Pyrazon (5-amino-4-chloro-2-phenyl-3(2H)-pyridazinone) labelled with tritium in the phenyl ring, was applied to both the root and shoot of seedlings of red beet (Beta vulgaris L., cv. "Detroit Dark Red" ), German millet (Setaria italica L.). and tomato (Lycopersicon esculentum Mill., cv. Heinz 1350). Of these species, red beet is tolerant, millet is moderately tolerant, and tomato is susceptible to pyrazon. When the roots were exposed to 1.325 ppm 3H-pyrazon in the nutrient solution, the greatest root absorption and translocation to the shoot was observed in tomato and the least in red beet, while millet was intermediate. Applications to the first true leaf indicated that pyrazon was transported primarily in the xylem. A radioactive metabolite of 3H-pyrazon was detected in the shoots of red beet, but there was no evidence for its presence in the shoots of the other 2 species. Since this metabolite is radioactive, its structure must contain at least some altered form of the phenyl ring which contained the tritium in the parent molecule. This differentiates it from 5-amino-4-chloro-pyridazinone, the previously proposed degradation product of pyrazon, since this compound lacks the phenyl ring. On the basis of these results, it is hypothesized that pyrazon will be phytotoxic if a sufficient concentration of the unaltered chemical reaches the site of herbicidal action in the shoot.
