The application of an automated dialysis and colorimetric procedure to the assay of NO2− and NO3− in plant extracts is described. Nitrite analysis was used for the estimation of nitrate reductase activity in extracts from etiolated Pisum arvense L. and Zea mays L. The advantages of this procedure include its convenience and suitability for performing analyses on a large number of samples. The procedure allowed the analysis of 15 assays (enzyme plus blank) per hour and had a lowest detection level of 6 nmol NO2−∙g FW−1∙h−1 (10× range expansion) for nitrate reductase activity. Dialysis serves to clarify the assay solution and also eliminate the interference of the enyzmatically unconsumed reducing cofactor during color development. Nitrate, an inducer of nitrate reductase, is conveniently estimated in the same extract as used for enzyme assay without further treatment.
