Submitted to: Weed Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/23/2007
Publication Date: 5/19/2008
Citation: Nandula, V.K., Reddy, K.N., Poston, D.H., Rimando, A.M., Duke, S.O. 2008. Glyphosate-tolerance mechanism in Italian ryegrass (Lolium multiflorum) from Mississippi. Weed Science 56:344-349. Interpretive Summary: Italian ryegrass is becoming a troublesome weed, especially as recently several populations with increased resistance to the herbicide glyphosate have been found in Mississippi. Scientists from the Mississippi State Delta Research Center and the Agricultural Research Service Southern Weed Science Research Unit and Natural Products Research Unit have studied why two populations of Italian ryegrass T1 and T2 are resistant to this herbicide. The herbicide is not metabolized in these populations, however the herbicide remains where applied as does not move in the plant. These studies indicate that the reduced uptake and movement of the herbicide is why these populations are tolerant to glyphosate.
Technical Abstract: A 3-fold glyphosate tolerance was identified in two Italian ryegrass populations, T1 and T2, from Mississippi. Laboratory experiments were conducted to characterize the mechanism of glyphosate tolerance in these populations. The T1 population absorbed less 14C-glyphosate (43% of applied) compared to the susceptible (S) population (59% of applied) at 48 h after treatment (HAT). The T2 population absorbed 14C-glyphosate at levels (56% of applied at 48 HAT) that were similar to both T1 and S populations, but tended to be more comparable to the S population. The amount of 14C-glyphosate that remained in the treated leaf was significantly higher in both T1 (67% of absorbed) and T2 (65% of absorbed) populations compared to the S population (45% of absorbed) at 48 HAT. Amount of 14C-glyphosate that moved out of treated leaf to shoot and root was lower in both T1 (25% of absorbed in shoot and 9% of absorbed in root) and T2 (25% of absorbed in shoot and 11% of absorbed in root) populations compared to the S population (40% of absorbed in shoot and 16% of absorbed in root) at 48 HAT. There were no differences in epicuticular wax mass among the three populations. Treating a single leaf with glyphosate solution at the field use rate (0.84 kg ae ha-1) as ten 1-'l droplets killed the S plant but not the T1 and T2 plants (33 and 55% shoot fresh weight reduction, respectively). Shikimic acid accumulated rapidly at higher levels in glyphosate-treated leaf segments of the S population compared to the T1 population up to 100 'M glyphosate. However, above 500 'M glyphosate, the levels of shikimate were similar in both the S and T1 populations. Furthermore, shikimic acid content was three to six-fold more in whole plants treated with 0.22 kg ae ha-1 glyphosate of the S population compared to the T1 and T2 populations. No degradation of glyphosate to aminomethylphosphonic acid was detected among the tolerant and susceptible populations. These results indicate that tolerance to glyphosate in the T1 population is partly due to reduced absorption and translocation of glyphosate and in the T2 population is partly due to reduced translocation of glyphosate.