Objective The pollution effects of microplastics in agricultural ecosystems have attracted increasing attention, yet the in-situ field impacts of microplastics with different particle sizes on crop growth remain unclear. This study aimed to clarify the effects of polyethylene (PE) microplastics with different particle sizes on the agronomic traits and physiological characteristics of tobacco, providing a basis for ecological risk assessment of microplastic pollution in farmland.

Method Field in situ experiments were conducted in four tobacco-growing regions: Kaiyang (Guizhou), Shizhu (Chongqing), Chengjiang (Yuxi), and Gulin (Sichuan). Treatments included PE microplastics with particle sizes of 20 nm and 100 μm, along with a control group. Plant height, leaf length, leaf width, and effective leaf number were measured at the rosette, vigorous growth, and pre-harvest stages. Physiological and biochemical indicators, including superoxide dismutase (SOD), catalase (CAT) activity, malondialdehyde (MDA), hydrogen peroxide (H₂O₂), ascorbic acid (AsA), glutathione (GSH), soluble sugars, and free proline (fPro) contents, were analyzed in leaves collected before harvest. A comprehensive analysis was conducted to characterize the effects of different particle sizes.

Result The effects of PE microplastics on tobacco growth showed both particle size-dependent and regional differences. The 100 μm PE microplastics treatment significantly inhibited tobacco growth in the Kaiyang and Shizhu regions, reducing plant height by 6%-17% before harvest (e.g., from 99.01 cm to 82.12 cm in Shizhu) and decreasing maximum leaf length and width by 5%-12%. Its inhibitory effect was stronger than that of the 20 nm PE microplastics treatment. The 20 nm PE microplastics treatment did not significantly affect agronomic traits of tobacco in any region during the early growth stages. However, by the pre-harvest stage, plant height of tobacco in the Kaiyang region decreased by 6% compared to the control, suggesting that nano-sized microplastics may exhibit biological accumulation effects. At the physiological level, both 20 nm and 100 μm PE microplastic treatments across all regions increased MDA and H₂O₂ contents by 10%-25% and enhanced SOD activity along with AsA and fPro contents by 8%-20%, indicating that tobacco experienced oxidative stress and activated its antioxidant defense system.

Conclusion Under in-situ field conditions, PE microplastics of different particle sizes affect tobacco growth and physiological metabolism through distinct pathways. Their ecological effects are jointly regulated by particle size characteristics and regional environmental factors.