Changes in antioxidant substances and antioxidant enzyme activities in raspberry fruits at different developmental stages

Raspberry is a small berry shrub with a relatively recent cultivation history in China, and its fruits have remarkable antioxidant activity. In this study, the raspberry cultivars 'Clode Summit', 'Heritage' and 'Bristol' were used as experimental materials to determine the color parameters, dry and fresh weights, ROS, MDA, ASA and GSH contents, activities of SOD, CAT and POD, and total antioxidant activity of fruits in different harvest periods. In addition, we analyzed the relative gene expression levels of SOD, CAT, POD and related transcription factors in the flavonoid synthesis pathway during fruit development. The results showed that the production rate of O2− and the content of MDA increased gradually, while the activities of CAT and POD decreased gradually. The growth and development of raspberry fruits activated the SOD enzyme system, but the change trend of SOD activity was different among different cultivars. Principal component analysis and correlation analysis showed that the antioxidant capacity of raspberry mainly came from ASA, GSH and anthocyanins. The content of ASA in green raspberry fruits was generally higher than that in red raspberry and black raspberry fruits. Black raspberry had the highest content of GSH and anthocyanin, as well as antioxidant activity. Correlation analysis showed that POD activity was significantly correlated with the expression level of RuPER17. The expression levels of RuPAL, RuFLS, RuMYB6, RuMYB306 and RuWRKY24 in the three raspberry fruits first increased and then decreased. The results provide a reference for further exploration of the mechanism of antioxidation and for regulating the biosynthesis of related enzymes in raspberry fruits.