The fate of [3-14C]metamitron in sugar beets after preemergence application in a lysimeter study

In a lysimeter experiment, 3-¹⁴C]metamitron was sprayed in a preemergence treatment of sugar beets, corresponding to approx 4.9 kg metamitron (7 kg Goltix)/ha. After 6 months, the beets contained metamitron equivalents amounting to 0.1 mg/kg fresh wt, calculated on the basis of the specific radioactivity of the 3-¹⁴C]metamitron employed. Radioactivity was also detected in the pure sugar isolates. The ¹⁴C activity represented approx 0.2 mg metamitron equivalent/kg pure sugar. Since the specific radioactivities of the sugar fractions were too low to employ physicochemical methods, a microbial degradation was used to investigate whether the radiocarbon was incorporated in the sucrose molecule. Microorganisms (Proteus vulgaris) degraded U-¹⁴C] sucrose and the sugar isolates at the same ¹⁴CO₂ release rates under strictly controlled experimental conditions. This result indicates that about one fourth of the carbon from the C-3 position of the triazine ring of the metamitron, found in the sugar beets at harvest time, is partly being used as a substrate in the production of sucrose possibly via assimilation of mineralized ¹⁴CO₂.