Two Alisma plantago-aquatica biotypes resistant to bensulfuron-methyl were detected in rice paddy fields in Portugal's Mondego (biotype T) and Tagus and Sorraia (biotype Q) River valleys. The fields had been treated with bensulfuron-methyl-based herbicide mixtures for 4-6 years. In order to characterize the resistant (R) biotypes, dose-response experiments, absorption and translocation assays, metabolism studies and acetolactate synthase (ALS) activity assays were performed. There were marked differences between R and susceptible (S) biotypes, with a resistance index (ED50R/S) of 500 and 6.25 for biotypes Q and T respectively. Cross-resistance to azimsulfuron, cinosulfuron and ethoxysulfuron, but not to metsulfuron-methyl, imazethapyr, bentazone, propanil and MCPA was demonstrated. No differences in the absorption and translocation of C-14-bensulfuron-methyl were found between the biotypes studied. Maximum absorption attained 1.12, 2.02 and 2.56 nmol g(-1) dry weight after 96 h incubation with herbicide, for S, Q and T biotypes respectively. Most of the radioactivity taken up by the roots was translocated to shoots. Bensulfuron-methyl metabolism in shoots was similar in all biotypes. The R biotypes displayed a higher level of ALS activity than the S biotype, both in the presence and absence of herbicide and the resistance indices (IC50R/S) were 20 197 and 10 for biotypes Q and T respectively. These data confirm for the first time that resistance to bensulfuron-methyl in A. plantago-aquatica is target-site-based. In practice, to control target site R biotypes, it would be preferable to use mixtures of ALS inhibitors with herbicides with other modes of action.
