Folcisteine Safeguards Maize Against Copper–Cadmium Stress by Boosting the Activity of Photosynthesis-Related Enzymes and Antioxidant Defense Systems, Mediating Ascorbate–Glutathione Pathways and Hormonal Regulation

With the rapid development of industry and agriculture, soil heavy metal pollution has become increasingly severe. Copper (Cu) and cadmium (Cd) often co-occur in soils, exerting combined stress on crops. As a major food and feed crop, maize was studied under CuCd stress to assess the mitigating effects of exogenous Folcisteine (NATCA). Two varieties with contrasting tolerance (Jiuyuan 15 and Longfuyu 6) were subjected to composite stress (80 mg·L^-1 CuSO₄ + 100 mg·L^-1 CdCl₂), with or without 20 mg·L^-1 NATCA. The impacts on photosynthesis, reactive oxygen species (ROS) metabolism, the ascorbate-glutathione cycle, and endogenous hormones were investigated. The results showed that CuCd stress reduced the activities of RUBPCase and PEPCase, inhibiting CO₂ fixation, while NATCA application enhanced their activities and improved photosynthetic efficiency. Stress also induced ROS accumulation (elevated O₂·^- and H₂O₂) and elevated electrolyte leakage, whereas NATCA reduced oxidative damage and stabilized membrane integrity. Additionally, NATCA boosted both enzymatic and non-enzymatic antioxidant capacity in the ascorbate-glutathione cycle, improving ROS scavenging. Stress disrupted endogenous hormone balance, decreasing IAA, GA, and ZR, and increasing ABA. NATCA application restored hormone levels toward balance, promoting growth and enhancing tolerance to CuCd stress. These findings demonstrate NATCA's role in improving maize resilience under heavy metal stress.