Uptake, Translocation, and Metabolism in Alfalfa (Medicago sativa) Selected for Enhanced Tolerance to Terbacil

Terbacil tolerance was investigated in field-selected alfalfa strain ‘OK 182.’ Growth response studies indicated a 78% increase in tolerance over the unselected parental cultivar ‘Cimarron.’ Photosynthetic electron transport was inhibited similarly in both cultivars when isolated thylakoids were exposed to a wide range of terbacil concentration indicating that tolerance was not due to reduced sensitivity at the site-of-action. ‘OK 182’ took up an average of 22% less 14C-terbacil over the 6-d labeling period. A similar proportion of total radiolabel was translocated in both strains indicating that differential translocation was not a factor in the tolerance mechanism. Once in the leaves, terbacil was rapidly metabolized to a glucosidic conjugate by both strains. Terbacil and total concentration of metabolites in leaf tissues were 17% and 33% lower in ‘OK 182’ than in ‘Cimarron,’ respectively. Enhanced tolerance to terbacil in ‘OK 182’ was attributed to decreased terbacil uptake.