Structural Changes and Cd Fixation Mechanisms of Hemicellulose and Cellulose in Roots and Leaves of Solanum nigrum under Cadmium (Cd) Stress

To investigate the structural changes of hemicellulose and cellulose under cadmium (Cd) stress and their impacts on Cd immobilization, this study examined the Cd enrichment capacity and polysaccharide property alterations of hemicellulose and cellulose in leaves and roots of Solanum nigrum. The results show that, overall, the Cd-binding capacity of the two polysaccharides in the leaf cell walls of the 100 μmol·L^-1 Cd treatment group was significantly increased by an average of approximately 1.21 times compared with that of the 50 μmol·L^-1 Cd treatment group; however, there was no significant difference in the Cd-binding capacity of the two polysaccharides in the roots (P>0.05). Hemicellulose played a more important role in the Cd immobilization process of the cell walls in both roots and leaves, with the average Cd enrichment in roots and leaves reaching 2.97 times that of cellulose. For hemicellulose, its content progressively increased with Cd stress intensity, accompanied by a reduction in xyloglucan (the primary backbone component) and increased proportions of galactose and mannose in polysaccharide composition. FTIR spectroscopy revealed peak shifts corresponding to hydroxyl and carboxyl groups under Cd stress, indicating their involvement in Cd immobilization. The elevated activities of hemicellulase/xyloglucanase under Cd stress exposed additional Cd-binding sites. Uronic acid analysis suggested extensive cross-linking between hemicellulose and other cell wall components, potentially providing additional Cd-binding sites. Regarding cellulose, hydroxyl groups on sugar chains demonstrated limited Cd immobilization capability, though with relatively low overall contribution. Under Cd stress, both cellulose content and polysaccharide molecular weight substantially increased in roots, primarily serving structural support and physical barrier functions.