Abstract: To assess the ecological risks of biodegradable polylactic acid microplastics (PLA-MPs) to crops, this study investigated the effects of PLA-MPs on seed germination and seedling functional traits of wheat (Triticum aestivum). Experiments were conducted using germination tests and hydroponic cultures. Wheat seeds and seedlings were exposed to two particle sizes (25 and 150 μm) of PLA-MPs at four concentrations (0, 50, 200, and 1 000 mg·L^-1).Results indicated that PLA-MPs significantly inhibited key germination parameters. Compared to the control, PLA-MPs reduced the final germination percentage (GP), germination vigor (GV) and germination index (GI), while prolonging the mean germination time (MGT). High-concentration treatments (1 000 mg·L^-1) induced a higher incidence of root rot and decreased the root-shoot ratio.At the seedling stage, PLA-MPs exposure elicited substantial physiological and morphological disturbances. PLA-MPs triggered oxidative damage, evidenced by the accumulation of reactive oxygen species (ROS) and increased malondialdehyde (MDA) content. Root system architecture was adversely affected, showing significant inhibition of root growth and a decline in root activity. Photosynthetic function was impaired, as reflected in reduced chlorophyll content and inhibited photosynthetic efficiency. Consequently, specific leaf area (SLA) increased significantly, while leaf dry matter content (LDMC) and the uptake of essential macronutrients (nitrogen, phosphorus and potassium) were markedly reduced.Two-way analysis of variance (ANOVA) revealed that concentration was the primary factor influencing wheat seed germination and seedling functional traits, with particle size exerting a secondary effect. Significant interactions between concentration and particle size were observed for only a minority of the measured indicators.Principal component analysis (PCA) further confirmed the significant particle size- and concentration-dependent effects of PLA-MPs on wheat. The phytotoxicity of PLA-MPs intensified with decreasing particle size and increasing concentration. Smaller particles (25 μm) and higher concentrations (200 and 1 000 mg·L^-1) consistently induced more pronounced detrimental effects across most measured parameters compared to larger particles (150 μm) and lower concentrations.These findings demonstrate that biodegradable PLA-MPs pose significant physiological and morphological threats to wheat, a staple crop. The toxicity is concentration-dependent and particle size-specific, with higher concentrations and smaller sizes causing greater harm. This study provides critical evidence for evaluating the ecological risks posed by biodegradable microplastics in agricultural systems and underscores the need for further investigation into their long-term environmental impacts.