In-vivo X-ray spectroscopy (XRF and XANES) probing the kinetics of ZnSO₄ and Zn-EDTA root-to-shoot transport on soybean plants (Glycine max)

In the context of plant nutrition, zinc (Zn) is an essential micronutrient that plays a role in a range of vital processes, such as enzymatic activity and carbohydrate metabolism. However, since around 50% of soils worldwide are Zn scarce, an external supply is required for the proper development of most crops. In this scenario, ZnSO₄, and chelated Zn-EDTA are two Zn sources commonly employed either for soil and foliar Zn supplementation. In this study, we evaluated the root-to-shoot transport of ZnSO₄ and Zn-EDTA through soybean (Glycine max (L.) Merill) stems. For this purpose, the roots of soybean plants at the V4 growth stage were exposed to a hydroponic solution containing ZnSO₄ and Zn-EDTA at 100 mg Zn L^-1, respectively. The stems were in vivo monitored along 48h using a handheld X-ray fluorescence spectrometer (XRF, Bruker, Tracer III SD, USA) for Zn uptake kinetics and then, by X-ray absorption near edge structure (XANES) spectrometry on the XAFS2 beamline of the Brazilian Synchrotron Light Laboratory (LNLS) for the Zn chemical speciation evaluation. At the end of the experiment, chemical μ-XRF (Orbis PC, EDAX, USA) maps of fresh stem tissues transversely cut and Zn quantification on root and leaves tissues were also performed. We observed that Zn content in the stems of the plants exposed to ZnSO₄(aq) was around 10-fold higher when compared with that with Zn-EDTA(aq) and above 50-fold higher than in the control plants. The Zn uptake presented an exponential increasing followed by a drop. Although present a similar uptake velocity for both treatments, the point at which the uptake rate starts decreasing was achieved faster for Zn-EDTA(aq) (9.4±1.3 h) than for ZnSO₄(aq) (29.5±1.7 h), showing that soybean plants were able to control the uptake of Zn-EDTA(aq) earlier than ZnSO₄(aq), resulting in the higher Zn content found in the plants exposed to ZnSO₄(aq). In the other hand, XANES spectra allow stating zinc is biotransformed in its way up to the shoot as different forms of zinc, regardless of exposing source. Since the XRF chemical maps show that Zn is found simultaneously in transport tissues (i.e xylem and phloem), as well cortex tissues, it is feasible to assume that it could be at different chemical species in each of these environments. (author)