Exogenous Melatonin Application Alleviates Microplastics and Cadmium‐Induced Phytotoxicity in Maize ( <i>Zea mays</i> L.) Plants: Insights From Physiological and Metabolomic Analyses

Soil contamination by microplastics (MPs) and heavy metals has become a global ecological and environmental issue and poses considerable threats to crop production and human health. In plants, melatonin (MT) functions as a powerful biostimulant, orchestrating vital physiological processes and enhancing stress tolerance. In this study, through controlled pot experiments, how exogenous MT (0.1 mmol L⁻¹) modulates maize responses to low-density polyethylene (LDPE) MPs, cadmium (Cd), and their combination was investigated. Simultaneous exposure to LDPE MPs and Cd exacerbated oxidative damage, inhibited chlorophyll biosynthesis, suppressed photosynthetic capacity, and reduced biomass in maize plants, alongside increasing shoot and root Cd²⁺ levels. Conversely, exogenous MT application reduced the malondialdehyde content by 12.5% under combined stress conditions, indicating a substantial reduction in oxidative damage. Additionally, MT inhibited the absorption and accumulation of Cd²⁺, increased the chlorophyll content, enhanced the photosynthetic efficiency, improved the plant height and stem diameter, thereby increasing maize plant biomass by 5.6%. MT also increased the activity of reactive oxygen species scavenging antioxidant enzymes and promoted the biosynthesis of non-enzymatic antioxidants such as proline and soluble sugars. Metabolomic analysis revealed that exogenous MT treatment significantly affected the levels of 210 metabolites. Notably, key metabolic pathways, including purine metabolism, phenylpropanoid biosynthesis, and tryptophan metabolism, were upregulated, indicating their pivotal role in the stress response mechanism of plants. These results reveal that exogenous MT effectively alleviates the synergistic phytotoxicity of PE MPs and Cd in maize plants, underscoring its promise as a practical strategy for enhancing crop resilience in contaminated environments.