Photosynthesis in corn (Zea mays L. ‘Pioneer 3369A’) was inhibited 25% by atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] after a 2-h root exposure in nutrient solution. Photosynthesis of the treated plants had completely recovered 21.4 h after removal from the treatment solution. Recovery of photosynthesis from second and third successive atrazine treatments required only 10.3 h and 4.0 h, respectively. Atrazine metabolism rates in corn plants after 1, 2, or 3 successive 4-h treatment periods, where each treatment period was followed by a 12-h recovery period, showed increased rates of metabolism of atrazine during each recovery time. An increased level of glutathione-S-atrazine was found following the first 4-h atrazine treatment and 12-h recovery. The enhanced production of GS-atrazine was maintained throughout subsequent exposures and recoveries. An inverse correlation (r2 = 0.992) was found between the increase in GS-atrazine production following each 4-h atrazine exposure and the time required for corn plants to recover from atrazine-induced photosynthesis inhibition. Enhanced detoxification of atrazine by corn plants pretreated with atrazine was indicated. No enhanced tolerance to other herbicides was observed from atrazine pretreatment. This phenomenon is similar to that described for the enhanced detoxification of several insecticides in animal systems.

Reaction of some wild oat (Avena fatua L. ♯4 AVEFA) populations to the herbicide triallate [S-(2,3,3-trichloroallyl)diisopropylthiocarbamate] was investigated in a controlled-environment chamber. Under specified conditions, 1.0 mg active ingredient of triallate incorporated in 1 kg of soil was a satisfactory diagnostic rate for 14-day-old seedlings. Five seedling growth parameters were studied. These parameters were emergence, survival to produce first leaf, mesocotyl length, first-leaf length, and seedling height. Of these, mesocotyl length was least affected by triallate treatment, and significant differences in variability for triallate reaction were found within and among historically triallate unexposed, as well as exposed populations. Within-population variation for seedling emergence and survival was higher in unexposed than in exposed populations. Populations with recurrent exposure to triallate were more tolerant to the herbicide under experimental conditions than historically unexposed populations. Triallate-tolerant lines were identified which were tolerant to two other common wild oat herbicides.