The role of low molecular weight ligands in nickel hyperaccumulation in Hybanthus floribundus subspecies floribundus

Abstract

The mechanisms responsible for nickel (Ni) hyperaccumulation in Hybanthus floribundus (Lindl.) F.Muell. subspecies floribundus are obscure. In this study, organic acids and free amino acids (AAs) were quantified in 0.025 M HCl H. floribundus subsp. floribundus shoot extracts using HPLC and ultra performance liquid chromatography (UPLC). In a 20 week pot experiment, plants exposed to five levels of Ni (0–3000 mg kg–1 Ni) accumulated up to 3200 mg Ni kg–1 dry weight in shoots, and the shoot : root Ni concentration ratios were >1.4. Concentration of organic acids followed the order malic acid > citric acid > oxalic acid. Citric acid concentration significantly increased upon Ni exposure, with concentrations between 2.3- and 5.9-fold higher in Ni treated plants that in control plants. Molar ratios of Ni to citric acid ranged from 1.3 : 1 to 1.7 : 1 equivalent to >60% of the accumulated Ni. Malic acid concentration also increased upon exposure to applied Ni. However, concentrations were statistically at par across 0–3000 mg kg–1 Ni treatments, suggesting that the production of malic acid is a constitutive property of the subspecies. Total AA concentrations were stimulated upon exposure to external Ni treatment, with glutamine, alanine and aspartic acids being the predominant acids. These AAs accounted for up to 64% of the total free AA concentration in control plants and up to 75% for the 2000 mg kg–1 Ni treatment plants. These results suggest that citric acid in addition to the aforementioned AAs are synthesised in H. floribundus subsp. floribundus plants following exposure to elevated concentrations of Ni and may act as potential ligands for detoxification and possibly storage of accumulated Ni.