Shikimate Accumulates in Both Glyphosate-Sensitive and Glyphosate-Resistant Horseweed (Conyza canadensis L. Cronq.)

Abstract

Horseweed (Conyza canadensis) is a cosmopolitan weed that commonly grows throughout North America. Horseweed that is not completely controlled by normal applications of glyphosate has been reported in western Tennessee. This research had three objectives: (1) to develop and validate an analytical procedure for the quantitative determination of shikimate, an important indicator of glyphosate activity in plants; (2) to confirm resistance to glyphosate in a horseweed population; and (3) to examine the accumulation of shikimate in both glyphosate-resistant and glyphosate-susceptible horseweed plants. The analytical procedure to determine shikimate used extraction with 1 M HCl for 24 h, followed by liquid chromatography using photodiode array detection, and shikimate recoveries were >or=82%. Glyphosate applications of both 0.84 kg ae/ha (the standard application rate) and 3.8 kg ae/ha to susceptible plants caused complete plant death. The same glyphosate applications to putative resistant populations caused less than 15% growth reduction as determined by visual evaluations, and fresh weights of these resistant plants 17 days after glyphosate treatment (DAT) were reduced an average of 45% in one population and were not affected in a different population. This direct comparison conclusively confirms that horseweed plants collected in western Tennessee in 2002 are resistant to 4 times the normal application dosage of glyphosate. The glyphosate-resistant horseweed biotypes still exhibited some herbicidal effects from the glyphosate, such as yellowing in the most actively growing, apical shoot meristems. The yellowing in the shoot apexes was transitory, and the plants recovered from this damage. Shikimate concentrations in all untreated horseweed plants were less than 100 microg/g, which was significantly less than that in all plants which had been treated with 0.84 kg ae/ha of glyphosate. Unexpectedly, shikimate accumulated (>1000 microg/g) in both resistant populations and the susceptible population. However, there were differences in shikimate accumulation patterns between resistant and susceptible horseweed biotypes. Shikimate concentrations in resistant populations declined about 40% from 2 to 4 DAT, while shikimate concentrations in the susceptible horseweed plants increased about 35% from 2 to 4 DAT. The confirmed resistance of a widespread weed implies that alternative control strategies for glyphosate-resistant horseweed will be needed in those no-tillage production systems where it commonly occurs.

Glyphosate influence on the physiological parameters of Conyza bonariensis biotypes

“…These results are similar to those found in the literature, in which resistant and susceptible biotypes of C. canadensis to glyphosate present accumulation of shikimic acid (Mueller et al, 2003;Feng et al 2004). In Conyza sp.…”

“…stalled the growth of the susceptible biotype by 57%, while for the resistant biotype DM was not affected (Table 1). A similar result was observed by Mueller et al (2003), when resistant biotypes of Conyza canadensis showed stoppage of increment of DM by about 40% after application of 840 g ha -1 and glyphosate e.a., while the susceptible biotypes showed a drop of more than 80% at 17 DAH.…”

Microwave-assisted solvent extraction and analysis of shikimic acid from plant tissues

“…Singh & Shaner (1998) and Mueller et al (2003) reported recoveries higher than 80% of spiked shikimic acid in crude extracts of soybean, maize and horseweed. Recoveries of 100% of shikimic acid in spiked samples of cotton, and 85% to 104% in fortified wheat tissue were also reported by Pline et al (2002) and Anderson et al (2001), respectively.…”

“…In contrast, other shikimic acid methods use various reagents such as sulfuric acid (Harring et al, 1998), HCl (Mueller et al, 2003) and alcohols (Anderson et al, 2001;Jaroszynska 2003). Our method provides close to 100% extraction efficiency and a high level of sensitivity without interference with other compounds.…”

“…Current extraction procedures require long extraction times and the use of high amounts of reagents such as methanol, ethanol, acetone, HCl, and sulfuric acid (Harring et al, 1998;Anderson et al, 2001;Jaroszynska, 2003;Mueller et al, 2003;Li et al, 2007). More efficient extraction methodologies would give rise to less solvent waste.…”