Biogeochemical survey of some ophiolitic outcrops in Tuscany.

The mineral composition of plants and soils collected in three localities of the ophiolitic outcrops located in the ''Metalliferous Hills'' (Tuscany) has been assessed. The majority of the plants show an higher than normal Ni concentration; the element is generally accumulated in the roots, but with the exception of Alyssum bertolonii DESV. which hyperaccumulates Ni (0.5-1.6% DW), som species such as Centaurea aplolepa MORETTI spp. carueliana (MICHELETTI) DOSTAL show relatively high Ni foliar concentrations (50 - 100 mug g-1 DW). Low concentrations of Co and Cr were always detected in the plant samples. Notwithstanding the soil unfavourable Ca/Mg quotient (< 1), the plants, with few exceptions, take up more Ca than Mg. In one locality a high soil Ca content is associated to a high frequency of species with low Ni levels; this seems to confirm the hypothesis that Ca reduces the toxic effects of metals by limiting their uptake.