Comparative effects of polystyrene nanoplastics with different surface charge on seedling establishment of Chinese cabbage (Brassica rapa L.)

Micro- and nano-plastics are common emerging pollutants of great interest. However, the impacts of them on terrestrial plants were still poorly understood. In this study, comparative effects of exposure of polystyrene nanoplastics (PS) and amino-modified polystyrene nanoplastics (PS-NH) on Chinese cabbage (Brassica rapa L.) plants at different growth stages were investigated. Hydroponically cultured seedlings were exposed to PS and PS-NH at 0, 1, 10, and 100 mg/L at skotomorphogenesis stage for 48 h, photomorphogenesis stage for 18 h, and the whole stage, respectively. Results showed that both PS and PS-NH had no discernible effect on radicle elongation at the skotomorphogenesis stage whereas significantly (P < 0.05) reduced photosynthetic pigment contents in varying degrees (18.06%-28.52%, 22.46%-36.86%) at the photomorphogenesis stage and the whole stage. Moreover, there was no significant difference between PS treatments and control except the 26.52% decline of chlorophyll a content at 1 mg/L at photomorphogenesis, while PS-NH significantly (P < 0.05) decreased photosynthetic pigment contents except the chlorophyll b content at 10 mg/L at photomorphogenesis. The content of chlorophyll a decreased by 26.68% for the PS-NH-treated group and 22.46% for the PS-treated group at 1 mg/L during the whole stage. Results manifested that less negatively charged PS-NH seemed to show more severe phytotoxicity both at the photomorphogenesis stage and the whole stage. Notably, the surface charge of nano-plastics and the integrity of seedling establishment could be the main factors impacting the above difference. These findings are expected to improve our understanding of the effects of PSNPs on crop plants.