Abstract
The efficacy of auxinic herbicides, a valuable weed control tool for growers worldwide, 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 herbicides are desirable when applications must be made around the clock. 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, respectively, are required to inhibit translocation by 50% at simulated sunrise compared to mid-day. Gas chromatographic analysis displayed that ethylene evolution in A. palmeri was 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 (TIBA) 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. Dawn applications of these herbicides were associated with increased expression of a putative 9-cis-epoxycarotenoid dioxygenase biosynthesis gene NCED1, 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. Overall, this research indicates that translocation is differentially regulated via specific protein-level mechanisms across times of application, and that ethylene release, a chief phytotoxic process involved in the response to auxinic herbicides, is related to translocation. Furthermore, transcriptional regulation of abscisic acid involvement in phytotoxicity and/or translocation are suggested.
Highlights
Growers are frequently challenged by variation in herbicide efficacy across the time of day when an application is made
Relative expression of NCED1 was approximately three-fold higher when herbicide applications were made at 8 am compared to 1 pm. These results demonstrate a clear association of application time and NCED1 activity, indicating some degree of diurnal control on herbicide-inducible abscisic acid (ABA) biosynthesis at the transcriptional level
Since no significant time of application effects were detected, there may be another mechanism that causes the reported increase in ethylene production with dicamba applications at mid-day compared to dawn. Another member(s) of the acid synthase (ACS) gene family not analyzed here may play a major role in this difference. It appears that reductions in auxinic herbicide activity at dawn coincides with an increase in NCED1 expression and potentially increased ABA concentrations in the plant cell
Summary
Growers are frequently challenged by variation in herbicide efficacy across the time of day when an application is made. Reduced weed control resulting from this variation is detrimental from a financial standpoint, and has serious implications on the evolution of resistance as reduced herbicide efficacy is consistently linked to selection for resistance-conferring traits [1,2,3]. Watson), a weed species that produces a large amount of genetic variability in offspring due to massive seed production and obligate outcrossing [4]. This characteristic coupled with a high growth rate, and minimized time required for reproduction, allows for accelerated evolution of herbicide resistance in the presence of overreliance on certain herbicide mechanisms of action [5,6]. The resistance of A. palmeri to glyphosate in particular has become extremely widespread and problematic for growers [1]
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