Mycorrhizal network: A bidirectional pathway between green-leaved terrestrial orchids and pine trees

Jianxin Chen¹Xiang Ma²Fengjinglin Wu¹Na Qiao³Xingliang Xu³Jianrong Wu^1*

• ¹Southwest Forestry University, Kunming, China
• ²University of Electronic Science and Technology of China, Chengdu, China
• ³Institute of Geographic Sciences and Natural Resources Research, Beijing, China

Increasing evidence demonstrates that plant roots can be connected via mycorrhizal networks. Such networks in roots play key roles in plant physiology and ecosystem functioning, but it remains debated whether bidirectional transfers of resources can occur simultaneously inside the network. We constructed a system to test for bidirectional transfer of carbon and nitrogen, using 13C and 15N dual labelling, between terrestrial orchids (Cymbidium goeringii, C. goeringii var. serratum, and C. faberi) and Pinus yunnanensis seedlings linked via one Ceratobasidium sp.. A unidirectional transfer of carbon and nitrogen was observed between C. goeringii and pine seedlings. In contrast, simultaneous bidirectional transfer of both elements occurred between the other orchids and pine seedlings; 1.0-3.7% of assimilated carbon and 0.20-12.2% of acquired nitrogen were transferred through the network. The net carbon transfer was always directed from pine seedlings to orchid. Nitrogen transfer showed three distinct patterns: (i) unidirectional from C. goeringii to pine seedlings, (ii) bidirectional but no net transfer between C. faberi and pine seedlings, and (iii) bidirectional with a net transfer from C. goeringii var. serratum to pine seedlings. The divergent transfer patterns among orchid species suggest that mycorrhizal networks may play species-specific roles in forest understories, potentially influencing orchid recruitment and nutrient dynamics.