Mechanism of herbicidal activity of a new cyclohexane-1,3-dione, tepraloxydim to Poa annua L.

Tepraloxydim [(EZ)‐(RS)‐2‐{1‐[(2E)‐3‐chloroallyloxyimino]propyl}‐3‐hydroxy‐5‐perhydropyran‐4‐ylcyclohex‐2‐en‐1‐one] showed high activity against annual bluegrass (Poa annua L.), which is relatively tolerant to sethoxydim [(±)‐2‐(1‐ethoxyiminobutyl)‐5‐[2‐(ethylthio)propyl]‐3‐hydroxycyclohex‐2‐en‐1‐one]. Absorption and translocation rates of tepraloxydim and sethoxydim were higher in P. annua than in Setaria faberi, but the absorption and translocation patterns of tepraloxydim in the two plants were similar to those of sethoxydim. Metabolic rates of tepraloxydim and sethoxydim in P. annua and S. faberi were found to be similar. The concentration for 50% inhibition (I₅₀) of acetyl‐coenzyme A carboxylase (ACCase) with tepraloxydim was approximately 3 × 10^?6 mol L^?1 for P. annua and 7 × 10^?7 mol L^?1 for S. faberi. For sethoxydim, the I₅₀ was found to be 2 × 10^?6 mol L^?1 with the enzyme of S. faberi, while sethoxydim showed a slight effect on ACCase from P. annua activity, even at 10^?4 mol L^?1. The strong inhibition of ACCase with tepraloxydim is considered to be the major factor contributing to the high herbicidal activity against P. annua. Measuring the whole plant growth response, the ratio of the tepraloxydim I₅₀ dose of P. annua to that of S. faberi (P/S) was found to be 2.4, while the P/S ratio of sethoxydim and a tepraloxydim analog with a propyl chain at R₂ were 56.3 and 73.3, respectively. The herbicidal activity against P. annua was remarkably influenced by the length of the R₂ alkyl chain, while the effect on S. faberi was not affected. Acetyl‐coenzyme A carboxylase from P. annua also exhibited a higher resistance to the tepraloxydim analog with a propyl chain than to tepraloxydim. These results suggest that a binding site structure of cyclohexane‐1,3‐diones in the ACCase differs between P. annua and S. faberi.