Innovative postharvest strategies for maintaining the quality of kiwifruit during storage: An updated review

Abstract

Being a respiratory climacteric fruit, kiwifruit is susceptible to age and decay rapidly in the postharvest stage. Therefore, the development of efficient postharvest methods to maintain the kiwifruit quality has been a long‐standing goal. This review summarizes the preservation and disease control methods of kiwifruit conducted over the past 5 years, and the characteristics, advantages, and action mechanisms of various methods are thoroughly discussed. Physical, chemical, and biotechnological methods, such as low‐temperature, essential oil, and endophytic yeast treatment, can enhance postharvest kiwifruit quality to a certain extent by controlling disease, delaying chilling injury, alleviating oxidative damage, inhibiting oversoftening and off‐flavor development. However, all these techniques have limitations per se, such as the inability to prevent secondary infections and potential side effects on human health. Novel approaches such as pulsed light and cold plasma or a synergistic application of several techniques may be the future direction for kiwifruit postharvest preservation.

INTRODUCTION

Kiwifruit (Actinidia deliciosa), a fruit native to China, belongs to the Actinidiaceae family. According to the Food and Agriculture Organization (FAO, 2022), the worldwide annual production rate of kiwifruit is nearly 4.5 million metric tons from the whole planted area of 286.1 thousand hectares. Green-fleshed kiwifruit has dominated the global market even though kiwifruit encompasses a variety of species and has a wide range of flesh and skin colors. However, red-flesh and yellow-flesh kiwifruit have gained popularity in recent years. Kiwifruit is favored by consumers for its flavor and nutritional qualities, as it boasts high levels of vitamin C and a well-balanced nutritional composition of minerals, carbs, dietary fibers, omega-3 fatty acids, and antioxidants (Wang, Vanga et al., 2019).

Kiwifruit is a typical respiratory climacteric fruit with noticeable physiological characteristics, such as high respiration and ethylene production rates, softening, and wrinkling of fruits after the ripening process. Kiwifruit is sensitive to low temperatures; therefore, postharvest low-temperature operations that induce chilling injuries cause losses to the fruit at both quantitative and qualitative scales (Yang, He et al., 2023). Kiwifruit is highly susceptible to the infection of pathogenic fungi, such as Botryosphaeria sp., Phomopsis sp., and Botrytis cinerea. In addition, oxidative damage, oversoftening, and off-flavor development also shorten the shelf life of kiwifruit and cause substantial quality loss, which reduces revenues and restricts the kiwifruit industry expansion (Huan et al., 2020).

The widespread application of synthetic fungicides has led to the emergence of resistant fungi strains. In response to the growing public attention to food safety, human health, and environmental protection, it is crucial to explore safe, effective, and environment-friendly alternative or integrative approaches. In recent years, some feasible treatments have been applied to delay kiwifruit senescence, which is generally grouped into four categories, namely, physical-based, chemical-based, biopreservation, and combined technologies (Figure 1). Cold storage is currently a commonly employed physical technique to improve kiwifruit shelf-life, which exhibits high efficiency but also demands significant energy consumption (Yang, He et al., 2023). Other emerging physical, such as controlled atmosphere storage, electron irradiation, and hypobaric treatment, along with the application of spraying and soaking chemical agents like 1-methylcyclopropene (1-MCP), essential oil (EO), and natural compounds, have also been employed to preserve the freshness and extend the shelf-life of kiwifruit (Huang et al., 2022; Li, Yang et al., 2021). Novel biopreservation techniques, including antagonistic microorganisms and endophyte microorganisms, can effectively control postharvest diseases while minimizing negative impacts on nutritional and sensory properties (Qiu et al., 2022).

[IMAGE OMITTED. SEE PDF]

The previous reviews mainly focused on kiwifruit postharvest fungal decays and their management strategies (Dai et al., 2022; Li, Zeng et al., 2022). Other postharvest problems, such as chilling injury, oxidative damage, oversoftening, and off-flavor development, as well as associated management strategies, are still not fully understood, which could lead to significant economic loss during fruit storage, transportation, and marketing. This review retrieved relevant literature published in the last 5 years from the Web of Science, PubMed, and Scopus databases. Advances in postharvest procedures to improve kiwifruit storage and shelf life are summarized and discussed in this study, along with their potentials and limits, particularly the mechanism of these treatments. Furthermore, the possibility of additional research is highlighted to direct future research.

FACTORS INFLUENCING KIWIFRUIT QUALITY DURING POSTHARVEST STORAGE

Kiwifruit deteriorates fast during postharvest storage mainly due to the damages caused by chilling injury and microbial spoilage, and a series of typical aging symptoms, such as tissue softening, oxidative damage, and off-flavor, also shorten the shelf life of kiwifruit.

Chilling injury

The most extensively utilized kiwifruit preservation technology for long-term storage is low-temperature storage, which can successfully limit flesh softening and extend kiwifruit storage life. However, kiwifruit is susceptible to low temperatures and exhibits chilling injury when stored at their freezing point for long periods, resulting in brown tissue and grainy in the outer pericarp, surface pitting, and pathogen proliferation, these physiological disorders become most severe after transferring to ambient temperature (Jin et al., 2021). Chilling injury is related to the changes of the membrane phospholipids, and low temperature induces the change of cell membrane-saturated lipids from a liquid-crystalline to a solid-gel state, thus resulting in an elevation of membrane permeability and ion leakage. Furthermore, the oxidation of membrane lipids and a decrease in the degree of membrane unsaturation may be responsible for skin browning and increased electrolyte leakage. Kiwifruit is unable to develop adaptation mechanisms at low temperatures, leading to fruit physiological disorders as well as rapid softening during cold storage (Liang et al., 2020). The critical temperature for chilling injury varies among different kiwifruit varieties. For example, Ruiyu and Hayward exhibit lower supercooling points at −3.4 and −3.2°C, respectively. Xuxiang, Jinfu, and Cuixiang have similar supercooling points at approximately −2.0, −1.7, and −1.7°C, respectively, while Hongyang has the highest supercooling point at −1.4°C (Li, Wang et al., 2021). The development of cold chains during fruit transportation necessitates alternate treatments for kiwifruit, a cold-sensitive species, to minimize postharvest low-temperature damage.

Microbial spoilage

Decay is another prominent factor for postharvest loss, which impeds the storability of kiwifruit, especially when storage temperature is higher. Kiwifruit is highly vulnerable to various fungal pathogens, including B. cinerea, Botryosphaeria dothidea, Alternaria alternata, Penicillium expansum, and Diaporthe phaseolorum, with B. cinerea being the most detrimental pathogen (Liu et al., 2020). Gray mold induced by B. cinerea typically begins at the stem end of the undamaged fruit and spreads uniformly to the distal end. The affected area of the fruit seems darker than the healthy area, and white to gray mycelium and sporulation growth might appear. Internally, the infected tissue becomes saturated with water and turns dark green, conidiophores with sclerotia and conidia can develop into infected fruits. In the later stages of the disease, mycelium links neighboring fruits to form disease nests. However, it has been noticed that immature fruits tend not to rot immediately. In contrast, as the fruit matures during the ripening and storage period, the pathogen starts to grow rapidly (Ge et al., 2020). On average, the fruit rot rate remains around 20% owing to gray mold, but in extreme circumstances, it can reach 30%, resulting in huge economic losses during postharvest storage, shipping, and marketing, particularly in developing countries (Di Francesco et al., 2018). Therefore, there is an urgent need to develop effective measures to control kiwifruit postharvest losses.

Oxidative damage

Reactive oxygen species (ROS) are the main cause of fruit aging. ROS, such as hydrogen peroxide, hydroxyl radicals, superoxide anion, and singlet oxygen, are highly reactive molecules that may cause oxidative damage to kiwifruit biomacromolecules, including DNA, proteins, and lipids. ROS are primarily produced by mitochondria, which are complex organelles in eukaryotic cells that play key roles in a variety of cellular processes, including adenosine triphosphate synthesis, apoptosis, calcium homeostasis, β-oxidation, and cell signaling. Therefore, mitochondrial components, such as proteins, exhibit heightened susceptible to oxidative damage, leading to the dysfunction of various mitochondrial components and ultimately accelerating aging (Li, Yang et al., 2021). Furthermore, when the fruit is attacked by fungal pathogens, one of the early host reactions is an oxidative burst, with significant amounts of ROS produced by several host enzyme systems (Zhu et al., 2008). Given the significance of ROS in fruit aging and fungal pathogenicity, it is crucial to maintain a balance between ROS production and antioxidant systems, particularly by enhancing the activity of low-molecular-mass antioxidants, ROS-scavenging enzymes, and enzymes regenerating the active forms of antioxidants (Xia et al., 2016).

Softening

The decline in firmness is a prominent manifestation of fruit softening, and the softening mode of kiwifruit has long been characterized as sigmoidal, with an initial slow phase, a rapid second phase, and a final slow phase. Fruit softening is initiated and sustained by developmental and hormonal signals and caused by changes in various physiological and metabolic activities, such as cell wall structural disintegration, water loss, pectin degradation, and membrane damage. The decomposition of cell wall structure, including a series of polysaccharide components like cellulose, pectin, and hemicellulose, characterizes fruit softening (Burdon et al., 2017). Early and quick fruit softening has a negative impact on fruit quality, shelf life, resistance to postharvest diseases, transportation capability, and consumer acceptability, ultimately lowering the fruit's commercial worth.

Off-flavor

The occurrence and perception of off-flavors in kiwifruit are closely related to the over-accumulation of sugar fermentation-related metabolites such as ethanol and acetaldehyde. In addition, other acyl-CoAs ethanol may also act as a substrate for subsequent esterification reactions catalyzed by alcohol acyl transferases, leading to the accumulation of ethyl ester volatiles (Schwab et al., 2008). The off-flavor volatiles typically accumulate at low levels during fruit ripening and play a pivotal role in fruit aroma volatile biosynthesis. However, following exposure to high CO₂/low O₂, long-term storage, and low-temperature period, the high accumulation of these metabolites leads to the perception of off-flavors, toxicity, and physiological disorders related to flesh browning (Ali et al., 2019). Therefore, regulating the metabolism of sugar fermentation to control the accumulation of acetaldehyde and ethanol is beneficial for maintaining postharvest kiwifruit quality.

INNOVATIVE POSTHARVEST STRATEGIES FOR KIWIFRUIT QUALITY MAINTENANCE

Physical treatment

Physical preservation technology refers to the method of manipulating environmental factors such as temperature, pressure, and gas composition to extend the shelf life, which has gained worldwide attention in shelf life extension of kiwifruit due to their complete absence of residues as well as minimal health and environmental impacts.

Low-temperature storage

Low-temperature storage is widely used to extend kiwifruit postharvest life and maintain nutritional value, as low temperature slows cell metabolic activities, particularly ethylene production and respiration rates. Such key processes are related to metabolism and maturation, involved in regulating fruit ripening and senescence. Cold storage has a significant impact on the physicochemical and quality characteristics of kiwifruit. Under low-temperature storage, flesh firmness, dry matter content, and starch decreased, soluble solids content (SSC) and total sugar contents (TSC) first increased sharply and then maintained at a generally stable level. Moreover, vitamin C content generally increased at first and then declined slightly (Shahkoomahally & Ramezanian, 2015). Both the weight loss and decay rate of “Donghong” kiwifruit under cold storage demonstrated an incredibly slow increasing trend when compared to storage at room temperature. Besides, refrigerated “Donghong” kiwifruit samples had a higher aroma and flavor intensity and total phenolic content than that of room-temperature ripened samples (Huang et al., 2019).

Storage temperature plays a pivotal role during low-temperature storage. Because kiwifruit is extremely sensitive to freezing points and exhibits damaging effects when stored at an improper temperature. The expression of genes involved in ethylene biosynthesis (e.g., AdACS, AdACO1, and AdACO2) and ethylene transcription factors (e.g., AdETR1 and AdERF1) after 12 weeks of storage at 0°C (fruit core) was significantly lower than their expression at −0.8 and −0.5°C, indicating that the increase in ethylene production is related to the increase in chilling injury. Therefore, a temperature of 0°C is the optimal temperature for long-term storage of kiwifruit (Afshar-Mohammadian et al., 2019). For many varieties, it is still necessary to investigate more efficient preservation techniques because chilling injury cannot be completely eradicated despite several attempts to diminish it in low-temperature storage.

Controlled atmosphere storage

Storing kiwifruit in a controlled environment (CA) is a common practice for expanding marketing windows; specifically, an elevated CO₂ concentration is synergistic with low O₂ and is considered an effective complement to proper temperature management. The optimal CA condition (4.2%–4.8% CO₂ and 14.0%–14.8% O₂) effectively enhanced the activity of antioxidant enzymes, inhibited malondialdehyde (MDA) increase, suppressed ethylene production and lipoxygenase (LOX) activity, ultimately alleviating chilling injury and browning of “Cuixiang” kiwifruit. Moreover, the mitigation of browning was concomitant with a delay in polyphenol oxidation, which was achieved by inhibiting polyphenol oxidase (PPO) activity (Jiao et al., 2021). The short-term anoxic treatment (24 h) can impede maturation by sustaining elevated levels of enzymatic antioxidants, including superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX), and increasing the level of nonenzymatic antioxidants such as total phenolics, flavonoids, anthocyanins, and ascorbic acid (AsA) (Ying et al., 2024). However, care must be taken when stored in overly high levels of CO₂ or even low levels of O₂, as there is a risk of reducing storage life due to the development of anaerobic metabolites such as acetaldehyde, ethanol, and lactic acid (Botondi et al., 2012). Although it is an optimal physical storage method, the application of CA storage is limited due to high technical requirements and large capital investment.

Ozone treatment

The Food and Drug Administration (FDA) has approved ozone as an antibacterial agent for direct contact with foods due to its ability to degrade the cell wall and plasma membrane of pathogenic fungi, as well as to kill some pathogens, thereby reducing the presence of pathogens in fruits and vegetables. Kiwifruit exposed to 79.4 ppm gaseous ozone for 1 h per day for 7 days exhibited significant inhibition of spore germination and mycelial development in both B. cinerea and P. expansum. Furthermore, the ozone-treated kiwifruit maintained elevated levels of titratable acidity (TA) and fruit firmness while also experiencing remarkable enhancement in defense-related enzyme activity, such as phenylalanine ammonia-lyase (PAL), PPO, POD, β-glucanase (GLU), and chitinase (CHI) (Luo et al., 2019). Furthermore, the ozone-treated fruit demonstrated significant reductions in cell wall swelling and softening by inhibiting the activity of cell wall-modifying enzymes, downregulating the expression of related genes involved in cell wall metabolism, maintaining a high level of cellulose and protopectin content, and keeping a low level of water-soluble pectin (Wang et al., 2024). The application of ozone also suppressed the excessive generation of ROS and the biosynthesis of MDA treatment by enhancing the activity of enzymes involved in enzymatic antioxidant metabolism, thereby effectively activating kiwifruit's protective mechanism against oxidative stress during cold storage (Wang et al., 2023).

Heat treatment

Heat treatment is another effective non-chemical approach to managing postharvest decay in various types of fruits. Fungi subjected to heat stress exhibit a time/dosage-dependent accumulation of intracellular ROS, resulting in oxidative damage to protein and lipids. Additionally, another mode of action has been reported to involve the activation of host defense responses, including the induction of pathogenesis-related proteins, antioxidant enzymes, and antifungal compounds (Zhao et al., 2014). Both hot water and hot air treatments at 70°C for 3 min were proved to be the most efficacious conditions in inhibiting mycelial (88% and 71%, respectively) and conidial growth (100% and 91.3%, respectively) of Cadophora luteo-olivacea isolates, with a significant reduction observed in pectinase and xylanase enzyme activities. Moreover, hot air treatment is exhibited as a more efficient method in reducing the incidence of skin pitting, exhibiting an average efficacy rate of 46.6% (Di Francesco et al., 2022).

Electron irradiation treatment

Electron irradiation is a novel, nonthermal, and eco-friendly processing technology that holds great promise as a physical preservation method. Its principle is to irradiate the surface of fruits or vegetables with an electron beam generated by an accelerator, and it can trigger a series of physical and chemical reactions to effectively eliminate microorganisms, inhibit physiological processes, and extend the shelf life of harvested products (Ye et al., 2024). According to the International Atomic Energy Agency, the approved irradiation dose for fresh fruits is ≤1 kGy. Electron-beam irradiation retarded the decline in firmness and the increase of SSC in “Xuxiang” kiwifruit, 500 Gy electron-beam irradiation exhibited optimal preservation effects by significantly inhibiting ROS metabolism and membrane lipid oxidation (Li, Yang et al., 2021). The 400 Gy electron-beam irradiation effectively suppressed the increase of weight loss, respiration rate, SSC, MDA accumulation, chlorophyll decomposition, and ethylene production, slowed down TA and moisture content loss, and significantly enhanced the activities of PAL and POD. Compared to untreated kiwifruit, electron-beam irradiation treatment at a dose of 400 Gy extended the shelf life for 20 days (Yang, Li et al., 2022; Ye et al., 2023). Although electron irradiation appears to be one of the most promising alternatives to chemical treatments due to its penetrating power, uniformity, and effectiveness in pathogen inactivation, in addition to safety concerns, irradiated products present issues such as the emergence of resistant strains, increased costs, and reduced nutrient content in treated products.

Hypobaric treatment

Hypobaric treatment is a promising preservation technology that maintains fruit quality during postharvest storage by increasing the pressure differential between internal and external environments to accelerate the outward diffusion of gases in fruit tissue, such as ethylene, ethanol, and CO₂. Application of the hypobaric treatment (25 ± 5 kPa for 30 min) twice on “Bruno” kiwifruit effectively retarded weight loss and fruit decay, preserved organic acids and total phenolic concentrations, and suppressed off-flavor development during storage at an ambient temperature. This is attributed to the enhancement of aerobic metabolism through increasing energy levels and activities of succinic dehydrogenase, cytochrome c oxidase, and H⁺-adenosine triphosphatase. Additionally, it inhibits anaerobic metabolism by reducing acetaldehyde and ethanol concentrations as well as the activities of pyruvate decarboxylase (PDC) and alcohol dehydrogenase (ADH) (Huan et al., 2021).

Among the physical methods, electron irradiation, heat, and ozone treatment showed remarkable application perspectives due to their direct and indirect effects against pathogens. Unfortunately, physical treatments cannot stop secondary infections of diseases despite having the power to directly manage or kill microorganisms.

Chemical treatment

Chemical preservation is achieved by spraying, soaking, or immersing chemical agents, such as 1-MCP, EO, and melatonin. Their main beneficial effects and related molecular mechanisms are discussed in detail below.

1-MCP

The ripening process of kiwifruit is primarily influenced by changes in ethylene-regulated gene expression. The application of 1-MCP, an effective ethylene sensing inhibitor that binds irreversibly to ethylene receptors, has been reported to significantly retard fruit ripening and senescence. Besides, 1-MCP is nontoxic, with negligible residue and high activity at low concentrations, providing high margins of safety for consumers (Xia et al., 2021). For the hardy kiwifruit variety “Cheongsan,” treatment with 1-MCP resulted in a downregulation of senescence-related genes AcACO, AcACS, and AcLOX (Figure 2), leading to reduced respiration and ethylene production (Lim et al., 2016). However, ethylene production and related genes varied among different parts of 1-MCP-treated kiwifruit, the expressions of AcACS1 and AcETR2 genes were found to be highest in the core, whereas those of AcACO2 and AcACO3 genes were observed to be the highest in the seedless pulp during harvest. Ethylene was detected in the seeded and core pulp after 3 and 9 days of storage, respectively, while remaining undetected in the seedless pulp (Liu, Chen et al., 2021). During storage, 1-MCP had a persistent anti-ethylene action that was equally effective in the outer, inner pericarp, and core tissues (Gong et al., 2020).

[IMAGE OMITTED. SEE PDF]

1-MCP treatment induces a wide range of physiological responses that not only retard ripening and senescence but also improve storage quality. A suitable concentration of 1-MCP can maintain high SSC, inhibit the decline of TA and vitamin C content, delay chlorophyll degradation, and reduce the water loss rate (Ruiz-Aracil et al., 2023; Tang et al., 2019). 1-MCP treatment resulted in the suppression of starch and sucrose metabolism during storage, thereby reducing nutrients and metabolic substrate consumption. This effect can be attributed to the inhibition of key enzymes involved in carbohydrate metabolism in kiwifruit treated with 1-MCP, including β-amylase (β-AMY), acid invertase (AI), neutral invertase (NI), sucrose phosphate synthase (SPS), sucrose synthetase (SS), phosphofructokinase, and sorbitol dehydrogenase (Xiong et al., 2023). 1-MCP treatment at a concentration of 0.8 µL/L enhanced the accumulation of total phenolics and flavonoids, as well as increased the activities of defense-related enzymes PPO and PAL, contributing to defense responses against postharvest pathogens (especially Phomopsis sp.) (Xia et al., 2021). Additionally, the application of 1-MCP (1.0 µL/L) to “Hayward” kiwifruit exhibited a remarkable ability to maintain flesh firmness for up to 180 days during both conventional cold storage (0°C) and shelf life (20°C) (Quillehauquy et al., 2020). The expressions of key regulators AcNAC3 and AcNAC5 genes were significantly suppressed by the application of 1-MCP, leading to a further reduction in the ripening-regulated expression of cell wall-related genes, such as polygalacturonase (PG), pectate lyase (PL), and expansin (EXP) (Figure 2). PG and PL have been identified as responsible enzymes for the depolymerization and solubilization of pectic backbones in cell wall polysaccharides. Specifically, PL cleaves de-esterified pectins through a b-elimination reaction, whereas PG hydrolytically cleaves unesterified pectin's α-1,4-galacturonosyl linkages. EXP is a nonenzymatic cell wall active protein that induces extension and stress relaxation of plant cell walls. Together, these factors inhibit wall hydrolysis (Mitalo et al., 2019). Notably, the degradation of cell wall components (cellulose, hemicellulose, and pectin) was also delayed by 1-MCP through the reduction of cell-wall-modifying enzyme activity, including cellulase (Cx), GLU, α-galactosidases, and β-galactosidases (β-Gal) (Xiong et al., 2023).

1-MCP also exerts a significant impact on the production of fruit volatiles and the development of off-flavors. The aroma volatiles (especially esters) are regulated by the endogenous ethylene signaling pathway, AcNAC5 expression correlated well with aroma volatile biosynthesis and the regulation of associated genes (Figure 2). 1-MCP treatment (1.0 µL/L) inhibited the formation of kiwifruit characteristic aroma by inhibiting the synthesis of ester, such as methyl butanoate, ethyl butanoate, E-2-hexanal, and hexenal, attributing to the inhibition of AcLOX, AcHPL, and AcAAT during fruit ripening (Wang et al., 2021). In contrast, treatment with 0.5 µL/L of 1-MCP not only preserved the fresh and fruity aromas but also did not hinder the development of kiwifruit fruity aroma throughout storage (Chen et al., 2020). Furthermore, 1.0 µL/L of 1-MCP treatment avoided the alcohol off-flavor development by inhibiting key enzymes involved in ethanol fermentation and γ-aminobutyric acid (GABA) shunt pathways, such as ADH, PDC, GABA-transaminase, and glutamate decarboxylase (Ali et al., 2021). Besides, 1-MCP treatment improved mitochondrial energy metabolism and functional tricarboxylic acid cycle, specifically regulating oxidative phosphorylation to maintain higher levels of adenosine triphosphate content and energy charge, which contributed to the prevention of ethanol fermentation and accumulation (Ali et al., 2020).

Besides, 1-MCP exerted a favorable impact on the inhibited production of ROS by enhancing antioxidant metabolism (Figure 2), thereby mitigating oxidative stress damage caused on the membrane, which includes lipid peroxidation, protein damage, DNA damage, and finally cell death. The activities of antioxidant enzymes, including APX, SOD, and CAT, were higher in kiwifruit treated with 1-MCP (Xu et al., 2021). 1-MCP treatment has been shown to enhance the activities of monodehydroascorbate reductase (MDHAR), glutathione reductase (GR), and dehydroascorbate reductase (DHAR), attributing to maintaining a strong reduced peroxidized ratio of AsA and glutathione (GSH), which is necessary for efficient scavenging of ROS within cells (Mittler, 2002). Moreover, the increase in AsA level is attributed to the upregulation of genes involved in AsA biosynthesis, particularly through the l-galactose pathway, key genes including AdGME, AdGalDH, and AdGalLDH involved in this pathway were significantly upregulated in 1-MCP-treated kiwifruit. The upregulation of AsA accumulation in kiwifruit by 1-MCP was demonstrated to include the d-galacturonic acid pathway, as evidenced by the enhanced expression of AdGalUR (Zhang et al., 2021).

Moreover, 1-MCP has been applied to prevent the occurrence of chilling injury in harvested kiwifruit. The application of 0.3 µL/L 1-MCP in combination with 1.0 µL/L ethylene mitigated the chilling injury of “Xuxiang” kiwifruit by regulating ethylene biosynthesis, reducing electrolyte leakage and MDA content, and enhancing the activities of CAT and APX (Liu, Pei et al., 2021). The chilling injury resulted in a more severe water-soaked appearance and increased lignification in the pulp tissue. 1-MCP also reduced and delayed the accumulation of lignin in flesh tissues, and more pronounced lignification was observed in core tissues. PAL and POD activities involved in lignin synthesis were increased in both flesh and core tissues, whereas cinnamyl ADH activity was enhanced in the core but decreased in the flesh (Suo et al., 2018).

Nitric oxide

Nitric oxide (NO) is a crucial signaling compound in eukaryotes, playing a vital role in various plant growth and development processes, such as acting as a chemical elicitor capable of inducing disease resistance in postharvest fruits. The 15 µL/L of NO treatment mitigated disease symptoms in postharvest kiwifruit inoculated with B. dothidea, and the disease incidence was about 50% of untreated fruit. The differentially expressed genes involved in phenylalanine metabolism, mitogen-activated protein kinase signaling pathway, calcium signal decoding, plant hormone signal transduction, and pathogenesis-related protein may jointly promote the resistance of kiwifruit to soft rot (Yang, Wang et al., 2022). NO also inhibited cell wall degradation by suppressing the expression of pectinesterase, PG, PL, and β-Gal, thus maintaining cell structure integrity. The upregulation of cellulose synthase genes and downregulation of starch degradation genes (β-AMY) may also be the cause of delayed kiwifruit softening. NO may also impede the expression of ACO-related genes as well as reduce the levels of ethylene receptors and ethylene-responsive transcription factors, thereby inhibiting both ethylene biosynthesis and signal transduction pathways (Yang et al., 2021). Sodium nitroprusside, an exogenous donor of NO, can enhance kiwifruit quality and disease resistance. The 100 µmol/L sodium nitroprusside treatment decreased the chilling injury index, elevated firmness, and activated phosphoinositide-specific phospholipase C activity, resulting in the induction of 1,4,5-trisphosphate production. Sodium nitroprusside treatment also resulted in a reduction in electrolyte leakage and MDA content as well as LOX activity and gene expression. Additionally, it induced gene expression of transcription factors, including C-repeat binding factor1 (CBF1), WRKY1, and NAC5 (Jiao, 2021).

Essential oil

EOs are intricate natural blends of volatile secondary metabolites possessing a strong sensorial impact, and the main constituents are mono and sesquiterpenes, along with alcohols, ethers, carbohydrates, ketones, and aldehydes. The high hydrophobicity and antimicrobial nature of EOs make it possible to apply them as green food preservatives. Precisely, EOs hydrophobicity permits them to alter the structures of cell membranes and mitochondria lipids, resulting in the loss of ions and other cellular contents, and the excessive leakage or loss of critical molecules and ions ultimately leads to the death of bacterial and fungal cells (Nazzaro et al., 2013). Most EOs components exhibit varying degrees of antifungal activity against postharvest kiwifruit except for farnesol. The carbon chain structures and length exert significant influence on the antifungal activities of EOs components. The presence of aldehyde, phenolic hydroxyl, and C2 = C3 groups significantly enhances the antifungal potency, but an increase in the carbon chain length of saturated fatty aldehyde results in a decrease in antifungal activity. Among all the components of EOs, carvacrol exhibits the most potent antifungal activity on B. dothidea (EC₅₀ = 12.6 µL/L) and Diaporthe fusicola (EC₅₀ = 11.2 µL). Carvacrol had strong oxidation-reduction properties that disrupt redox homeostasis in fungal cells, leading to the inhibition of respiration. The cytoplasmic and mitochondrial membranes were also the primary targets for carvacrol against the pathogen, carvacrol inhibited the formation of lipid components on the membrane, increased the permeability of cell membrane, and destroyed intracellular homeostasis including ion and biomacromolecules, thus leading to the pathogen cell death (Li, Fu et al., 2021).

However, EOs application is greatly limited for their strong volatility and susceptibility to oxidation. Encapsulation of EOs into nanoemulsions could effectively prevent oxidative deterioration and achieve slow release for prolonged action time, which may greatly promote the practical application of EOs for kiwifruit preservation. Lemon EO-based nanoemulsion exhibited significant inhibitory effects on spore germination and mycelia growth of kiwifruit Phomopsis sp., and the enhancement of intracellular antioxidant enzyme activity, ROS buildup, and cell death are all possible explanations for this inhibitory action. In addition, the development of soft rot in kiwifruit was inhibited by a lemon EO-based nanoemulsion in a dose-dependent manner, with the best effect being seen at a 1% loading quantity (Meng et al., 2022). In summary, compared with traditional chemical fungicides, EOs are considered to be safe, biodegradable, environmentally friendly, and non-phytotoxic, which renders them promising natural agents for postharvest disease control in kiwifruit. However, strict concentration control is necessary for large-scale commercial use of EOs as antifungal agents due to potential issues related to unpleasant odors, phytotoxicity, or limited technological implementation.

Coating

Applying coatings on kiwifruit creates a partial barrier to water movement, reducing moisture loss from the fruit surface. Additionally, it also establishes a modified atmosphere surrounding the fruit that retards respiration and senescence. Most importantly, it controls physiological diseases and reduces the growth of microbes (Zhang, Li et al., 2020). Natural-based edible coatings can maintain sensory characteristics without the addition of potentially hazardous chemicals to human health and the environment, three different coatings, including grape, berries, and dates juice solutions, were applied to kiwifruit samples by the quasi-static load, and the results showed that 5–10 days in dark conditions with the grape juice coatings was the best way for kiwifruit storage because grape juice treatment resulted in the highest phenolic content (99.3 mg/100 g), the lowest pH (2.8), and the highest stiffness (15.0 N) during storage for 10 days (Azadbakht et al., 2021). Lacquer wax, a significant fatty resource extracted from the mesocarp and seed of Toxicodendron vernicifluum berries, contains eicosanoic, hexadecanedioic, and dodecanedioic acid. The 2% lacquer wax-coated kiwifruit exhibited a slower ripening process compared to uncoated samples by inhibiting the loss of firmness, organic acids, and antioxidant activity, decreasing respiratory rate, as well as delaying the increase in the levels of ethylene, MDA, and TSC (Hu et al., 2019).

As a linear cationic polysaccharide (poly-β-(1→4)-N-acetyl-d-glucosamine) obtained through partial deacetylation of chitin alkali medium to varying degrees, chitosan is generally deemed safe by the FDA. Chitosan-based coatings exhibit promising due to their edible, low-cost, biodegradable, biocompatible, antimicrobial, film-forming, and anti-oxidative activities without toxicity. The treatment with 500 mg/L chitosan maintained the quality and prolonged postharvest life of “Garmrok” kiwifruit, possibly by the downregulating genes associated with ethylene biosynthesis and cell wall modification during the early maturation period, whereas upregulating genes related to lignin metabolism in later stages of ripening (Kumarihami et al., 2021). Moreover, the degree of defense response induction in kiwifruit is also influenced by the molecular weight of chitosan. Compared to high molecular weight chitosan (136.8–342.0 kDa), low molecular weight chitosan (3.4–51.3 kDa) exhibited greater efficacy in inhibiting spore germination and mycelial growth of B. cinerea, owing to its superior ability to penetrate kiwifruit's epidermal cell walls and subsequent recognition by the chitin receptor CERK1, resulting in a stronger immune response including the accumulation of ROS, callose deposition, induction of pathogenesis-related gene expression, and apoptosis (Hua et al., 2019).

In addition, chitosan exhibits high potential as a carrier for targeted additives (e.g., antimicrobials, anti-browning agents, antioxidants, and nutrients) to retard discoloration and microbial growth and extend the postharvest life of products. Curcumin and facilitated iron-functionalized cellulose nanofibers as components of chitosan edible coating result in a synergistic effect. The curcumin coating reduced microbial growth, and the biopolymeric nanofiber formed a network to improve the inherent properties of chitosan. This edible coating material effectively reduces the respiration rate, mass loss, firmness loss, and microbial count of the kiwifruit during storage (10 days at 10°C) (Ghosh et al., 2021). To efficiently inhibit Phomopsis sp. and B. dothidea, a chitosan composite membrane containing ferulic acid, dextrin, and calcium was successfully fabricated. Ferulic acid is a plentiful cinnamic acid derivative with excellent antibacterial performance, whereas calcium is essential for the quality, development, and storability of kiwifruit. Chitosan composite film enhanced defense responses by increasing resistance compounds and activating defense enzymes, such as SOD, PAL, PPO, and POD (Zhang, Long et al., 2020).

The successful utilization of chitosan implies that other polysaccharides may also be exploited similarly to maintain postharvest quality. Alginate oligosaccharides (AOS), extracted from the brown algae cell wall, are degradation products of sodium alginate, which is linked by α-l-mannuronic acid and β-d-guluronic acid via a 1,4-glycosidic bond. AOS effectively controlled the postharvest disease of blue mold (P. expansum), gray mold (B. cinerea), and black rot (A. alternata). AOS was found to be linked with the inhibition of cell wall degradation enzyme activity of pectin methyl esterase (PME) and PG while also increasing levels of antioxidant enzymes (e.g., CAT and SOD) and antioxidant compounds (e.g., phenolics and flavonoids). Besides, 50 mg/L AOS resulted in increased firmness and decreased SSC while enhancing the activities of PPO, PAL, and GLU related to pathogen defense, thus improving the total antioxidant capacity of the “Bruno” kiwifruit (Zhuo et al., 2022).

Plant hormones

Melatonin (N-acetyl-5-methoxytrypt-amine), an indole derivative of tryptophan, plays a pivotal role in regulating fruit growth, development, and ripening as well as orchestrating multiple biotic and abiotic stress responses. The 0.1 mmol/L melatonin treatment significantly alleviated soluble protein degradation and attenuated the level of membrane lipid peroxidation and MDA accumulation of yellow-fleshed kiwifruit “Jingshi No 1”. Moreover, melatonin boosted antioxidant capacity by increasing the accumulation of antioxidants like phenolics and flavonoids (Wang, Liang et al., 2019). Melatonin treatment resulted in a reduction of fruit decay and respiration rates while maintaining high levels of SSC, TA, and AsA. The upregulation of key genes in the l-galactose pathway during the early stage and circulating transformation genes in the later stage maintained AsA content in postharvest kiwifruit. Meanwhile, melatonin treatment promoted the expression of biosynthesis (e.g., AcGalLDH, AcGalDH, and AcGME2) and regeneration (e.g., AcGR, AcDHAR, and AcMDHAR1) genes and inhibited the expression of degradation gene AcAO (Luo et al., 2022). Melatonin suppressed the activities of cell-wall degrading enzymes (e.g., Cx, PME, PG, and β-Gal), delayed the degradation of pro-pectin, cellulose, and hemicellulose, while increasing water-soluble pectin content, resulting in a deceleration of kiwifruit softening and an enhancement of preserved fruit quality (Cao, Qu et al., 2021). Besides, melatonin delays softening by inhibiting amylase activity and upregulating the transcript levels of hexokinase and fructokinase genes to decelerate starch decomposition. Exogenous melatonin had certain effects on sugar metabolism in kiwifruit under cold storage by regulating the synthesis of sucrose and its dissociation into fructose and glucose in diverse cellular compartments through SS, SPS, AI, and NI. Overall, melatonin treatment increased the levels of soluble sugar components (e.g., glucose, sucrose, and fructose) by regulating related enzyme activities as well as their transcript levels. The accumulation of total chlorophyll, carotenoid, and ascorbate was increased, and fruit appearance was improved by melatonin treatment (Zhang et al., 2023). The 100 µmol/L melatonin application repressed the respiration and ethylene production in “Bruno” kiwifruit during storage, mainly attributing to reducing ADH and PDC activities along with suppressing the expression of AdPDC2, AdPDC1, and AdADH1, and possibly related to the genes of Achn226291, AdERF4, AdERF74, and AdERF75 (Cheng et al., 2022). Melatonin effectively alleviated postharvest kiwifruit chilling injury symptoms, including ameliorating water-soaking and lignification damage. The inhibition of water-soaking can be ascribed to its ability to enhance oxidation resistance through increasing antioxidant enzyme (e.g., SOD, CAT, APX, and GR) activities and levels of antioxidant substance (e.g., AsA and GSH) accumulation. Due to the inhibited activity of lignin metabolism enzymes (e.g., PAL, 4-coumarate-CoA ligase, and cinnamate 4-hydroxylase) and the expression of structural genes, the chilling injury-induced lignification was also alleviated under melatonin treatment (Jiao et al., 2022).

The intricate metabolic alterations that occur during fruit maturation are under the regulation of multiple hormones, including auxin, abscisic acid (ABA), gibberellins, and brassinosteroid (Table 1), which are also activated in response to pathogens infection in kiwifruit (Li et al., 2020). The application of 20 mg/L ABA decreased the number of lignin cells with 40–80 µm diameter, thus reducing the lignification chance in the “Hongyang” kiwifruit pulp. Furthermore, ABA treatment mitigated symptoms of the black patch, enhanced lignin content in the peel, and maintained a higher total antioxidant capacity of the edible part without compromising fruit quality (Jin et al., 2021). Exogenous indole-3-acetic acid (IAA) treatment, particularly at a concentration of 50 µg/mL, enhanced kiwifruit resistance against B. cinerea through a reduction in disease incidence and lesion area, accompanied by increasing the activity of pathogen resistance-related defense enzyme (e.g., POD, SOD, and CAT). The induction of resistance in kiwifruit through IAA treatment is closely associated with phenylpropanoid activation, including phenols and flavonoids, carbohydrate metabolism, terpenoids, and hormone signaling pathways (Li et al., 2023). Exogenous gibberellins can effectively and comprehensively regulate the postharvest ripening of kiwifruit, including retarding the decrease in firmness, starch, and cell-wall components (e.g., pectin, cellulose, and hemicellulose), as well as inhibiting the increase in total soluble solids, ethylene, and ester production (Yang, Li et al., 2023). The steroidal phytohormone 24-epibrassinolide (EBR) boasts a favorable safety profile that can retard firmness decrease, weight loss, and SSC increase in kiwifruit, as well as inhibit membrane permeability increase and MDA accumulation. The degradation of starch into soluble sugars is postponed after EBR treatment due to amylase inactivation. EBR treatment also suppressed the activity of AI, NI, SPS, sucrose synthase, hexokinase, and fructokinase, subsequently leading to decreased levels of sucrose, glucose, and fructose contents (Lu et al., 2019). Methyl jasmonate (MeJA), crucial endogenous signaling molecules that respond to both abiotic and biotic stresses in plants, could be used as an effective postharvest tool for maintaining kiwifruit flesh firmness and phytochemical compounds. Kiwifruit treated with 1.0 mmol/L MeJA has the highest vitamin C, total flavonoids, and antioxidant activity, whereas those treated with 0.3 mmol/L MeJA had the highest total phenolic content (Öztürk & Yücedağ, 2021). The 0.1 mmol/L MeJA treatment conferred disease resistance against B. dothidea-induced soft rot by inducing the activities of antioxidant and defense-related enzymes, namely, POD, SOD, CAT, PPO, and CHI, as well as total phenolic accumulation, while simultaneously reducing membrane lipid peroxidation (Pan et al., 2020).

TABLE 1 Effects of chemical treatments on kiwifruit quality.

Chemicals	Cultivar	Treatment	Main effects	References
Abscisic acid (ABA)	Hongyang	Kiwifruits were immersed in 20 mg/L ABA solution for 15 min	ABA accelerated lignin synthesis in the peel and maintained fruit quality with less black patches symptoms on the peel and lignin accumulation in pulp	Jin et al. (2021)
Indole-3-acetic acid (IAA)	Hongyang	Kiwifruits were wounded and then treated with 10 µL 50 µg/mL IAA	IAA enhanced kiwifruit's resistance to postharvest diseases by activating phenylpropanoids, defense enzymes, and hormone signaling pathways	Li et al. (2023)
Gibberellins	Hongyang	Kiwifruits were sprayed with 0.3 g/L GA solution once every 8 h, three times in total	GA delayed postharvest kiwifruit ripening by retarding the decreases in firmness, cell-wall material, and starch, as well as inhibiting the increase in ethylene, total soluble solids, and esters production	Yang, Li et al. (2023)
24-epibrassinolide (EBR)	Huayou	Kiwifruits were immersed in 5 µmol/L EBR solution for 10 min	EBR-mediated senescence delay may be derived from its ability to maintain membrane lipid integrity and inhibit starch conversion	Lu et al. (2019)
Methyl jasmonate (MeJA)	Hayward	Kiwifruits were immersed in 0.25, 0.5, and 1.0 mmol/L MeJA solutions	MeJA decreased weight loss, minimized the losses in vitamin C, total phenolics, and total flavonoids, and contributed to antioxidant activity throughout the shelf life of kiwifruit	Öztürk & Yücedağ (2021)
Methyl jasmonate (MeJA)	JinKui	Kiwifruits were exposed to 0.1 mmol/L MeJA solutions	MeJA effectively induced postharvest kiwifruit disease resistance to soft rot caused by B. dothidea, which is related to inducing defense responses through the enhancement of defense-related gene expression levels and enzyme activities	Pan et al. (2020)
p-coumaric acid (p-CA)	Xuxiang	Kiwifruits were plunged into 0.5 mmol/L p-coumaric acid solution for 10 min	p-CA treatment increased total phenolics accumulation and ASA-GSH cycle activity and maintained a balance between kiwifruit antioxidant status and ROS production, which preserved membrane integrity and delayed fruit senescence	Cao, Wang et al. (2021)
Quercetin	Qinmei	Kiwifruits were injected with 25 µL 0.5 mg/mL quercetin	Quercetin significantly hindered P. expansum growth by its toxicity against fungal pathogens	Zhang et al. (2018)
Chlorogenic acid	Xuxiang	Kiwifruits were immersed in 3 g/L chlorogenic acid solution for 1 min	Chlorogenic acid inhibited Diaporthe sp. growth by inducing mitochondrial oxidative stress, calcium influx, and defense-related enzyme activity in kiwifruit	Zhang, Bi et al. (2020)
Citral	Jinkui	Kiwifruits were dipped in 0.6 µL/mL citral for 15 min	Citral inhibited fungal pathogens such as B. dothidea, Phomopsis macrospore, and B. cinerea by increasing membrane permeability and cell membrane damage. It also enhanced the antioxidant capacity of harvested kiwifruit	Wei, Chen, Chen et al. (2021) and Wei, Chen, Wan et al. (2021)
(E)-2-Hexenal	Haegeum	Kiwifruits were fumigated with 100 µmol/L (E)-2-hexenal for 24 h at 20°C	(E)-2-hexenal fumigation boosted resistance against B. cinerea infection by promoting the signaling network from pathogen recognition to defense gene expression and facilitating secondary metabolite accumulation	Hyun et al. (2022)
γ-aminobutyric acid (GABA)	Hongyang	Kiwifruits were soaked in 10 mmol/L GABA solution for 10 min	GABA treatment increased AsA levels by regulating key genes in biosynthetic and degradable pathways, thus inducing chilling tolerance and maintaining kiwifruit membrane integrity	Liu et al. (2023)
Hydrogen gas (H2)	Xuxiang	Kiwifruits were exposed to 4.5 µL/L H2 for 24 h	H2 decreased ACS and ACO activities, ACC accumulation, and ethylene production. H2-induced also reduced disease incidence through an increase in the antioxidant ability of kiwifruit	Hu et al. (2018)
Hydrogen sulfide (H2S)	Jinkui	Kiwifruits were dipped into 45 and 90 µmol/L H2S for 30 min	H2S inhibited ethylene production, enhanced protective enzyme activities, and reduced ROS accumulation, thus delaying kiwifruit maturation and senescence	Lin et al. (2020)
H2S	Jinyan	Kiwifruits were fumigated with 20 µL/L H2S for 30 min	H2S enhanced kiwifruit resistance through microbicidal action and amino acid metabolism involved in postharvest disease	Duan et al. (2022)
Chlorine dioxide (ClO2)	Daebo	Kiwifruits were fumigated with 30 mg/L ClO2 for 30 min	ClO2 fumigation significantly reduced kiwifruit decay incidence and microorganisms growth, as well as retarded the ripening process through the maintenance of fruit quality attributes such as firmness, SSC, and TA	Park et al. (2019)
Silver nanoparticles (AgNPs)	Jinmei	Kiwifruits were sprayed with 13.8 ppm AgNPs	AgNPs were effectively employed for controlling kiwifruit postharvest rot and were safely applied on kiwifruit without leaving any residue	Li, Pan et al. (2022)
Natamycin	Jinyan	Kiwifruits were punctured and then added 10 µL 500 mg/L natamycin	Natamycin effectively reduced postharvest soft rot by inhibiting mycelial growth and spore germination, inducing antioxidant enzymes and antioxidant compounds, reducing MDA levels, and repressing cell wall degrading enzyme activity	Pan, Zhong, Xia et al. (2022)

Natural compounds

Natural compounds, including organic acids, flavonoids, phenols, terpenoids, aldehydes, and amino acids, have been assessed for their potential to manage postharvest diseases and maintain harvested kiwifruit quality (Table 1). p-Coumaric acid (p-CA) is a naturally occurring phenolic acid with a wide diversity of biological activity. p-CA pretreatment delayed softening, senescence, respiration intensity, and the accumulation of soluble sugars, MDA, and hydrogen peroxide in kiwifruit. p-CA-pretreated kiwifruit presented a higher antioxidant activity resulting from the increased ASA-GSH cycle-related enzyme activity (e.g., DHAR, MDHAR, APX, and GR). The augmented endogenous p-CA content, along with the activation of shikimate dehydrogenase and PAL activity, synergistically contribute to higher total phenolics in kiwifruit (Cao, Wang et al., 2021). Quercetin is an important anti-fungal flavonoid, which at a concentration of 0.25 mg/mL caused significant damage to the mycelial structure of P. expansum. With the quercetin treatment, kiwifruit infected with P. expansum showed increased activity and gene expression of CHI, GLU, PAL, PPO, and POD (Zhang et al., 2018). Chlorogenic acid is the predominant phenolic compound found in various crops and has antifungal activity and beneficial properties. An influx of Ca²⁺ mediated the chlorogenic acid-induced burst of mitochondrial ROS in Diaporthe sp. mycelia, resulting in mitochondrial dysfunction and triggering apoptosis in fungal cells. The concentration-dependent inhibitory effects of chlorogenic acid on kiwifruit postharvest diseases were observed without negatively affecting fruit quality (Zhang, Bi et al., 2020). Citral is a terpenoid isolated from Cymbopogon citratus and Litsea cubeba (lemongrass), which has inhibitory effects against various fungal pathogens such as B. dothidea, Phomopsis macrospore, and B. cinerea with a minimum inhibitory concentration of 0.4 µL/mL. Citral treatment inhibited the growth of B. cinerea hyphal, altered hyphae morphological characteristics, damaged the permeability of cell membrane, enhanced cellular content leakage, and reduced the activities of TCA-related enzymes (e.g., succinate dehydrogenase and malate dehydrogenase) (Wei, Chen, Chen et al., 2021). Besides, citral treatment resulted in a significant reduction of fruit weight loss and delayed softening. Postharvest senescence alleviation in citral-treated kiwifruit might be ascribed to higher levels of AsA, flavonoids, and phenolics and by induced activation of antioxidant enzymes, such as SOD, POD, and CAT, for maintaining the high antioxidant capacity (Wei, Chen, Wan et al., 2021). The (E)-2-hexenal, a natural volatile compound emitted by green plants, can effectively reduce the development of kiwifruit postharvest disease against B. cinerea. The (E)-2-hexenal treatment induced upregulation of genes encoding pattern signal transducer proteins, recognition receptors, and pathogenesis-related proteins, including CHI. Furthermore, the treatment resulted in an upregulation of jasmonic acid and flavonoid biosynthesis (Hyun et al., 2022). GABA is an important inhibitory neurotransmitter and is generally recognized as a safe compound. Kiwifruit treated with 10 mmol/L GABA displayed higher AsA content by increasing gene expression involved in AsA anabolic and regeneration while downregulating related catabolic genes. GABA treatment also upregulated the transcripts of candidate transcription factors involved in AsA biosynthesis, such as bHLHs and HZ1. A higher level of AsA in kiwifruit is beneficial for protecting cell membranes and alleviating oxidative damage induced by chilling (Liu et al., 2023).

Other compounds

Molecular hydrogen (H₂) was proposed as a multifunctional signaling gas. Fumigation with H₂ not only elevated the endogenous H₂ concentration but also delayed flesh softening and cell wall disassembly. H₂ also inhibited ethylene production by reducing the concentration of 1-amino cyclopropene-1-carboxylate (ACC), inhibiting the activities of ACC synthase and ACC oxidase, and downregulating the corresponding gene transcripts. Further, H₂ blocked ACC-induced ethylene production and ripening and reduced the incidence of natural decay (Hu et al., 2018). Hydrogen sulfide (H₂S), as a third endogenous gaseous transmitter, is a gaseous plant growth regulator. By regulating the genes encoding cell wall degrading enzymes, modulating the ethylene signal transduction pathway, and reducing ROS accumulation, kiwifruit maturation and senescence can be delayed after H₂S treatment. H₂S also maintained higher TA and Vc content of kiwifruit and inhibited firmness reduction and increase in SSC (Lin et al., 2020). In addition, H₂S (20 µL/L) significantly controlled kiwifruit soft rot caused by B. dothidea, as well as upregulated the expression of AcDHQS, AcSK, AcPAL, AcSDH, AcCAD, and AcCHS genes and accumulation of different metabolites, such as shikimic acid, tyrosine, phenylalanine, tryptophan, total flavonoids, phenol, and lignin (Duan et al., 2022). Chlorine dioxide (ClO₂) is increasingly utilized as a disinfectant to control microbiological growth in harvested fruits and vegetables. The 30 mg/L ClO₂ treatment resulted in a reduction of decay incidence and microorganisms population on the fruit surface and helped to maintain the firmness, SSC, and TA, thus delaying the ripening process of the hardy kiwifruit (Park et al., 2019). Silver nanoparticles (AgNPs) have gained significant attention in recent years for plant disease control due to their remarkable efficiency, broad-spectrum activities with minimal resistance, antimicrobial properties, and excellent safety profile. The growth of mycelium and germination of spores in four kiwifruit rot pathogens, namely, A. alternata, Pestalotiopsis microspora, Diaporthe actinidiae, and B. dothidea, were effectively inhibited by AgNPs. Additionally, the symptoms of kiwifruit rot were alleviated probably by enhancing the permeability of mycelial cell membranes without any residual Ag⁺ on the peel and flesh (Li, Pan et al., 2022). Natamycin, a natural antimicrobial agent produced by Streptomyces natalensis, efficiently inhibits molds and yeasts. Natamycin was dose-dependent on the inhibition of B. dothidea with a significant decrease in soft rot incidence to 35% observed at a concentration of 500 mg/L. Natamycin suppressed the mycelial growth and spore germination of B. dothidea while mitigating oxidative damage to hyphae. Additionally, it elicited disease resistance in kiwifruit tissue by activating antioxidant enzymes CAT and SOD, augmenting the total phenolic content of antioxidants, maintaining low levels of MDA, and repressing cell wall degrading enzyme activities, such as β-Gal, PME, PG, and PL (Pan, Zhong, Xia et al., 2022).

Chemical treatment exhibits prolonged residual activity, effectively preventing both primary and secondary infections, but it may result in a visible residue on the kiwifruit, which might affect its marketability. The efficacy of chemical treatment has been demonstrated in laboratory and small-scale practical experiments. However, further validation is required under scale-up and commercial conditions to ensure its efficacy. Additionally, safety concerns also need to be addressed.

Biological technology

Biological technology refers to the utilization of introduced living or organisms to suppress the activities of pathogens. This innovative and environmentally friendly approach, such as antagonistic microorganisms and endophytes, has been reported as effective agents for managing postharvest decay in kiwifruit.

Antagonistic microorganism

Antagonistic microorganism can effectively manage different postharvest diseases, which were attributed to the ability to survive and colonize in host tissues, thus effectively competing for nutrients. Meyerozyma caribbica can exhibit stable growth on kiwifruit at both 20 and 4°C, while also demonstrating the capacity to form biofilm. The yeast exhibited a dose-dependent reduction in both spore germination rate and germ tube length while simultaneously inducing an increase in defense enzyme activities (e.g., PPO, POD, APX, SOD, and PAL) of kiwifruit and the biosynthesis of antibacterial-related secondary metabolites (e.g., total phenolics, flavonoids, and lignin) to enhance host disease resistance (Qiu et al., 2022). Meyerozyma guilliermondii 37 significantly suppressed kiwifruit soft rot caused by B. dothidea and D. actinidiae while maintaining the soft-ripe quality. Moreover, the yeast strain 37 enhanced kiwifruit resistance through the upregulation of defense-related enzyme activity (e.g., PPO, CAT, and PAL), the augmentation of antioxidant substance levels (e.g., GSH, total phenolics, and flavonoids), and the inhibition of cell wall degrading enzyme activity (e.g., β-Gal and PG) (Pan, Zhong, Wang et al., 2022). Wickerhamomyces anomalus demonstrated a robust capacity for biofilm formation and the production of volatile organic compounds, particularly 2-phenethyl acetate, 1-butanol, and 3-methyl-butanol, which exhibited potent biocontrol activity against blue and gray mold in kiwifruit (Zhao et al., 2023). As an antagonistic yeast, Trichoderma harzianum exhibited the most potent inhibitory activity against C.luteo-olivacea isolates through volatile, nonvolatile, and dual culture assay, with inhibitory rates of 90.0%, 70.6%, and 78.8%, respectively, and concerning Aureobasidium pullulans (L1 and L8 strains) by 23.3% and 25.8%, 50.0% and 34.7%, and 22.5% and 23.6%, respectively (Di Francesco et al., 2021). Antagonistic microorganisms produce secondary metabolites with antagonistic activities, which exhibit a potent antifungal effect. The cell-free supernatant of Bacillus velezensis A4 markedly inhibited the growth of hyphae, the rate of spore germination, and the ability of hyphae penetration in B. cinerea. The excessive accumulation of ROS seriously damages hyphae and results in the peroxidation of the membrane, compromised membrane integrity, and leakage of cell constituents, resulting in decreased hyphal growth and virulence (Zhao et al., 2022). The integration of antagonistic yeasts and elicitor treatments exhibited more effective in managing postharvest diseases than each single treatment. Hanseniaspora uvarum (an antagonistic yeast) combined with β-aminobutyric acid (a hypersensitive response elicitor), and Candida oleophila (an antagonistic yeast) combined with oligogalacturonide (a hypersensitive response elicitor) also significantly reduced kiwifruit black rot (A. alternata) and gray mold (B. cinerea), which may be attributed to the induction of gene expression and enzyme activities of CHI, GLU, POD, and PAL in kiwifruit (Cheng et al., 2019; Gao et al., 2021).

Endophyte

Endophytes refer to all microorganisms that inhabit plants. These endophytes establish a symbiotic relationship with their plant hosts, making antagonistic yeast a promising candidate for combating pathogenic fungi. The endophytic yeast Vishniacozyma victoriae extracted from kiwifruit and isolated from kiwifruit, demonstrated significant potential due to its low toxicity. It utilized the organic acids and energy of the host to colonize the wounds, thereby effectively preventing contact between B. cinerea and the host (Nian et al., 2023). The strain Fusicolla violacea J-1, isolated from the “Hongyang” kiwifruit, demonstrated strong antifungal activity against A. alternata both in vitro and in vivo. The aseptic filtrate of the strain J-1 exhibited significant inhibitory effects on mycelial growth and conidia germination of A. alternata, inducing morphological changes in bmycelial, enhanced crucial cell wall enzyme activity, disrupted cell membrane integrity, and resulting in nucleic acids, proteins, and other substances leakage, thus leading to cell demise. The aseptic filtrate of F. violacea J-1 also exhibited a broadspectrum of antifungal activities against five pathogens, namely, Diaporthe eres, Fusarium graminearum, Epicoccum sorghinum, Phomopsis sp., and B. dothidea (Li, Long et al., 2021). The endophytic bacterium, Bacillus amyloliquefaciens strain M9, was discovered in healthy kiwifruit, its culture filtrate exhibited a strong antifungal activity (a 73.1% decline in decay rate) against B. dothidea, as well as successfully extending the shelf life and enhancing the quality of stored kiwifruit (Pang et al., 2021).

It is interesting to note that postharvest biocontrol research has grown during the last 5 years. However, in order to accomplish large-scale manufacturing and deployment of biocontrol products in the management of postharvest diseases of kiwifruit, further research is needed to address the critical issues of high levels of inoculum, poor storage of the biocontrol product, biosafety, cost-effectiveness, and improper application.

Integrated management

It is not always possible to reduce microbial load without compromising sensory or nutritional quality using only one technique; therefore, some combined applications of these techniques have been shown to be extremely effective in extending shelf life and microbial inhibition. The synergistic effects of combining food additives as inducers with physical methods contribute to reducing disease occurrence. A combined treatment of 5 g/L potassium sorbate with hot water at 48°C resulted in a significant increase in total phenolic content, a decrease in MDA content, an improvement in defense-related enzyme activities (e.g., CHI, GLU, PAL, PPO, and POD), and an increase in transcript levels of CHI and GLU compared to a potassium sorbate and hot water treatment alone. The combined treatment also suppressed mycelial growth, germ tube elongation, and spore germination of B. cinerea while maintaining a higher level of fruit firmness and reducing weight loss (Ge et al., 2020). The hurdle technology combining biocontrol and physical controls may offer a promising approach for the management of gray mold on kiwifruit. The combined treatment of M. guilliermondiiand UV-C radiation induced the upregulation of defensive gene expression (e.g., CHI and GLU) and increased total phenolic content in kiwifruit, which provided greater control efficacy than individual treatment (Cheng et al., 2023). The compatibility of an effective antagonist with postharvest conditions is essential. In CA storage (2.0% O₂ and 4.5% CO₂), the efficacy of kiwifruit treated with A. pullulans (L1 and L8 strains) was only 30% and 60%, respectively. However, under normal refrigeration (−1°C), both L1 and L8 strains of A. pullulans exhibited a reduction in B. cinerea incidence by over 80%. The combined treatment significantly elevated the levels of aspartic and glutamic acids and promoted the biosynthesis of new amino acids in kiwifruit (Di Francesco et al., 2018). Lysozyme coatings or 1-MCP treatment led to the inhibition of ethylene production and respiratory rate, as well as delayed increases in decay incidence, weight loss, SSC, and total bacterial count. Additionally, the firmness, chlorophyll content, TA, AsA content, and activities of antioxidant enzymes, such as APX, SOD, and CAT, were enhanced during storage. The combined of lysozyme (0.08%, 2 min) and 1-MCP treatment (0.9 µL/L, 22 h) exhibited superior efficacy compared to the individual treatment with lysozyme or 1-MCP, which may be a more practical and effective approach to delaying ripening and enhancing the postharvest quality of kiwifruit during storage (Xu et al., 2019). Low-temperature treatment upregulated the levels of key antioxidant enzymes of kiwifruit, such as APX, SOD, and CAT, but easily led to cold damage to kiwifruit. The combination of gradual cooling (decreasing 3°C every day from 26 to 2°C) and ozone (300 ppb) treatment could improve chilling tolerance, alleviate chilling injuries, and preserve membrane integrity by inhibiting LOX activity and MDA accumulation (Goffi et al., 2020). The rot disease of kiwifruit rapidly accelerated when transferred to room temperature after long-term controlled atmosphere storage. The technology of 1000 µL/L ethrel combining with 0.5 mg/L ozone flow microcirculation could inhibit the growth of microorganisms on the “Qinmei” kiwifruit. Such combined treatment greatly prevented ethylene production as well as lowered oxidative damage and reduced physiological disorders via inducing the activities of the defense-related enzyme (e.g., POD, SOD, and CAT). Furthermore, this treatment efficiently improved CHI and GLU activities and mitigated the extent of membrane lipid peroxidation, thus enhancing disease resistance and maintaining firmness (Ran et al., 2022).

CONCLUSIONS AND PERSPECTIVES

As a typical respiratory climacteric fruit, kiwifruit is suffering severe postharvest losses. This article provides an overview of the causes and symptoms of postharvest kiwifruit common issues, including fungal decays, chilling injury, oxidative damage, oversoftening, and off-flavor development. Physical, chemical, and biological technologies have been proven effective in managing postharvest kiwifruit over the past 5 years, which was attributed to the following: (1) enhancing the antioxidant system; (2) maintaining membrane structure; (3) reducing cell metabolic activities and respiration rates; (4) inhibiting mycelial and spores growth; (5) degrading the cell wall membrane of pathogenic fungi; and (6) regulating genes and enzymes related to off-flavor production. The implementation of these postharvest technologies requires careful consideration of several crucial factors, such as the duration and temperature of heat treatment, the dosage of electron irradiation treatment radiation, as well as the storage methods for biological control agents. However, all these techniques have their limitations. For instance, physical methods are incapable of preventing secondary infections of diseases; chemical methods may potentially result in high effective doses and residues; and biological control may have potential side effects on human health and the environment. To address the need for efficacy, cost-effectiveness ratio, and convenient manipulation, future research should be directed toward novel approaches such as pulsed light, cold plasma and electromagnetic fields, and monitor potential nutritional losses and deterioration of sensory characteristics of kiwifruit. Precise combinations of several techniques also can exhibit synergistic or complementary effects, enabling effective control of postharvest fungal diseases while maintaining the kiwifruit quality. However, the application of these technologies only exists at the lab level currently, it is imperative to further extend them to pilot and industrial-scale trials.

AUTHOR CONTRIBUTIONS

Yu Xia: Conceptualization; writing—original draft; writing—review and editing; visualization. Ding-Tao Wu: Writing—review and editing. Maratab Ali: Writing—review and editing. Yi Liu: Writing—review and editing. Qi-Guo Zhuang: Writing—review and editing. Syed Abdul Wadood: Writing—review and editing. Qiu-Hong Liao: Writing—review and editing. Hong-Yan Liu: Conceptualization; writing—review and editing; supervision. Ren-You Gan: Conceptualization; writing—review and editing; supervision.

ACKNOWLEDGMENTS

The study was supported by the Local Financial Funds of National Agricultural Science and Technology Center (NASC2021TD01 and NASC2023ST04), the Agricultural Science and Technology Innovation Program (ASTIP2024-34-IUA-09), and the Sichuan Science and Technology Program (2022JDJQ0063).

CONFLICT OF INTEREST STATEMENT

The authors declare no conflicts of interest.

ETHICS STATEMENT

None declared.

References

Afshar‐Mohammadian, M., Fallah, S. F., & Rezadoost, M. H. (2019). Different expression of kiwifruit ethylene‐related genes during low storage temperatures. Journal of Consumer Protection and Food Safety, 14, 113–120. [DOI: https://dx.doi.org/10.1007/s00003-018-1205-6]