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.

The relative phytotoxicity of sorgoleone as measured by seed germination and seedling growth of selected crop and weed species and inhibition of photosynthetic oxygen evolution in atrazine-resistant and -susceptible cell cultures of potato (Solanum tuberosum L.) and common groundsel (Senecio vulgaris L.) were investigated. Relatively little or no effect of sorgoleone was observed on radicle elongation at concentrations less than 500 μM in Petri dish bioassays. Sorgoleone was very phytotoxic to large crabgrass (Digitatia sanguinalis), with a GR50 of 10 μM for shoot growth in a hydroponic culture bioassay. Inhibition of shoot and root growth of velvetleaf (Abutilon theophrasti) and barnyardgrass (Echinocloa crus-galli) was also observed at higher concentrations ranging from 10 to 200 μM, but ivyleaf morningglory (Ipomea hederacea) was tolerant. Sorgoleone inhibited photosynthetic oxygen evolution in both susceptible and resistant cell cultures of potato and common groundsel, and the effect was similar to that of diuron, a strong inhibitor of PS II electron transport. Chlorophyll fluorescence response to sorgoleone in both resistant and susceptible cell cultures was nearly the same. Grain sorghum (Sorghum bicolor L. Moench) genotypes varied considerably in the amount of sorgoleone produced. Root exudates generally contained 85−90% pure sorgoleone on the basis of HPLC analysis. These data indicate that sorgoleone is phytotoxic at micromolar concentrations, exhibits marked selectivity, and inhibits photosynthetic electron transport similar to diuron. Keywords: Sorgoleone; bioassay; hydroponics; inhibition; phytotoxicity; photosynthetic oxygen evolution; electron transport; root exudate

Effects of the alleochemical sorgoleone on photosynthetic electron transport by oxygen-evolving chloroplast thylakoids and Triton X-100-prepared Photosystem II (PSII) membranes were analyzed. The Hill activity of the thylakoids proved to be at least as sensitive to inhibition by sorgoleone as it was to DCMU [3-(3,4-dichlorophenyl)-1,1-dimethylurea], a potent herbicidal inhibitor of PSII. However, a Photosystem I (PSI) partial reaction was not affected by a 10-fold greater concentration of sorgoleone than is required for complete inhibition of Hill activity. Measurements of flash-induced chlorophyll a variable fluorescence showed that sorgoleone neither dissipated excitation energy nor diminished the amplitude of chlorophyll a variable fluorescence. However, it inhibited the decay of variable fluorescence as effectively as DCMU, which blocks the oxidation of the PSII-reduced primary electron acceptor, Q- A, by the PSII secondary electron acceptor, QB, by displacing QB from the D1 protein. Additionally, sorgoleone competitively inhibited the binding of [14C]atrazine to the QB locus. Increasing durations of trypsin proteolysis of the PSII membranes or thylakoids and of the QB-binding niche itself caused parallel losses of inhibition of O2 evolution from sorgoleone and DCMU, as well as from bromoxynil, a phenol-type herbicide also binding to the QB locus. Keywords: Allelopathy; sorgoleone; photosystem II; electron transfer; herbicide