Five photosynthesis inhibiting herbicides were compared for their mobility in soil columns used as the penetration model. The vertical movement of these compounds in soil was analyzed by a newly developed technique in which herbicide distribution was determ ined by the intensity of chlorophyll fluorescence in Chlorella cells suspended in aqueous extracts of soil samples from various depths. With the aid of a fluorescence microplate scanner, measurement of chlorophyll fluorescence was performed with great efficiency. Photosynthesis inhibiting activity of tested herbicides was determined beforehand by the same method and served in calculating herbicide concentrations in sample extracts. Among tested compounds, hexazinone showed the greatest mobility and diuron the smallest, suggesting water solubility is one of the factors which govern the soil mobility of herbicides.
Ruling factors governing pre-emergence herbicidal activity were analysed for 16 photosynthesis-inhibiting 5-hydroxy-3-methyl-2-0x0imidazolidine-1-carboxamide derivatives. Herbicidal performance was quantified by the reduction in area of experimental weed vegetation, measured by a computer-aided image analysis system. A system for fluorometric estimation of photosynthesis inhibitor concentration in aqueous solution greatly facilitated determination of the soil adsorption coefficients (K,). Maximum herbicidal performance was found for N-sec-butyl-5-hydroxy-3-methyl-2-oxo-imidazolidine-1carboxamide, a compound with the second lowest soil adsorptivity and average photosynthesis-inhibiting activity. A multiple regression analysis suggested that herbicidal performance of the soil-applied imidazolidine derivatives was determined by a balance between K, and photosynthesis-inhibiting activity. In the present experimental system, however, the main influence was attributed to K,.
An attempt was made to account for quantitatively measured herbicidal performance of foliage-applied 5-hydroxy-3-methyl-2-0x0-imidazolidine-1carboxamide derivatives by their photosynthesis-inhibiting activity and systemicity. Photosynthesis-inhibiting activity was estimated from the increase of chlorophyll fluorescence intensity in Chlorella vulgaris Beijer cells measured by a microplate scanner, and systemicity was also evaluated by computer-aided chlorophyll fluorescence imaging. The highest herbicidal performance was recorded for N-cyclohexyl-5-hydroxy-3-methyl-2-oxo-imidazo~idine-l-carboxamide, a compound with the second strongest photosynthesis-inhibiting activity and intermediate systemicity. Though neither photosynthesis-inhibiting activity nor systemicity showed significant correlation with the actual herbicidal performance in simple regression analyses, a high predictability was found for a multiple regression on both parameters as two independent variables, suggesting that these two factors work cooperatively in the field performance.
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