How does UV-B radiation influence the photosynthesis and secondary metabolism of Schisandra chinensis leaves?

Schisandra chinensis (Trucz.) is a medicinal plant with a rich profile of physiologically active substances, including lignans. The levels of these substances can be affected by external factors such as light exposure, temperature, and humidity. The current study aimed to explore the impacts of UV-B radiation on the photosynthetic properties and biochemical parameters as well as the contents of major lignans and phenolic compounds in two-year-old S. chinensis plants. The results revealed that exposure to UV-B radiation (10.0 μW cm−2, 30 min per day) did not cause any significant changes in the photosynthetic properties of S. chinensis plants until the 7th or 15th days of treatment. However, a significant reduction in photosynthetic capacity was observed after 30 days of UV-B exposure. The contents of photosynthetic pigments including chlorophyll a, b, total chlorophyll, and carotenoid decreased under UV-B treatment. Besides, we found that the highest level of lignan synthesis, a major bioactive compound in S. chinensis, occurred after seven days of UV-B treatment. However, lignan levels gradually decreased over time, with most individual lignans either having similar or lower contents than the control at 30 days. Furthermore, a comprehensive analysis of 35 phenolic compounds present in S. chinensis leaves showed that quercetin, isoquercetin, and 4-hydroxycinnamic acid were critical phenolic compounds responsible for moderating the plant's response to ultraviolet stress. Consistent with the trend of lignan content, these phenolic compounds exhibited the highest levels after seven days of UV-B treatment and gradually decreased over time. Our findings demonstrate that an appropriate dose of UV-B exposure can stimulate the synthesis of bioactive compounds such as lignans and phenolics via activation of the phenylpropanoid pathway in S. chinensis, enhancing its medicinal value.