Identification and quantification of chelidonic acid and other ligands with potential for Ni uptake in the hyperaccumulator, Berkheya coddii.
B. coddii is a known nickel hyperaccumulator, a plant that is able to take up high levels of Ni (< 37 000 mg kg -1) without exhibiting toxic effects. It grows in serpentine soil and is endemic to the Barberton Region, Mpumulanga Province, South Africa. One of the key factors governing hyperaccumulators is the mechanism of uptake. This study investigates the uptake mechanism of B. coddii. Plants and soil from this region were collected for analysis and uptake studies (Ni spiking). A characterisation of amino and organic acids, both commonly associated with uptake in hyperaccumulators, was carried out using a range of chromatographic techniques namely, reverse phase-high performance liquid chromatography (RP-HPLC) and liquid chromatography-mass spectrometry (LC-MS). Amino acid analysis results showed a high proline concentration (12 400 mg kg-1) in younger leaves of B. coddii, and organic acid results showed high levels of ascorbic acid (2100 mg kg-1) present in the stem, indicating a stress response to high levels of Ni. Analysis carried out on B. coddii, using methods of size exclusion chromatography (SEC), high resolution-mass spectrometry (HR-MS) and HPLC indicated that the ligand responsible for Ni uptake is the organic acid, chelidonic acid. This study has shown that a leaf-water extract contains chelidonic acid in a 3:1 molar ratio to Ni. Nickel uptake studies were performed to further investigate the relationship between Ni and chelidonic acid. Upon treatment of B. coddii with soluble Ni the concentrations of Ni and chelidonic acid increased. This resulted in an increase of the molar ratio between chelidonic acid and Ni to 6.76:1.