Effect of temperature on the physiology and phytoremediation capacity of Spirodela polyrhiza exposed to atrazine and S-metolachlor

Environmental toxicity of pesticides to aquatic plants can vary with temperature, as temperature affects plant metabolic processes. We exposed the globally distributed duckweed Spirodela polyrhiza to environmentally relevant concentrations (40 µg/L) of atrazine and S-metolachlor at temperatures typical of surface freshwater in temperate zones (10, 15, and 21 °C). Our objective was to assess the effects of low temperatures and herbicide concentration, and their interactions, on growth, photosynthesis, pigments, antioxidant enzymes, and phytoremediation capacity. Lower temperatures (10 °C) intensified the adverse effects of both herbicides on the quantum yield of photosystem II in S. polyrhiza, with photosynthesis being a more sensitive endpoint than biomass growth rate. Both in the control and herbicide treatments, plants exposed to 10 °C exhibited lower concentrations of photosynthetic pigments (chlorophylls and carotenoids) and reduced ascorbate peroxidase activity, which may have contributed to the intensified negative effects on photosynthesis at this temperature. The removal of S-metolachlor was lower at 10 and 15 °C (3-8 %) compared to 21 °C (17 %), while no difference was observed between the three tested temperatures for atrazine (2-8 %). Our findings suggest that conducting pesticide toxicity tests at around 25 °C may underestimate the contaminants' inhibitory effects on aquatic plants during colder seasons and in temperate regions. Additionally, lower temperatures pose a challenge to the effectiveness of atrazine and S-metolachlor phytoremediation in aquatic environments.