Responses of glyoxalase system, ascorbate-glutathione cycle, and antioxidant enzymes in Pontederia cordata to lead stress and its capacity to remove lead

A hydroponic experiment was conducted to investigate the variations in membrane permeabilities, chlorophyll contents, antioxidase activities, the ascorbic acid (AsA)-glutathione (GSH) cycle, and the glyoxalase system in the leaves of Pontederia cordata with 0 ∼ 15.0 mg L^-1 lead ion (Pb²⁺) exposure. The concentrations of Pb²⁺ accumulated in the plant roots, stems, and leaves were also evaluated. After 7 days of exposure, the plants maintained normal growth, and there was a significant increase in ascorbate peroxidase and dehydroascorbate reductase activities. With 5.0 mg L^-1 Pb²⁺ exposure for 28 days, nearly 66.36% of Pb²⁺ accumulated in the roots, while excess Pb²⁺ immobilized in the leaves was not observed. Exposure to 10.0 and 15.0 mg L^-1 Pb²⁺ for 28 days significantly increased Pb²⁺ contents in the leaves. This led to decrease in chlorophyll a, b, and carotenoid contents, and to increase in the methylglyoxal content in the leaves. With 10 and 15 mg L^-1 Pb²⁺ exposure, NPT and PCs contents in leaves increased. however, the glyoxalase system did not function well in the plant tolerant to Pb²⁺ at higher concentrations. The AsA-GSH cycle did not cooperate with the glyoxalase system in the plant defense against Pb²⁺ exposure in the present investigation.