Evaluation of a rapid semi-quantitative assay for cyanogens in cassava

A rapid qualitative test for hydrogen cyanide, involving color changes on paper pre-treated with the compound tetra base (4,4'-Methylene bis [N,N-dimethylaniline]), was recently modified. The modified method was subsequently proposed as a semi-quantitative screening assay for the cyanogen content of cassava root parenchymal tissue (Bradbury and Egan, in press). This test is being considered specifically for use by cassava breeding programs as a replacement for the now updated picrate test. Research is currently being undertaken at CIAT to evaluate the proposed assay, using a variety of cassava cultivars with widely varying cyanogen contents. A quantitative enzymic assay is being used as a control. Work to date has indicated that a broad differentiation of samples according to cyanogen content may be obtained. This differentiation was more readily obtained on the basis of the time taken for color development, than according to differences in colors obtained within a fixed test-time. No significant temperature-related differences were indicated in the assay, within the range 20-35 degrees C. The reliability of the sampling method used in the assay has also been partially investigated. The sampling method employed involves the removal of a central disc from the root being assayed. The cyanogen content of the central disc was compared against that of the remainder of the parenchyma. An acceptable degree of correlation was indicated, implying that the sampling procedure is acceptable within the context of a semi-quantitative assay. The aims of further work being undertaken include the continued investigation of possible effects of temperature upon the assay as well as a closer definition of its accuracy reproducibility and limitations. The endogenous enzyme linamarase is responsible for the evolution of hydrogen cyanide in a given sample of cassava parenchymal tissue. The tetrabase assay depends on this function of the enzyme. Further work is planned to investigate possible assay-related effects of variation in sample linamarase activity