Boron Mitigates Cadmium Toxicity by Reducing Cadmium Accumulation, Enhancing Cell Wall Immobilization and Regulating Gene Expression in Malus Rootstock Under Hydroponic Conditions

To investigate the mitigating role and underlying mechanisms of exogenous boron (B) in cadmium (Cd)-stressed woody fruit trees, a hydroponic study was conducted using Malus hupehensis Rehd. seedlings treated with different B concentrations (0, 12.5, 50, and 150 μM H3BO3). Cd stress significantly inhibited plant growth and reduced photosynthetic parameters, pigment content, biomass, and root activity, but induced reactive oxygen species (ROS) accumulation. In contrast, the 50 μM B treatment (B2) effectively alleviated Cd toxicity. This treatment significantly decreased Cd accumulation, bioconcentration factor, and translocation factor across tissues. The B2 treatment enhanced Cd immobilization in root cell walls by increasing pectin content and pectin methylesterase activity. Additionally, it shifted Cd chemical forms toward lower-toxicity forms—increasing pectin- and protein-bound, phosphate-bound, and oxalate-bound Cd—while reducing inorganic and water-soluble Cd fractions. The B2 treatment elevated the activities of superoxide dismutase and peroxidase, and increased free proline and ascorbic acid, thereby reducing ROS and malondialdehyde accumulation. The B2 treatment also downregulated key genes including ZIP6 and IRT1 involved in Cd uptake. In conclusion, the most effective concentration among those tested (50 μM) alleviated Cd stress in Malus hupehensis Rehd., accompanied by modified Cd uptake and translocation, enhanced cell wall fixation, altered Cd chemical forms, improved antioxidant responses, and changes in stress-related gene expression.