%0 Journal Article
%T Physiology of Glyphosate-Resistant and Glyphosate-Susceptible Palmer Amaranth (Amaranthus palmeri) Biotypes Collected from North Carolina
%A Jared R. Whitaker
%A James D. Burton
%A Alan C. York
%A David L. Jordan
%A Aman Chandi
%J International Journal of Agronomy
%D 2013
%I Hindawi Publishing Corporation
%R 10.1155/2013/429294
%X Glyphosate-resistant (GR) biotypes of Palmer amaranth are now commonly found across the southern United States. Experiments were conducted to characterize physiological differences between a GR biotype and a glyphosate-susceptible (GS) biotype from North Carolina. The GR biotype had an 18-fold level of resistance based upon rates necessary to reduce shoot fresh weight 50%. Shikimate accumulated in both biotypes following glyphosate application, but greater concentrations were found in GS plants. Absorption and translocation of 14C-glyphosate were studied in both biotypes with and without an overspray with commercial glyphosate potassium salt (840£¿g£¿ae£¿ha£¿1) immediately prior to 14C-glyphosate application. Greater absorption was noted 6£¿h after treatment (HAT) in GS compared with GR plants, but no differences were observed at 12 to 72 HAT. Oversprayed plants absorbed 33 and 61% more 14C by 48 and 72 HAT, respectively, than plants not oversprayed. 14C distribution (above treated leaf, below treated leaf, roots) was similar in both biotypes. Together, these results suggest that resistance in this biotype is not due to an altered target enzyme or translocation but may be in part due to the rate of glyphosate absorption. These results also are consistent with resistance being due to increased gene copy number for the target enzyme. 1. Introduction Palmer amaranth is the most troublesome weed for cotton (Gossypium hirsutum L.) and soybean (Glycine max (L.) Merr.) producers in much of the southern United States [1]. Glyphosate has traditionally been effective in controlling Palmer amaranth [2, 3], and excellent control has been achieved in glyphosate-only systems [4¨C6]. Growers rapidly adopted GR crop technology for reasons discussed by Culpepper and York [4], and 73, 80, and 93% of the US corn (Zea mays L.), cotton, and soybean crops were planted to GR cultivars and hybrids in 2012 [7]. In the late 1990s, weed resistance to glyphosate was considered unlikely because of unique properties of the herbicide, such as its mechanism of action, absence of metabolic degradation in plants, and lack of residual activity in soil [8]. However, with widespread planting of GR crops and extensive reliance on glyphosate, resistant biotypes evolved. Today, resistance to glyphosate has been confirmed in 24 weed species [9]. The first confirmation of resistance to glyphosate in an Amaranthus species occurred with Palmer amaranth in Georgia in 2005 [10]. By 2012, GR Palmer amaranth had been confirmed in Alabama, Arizona, Arkansas, California, Delaware, Georgia, Illinois, Kansas,
%U http://www.hindawi.com/journals/ija/2013/429294/