A Simple Bioassay for Herbicidal Substances of Microbial Origin by Determining de novo Starch Synthesis in Leaf Segments

A simple and sensitive assay system was developed in the search for new herbicidal substances. The system is based on the following two phenomena observed in photosynthesis: (1) de novo starch synthesis, determined in excised leaf segments of barnyard millet (Panicum crus-galli), a C₄ plant, and Italian ryegrass (Lolium multiflorum Lam.), a C₃ plant; and (2) oxygen evolution in the cells of Scenedesmus obliquus, detected by using an oxygen electrode. The system can detect photosynthesis-inhibiting herbicides at a concentration as low as 0.1 ppm. After assaying 6,500 microbial culture broths, 6 culture filtrates were selected for further study.