Does the effect of herbicide pulse exposure on aquatic plants depend on KOW or mode of action?

Department of Agricultural Sciences, The Royal Veterinary and Agricultural University (KVL), Højbakkegård Allé 9, DK-2630 Taastrup, Denmark.
Aquatic Toxicology (Impact Factor: 3.45). 03/2005; 71(3):261-71. DOI: 10.1016/j.aquatox.2004.11.010
Source: PubMed


The highest concentrations of herbicides measured in flowing surface waters are often only present for short periods of time. These herbicide pulses can reach concentrations that would affect aquatic plants if present over a long time. The aim of this study was to assess the effect of a 3-h herbicide pulse relative to the effects of long-term (4 and 7 days) exposure of six herbicides with different sites of action and different K(ow) on the growth of the floating macrophyte Lemna minor. The herbicides were the two photosynthetic inhibitors: diquat and terbuthylazine, the inhibitors of acetolactate syntase (ALS), imazamox and metsulfuron-methyl and the microtubule assembly inhibitors propyzamide and pendimethalin. The log K(ow) ranged from -4.6 to 5.2. For imazamox, metsulfuron-methyl, propyzamide and pendimethalin a 3-h pulse induced the effect on area-specific growth as did a 4-day exposure at an approximate 10-fold higher concentration. For diquat and terbuthylazine a concentration closer to a factor of 100 or more was needed for a 3-h pulse to induce an effect similar to that of a 4-day exposure. For diquat, the low pulse-effect was most likely due to a slow uptake of the hydrophilic ion (log K(ow) = -4.6), as no effect was observed on chlorophyll fluorescence within 8 h after exposure. The chlorophyll fluorescence parameters are expected to respond quickly to a PSI inhibitor as diquat. For terbuthylazine, fluorescence measurements showed an effect on photosynthesis within 1h of exposure, and reached a minimum after 3 h. Recovery was fast, and initial fluorescence was restored within 24 h. Hence, the small pulse effect on area-specific growth was due to rapid recovery of photosynthesis. In contrast to terbuthylazine, the stop in area-specific growth observed for the ALS-and microtubule assembly inhibitors, took up to 4 days to recover from. Such a long recovery time after a pulse of only 3 h indicate that at realistic pulse exposures of up to a day or two, pulse-effects will approach the effects obtained in long-term studies. When investigating the effects of pulse exposures on aquatic plants, we should therefore focus more on non-photosynthetic inhibitors, which might not appear in pulses in as large concentrations as the PSII inhibitors investigated up till now, but whose effect, even in a shorter pulse, can be more damaging.

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Available from: Nina Cedergreen, Mar 10, 2014
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    • "Among these, hydrophobicity is one of the most important properties explaining the removal potential of plants. Organic chemicals with an octanol–water partition coefficient (logK ow ) between 0.5 and 3 are considered hydrophobic compounds able to move through the lipid bilayer of membranes, but still water-soluble enough to travel into the cell fluids (Cedergreen et al. 2005). Dimethomorph, with a log K ow of 2.63, can enter plants rapidly, explaining why removal was almost maximum after 96 h. "
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    • "Most laboratory toxicity tests performed with herbicides and macrophytes are characterized by either a constant or regular pulsed (static or semi-static) exposure regime (Cedergreen et al., 2004a,b; Belgers et al., 2007; Turgut, 2007; Arts et al., 2008; Huiyun et al., 2009). These tests usually do not provide insight in the effects of time–variable exposures, which are more often the rule rather than the exception in edge-of-field surface waters (Handy, 1994; Cedergreen et al., 2005; Brock et al., 2010). In the current (EC, 2002) and recently proposed (Maltby et al., 2010) European pre-registration risk assessment procedures for herbicides and aquatic vascular plants the predicted peak concentration (PEC max ) for edge-of-field surface waters is compared with the laboratory EC 50 of test species like Lemna sp. and Myriophyllum sp (Brock et al., 2010). "
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