Metabolism of the 4-Hydroxyphenylpyruvate Dioxygenase Inhibitor, Mesotrione, in Multiple-Herbicide-Resistant Palmer amaranth (Amaranthus palmeri)

Metabolic resistance to the maize-selective, HPPD-inhibiting herbicide, mesotrione, occurs via Phase I ring hydroxylation in resistant waterhemp and Palmer amaranth; however, mesotrione detoxification pathways post-Phase I are unknown. This research aims to (1) evaluate Palmer amaranth populations for mesotrione resistance via survivorship, foliar injury, and aboveground biomass, (2) determine mesotrione metabolism rates in Palmer amaranth populations during a time course, and (3) identify mesotrione metabolites including and beyond Phase I oxidation. The Palmer amaranth populations, SYNR1 and SYNR2, exhibited higher survival rates (100%), aboveground biomass (c.a. 50%), and lower injury (25-30%) following mesotrione treatment than other populations studied. These two populations also metabolized mesotrione 2-fold faster than sensitive populations, PPI1 and PPI2, and rapidly formed 4-OH-mesotrione. Additionally, SYNR1 and SYNR2 formed 5-OH-mesotrione, which is not produced in high abundance in waterhemp or naturally tolerant maize. Metabolite features derived from 4/5-OH-mesotrione and potential Phase II mesotrione-conjugates were detected and characterized by liquid chromatography-mass spectrometry (LCMS).

Germination and seed persistence of Amaranthus retroflexus and Amaranthus viridis: Two emerging weeds in Australian cotton and other summer crops

Redroot pigweed (Amaranthus retroflexus L.) and slender amaranth (Amaranthus viridis L.) are becoming problematic weeds in summer crops, including cotton in Australia. A series of laboratory and field experiments were performed to examine the germination ecology, and seed persistence of two populations of A. retroflexus and A. viridis collected from the Goondiwindi and Gatton regions of Australia. Both populations of A. retroflexus and A. viridis behaved similarly to different environmental conditions. Initial dormancy was observed in fresh seeds of both species; however, germination reached maximum after an after-ripening period of two months at room temperature. Light was not a mandatory prerequisite for germination of both species as they could germinate under complete darkness. Although both species showed very low germination at the alternating day/night temperature of 15/5 C, these species germinated more than 40% between ranges of 25/15 C to 35/25 C. Maximum germination of A. retroflexus (93%) and A. viridis (86%) was observed at 35/25 C and 30/20, respectively. Germination of A. retroflexus and A. viridis was completely inhibited at osmotic potentials of -1.0 and -0.6 MPa, respectively. No germination was observed in both species at the sodium chloride concentration of 200 mM. A. retroflexus seedling emergence (87%) was maximum from the seeds buried at 1 cm while the maximum germination of A. viridis (72%) was observed at the soil surface. No seedling emergence was observed from a burial depth of 8 cm for both species. In both species, seed persistence increased with increasing burial depth. At 24 months after seed placement, seed depletion ranged from 75% (10 cm depth) to 94% (soil surface) for A. retroflexus, and ranged from 79% to 94% for A. viridis, respectively. Information gained from this study will contribute to an integrated control programs for A. retroflexus and A. viridis.

Influence of flooding period and seed burial depth on Palmer amaranth (Amaranthus palmeri) seed germination

BACKGROUND

Flooding throughout fall and winter months is an effective practice for rice (Oryza sativa L.) straw decomposition, soil seedbank depletion, and waterfowl habitat in Mississippi. Nevertheless, limited research is available regarding the effects of fall-winter flooding and seed burial depth on Palmer amaranth (Amaranthus palmeri S. Wats.) seed germination. The objective of this study was to evaluate the effect of flooding period and seed burial depth on A. palmeri seed damage and germination in three different soil textures in Mississippi.

RESULTS

Amaranthus palmeri seed damage was greater when seeds were buried in sandy loam compared to silt loam soil textures. An interaction between flooding period and seed burial depth was present for A. palmeri seed germination. Flooding periods of 1-month (at 0 and 15 cm burial depth) and 2 months (at 0 cm burial depth) provided similar A. palmeri seed germination compared to no-flooding (at 0 cm burial depth). In addition, flooding periods of 3, 4, and 5 months reduced A. palmeri seed germination by 10, 10 and 14 percentage points at 0 cm burial depth, and 36, 40, and 41 percentage points when seeds were buried at 15 cm, respectively, across all soil textures.

CONCLUSION

This research demonstrates that flooding for 3, 4, and 5-months throughout fall and winter is an effective cultural practice to increase soil seedbank depletion through reduced germination potential to help manage herbicide-resistant A. palmeri populations in sandy loam, silt, and silt loam soil textures. © 2020 Society of Chemical Industry.