Effects of different intermediate rootstocks on epidermal wax and cuticle of Fuji apple fruits during storage

Abstract:【Objective】Apple skin wrinkling, caused by water loss during storage, is a critical factor that impacts the quality and taste of the fruit. The cuticle and wax layer of apple fruits play an important role in inhibiting water transpiration and respiration. Therefore, the qualitative and quantitative determination of wax and cuticle fractions in apple fruits during storage can help enhance fruit storage practices in the future. Although studies have been conducted on the effects of various rootstocks on apple fruit quality, antioxidant activity and disease resistance, the impact of different rootstocks on fruit wax and cuticle during apple storage has not been reported. SH6 and M26 are two types of dwarf interstocks commonly utilized in the cultivation of Fuji apples in Northwest China. The former was selected by Shanxi Pomology Institute, and the latter was chosen by East Malling Experimental Station in Britain. In this study, Fuji apple trees on SH6 and M26 intermediate rootstocks were used as test materials. Com-parison of differences in weight loss, respiration intensity, and epidermal wax morphology and structure during storage of Fuji apples with SH6 and M26 as intermediate rootstocks was carried out.【Methods】 In this study, the weight loss of fruits growing on two types of rootstocks during storage was measured using the gravimetric method. The changes in respiration rate of fruits growing on two types of rootstocks during ambient storage were measured using a fruit and vegetable measuring instrument. Morphology of the epidermal wax layer was observed under a scanning electron microscopy. The waxy layer and cuticle of the fruit epidermis were extracted using organic solvents like chloroform and methanol. The total amount of cutin and wax, as well as the content of various monomer components, were analyzed using GC- MS. In order to compare the storage characteristics of fruits, the differences in horny and wax morphology and composition between the two rootstocks were examined.【Results】The results showed that the weight loss and respiration rate of SH6 fruits were significantly lower than those of M26 fruits. There were significant differences in surface wax morphology and thickness between SH6 and M26. The wax on the surface of SH6 fruits was relatively smooth with a few wide gaps and depressions. The surface of M26 fruits was densely covered with waxy roughness and depressions. With the prolongation of storage time, the wax on the surface of SH6 fruits became coarser, and the gaps and depressions increased while the gap spacing decreased. However, the waxy depression on M26 fruit epidermis was more pronounced, with larger and wider gaps visible. The thickness of the epidermal wax layer, total wax and total cuticle were also significantly higher than those of M26 fruits during ambient storage. 28 common cuticle components included 14 fatty acids, 7 hydroxy fatty acids, 2 dicarboxylic acids and 5 alcohols. 34 common wax components included 5 alkanes, 6 alcohols, 4 fatty acids, 12 esters, 2 aldehydes and 4 other substances. In addition, SH6 fruits contained four unique components: arachidonic acid, 18- hydroxy- 9,12- octadetadienoic acid, ursolic acid methyl ester and butyl eicosanate. M26 fruits contained three unique components: octadecanoic acid, 7,10,13-eicosatrienoic acid and behenic acid ethyl ester. Correlation analysis revealed a significant negative correlation between the weight loss rate of fruit during storage and the thickness of the wax layer. This suggested that fruits with thicker wax and cuticle layers experienced lower rates of weight loss. The weight loss rate was positively correlated with hydroxyl fatty acids and esters, indicating that substances with higher hydrophilicity facilitated water evaporation more effectively. Respiratory intensity was negatively correlated with the thickness of epidermal wax and cuticle layer. This suggested that the thicker the wax layer and cuticle, the lower the respiratory intensity of fruits. Respiratory intensity was positively correlated with the levels of hydroxyl fatty acids and dicarboxylic acids, suggesting that these compounds were advantageous for gas exchange both within and outside the fruit epidermis.【Conclusion】Compared with M26 fruits, SH6 fruits had lower weight loss and respiration rate during ambient storage, and the surface morphology of the epidermic wax layer was smoother. SH6 fruits had a thicker epidermal wax layer and a richer cuticle and wax content, resulting in lower weight loss and respiration rate during storage.