The Role of Organic Acids and Thiol Compounds in Detoxification and Tolerance of Zn Stress in Two Populations of Harmel

The present study aimed to investigate the effects of Zn exposure (0, 1, 5, 15, 30 ppm Zn) on harmel seedlings. Two populations (metallicolous and non-metallicolous) were compared in terms of the role of soluble sugars, proline, anthocyanin, reducing sugars, cysteine, total free amino acids, ascorbate, dehydroascorbate, glutathione, hydrogen peroxide (H 2 O 2 ), lipid peroxidation, thiol compounds, organic acids, biomass and Zn concentration. The Zn concentration in plants from metallicolous and non-metallicolous populations was similar, not significantly different. The results of hydroponic culture showed that the increase of Zn concentrations in the nutrient solution increased soluble sugars, proline, anthocyanin, reducing sugars, cysteine, total free amino acids, ascorbate, dehydroascorbate, glutathione, thiol compounds, and organic acids. In other words, the contents of H 2 O 2 and lipid peroxidation in metallicolous populations were lower than non-metallicolous populations under Zn stresses. However, with increasing Zn stresses, the concentration of non-protein thiols increased compared to the concentration of glutathione, which indicates the accumulation of phytochelatin. Also, plants exposed to Zn showed a significant increase in malate, citrate, and oxalate but fumarate, and acetate were significantly reduced. These results indicated that the metallicolous population of harmel had a greater capacity than the non-metallicolous population to adapt to oxidative stress caused by Zn stress, and antioxidative defense in the metallicolous population of harmel might play a key role in detoxification and tolerance of Zn. In conclusion, the above results show that harmel may have a detoxification mechanism to counteract high concentrations of Zn.