Zinc, iron, and copper isotopic fractionation in Elsholtzia splendens Nakai: A study of elemental uptake and (re)translocation mechanisms

Abstract

Zinc, Fe and previously reported Cu isotopic compositions were applied in a systematic investigation of uptake and translocation in Cu-tolerant Elsholtzia splendens Nakai. They exhibit characteristic isotopic fractionation during uptake: non-specific uptake for Zn, direct uptake as Fe(Ш)-complexes as strategy II species and a prerequisite of Cu(II) reduction to Cu(I) before uptake of Cu. There are, however, differences during translocation from stem and branch to leaf and flower. Light Zn and Fe isotopic compositions of flowers are dependent on their phloem pool, with enrichment in branch and stem. The similarity of Zn and Fe isotopic compositions in flowers, regardless of height, implies uniform mobility in phloem. Cu isotopic composition displays marked variations in leaves and flowers with height. Re-translocation from old tissue to new tissue has a significant influence on metal isotopic compositions in leaves and flowers during the late life stage. Light Zn and Fe isotopes are exported from leaves by transporters, leaving heavy isotopes in the remaining fraction. Cu readily forms complexes, and its heavy isotope is re-translocated preferentially and then light isotope by transporter proteins from old leaves dependent on the degree of remobilization. The remobilization efficiency follows the trend Zn < Fe < Cu, opposite to the trend of metal mobility in xylem and phloem (Zn > Fe > Cu).