Spatial distribution and speciation of copper in root tips of cucumber revealed by μ-XRF and μ-XANES

The localization, biotransformation, and chemical speciation of copper in root tips of cucumber (Cucumis sativus) were investigated using synchrotron-based micro X-ray fluorescence (mu-XRF) and micro X-ray absorption near edge structure (mu-XANES). The highest content of Cu was found in root cap and meristematic zone whereas low Cu content in elongation and maturation zone. There was a dramatic increase of Cu content in root cap and meristematic zone after treatment with 100 mu M CuSO4 for 72 h. The mu-XANES analysis revealed that most of Cu in root tip was bound with alginate, citrate, and cysteine-like ligands whereas rarely deposited in form of CuO. From root cap to maturation zone, the proportion of Cu bound with alginate-like ligands increased whereas that bound with citrate-like ligands decreased. The proportion of Cu bound with cysteine-like ligands increased from root cap to elongation zone but sharply declined in maturation zone. The results suggested that Cu was chelated by S ligands in the cell walls which protect protoplasm against possible damage caused by Cu excess.