PHYTOTOXICITY AND DISTRIBUTION OF SORGOLEONE IN GRAIN-SORGHUM GERMPLASM

The relative phytotoxicity of sorgoleone as measured by seed germinati on and seedling growth of selected crop and weed species and inhibitio n of photosynthetic oxygen evolution in atrazine-resistant and -suscep tible cell cultures of potato (Solanum tuberosum L.) and common ground sel (Senecio vulgaris L.) were investigated. Relatively little or no e ffect of sorgoleone was observed on radicle elongation at concentratio ns less than 500 mu M in Petri dish bioassays. Sorgoleone was very phy totoxic to large crabgrass (Digitatia sanguinalis), with a GR(50) of 1 0 mu M for shoot growth in a hydroponic culture bioassay. Inhibition o f shoot and root growth of velvetleaf (Abutilon theophrasti) and barny ardgrass (Echinocloa crus-galli) was also observed at higher concentra tions ranging from 10 to 200 mu M, but ivyleaf morningglory (Ipomea he deracea) was tolerant. Sorgoleone inhibited photosynthetic oxygen evol ution in both susceptible and resistant cell cultures of potato and co mmon groundsel, and the effect was similar to that of diuron, a strong inhibitor of PS II electron transport. Chlorophyll fluorescence respo nse to sorgoleone in both resistant and susceptible cell cultures was nearly the same. Grain sorghum (Sorghum bicolor L. Moench) genotypes v aried considerably in the amount of sorgoleone produced. Root exudates generally contained 85-90% pure sorgoleone on the basis of HPLC analy sis. These data indicate that sorgoleone is phytotoxic at micromolar c oncentrations, exhibits marked selectivity, and inhibits photosyntheti c electron transport similar to diuron.