Xenobiotics translocate in aquatic plants: a case study using three aquatic herbicides

Abstract

When invasive aquatic weeds dominate aquatic ecosystems there are numerous negative impacts. Milfoil (Myriophyllum spp.) and hydrilla [Hydrilla verticillata (L.f.) Royle] are the most costly aquatic plants to manage in the U.S. per year. These invasive plants form extensive surface canopies that negatively affect water quality and native plant communities, and can also impact recreational uses such as swimming, fishing, and boating. Synthetic auxins, such as 2,4-dichlorophenoxyacetic acid (2,4-D), have been widely used for selective control of milfoil since 1959. Since then, several populations of hybrid watermilfoil (M. sibiricum x M. spicatum; HWM) have showed lower sensitivity to this herbicide. In 2015, a HWM population with lower sensitivity to 2,4-D was found in Idaho, USA. Using the same 2,4-D-resistant population and a known susceptible Eurasian watermilfoil (M. spicatum; EWM) population from Colorado, the mechanism of 2,4-D resistance was examined by conducting 14C-2,4-D absorption, translocation, desorption, and metabolism experiments. 2,4-D resistance in HWM is not due to non-target-site resistance as no differences in herbicide absorption, translocation, desorption and/or metabolism were identified; therefore, target-site resistance is the most likely resistance mechanism. More research is needed to identify the molecular basis for the 2,4-D-resistant trait in HWM. Herbicide combinations are widely recommended to alleviate the evolution of herbicide resistance. The aquatic herbicide endothall is often used in combination with 2,4-D for HWM management as an effective control option and a resistance management strategy, but it is still unknown how combining herbicides might impact the behavior of each herbicide. Experiments combining radiolabeled with non-radiolabeled herbicides were conducted to evaluate herbicide absorption, accumulation, and translocation from shoots to roots in HWM. Endothall accumulation was not impacted when these herbicides were applied in combination, but its translocation from shoots to roots was reduced by 50% when applied in combination with 2,4-D. When 2,4-D, was applied in combination with endothall shoot absorption increased by 80%; however, 2,4-D movement from shoots to roots was reduced from 24.8% ± 2.6 to only 3.93% ± 0.4 when in the presence of endothall. The overreliance on a single mode of action resulted in evolved fluridone resistance in hydrilla in the late 1990s. 2,4-D is not effective for hydrilla control at label rates, but the most recently registered auxinic herbicide, florpyrauxifen-benzyl, is highly active against hydrilla. Where fluridone-resistant hydrilla is present, endothall is being used in combination with florpyrauxifen-benzyl for its control. In order to test experiments combining radiolabeled and non-radiolabeled endothall and florpyrauxifen-benzyl were conducted to evaluate herbicide absorption, accumulation, and translocation in two hydrilla biotypes, monoecious (MHV) and dioecious (DHV). Herbicide accumulation in both biotypes was not impacted when these herbicides were applied in combination. Endothall translocation from shoots to roots in DHV did not appear to be impacted (alone = 18.7% ± 1.4; combination = 23.2% ± 2.2); however, endothall shoot-to-root translocation in MHV was reduced from 16.2% ± 1.3 to 2.2% ± 0.1 when applied in combination with florpyrauxifen-benzyl. Florpyrauxifen-benzyl shoot-to-root translocation was reduced by 16 and 6 times in DHV and MHV when applied in combination with endothall, respectively. These data highlight differences in herbicide behavior when herbicides are applied in combination. Future research is needed to determine if these differences negatively impact the operational effectiveness when herbicides are applied in combination. Lastly, endothall and 2,4-D have been used to control aquatic weeds for more than 60 years, and still there is very little information available about the in planta behavior of these herbicides in aquatic weed species. 2,4-D is purportedly systemic in aquatic plants based almost entirely on its behavior in terrestrial plants. It was demonstrated in this dissertation that radioactive 2,4-D and endothall can translocate from shoots to root systems; however, it was not determined if the radioactivity in the roots was parent herbicide or a metabolite(s). Therefore, the last chapter of this dissertation used multiple analytical methods to answer the question if 2,4-D and endothall are truly systemic in aquatic plants. The intact 2,4-D detected in HWM shoots was 1.31 µg g-1 dry weight (DW) and 0.11 µg g-1 DW was detected in the roots. For endothall, 1.08 and 0.12 µg g-1 DW was detected in DHV shoots and roots, respectively. We therefore conclude that 2,4-D and endothall have similar in planta behavior, with about 8-10% of absorbed intact active ingredient translocating to the roots of these aquatic plants.