TERBACIL AND BROMACIL CROSS-RESISTANCE IN POWELL AMARANTH (AMARANTHUS-POWELLII)

A triazine-resistant Powell amaranth biotype collected in Idaho was approximately six times more resistant to terbacil and sixteen times more resistant to bromacil than a normal susceptible biotype when planted into terbacil- or bromacil-treated soil. The concentration of terbacil required to reduce photosystem II activity by 50% (150) in isolated thylakoids was 0.24 and 13.33 muM for the susceptible and resistant biotypes, respectively. Likewise, the 150 values for bromacil were 0.33 and 18.4 muM for the susceptible and resistant biotypes, respectively. More C-14-terbacil was bound to isolated thylakoids of the susceptible than the resistant biotype with binding constants (Kb) of 0.26 and 12.9 muM, respectively, indicating that resistance was at the chloroplast level.