Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/20/1999
Publication Date: N/A
Citation: Interpretive Summary: Weed control by the herbicide Staple, has been inconsistent since its commercial introduction in 1996. This variability has been observed under field conditions. The influence of environmental temperature on this variability was investigated. Results from field studies performed over tow growing seasons identified plant and air temperatures at the time of herbicide application that correlated with whole plant efficacy differences. These results were used to create recommendations for Staple application that have the potential to improve the effectiveness of the herbicide under production conditions. The effect of temperature on the plant processes (the synthesis of amino acids) that are responsible for the herbicidal action were investigated in the laboratory using plant extracts. It was found that the plant processes responsible for herbicide effectiveness responded to temperature in a manner that is similar to the herbicide in the field. Such investigations of the temperature dependence of plant processes may provide another means of understanding how temperature in the field affects herbicide effectiveness and provide a new approach for predicting herbicide effectiveness at the whole plant level.
Technical Abstract: Variability in weed control following pyrithiobac applications has been observed under field conditions. The influence of temperature on this variability was investigated. Results from field studies performed over two growing seasons identified plant and air temperatures at the time of herbicide application that correlated with whole plant efficacy differences. To investigate a potential source of thermal limitations on pyrithiobac efficacy, the thermal dependence of in vitro inhibition of acetolactate synthase (ALS), the site-of-action for pyrithiobac, was examined. A crude leaf extract of ALS was obtained from Palmer amaranth. Relative inhibitor potency (I_50) values were obtained at saturating substrate conditions for temperatures from 10 to 50 degrees C. Regression analysis of field activity against I_50 values showed the two data sets to be highly correlated (R^2=0.88). Based on the field activity data, pyrithiobac activity was optimal at application temperatures of 20 to 34 degrees C. The thermal dependence of enzyme/herbicide interactions may provide another means of understanding environmental factors limiting herbicidal efficacy and predicting herbicide inhibition at the whole plant level.