Incubation Time and Size Effects of Biodegradable Mulch Microplastics on Lettuce Plantlets In Vitro

The use of biodegradable mulch films (BDM) in agriculture has raised concerns about the potential impact of the microplastics (MPs) they release over time, after the BDM’s useful life. The effects of BDM MPs have been explored through a diversity of assays, with still poorly understood and frequently contrasting results. Furthermore, the impact on plants as the MPs evolve in size and as a function of residence time in the soil remains largely unexplored. Through a controlled in vitro lettuce culture, this study explores the effect of BDM MPs size, using fractions 5 to <0.2 mm and pre-incubation times of 0 to 8 weeks, on plant development. Short incubation times, of 1 and 2 weeks, and freshly adding the BDM MPs inhibited plantlet growth, with smaller MPs inducing stronger effects. In contrast, longer MPs incubation, of 8 weeks, promoted plantlet development, enhancing leaf and particularly root elongation while reducing lateral root branching. The effects on roots were more pronounced, as the MPs size decreased. Germination and photosynthetic pigments were unaffected by any treatment. Overall, BDM MPs’ impact on plants was mainly driven by particle size and incubation time in the medium prior to seeding, with adverse effects on plant development observed at short incubation times that were no longer present when incubation was extended. These findings highlight the need to unravel the dynamic and temporal nature of the BDM MPs’ interaction with plants.