Serendipita indica Enhances Drought Tolerance in Phoebe sheareri Seedlings by Improving Photosynthetic Efficiency, Stimulating the Antioxidant Defense System, and Modulating Hormone Synthesis

In the context of contemporary climate change, drought is widely recognized as a major stressor affecting plant growth. While numerous studies have demonstrated that <i>Serendipita indica</i> enhances stress resistance in host plants and is widely used in agriculture, research on its symbiotic interactions with woody plants for improving drought tolerance remains limited. This study investigated the effects of <i>S. indica</i> inoculation on the growth of <i>Phoebe sheareri</i> seedlings under varying drought conditions-well-watered (WW), moderate drought (MD), and severe drought (SD)-and explored the physiological mechanisms underlying improved drought resistance. The results showed that under WW conditions, <i>S. indica</i> inoculation promoted seedling growth and development. Under MD and SD conditions, although drought stress inhibited growth, inoculation significantly increased plant biomass, root parameters, chlorophyll content, and photosynthetic efficiency. Additionally, it alleviated drought-induced damage by reducing REC, MDA, H₂O₂, and O₂^- levels, while enhancing SOD, POD, and CAT activities, and increasing root ABA, GA, IAA, and CTK content. Under MD stress, adaptive changes in root architecture and hormone levels were observed, including increases in total root length, surface area, volume, average diameter, and elevated IAA and CTK levels-all of which were further enhanced by <i>S. indica</i> inoculation. In conclusion, symbiosis with <i>S. indica</i> improved drought tolerance in <i>P. sheareri</i> seedlings likely through enhanced photosynthesis, antioxidant enzyme activity, and hormone regulation.