Exogenous Selenium Enhances Cadmium Stress Tolerance by Improving Physiological Characteristics of Cabbage (Brassica oleracea L. var. capitata) Seedlings

In recent years, the levels of cadmium (Cd) in agricultural soils have been increasing. Cd is highly toxic and can enter the human body through the food chain, threatening human health, therefore, reducing the Cd content in vegetables and producing green and non-polluting food has become a common concern in society. However, the physiological properties of exogenous selenium in alleviating Cd stress in cabbage seedlings have not been thoroughly investigated. In this study, exogenous Se (10 mu Mol/L) was applied under Cd (25 mu Mol/L) stress and the physiological characteristics such as biomass, photosynthetic fluorescence parameters, Se and Cd contents, chloroplast ultrastructure, leaf membrane esterification, and antioxidant enzyme activities were determined. The results showed that the exogenous application of Se could effectively alleviate the decrease in growth, photosynthetic pigment, and the gas exchange characteristics of the cabbage seedlings under Cd stress, improve cabbage root vitality, reduce root leaf Cd content, and alleviate the Cd stress-induced damage. Ultrastructural observation showed that the Cd stress caused the disruption to the chloroplasts' internal structure in the cabbage leaves, while an exogenous Se treatment alleviated the chloroplast damage to some extent, improved the stability of the inner capsule membrane, and alleviated the Cd stress-induced damage to the photosynthetic organs. Cd stress also caused oxidative damage and the excessive accumulation of reactive oxygen species (ROS) in the leaves of cabbage seedlings, as evidenced by the significant accumulation of superoxide anion (O2-), hydrogen peroxide (H2O2), malondialdehyde (MDA), and electrolyte leakage. On the other hand, after the exogenous Se treatment, the Cd stress-induced oxidative damage could be reduced by up-regulating the activities of the antioxidant enzymes, such as superoxide dismutase (SOD), peroxidase (POD), and ascorbate peroxidase (APX). At the same time, Cd stress significantly increased glutathione (GSH) levels, and the exogenous Se treatment further increased the GSH levels, thereby increasing the tolerance of the cabbage to Cd stress. In conclusion, exogenous Se can further improve the Cd tolerance of the cabbage seedlings by protecting the photosynthetic system, eliminating excessive accumulation of reactive oxygen species under Cd stress, alleviating oxidative stress, and reducing Cd levels in plants, among other physiological properties.