Integrated physiological and anatomical analyses reveal the mycorrhizal symbiosis efficiency of Heptacodium miconioides under different nitrogen conditions.

Arbuscular mycorrhizal fungi (AMF) have been shown to play a role in protecting endangered plants from environmental stresses. However, the adaption mechanisms by which AMF symbiosis influences the physiological and anatomical traits of the endangered species Heptacodium miconioides under varying nitrogen (N) conditions remain unclear. Here, we examined the effects of three types of AMF inoculation treatments-Rhizophagus intraradices, Glomus versiforme, and a mixture of the two-on the growth, photosynthesis, antioxidant enzyme activity, and stem and leaf structure of H. miconioides seedlings under low nitrogen (LN) and normal nitrogen (NN) conditions. Findings indicated that LN conditions significantly restricted the growth, physiological and anatomical properties of non-inoculated seedlings. In contrast, AMF inoculation improved dry weight, net photosynthetic rate, chlorophyll content, catalase activity, and peroxidase activity in seedlings under LN. Under LN conditions, AMF colonization-particularly a combination of R. intraradices and G. versiforme-significantly increased stomatal size and aperture by 23.00%, 64.90%, respectively, while significantly reducing stomatal density by 25.00%. Furthermore, treatment with mixed AMFs resulted in substantially greater palisade tissue thickness, stem phloem, stem xylem, stem mean vessel diameter, and stem xylem/phloem ratio, which increased by 41.17%, 20.34%, 46.00%, 14.30%, and 21.62% respectively. These findings indicate that mixed AMF inoculation exhibits superior mycorrhizal efficiency for the host plant, enhancing photosynthetic efficiency, antioxidant enzyme activity, and improving stomatal traits, leaf assimilative tissues, and stem conductive tissues under LN conditions. The study also suggests the potential use of AMF in the cultivation and protection of H. miconioides under N-poor habitats.