Impacts of Various Microplastic Pollutants on the Functional Quality and Phytochemical Profile of Mulberry (Morus alba L.) Leaves

Mulberry (Morus alba L.) leaves are widely used as functional food and medicinal-food raw materials, yet the influence of soil microplastic contamination on their quality formation remains insufficiently understood. In this study, a 45-day pot experiment with 3–4 biological replicates was conducted to compare the effects of conventional polyethylene (PE) and biodegradable poly (butylene succinate) (PBS) microplastics at graded soil application rates on mulberry seedling growth, tissue ultrastructure, antioxidant-related quality attributes, and metabolic profiles. PE and PBS suppressed mulberry height, stem diameter and biomass in a dose-dependent way (p < 0.05). At 400 mg/kg, PE reduced plant height, stem diameter and fresh biomass by 34.7%, 30.8% and 31.4%, while PBS caused larger declines of 45.5%, 39.8% and 42.5%; total dry biomass dropped maximally by 34.2% under PE and 46.8% under PBS. Both treatments triggered 31.8% compensatory root elongation potentially for nutrient capture, yet PBS induced more severe root pore blockage and leaf cuticle damage than PE across tested concentrations. MP exposure raised leaf total flavonoids and phenolics markedly. At 400 mg/kg, PE and PBS boosted flavonoids by 82.8% and 90.9%, phenolics by 90.1% and 100.8%. Meanwhile, 400 mg/kg PE and PBS increased DPPH activity by 65.9% and 73.5%, ABTS activity by 66.3% and 74.4% relative to the control. Non-targeted metabolomics demonstrated pronounced shifts in the leaf metabolic profiles, characterized by the accumulation of coumaric acid, quercetin, and rutin, while glutathione levels declined. KEGG analysis identified phenylpropanoid and flavonoid biosynthesis as core responsive pathways; high PBS levels strongly disturbed lipid and stress metabolism. Overall, microplastic stress induced a trade-off between vegetative growth and antioxidant-oriented secondary metabolism in mulberry leaves. These findings suggest that biodegradable PBS residues can induce prominent physiological variations during the initial crop production stage, which highlights the need to closely track their temporal effects on functional raw materials.