Published online by Cambridge University Press: 12 June 2017
Inhibition of net photosynthesis of jointed goatgrass and downy brome protoplasts by metribuzin and its ethylthio analog (ethyl-metribuzin) was greater at 25 than at 10 C. As temperature increased from 10 to 25 C, the concentration of ethyl-metribuzin required to inhibit net photosynthesis 50% (I50) decreased by a factor of 3.5 and 4.3, respectively, in jointed goatgrass and downy brome. I50 values for metribuzin decreased by a factor of 1.5 and 2.5 in jointed goatgrass and downy brome, respectively, for the same 15 C increase in temperature. Based on I50 values at 10 C, metribuzin was nine times more inhibitory than ethyl-metribuzin in jointed goatgrass and eight times more inhibitory in downy brome. At 25 C, metribuzin was only 4.7 and 3.9 times more inhibitory than ethyl-metribuzin in jointed goatgrass and downy brome, respectively. Thus, cold temperatures reduced the activity of ethyl-metribuzin more than metribuzin. The activity of both herbicides was reduced less in protoplasts of jointed goatgrass than in protoplasts of downy brome over the 15 C range.