Differential physiological and biochemical responses of lettuce cultivars to polyethylene microplastic stress

Abstract

Polyethylene microplastics are widespread soil contaminants that may threaten agroecosystem stability and crop quality. However, plant physiological and biochemical responses to microplastic exposure, particularly cultivar-specific responses, remain insufficiently understood. This study evaluated the ecotoxicological effects of polyethylene microplastics on two lettuce (Lactuca sativa L.) cultivars, ‘Little Gem Red’ (LGR) and ‘Little Gem Green’ (LGG), and examined their biochemical defense responses under stress conditions. A 35-day pot experiment was conducted using microplastic concentrations of 0, 50, 250, and 1000 µg/mL. Growth parameters, photosynthetic pigments, oxidative stress markers, antioxidant enzyme activities, total antioxidant capacity, and key phytochemicals were analyzed. Microplastic exposure induced non-linear and cultivar-dependent responses. In LGR, antioxidant enzyme activity increased rapidly under low microplastic exposure, with superoxide dismutase and ascorbate peroxidase reaching peak values of 17.03 ± 0.11 and 2.28 ± 0.039 U/mg protein, respectively, at 50 µg/mL. In LGG, maximum enzyme activities occurred at 250 µg/mL, with superoxide dismutase and ascorbate peroxidase reaching 21.76 ± 0.33 and 0.67 ± 0.003 U/mg protein, respectively. LGG also showed sustained increases in phenolics and flavonoids under higher microplastic exposure, indicating greater involvement of non-enzymatic antioxidant pathways. These findings demonstrate that lettuce cultivars employ distinct biochemical adjustment strategies in response to polyethylene microplastics and highlight the importance of considering intraspecific variation when assessing crop responses to microplastic-contaminated soils.