Abstract: Root cell wall is the first barrier to the transfer and translocation of Cd from soil to plants. To understand the mechanism of Cd transfer and translocation by tobacco cultivars affected by subcellular Cd accumulation and binding forms in root cell walls, we investigated the bioconcentration factor (BCF) and transfer factor (TF) of Cd in roots, shoots and leaves of twelve tobacco cultivar species in pot experiments. We determined the subcellular Cd accumulation characteristics and binding forms in root cell walls using in vivo bioseparation technology. We then studied the relationship between Cd uptake and translocation by tobacco plants and subcellular Cd accumulation and binding forms in root cell walls. The results showed significant difference (P<0.05) in BCFs of Cd in roots, shoots and leaves among the tested tobacco cultivars. For example, the Cd concentrations in roots ranged from 3.13 to 7.10 mg/kg, a maximum 2.27 fold variation; the Cd concentrations in tobacco leaves ranged from 3.29 to 9.93 mg/kg, a maximum 3-fold variation. Similarly, the TFs of Cd from root to leaf varied from 0.50 to 3.17, a maximum 6.34 fold change among cultivars. Our analysis indicated that the insoluble forms of Cd, binding to phosphate (Cd(PO4)n)or hemicellulose (FHAC-Cd), were the main binding forms of Cd in plant roots. The results further showed that Cd concentrations in root cell walls increased significantly after removing pectin while decreased sharply after removing hemicelluloses. We found negative correlation (P<0.05) between root-to-leaf TFs and Cd contents of cell walls after removing pectin; e.g., cultivar K346 had the highest Cd content but lowest root-to-leaf TF among the cultivars. The results implied that hemicelluloses bind most of insoluble forms of Cd in root cell walls, impeding the long distance migration and translocation of Cd from soil through xylem to the above ground parts of tobacco plants.

Key words: tobacco cultivars, cadmium, translocation factors, binding forms, subcellular distribution