Anatomical Microstructure Characteristics and Dynamic Contents of Soluble Sugars, Soluble Proteins, and Tannins at the Graft Union During Healing of Different Crabapple Rootstock–Scion Combinations

Grafting is the primary vegetative propagation technique for ornamental crabapple, and rootstock–scion compatibility critically determines graft healing efficiency and seedling quality. Selecting suitable rootstocks is essential for the popularization of new crabapple cultivars, yet the healing performance of the newly bred cultivar Malus ‘Huabiao’ grafted onto different local Malus rootstocks remains unclear. This study evaluated graft compatibility for M. ‘Huabiao’ grafted onto three rootstocks (M. hupehensis, M. robusta, and M. baccata) through histological observation and dynamic detection of soluble sugar, soluble protein, and tannin during graft healing. The results indicated that M. ‘Huabiao’/M. hupehensis and M. ‘Huabiao’/M. robusta achieved higher survival rates (80.0% and 83.3%, respectively) and complete vascular reconstruction within 28 days, whereas the M. ‘Huabiao’/M. baccata combination exhibited a lower survival rate (63.0%) and obvious healing lag. All combinations showed consistent physiological variation trends, while the M. baccata group maintained higher soluble sugar content, greater protein fluctuations, and significantly elevated tannin levels (p < 0.05). Physiological substance accumulation was closely correlated with healing performance, with higher soluble sugar and protein levels associated with better wound recovery, and excessive tannin accumulation correlated with retarded tissue regeneration capacity. Distinct healing asynchrony was observed among combinations, with incomplete vascular connection detected in the M. baccata group at day 28. This study confirms that M. hupehensis and M. robusta are superior compatible rootstocks for M. ‘Huabiao’, and the dynamic levels of the three physiological indices can serve as potential indicators for evaluating crabapple graft compatibility.