microstructure, showing intact and thick exocarp tissue with a dense layer of epicuticular wax. The mesocarp tissue showed a normal and clear cellular framework with well organized and arranged cells, after 28 days storage at 4 °C. Based on these results, we can conclude that application of the dual treatment (Mt + Mj) at 50 ppm is a promising way to prolong the storability of date palm fruits and maintain their quality during storage periods.

Inhibitory Effects of CaCl2 and Pectin Methylesterase on Fruit Softening of Raspberry during Cold Storage

Quality of raspberry fruit experiences a rapid decline after harvest due to its vulnerable texture and high moisture content. Application of calcium chloride (CaCl2) combined with pectin methylesterase (PME) is efficient in delaying fruit softening. In this study, the effects of exogenous CaCl2 alone or in combination with PME on the structure of the cell wall, the molecular properties of pectin, and the amount of free water of raspberry during postharvest storage were investigated. The results showed that CaCl2 combined with PME treatment could maintain fruit firmness and inhibit weight loss. The treatment of CaCl2+PME maintained the cell wall structure via sustaining middle lamella integrity and reducing the activities of cell wall-degrading enzymes, such as polygalacturonase, pectin methylesterase, β-galactosidase, α-L-arabinofuranosidase, and β-xylosidase. In addition, CaCl2+PME treatment could effectively increase the content of chelate-soluble pectin (CSP) and develop a cross-linked structure between Ca2+ and CSP. Moreover, CaCl2+PME treatment was of benefit in maintaining free water content. CaCl2 in combination with PME treatment could be a promising method for inhibiting softening and maintaining the quality of postharvest raspberry during cold storage.

Geographical and Varietal Traceability of Chinese Jujubes Based on Physical and Nutritional Characteristics

Chinese jujube fruits are known for their high nutritional and functional values. To protect advantageous regional jujube fruits, it is important to monitor quality indicators and trace the origin and variety. In this study, 31 quality indicators of Chinese jujubes collected from 6 main producing areas were determined. According to different origins and varieties, Chinese jujube fruits were divided into five and six categories, respectively. To simplify the parameters, eight of the main characteristics, namely, soluble sugar content, fresh mass, edible rate, Na, Mg, K, Zn, and cyclic adenosine monophosphate (cAMP), were screened based on multiple comparison, correlation analysis, and principal component analysis (PCA). According to the eight main parameters, it was found that that both the categorical and cross-validated classification accuracy of linear discriminant analysis (LDA) were 100%. The discrimination accuracy of the testing set samples based on the orthogonal partial least squares-discriminant analysis (OPLS-DA) model were 90 and 93% for geographical and varietal classification, respectively. This indicated that the eight main parameters could be used as the characteristic parameters for the origin and variety traceability of Chinese jujubes.

Melatonin Treatment Delays Senescence and Maintains the Postharvest Quality of Baby Mustard (Brassica juncea var. gemmifera)

The effect of melatonin treatment on the visual quality and content of health-promoting compounds in baby mustard (Brassica juncea var. gemmifera) at 20°C was investigated in this study. Application of 100 μmol L^-1 melatonin was the most effective in prolonging the shelf life of baby mustard among all of the concentrations tested (1, 50, 100, and 200 μmol L^-1). The 100 μmol L^-1 melatonin treatment also delayed the increase in weight loss and the decrease in sensory parameter scores; retarded the decline of chlorophyll content; slowed the decline in antioxidant capacity by maintaining the content of carotenoids and ascorbic acid, as well as increasing the levels of total phenolics; and increased the content of individual and total glucosinolates in the lateral buds of baby mustard. These findings indicate that melatonin treatment is effective for maintaining the sensory and nutritional qualities of postharvest baby mustard.