Antioxidative enzymes activity and thiol metabolism in three leafy vegetables under Cd stress

The enrichment of Cadmium in vegetables is threatening human health. The study aimed to screen Cd low-enriched leafy vegetables and explore whether antioxidative enzymes and heavy metal chelators are synergistic defensive mechanisms. In this paper, the Cd accumulation and translocation of garland chrysanthemum (Chrysanthemum coronarium L.), spinach (Spinacia oleracea L.), and lettuce (Lactuca sativa L.) were examined by soil pot culture and hydroponic experiments. The responses of oxidative stress markers, antioxidative enzymes activity, and thiol pool (cysteine, γ-glutamylcysteine, glutathione, and phytochelatins) content to Cd stress were assayed. The results showed that Garland chrysanthemum was Cd low-uptake species. The soil Cd safety thresholds for spinach, lettuce, and garland chrysanthemum were 0.41, 0.49, and 9.10 mg kg^-1, respectively. The order of root phytochelatins (PCs) concentration was consistent with that of plant tolerance index (TI): garland chrysanthemum > spinach > lettuce. While the order of the ratio of shoot Cd to root Cd (SR ratio) was exactly the opposite of that of TI. In lettuce root, activity of superoxide dismutase, peroxidase, and catalase decreased significantly under Cd stress. Nevertheless those parameters in the roots of spinach and lettuce maintained steady, or even enhanced. In conclusion, the Cd translocation and partition in plant, antioxidative defense, and PCs homeostasis played an important role in the Cd tolerance of vegetables.