Root absorption of 14C-5 (ring)-metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one] from hydroponic culture and its subsequent translocation in tolerant (‘Russet Burbank’) and susceptible (‘Chipbelle’) potato (Solanum tuberosum L.) cultivars were measured. Differences in root absorption did not appear to be as important to differential tolerance as did translocation differences. Total available metribuzin absorbed was 6 and 7%, 18 and 29%, and 31 and 45% after 1, 4, and 8 days for Russet Burbank and Chipbelle, respectively. Radioactivity was concentrated in stems, petioles, and leaf veins in the tolerant cultivar, whereas interveinal leaf tissue was the major accumulation site for 14C in the susceptible cultivar. Of the total radioactivity in the plants after 8 days, 82 and 87% were in the foliage for the tolerant and the susceptible cultivar, respectively. Translocation was apoplastic. Metribuzin tolerance by Russet Burbank is due in part to restricted translocation to the leaf blades. A model for differential transport between cultivars is proposed.

Root and foliar absorption and translocation of 14C-metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one] were determined in tolerant (‘Steptoe’) and susceptible (‘Morex’) barley (Hordeum vulgare L.) cultivars grown in nutrient solution culture under greenhouse conditions. Root-applied metribuzin toxicity to the two barley cultivars was also examined. A 50% reduction in growth occurred at 0.22 and 0.72 μM metribuzin for Morex and Steptoe, respectively. Root absorption was similar for both cultivars at 1 day, but Steptoe absorbed about two times more metribuzin 4 and 8 days after application than did Morex. Steptoe absorbed 19% of the total applied metribuzin by 8 days. Root absorption was positively correlated with water uptake (r≥ 0.87). Leaf absorption was three times greater for Morex than for Steptoe after 4 days. Transport to leaves from root application was rapid, and, by 1 and 8 days, leaves of Steptoe contained 71 and 82% and leaves of Morex 78 and 84% of the total absorbed 14C, respectively. Translocation was apoplastic following both root and leaf absorption and was similar for both cultivars. Differential tolerance could partially be accounted for by differences in foliar absorption but not by differences in root absorption.

Twenty-six cultivars of tomatoes from several countries were divided into four metribuzin [4-amino-6-tert-butyl-3-(methylthio)-1,2,4-triazin-5(4H)-one] tolerance groups: tolerant, semi-tolerant, semi-susceptible, and susceptible. Early maturity and tolerance to metribuzin were correlated (r = −0.78) in the field experiment and (r = −0.72) in a sinking leaf-disc bioassay. New super early cultivars were especially sensitive to metribuzin. Chlorophyll a and b content were not highly correlated with tolerance to metribuzin. Metribuzin rates used for weed control are close to toxic concentrations for very early cultivars of tomatoes. Plants in the 9-leaf stage were less sensitive than plants in the 5- or 7-leaf stage. Metribuzin tolerance came at later stages in early maturing cultivars than in later maturing cultivars.

Whole-plant, negative cross-resistance was studied in Conyza canadensis and Echinochloa crus-galli, important global weeds. Negative cross-resistance can be a most useful preemptive, cost-effective tool for delaying the evolution of resistance, as well as for resistance management, after resistant populations evolve. Seeds of triazine-resistant and -susceptible biotypes were collected in or near orchards that had been continuously treated with atrazine for more than 10 yr. Plants grown from the seeds were treated, in a greenhouse, with herbicides from the following chemical families: triazine, benzothiadiazole, phenyl-pyridazine, arylophenoxy-propionate, cyclohexanedione, phenoxycarboxylic acid, pyridine carboxylic acid, phosphinic acid, glycine phosphate, chloroacetamide, sulfonylurea, and bipyridylium. Eleven of the 18 herbicides tested exerted significant negative cross-resistance against atrazine-resistant weeds, ranging from 0.03 to 0.67 of the concentration required to affect the triazine-sensitive type. No synergism was found between bentazon and fluroxypyr in mixture on Conyza, even though both separately exerted negative cross-resistance. Using a mixture with half the amount of each component lowers the environmental effect of each component while controlling a broader spectrum of other weeds.