Uptake of microplastics and impacts on plant traits of savoy cabbage

Anthropogenic influences such as plastic pollution are causing serious environmental problems. While effects of microplastics on marine organisms are well studied, less is known about effects of plastic particles on terrestrial organisms such as plants. We investigated the effects of microplastic particles on different growth and metabolic traits of savoy cabbage (Brassica oleracea var. sabauda). Sections of seedlings exposed to polystyrene particles were analysed by coherent Raman scattering microscopy. These analyses revealed an uptake of particles in a size range of 0.5 µm to 2.0 µm into cells of the hypocotyl. Furthermore, plants were grown in substrate amended with polyethylene and polystyrene particles of different sizes (s₁: 200-500 µm; s₂: 100-200 µm; s₃: 20-100 µm; s₄: < 100 µm, with most particles < 20 µm; s₅: < 20 µm) and in different concentrations (c₁ = 0.1%, c₂ = 0.01%, c₃ = 0.001%). After several weeks, shoot and root biomass were harvested. Leaves were analysed for their carbon to nitrogen ratio, while amino acid and glucosinolate composition were measured using high performance liquid chromatography. Plastic type, particle size and concentration showed distinct effects on certain plant traits. Shoot biomass was interactively influenced by size and concentration of polyethylene, while root biomass was not modified by any of the plastic exposure treatments. Likewise, the composition and total concentrations of leaf amino acids were not affected, but the leucine concentration was significantly increased in several of the plastic-exposed plants. Glucosinolates were also slightly altered, depending on the particle size. Some of the observed effects may be independent of plastic uptake, as larger particles were not taken up but still could affect plant traits. For example, in the rhizosphere plastic particles may increase the water holding capacity of the soil, impacting some of the plant traits. In summary, this study shows how important the plastic type, particle size and concentration are for the uptake of microplastics and their effects on plant traits, which may have important implications for crops, but also for ecosystems.