Effect of Flurazole and Other Safeners for Chloroacetanilide Herbicides on Cysteine Synthase in Sorghum Shoots

The effects of selected herbicide safeners on shoot growth inhibition and on cysteine synthase [CS; EC 4.2.99.8] activity of sorghum were examined. Seed treatment (1.25 g/kg seed) of flurazole [phenylmethyl 2-chloro-4-(trifluoromethyl)-5-thiazolecarboxylate] protected seedlings from growth inhibition by alachlor [2-chloro-N-(2,6-diethylphenyl)-N(methoxymethyl)acetamide]. Flurazole seed treatment (1.25 g/kg) increased the specific activity (mumol/min/mg protein) and total activity (mumol/min/g fresh weight) of CS from 1.13to 1.41-fold and from 1.40- to 1.75-fold at 48 and 72 h after planting, respectively. CS activity at 48 h after planting increased as the dosage of flurazole increased from 0.01 to 2.5 g/kg seed. Protection from growth inhibition by alachlor (100 muM) also increased as the flurazole dosage increased from 0.01 to 0.625 g/kg seed. Flurazole had little inhibitory effect on CS activity between 3 and 30 muM in the in vitro enzyme assay, suggesting that increased CS activity by this safener was not due to the activation of constitutive CS. Protection from growth inhibition by alachlor was also observed for the seed treatments of fluxofenim [O-(1,3dioxolan-2-yl)-2,2,2-trifluoro-4ʹ-chloroacetophenone-oxime; 0.1-0.4 g/kg seed], naphthalic anhydride [1H, 3H-naphtho(1,8-cd)-pyran-1,3-dione; 0.625-2.5 g/kg seed], benoxacor [4-(dichloroacetyl)-3,4-dihydro-3-methyl-2H-1,4-benzoxazine; 0.625 -2.5 g/kg seed], and dichlormid [2,2-dichloro-N-Nʹ-di-2-propenylacetamide; 0.625 -2.5 g/kg seed]. The safening efficacy of dichlormid was the lowest for the compounds tested. Fluxofenim (0.4 g/kg seed), naphthalic anhydride (2.5 g/kg seed), benoxacor (2.5 g/kg seed), and dichlormid (2.5 g/kg seed) increased CS activity (mumol/min/g fresh weight) by 36, 38, 61, and 22%, respectively at 48 h after planting. However, the safening efficacy by these compounds and the increase in CS activity were not clearly correlated. These results suggest that safener treatments increase the extractable CS activity in sorghum shoots.