The alleviating effect on the growth, chlorophyll synthesis, and biochemical defense system in sunflower seedlings under cadmium stress achieved through foliar application of humic acid

Abstract Background: With the progress of industrialization and urbanization, cadmium (Cd) pollution in farmland is increasingly severe, greatly impacting both the ecological environment and human health. Consequently, sunflowers possess high resistance to Cd stress and great potential for phytoremediation of Cd-contaminated soil. Previous studies have shown that humic acid (HA) effectively mitigates plant damage induced by Cd; however, its alleviating effects on sunflower seedlings under Cd stress remain largely unknown. Results: In this study, we used four different concentrations of HA (50, 100, 200, and 300 mg/L) to investigate the effects of Cd stress on sunflower seedlings' growth, chlorophyll synthesis, and biochemical defense system through foliar spraying. The results revealed that Cd stress not only reduced plant height, stem diameter, fresh and dry weight, and chlorophyll content in sunflower seedlings but also altered their chlorophyll fluorescence characteristics compared to the control group. After Cd stress, the photosynthetic structure was damaged and the number of PSII reactive centers per unit changed. Application of 200 mg/L HA promotes sunflower seedling growth and increases chlorophyll content. HA significantly enhances antioxidant enzyme activities (SOD, POD, CAT, and APX) and reduces ROS content (O₂⁻, H₂O₂ and ⁻OH). Conclusion: The foliar spraying of 200 mg/L HA was found to be the most effective in alleviating Cd stress in sunflower seedlings, improving light energy utilization and antioxidant enzyme activities, while reducing ROS contents. These results may provide a theoretical basis for using HA to alleviate Cd stress in sunflowers.