Toxic Responses of Palladium Accumulation in Duckweed (Lemna minor): Determination of Biomarkers

Palladium (Pd) is a trace metal of the platinum group elements, representing an emerging contaminant for the environment. It is of great interest to characterize the bioaccumulation and toxicity of Pd to improve our toxicological knowledge for this contaminant. Under standardized toxicity testing conditions, we analyzed Pd accumulation and toxicity effects on the duckweed Lemna minor exposed to nominal concentrations from 2 to 50 mu M. The inhibitory effect was significant (p < 0.05) from 8 mu M of Pd, starting with 9.5% of growth inhibition and a decrease of 1 cm for the root size. Under 12.5 mu M of Pd, the bioaccumulated Pd of 63.93 mu g/g fresh weight inhibited plant growth by 37.4%, which was caused by a strong oxidative stress in the cytosol and organelles containing DNA. Under 25 and 50 mu M of Pd, bioaccumulated Pd was able to deteriorate the entire plant physiology including chlorophyll synthesis, the photosystem II antenna complex, and the photochemical reactions of photosynthesis. In fact, plants treated with 50 mu M Pd accumulated Pd up to 255.95 mu g/g fresh weight, causing a strong decrease in total biomass and root elongation process. Therefore, we showed several growth, physiological, and biochemical alterations which were correlated with the bioaccumulation of Pd. These alterations constituted toxicity biomarkers of Pd with different lowest-observed-effect dose, following this order: root size = growth inhibition < catalase activity = carotenoid content = reactive oxygen species production = total thiols < chlorophyll a/b = variable fluorescence to maximal fluorescence intensity ratio = absorbed-light energy transfer from the chlorophyll a antenna to the photosystem II reaction center = performance index of photosystem II activity < V-J. Therefore, the present study provides insight into the toxicity mechanism of Pd in L. minor plants under standardized testing conditions. Environ Toxicol Chem 2021;00:1-9. (c) 2021 SETAC