Background <p>Microplastic (MPs) pollution is increasingly recognized as an emerging environmental factor affecting soil functionality, nutrient dynamics, and plant stress physiology. However, the influence of MPs particle size and dose on boron (B) toxicity in soil-plant systems remains poorly understood. This study investigated the effects of polyethylene MPs with different particle sizes (100, 250, and 500&#xa0;μm) and doses (0.5, 1, and 2%, w/w) on soil B availability, B accumulation, biomass production, oxidative stress, and antioxidant responses in wheat (<i>Triticum aestivum</i> L.) grown under toxic B conditions (20&#xa0;mg kg^− 1).</p> Results <p>MPs applications significantly altered soil B dynamics and plant physiological responses depending on particle size and dose. Fine MPs (100&#xa0;μm) reduced extractable soil B concentration from 31.0 to 21.6&#xa0;mg kg^− 1, while increasing root and shoot B concentrations up to 94.7 and 236&#xa0;mg kg^− 1, respectively. Increased tissue B accumulation was closely associated with elevated lipid peroxidation and chlorophyll degradation. The highest malondialdehyde (MDA) level (13.3 nmol MDA g^− 1 FW) and the lowest superoxide dismutase (SOD) activity (108 U g^− 1 FW) were determined in the 100&#xa0;μm-2% MPs treatment. In contrast, large MPs (500&#xa0;μm) promoted higher catalase (CAT) activity and SH-group accumulation, indicating activation of alternative antioxidant defense pathways. Biomass production decreased under high MPs doses, particularly in fine MPs treatments.</p> Conclusions <p>MPs particle size exerted a stronger effect on B behavior and redox regulation than MPs dose. Fine MPs intensified B toxicity and oxidative stress by enhancing B accumulation and disrupting antioxidant coordination. These findings indicate that small-sized MPs may function as secondary stress enhancers in B-affected agricultural soils, highlighting the ecological importance of MPs fragmentation in agroecosystems.</p>