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item Koger Iii, Clifford
item Reddy, Krishna

Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/23/2004
Publication Date: 1/1/2005
Citation: Koger III, C.H., Reddy, K.N. 2005. Role of absorption and translocation in the mechanism of glyphosate resistance in horseweed (conyza canadensis). Weed Science 53:84-89.

Interpretive Summary: Glyphosate (Roundup®) is globally the most widely used herbicide, especially since the introduction of transgenic glyphosate-resistant crops such as Roundup Ready canola, corn, cotton, and soybean. Repetitive use of glyphosate has led to the development of glyphosate-resistant horseweed in ten states of the eastern U.S. including Mississippi. Information regarding the mechanism responsible for glyphosate resistance in horseweed is lacking. Thus, greenhouse and laboratory studies were conducted at the Southern Weed Science Research Unit, Stoneville, MS to examine absorption and translocation of glyphosate in resistant and susceptible horseweed biotypes from Mississippi, Arkansas, Tennessee, and Delaware. A non-destructive leaf-dip assay was developed and used to confirm resistance and susceptibility of test plants. Glyphosate absorption was similar (47 to 54%) between plants and the amount of glyphosate translocated from the treated leaf was reduced (28 to 48%) in resistant plants compared to susceptible plants. These results suggest that reduced translocation of glyphosate plays a major role in glyphosate resistance in resistant biotype of horseweed.

Technical Abstract: Greenhouse and laboratory experiments were conducted to investigate mechanisms of glyphosate resistance in horseweed populations from Mississippi, Arkansas, Delaware, and Tennessee. Seeds of suspected resistant (R) and known susceptible (S) biotypes were collected from each state. A non-destructive leaf-dip bioassay was developed to confirm resistance and susceptibility in individual test plants. A single leaf was excised from each plant and the petiole and bottom one-fourth of leaf was dipped in a 0.11 kg ha-1 glyphosate solution for 2 d followed by visually estimating injury on a scale of 0 to 10. Plants were classified as resistant if the score was 2 to 3 and susceptible if the score was 5 to 6. 14C-glyphosate solution was applied on the adaxial surface of a fully expanded leaf of the second whorl of four whorl rosette plants. Plants were harvested 48 h after treatment and radioactivity was determined in treated leaves, other leaves, crown, and roots. Absorption of 14C-glyphosate was similar (47 to 54%) between R and S plants from within and among the four states suggesting absorption is not involved in glyphosate resistance. The amount of radioactivity translocated from the treated leaf was reduced in R plants compared to S plants. The reduction in translocation of 14C-glyphosate ranged from 28% in Mississippi R biotype to 48% in Delaware R biotype compared with their respective S biotypes. Radioactivity was distributed throughout the plant, but less radioactivity was accumulated in other leaves, crown, and roots of R biotype compared to respective S biotype. Epicuticular wax mass ranged from 6 to 80 µg cm-2 among horseweed biotypes, with no differences between R and S biotypes within each state. Treating two leaves with glyphosate solution at the field use rate (0.84 kg ae ha-1) killed S plant but not R plant (38 to 58% control) regardless of state origin. These results suggest that a simple bioassay can be used to screen biotypes for suspected resistance and that reduced translocation of glyphosate plays a major role in glyphosate resistance in R biotypes of horseweed.