An accession of smooth crabgrass [Digitaria ischaemum (Schreb.) Muhl] exhib
iting resistance to the herbicide fenoxaprop was discovered recently in New
Jersey. This accession was highly resistant to fenoxaprop-ethyl (approxima
te R/S GR(50) ratio of 102) and moderately resistant to quizalofop-ethyl (a
pproximate R/S GR(50) ratio of 16.3), but exhibited low resistance to cyclo
hexanediones such as sethoxydim (R/S GR(50) ratio of 1.3). The potential me
chanism of resistance was investigated by evaluating the effect of fenoxapr
op on acetyl-CoA carboxylase (EC 6.4.1.2, ACCase) activity extracted from s
hoots of resistant and susceptible plants and comparing the absorption, tra
nslocation, and metabolism of radiolabeled fenoxaprop in resistant and susc
eptible plants. The patterns of absorption, translocation, and metabolism o
f fenoxaprop were similar in resistant and susceptible smooth crabgrass. AC
Case activity from susceptible plants was very sensitive to fenoxaprop-ethy
l and fenoxaprop acid with I-50 values of 2 and 4.9 mu M, respectively. ACC
ase activity from resistant plants was very resistant to fenoxaprop-ethyl (
I-50 > 182 mu M) and moderately resistant to fenoxaprop acid (I-50, 29 mu M
). ACCase activity from resistant smooth crabgrass was 50-fold less sensiti
ve to quizalofop-ethyl than that extracted from susceptible smooth crabgras
s. ACCase activity extracted from either resistant or susceptible plants wa
s inhibited strongly by sethoxydim (I-50 of 7.4 mu M in R and 3.4 mu M in S
). These results suggest that a less sensitive form of the target enzyme, a
cetyl-CoA carboxylase, confers a high degree of resistance to smooth crabgr
ass toward fenoxaprop and moderate resistance to other aryloxyphenoxypropio
nate herbicides. (C)1999 Academic Press.