The development of agricultural technologies has intensified the use of plastic in this sector. Products of plastic degradation, such as microplastics (MPs), potentially threaten living organisms, biodiversity and agricultural ecosystem functioning. Thus, biodegradable plastic materials have been introduced to agriculture. However, the effects of biodegradable plastic substitutes on soil ecosystems are even less known than those of traditional ones. Here, we studied the effects of environmentally relevant concentrations of MPs prepared from a biodegradable plastic (a starch-polybutylene adipate terephthalate blend, PBAT-BD-MPs) on the growth and defense mechanisms of lettuce (Lactuca sativa) in CLIMECS system (CLImatic Manipulation of ECosystem Samples). PBAT-BD-MPs in the highest concentrations negatively affected some traits of growth, i.e., dry weight percentage, specific leaf area, and both C and N contents. We observed more profound changes in plant physiology and biochemistry, as PBAT-BD-MPs decreased chlorophyll content and triggered a concerted response of plant defense mechanisms against oxidative stress. In conclusion, exposure to PBAT-BD-MPs induced plant oxidative stress and activated plant defense mechanisms, leading to oxidative homeostasis that sustained plant growth and functioning. Our study highlights the need for in-depth understanding of the effect of bioplastics on plants.

Amsterdam Institute for Life and Environment Faculty of Science Vrije Universiteit Amsterdam De Boelelaan 1085 1081 HV Amsterdam the Netherlands

Institute of Environmental Sciences Leiden University P O Box 9518 2300 RA Leiden the Netherlands

Natural Resources Institute Finland Latokartanonkaari 9 00790 Helsinki Finland

Norwegian Institute for Water Research Økernveien 94 0579 Oslo Norway

Norwegian Institute for Water Research Økernveien 94 0579 Oslo Norway; Research Centre for Toxic Compounds in the Environment Masaryk University 62500 Brno Czech Republic

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