Metal uptake and thiol production in Spirodela polyrhiza (L.) SP20

Abstract

Spirodela polyrhiza (L.) SP20 was hypersensitive to Cu (1-100 μmol/L) compared with Ni (25-500 μmol/L). Lower Cu concentration (1 μmol/L) or Ni (25 μmol/L) stimulated general growth of the test organism compared with toxicity at elevated concentrations of both of the cations. Cu uptake was very fast during the first 10 min and remained stable irrespective of the extension of incubation. Ni uptake also showed concentration- and time-dependence, with saturation only at 40 h. If compared at equimolar concentration (100 μmol/L, each), Cu accumulated to the extent of 2.7 mg g-1 dw, a value in close proximity with that of Ni (2.84 mg g-1 dw). At lower concentrations, both of the cations showed high accumulation in root; however, the root/top ratio for either Ni or Cu decreased at elevated concentrations, indicating that the metal(s) could be translocated only after a threshold limit of accumulation in the root. The hypertoxic Cu was also the effective inducer of thiol biosynthesis in the test plant as 10 μmol/L Cu increased it to 15 μmol thiol g-1 dw (10 h) over the metal-less control (10.26 μmol). A 10-fold higher Ni concentration (100 μmol/L) was, however, needed to achieve the same level of thiol. Thiol biosynthesis was also correlated with the intracellular buildup of Cu or Ni.