Postharvest UV-B radiation increases enzyme activity, polysaccharide and secondary metabolites in honeysuckle (Lonicera japonica Thunb.)

Postharvest ultraviolet-B (UV-B) radiation can modulate the accumulation of bioactive compounds with many pharmacological effects in plants. Honeysuckle (Lonicera japonica Thunb.) is a UV-B tolerant crop which has a high medical value. The effects of UV-B on bioactive compounds in its flowers have been reported, while very few studies focused on the leaves and stems. Therefore, the effects of postharvest UV-B radiation on basic physiological traits and bioactive compounds (polysaccharides and secondary metabolites) in the leaves, stems and flowers of honeysuckle were investigated in this study. In this study, the leaves, stems and flowers of honeysuckle were exposed to UV-B radiation (0, 8.4 and 22.4 μW cm⁻²) for different times (2, 4, 6 and 8 h), and variables were detected after 24 h after they were able to get a repair time. The results showed that the contents of chlorophyll, carotenoid, soluble sugar, and the activities of catalase (CAT) and superoxide dismutase (SOD) in postharvest leaves were increased after UV-B treatment. But the malonaldehyde (MDA) content in leaves decreased when the duration of UV-B treatment lasted for 4 h. Besides, the contents of polysaccharide, total polyphenols, total flavonoid, and chlorogenic acid in different organs all increased significantly and reached a peak under the UV-B₁ (8.4μW cm⁻²) treatment for 2 h, whereas they reached the lowest point under the 6 h of UV-B₂ (22.4μW cm⁻²) exposure according to the heat map analysis. Interestingly, the Pearson correlation analysis revealed that the total flavonoid content was positively correlated with the chlorogenic acid content in the flowers of honeysuckle, and the total flavonoid content was negatively correlated with the contents of chlorophyll, CAT and carotenoid in plant leaves. In a word, the secondary metabolites and polysaccharide of honeysuckle can be increased by postharvest UV-B, and the quality can be improved by adjusting its physiological traits. In addition, the content of bioactive substances is correlated with the physiological traits. The results will provide a basis for improving the medicinal values of honeysuckle by postharvest UV-B treatment.