Study on the Physiological and Metabolic Mechanisms of Graft Union in Camellia pyxidiacea var. rubituberculata

This study used Camellia pyxidiacea var. rubituberculata as the rootstock and the improved cultivar Camellia oleifera ‘Qianyou 2’ as the scion for heterografting (CO group), with homografting (using C. pyxidiacea var. rubituberculata as both rootstock and scion) as the control. The physiological and metabolic changes during graft healing were investigated by measuring enzyme activities, targeted analysis of endogenous hormones, and non-targeted metabolomics via LC-MS/MS. The results indicate that, compared to homologous grafting, an integrated response pattern characterized by “defense prioritization and growth suppression” was observed in heterologous grafting combinations. At the hormonal level, heterografting significantly inhibited growth-promoting hormones (e.g., zeatin riboside), while defense-related hormones (salicylic acid SA, jasmonic acid JA) and their metabolite, capsidiol, were continuously accumulated and upregulated. At the metabolic pathway level, phenylpropanoid metabolism was redirected: the activity of phenylalanine ammonia-lyase (PAL) was suppressed, shifting synthesis toward soluble defense compounds like coumarins. Enzymatically, the activities of polyphenol oxidase (PPO) and peroxidase (POD) were significantly enhanced in the mid-to-late stages of healing, promoting the oxidative crosslinking of phenolic compounds and the formation of lignin-related physical barriers. Thus, these three mutually corroborating levels—hormonal signaling, dynamic enzyme activities, and global metabolic networks—collectively point to a healing mode characterized by defense prioritization and growth suppression. This study provides theoretical guidance for the grafting utilization of C. pyxidiacea var. rubituberculata.