F. Bravin et al., Resistance to diclofop-methyl in two Lolium spp. populations from Italy: studies on the mechanism of resistance, WEED RES, 41(5), 2001, pp. 461-473
The mechanisms of herbicide resistance were investigated in two diclofop-me
thyl-resistant Lolium spp. populations from central Italy, Roma '94 and Tus
cania '97. These two populations were compared with two susceptible Italian
populations (Vetralla '94, Tarquinia '97) and a resistant and a susceptibl
e population from Australia, SLR31 and VLR1. The activity of acetyl Co-A ca
rboxylase (ACCase) extracted from susceptible (S) or resistant (R) individu
als from the Italian populations was inhibited by both aryloxyphenoxypropan
oate (diclofop acid and fluazifop acid) and cyclohexanedione (sethoxydim) h
erbicides. Diclofop-methyl was rapidly de-esterified to diclofop acid at a
similar rate in both R and S populations. In all populations, diclofop acid
was subsequently degraded to other metabolites. The rate of degradation of
diclofop acid was not significantly faster in R than in S populations; how
ever, diclofop acid was degraded more completely in Roma '94 and Tuscania '
97 compared with the S populations. Application of the mixed-function oxida
se inhibitor 1-aminobenzotriazole (ABT) significantly enhanced diclofop-met
hyl toxicity towards both R populations, but not in S populations. However,
enhanced herbicide metabolism does not completely account for the measured
resistance level. A mechanism other than an altered ACCase and enhanced he
rbicide metabolism appears to be responsible for resistance to diclofop-met
hyl in Roma '94 and Tuscania '97.