Zinc uptake from ZnSO4 (aq) and Zn-EDTA (aq) and its root-to-shoot transport in soybean plants (Glycine max) probed by time-resolved in vivo X-ray spectroscopy.

This study investigated the dynamic of zinc (Zn) uptake and the root-to-shoot Zn-transport when supplied as ZnSO_{4 (aq)} or Zn-EDTA _(aq) in soybean seedlings using in vivo X-ray fluorescence (XRF) and X-ray absorption spectroscopy (XANES). The time-resolved X-ray fluorescence showed that plants absorbed ca. 10-fold more Zn from ZnSO_{4 (aq)} than from Zn-EDTA _(aq). However, the uptake velocity did not influence the amount of Zn in the stem. It let furthermore appear that the plants were able to reduce the absorption of Zn from Zn-EDTA _(aq) earlier than ZnSO_{4 (aq)}. Thus, the entrance of Zn²⁺ into the roots is not necessarily accompanied by SO₄^2-_(aq). Regardless the source, the Zn distribution and its transport in the stem were spatially correlated to the bundles and cortex nearby the epidermal cells. Its chemical speciation showed that Zn is neither transported as ZnSO_4(aq) nor as Zn-EDTA_(aq), indicating that these compounds are retained in the roots or biotransformed on in the root-solution interface. Zn²⁺ was long-distance transported complexed by organic molecules such as histidine, malate, and citrate, and the proportion of ligands was affected by the concentration of Zn²⁺ in the stem rather than by the type of Zn source.