The influence of 1, 2-alkylene-l, 2, 4-triazolidines and 3, 4-alkylene-1, 3, 4-thiadiazolidines on peroxidizing phytotoxicities was investigated using corn etioplast protoporphyrinogen-IX oxidase (Protox), autotrophic Scenedesmus acutus cells and Echinochloa utilis. Both types of compounds inhibited Protox, caused ethane formation, decreased chlorophyll content and inhibited growth of Scenedesmus and root-growth of Echinochloa, like peroxidizing herbicides such as p-nitrodiphenyl ethers and cyclic imides. All compounds were identified as peroxidizing herbicides. Among the 1, 2-alkylene-l, 2, 4-triazolidines, tetramethylene-triazolidines exhibited the strongest peroxidizing phytotoxicity, trimethylene-triazolidines and pentamethylene-triazolidines were found to be less active peroxidizers. The tetramethylene moiety of the triazolidine-type compounds appears to be essential for producing active peroxidizing compounds.Phytotoxicities by triazolidin-one-thiones and triazolidine-dithiones were more than 10 times stronger than those of triazolidine-diones.Replacing carbonyl groups in the five-membered moiety by at least one thiocarbonyl group produces stronger peroxidizing phytotoxities. Although the Protox inhibition of triazolidine-peroxidizers was 10-70 times stronger than that of the corresponding thiadiazolidineperoxidizers, the level of other phytotoxic activities was quite similar with both types of peroxidizers. This fact may be caused by isomerization of thiadiazolidines to triazolidines.

Using autotrophic Scenedesmus cells and maize protoporphyrinogen-IX oxidase, twelve pyrazole compounds recently synthesized were assayed with respect to peroxidizing phytotoxic activity. Three pyrazoles, namely 1-(4-chloro-2-fluoro-5-propargyloxyphenyl)-3, 5-dimethyl-4-nitropyrazole, 5-amino-4-cyano-l-(2, 6-dichloro-4-trifluoromethylphenyl)pyrazole and 5chloro-l-(4-chloro-5-difluoromethoxy-2-fluorophenyl)-3, 4-tetramethylenepyrazole, strongly inhibited growth of the Scenedesmus cells and caused severe chlorophyll deficiency (bleaching activity) at a 10-6 M concentration. The three pyrazoles mentioned were selected for a further quantitative study to determine whether the chlorophyll decrease observed was due to inhibition of pigment biosynthesis or to peroxidative destruction of existing chlorophyll. The pyrazoles inhibited protoporphyrinogen-IX oxidase, caused not only accumulation of protoporphyrin-IX but also light-induced ethane formation. Thus, the three pyrazoles (1, 11 and 12) were identified as peroxidizing herbicides. A 1-(4-chloro-2-fluoro-5-substituted)phenyl moiety at the pyrazole ring appears to be essential for producing active peroxidizing compounds.

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