Effects of Exogenous Organic Acids on the Growth and Antioxidant System of Cosmos bipinnatus under Cadmium Stress

Cadmium pollution in China's urban green spaces is extremely severe, causing a significant increase in the risk of urban environmental pollution. Applying organic acids under Cd stress conditions improves the activity of the major antioxidant enzymes in the AsA-GSH cycle signi-ficantly. In this study, Cosmos bipinnatus seedlings were cultured in a nutritive medium containing 40 mu molmiddotL-1 Cd. They were subsequently supplemented with different concentrations of citric acid and malic acid (0,0.5,1.0,2.0 mmol L-1) to analyze the roles of the organic acids in regulating the AsA-GSH cycle involved in plant resistance to Cd stress. Exogenous organic acids inhibited the growth of C. bipinnatus in a dose-dependent manner. AsA, GSH, and GR in the leaves decreased dramatically at high malic acid concentrations (2 mol L-1). The contents of GSH and GR and the APX activities showed a downward trend when the seedlings were treated with 0.5-2 mmolmiddotL-1 of citric acid. However, AsA exhibited contrasting results under these conditions. Notably, both the antioxidant and enzyme activities were reduced considerably to a minimum when the citric acid concentration was 2 mmolmiddotL-1. Low concentrations of exogenous acids increased the activities of APX and GR in the leaves of C. bipinnatus, thus increasing the contents of ASA and GSH. Such increases boost the efficiency of the ASA-GSH cycle in relieving the toxic effect of cadmium on C. bipinnatus.