UPLC-MS/MS-based analysis of the effect of bagging on polyphenols in apple varieties and metabolomic characterization before and after bag removal.

With the increasing cost of bagging cultivation, non-bagging cultivation has become an important direction for green and sustainable development. In this study, the most representative 'Fuji' and 'Ralls' apple varieties were used as test materials to investigate bagging's effect on apple polyphenols and post-removal metabolic mechanisms using UPLC-MS/MS and metabolomics. For the first time, the feasibility of non-bagging apple cultivation was explored from the perspective of nutritional quality regulation and polyphenol antioxidant properties. Results showed that the total phenol contents of 'Fuji' and 'Ralls' varieties significantly decreased under bagging treatment at maturity. After bagging, 'Fuji' had higher flavanols, flavonols and anthocyanins but lower dihydrochalcones and phenolic acids; 'Ralls' showed higher dihydrochalcones and anthocyanins but lower phenolic acids. Comprehensive evaluation indicates that bagging treatment significantly reduces the antioxidant activity of apple fruit. Metabolomics identified 31 and 19 differential metabolites in 'Fuji' and 'Ralls', with shared enriched pathways including flavonoid biosynthesis. In summary, these findings provide new insights into non-bagging apple cultivation and offer a theoretical basis for regulating the nutritional quality of apple fruit.