Pb accumulation, Pb tolerance, antioxidants, thiols, and organic acids in metallicolous and non-metallicolous Peganum harmala L. under Pb exposure

• Metallicolous (M) and non-metallicolous (NM) Peganum harmala were compared for Pb tolerance and accumulation, oxidative stress markers, antioxidant capacities, thiols, and organic acids.
• M and NM plants were equally tolerant to Pb in the nutrient solution, but M plants accumulated and tolerated more Pb in roots and shoots.
• M plants showed slightly higher (<1.2-fold) antioxidant capacities and slightly lower (>0.8-fold) levels of oxidative stress markers under Pb exposure.
• The quantitatively most pronounced differences between M and NM plants were the Pb-induced increases of protein thiol (1.5-fold higher in M) and citrate (2-fold higher in M).

The aim of the present study was to compare the effect of lead (Pb) exposure (0–50 mg L−1 Pb) on Pb accumulation, growth, and the concentrations of thiol compounds, organic acids, antioxidant enzyme activities and other biochemical parameters between a non-metallicolous and a metallicolous harmal (Peganum harmala L.) population. Seedlings were grown in hydroponics, and exposed to 0, 5, 10, 25 and 50 mg L−1 Pb for 14 days. Lead decreased dry weight to the same extent in both populations, in a concentration-dependent way. Pb accumulation was consistently higher in the metallicolous population than in the non-metallicolous one, both in roots and shoots, showing that the metallicolous plants were more tolerant to plant-internal Pb. Under Pb exposure the concentrations of malondialdehyde (MDA) and H2O2, as well as the LOX and GPX activities, were higher in the non-metallicolous population, albeit inconsiderably. The activities of antioxidant enzymes such as superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT), as well as the concentrations of the non-enzymatic antioxidants GSH, ascorbic acid, and proline, were enhanced under Pb exposure, and overall higher in the metallicolous population, though only significantly so under Pb exposure. The concentrations of total thiols, non-protein thiols and protein bound thiols were also increased under Pb exposure. In particular, Pb strongly enhanced the average protein cysteine content in both populations, but much more so in the metallicolous plants than in the non-metallicolous ones. Furthermore, Pb-exposed plants showed significantly increased concentrations of organic acids. Citrate accumulation was strongly induced under Pb exposure, particularly in the metallicolous plants, suggesting that citrate is the most obvious candidate ligand for Pb in the vacuole. Overall, the quantitatively most remarkable differences in Pb response between the populations lie in the degrees of Pb-induced accumulation of proline and protein thiol.