Role of chelant on Cu distribution and speciation in Lolium multiflorum by synchrotron techniques

Highlights: • EDDS alleviated the deposition of Cu in the root meristem of root apex and the junction of lateral root zone. • EDDS formed complex with Cu, decreased the root sequestration of Cu, and facilitated Cu translocation as CuEDDS. • In spite of CuEDDS, a proportion of Cu-histidine and Cu(I)-glutathione like species was observed in ryegrass tissues. • A conceptual model was developed on Cu uptake and transport mechanisms in ryegrass. Chelants are known to enhance metal translocation in plants; however, the underlying mechanisms are still not fully understood. This study aimed to elucidate the distribution and speciation of Cu in ryegrass (Lolium multiflorum) in both absence and presence of the biodegradable chelant [S,S′]-ethylenediamine disuccinic acid (EDDS). The results showed that EDDS increased the Cu translocation factor from root to shoot by 6–9 folds under CuEDDS in comparison with free Cu (50–250 μM). Synchrotron-based microscopic X-ray fluorescence (μ-XRF) mapping revealed that EDDS alleviated Cu deposition in the root meristem of root apex and the junction of lateral root zone, and facilitated Cu transport to root stele for subsequent translocation upwards. X-ray absorption near edge structure (XANES) analysis found that free Cu was sequestered in plants as a mixture of Cu-organic ligands. In the EDDS treatment, Cu was primarily present as CuEDDS (49–67%) in plants with partial chemical transformation to Cu-histidine (21–36%) and Cu(I)-glutathione (0–24%). These results suggest that EDDS improves internal Cu mobility through forming CuEDDS, thus decreasing the root sequestration of Cu, and ultimately facilitating Cu transport to plant shoots.