Relationship between temperature and triazinone herbicide activity: I. Herbicide binding to thylakoid membranes

Abstract

The binding rate of metribuzin, 4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4 H)-one, and its ethylthio analog (ethyl-metribuzin), 4-amino-6-(1,1-dimethylethyl)-3-(ethylthio)-1,2,4-triazin-5(4 H)-one to thylakoid membranes of downy brome ( Bromus tectorum L.), jointed goatgrass ( Aegilops cylindrica Host), and winter wheat ( Triticum aestivum L.) increased as temperature increased from 10 to 30°C. However, total herbicide bound was approximately 20% less at 30°C than at 10 or 20°C. At concentrations ≤0.316 μ M, more metribuzin than ethyl-metribuzin was bound to thylakoid membranes of all three species at 10 to 30°C. Binding constants ( K b ) for ethyl-metribuzin were 1.8 to 2.1 times greater than those for metribuzin for all three species. The reduced binding affinity of ethyl-metribuzin, compared with metribuzin, partially explains its reduced activity at the protoplast and whole plant level. In competitive binding studies, atrazine, 6-chloro- N-ethyl- N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine, metribuzin, and ethyl-metribuzin all competed for a common binding site.