Cadmium causes oxidative stress in mung bean by affecting the antioxidant enzyme system and ascorbate-glutathione cycle metabolism

Ascorbate (AsA)-glutathione (GSH) cycle metabolism is an essential mechanism for the resistance of plants under stress conditions. In a greenhouse pot experiment, the influence of cadmium (Cd) (25, 50, and 100 mg/kg soil) on plant dry weight and leaf area, photosynthetic parameters (net photosynthetic rate (P-N) and chlorophyll (Chl) content) and oxidative stress, and the possible protective role of AsA-GSH cycle metabolism was studied in two mung bean (Vigna radiata (L.) Wilczek.) cvs. Pusa 9531 (Cd-tolerant) and PS 16 (Cd-susceptible) at 30 days after sowing. The contents of thiobarbituric acid-reactive substances (TBARS), H2O2, and the leakage of ions were the highest at 100 mg Cd/kg soil, and the effect was more pronounced in cv. PS 16 than in cv. Pusa 9531. This was concomitant with the strongest decreases in P-N, plant dry weight, and leaf area. The changes in the AsA-GSH redox state and an increase in AsA-GSH-regenerating enzymes, such as glutathione reductase, monodehydroascorbate reductase, dehydroascorbate reductase, and other antioxidant enzymes, such as superoxide dismutase and ascorbate peroxidase, strongly supported over-utilization of AsA-GSH in Cd-treated plants. However, the oxidative stress caused by Cd toxicity was partially overcome by AsA-GSH-based detoxification mechanism in the two genotypes studied because an increases in lipid peroxidation (TBARS, ion leakage) and H2O2 content were accompanied by a corresponding decrease in reduced AsA and GSH pools. Thus, changes in AsA-GSH pools and the coordination between AsA-GSH-regenerating enzymes and other enzymatic antioxidants of the leaves suggest their relevance to the defense against Cd stress.