Sensitivity of Sorghum (Sorghum saccharatum) and Mustard (Sinapis alba) to Soil Levels of Bio-Based Microplastics

(1) Background: Bio-based plastics are an alternative for commonly used petroleum-based plastics, and their production will increase in the coming decades. In this work, two innovative bio-based plastics, i.e., polylactide-based (PLA-based) and polyhydroxybutyrate-based (PHBV-based), were studied with regard to their effect on the growth of higher plants (Sorghum saccharatum, Sinapsis alba) in the soil environment. (2) Methods: The experiments were conducted in pots filled with the Organisation for Economic Co-operation and Development (OECD) reference soil with or without one of the bioplastics at concentrations from 0.1% w/w to 12.5% w/w. This study is one of few works in which soil instead of another medium (e.g., deionised water) was used for the evaluation of the impact of microplastics on plant growth. (3) Results: Mustard (Sinapsis alba) was more sensitive to the presence of microplastics in the soil than sorghum (Sorghum saccharatum). The length of mustard shoots exposed to PLA-based plastic were shorter from 25% to about 56% than those in the control tests, while in the case of PHBV-based plastic, the decrease of mustard shoot length varied from 6% to 26%. The presence of the bioplastics studied, in particular the PLA-based one, at the levels of 2.5% w/w and higher contributed to reduced germination and shoot length and to the decrease in the relative chlorophyll content. (4) Conclusions: These three endpoints occurred to be more sensitive than the dry weight or elemental composition of plant biomass. They are recommended to be used in the evaluation of phytotoxicity of microbioplastics to study how to maintain the sustainability of the soil environment.