Localization and speciation of cobalt and nickel in the leaves of the cobalt-hyperaccumulating tree Clethra barbinervis

Key message The accumulation and tolerance mechanisms for Co are clearly different from those for Ni in the leaves of C. barbinervis in terms of both the distribution and speciation. Abstract Clethra barbinervis is a Co-hyperaccumulating tree and also accumulates Ni at high concentrations. The mechanism and role of accumulation in tree physiology remains unclear. The aim of this study was to determine the localization and speciation of Co and Ni in the leaves of C. barbinervis to reveal the mechanisms behind its tolerance to high concentrations of these elements. C. barbinervis seedlings were grown for 3 years under treatments with Co or Ni in the rhizosphere. X-ray fluorescence (XRF) and X-ray absorption near edge structure (XANES) analyses were then used to evaluate the distribution and chemical states of Co, Ni, and S in the adaxial leaf epidermis. In addition, the treated leaves were cut into several parts according to the XRF imaging results on Co or Ni, and the concentrations of elements, sulfate, and organic acids were determined in each part by chemical analyses. XRF images showed that Co was present at the tip of the leaf at a high concentration, whereas Ni was mainly distributed around the leaf edge. Results of chemical analyses on leaf parts containing Co or Ni indicated that sulfate acts as a counter ion for Co and that Ni combined with succinic and/or oxalic acid. In addition, XANES analysis showed that sulfate tended to be reduced and glutathione was generated in the tip of the leaf. Our results indicate that C. barbinervis distinguishes Co and Ni and translocates them to different parts of the leaf.