Comparative Analysis of Physiological and Biochemical Responses Between Compatible and Incompatible Graft Combinations of Cyclocarya paliurus

Cyclocarya paliurus (Batal.) Iljinsk is a multipurpose tree species with great potential for development. To identify indicators of grafting compatibility during the healing process, two cloned genotypes, CR4 and CR5, were used as scions, grafted onto seedling rootstocks derived from the Guangxi provenance. Following branch grafting, samples from the graft union were collected at 0, 20, 40, and 60 days. Physiological–biochemical indicators, including soluble sugars, soluble proteins, starch, peroxidase (POD) activity, polyphenol oxidase (PPO) activity, phenylalanine ammonia-lyase (PAL) activity, as well as flavonoid and phenylpropanoid contents, were analyzed. The graft survival rate of the CR4 combination (60%) was significantly higher than that of CR5 (35%). Significant differences in key physiological indicators were observed between the two scion–rootstock combinations. Analysis revealed that soluble sugar and soluble protein levels, along with POD and PAL activities in the scion during the early grafting stage, significantly influenced the final survival rate. Furthermore, lignans, lignin precursors, and several flavonoid compounds were found to accumulate preferentially at the graft union of the CR4 combination, which exhibited higher compatibility. These findings provide a physiological and biochemical foundation for selecting compatible scions and advancing clonal cultivation of C. paliurus.