Cold storage is a common method of preserving horticultural products after harvest. However, for cold-sensitive horticultural products, such as cucumber (Wang et al., 2018; Wang et al., 2020a), peach (Song et al., 2022), and kiwifruit (Liu et al., 2023), low temperatures can cause chilling injury (CI). CI damages the permeability of the fruit cell membrane, leading to cytoplasmic leakage. When cucumber fruit is transferred from cold storage to room temperature, the symptoms of CI appear quickly, manifested as watery spots on the fruit epidermis and an unpleasant odor, which are generally deemed unacceptable by consumers (Tsuchida et al., 2010; Wang et al., 2018).

Cold stress induces the accumulation of reactive oxygen species (ROS) in plant tissues, leading to a series of oxidative damage events, such as membrane lipid peroxidation, and eventually inducing cell death (You and Chan, 2015; Mittler et al., 2022). The plant ROS-scavenging system consists of an enzymatic defense system and a non-enzymatic antioxidant defense system (Gill and Tuteja, 2010). The former includes peroxidase (POD), catalase (CAT) and superoxide dismutase (SOD). SOD can remove excess superoxide anions ( O 2 − ) in plants and reduce the damage caused by O 2 − to the cell membrane. In turn, POD and CAT catalyze the decomposition of hydrogen peroxide (H₂O₂) to maintain H₂O₂ at a low level, minimize the production of hydroxyl radicals (-OH), and protect cell membrane structure (Yu et al., 2022). The ascorbic acid-glutathione (AsA-GSH) cycle is an important component of the non-enzymatic antioxidant system. The AsA-GSH cycle comprises ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), and glutathione reductase (GR), which can directly remove H₂O₂ from plant tissues. The AsA-GSH cycle promotes the production of two non-enzymatic antioxidants (AsA and GSH) to maintain the balance of redox substances in plants (Peng et al., 2022). The ROS-scavenging system has been shown to be critical for the enhancement of cold resistance in many horticultural products, such as loquats (Cai et al., 2011), bell peppers (Endo et al., 2019), okra (Li et al., 2023), and bananas (Chen et al., 2021b). Therefore, improving antioxidant capacity is a promising approach for alleviating low temperature-induced oxidative damage in horticultural products.

Mitochondria are important sites for respiration and the production of ATP in living tissues, thus providing energy for all cellular activities (Chen et al., 2021a). H⁺-ATPase, Ca²⁺-ATPase, cytochrome c oxidase (CCO), and succinate dehydrogenase (SDH) are key enzymes involved in energy synthesis in the mitochondrial membrane (Li et al., 2021a). The decrease or loss of activity of these enzymes can lead to insufficient energy supply to cells, which in turn leads to cellular senescence and death. Numerous studies have shown that the occurrence of CI during the post-harvest cold storage of horticultural products, such as strawberries (Li et al., 2018), pears (Tao et al., 2019), and peaches (Jin et al., 2013), is related to energy metabolism. Energy loss leads to the accumulation of ROS, cell membrane damage, impaired metabolism, and a slow response to low temperature signals, thus accelerating the occurrence of CI.

Numerous preservation techniques, such as brief exposure to hot water (Nasef, 2018), pre-storage cold acclimation (Wang et al., 2018; Wang et al., 2021a), or treatment with hydrogen sulfide (Wang et al., 2022), melatonin (Madebo et al., 2021), or glycine betaine (Zhang et al., 2008), have been shown to extend the shelf life of cold-stored cucumber fruits. Polysaccharide coatings have also been used to improve cold resistance after harvest; for example, chitosan has been shown to improve cold resistance in cucumber fruit (Ru et al., 2020), and treatment with Astragalus polysaccharides inhibited the occurrence of CI in bananas (Tian et al., 2022). Fucoidan is a type of fucose-enriched and sulfated polysaccharide that extracted from different kinds of brown seaweed, which contains L-fucose and sulfate groups (Zhao et al., 2004). As a nontoxic natural heteropolysaccharide, fucoidan has multiple biological functions, such as anti-coagulation, anti-tumor, anti-thrombus, anti-virus, anti-oxidation and enhance the body’s immune function (Senthilkumar et al., 2013; Wu et al., 2016). In addition, fucoidan can be used as a preservative for storing fruit and vegetables after harvest. Some studies have shown that fucoidan improves antioxidant capacity and maintains post-harvest fruit quality; indeed, fucoidan can extend the shelf life of some fruits, such as mangos (Xu and Wu, 2021), and strawberries (Duan et al., 2019; Luo et al., 2020). However, to date, the effect of fucoidan on CI in cucumber fruits has rarely been reported. Therefore, here, we investigated whether fucoidan treatment can improve cold resistance in cucumbers stored at low temperature, with a focus on ROS homeostasis and energy metabolism. Our findings support the use of fucoidan as a new preservative for cucumber cold storage.

CI easily occurs in cold-stored cucumber fruits after harvest and leads to a decrease in fruit quality and commodity value (Wang et al., 2018; Madebo et al., 2021; Wang et al., 2022). Polysaccharide coatings are environment friendly materials for preservation and improving cold resistance of horticultural products. Carboxymethyl cellulose (CMC) coating help to reduce CI in pomegranate (Ehteshami et al., 2019) and mandarin fruit (Ali et al., 2021) during cold storage. Astragalus polysaccharides delayed the CI and attenuated postharvest peel browning of banana fruit (Tian et al., 2022). In vegetables, the coating of polysaccharide has a similar preservation effect to that of fruit. CMC-based water barrier coating is promising to control the water loss and further maintain the whole quality of pakchoi (Yu et al., 2022). Chitosan induced chilling resistance in cucumber fruit (Ru et al., 2020). Fucoidan has a wide range of biological activities (Zhao et al., 2004)., which has been used to maintain the quality of some fruits. Fucoidan treatment can effectively extend the shelf life of mangoes (Xu and Wu, 2021). Fucoidan treatment improved the cold resistance of strawberry fruit during cold storage (Duan et al., 2019; Luo et al., 2020). In this study, different concentrations of fucoidan (5, 10, and 15 g/L) effectively inhibited CI in cucumber fruits and delayed the appearance of CI symptoms. With the increase of fucoidan concentration, the CI index is lower, the treatment with 15 g/L fucoidan achieved the best results in this study. However, the best result of 15 g/L fucoidan is based on this work, may not the optimum concentration. Thus, the optimum concentration of fucoidan for reducing the CI in cucumber will be further explored.

Cold stress-induced plant damage typically involves an imbalance between ROS production and depletion (Duan et al., 2012); excessive accumulation of ROS, such as H₂O₂ and O 2 − , can irreversibly damage organelles, including chloroplasts and mitochondria. Protecting cells from oxidative damage under stress is considered a major mechanism of plant resistance to low temperatures, and this resistance may depend on the antioxidant system (Mittler et al., 2022). The enzymatic antioxidant system is the main mode of ROS scavenging, and SOD, CAT, and POD are the key antioxidant enzymes involved in this process (Tan et al., 2020). These three antioxidant enzymes work together to reduce the oxidative damage to the cell membrane caused by H₂O₂ and O2^-, thus maintaining its integrity and minimizing CI (Cao et al., 2018; Mirshekari et al., 2020). In this study, fucoidan treatment significantly enhanced the SOD activity of cucumber fruits, effectively reduced the O2^- production rate, and maintained high CAT and POD activities, thereby inhibiting the accumulation of H₂O₂. Similarly, previous studies have reported that fucoidan treatment enhances antioxidant activity and improves the cold tolerance of strawberries (Duan et al., 2019; Luo et al., 2020). Thus, we conclude that fucoidan treatment is beneficial in regulating the redox state, and improves the antioxidant capacity of cold-stored cucumber fruits.

The AsA-GSH cycle is an important H₂O₂ scavenging system in plants. AsA and GSH are important non-enzymatic antioxidants in this cycle, which can scavenge excessive ROS produced by plant cells in response to low-temperature stress injury (Li et al., 2010). The AsA/DHA and GSH/GSSG ratios reflect the redox capacity of plants. Generally, the higher the content of reducing substances, the stronger the stress tolerance of plants (Li et al., 2023). In this study, fucoidan treatment consistently maintained higher AsA and GSH contents and AsA/DHA and GSH/GSSG ratios in cucumber fruits, consistent with results from tomatoes (Li et al., 2019), and bell peppers (Endo et al., 2019). GSH/GSSG ratios play an important role in cold tolerance of tomatoes (Li et al., 2019). Hot water treatment enhanced AsA-GSH cycle to alleviate CI in pepper fruit during postharvest cold storage (Endo et al., 2019). APX, MDHAR, DHAR, and GR are key enzymes involved in AsA-GSH metabolism that cooperate to scavenge excess H₂O₂ in cells and convert it into H₂O and O₂- (Li et al., 2010). Enhanced gene expression and activity of APX, MDHAR, DHAR, and GR has been shown to improve fruit cold tolerance and reduce the occurrence of CI in bell peppers (Wang et al., 2016; Endo et al., 2019), peaches (Song et al., 2016), okra (Li et al., 2023), and bananas (Chen et al., 2021b). These results support our findings that fucoidan treatment can regulate the activities of key enzymes and the expression of related genes in the AsA-GSH cycle of cucumber fruits during cold storage, thereby improving H₂O₂scavenging, reducing the degree of membrane lipid peroxidation, and delaying the occurrence of CI in cucumber fruits.

Energy molecules, such as ATP, play an important role in maintaining the integrity of cell membranes (Saquet et al., 2003). Energy deficits can impair the integrity of cell membranes and lead to a decline in the post-harvest quality of horticultural products (Wang et al., 2020c; Li et al., 2021a). ATP and ADP contents in loquats (Jin et al., 2015), kiwifruit (Wang et al., 2020b), and peaches (Hu et al., 2023), have been reported to drop rapidly and remain at low levels during cold storage. In this study, ATP, ADP, and energy charge declined rapidly in cucumber fruits during cold storage, and the symptoms of fruit CI were aggravated. Fucoidan treatment maintained higher levels of ATP, ADP, and energy charge in cucumber fruits, thereby reducing the ROS-induced membrane damage and alleviating CI. Mitochondria are responsible for energy synthesis; SDH, CCO, H⁺-ATPase, and Ca²⁺-ATPase are key respiratory enzymes located in the inner mitochondrial membrane, and their activities reflect the energy production status of mitochondria (Li et al., 2021b; Wang et al., 2021b). H⁺-ATPase catalyzes ATP synthesis by establishing an H⁺ electrochemical gradient on both sides of the cell membrane to generate transmembrane proton propulsion (Azevedo et al., 2008). Ca²⁺-ATPase is a Ca²⁺ pump responsible for transporting Ca²⁺ into the mitochondria, and a decrease in Ca²⁺-ATPase activity leads to the accumulation of Ca²⁺ in the cytoplasm, leading to the destruction of membrane structural integrity (Chung et al., 2000). SDH catalyzes the conversion of succinic acid to fumaric acid in the tricarboxylic acid cycle, leading to the production of H⁺ and ATP (Ackrell et al., 1978). CCO provides energy to cells mainly through oxidative phosphorylation and is a key enzyme in the oxidative phosphorylation process in the mitochondrial respiratory chain (Millar et al., 1995). Cold stress affects the production of ATP by affecting mitochondrial structure and the activity of these enzymes, and the loss of ATP impairs metabolism and destroys cell structure; this further aggravates CI, resulting in a decline in the quality of horticultural products (Jin et al., 2015; Wang et al., 2020b; Hu et al., 2023). Altogether together, the results of this study demonstrate that fucoidan treatment can maintain mitochondrial function by improving the activities of relevant enzymes in cucumbers during cold storage, thereby ensuring continual energy synthesis and ultimately reducing the occurrence of CI.

Fucoidan treatment effectively reduced CI in cucumber fruits by modulating ROS scavenging and energy metabolism. Fucoidan treatment can reduce the accumulation of H₂O₂ and O 2 − and the degree of membrane lipid peroxidation by increasing the activity of antioxidant enzymes and the expression of key genes. Furthermore, fucoidan treatment helps maintain high energy levels by regulating the activities of enzymes related to mitochondrial metabolism, thus effectively improving the resistance of cold-stored cucumber fruits to low-temperature stress and reducing the symptoms of cold injury.

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

DL: Conceptualization, methodology, software, writing—original draft preparation. RY: validation, formal analysis, data curation, writing—original draft preparation. MX: investigation, resources. SL: visualization, data curation. JC: writing—review and editing, supervision. ZG: writing—review and editing, project administration, funding acquisition. All authors contributed to the article and approved the submitted version.