Histological and ultrastructural evidence for zinc sequestration in Solanum nigrum L.

The accumulation of contaminants in the environment due to anthropogenic activities is a matter of global concern. Solanum nigrum L. plants, able to accumulate zinc and hyperaccumulate cadmium, were challenged with 0.025 g Zn L-1 during 35 days. The localization of Zn in roots, stems and leaves of S. nigrum plants was evaluated by autometallography (AMG) in order to determine the specific tissue, cell and subcellular compartments of Zn sequestration. This Zn concentration resulted in stunted plant growth but no other symptoms of Zn toxicity. Zinc concentration in the plants was highest in the roots, 666 mg Zn kg(-1) fresh weight (f.w.), and lower in the stems, 318 mg Zn kg(-1) f.w., and leaves, 101 mg Zn kg(-1) f.w. Roots of Zn-treated plants showed an underdeveloped structure but additional layers of proliferating cortical parenchyma cells. AMG of S. nigrum roots, stems and leaves revealed a generalized Zn distribution associated with the cell walls in all tissues. In the vasculature (xylem and phloem), Zn was observed at the plasma membrane-cell wall complex of vascular parenchyma cells and conducting elements. Conspicuous Zn deposits were detected in the vacuoles of cortical parenchyma and starch sheath, as well as in the tonoplast of the mesophyll cells. Our results suggest that Zn flux through the plant occurs via the xylem and phloem and associated parenchyma until it is conducted to permanent storage sites, namely, the apoplast and vacuoles of cortical parenchyma cells of the root, stem and the leaf mesophyll.