Translocation of Ni and Zn in Odontarrhena corsica and Noccaea caerulescens: the effects of exogenous histidine and Ni/Zn interactions

Aims: Nickel (Ni) and zinc (Zn) interactions during their uptake and root-to-shoot translocation and the potential role of histidine therein were studied in different populations of the Ni/Zn hyperaccumulator Noccaea caerulescens and the Ni hyperaccumulator Odontarrhena corsica. Methods: The effect of exogenous L-histidine supply on Ni and Zn uptake and translocation in N. caerulescens and O. corsica, and xylem loading in shoot-excised root systems of different N. caerulescens populations, were studied under separate and combined exposure. Results: In O. corsica, Zn inhibited both the translocation and the uptake of Ni, whereas Ni did not significantly affect Zn uptake or translocation. In N. caerulescens, both in intact plants and shoot-excised root systems, Zn usually inhibited the uptake, but not the translocation of Ni, whereas Ni did not affect the uptake, but inhibited the translocation of Zn, though not in two populations with low Zn xylem loading capacity. Exogenous histidine supply did not significantly affect Zn translocation in O. corsica and intact plants of an ultramafic population of N. caerulescens, but enhanced Zn xylem loading in two calamine populations of N. caerulescens and Ni translocation in all of them. Conclusions: High free L-histidine concentrations in roots might promote Ni hyperaccumulation in obligate Ni hyperaccumulators, such as O. corsica, in nature. The high histidine concentration in roots of N. caerulescens, which is primarily a Zn hyperaccumulator, might not only explain its species-wide conserved capacity to hypertranslocate Ni, but may also contribute to its Zn translocation capacity, at least in non-ultramafic populations.