Metribuzin metabolism as the basis for tolerance in barley (Hordeum vulgare L.)*

Metabolism of ¹⁴C-5 (ring)-metribuzin was studied in Steptoe (tolerant) and Morex (susceptible) barley (Hordeum vulgare L.) cultivars, 1, 4, and 8 days following a single application to roots. Both cultivars contained similar ether-soluble metribuzin metabolites and five water-soluble metabolites. Water-soluble compounds increased from 12 to 53% of the total ¹⁴C recovered for Steptoe and 5–17% for Morex between 1 day and 8 days, respectively, whereas the percentage of ether-soluble metabolites decreased. Ninhydrin reacting compounds were the major water-soluble metabolites in the leaf blades 8 days after treatment. On a d.p.m. mg^?1 dry weight basis, Steptoe leaves had five times more water-soluble material than Morex leaves and half the quantity of ether-soluble compounds. Metribuzin comprised 83 and 89% of the ether-soluble compounds in leaves of Morex and Steptoe, respectively, at 8 days. Terminal radioactivity comprised between 19% and 26% of total radioactivity for both cultivars as early as 1 day after application, with little change over 8 days. Rapid metabolism of metribuzin to non-phytotoxic water-soluble conjugates and terminal residues was the major mechanism responsible for the differential tolerance between these two barley cultivars.