HEAVY-METAL TOLERANCE AND ACCUMULATION IN METALLICOLOUS AND NON-METALLICOLOUS POPULATIONS OF THLASPI-CAERULESCENS FROM CONTINENTAL EUROPE

In continental Europe, the heavy metal hyperaccumulator Thlaspi caerul escens occurs both on heavy-metal polluted soils (subsp. calaminare) a nd on soils with normal heavy metal content (subsp. caerulescens). In order to assess the extent and partitioning of variation in heavy meta l tolerance and foliar mineral composition, twelve families from two p opulations of each subspecies were grown in pots in four soil treatmen ts differing in heavy metal (Zn, Pb) and macronutrient concentrations. The two subspecies differed systematically in many respects. Subsp. c alaminare had a higher survival at high levels of heavy metals and a h igher tolerance index in all treatments. It also had three times lower foliar zinc and lead concentrations when grown at moderate levels of heavy metals. This, together with a negative correlation of foliar Pb concentration with growth in subsp. caerulescens, suggests that heavy metal accumulation per se is not a mechanism of tolerance in this spec ies. Variation among families within populations accounted for a large r proportion of total variance in growth and mineral composition than variation between populations. Additionally, within population variati on in heavy metal tolerance and accumulation was significantly lower i n subsp. calaminare. This suggests that, adding to a background consti tutive tolerance at the species level, natural selection has increased heavy metal tolerance in metallicolous populations of Thlaspi caerule scens.