Wild poinsettia (Euphorbia heterophylla) is an important weed in more than 56 countries, including Brazil, where resistant biotypes to ALS inhibiting herbicides have been reported in soybean crops. The objectives of this research were to evaluate the effect of imazaquin on the activity of the acetolactate synthase (ALS), extracted from two corn varieties and several biotypes of E. heterophylla from Brazilian agricultural areas. Plants were grown under greenhouse conditions for 21 days. The ALS activity of wild poinsettia has shown resistance to imazaquin herbicide in soybean crops. In vitro acetolactate synthase (ALS) activity of imazaquin-resistant wild poinsettia collected in different soybean production areas from Brazil and the two corn populations was compared to that of imazaquin-sensitive populations. The activity of the ALS extracted from the leaves was measured with different imazaquin rates. The ALS purification process was substituted by centrifugation at 2800 rpm during two minutes. Linear regression equation for specific light length absorbance as a function of the log concentration of imazaquin was built to obtain I50 value (50% inhibition of the enzyme activity). The I50 values of Pioneer 3162 IR (I50 260 µM) corn genotype were 4,333 times higher than BRS 473-sensitive (0,06 µM). The wild poinsettia biotypes from the state of Rio Grande do Sul had I50 values of 1,961.3 µM and 13.8 µM for the resistant and sensitive biotypes, respectively. The resistant populations from Cafelândia, Maringá (PR) and Viçosa (MG) had I50 values higher than 5,000 µM, while the sensitive population had values higher than 1,000 µM. The I50 value for the wild poinsettia population collected from fields that had never been sprayed with imazaquin was of 12.2 µM. Therefore, the in vitro ALS activity was a sensitive method to detect wild poinsettia-resistant imazaquin and ALS purification can be substituted for a method involving a quick centrifugation process.
herbicide resistance; ALS; soybean; corn
