Mechanisms Of Graphene Toxicity To Plants

Graphene, a class of 2D carbon nanoparticles, has showed great promise due to its uniqe properties in a wide range of fields including energy storage, nanoelectronic devices, batteries, transparent conductors, environmental protection and many others. With their increasing use, they might ultimately be released into the aquatic, terrestrial, and atmospheric environments, where their ecological risks are largely unknown. The goals of the present study were to characterize the impacts of graphene on green algae Scenedesmus obliquus and wheat (Triticum aestivum L.), thereby addressing a key gap in the understanding of the ecological risks of graphene on plants. The main research includes three parts as follows:(1) Results showed that after 72 h expousure of graphene at the concentrations ranged from 0,10,20,50,100,150,200,250 to 300 mg L-1, the growth of S. obliquus was notably inhibited in a dose- and time- dependent manner. The 72-h IC50 values of graphene evaluated using the logistic and Gompertz models were 148 and 151 mg·L-1, respectively. In order to understand the toxicity mechanisms of graphene to S. obliquus, the surface morphology and cellular ultrastructure of cells were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In 150 mg L-1 graphene treatment, we found that graphene adhered to the surface of algal cells as a semitranslucent coating. On the other hand, compared with the intact cellular structure illustrated in TEM images of control, damaged cell walls occurred in graphene-exposed cells. Two other major physiological changes that were apparent in graphene-exposed cells were clumping of nuclear chromatin into more densely packed material and increased number of starch grains These results implied that graphene inhibited the growth of algal cells might be associated with the extracellular shading and intracellular damage. Besides, we also found that the cell permeability increased with the increasing concentrations of graphene, inferring the cells membrane integrity loss occurred in graphene treatments. Moreover, graphene promoted the generation of reactive oxygen species (ROS) and malondialdehyde (MDA) in algal cells, which were considered to the typical signals of oxidative effects. Finally, significant decreases were also observed in chlorophyll a, chlorophyll b and chlorophyll alb levels in algal cells under graphene expousure, indicating photosynthetic activity in graphene-exposed cells might be affected. Chlorophyll fluorescence parameters analysis showed that yield of photochemical quantum [Y(Ⅱ)], photochemical quenching coefficient (qP), non-photochemical quenching coefficient (NPQ) and relative electron transport ratio (rERT) were all decreased in graphene-exposed cells, as compared to control. These results inferred that graphene significantly down-regulated photosystem Ⅱ (PSⅡ). Taken together, the toxicity effect of graphene on algae cells might due to extracellular shading and intracellular oxidative damage, thus play an inhibitory effect on algae photosynthesis which ultimately suppressed the growth of algal cells.(2) Results showed that the root elongation rates increased by 69%, 221%,208% and 288% in wheat plants after 48 h exposure of 250,500, 1000 and 1500 mg L-1 graphene, respectively, as compared with control. Accompanied by microscope observation, we suggested that the graphene induced-root elongation might attribute to cells elongation. Besides, graphene induced several changes in graphene exposed-roots:short and sparse root hairs; high MDA levels; intense staining of NBT and Evans blue stainings also implied that roots in graphene-exposed wheat were suffered in oxidative stress. Meanwhile, both superoxide dismutase (SOD) and peroxidase (POD) activities increased in the low dose-graphene treatments while the opposite was true in high dose-treatments. These findings suggested that graphene supplied might lead to disorders of the oxidative stress defense system. Combining all of these findings together, we inferred that graphene could induce oxidative stress and retardation of root hairs development.(3) Results showed that 30 days-incubation of 500 mg L-1 graphene significantly decreased fresh weight of shoot in wheat plants. Meanwhile, chlorophyll contents in leaves of graphene-exposed plants also decreased. These phenomena indicated that long-term of graphene incubation could inhibit plant growth. In order to investigate the alterations of the nutrient contents in wheat plants with graphene exposure, we firstly focus on the contents of macroelements including nitrogen, phosphorus and potassium. Results indicated that the contents of nitrogen and potassium in the leaves of graphene-exposed plants were significantly decreased, whereas the total phosphorus contents remained unchanged. Moreover, calcium and magnesium contents in leaves also greatly declined in graphene treatments, while sodium contents remained unchanged. Similarly, the contents of trace elements such as zinc, iron and copper were markedly declined by graphene supplied whereas manganese contents were increased. These results showed that graphene could disturb the balances of nutrient elements. In conclusion, the effects of graphene on nutrients metabolism might be another cytotoxicity characterization.