Histidine-Mediated Nickel and Zinc Translocation in Arabidopsis thaliana and Lepidium ruderale

In this work, the effect of exogenous histidine supply on zinc (Zn) and nickel (Ni) translocation in shoot-excised root systems of the non-accumulating species Arabidopsis thaliana (L.) Heynh. and Lepidium ruderale L. was studied. Intact 7-week-old plants of A. thaliana and L. ruderale were pretreated for 4 h (13:00 till 17:00) with MES/KOH buffered 1 mM L-histidine, L-alanine or demineralized water. After the pretreatment, the leaf rosettes were cut off with a razor blade and the root systems were transferred to a fresh nutrient solution amended with 25 µM Ni(NO 3 ) 2 or Zn(NO 3 ) 2 . Root pressure exudates were collected overnight (till 11:00 AM). The Ni and Zn concentrations in the roots and root pressure exudates were determined by atomic absorption spectrophotometry. The amount of Ni or Zn loaded into the xylem exudate (‘total amount of Ni or Zn exudated’) was calculated as the product of the metal concentration and the volume of root pressure exudate, expressed on a root dry weight basis. The ‘total Zn or Ni amount’ present in the root system and the root pressure exudate together (‘total uptake’), at the end of the experiment, was calculated as the sum of the total amount of Zn or Ni present in the root pressure exudates and the remaining amount in the root system after sap collection, and expressed on a root dry weight basis. Zn or Ni xylem loading was then recalculated as a percentage of the total Zn or Ni uptake. Pretreatment with L-histidine caused a significant increase in the xylem loading and the total amount of exudated Ni and Zn in L. ruderale and Zn in A. thaliana . No increase in Ni xylem loading, Ni concentration in the root pressure exudate, or total amount of Ni exudated was observed in A. thaliana after pretreatment with L-histidine. No decrease in the volume of root pressure exudates was observed in histidine-pretreated plants, indicating that the increase in metal concentration in the root pressure exudates is certainly not attributable to decreased root pressure exudation in any of the treatments. In contrast to L-histidine, pretreatment with L-alanine did not increase the metal concentrations in the root pressure exudates or the percentage of metal loaded into the xylem. The total uptake of Ni and Zn in A. thaliana was significantly higher than in L. ruderale , which is consistent with the higher concentrations of both metals in the roots of A. thaliana . Exogenous L-histidine and L-alanine did not affect the uptake of metals and