## 1 Introduction
Biotests and ecotoxicological methods to assess the risk of MP to the environment are becoming increasingly useful, and a number of plant species are being used. For example, seed germination and root growth of cress (_Lepidium sativum_) showed a size-dependent response to MP, but only in the first 24 h, which was attributed to physical clogging of the pores in the seed capsule by MP, while MP accumulated in the root tips at later stages [5]. In common bean [31], maize [32], and onions [33], limited or no effects of polyethylene MP (LDPE or PEHD) on growth parameters were reported, possibly due to the fact that the chemical structure of polyethylene MP (C\({}_{2}\)H\({}_{4}\))\({}_{n}\), which is structurally stable and does not contain nutrients that could affect soil nutrient dynamics, has less pronounced effects on changes in soil properties. Negative effects were observed with the use of biodegradable MP (PBAT, PLA, starch-based MP), as evidenced by reduced root and shoot biomass of beans, maize, and wheat. In their work, Ref. [34] suggested that the altered bacterial communities in the rhizosphere and the increase in volatile compounds such as dodecanal could be responsible for the decline in total biomass. The intermediate and final metabolites degraded by PLA-MPs could have direct or indirect effects on soil properties, soil biota, and nutrient availability [34]. PBAT could increase the growth of rhizobacteria in the soil and thus compete with plant roots for nutrients [35]. Polyester microfibers at concentrations of 0.1% and 1% (\(w/w\)) acted as stressor on _Sinapis alba_ by altering chlorophyll fluorescence levels, reducing the number of flowers and decreasing the pod-to-seed ratio [23]. The addition of polypropylene (PP) plastic, which is often used for the production of bottles and packaging, led to contradictory results in plant growth parameters. Negative effects were observed for the germination of tomatoes [36] and garden cress [37], as well as for the shoot growth of pumpkin (_Cucurbita pepo_) [38], while PP contamination of the soil significantly increased the root biomass and length of green onion as well as the total biomass of wild carrot (_Daucus carota_) [33; 39]. Under hydroponic conditions, PP-MP had no effect on seed germination, but increased shoot and root elongation of tomato and cherry tomato seedlings. Nutrient uptake was also affected by MP, but the effect varied depending on the element and plant species as well as the duration of MP exposure, suggesting that long-term exposure to MPs may lead to nutrient imbalance in plants [40]. The objective of this study was to investigate the effects of different concentrations of low-density polyethylene (LDPE) MP on the germination and growth of a model plant species. Specifically, the impact on seed germination, root development, and shoot growth of white mustard (_Sinapis alba_ L.) was assessed. LDPE dominates the market due to its durability, puncture resistance, tensile strength, and stability under UV radiation and low temperatures [41], making it ideal for protecting crops and enhancing soil conditions [42; 43]. However, the poor end-of-life management of LDPE films often leads to their fragmentation and incorporation into soils during ploughing, contributing to microplastic accumulation [44]. Given its widespread use, environmental persistence, and high hydrophobicity, LDPE was selected in this study as a representative conventional plastic to assess its potential impacts on seed germination and shoot growth. Based on previous findings on MP-induced stress in plants [5; 14; 15; 45; 46; 47], the following hypotheses were proposed: (i) exposure to LDPE microplastics would reduce seed germination and inhibit early seedling growth of _Sinapis alba_ L. in a concentration-dependent manner under laboratory conditions; (ii) LDPE MPs would impair root and shoot development of _S. alba_ L. under controlled pot experiment conditions. Both laboratory and pot experiments were conducted using white mustard seeds and LDPE MP particles.