Four substituted pyridazinone compounds inhibited the Hill reaction and photosynthesis in barley (Hordeum vulgareL., var. Dayton C.I. 9517). These inhibitions appeared to account for the phytotoxicity of 5-amino-4-chloro-2-phenyl-3(2H)-pyridazinone (pyrazon). The pyridazinone chemicals were weaker inhibitors than 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine). Two substitutions onto the molecular structure of pyrazon result in a new experimental herbicide, 4-chloro-5-(dimethylamino)-2-(α,α,α-trifluoro-m-tolyl)-3(2H)-pyridazinone (hereinafter referred to as 6706), which retains the action mechanism of pyrazon but also has two additional biological properties. It is resistant to metabolic detoxication in plants, and it possesses a second mode of action involving interference with chloroplast development. The second action is like that expressed by 3-amino-s-triazole (amitrole) and by 3,4-dichlorobenzyl methylcarbamate (dichlormate). However, the new chemical is 100 to 1000 times more effective. The trifluoromethyl substitution on the phenyl ring and the dimethyl substitution on the amine are both required to give either of the two additional physiological properties. Analogs with only one of the two substitutions behave like pyrazon rather than like 6706.
Differences in levels of resistance toSeptoria tritici blotch were observed in plants with a specific height-reducing gene. When the gene 'Rht 2' was present either as an isoline or in the progeny, a higher degree of resistance was found. The most susceptible plants were observed in populations carrying the 'Rht 1' gene. Associations, as determined by phenotypic correlations, were detected betweenSeptoria tritici blotch and tall stature, late heading, and maturity. Plants having short stature, early heading, early maturity, and acceptable levels of resistance were identified in the F2 population whenRht 2 was present. Results of this study indicated that wheat breeders must select the appropriate dwarfing source that may confer resistance and grow large F2 populations, in order to increase the probability of obtaining desired genotypes.
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