*4.2. Preharvest Application of Chitosan Is Associated with the Reduced Production of Ethylene and Rate of Respiration during Cold Storage*

During the cold storage, we observed a noticeable effect of preharvest chitosan application on the reduction of ethylene production and respiration rate of 'Garmrok' kiwifruit. The reduced ethylene production and respiration rate can be explained by the film-forming property of chitosan [17,19]. Chitosan has excellent selective permeability to the respiratory gases, particularly by blocking the oxygen that reduces respiration [20]. Kiwifruit is a climacteric fruit and the ripening process is largely regulated by the plant hormone ethylene. Ethylene production in climacteric fruit is mainly governed by the activity of two enzymes, i.e., 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS) and ACC oxidase (ACO) [37,38]. The conversion of ACC to ethylene by ACO is O2-dependent [37,38]. The enzyme activity of ACO is suppressed by low O2 and high CO2, while high CO2 concentrations prevent the auto-induction of ACS [37–39]. The effect of low O2 and high CO2 concentrations during storage on ethylene production and ripening for the shelf-life has been well reported in *A. deliciosa* 'Hayward' kiwifruit [39,40] and also in *Pyrus communis* 'Conference' pear [41].

The application of preharvest chitosan that acted as a barrier of oxygen might have affected the low production of ethylene in the kiwifruit by suppressing the key genes in the ethylene biosynthesis. Recently, He et al. [42] also observed that chitosan oligosaccharides (COS) suppressed the expression of genes involved in the ethylene biosynthesis (*FaACS* and *FaACO*), which reduced the softness and increased the shelf-life of strawberry fruit (*Fragaria ananassa* 'Qingxiang'). In addition, the resultant reduced production of ethylene and respiration rates might have positively influenced the retention of various quality attributes that prolonged the postharvest life of 'Garmrok' kiwifruit.
