Nickel and zinc accumulation capacities and tolerance to these metals in the excluder Thlaspi arvense and the hyperaccumulator Noccaea caerulescens

Representatives of Brassicaceae species—the hyperaccumulator Noccaea caerulescens F.K. Mey and the metal excluder Thlaspi arvense L.—were compared in terms of their ability to accumulate nickel (Ni) and zinc (Zn) and their tolerance to these metals. Four ecotypes of N. caerulescens were used: the ecotypes La Calamine (LC, Belgium) and Saint Felix de Palliéres (SF, France) grow naturally on calamine soils rich in Zn, Cd, and Pb; the ecotype Monte Prinzera (MP, Italy) originates from serpentine soils rich in Ni, Co, and Cr; and the ecotype Lellingen (LE, Luxembourg) inhabits non-metalliferous soils. The plants of N. caerulescens were grown for 8 weeks in a half-strength Hoagland solution supplemented with 25, 100, 200, 300, and 400 μM Ni(NO₃)₂ (ecotypes LC, SF, MP, LE) or 100, 200, 400, 800, and 1000 μM Zn(NO₃)₂ (ecotypes LC, SF, LE); the plants of T. arvense were grown in the presence of 10, 20, 25, and 30 μM Ni(NO₃)₂ or 40, 50, 60, 70, 80 μM Zn(NO₃)₂. The toxic effect of Ni and Zn was assessed from changes in dry matter of roots and shoots of treated plants compared to untreated. The content of metals in roots and shoots was determined by means of atomic absorption spectrophotometry. The Ni-accumulating capacity of N. caerulescens ecotypes increased in the order: LC < SF < LE < MP, and the Zn-accumulating capacity increased in the row: LC < SF < LE. In the hyperaccumulating plant N. caerulescens, the increments of biomass started to decrease at a lower metal content in roots than in shoots, whereas the opposite pattern was observed in the metal excluder T. arvense. Since T. arvense plants accumulated Ni and Zn in roots, whereas N. caerulescens accumulated these metals in shoots, one may assume that the greater sensitivity of root growth compared with shoots in N. caerulescens was determined by more effective mechanisms of metal detoxification in shoots. Conversely, the higher sensitivity of shoot growth compared to root growth in T. arvense was determined by more effective mechanisms of metal detoxification in roots. Being more tolerant to Ni and Zn than T. arvense plants, the N. caerulescens ecotypes differed substantially in terms of metal-accumulating capacity and their tolerance to heavy metals. The ecotype originating from non-metalliferous soils (LE) accumulated larger amounts of Zn, but was less tolerant compared with ecotypes growing naturally on calamine soils (SF and LC), whereas the ecotype occurring on serpentine soils (MP) exhibited a markedly greater tolerance to Ni, compared with other ecotypes examined, as well as the largest accumulation of this metal. The results indicate the existence of different mechanisms responsible for plant tolerance to Ni and Zn; the study of these mechanisms is a promising direction for future research.