Comparison of cadmium distribution in the root tissues of Solanum melongena and Solonum torvum, which have different abilities for cadmium transport from the root to shoot
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.