Phytotoxicity assessment of conventional and biodegradable plastic bags using seed germination test

A large fraction of plastic litter found in natural environments is constituted by conventional not biodegradable plastic bags, and their adverse effects via ingestion or entanglement on terrestrial and marine organisms are largely documented. Biodegradable and compostable shoppers have been recently developed as alternative to traditional ones. These bags are specifically designed to degrade in composting facilities and generate a product devoid of toxicity to soils and crops. However, very little is known on the effects of bag leaching, i.e. the transfer of chemicals from plastic into natural environments, on vegetation. Some plant species are highly sensitive to a variety of chemicals, and seedling growth is generally the most affected life history stage. In this study we assessed the potential effects of conventional (high-density polyethylene, HDPE) and compostable (Mater-bi®, MB) bags, when left in natural environments, on water quality and plant development. To this end, seeds of Lepidium sativum L., a terrestrial plant commonly used in phytotoxicity standard tests, were exposed to leachates obtained from different amount of HDPE and MB bags, simulating various pollution degrees occurring in nature, for 72 h. Both not-exposed (or virgin) bags and natural weathering exposed bags were used. Variations of chemical-physical characteristics of extracts were used as indicative of water quality deterioration, while alterations of seed gemination and seedling radicle and hypocotyl length were considered as indicative of phytotoxicity. A chemical qualitative screening of the leachates was also performed to identify the compounds with potential phytotoxicity. Both types of bags affected water characteristics (pH, salinity and total dissolved solids) relevant to plants, and released into water intentionally added chemicals, such as the noxious bisphenol A, and other phytotoxic substances probably generated during bag manufacturing. Leachates from both bag types did not affect seed germination. But, a significant number of seedlings showed developmental abnormalities or reduced seedling growth. The hypocotyl was the most sensible seedling organ to HDPE bag leachates while the radicle was the most vulnerable to MB ones. These findings indicate that plastic bags, including those that meet biodegradability and compostability standards, represent a potential threat to plants, if left in natural environments. Therefore, people and managers should be adequately informed about the potential environmental impact of an incorrect bag disposal. Simple, rapid standard phytotoxicity tests, such as the L. sativum bioassay, applied to bag leachates could be used in the future to select not noxious additives so to develop more eco-friendly bags.