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Abstract
The efficacy of auxinic herbicides has been shown to vary with the time of day in which applications are made, however little is known about the mechanisms causing this phenomenon. Investigating the differential in planta behavior of these herbicides across different times of application may grant an ability to advise which properties of auxinic herbicide activity are desirable when applications must be made around the clock. Furthermore, determining under which environmental conditions this phenomenon is most prone to occurring may grant growers valuable information to inform future application strategies. Radiolabeled herbicide experiments demonstrated a likely increase in ATP-binding cassette subfamily B (ABCB)-mediated 2,4-D and dicamba transport in Palmer amaranth (Amaranthus palmeri S. Watson) at simulated dawn compared to mid-day, as dose response models indicated that many orders of magnitude higher concentrations of N-1-naphthylphthalamic acid (NPA) and verapamil are required to inhibit translocation by 50% at simulated sunrise compared to mid-day. Gas chromatographic analysis displayed that the log of ethylene evolution in A. palmeri was 0.32 L kg FW-1 h-1 higher when dicamba was applied during mid-day compared to sunrise. Furthermore, it was found that inhibition of translocation via 2,3,5-triiodobenzoic acid resulted in an increased amount of 2,4-D-induced ethylene evolution at sunrise, and the inhibition of dicamba translocation via NPA reversed the difference in ethylene evolution across time of application. Molecular experiments showed that expression of 9-cis-epoxycarotenoid dioxygenase biosynthesis gene NCED1 was increased with dawn applications, while there was a notable lack of trends observed across times of day and across herbicides with ACS1, encoding 1-aminocyclopropane-1-carboxylic acid synthase. Growth chamber studies indicated that temperature and day/night temperature differential affect percent control of A. palmeri with 2,4-D and dicamba applications more than humidity. Time of application effects were observed with 2,4-D across different temperature regimens, but not dicamba. Across different temperature differentials, a time of application effect observed with dicamba was eliminated upon increasing the rate. The formation of H2O2 was closely associated with phytotoxicity resulting from 2,4-D and dicamba applications.