Effect of shading on saponin content and biochemical indexes of Paris polyphylla Smith var. chinensis (Franch.) Hara in northern Zhejiang

Paris polyphylla Smith var. chinensis (Franch.) Hara is a rare traditional herb, the rhizome of which (named Paridis Rhizoma) is commonly used for its antitumor and anti-inflammatory effects. P. polyphylla mainly comprises saponins, the production of which is affected by the intensity of light. Therefore, the determination of the most suitable shading degree for this herb plant in northern Zhejiang can lay the foundation for the promotion of its’ artificial cultivation. In this study, the effects of different degrees of shading on the accumulation of saponins in P. polyphylla and on changes in their biochemical indices were explored. The plants were maintained at the arboretum test site of Zhejiang Agriculture and Foresty University, China (30°15ʹN, 119°43ʹE) from June to September 2019. Black shading nets were used to create different shading conditions (70%, 80%, and 90%). Six-year-old P. polyphylla plants were used as the test materials, and their contents of saponins, photosynthetic pigments, malondialdehyde (MDA), and osmotic adjustment substances as well as antioxidative enzymes activities were measured to determine their response to shading conditions.The results showed that the saponin content was negatively correlated with the superoxide dismutase (SOD) activity, peroxidase (POD) activity, and MDA content (P<0.01), but positively correlated with the chlorophyll a/b ratio, catalase (CAT) activity, and soluble protein content (P<0.05). The 80% shading condition was most beneficial to saponin accumulation, as evidenced by the highest total saponin content (7.19 mg·g⁻¹) in the plant compared with that yielded by the 70% (3.79 mg·g⁻¹) and 90% (1.85 mg·g⁻¹) shading conditions. The contents of all types of saponins were the highest under the 80% shading condition (polyphyllin Ⅰ: 1.45 mg·g⁻¹; polyphyllin Ⅵ: 2.52 mg·g⁻¹; and polyphyllin Ⅶ: 3.22 mg·g⁻¹). Additionally, the contents of photosynthetic pigments (total chlorophyll: 4.72 mg·g⁻¹) and soluble proteins (11.28 mg·g⁻¹) were higher in plants under 80% shading than in those under 70% (total chlorophyll: 2.94 mg·g⁻¹; soluble protein: 9.23 mg·g⁻¹) and 90% shading (total chlorophyll: 3.81 mg·g⁻¹; soluble protein: 7.72 mg·g⁻¹). However, the activities of POD (2.89 U·g⁻¹·min⁻¹) and SOD (105.01 U·g⁻¹·min⁻¹), contents of MDA (11.90 μmol·g⁻¹) and proline (8.88 mg·g⁻¹) in plants under 80% shading were lower than those in plants under 70% (POD: 3.20 U·g⁻¹·min⁻¹; SOD: 123.38 U·g⁻¹·min⁻¹; and MDA: 13.89 μmol∙g⁻¹; proline: 11.48 mg·g⁻¹) and 90% shading (POD: 4.90 U·g⁻¹·min⁻¹; SOD: 165.09 U·g⁻¹·min⁻¹; and MDA: 15.46 μmol∙g⁻¹; proline: 12.68 mg·g⁻¹). Moreover, compared with the plants treated with 80% shading, those treated with 70% and 90% shading showed significantly lower contents of total saponins. In summary, different degrees of shading significantly influenced the P. polyphylla saponin content and primary metabolism. The data from this study provide a theoretical reference for the artificial cultivation and quality improvement of P. polyphylla in northern Zhejiang.