The downward mobility

The relationship between soil moisture and concentration of soil-applied thiobencarb [S-(4-chlorobenzyl) diethyl thiocarbamate] in soil solution was investigated. When thiobencarb was applied to soil (low-humic andosols) at the same dosages based on air-dried weight, its concentration in soil solution was similar at any soil moisture level tested (45-75%, moisture weight percentage). Thus, the total amount of thiobencarb in soil solution increased as soil moisture increased regardless of the dosage applied. It was found that the partition coefficient of thiobencarb was inversely proportional to the application dosage at a given soil moisture condition in the soil. Bioassay with soil solution which was extracted by centrifugation with a double tube under various soil moisture conditions (45-75%) showed little difference in shoot growth of Echinochloa crus-galli (L.) Beauv. var. praticola Ohwi and Digitaria ciliaris Koeler. The rate of growth inhibition was almost identical with that of the freshly prepared thiobencarb solution. This supports that the concentration in soil solution is closely related to the herbicidal activity of soil-applied thiobencarb, although most of the thiobencarb was contained in the soil phase. Therefore, it was concluded that thiobencarb concentration in soil solution was almost constant under different soil moisture conditions.

In growth response to pyrazosulf uron-ethyl [ethyl 5-[[3-(4, 6-dimethoxypyrimidin-2-yl) ureido] sulf onyl]-1 H-1-methylpyrazole-4-carboxylate], indica rice cultivars were more tolerant than japonica cultivars in a manner similar to bensulfuronmethyl [methyl a-[[3-(4, 6-dimethoxypyrimidin-2-yl) ureido] sulf onyl]-o-toluate]. The growth inhibition by pyrazosulf uronethyl and bensulfuron-metyl was reduced by dimepiperate[S-(a, a-dimethylbenzyl) piperidine-l-carbothioate] in indica and japonica rice cultivars but not in Cyperus serotinus. The mechanism of differential tolerance of rice cultivars to pyrazosulfuron-ethyl was discussed in relation to the safening effect of dimepiperate on the growth inhibition by the herbicides.

Effects of pyrazosulfuron-ethyl [ethyl 5-[[3-(4, 6-dimethoxypyrimidin-2-yl) ureido] sulf onyl]-1H-1-methylpyrazole-4-carboxylate] on the growth and activity of acetolactate synthase in rice (Oryza sativa) and C. serotinus plant were investigated. The growth of root and shoot was suppressed more remarkably in C. serotinus than in rice. In vitro, the enzyme activity from each organ of both plants was highly and similarly sensitive to pyrazosulfuron-ethyl; however, in vivo the activity was reduced more remarkably in C. serotinus organs after treatment with pyrazosulf uron-ethyl. Only in rice shoot was the reduction of enzyme activity in vivo remarkably recovered as a function of time after treatment. It is presumed that the primary site of action of pyrazosulf uron-ethyl is acetolactate synthase, and that its selectivity depends on the difference in degree of inhibition on acetolactate synthase activity in vivo as a function of time.

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