Separation and determination of heavy metals associated with low molecular weight chelators in xylem saps of Indian mustard (Brassica juncea) by size exclusion chromatography and atomic absorption spectrometry

To elucidate the role of low molecular weight chelators in long-distance root-to-shoot transport of heavy metals in Indian mustard, an "off-line" size exclusion high-performance liquid chromatography- graphite furnace atomic absorption spectrometry was developed to investigate heavy metals associated with low molecular weight chelators in xylem saps of Indian mustard (Brassica juncea). The size exclusion chromatogram presented only the peaks with molecular weight for all xylem saps and directly indicated the long-distance transport of phytochelatins (PCs) of Indian mustard under Cd stress. In the absence of Cd stress, only organic acids and inorganic anions participated in the long-distance transport of Cd, but organic acids, inorganic anions, glutathione (GSH), and cysteine might relate to the long-distance transport of Cu or Zn. In the presence of Cd stress, PCs were induced, and Cd ions in xylem saps were associated with the induced PCs. As the Cd levels in nutrient solution increased, more Cd in xylem saps adopted the form of PC-Cd. Although PCs might participate in the long-distance transport of Cd under Cd stress, the majority of Cd was still transported by organic acids and inorganic anions in xylem vessels. Moreover, results indicated the existence of complexation competition for GSH and cysteine between Cd and Cu (or Zn) and complexation competition for Cd between PCs and GSH (or cysteine) in xylem vessels. Our work might be very useful for understanding the mechanism of long-distance transport of heavy metals in hyperaccumulator.