A cosegregation analysis of zinc (Zn) accumulation and Zn tolerance in the Zn hyperaccumulator Thlaspi caerulescens

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.