| Summary: | Cyanazine activity against wheat, blackgrass and turnip seedlings, grown in soil, sand or liquid culture, was investigated under glasshouse conditions. Post-emergence foliar applications of cyanazine were seriously phytotoxic to turnip alone, but regular rewetting of foliar deposits significantly increased phytotoxicity to wheat. Cyanazine applied postemergence to the soil surface entered wheat via the roots alone, whereas in turnip entry occurred both through the root and sub-surface shoot. Entry of soil-applied cyanazine into blackgrass was largely confined to adventitious roots developed close to the soil surface, Selectivity was related to differences in intrinsic tolerance and rate of uptake from soil. Wheat tolerated tissue concentrations five to six times those tolerated by blackgrass or turnip, and unlike them, was incapable of absorbing cyanazine from soil above seed level. However, due to its high transpiration rate, wheat rapidly accumulated high concentrations of cyanazine when the roots were exposed to the herbicide. Increasing the soil water content or reducing the seed depth encouraged exposure of the root system and considerably reduced selectivity. Selectivity was unrelated to differences in translocation, but inactivation within the tissues was slower in wheat than the other species. Phytotoxicity was related to stage of growth at treatment. Susceptibility to soil-applied cyanazine increased temporarily as photosynthetic tissues were first formed, but decreased steadily thereafter. Susceptibility of turnip, to foliar applications, decreased slightly as foliage density increased, reducing the percentage of leaf area exposed, but that of wheat and blackgrass increased as the leaves became larger and less erect. Selectivity was greatest at the very early seedling stages, and decreased steadily thereafter. Measurements were made of persistence and movement of cyanazine in soil, and of its effect on transpiration.
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