Physiological characteristics and transcriptomic insights into rootstock-scion interactions in apple under cadmium stress

Cadmium (Cd) contamination of orchard soils threatens tree growth, fruit quality, and food safety, and strategies are needed to limit Cd accumulation in apple. Grafting is widely used in apple production, but how rootstock-scion interactions regulate Cd uptake and detoxification remains unclear. This study aimed to elucidate the physiological and molecular mechanisms by which different apple graft combinations modulate Cd accumulation and tolerance. Four graft combinations comprising 'Hanfu' (HF) or 'Fuji' (FJ) scions grafted onto Malus baccata (Mb) or M. micromalus (Mm) rootstocks, were grown in nutrient solution with or without 50 μM CdCl₂. Plant growth, Cd2+ influx, Cd accumulation and localization, antioxidant capacity, cell wall composition, and root and leaf transcriptomes were analyzed. Cd stress reduced biomass, root system development, and PSII efficiency but generally increased non-enzymatic antioxidant capacity. Mb rootstocks showed lower root net Cd2+ influx, lower Cd accumulation in roots and leaves, and weaker Cd signals in xylem than Mm, indicating a greater ability to restrict Cd uptake and transport. Across combinations, root cell walls were the major Cd sink, and Cd exposure increased pectin, hemicellulose, and lignin contents. Transcriptome analyses revealed rootstock and scion specific Cd responses, with distinct enrichment of genes related to photosynthesis, oxidative stress, and cell wall metabolism. These findings provide a physiological and molecular basis for selecting low-Cd apple graft combinations.