Sorghum cultivars were compared for their root development and production of the witchweed germination stimulant sorgoleone. In pot experiments, the resistant cultivar IS-7777 yielded a greater root biomass and root length, but a smaller root length/biomass ratio than the highly susceptible cultivars CK-60B and Bimbiri. Seedlings of these latter two cultivars exuded a high concentration of sorgoleone, while the low-susceptible cultivar IS-14825 exhibited a low production of this compound. In contrast, the low-susceptible cultivar Seguetana Niarabougou exhibited a high production of sorgoleone, while the resistant cultivars IS-7777 and Framida exuded a moderate amount of this compound. These results indicate that witchweed seed avoidance by means of reduced root growth is unlikely to be an important factor involved in the resistance of IS-7777. Low production of sorgoleone could partly explain low susceptibility of IS-14825, but other compounds may play a more significant role in the stimulation of witchweed germination.

The root exudates produced by sorghums contain a biologically active constituent known as sorgoleone. Seven sorghum accessions were evaluated for their exudate components. Except for johnsongrass, which yielded 14.8 mg root exudate/g fresh root wt, sorghum accessions consistently yielded approximately 2 mg root exudate/g fresh root wt. Exudates contained four to six major components, with sorgoleone being the major component (> 85%). Three-dimensional structure analysis was performed to further characterize sorgoleone's mode of action. These studies indicated that sorgoleone required about half the amount of free energy (493.8 kcal/mol) compared to plastoquinone (895.3 kcal/mol) to dock into the QB-binding site of the photosystem II complex of higher plants. Light, cryo-scanning, and transmission electron microscopy were utilized in an attempt to identify the cellular location of root exudate production. From the ultrastructure analysis, it is clear that exudate is being produced in the root hairs and being deposited between the plasmalemma and cell wall. The exact manufacturing and transport mechanism of the root exudate is still unclear. Studies were also conducted on sorgoleone's soil persistence and soil activity. Soil impregnated with sorgoleone had activity against a number of plant species. Recovery rates of sorgoleone-impregnated soil ranged from 85% after 1 h to 45% after 24 h. Growth reduction of 9 14-d-old weed species was observed with foliar applications of sorgoleone.