Antioxidant system activation by mercury in Pfaffia glomerata plantlets

Oxidative stress caused by mercury (Hg) was investigated in Pfaffia glomerata plantlets grown in nutrient solution using sand as substrate. Thirty-day-old acclimated plants were treated for 9 days with four Hg levels (0, 1, 25 and 50 mu M) in the substrate. Parameters such as growth, tissue Hg concentration, toxicity indicators (delta-aminolevulinic acid dehidratase, delta-ALA-D, activity), oxidative damage markers (TBARS, lipid peroxidation, and H2O2 concentration) and enzymatic (superoxide dismutase, SOD, catalase, CAT, and ascorbate peroxidase, APX) and non-enzymatic (non-protein thiols, NPSH, ascorbic acid, AsA, and proline concentration) antioxidants were investigated. Tissue Hg concentration increased with Hg levels. Root and shoot fresh weight and delta-ALA-D activity were significantly decreased at 50 mu M Hg, and chlorophyll and carotenoid concentration were not affected. Shoot H2O2 concentration increased curvilinearly with Hg levels, whereas lipid peroxidation increased at 25 and 50 mu M Hg, respectively, in roots and shoots. SOD activity showed a straight correlation with H2O2 concentration, whereas CAT activity increased only in shoots at 1 and 50 mu M Hg. Shoot APX activity was either decreased at 1 mu M Hg or increased at 50 mu M Hg. Conversely, root APX activity was only increased at 1 mu M Hg. In general, AsA, NPSH and proline concentrations increased upon addition of Hg, with the exception of proline in roots, which decreased. These changes in enzymatic and non-enzymatic antioxidants had a significant protective effect on P. glomerata plantlets under mild Hg-stressed conditions.