SUSCEPTIBILITIES OF DIFFERENT TEST SYSTEMS FROM MAIZE (ZEA-MAYS), POAANNUA, AND FESTUCA-RUBRA TO HERBICIDES THAT INHIBIT THE ENZYME ACETYL-COENZYME A CARBOXYLASE

The susceptibilities of maize (Zea mays cv. Champ) and two graminicide -resistant grass species, Poa annua (annual meadow grass) and Festuca rubra (red fescue), to two aryloxyphenoxypropionates (quizalofop and f luazifop) and a cyclohexanedione (sethoxydim) graminicide were evaluat ed in leaf blades and isolated chloroplasts, and by assaying acetyl-co enzyme A carboxylase (ACCase) in desalted leaf homogenates. The gramin icide resistance of P. annua and F. rubra appeared to be at the level of ACCase. Festuca rubra ACCase was highly insensitive and P. annua AC Case was partially insensitive to the graminicides that were tested. F atty acid synthesis in isolated maize chloroplasts was more susceptibl e to inhibition than was ACCase activity from whole leaves. There was a smaller difference in graminicide sensitivity between these two test systems in P. annua. The developmental pattern of ACCase specific act ivity and its inhibition by quizalofop was measured in maize and P. an nua leaf blades. There was an age-dependent increase in the sensitivit y of maize leaf ACCase activity to inhibition by quizalofop. Together with the greater susceptibility of chloroplasts compared with leaf hom ogenates this could imply that a graminicide-insensitive (extrachlorop lastic) ACCase isoform is less highly expressed in older leaves. Poa a nnua ACCase did not significantly alter in sensitivity as leaves aged, consistent with the smaller difference in the level of inhibition bet ween chloroplasts and leaf homogenates in this species. A small pyruva te carboxylase activity was detected in maize leaves after 9 days, By 38 days, when leaves were senescing, pyruvate carboxylase activity pre dominated over ACCase. (C) 1996 Academic Press, Inc.