Influence of Exposure Concentration and Duration on Effects and Recovery of Lemna minor Exposed to the Herbicide Norflurazon

Abstract

This study evaluated the effects and potential recovery of the surrogate aquatic macrophyte Lemna minor exposed to the herbicide norflurazon for 10 days under controlled conditions. Decreases in frond production occurred as early as 2 days after treatment (DAT) at concentrations ≥250 μg/L. The observed no observable-adverse effects and lowest observable-adverse effects concentrations during the 2-, 6-, and 10-day exposure periods were 100/250, 10/25, and 10/25 μg/L, respectively, for total frond production. The estimated EC₅₀ value for total frond production was 24.9 ± 4.1 μg/L (6 days of exposure). Symptoms of norflurazon toxicity (bleaching of foliage) were apparent within 2 DAT for concentrations ≥25 μg/L with 30–39 % of the fronds within each treatment exhibiting symptoms of toxicity. After 6- and 10-day exposures, 69–77 and 80–95 % of these plants showed toxic symptoms, respectively. Symptoms of toxicity for the 10 μg/L treatment first appeared at 4 DAT (51 % of fronds were symptomatic), peaked at 91 % 8 DAT, and were only 2 % at 10 DAT, thus indicating recovery. Norflurazon toxicity was eventually reversible at all concentrations once it was removed from the nutrient solutions. After 17 days of recovery (27 DAT), growth rates for all concentrations ≤250 μg/L were similar to those of the controls. Growth rates for all treatment concentrations recovered to control levels after 28 days of recovery (38 DAT).