CORN (ZEA-MAYS) ACETOLACTATE SYNTHASE SENSITIVITY TO 4 CLASSES OF ALS-INHIBITING HERBICIDES

In vitro acetolactate synthase (ALS) activity from three commercial im idazolinone-resistant corn hybrids (ICI 8692 IT, Pioneer 3751 IR, and Ciba 4333 IMR) was compared to imidazolinone-sensitive isogenic hybrid controls for sensitivity to 11 herbicides representing four classes o f ALS-inhibiting herbicide chemistry, Acetolactate synthase activity f rom Pioneer IR and Ciba IMR was cross-resistant to all four classes of ALS inhibitors, ranging from 48- to 5,000-fold. The ICI IT hybrid dis played only four-to eightfold resistance to the six imidazolinone herb icides and the pyrimidinylthiobenzoate herbicide, pyrithiobac, but no cross-resistance to the sulfonylurea and triazolopyrimidine sulfonanil ide herbicides. The four-to eightfold enzyme resistance to imidazolino ne herbicides provides whole-plant resistance; however, the sevenfold enzyme resistance to pyrithiobac was insufficient to afford whole-plan t protection to a field application rate of the herbicide. A second im idazolinone-specific resistance allele, XT-12, currently under commerc ial development, was examined for the level of dominance at the enzyme level. In the heterozygous state, imazethapyr resistance was fivefold , compared to 250-fold in the homozygous condition, indicating XI-12 i s a semidominant trait. No cross-resistance to nicosulfuron or primisu lfuron was observed in the heterozygous XI-12 hybrid extracts nor to n icosulfuron in tile XT-12 homozygote; however, a fivefold resistance r o primisulfuron was detected in the XI-12 homozygote.