Cadmium enrichment influences cell wall properties and leaching resistance of tension and opposite wood in poplar

Poplar is a high-biomass species and is widely used for the phytoremediation of soils contaminated with cadmium. It is often subjected to heterogeneous soil conditions and associated stresses that promote reaction wood formation during growth. However, there is limited understanding of the anatomical and chemical responses of its reaction wood to Cd stress. This study examined the structural and physicochemical properties of tension wood (TW) and opposite wood (OW) formed in inclined Cd-enriched poplars. Varying Cd concentrations (0–100 mg/kg) were applied, and vessel wall thickness, cellulose crystallinity, microfibril angle (MFA), functional group composition, and Cd leaching behavior were systematically assessed. The results revealed non-linear crystallinity and vessel wall thickness responses in TW and OW, with the lowest values occurring at moderate Cd levels (20 mg/kg). MFA increased monotonically in OW, while TW showed a biphasic pattern. FTIR analysis indicated distinct chemical signatures between TW and OW under Cd exposure. Sequential extraction revealed that Cd was predominantly retained in acid-extractable and reducible forms. Significant Cd leaching occurred only during aqueous immersion, whereas retention remained stable under 75 % relative humidity. This study reveals how cadmium stress differentially alters the structure and chemistry of tension and opposite wood in poplar, providing critical insights for the safe utilization of Cd-enriched biomass in phytoremediation. • Microstructure changed significantly when Cd exceeded 20 mg/kg. • Chemical composition differed between tension and opposite wood. • Cd forms in cell walls ranked: acid-extractable > reducible > water-soluble. • Cd leached significantly after cyclic water soaking. • Cd content in wood remained stable under humid conditions.