Comparison of Growth and Quality between Hydroponically Grown and Soil-Grown Lettuce under the Stress of Microplastics

There is growing concern of microplastics (MPs) contamination in farmlands and freshwater systems. This research investigated the impact of polyethylene (PE)-MPs (22–48 μm) on Romaine lettuce under a 30-day exposure at 50, 250, and 1000 (mg/kg or mg/L) in soil or hydroponic conditions. Growth parameters (plant height, root length, and fresh weight of shoots and roots) of hydroponic lettuce were lower with two lower doses of PE-MP treatments, but not affected by the highest dose. The growth parameters of soil-grown lettuce decreased by all PE-MP treatments. Moisture content was decreased by PE-MPs in hydroponic lettuce but increased in soil-grown lettuce. Ash followed an opposite trend to moisture. Ascorbic acid was lower in both groups of lettuce. Chlorophylls, total phenolics, and antioxidant capacity were increased in hydroponic lettuce but decreased in soil-grown lettuce. Lettuce firmness increased in both groups, which positively correlated to the increased lignin in plant cell walls. All PE-MP-treated lettuce experienced greater weight loss and accelerated color change during the 30-day storage at 4 °C, suggesting a shortened shelf life. Findings indicate that plant growth and quality were affected by the MP dose, growth systems, and their interactions, which should raise awareness of the risks of MPs in different agroecosystems.