Distribution of heavy metals in Metallophytes

Project aims of the planned research plan are anatomical and physiological studies on east alpine heavy-metal plants (Metallophytes) for the accumulation of heavy metals. The constant settlement of east alpine heavy metal locations ( calamine and copper ) is left up to some few plant species. The life cycle of these specialists ("Eumetallophyten") is stamped by a number of environmental physiological stress factors and the partly very high heavy metal content of the substratum. For the mastering the heavy metal stress the plants have developed a number of strategies to be able to survive on these partly very steep mine dumps. Among other things the ability of heavy-metal-uptake by plants and its fixation internally through different metabolic reactions is characterized often by the withdrawal of heavy metals from the current metabolism of these plants. Furthermore some species have the ability by permanently growing up new shoot organs above ground to create an additional storage capacity ( by new cells ) for heavy metals. This strategy has to be watched possibly at plants with rosette leaves and cushion habit. Main emphasis of the research project is one question where and how east alpine metallophytes accumulate heavy-metals or in which way plants do deposite and immobilize heavy-metals in various organs. Furthermore the expected results ( that is the more exact knowledge of the plant deposition in the organs above ground ) should provide an improved basis for the strategies to the renovation of the rhizosphere ( soil/ root area ) by " phytoremediation ".

Over the past 10 years, interest in plant metal accumulation has increased considerably. Metal hyperaccumulators are plants that are capable of extracting metals from the soil and accumulating them to extraordinary concentrations in aboveground tissues. Worldwide, about 400 species have been identified that can accumulate heavy metals (Ni, Zn, Cu, Pb, Cd, Co). The study objects of the present work were primarily representatives the Brassicaceae, since this group constitutes most of the forms (11 genera and 87 species) known to hyperaccumulate heavy metals. Thlaspi goesingense, a Ni-hyperaccumulator from an ultramafic site, was studied. Additional species from calamine (Pb, Zn)-soil were Thlaspi minimum and Arabidopsis halleri as well as Linaria alpina, a Scrophulariaceae. The anatomical structure of the plants as well as the uptake and distribution of heavy metals were investigated. The Ni concentration was highest in the rosette leaves of Thlaspi goesingense. This value was more than 5-times higher than in the roots. Although the Zn concentration in the soil was much lower than the Ni concentration, the values of the two heavy metals differed only little in the plant. This points to an active transport mechanism especially for Zn, because a passive influx alone would lead to greater differences in the concentration of the two metals in the plant tissue. Both Ni and Zn were identified mainly in the large epidermal cells, with no difference between upper and lower epidermis. The heavy metal concentrations were significantly lower in the other tissues. The heavy metals were predominantly localized in the vacuoles within the cells. In Thlaspi minimum, zinc was mainly transported to the rosette leaves and the leaves on the inflorenscense axis or flower shoots. In the aboveground organs of Arabidopsis halleri, Zn was also mainly located in the leaves on the inflorenscense axis and the leaves. Within the rosette leaves, the parenchyma cells of the chlorenchyma were favoured sites of heavy metal deposition. In both species - Thlaspi minimum and Arabidopsis halleri - the highest metal levels were found in cells with enlarged vacuoles. The biggest vacuoles are in the epidermis of T. minimum, where large elongated cells are situated between the normal cells that surround the stomata. The largest vacuoles in A. halleri are located in the parenchyma of the leaf. Linaria alpina showed the highest zinc concentration in the root tissue; this metal was hardly transported into the aboveground organs. This species is therefore a typical representative of the excluder plants. In all species on calamine substratum, which had high lead concentrations, lead was found in only low amounts in the aerial parts of the plants. In all species, lead was fixed predominantly in the underground biomass. By being bound to the cell walls of the root cortex (probably by an apoplastic barrier), this heavy metal is largely excluded from transport into the shoots.