**1. Introduction**

Kiwifruit (*Actinidia* sp.) is widely accepted by consumers for its organoleptic and nutritional properties. It is an excellent source of vitamin C (L-ascorbic acid) [1,2], and is generally known as "Chinese gooseberry", "China's miracle fruit", and "the horticultural wonder of New Zealand" [3]. The genus *Actinidia* contains more than 60 species with a wide diversity, but only two of them (*A. deliciosa*, and *A. chinensis*) are being produced commercially [2,4,5]. Due to their high nutritional value and desirable taste, the demand for kiwifruits has been increasing. Year-round production and supply of kiwifruit to meet the demands inevitably requires proper postharvest management.

**Citation:** Kumarihami, H.M.P.C.; Kim, J.G.; Kim, Y.-H.; Lee, M.; Lee, Y.-S.; Kwack, Y.-B.; Kim, J. Preharvest Application of Chitosan Improves the Postharvest Life of 'Garmrok' Kiwifruit through the Modulation of Genes Related to Ethylene Biosynthesis, Cell Wall Modification and Lignin Metabolism. *Foods* **2021**, *10*, 373. https://doi.org/10.3390/ foods10020373

Academic Editors: Eleni Tsantili and Jinhe Bai Received: 19 January 2021 Accepted: 6 February 2021 Published: 9 February 2021

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Kiwifruit is extremely perishable, with a typical climacteric ripening pattern [1,5]. It is harvested at an unripe but physiologically mature stage [5]. After harvest, the physicochemical properties of fruit decline rapidly due to the influence of internal biochemical reactions and the external environment [6,7]. The postharvest performance of kiwifruit is strongly affected by the maturity or physiological state of the fruit at harvest, in conjunction with the applied postharvest management. In particular, the postharvest storage life of kiwifruit is mostly limited by its relatively high metabolic activity and extreme sensitivity to ethylene during storage [5–8]. Accordingly, the conditions for the preservation of kiwifruit for prolonged periods are undoubtedly important. The kiwifruit industry highly relies on low temperature [5,9] in combination with controlled/modified atmospheres [7,10] to extend their postharvest life. Besides, several other preservation technologies including edible coatings [7,10,11], and treatments with chemical agents such as methyl jasmonate [12], salicylic acid [6], and 1-methyl cyclopropane (1-MCP) [12] have been trying to extend the kiwifruit postharvest life. Among these, the application of edible coatings has been reported as one of the novel technologies with great potential for extending the postharvest life of kiwifruit.

The edible coatings on fresh fruit serve as an alternative to modified atmosphere packaging, as they reduce the quality changes and quantity losses through control of the internal atmosphere of the individual fruit [13]. It has been shown that edible coatings have the potential to reduce moisture loss, rate of respiration, ethylene production, and ripening, while they maintain quality along with storability [7,11,13,14]. In postharvest management, polysaccharide-, protein-, and lipid-based solutions have been demonstrated as applicable edible coatings that prolonged the postharvest life of the whole kiwifruit or other fruits [1,7,10,11,14–16].

Recently, the natural compound chitosan (poly *β*-(1,4) *N*-acetyl-d-glucosamine) has been widely used as an edible coating. It is a polysaccharide derived from the deacetylation of chitin [6,17–20]. It has been shown that the use of chitosan as an edible coating enhances the quality and postharvest life of various fruits owing to its excellent film-forming, nontoxic, biocompatible, biodegradable, and antifungal properties [2,8,18,20–22]. Besides, several studies have demonstrated that the postharvest use of chitosan as an edible coating maintains quality and extends the postharvest life of kiwifruits such as green-fleshed (*A. deliciosa*) kiwifruit [3,18,23], yellow-fleshed kiwifruit (*A. chinensis*) [6,24], red kiwifruit (*A. melanandra*) [2], hardy kiwifruit (*A. arguta*) [13], and arctic kiwifruit (*A. kolomikta*) [21]. Although chitosan can be applied in either preharvest or postharvest treatments, the reports on preharvest treatments of chitosan and the effects on the postharvest control are still limited. Nonetheless, the preharvest application of chitosan is highly feasible and can be applied on the fruit around harvest time [19].

In our previous studies, we evaluated the effectiveness of the application of preharvest chitosan in combination with calcium chloride (Ca-chitosan) that resulted in the enhancement of fruit quality and postharvest life of hardy kiwifruit (*A. arguta* 'Saehan') [25] and green-fleshed kiwifruit (*A. deliciosa* 'Garmrok') [8]. In a separate study, Zhang et al. [22] reported that the preharvest spraying of chitosan composite films (chitosan, calcium, dextrin, ferulic acid, and auxiliaries) had several positive effects on the postharvest quality and control of diseases in kiwifruit (*A. deliciosa* 'Guichang'). Although these results suggest the efficacy of preharvest treatment of chitosan in maintaining the postharvest properties of kiwifruit, the physiological changes and underlying molecular events involved in maintaining fruit physicochemical properties need to be further investigated. In the current study, we show that the preharvest application of chitosan maintains the fruit quality and extends the postharvest life of 'Garmrok' kiwifruit possibly through the modulation of genes related to ethylene biosynthesis, cell wall modification, and lignin metabolism.
