Non-target-site herbicide resistance was evaluated in commercially available herbicides labeled for the management of Palmer Amaranth (Amaranthus palmeri). Seven treatments (T1: Control, T2: Flexstar (22.1%), T3: Buccaneer 5 Extra (53.8%), T4: Flexstar (5.88%) GT3.5 with Glyphosate (22.40%), T5: Defy LV-6 (88.4%): 2,4-D, T6: Enlist Duo (Glyphosate: 22.1% 2,4-D: 24.4%), and T7: Dicamba were applied at recommended rates. Glutathione-S-Transferase (GST) and Glutathione Reductase (GR) activities were measured in leaf samples after 15 minutes of herbicide exposure. Glyphosate exhibited the highest GST activity, followed by Fomesafen > Fomesafen Glyphosate > 2,4-D > Dicamba > 2,4-D Glyphosate. Treatments with a single mode of action exhibited higher GST activity, while mixed-mode treatments showed lower GST levels. Interestingly, an inverse relationship between GST and GR activity was observed, suggesting a compensatory mechanism. When GST activity was low, GR activity increased, indicating that the plant may enhance glutathione regeneration through GR to sustain detoxification capacity and manage oxidative stress. This biochemical compensation could enable the plant to survive herbicide exposure, even when direct detoxification (via GST) is limited. Such adaptability might contribute to the gradual development of non-target-site resistance, as the plant's defense system finds alternative pathways to mitigate herbicidal damage. These findings highlight that herbicide with a single mode of action, which trigger higher GST activity, may accelerate resistance evolution. In contrast, mixed-mode herbicides, which induce lower GST activity and potentially limit compensatory responses, can slow the progression of resistance. Thus, diversified herbicide strategies are essential for sustainable and effective management of Palmer Amaranth.
