Influence of light intensity on the toxicity of atrazine to the submerged freshwater aquatic macrophyte Elodea canadensis

Abstract

Light intensity can have a profound influence on the degree of phytotoxicity experienced by plants exposed to photosystem II (PSII) inhibiting herbicides. This relationship was evaluated in the submerged aquatic macrophyte Elodea canadensis exposed to three different concentrations of atrazine (510, 1000 and 2000μga.i./L) plus an untreated control at three different light intensities (0, 500 and 6000lx) under static-renewal conditions for 14 days. Under 500lx light intensity, control plants demonstrated a rapid increase in shoot length but minimal increase in dry shoot weight, suggesting limited photosynthesis. Based on shoot-length and biomass, growth was not affected by any atrazine exposure relative to controls under dark conditions (0lx). Under low-light conditions at 500lx, exposures to 510, 1000 and 2000μga.i./L atrazine significantly decreased net shoot lengths by 34%, 38% and 35%, respectively, relative to corresponding (500lx) controls. However, atrazine exposure under this light condition did not significantly decrease biomass (dry shoot weight). Compared to 6000lx, only approximately 8% of photosynthetically active radiation (PAR) was measured under 500lx intensity, indicating that minimal PAR was available for photosynthesis. Under optimal light conditions (6000lx), net shoot lengths significantly decreased in the treated atrazine groups by 48%, 51% and 68%, and net dry shoot weights (biomass) were significantly decreased by 79%, 81% and 91%, respectively, relative to corresponding (6000lx) controls. These data show that under low light conditions, atrazine-induced effects on dry shoot weight (biomass) are dependent on available PAR and active photosynthesis.