Activity and persistence of sorgoleone, a long-chain hydroquinone produced by Sorghum bicolor

Sorghum bicolor (L.) Moensch and other related Sorghum species produce sorgoleone - a long chain hydroquinone compound (MW = 358), which is exuded by living root systems. This compound exhibits potent activity as a photosynthetic inhibitor. A diverse group of sorghum germplasm was evaluated for sorgoleone production. Production was quite variable, with certain accessions producing up to 15 mg sorgoleone/g fresh root weight. The root exudate composition among accessions was less variable, with sorgoleone being the major constituent (76-99%) within the extract. The potential binding of sorgoleone to the D1 protein of the PSII complex was evaluated in triazine resistant and susceptible redroot pigweed (Amaranthus retroflexus L.) thylakoids. Sorgoleone, metribuzin and diuron exhibited competitive binding with atrazine in susceptible thylakoids, while no competition was evident in resistant thylakoids. Sorgoleone has an intermediate affinity between that of diuron and metribuzin from estimated binding constants. Computer-aided design programs have proven useful to further evaluate structural activity relationships for sorgoleone, and PSII inhibitors. Sorgoleone at concentrations up to 40 ppmw significantly reduced shoot fresh weight in several broadleaf weed species when incorporated in soil, but had limited effect on root development. Sorgoleone at the same concentrations applied postemergence also caused significant growth reduction to a number of weed species. Recovery of soil impregnated with sorgoleone was most effective (up to 85%) when acetonitrile:water (80:20 v/v) was used as the extractant, in comparison to methanol:water or water alone. Recovery declined over a 42 day period after incorporation, but detectable sorgoleone levels were observed after 7 weeks.