Gradient Dependent Chronic Toxicity of Polyethylene Microplastics (PE-MPs) on Tomato

Despite growing concern about microplastic contamination in terrestrial ecosystems, there have been few researches on the possible consequences of various microplastics on terrestrial plants. In this study, the toxicity of Polyethylene microplastics (PE-MPs) with different concentrations (0, 0.25, 0.50, 0.75 and 1.00%) to tomato (Solanum lycopersicum) were studied. Results showed a concentration-dependent negative impacts on the physiological, biochemical, growth, yield attributes and fruit quality of tomatoes. Amongst various concentrations, the maximum effect was observed with 0.75 and 1.00% PE-MPs and among the various growth stages studied (vegetative, flowering, fruiting and harvest), the highest impact was observed during the fruiting stage. Besides exhibiting a deleterious effect on physiological traits, it was also confirmed that PE-MPs could exert oxidative stress in plants thereby delaying the days to first flowering. Moreover, exposure to PE-MPs at 1.00% recorded the maximum reduction in the number of flowers per cluster (17.67%), number of flower clusters per plant (18.27%), number of fruits per cluster (32.76%), number of fruits per plant (56.33%) and single fruit weight (47.57%). Furthermore, 1.00% PE-MPs exposure was also observed to significantly reduce the fruit quality like titrable acidity, ascorbic acid and lycopene content by 22.32, 21.09 and 23.96% respectively. Additionally, the application of 1.00% PE –MPs decreased the soil bulk density, available phosphorus and potassium; whereas, the EC, organic carbon, microbial biomass carbon (8%), NO₃–N (22%), NH₄–N (23%) significantly increased. Moreover, the presence of PE-MPs in soil also had a significant influence on the soil enzyme activities. Metagenomic analysis (16 s) reveals that control had higher diversity, richness and evenness, compared to rhizosphere soil with 1.00% PE-MPs. These results can provide a theoretical basis and data support for further investigation on the toxicity of microplastics to tomatoes, and contribute to understanding the type specificity of microplastics’ toxic effects on plants.