Microplastic particles (MPs), as waste and degradation products of larger plastics, are widespread pollutants with potential environmental impact. Polystyrene (PS) and poly(methyl methacrylate) (PMMA) plastics are widely used in industry and can accumulate in the environment, so it is important to investigate their possible effects on plants. In this thesis, I investigated the effects of different concentrations (10, 50 and 100 mg L-1) of PS- and PMMA-MPs on the growth and photosynthesis of common duckweed (Lemna minor L.), a model organism used for ecotoxicological research and phytoremediation. PS-MPs caused an increase in the number of fronds and total area, while PMMA-MPs, particularly at the highest concentration, caused a decrease. However, these changes were not statistically significant. Neither type of MPs had a significant effect on the maximum quantum yield of PSII (Fv/Fm), although the photosynthetic performance index (PIABS) was slightly higher for the PS-MP treatments than for PMMA-MPs. PMMA-MPs also led to a greater reduction in pigment content, especially chlorophyll a, while lower concentrations of PS-MP appeared to have a positive effect on the content of chlorophyll a, chlorophyll b and carotenoids compared to the control. Both types of MPs reduced the expression of RuBisCO protein, with PMMA-MPs having a more pronounced impact. In conclusion, although both MPs affected plant physiology, PMMA-MPs appear to be more harmful to L. minor than PSMPs, and it is advisable to continue research into its effects and possible plant defense mechanisms.