Submitted to: American Society of Plant Physiologists Meeting
Publication Type: Abstract Only
Publication Acceptance Date: June 27, 1998
Publication Date: N/A
Technical Abstract: An effective herbicide should function over a range of temperatures. Thermal dependence can be determined empirically and may require several growing seasons for accurate prediction. Pyrithiobac is a postemergence topical herbicide which is used to control broadleaf weeds. Its mechanism of action is inhibition of acetolactate synthase, an obligatory enzyme in the synthesis of branched chain amino acids. Under high temperatures the efficacy of pyrithiobac is reduced. The goal of this study was to determine the extent to which the thermal dependence of field efficacy is related to the thermal dependence of the inhibition kinetics of acetolactate synthase/pyrithiobac interactions. The thermal dependence of the inhibition constant, Ki, for the enzyme-herbicide reaction was determined over a 10 to 50 deg C range. The results indicate that the Ki is a function of temperature with a 20-fold variation. The Ki was minimal, indicating "optimal inhibition", between 20 and 33 deg C and increased rapidly at temperatures above and below that range. In a separate study of field efficacy, it was demonstrated that the thermal dependence of efficacy was highly correlated with that of the Ki. These results suggest that the thermal dependence of enzyme inhibition by a herbicide may provide a means of predicting the weed control under production environments. The enzyme/herbicide kinetic data was used with plant temperature data sets collected over several growing seasons to make an assessment of the probability, duration, and frequency of temperatures that limit pyrithiobac efficacy. Computer visualization of seasonal patterns were developed to provide direction for mathematical quantification and description of efficacy.