AbstractDiuron, a substituted urea herbicide, was supplied to sugarcane in factorial combination with variable nitrate. Employing the sand-culture technique, Diuron was given at rates of 0, 0.05, and 0.50 p.p.m., and nitrate at 1.5, 4.5, and 13.5 meq./liter. There were three objectives: 1, To confirm a theory that Diuron is capable of retarding sucrose production; 2, that Diuron rather than soil factors directly influence enzymes related to sugar level; and 3, to study persistence of Diuron-sugar-enzyme relationships under differing conditions of nitrogen status. Plant growth was not affected by Diuron, but it increased markedly between 1.5 and 4.5 meq./liter of nitrate. Leaf analyses gave an average of about 2.5 and 14 µg. of Diuron per gram of tissue, respectively, for the medium and high Diuron treatments. Variable nitrate did not affect Diuron content. Diuron suppressed leaf-sucrose content and increased fructose as main effects. However, at the medium nitrate level, sucrose was increased by the 0.05-p.p.m. Diuron treatment. Both Diuron and increasing nitrate suppressed meristem sucrose. Thus, both the ability of Diuron to suppress sucrose and a dependence of Diuron action upon nitrogen status was demonstrated. Phosphatase and peroxidase were activated by Diuron, confirming earlier observations with ratoon samples from Diuron-treated soil. Amylase was moderately increased by Diuron. Meristem amylase reflected a marked interaction between Diuron and nitrogen. Polyphenol oxidase was greatly stimulated by decreasing nitrate, regardless of Diuron supply.
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