Subcellular Distribution of Metals within Brassica chinensis L. in Response to Elevated Lead and Chromium Stress

Differential centrifugation and synchrotron radiation X-ray fluorescence spectroscopy (SRXRF) microprobe were used to study the distribution of the elements in tissue cross sections of pakchoi (Brassica chinensis L.) under stress of elevated Pb and Cr. Subcellular fractionation of the different tissues grown in a nutrient solution containing 200 mg L–1 Pb or 5 mg L–1 Cr showed that 86.7 and 76.3% of the Pb that accumulated in the roots and shoots, respectively, was contained in the cell wall and vacuoles in those areas. Whereas 75.0% of the Cr that accumulated in the root was contained in the cell wall, 63.1% of the Cr that accumulated in the shoot was found in the vacuoles and cell wall. SRXRF analysis revealed that, when pakchoi seedlings were placed under excess Pb stress, the Pb, Ca, Cu, and Zn were concentrated in the cortex and vascular bundle of the root and mixed Fe–Mn plaques were seen on the surface of the pakchoi root. In the Cr treatment group, Cr, Ca, Mn, and Zn were mainly located in the cortex of the root, whereas in the stem, only Ca, Cu, and Zn were detected at higher levels in the cortex area. Thus, this study provides evidence that, in response to Pb and Cr stress, pakchoi uses cell walls and vacuoles to reduce the transport of these heavy metals through the plant, as well as restrict transport from the root to the stem.