Integrative study of subcellular distribution, chemical forms, and physiological responses for understanding cadmium tolerance in two garden shrubs

Background and aims Urban ornamental shrubs have significant potential for restoring cadmium (Cd)-contaminated soil. Simulated pot pollution was applied to Buxus sinica and Ligustrum × vicaryi to study their Cd enrichment characteristics and tolerance mechanisms. Methods Cd content and accumulation were analyzed in different plant organs, subcellular distribution and chemical forms of Cd in the roots, and the effects of Cd on the ultrastructure of root cells under various Cd concentrations (0, 25, 50, 100, and 200 mg·kg⁻¹). Results (1) With increasing Cd treatment levels, the total biomass of B. sinica gradually decreased, while L. × vicaryi exhibited a stimulation effect at low Cd concentrations and inhibition at high Cd concentrations. (2) The Cd content in different organs of both shrubs increased with rising Cd levels, with L. × vicaryi showing a significantly higher increase than B. sinica, indicating a stronger Cd accumulation capability in L. × vicaryi . (3) Cd in the root of both shrubs was primarily present in NaCl-extractable forms, and was majorly bound to the cell wall. (4) Excessive Cd caused damage to the cellular structure of B. sinica leaves, while the cells of L. × vicaryi leaves maintained normal morphology. (5) In both shrubs, Cd primarily binds to the cell wall through hydroxyl, amino functional groups, and soluble sugars. Conclusion Converting Cd to less active forms, immobilizing Cd in the cell wall, and providing binding sites through functional groups may be crucial resistance mechanisms for both shrubs in response to Cd stress.