Tolerance and phytoremediation capacity of atrazine and S-metolachlor by two duckweeds

The phytotoxicity and removal of atrazine and S-metolachlor in sterile duckweed systems were estimated in this study. Herbicides were added at environmentally relevant ranges: 0-400 µg/L for atrazine or 0-200 µg/L for S-metolachlor in systems with Spirodela polyrhiza or Lemna minor. Toxicity biomarkers, i.e., changes in plant biomass, surface area, chlorophyll fluorescence parameters, pigments, lipid peroxidation, protein concentration, and antioxidative enzyme activities in plants were estimated after 7 days. S. polyrhiza (RGR_biomass-EC₅₀ = 164.8 µg/L) was more tolerant to atrazine than L. minor (RGR_biomass-EC₅₀ = 101.0 µg/L). Atrazine caused damage to photosystem II (Φ_M), a reduction in electron transport between PSII and PSI (Φ'_M), as well as disruption in energy distribution pathways (decrease in qPrel and increase in UQFrel), most prominently in L. minor. However, L. minor (RGR_biomass-EC₅₀ = 128.9 µg/L) was more tolerant to S-metolachlor than S. polyrhiza (RGR_biomass-EC₅₀ = 15.5 μg/L). The highest sensitivity of S. polyrhiza to S-metolachlor was attributed to a decrease in absorbed energy used in photochemistry (qPrel) and an increase in lipid peroxidation, indicating that S. polyrhiza plants were experiencing oxidative stress. Residual pesticide analysis in the water after seven days allowed us to conclude that plants were responsible for reducing up to 16.5% of atrazine and 28.7% of S-metolachlor in the duckweed system. S. polyrhiza showed higher atrazine phytoremediation capacity than L. minor. S. polyrhiza was more efficient at an environmentally relevant concentration of S-metolachlor (25 μg/L) and L. minor at higher concentrations (200 μg/L).