Phytochemical analysis reveals an antioxidant defense response in Lonicera japonica to cadmium-induced oxidative stress

Cadmium (Cd), though potentially beneficial at lower levels to some plant species, at higher levels is a toxic metal that is detrimental to plant growth and development. Cd is also a carcinogen to humans and other contaminated plant consumers, affecting the kidneys and reducing bone strength. In this study we investigated responses of growth, chlorophyll content, reactive oxygen species levels, and antioxidant responses to Cd in honeysuckle leaves (Lonicera japonica Thunb.), a potential Cd hyperaccumulator. Results indicated that plant height, dry weight, leaf area, and chlorophyll content increased when honeysuckle was exposed to 10 mg kg−1 or 30 mg kg−1 Cd (low concentration). However, in response to 150 mg kg−1 or 200 mg kg−1 Cd (high concentration) these growth parameters and chlorophyll content significantly decreased relative to untreated control plant groups. Higher levels of superoxide radical (O2·−) and hydrogen peroxide (H2O2) were observed in high concentration Cd groups. The activities of ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), and glutathione reductase were enhanced with exposure to increasing levels of Cd. Additionally, the Ascorbate–Glutathione (AsA–GSH) cycle was activated for the removal of H2O2 in honeysuckle in response to elevated Cd. The Pearson correlation analysis, a redundancy analysis, and a permutation test indicated that proline and APX were dominant antioxidants for removing O2·− and H2O2. The antioxidants GSH and non-protein thiols (NPTs) also increased as the concentration of Cd increased.