A new phytotoxin, pyridazocidin, was produced in shake-flask cultures of a Streptomyces sp. soil isolate. A combination of solvent partitioning, size-exclusion chromatography, and high-pressure liquid chromatography (HPLC) was used to isolate the material. The structure was assigned based on mass spectroscopy and nuclear magnetic resonance (NMR) experiments. Greenhouse evaluations indicated rapid necrosis and chlorosis of treated weeds, particularly of giant foxtail. Isolated chloroplasts consumed oxygen in the presence of pyridazocidin over a concentration range also inhibiting plant growth. Pyridazocidin represents the first reported natural product that appears to act via reversible oxidation/reduction linked to photosynthetic electron transport.

Isoxaben is an inhibitor of the synthesis of cellulose from glucose. Some dicot weed species are relatively insensitive to isoxaben inhibition. This study investigates mechanisms by which decreased sensitivity may occur in three dicot weed species: catchweed bedstraw, redroot pigweed, and velvetleaf. Dose response curves were generated to determine I50 values for the inhibition of glucose incorporation into cellulose by isoxaben and compared to that of a sensitive species, mouse-ear cress. Metabolic detoxification and uptake rates were measured and the degree of tolerance conferred by these mechanisms was calculated. In all cases, metabolic detoxification was negligible. Lower uptake rates were significant but minor components of tolerance in all species. It is suggested that the principal cause of isoxaben tolerance in these dicot weed species is decreased sensitivity at the target site.