Abstract: To better understand the effects and internal mechanism of the combined contamination of cadmium (Cd) and polystyrene microplastics (PS-MPs) on the growth, physiological characteristics, and canopy temperature of the Chinese cabbage pakchoi (Brassica chinensis), hydroponic experiments were conducted with Cd and two particle sizes of PS-MPs (100 and 1 000 nm). Changes in the growth, photosynthetic pigments, antioxidant enzyme activities, nutritional quality, anatomical structure, canopy temperature, and Cd absorption and transport characteristics were measured and analyzed. Results show that stress from either Cd or PS-MPs greatly inhibited the cabbage's growth and development and the process of photosynthetic physiological. This stress greatly reduced phenotypic indicators and photosynthetic pigments. Moreover, Cd and PS-MPs stress enhanced the oxidative stress response and decreased the vitamin, soluble sugar and protein contents in cabbage leaves. These changes also decreased the thickness of leafy, spongy and palisade parenchym tissues and significantly increased the temperature in the plant canopy. A low PS-MPs concentration (1 mg·L^-1) effectively alleviated the toxicity of Cd to the cabbage caused by combined stress of Cd and PS-MPs. A high PS-MPs concentration (20 mg·L^-1) combined with Cd resulted in a compound stress effect, which adversely affected the growth and development of the cabbage. These results can provide basis for ecological risk assessment of Cd and microplastics pollution in agricultural soil.

Key words: Brassica chinensis, cadmium, polystyrene microplastics (PS-MPs), physiological characteristics, canopy temperature