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OUR efforts to isolate and characterize the Striga germination stimulant in root exudates of maize were accompanied by testing more than 270 compounds for their capacity to stimulate germination. A previous paper1 reported the stimulation of Striga asiatica (witchweed) seed germination and described morphogenic effects on the seedlings induced by kinetin and certain other 6-(substituted) purines. This communication reports the results obtained with 41 coumarin derivatives, two of which stimulated germination of witchweed seeds.

CERTAIN 1,1-dimethyl-3-phenylureas are potent herbicides1–3. Their herbicidal action is attributed in part to inhibition of chloroplast electron transport (the Hill reaction) in photosynthesis4–7. The corresponding mono-methylureas differ only slightly in herbicidal activity2,8 and in inhibition of the Hill reaction7. Nonmethylated phenylureas are, however, relatively inactive both as herbicides2,9 and Hill inhibitors7. In studies of the relationship between structure, herbicidal activity and inhibition of the Hill reaction, it seemed desirable to prepare and test 4-phenylsemicarbazides with varied methyl substitution in the 1 and 2 positions. The 4-(3,4-dichlorophenyl) analogues shown in Table 1 were chosen as models because of the high herbicidal and Hill inhibitory activities of diuron [I,3-(3,4-dichlorophenyl)-1,1-dimethylurea] and nordiuron [II, 3-(3,4-dichlorophenyl)-1-methylurea].