Grafting the eggplant Solanum melongena onto Solanum torvum has been reported to be an effective method for reducing cadmium (Cd) concentration in the fruits of eggplant. Although Cd concentration in the roots of both species of eggplant was almost identical, it was observed to be higher in the shoot and xylem sap of S. melongena. We thus assumed that the translocation of Cd into the shoot of S. torvum was inhibited by a barrier function in its root tissues. Using synchrotron micro X-ray fluorescence (SR μ-XRF), we attempted to elucidate the role of root tissues in the inhibition of Cd transport to the shoot by comparing the microscale distribution pattern of Cd in the root tissues of S. melongena and S. torvum. Cd was localized in the central cylinder and epidermis in the transverse sections of S. melongena, regardless of the distance from the root apex. In the root section of S. torvum, the intensity of X-ray fluorescence from Cd, which was evenly distributed at 10 mm from the root apex, was very weak; in contrast, remarkable localization of Cd in the endodermis was observed in the root section at 40 mm. These contrasting distribution patterns of Cd explained the higher Cd concentration in the xylem sap of S. melongena than in the xylem sap of S. torvum. In conclusion, radial transport of Cd toward the central cylinder was restricted by an endodermal barrier—Casparian band or suberin lamella—present in the root of S. torvum. This barrier function of root tissue is one of the mechanisms responsible for low Cd loading to the xylem of S. torvum as compared to S. melongena.

Cadmium (Cd) concentration in eggplant (Solanum melongena) fruits can be drastically reduced by grafting them with Solanum torvum rootstock. However, mechanisms of Cd uptake in roots and translocation from roots to shoots in both plants are not understood well. We thus characterized symplastic Cd uptake in roots and translocation from roots to shoots via xylem loading process of both plants. Additionally, we also examined effect of other elements in roots uptake and translocation process with the increase of Cd concentration in the medium. Cd concentration in roots of S. torvum was almost similar to that of S. melongena for 7 days. On the other hand, Cd concentrations in the shoots and xylem sap were higher in S. melongena than in S. torvum. A concentration dependent experiment in roots indicated that Km values were almost the same for species, but the Vmax values were 1.5-fold higher in S. melongena than in S. torvum. A concentration dependent experiment in xylem loading process indicated that Km values were approximately 7-fold higher in S. torvum than in S. melongena. These results suggest that xylem loading process is a critical factor for determining Cd accumulation in the shoots of both plants. Several elements concentration in roots and xylem sap of both plants decreased with the increase of Cd concentration in the medium, suggesting that Cd uptake and translocation process are partly mediated by similar transporters involving these elements transport.