Uptake and localization of cadmium by Biscutella laevigata, a cadmium hyperaccumulator

Biscutella laevigata has recently been recognized as a species able to accumulate large amounts of cadmium. The experiments reported in this paper were conducted on two geographically isolated populations of B. laevigata in Poland. Both populations grow on metalliferous soils: a lead-zinc (calamine) waste heap in Boleslaw near Olkusz (189 mg Cd/kg d.m.) and limestone rock in the West Tatra Mts (1.4-6.1 mg Cd/kg d.m.). The two populations were compared after cultivating them in medium containing cadmium salt (2-120 mg/dm(3)) for 3-30 days. Root-to-shoot transport of cadmium was higher in the waste-heap population than in the mountain population. In the waste-heap population, large amounts of cadmium were transported to the oldest leaves, reaching levels even twice those of the mountain population. This shows that the ability to hyperaccumulate metals may be a property of a population, not an entire species, and that the ability to accumulate cadmium in the oldest (withering) leaves may be a way the plant eliminates the toxic metal. Histochemical detection of cadmium (with dithizone) in tissues showed that it was taken up by the root hairs and then transported through vascular bundles to the leaves. The surface cells of the leaves, the epiderm and hairs accumulated particularly large amounts of cadmium. The leaves of the B. laevigata waste-heap population are much more thickly covered by hairs than those of the mountain population; we suggest that the ability to accumulate cadmium in leaf hairs may be a mechanism of detoxifying and hyperaccumulating cadmium in the shoots of that population.