Effects Of Differentially Charged Polystyrene Nanoplastics On Seedling Establishment Of Chinese Cabbage (Brassica Rapa L.)

Plastics have been widely used in daily production and life due to their remarkable properties in durability and low cost.However,in the process of production,application,treatment and disposal of plastic products,plastics will inevitably be released into the soil,water and air environment,causing"white pollution"problem.Meanwhile,microplastics（MPs）even nanoplastics（NPs）present in the environment,which were gradually produced by the aging of macroplastics plastics through different pathways,which may have unpredictable impacts on the ecosystem.Chinese cabbage（Brassica brassica L.）is an important vegetable crop in our daily life.Micro-and nano-plastics entering the environment will be absorbed by Chinese cabbage and accumulate in edible parts,which can have an impact on human health.Microplastics and nanoplastics are now gaining steadily increasing attention in the field of ecotoxicology.Many researchers have reported the single/combined phytotoxicity of micro/nano-materials and other common pollutants including heavy metals and organic contaminants in the seedling growth stage during the last couple of years.However,the researches on the interrelation between MPs/NPs and plants at the early seedling establishment stage,generally consisting of skotomorphogenesis and photomorphogenesis are quite scarce because it lasts so short to be neglected.In our study,we selected Chinese cabbage as a tested plant to explore the effects of pristine polystyrene nanoplastics（PS）and amino polystyrene nanoplastics（PS-NH₂）at the concentration of 1mg/L,10 mg/L and 100 mg/L on the plant skotomorphogenesis,photomorphogenesis and the whole stage of by hydroponic experiment.The main contents and results include the following parts:The first part mainly evaluates the basic properties of two kinds of nanoplastics.The intrinsic properties of plastics pollutants were evaluated by means of sanning electron microscopy,particle size distribution,Fourier transform mirco-infrared spectrometer,pH meter,Zeta potential and hydrodynamic diameter.The results showed that both of the pollutants is spherical and anionic and the presence of amino functional group on the surface of PS-NH₂ makes its surface charge less negative than PS,which provides a possibility for it to exhibit stronger phytotoxicity.The toxic effects of different doses of polystyrene nanoplastics（PS,PS-NH₂）（1,10,100mg/L）on Chinese cabbage during skotomorphogenesis of early seedling establishment were studied in the second part.The results showed that PS and PS-NH₂had no significant effect on radicle elongation during skotomorphogenesis stage of Chinese cabbage seedlings.This can be explained from the perspective that the two negatively charged nano-plastic pollutants stayed in the root cells for a relatively short time due to electrostatic repulsion with the root cells and had no significant effect on the radicle.On the basis of the secobd part,the third part continued to explore the toxic effects of different doses of polystyrene nanoplastics（PS,PS-NH₂）（1,10,100 mg/L）on Chinese cabbage during photomorphogenesis stage at the late seedling stage.The results showed that the content of photosynthetic pigments in cotyledons of Chinese cabbage seedlings decreased in different degrees（18.06%-28.52%）when exposed to photomorphogenesis stage.In addition,compared with PS,PS-NH₂ showed stronger phytotoxicity during photomorphogenesis stage.Combining with the characterization resluts in the first part,the experimental phenomena of this part can be explained as follows:because the presence of amino functional group on the surface of PS-NH₂ makes its surface charge less negative than PS,the electrostatic repulsion between negatively charged PS-NH₂ NPs and negatively charged leave surface was weakened accordingly,and thus PS-NH₂ aggregates were prone to form and continuously growing aggregates blocked upward translocation of water and nurtients.On the basis of the second and third parts,we continued to explore the toxic effects of different doses of polystyrene nanoplastics（PS,PS-NH₂）（1,10,100 mg/L）on Chinese cabbage during whole seedling establishment period in the third part.The results showed that the content of photosynthetic pigments in cotyledons of Chinese cabbage seedlings decreased by 22.46%-36.86%when exposed to the whole stage of seedling establishment,and the reduction was greater than that when only exposed to photophogenesis（in the third part of the experiment）.Similarly,PS-NH₂ showed higher phytotoxicity than PS during the whole seedling establishment stage.From the experimental results of the above several parts,it can be seen that the amino functional group of PS-NH₂ is the main reason for the photosynthetic toxicity of Chinese cabbage seedlings.The exposure of PSNPs to skotomorphogenesis of Chinese cabbage seedlings was an important factor affecting the photosynthetic toxicity.The conclusion of the present study improves the understanding of plant toxicology in seedling establishment stage and provides important implications for the effect of PSNPs on the early growth of vegetable crops.