Mechanistic insights into the size-dependent bioaccumulation and phytotoxicity of polyethylene microplastics in tomato seedlings

Microplastic (MP) contamination in agricultural soils poses a growing threat to crop health, yet the size-dependent mechanisms governing their uptake and phytotoxicity remain poorly understood. This study investigated the physiological and biochemical responses of tomato (<i>Solanum lycopersicum L</i>.) seedlings exposed to polyethylene (PE) microplastics of four distinct particle sizes at a constant mass concentration (1% w/w): T1 (1-2 mm), T2 (0.2-1 mm), T3 (50-200 μm), and T4 (1-50 μm). Results showed that PE exposure significantly inhibited plant growth in a size-dependent manner. The T4 treatment (1-50 μm) caused the most severe phytotoxicity, reducing shoot fresh weight by 42.3% and total root length by 55.1% compared to the control, indicating that micro-sized particles severely restrict root system expansion. This growth retardation was accompanied by aggravated oxidative stress, evidenced by a 263.4% surge in malondialdehyde (MDA) content (reaching 29.8 nmol/g FW) in the T4 group. To mitigate this stress, the antioxidant defense system was significantly activated, with SOD, POD, and CAT activities increasing by 122.2%, 194.1%, and 177.8%, respectively. The bioaccumulation of PE in plant tissues was highly non-linear and fitted well to the Freundlich isotherm model (R² > 0.97). Notably, the uptake and mobility of MPs were strongly governed by particle size, as reflected by the bioconcentration factor (BCF) and translocation factor (TF). Both indices exhibited a sharp increase as particle size decreased: root BCF surged from 0.016 (T1) to 0.840 (T4), while TF rose from 0.125 to 0.286, confirming the exponentially higher bioavailability and upward translocation potential of micro-sized particles (T4). Physiologically, small-sized MPs (T3, T4) induced non-stomatal limitations to photosynthesis. Furthermore, Variance Partitioning Analysis (VPA) revealed a distinct mechanistic shift around a critical size threshold: while growth inhibition under large-sized MP exposure (T1, T2) was primarily driven by disruptions in indole-3-acetic acid (IAA) homeostasis (explaining 32.4% of variation), the toxicity of small-sized MPs was predominantly governed by oxidative stress responses (explaining 38.6% of variation). These findings highlight that environmental risk assessments based solely on mass concentration may underestimate the hazards of micro-sized fragments, which exert toxicity through fundamentally different physiological pathways compared to larger particles. Ultimately, the identified shifts in hormonal balance and oxidative status provide a valuable mechanistic framework for evaluating the potential impacts of microplastic stress on the nutritional composition and overall quality of tomato fruits in subsequent growth stages.