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 ( Solanum lycopersicum L .) 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 (R ² > 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.

The global shortage of freshwater, especially in agriculture, has become a serious threat to food security and safety; therefore, using brackish water with soil amendments in facility-grown vegetable production has drawn more attention recently. In this study, the effects of the combined use of peat or biochar at different rates (1%, 3%, and 5%) on soil properties, plant growth, and physiological responses were investigated in cherry belle radish (Raphanus sativus L.) irrigated with brackish water (5 g/L). The results revealed that peat significantly cut soil electrical conductivity (EC) by 42.61% and the pH value by 0.29, while biochar reduced soil EC by 32.44% but increased the pH value slightly. Both peat and biochar significantly increased the chlorophyll content and photosynthetic characteristics in cherry belle radish leaves and effectively promoted the accumulation of plant biomass. The net photosynthetic rate (Pn) of cherry belle radish in the soil amended with 5% peat or with 3% biochar increased by 89.06% and 85.94%, respectively, compared with CK, while the fresh weight of fleshy roots rose by 74.40% and 50.27%, respectively. This study further found that peat and biochar had effectively inhibited lipid peroxidation and proline accumulation in plants. The plasma membrane permeability was reduced by 41.18% and 39.90%, and the malondialdehyde (MDA) content decreased by 49.66% and 41.90%, respectively. In addition, peat and biochar significantly improved ion homeostasis in plants by increasing the ratios of K⁺/Na⁺ and Ca²⁺/Na⁺ in leaves, with the increment amplitudes reaching 90.21%/81.45% and 60.47%/79.15%, respectively. Nevertheless, biochar exhibited a superior effect compared to peat on balancing plant ions. In conclusion, a proper application of peat or biochar under brackish water irrigation has a significant potential to ameliorate soil properties and alleviate salt stress in plants, providing a safe approach for using brackish water with soil amendments in facility vegetable production in arid and semi-arid regions.