Effects of Herbicides on Lemna gibba and Recovery from Damage After Prolonged Exposure

Abstract

To determine the potential impact of contaminants on the aquatic vascular plants Lemna sp., toxicity tests are usually conducted for a 4- to 14-day exposure, and the toxicity is usually expressed as EC50. However, the effects of longer exposure and the recovery potential after exposure to chemicals are other important factors which should be considered. We present the relative risks of a variety of exposure scenarios and recovery potentials from damage, using herbicides with different modes of action. Toxicity was assessed on the basis of both EC50 and relative growth rate (RGR) compared with untreated controls in exposure and recovery. The EC50 of atrazine was found to be 89 ppb, and its phytostatic concentrations were 1600 and 800 ppb for exposure periods of 14 and 28 days, respectively, and no phytocidal effects were observed up to 3200 ppb for a 28-day exposure. The RGR in recovery was not affected by the RGR in exposure, and regrowth was possible even after complete inhibition of growth for 28 days at the highest concentration tested. Alachlor, with an EC50 of 31 ppb, was phytostatic at 400 ppb for a 14-day exposure and phytocidal at 200 ppb for 21- and 28-day exposures. Paraquat, with an EC50 of 31 ppb, showed phytocidal rather than phytostatic effects. All phytostatic fronds could not grow in the recovery period, and the phytocidal concentration decreased with exposure period, from 80 ppb for a 7-day exposure to 20 ppb for 21- and 28-day exposures. The RGR of alachlor and paraquat in recovery was dependent on the RGR in exposure. In the case of cyclosulfamuron, phytostatic concentrations were 100 and 50 ppb for 7- and 14-day exposures, respectively. In the case of exposures longer than 21 days, however, it exhibited phytocidal activity at 10 ppb. The results of this study suggest that it is important to examine the effects of chemicals over a longer exposure period as well as the recovery potential from damage for reliable ecological risk assessment.

Potential of macrophytes for removing atrazine from aqueous solution

“…The growth was inhibited after 7 days exposure, however atrazine was not lethal to L. gibba at 3200 ppb for 28 days exposure. (Mohammad et al, 2010).…”

“…Atrazine is a pre and post-emergence herbicide, photosyntesis disrupter, which have an impact on the electron acceptors around PSI and PSII. It blocks the electron transport in photosynthesis, leading to a reduction in photosynthetic oxygen production (Mohammad et al, 2010), affecting the carbon assimilation in the target plant.…”

New Concept for Evaluating Toxicity of Herbicide for Ecological Risk Assessment

“…No growth was observed at 1600 or 3200 ppb after a 7-day exposure. There were no significant changes in the color of the fronds at any concentrations at any stages of exposure (Mohammad et al, 2010). Atrazine disrupts photosynthesis, the most basic functionin the plant kingdom.…”

“…Due to the environmental significance, they recommended that if substantial inhibition is observed from a 4-or 5-day exposure to the test material, the long-term exposure toxicity and the recovery phase should be conducted, and the phytostatic and phytocidal concentrations should be determined as the primary responses, because the test procedure provides a better assessment of toxic effects on an aquatic plant population. In a study with an aquatic plant, Lemna gibba, the phytocidal concentrations were 2.6 to >36 times higher than the corresponding EC50 values depending on the type of the herbicides tested (Mohammad et al, 2006(Mohammad et al, , 2010. In another study with Scenedesmus quadricauda, the EC50 of paraquat decreased with an increasing of the exposure period, and paraquat caused algistatic rather than algicidal effects at the higher concentration (Saenz et al, 2001).…”

“…These chemicals inhibited growth moderately even at 1000 ppb, (2) atrazine showed moderate toxicity among the herbicides and inhibited growth completely at 1000 ppb, (3) simetryn, alachlor and diuron inhibited growth less than 16 % RGR at 100 ppb, (4) paraquat with 86% RGR in exposure at 10 ppb caused death at 100 ppb, and (5) bensulfuronmethyl and cyclosulfamuron showed higher toxicity with 24% RGR at 10 ppb and 48% RGR at 1 ppb, respectively. Based on the results, long term exposure effects were examined for the representative herbicides, atrazine, alachlor, paraquat and cyclosulfamuron with different mode action (Mohammad et al, 2006(Mohammad et al, , 2010 …”