The Physiological Mechanism of Low-Temperature Tolerance Following the UV-B Radiation of <i>Eucommia ulmoides</i> Oliver

<i>Eucommia ulmoides</i> Oliver with rich active components, such as flavonoids, lignans, polysaccharides, is used as a medicinal plant. Unfortunately, its popularization and cultivation are limited due to its low-temperature sensitivity. In this study, we aimed to explore the effect of different doses of ultraviolet-B (UV-B) radiation (UV-1, UV-2, and UV-3) and low-temperature (LT) stress, both applied individually and in combination, on the photosynthetic properties, biochemical parameters, and the contents of salicylic acid in <i>E. ulmoides</i> plants. The results showed that UV-B radiation alone significantly reduced photosynthetic performance and soluble total sugar content, as well as causing increases in soluble protein, proline, and superoxide anion content and antioxidant activity including SOD, POD, CAT, total phenol, and total flavonoid content. The leaf thickness and photosynthetic parameters significantly increased, as well as a significant decrease in SOD activity and soluble sugar, proline, and superoxide anion content after 14 days of none-UV-B radiation exposure. UV-B combined with LT significantly improved photosynthetic properties, <i>Chl</i> content, and soluble sugar content but significantly decreased proline content. Principal component analysis showed that salicylic acid was the key factor in improving LT tolerance, and UV-2 radiation showed the best LT resistance. We aim to provide new ideas and a theoretical basis for the directional cultivation and LT stress tolerance research of <i>E. ulmoides</i>. Our findings demonstrate that the combined effect was more positively helpful in improving the ability to resist LT tolerance via the improvement of photosynthetic ability and the increase in soluble sugar and salicylic acid content in <i>E. ulmoides</i>.