*4.6. Preharvest Application of Chitosan Maintained the Firmness of 'Garmrok' Kiwifruit during Cold Storage*

Loss of fruit firmness is a deleterious quality attribute that determines the postharvest life, fruit quality, commercial value, and consumer acceptability. In our study, preharvest application of chitosan exerted a beneficial effect on kiwifruit firmness during the cold storage. These results are in agreement with those obtained in our previous study on *A. deliciosa* 'Garmrok' kiwifruit [8] and Zhang et al. [22] on *A. deliciosa* 'Guichang' kiwifruit. It was considered that maintenance of firmness by chitosan coatings results from the formation of a semipermeable film on fruit surface that can act as a protective barrier to reduce respiration rate, thereby slowing the metabolic activity and textural changes [17,19,22,44,46,49]. As kiwifruit is a typical climacteric fruit, the reduced production of respiration and ethylene affected by chitosan treatment appear to be linked to the kiwifruit softening. Whether ethylene directly regulates expression of cell wall-modifying enzymes in kiwifruit, as observed in other climacteric, fruit remains to be investigated.

In addition, fruit firmness is closely associated with changes in the biochemical properties of the cell wall. Polygalacturonase (PG) is a key enzyme involved in cell wall modifications during fruit ripening, with its important functions in degradation of soluble pectin and depolymerization of solubilized pectin [50–52]. In kiwifruit, there are three polygalacturonase genes, namely, *PGA*, *PGB*, and *PGC*. Of these, *PGC* is a predominant gene expressed throughout the softening of kiwifruit [50,51,53]. The suppression of the *AdPGC* gene in preharvest chitosan-treated 'Garmrok' kiwifruit during the early phase of storage (i.e., 30 and 60 days) may be associated with the delayed kiwifruit softening and retained firmness. Expansins (EXPs) are non-enzymatic proteins that have been considered to play a role in loosening and extension of the cell wall (the cellulose-xyloglucan hydrogen bonds) [50,52,54]. Yang et al. [54] reported that two expansin genes *AdEXP1* and *AdEXP2* are actively involved in kiwifruit (*A. deliciosa* 'Bruno') ripening and softening. In our study, we observed a negative effect on expression of expansin genes (*AdEXP1* and *AdEXP2*) during the postharvest storage of 'Garmrok' kiwifruit. Thus, preharvest chitosan treatment was effective in delaying fruit softening, maintaining kiwifruit firmness, and extend their storage life by possibly inhibiting the activities of EXPs.

Further, a higher flesh firmness of 'Garmrok' kiwifruit treated with preharvest chitosan can be ascribed to the increase in lignin content. Lignin is one of the most abundant polyphenolic polymers in higher plants that mostly functions as a structural material which enhances the strength and rigidity of plant cells [12,55]. The increase of total lignin content in the chitosan-treated 'Garmrok' kiwifruit during the cold storage indicates that preharvest chitosan could induce lignin biosynthesis, as shown by He et al. [42] and Saavedra et al. [48]. Lignin biosynthesis involves the activities of three major enzymes (PAL, CAD, and POD), as well as the coordinated expression of these enzyme genes [12,32,33,55,56]. In our study, the higher expression of *AcPAL* during the early period of storage may have influenced the lignin accumulation in the preharvest chitosan-treated kiwifruit. In addition, the increased level of expression in *AcCAD* and *AcPOD2* at the later stage of storage is associated with the increased content of lignin in the preharvest chitosan-treated kiwifruit. Collectively, expression of lignin metabolism-related genes affected by preharvest chitosan treatment may have enhanced lignin metabolism that conferred greater firmness in 'Garmrok' kiwifruits.
