Leaf physiological and endophytic microbial community characteristics and interactions of different scions grafted onto <i>Malus sieversii</i>

Endophytic microbial communities in scion leaves substantially impact the growth efficiency of apple trees (Malus × domestica Borkh.); however, the underlying mechanisms remain underexplored. Herein, we grafted three varieties-Malus sieversii, Hanfu and Fuji-onto M. sieversii (Ledeb.) M. Roem rootstocks and employed high-throughput sequencing technology to investigate how physiological traits of scion leaves influence endophytic microbiota and apple tree growth. Compared with the M. sieversii scion, the aboveground (+49.28%) and root (+62.77%) biomass of juvenile trees grafted with the Hanfu scion significantly increased, with the net photosynthetic rate and stomatal conductance rising by 20.40% and 42.26%, respectively. Additionally, the leaves of the Hanfu scion exhibited a significant increase in sucrose synthase activity and carbon accumulation (CA) compared with the M. sieversii and Fuji scions, while the carbon content and carbon-to-nitrogen ratio (C/N) significantly decreased. Furthermore, through 16S rDNA and internal transcribed spacer high-throughput sequencing, we found that the diversity and abundance of endophytic bacteria and fungi in the leaves of the Hanfu scion were higher than in the M. sieversii and Fuji scions. Hanfu scion leaves were predominantly enriched with the phyla Firmicutes and Ascomycota and the genus Salinicoccus. A close association was observed between leaf endophytic bacterial and fungal communities and physiological traits, with particularly significant correlations in the fungal communities. Parameters such as leaf intercellular carbon dioxide concentration, chlorophyll b content, C/N and CA were implicated in enriching dominant endophytic microbial phyla and genera. Through partial least squares structural equation models, we confirmed that leaf photosynthetic properties and carbon and nitrogen metabolism significantly affect leaf carbon and nitrogen accumulation through the regulation of endophytic fungal diversity, thereby affecting apple tree growth. In conclusion, the interaction between leaf physiological properties of different scion varieties and the diversity and composition of endophytic microbial communities influences apple tree growth.