Time of Application Influences Translocation of Auxinic Herbicides in Palmer Amaranth (Amaranthus palmeri)

Abstract

The efficacy of WSSA Group 4 herbicides has been reported to vary with dependence on the time of day the application is made, which may affect the value of this mechanism of action as a control option and resistance management tool for Palmer amaranth. The objectives of this research were to evaluate the effect of time of day for application on 2,4-D and dicamba translocation and whether or not altering translocation affected any existing variation in phytotoxicity seen across application time of day. Maximum translocation (Tmax) of [14C]2,4-D and [14C]dicamba out of the treated leaf was significantly increased 52% and 29% to 34% in one of two repeated experiments for each herbicide, respectively, with application at 7:00 AM compared with applications at 2:00 PM and/or 12:00 AM. Applications at 7:00 AM increased [14C]2,4-D distribution to roots and increased [14C]dicamba distribution above the treated leaf compared with other application timings. In phytotoxicity experiments, dicamba application at 8 h after exposure to darkness (HAED) resulted in significantly lower dry root biomass than dicamba application at 8 h after exposure to light (HAEL). Contrasts indicated that injury resulting from dicamba application at 8 HAEL, corresponding to midday, was significantly reduced with a root treatment of 5-[N-(3,4-dimethoxyphenylethyl)methylamino]-2-(3,4-dimethoxyphenyl)-2-isopropylvaleronitrile hydrochloride (verapamil) compared with injury observed with dicamba application and a root treatment of verapamil at 8 HAED, which corresponded to dawn. Overall, time of application appears to potentially influence translocation of 2,4-D and dicamba. Furthermore, inhibition of translocation appears to somewhat influence variation in phytotoxicity across times of application. Therefore, translocation may be involved in the varying efficacy of WSSA Group 4 herbicides due to application time of day, which has implications for the use of this mechanism of action for effective control and resistance management of Palmer amaranth.

Time of Day Effects on Peanut Herbicide Efficacy

“…There were also no significant differences between programs or TOD (Tables 6 and 7). Reduced control of Palmer amaranth when applying 2,4-D, imazethapyr, dicamba, glufosinate, and bentazon late in the evening or at night, has been observed in other research (Doran and Andersen, 1976;Johnston et al, 2018;Montgomery et al, 2017;Stopps et al, 2013). However, these studies differ from ours in that a single herbicide was used, not a tank-mixture with multiple active ingredients.…”

Investigation of physiological and molecular mechanisms conferring diurnal variation in auxinic herbicide efficacy

“…The advances in herbicide-resistant crops thus warrants extensive study into application strategies that maximize their efficacy. Variation in auxinic herbicide efficacy across time of application has been observed, displaying the classical trend of reduced phytotoxicity near dawn and/or dusk that has been reported with other herbicides [ 16 , 22 , 23 ]. This has been specifically observed in A .…”

“…The causal mechanism(s) responsible for the diurnal variation in auxinic herbicide efficacy have yet to be conclusively established. An increase in translocation has been observed with dawn applications of 2,4-D and dicamba, therefore suggesting an inverse relationship between translocation with herbicidal activity [ 16 ]. This suggested relationship contrasts the previously established understanding of the relationship between translocation and phytotoxicity, which classically couples reduced translocation with reduced herbicidal activity [ 16 , 26 – 28 ].…”

“…An increase in translocation has been observed with dawn applications of 2,4-D and dicamba, therefore suggesting an inverse relationship between translocation with herbicidal activity [ 16 ]. This suggested relationship contrasts the previously established understanding of the relationship between translocation and phytotoxicity, which classically couples reduced translocation with reduced herbicidal activity [ 16 , 26 – 28 ]. The framework for the translocation of auxinic herbicides is formed by the activity of auxin transport mechanisms, as structural similarities render auxinic herbicides substrates to many of the same proteins involved in both intra- and intercellular endogenous auxin movement.…”

Evaluation of the time-of-day effect of herbicides applied POST on protoporphyrinogen IX oxidase–resistant and –susceptible Palmer amaranth (Amaranthus palmeri)

“…Palmer amaranth resistance to protoporphyrinogen IX oxidase (PPO)-inhibiting herbicides (WSSA Group 14) has complicated chemical control tactics in the mid-South (Giacomini et al 2017;Heap 2018;Johnston et al 2018;Schwartz-Lazaro et al 2017). Over the last decade, management of glyphosate and acetolactate synthase inhibitor-resistant Palmer amaranth has relied on PPO-inhibiting herbicides applied PRE and POST for control.…”