Heavy metal tolerance of Minuartia verna

Individuals of Minuartia verna ssp. hercynica, growing on heavy metal polluted soil of medieval mine dumps, accumulate remarkable concentrations of copper and zinc in their leaves. The cellular and intracellular distributions of heavy metals were investigated by conventional electron microscopy, EDX, ESI and EELS. Considerable amounts of Fe, Cu, Zn, and Al found on the leaf surface are excreted by hydrathodes. Intracellular spaces and cell walls of the leaf parenchyma contain Fe, Cu, Zn, and Pb, whereas no metals could be detected in the cytoplasm, vacuole or cellular organelles. High Si concentrations evident in vacuoles, cell walls and intercellular spaces are not directly involved in the detoxification of the heavy metals with one exception, zinc is precipitated as a Zn-silicate in the epidermal cell walls. The possible mechanisms of heavy metal tolerance in Minuartia and the role of Si in these processes are discussed.