Comparative physiology of xylem nickel loading in the hyperaccumulator Odontarrhena inflata and the non-accumulator Aurinia saxatilis

Background and aims: This study aimed to gain insight into the biochemical mechanisms of Ni movement in Odontarrhena inflata and Aurinia saxatilis. Methods: We examined the effects of Ni exposure on the concentrations of histidine and nicotianamine in roots, shoots, and in xylem sap of Odontarrhena inflata (as a Ni hyperaccumulator) and Aurinia saxatilis as a non-accumulator. Furthermore, we analysed the effects of Fe and Zn deficiency, and of an apoplastic pathway blocker, on the mobility of Ni. Results: In plants unexposed to Ni, root His and shoot NA concentrations were higher in O. inflata than in A. saxatilis. Ni exposure caused an increase in His in the xylem sap of O. inflata. Ni exposure caused concentration-dependent increases in shoot His and in root NA concentrations, which were similar in both species for His and distinct between the two species for NA. Fe deficiency, followed by a short-term Fe luxury and Ni exposure, led to enhanced Ni uptake and Ni flux from the root to the shoot of O. inflata. By contrast, we observed decreased Ni loading into the xylem in O. inflata subjected to Zn deficiency. An apoplastic pathway blocker resulted in a decrease in root Ni levels by almost 20%, and in decreased shoot Ni concentrations only under high Ni exposure. Conclusion: The processes enhanced in response to Fe deficiency can contribute to root uptake and xylem loading of Ni in the hyperaccumulator species. The contribution of the apoplastic pathway to root-to-shoot Ni flux is negligible under natural ecological conditions.