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BACKGROUND: Endothall has been used to control submersed aquatic plants since 1960, providing broad-spectrum control of aquatic weeds. Although endothall is considered a contact herbicide, many field observations suggest that it might have systemic activity. The goals of this research were to determine endothall's (i) absorption characteristics, (ii) translocation from shoots to roots, and (iii) potential for desorption in Eurasian watermilfoil (EWM), monoecious and dioecious hydrilla. RESULTS: Endothall absorption was linear in dioecious hydrilla up to 192 HAT, while in EWM and monoecious hydrilla absorptiondata best fit an asymptotic rise function. Endothall absorption in EWM, monoecious and dioecious hydrilla was 3.3, 6.6, and 11.0 times the external herbicide concentration determined by the plant concentration factor. Translocation to EWM roots reached 7.9% of total absorbed radioactivity by 192 HAT, while translocation to monoecious and dioecious hydrilla roots reached 17.8% and 16.4% by 192 HAT, respectively. For all three species, no more than 30% of absorbed endothall moved from the plant to clean water 96 HAT. CONCLUSION: Endothall is a very water soluble compound and yet it accumulated in these three important aquatic weeds at concentrations significantly higher than the external herbicide concentration. These data provide evidence that endothall could have systemic activity in these aquatic species. Following 14 C-endothall applications, more 14 C translocated from shoots to roots compared to the translocation of 14 C for other systemic aquatic herbicides. The final confirmation of endothall's systemic behavior requires that the radioactivity found in the root system of these aquatic plants is 14 C endothall.

Hydrilla [Hydrilla verticillata (L.f.) Royle] is often called the “perfect aquatic weed” as it has numerous physiological adaptations that make it highly aggressive and competitive. Hydrilla verticillata has historically been managed effectively using fluridone; however, the overreliance on this single mechanism of action (MOA) resulted in evolved fluridone resistance in the late 1990s. Where fluridone-resistant H. verticillata populations evolved, endothall became widely used for H. verticillata control. In 2018, florpyrauxifen-benzyl, a highly active auxin-mimic herbicide, was registered for H. verticillata control, and its use has increased since its introduction. Endothall and florpyrauxifen-benzyl provide two effective MOA for H. verticillata management, and combining these two MOA would be an effective strategy to delay further resistance evolution. The objective of this research was to determine if combining endothall and florpyrauxifen-benzyl would significantly impact the behavior of either herbicide in dioecious (DHV) or monoecious (MHV) H. verticillata compared to their behavior when applied alone. Endothall and florpyrauxifen-benzyl absorption and accumulation alone and in combination were measured over a 192-h time course. Translocation patterns were also determined. Herbicide accumulation in MHV and DHV was not impacted when these herbicides were applied in combination. Endothall translocation from shoots to roots in DHV was not 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 applied alone, to 2.2% ± 0.1 when applied in combination with florpyrauxifen-benzyl. Florpyrauxifen-benzyl shoot-to-root translocation was reduced in both MHV and DHV when applied in combination with endothall. Florpyrauxifen-benzyl translocation was reduced by 16- and 6-fold in DHV and MHV, respectively. These data do not suggest that there would be operational impacts from endothall and florpyrauxifen-benzyl mixtures. Still, there appear to be changes in herbicide behavior, primarily shoot-to-root translocation, when these two herbicides are applied in combination.

BACKGROUND: Hybrid watermilfoil is becoming more prevalent in many lakes where the invasive Eurasian (Myriophyllum spicatum, EWM) and native northern watermilfoil (M. sibiricum) co-occur. These Eurasian and northern watermilfoil hybrids (HWM) grow 30% faster and in many cases are less sensitive to 2,4-dichlorophenoxy acetic acid (2,4-D) than either parent. The mechanism(s) impacting 2,4-D tolerance in these hybrids was investigated by comparing the absorption, translocation, metabolism, and desorption of two 2,4-D formulations in EWM and HWM. RESULTS: 2,4-D absorption in EWM and HWM was 5.7 and 7.9 times the external herbicide concentration determined by the plant concentration factor, a metric used to determine herbicide bioaccumulation, and 2,4-D butoxyethyl ester absorption was 35.6 and 52.1 times the external concentration in EWM and HWM, respectively. Herbicide bioaccumulation was greater in HWM than in EWM. Herbicide translocation to HWM roots was limited at 192 HAT and herbicide desorption in HWM was slightly lower than EWM. No differences were found in herbicide metabolism between the two plant species.CONCLUSION: 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.

BackgroundResistance to 2,4‐Dichlorophenoxyacetic acid (2,4‐D) has been reported in several weed species since the 1950s; however, a biotype of Conyza sumatrensis showing a novel physiology of the rapid response minutes after herbicide application was reported in 2017. The objective of this research was to investigate the mechanisms of resistance and identify transcripts associated with the rapid physiological response of C. sumatrensis to 2,4‐D herbicide.ResultsDifferences were found in 2,4‐D absorption between the resistant and susceptible biotypes. Herbicide translocation was reduced in the resistant biotype compared to the susceptible. In resistant plants 98.8% of [14C] 2,4‐D was found in the treated leaf, whereas ≈13% translocated to other plant parts in the susceptible biotype at 96 h after treatment. Resistant plants did not metabolize [14C] 2,4‐D and had only intact [14C] 2,4‐D at 96 h after application, whereas susceptible plants metabolized [14C] 2,4‐D into four detected metabolites, consistent with reversible conjugation metabolites found in other 2,4‐D sensitive plant species. Pre‐treatment with the cytochrome P450 inhibitor malathion did not enhance 2,4‐D sensitivity in either biotype. Following treatment with 2,4‐D, resistant plants showed increased expression of transcripts within plant defense response and hypersensitivity pathways, whereas both sensitive and resistant plants showed increased expression of auxin‐response transcripts.ConclusionOur results demonstrate that reduced 2,4‐D translocation contributes to resistance in the C. sumatrensis biotype. The reduction in 2,4‐D transport is likely to be a consequence of the rapid physiological response to 2,4‐D in resistant C. sumatrensis. Resistant plants had increased expression of auxin‐responsive transcripts, indicating that a target‐site mechanism is unlikely. © 2023 Society of Chemical Industry.