Two agricultural ditch plant species are negatively affected by root-zone glyphosate exposure

Background/Question/Methods

Extensive use of herbicide-resistant crop varieties and widespread implementation of conservation tillage have contributed to making glyphosate the most commonly applied herbicide in the United States. Glyphosate is a water-soluble compound that is present in runoff from agricultural fields.  The primary intercept for agricultural runoff is a complex system of ditches, from edge-of-field low order ditches to higher order large drainages.  These ditches share many qualities in common with wetland ecosystems, and vegetation within ditches has the potential to improve water quality before entering surface waters. This study investigated the question of whether different species commonly found in agricultural ditches are affected by glyphosate present in runoff by exposing two plant species to root-zone glyphosate at environmentally relevant concentrations. Panicum hemitomon and Polygonum hydropiperoides plants were individually potted in sand and exposed to one of four glyphosate concentrations (0, 10, 1000, 10000 mg L^-1). Glyphosate solution (100 ml) was introduced to the top of the substrate, allowed to infiltrate for two hours, and then the substrate was rinsed with 500 ml deionized water. Leaf chlorophyll content was measured daily and root-to-shoot ratio and survival were quantified at the study’s termination. 

Results/Conclusions

Root-zone glyphosate exposure negatively affected plants of both species. Seven days after exposure, leaf chlorophyll content was significantly reduced in both P. hemitomon and P. hydropiperoides for all glyphosate treatments. Root-to-shoot ratio was affected by glyphosate exposure in both species but in different ways. For both species, root-to-shoot ratio increased for the 10 mg L^-1 glyphosate concentration treatment compared to controls, likely a hormesis effect. In P. hemitomon, root-to-shoot ratios for the 1000 and 10000 mg L^-1 treatments did not significantly differ from the control treatment, while in P. hydropiperoides the root-to-shoot ratios for these two treatments were significantly decreased compared to controls. Survival was significantly reduced in P. hemitomon only for the 10000 mg L^-1 treatment, while survival was significantly reduced in P. hydropiperoides for both the 1000 and 10000 mg L^-1 treatments.  The results of this study indicate that P. hemitomon is less susceptible to negative effects from root-zone glyphosate exposure as compared to P. hydropiperoides. Community dynamics of wetland vegetation in agricultural ditches may be influenced by differential tolerance to glyphosate present in runoff.