Summary• Here, Thlaspi caerulescens populations from contrasting soil types (serpentine, calamine and nonmetalliferous) were characterized with regard to tolerance, uptake and translocation of zinc (Zn), cadmium (Cd) and nickel (Ni) in hydroponic culture.• Results showed that high-level tolerances were apparently metal-specific and confined to the metals that were enriched at toxic levels in the soil at the population site.• With regard to metal accumulation, results suggested that, unlike Zn hyperaccumulation, Cd and Ni hyperaccumulation were not constitutive at the species level in T. caerulescens .• In general, the populations under study exhibited a pronounced uncorrelated and metal-specific variation in uptake, root to shoot translocation, and tolerance of Zn, Cd and Ni. The distinct intraspecific variation of these characters provides excellent opportunities for further genetic and physiological dissection of the hyperaccumulation trait.

Naturally occurring genetic variation for contents of cationic minerals in seeds of Arabidopsis thaliana was studied by screening a series of accessions (ecotypes) for Ca, Fe, K, Mg, Mn, Na, Zn, and for total contents of P. Variation was observed for all minerals and correlations between contents of various minerals were present, most noticeably between Ca and Mg, P and Mg, and P and Mn. The genetic basis of this variation was further studied by QTL analysis, using the Landsberg erecta (L er ) ¥ ¥ ¥ ¥ Cape Verde Islands (Cvi) recombinant inbred population. For all minerals, except Na, one or more QTL were detected, explaining up to 78% of the variation. The map positions of several QTL were confirmed by analysis of near isogenic lines, carrying small Cvi introgressions in L er background. Interesting co-locations of QTL suggest pleiotropic effects, due to physiological coupling of the accumulation of certain minerals or to linkage of different genes. By comparing the map positions of QTL with the positions of genes expected to play a role in cation translocation, several candidate genes are suggested.

This study reports changes in the plant's chemical composition and the decomposition of this plant material under enhanced solar UV-B radiation. Calamagrostis epigeios, a dominant grass species in the dune grassland in The Netherlands, was grown outdoor on an experimental field under ambient and enhanced solar UV-B (5 and 7.5 kJ m-2 day-l UV-BBE, respectively), corresponding to about 15% stratospheric ozone depletion. After one growing season aerial plant parts were harvested. The decomposition of this harvested leaf material was studied in a dune grassland and on the above mentioned experimental field under ambient (5 kJ m-2 day-l UV-BBE) and enhanced (7.5 kJ m -2 day-l UV-BBE) radiation, using litter bags. The chemical quality of the leaves grown under enhanced solar UV-B changed. There was an increase in the leaf content of lignin, while no significant changes occurred for the content of a-cellulose, hemicellulose and tannins under enhanced UV-B. In the field, the rate of decomposition ofleaf material grown under enhanced UV-B (with an increased content of lignin) was reduced. The content of lignin of the decomposing leaf material increased, but less under exposure to enhanced UV-B. The latter may be explained by photodegradation of the lignin. The consequences of enhanced UV-B radiation for carbon fluxes in the dune grassland ecosystem are discussed.

Summary• To analyse the relation between zinc (Zn) accumulation and Zn tolerance in the Zn hyperaccumulator, Thlaspi caerulescens , a cross was made between a plant from a nonmetallicolous population (LE: high accumulation, low tolerance) and one from a calamine population (LC: low accumulation, high tolerance).• More or less homogeneous F 3 lines with contrasting extreme accumulation phenotypes were selected and phenotyped for tolerance, using the threshold exposure level for chlorosis as a tolerance measure. Zn accumulation and tolerance segregated largely independently, although there was a significant degree of association between low accumulation and high tolerance.• Plants from an F 2 family were phenotyped for Zn tolerance and their Zn accumulation rates were compared. The plants with low Zn tolerance exhibited significantly higher Zn accumulation than did the more tolerant plants.• The results suggest that the superior Zn tolerance in LC plants compared with LE plants results from a superior plant-internal Zn sequestration capacity and, although to a lower degree, a reduced rate of Zn accumulation. It is argued that the relatively low Zn accumulation capacity levels found in LC and several other calamine T. caerulescens populations might represent an adaptive response to Zn-toxic soil.

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