Histidine-Mediated Nickel and Zinc Translocation in Intact Plants of the Hyperaccumulator Noccaea caerulescens

In this work, the effect of exogenous histidine supply on zinc (Zn) and nickel (Ni) translocation to the shoots in intact plants of the hyperaccumulator Noccaea caerulescens F.K. Mey was studied. Three series of experiments were carried out. (1) Intact N. caerulescens plants (St-Félix-de-Pallières population) were pretreated for 4 h (12:00 till 16:00) with a MES/KOH-buffered 1 mM L-histidine solution or demineralized water, then exposed overnight (20 h) to 5, 25 or 250 µM Ni or Zn and harvested. (2) Intact N. caerulescens plants of the same population were pretreated with 1 mM L-histidine solution or demineralized water overnight (20 h) and then exposed to 250 µM Ni or Zn for 8 h during the day (10:00 till 18:00) and harvested. (3) Intact N. caerulescens plants (the calamine populations St-Félix-de-Pallières (SF) and La Calamine (LC), and the ultramafic population Monte Prinzera (MP)) were exposed for 8 h (10:00 till 18:00) to 250 µM Ni or Zn and then to 1 mM L-histidine solution or demineralized water overnight (20 h) and harvested. The Ni and Zn concentrations in the roots and shoots were determined by atomic absorption spectrophotometry. The translocation factor (TF), expressed as the shoot to root metal concentration ratio, the total plant Ni or Zn content, and the percentage of the total Ni or Zn content present in the shoot (% translocated) were calculated. A 4 h pretreatment with L-histidine during the afternoon (before metal exposure overnight) significantly decreased the Ni and Zn concentrations in the root and increased the concentration of Ni, but not of Zn, in the shoot, significantly increased both TF and the % translocated for both metals, albeit much more strongly for Ni, and also slightly, but significantly, increased the total plant content of Ni, but not of Zn. Overnight pretreatment with L-histidine (followed by metal exposure during the day) of the same population (SF) had basically similar effects on Ni translocation, but significantly decreased the plant total Ni content, and was without significant effects on Zn translocation, but considerably decreased the root Zn concentration. The different populations under study (SF, MP, LC) showed significant differences in their Ni and Zn uptake and translocation capacities, but in general showed qualitatively similar responses to post-treatment with L‑histidine that strongly increased the TF and the % translocated for both metals in SF and MP, whereas in LC the effect was prominent only for Ni. Significant population × histidine treatment effect interactions were obtained for the root Zn concentration, and the TF and % translocated for Ni, which were largely explained by a relatively low responsiveness to the L-histidine treatment in LC, compared to SF and/or MP. It is concluded that the high endogenous L-histidine concentrations in N. caerulescens are probably functional in the hyperaccumulation of both Ni and Zn. The overall stronger effect of exogenous L-histidine supply on the translocation of Ni, compared to Zn, seems to result, at least in part, from the high Zn burdens at the start of the treatments, particularly in the shoots, which largely mask the apparent effects of exogenous L-histidine supply on the shoot Zn concentration and, to a lower degree, the % Zn translocated.