Getting More Ecologically Relevant Information from Laboratory Tests: Recovery of Lemna minor After Exposure to Herbicides and Their Mixtures

Abstract

Recovery after exposure to herbicides-atrazine, isoproturon, and trifluralin-their binary and ternary mixtures, was studied under laboratory conditions using a slightly adapted standard protocol for Lemna minor. The objectives of the present study were (1) to compare empirical to predicted toxicity of selected herbicide mixtures; (2) to assess L. minor recovery potential after exposure to selected individual herbicides and their mixtures; and (3) to suggest an appropriate recovery potential assessment approach and endpoint in a modified laboratory growth inhibition test. The deviation of empirical from predicted toxicity was highest in binary mixtures of dissimilarly acting herbicides. The concentration addition model slightly underestimated mixture effects, indicating potential synergistic interactions between photosynthetic inhibitors (atrazine and isoproturon) and a cell mitosis inhibitor (trifluralin). Recovery after exposure to the binary mixture of atrazine and isoproturon was fast and concentration-independent: no significant differences between relative growth rates (RGRs) in any of the mixtures (IC10, 25, and 50) versus control level were recorded in the last interval of the recovery phase. The recovery of the plants exposed to binary and ternary mixtures of dissimilarly acting herbicides was strictly concentration-dependent. Only plants exposed to IC10, regardless of the herbicides, recovered RGRs close to control level in the last interval of the recovery phase. The inhibition of the RGRs in the last interval of the recovery phase compared with the control level is a proposed endpoint that could inform on reversibility of the effects and indicate possible mixture effects on plant population recovery potential.

Growth inhibition and recovery patterns of common duckweed Lemna minor L. after repeated exposure to isoproturon

“…2, 3). Estimated EC 50 values in continuous exposure scenario based on standard endpoint average growth rate of frond number are comparable to values reported by Teodorović et al (2012), Knežević et al (2016) and Tunić et al (2015). Reduction of biomass, frond number, and growth rates in comparison to the controls indicated that inhibition of photosynthesis occurred at all tested exposure concentrations, but not to the extent that would result in plant death.…”

“…For continuous toxicity test, plants were treated with 50, 100, 150, and 200 µg L −1 of isoproturon in the nutrient solution. To provide live and growing population during the whole test, selected herbicide levels included a range of concentrations lower than the reported 7-day EC 50 value for common duckweed reported in the literature (Knežević et al (2016): EC 50 = 220 µg L −1 and Tunić et al (2015): EC 50 = 230 µg L −1 ). Duckweed plants were exposed to isoproturon at 8 different intended peak concentrations (ranging from 116.65 to 700 µg L −1 ) for the pulse exposure scenarios (Supplementary Table 1).…”

“…Inhibition of photosynthesis can lead to slow starvation of the plant followed by diminished growth (Varshney et al 2012) and increased production of reactive oxygen species in affected plants (Bi et al 2012;Wang et al 2015). Growth inhibition caused by isoproturon has been reported in studies with photosynthetic non-targeted aquatic organisms (Bi et al 2012;Tunić et al 2015;Knežević et al 2016).…”

“…Therefore, the present study aimed to investigate the effects of repeated exposures to isoproturon on growth, as a standard endpoint, and recovery potential of common duckweed, Lemna minor L. Tests were performed as a modified standard growth inhibition test in three repeated treatment cycles with isoproturon in a concentration range below reported 7-day effective concentration (EC 50 ) causing 50% inhibition of frond number growth rate (Knežević et al (2016): EC 50 = 220 µg L −1 ; Tunić et al (2015): EC 50 = 230 µg L −1 ). L. minor, a standard photosynthetic nontargeted aquatic organism has been chosen for this study due to favorable traits that enable population-level studies in laboratory conditions and a high potential for recovery from a single exposure to isoproturon (Varga et al 2019) and other types of herbicides (Wilson and Koch 2013;Burns et al 2015;Tunić et al 2015).…”

Herbicides in surface water bodies - behaviour, effects on aquatic organisms and risk assessment

“…This indirect impact on other species results from direct impact on plants, which mostly occurs in surface waters after deliberate use in order to control unwanted plant species in surface waters (Solomon et al, 2014). Adverse effects of herbicides on nontarget aquatic primary producers, both algae and plants, have been studied thoroughly (Knauer et al, 2008;Pereira et al, 2009;Dalton et al, 2013;Della Vechia et al, 2016;Knežević et al, 2016;Nagai et al, 2016;Stevanović et al, 2013Stevanović et al, , 2019Chamsi et al, 2019;Nagai, 2019). Arguments in favour of significant differences in sensitivity between flotant and rooted species have led to an extension of risk assessment framework for herbicides.…”