C. Preston et Sb. Powles, Amitrole inhibits diclofop metabolism and synergises diclofop-methyl in a diclofop-methyl-resistant biotype of Lolium rigidum, PEST BIOCH, 62(3), 1998, pp. 179-189
Biotype SLR 31 of Lolium rigidum has developed resistance and cross-resista
nce to a wide range of herbicides. Herbicide resistance in SLR 31 has been
demonstrated to be the result of multiple mechanisms of resistance. Despite
much speculation, the mechanisms endowing resistance to the acetyl-coenzym
e A carboxylase (ACCase)-inhibiting herbicide diclofop-methyl have not been
fully elucidated. Two subsets of this population were isolated based on ro
ot length of individuals germinated on agar containing 3 mu M of the ACCase
-inhibiting herbicide fluazifop-P-butyl. Those individuals with normal root
length were highly resistant to all ACCase-inhibiting herbicides and conta
ined a herbicide-resistant target enzyme, ACCase. Those individuals with hi
ghly stunted roots were resistant to diclofop-methyl and fluazifop-P-butyl,
but not to sethoxydim, and contained a herbicide-sensitive ACCase. Biotype
SLR 31 also has enhanced metabolism of diclofop acid. Amitrole, a non-ACCa
se-inhibiting herbicide, was demonstrated to be an inhibitor of diclofop ac
id metabolism in both SLR 31 and the susceptible biotype and synergised the
effect of diclofop-methyl an both biotypes. This demonstrates that enhance
d diclofop acid metabolism can confer substantial resistance to diclofop-me
thyl in L. rigidum. In contrast amitrole was unable to synergise the effect
of chlorsulfuron, an acetolactate synthase-inhibiting herbicide, to which
SLR 31 is resistant also due to enhanced herbicide metabolism. Amitrole als
o did not inhibit chlorsulfuron metabolism in SLR 31. Resistance to diclofo
p-methyl in biotype SLR 31 is due to at least two mechanisms: enhanced meta
bolism, which provides 26-fold resistance, and a less sensitive target site
, which provides 9-fold resistance. When the two mechanisms are combined, i
ndividuals are 239-fold resistant compared to the susceptible population. (
C) 1998 academic Press.