A BIOTYPE OF COMMON WATERHEMP (AMARANTHUS-RUDIS) RESISTANT TO TRIAZINE AND ALS HERBICIDES

A common waterhemp biotype that was not controlled by triazine or acet olactate synthase (ALS)-inhibiting herbicides was isolated from a fiel d in Bond County, IL, in the fall of 1996. Greenhouse and laboratory e xperiments determined resistance to atrazine and three ALS-inhibiting herbicides in this biotype. Based on whole-plant response, the Bond Co unty common waterhemp biotype required over 1,000 times more imazethap yr relative to a susceptible biotype to reduce growth 50%. Cross-resis tance to thifensulfuron, a sulfonylurea, and flumetsulam, a triazolopy rimidine sulfonanilide, was also detected. Based on in vivo enzyme ass ays, ALS in the Bond County common waterhemp biotype was 20-, > 8-, an d 68-fold less sensitive than ALS in the susceptible biotype to imazet hapyr, thifensulfuron, and flumetsulam, respectively. Whole-plant effi cacy trials also indicated that the Bond County common waterhemp bioty pe required more than 20 kg ha(-1) of atrazine to inhibit growth 50%. Chlorophyll fluorescence assays revealed that 100 nM atrazine inhibite d photosynthesis in the susceptible biotype, whereas 10 M did not affe ct photosynthesis in the resistant biotype. Regions of the genes encod ing ALS and D1 proteins were sequenced to determine the molecular basi s for the resistances. Triazine resistance was conferred by a glycine for serine substitution at residue 264 of the D1 protein, while ALS re sistance was conferred by a leucine for tryptophan substitution at res idue 569 of ALS.