Resistance to glyphosate in Lolium rigidum.: II.: Uptake, translocation, and metabolism

Experiments were conducted to determine potential mechanisms of glyphosate resistance in Lolium rigidum from Australia. C-14-Glyphosate uptake, translocation, and metabolism were compared between resistant (R) and sensitive (S) biotypes. The R seed (48118a) represented the F-1 progenies of plants having survived a 1.73-kg ae ha(-1) (4.8 L ha(-1)) application of a Roundup(R) formulation. The S seed was a sensitive biotype of L. rigidum from Australia. Plants (one to four tillers, 2 to 4 wk old) were presprayed with a high (1.26 kg ae ha(-1)) or a low (0.28 kg ae ha(-1)) dose of formulated glyphosate. The first leaf of the first tiller, which was shielded from the spray, was immediately treated with a C-14-glyphosate solution via manual application to the adaxial surface. Harvest was made 6 days after treatment (DAT), and glyphosate residues in the leaf wash, treated leaf roots, and sheets were quantified based on radioactivity as percentage of applied dose. The overall radioactivity recoveries were very good (90.2 to 97.3%, of applied dose). R and S plants showed comparable uptake at the high (79.2 vs. 78.0%) or the low (64.0 vs. 64.7%) doses of glyphosate. About one-half of the absorbed glyphosate in both R and S (32.9 to 38.3% appl.) was translocated into the plant and distributed almost equally into roots (13.6 to 16.0%, appl.) and shoots (18.1 to 22.6% appl.). Autoradiography studies demonstrated no difference in tissue localization of glyphosate between the R and S plants. For metabolism studies, tissues from individual plants were homogenized in water, and extracts were analyzed by anion exchange high-pressure liquid chromatography (HPLC) with radioactivity detection. There was little to no metabolism of glyphosate in extracts from various tissues of either R or S plants Based on these results, we conclude that neither uptake, translocation, nor metabolism play a major role in glyphosate resistance in L. rigidum.