A 5-d germination assay and a 14-d hydroponic trial are performed to evaluate the impacts of graphene quantum dots (GQDs) on lettuce. Results show that GQDs are toxic to lettuce plants and that the effects are highly dependent on particle surface functionalization and plant growth stage. The germination rate is not affected by aminated GQDs (N-GQDs) and carboxylated GQDs (C-GQDs) but is reduced by hydroxylated GQDs (O-GQDs) by 39-71%. During the hydroponic trial, N-GQDs (50 mg L-1 ) increase the root dry weight by 34%, while C-GQDs and O-GQDs reduce it by 39% and 43%, respectively. Shoot dry weight is not affected by N-GQDs but is reduced by C-GQDs (44%) and O-GQDs (36-55%) treatments. C-GQDs and O-GQDs cause oxidative damage, disruption of mineral and organic nutrients homeostasis, impairment of photosynthesis, and modulates the levels of phytohormones. Light-triggered reactive oxygen species generation and oxidation of antioxidants in plants are the critical reason for the phytotoxicity and explain the difference between the different functionalizations. These findings suggest that GQDs may not be as safe as expected. Future studies should consider the modulation of surface chemistry to achieve optimal safety of GQDs, and more plant species should be tested over a longer-term scale.
Keywords: graphene quantum dots; oxidative damage; phytohormones; phytotoxicity; surface functionalization.
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