Abstract: Microplastics, a type of new environmental pollutant, have received much attention for their negative effects on organisms and environment. We examined the effects of microplastics on seed germination and seedling physiological characteristics of spinach (Spinacia oleracea) under alkali stress, taking polystyrene microspheres with a diameter of 100 nm (200, 400, 800, 1600 mg·L^-1) as the microplastic treatment, and mixed NaHCO₃ and Na₂CO₃ as alkaline salt solution (5, 10, 20, 40 mmol·L^-1) according to the molar ratio of 1:1. The results showed that the presence of MPs (≥400 mg·L^-1) inhibited seed germination, and that the length of roots and shoots increased at low while decreased at high concentration of MPs. Different concentrations of alkali alone could inhibit seed germination, root and bud elongation. With the increases of MPs concentration, SOD activity of spinach seedlings gradually decreased, while POD activity firstly increased and then decreased, and chlorophyll content increased at low concentration (200 mg·L^-1) and decreased significantly at medium and high concentration (≥400 mg·L^-1). Different alkali stresses reduced chlorophyll content of spinach seedlings, and the effects on SOD and POD were ‘promotion at low concentration and inhibition at high’. In the treatments of microplastics (200, 800 mg·L^-1) and alkali (5, 20 mmol·L^-1) combined exposure, germination of spinach seeds was inhibited, and chlorophyll content decreased. The activities of SOD and POD in spinach seedlings were reduced under the combined exposure except the treatment of 200 mg·L^-1 MPs and 5 mmol·L^-1 alkali. Compared to the alkali stress, the combination of low concentration of MPs (200 mg·L^-1) and alkali could improve germination rate, germination index, germination vigor and vigor index of seeds, and significantly promoted the elongation of roots and shoots, while the addition of high concentration of MPs (800 mg·L^-1) reduced the germination rate, germination index, germination vigor and vigor index of seeds and inhibited the growth of roots and buds. The different concentrations of combined exposure inhibited the activities of SOD and POD and decreased the content of chlorophyll in spinach seedlings.

Key words: microplastics, alkali stress, spinach, seed germination, antioxidase, chlorophyll