CoDlection of aDlelopathic chemicals from the undisturbed plant root system is difficult because of their low concentrations and the high level of contaminants in growth media such as soil. A new approach for the continuous trapping of quantities of extracelular chemicals from donor plants is described. Bigalta limpograss (Hemarthria altissima), a tropical forage with aflelopathic activities, was established in sand culture. Nutrient solution was circulated continuously through the root system and a column containing XAD4 resin. ExtraceDlular hydrophobic metabolites were selectively adsorbed by the resin, while inorganic nutrients were recycled to sustain plant growth. Columns were eluted with methanol and the eluate separated into neutral, acidic, and basic fractions. Bioassays of trapped root exudates using lettuce seed combined with paper and thin layer chromatography showed that the inhibitors were mainly phenolic compounds. The active neutral fraction was methylated and analyzed by gas chromatography-mass spectrometry. Twelve compounds were identified, with two additional compounds tentatively identified. 3-Hydroxyhydrocinnamic, benzoic, phenylacetic, and hydrocinnamic acids were the major rhizospheric compounds with known growth regulatory activities.Inasmuch as the root system was undisturbed and the recovery of exudates was highly efficient compared to conventional solvent extraction methods, this trapping system should be useful for a wide range of studies that relate to the chemistry of the rhizosphere.In a recent review updating research on allelopathy, Rice (13) cited more than 400 references, with the majority published after 1970. This large volume of research activity reflects the growing awareness of the implications of allelopathic interactions in agricultural and ecological systems. Numerous observations have been recorded on the harmful effects of one plant species upon another grown in the same community (14). To prove that the effects were allelopathic, careful experimental design was used to differentiate them from the influences of competition for light, water, and nutrients. More detailed work (13,14) conditions. Rather than collecting the responsible extracellular inhibitors from the intact, living donor plants, tissue extracts from either fresh or dried plant materials were commonly used (8,14). Compounds identified in this manner, however, were not necessarily responsible for the observed allelopathic interaction. Efforts were made in the past to collect inhibitors from the donor root systems under undisturbed conditions. Samples were obtained by solvent extraction of the medium of hydroponic cultures (3, 5) or by the washings of sand (1) or gravel cultures (6). These methods were often tedious, and their success in the isolation of the allelochemics limited (14). Consequently, our knowledge of allelopathic chemistry remains inadequate, despite the increasing capability of modem instrumental analysis for trace organic molecules.To overcome difficulties in sample collection, we...

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The phytoalexin resveratrol (trans-3,5,4'-trihydroxy-stilbene), a natural component of resistance to fungal diseases in many plants, is synthesized by the enzyme trihydroxystilbene synthase (stilbene synthase, EC 2.3.1.95), which appears to be deficient or lacking in susceptible plants. Earlier workers isolated a stilbene synthase gene (Vst1) from grapevine (Vitis vinifera L.), which has subsequently been introduced as a transgene into a range of species to increase resistance of hosts to pathogens to which they were originally susceptible. Papaya (Carica papaya L.) is susceptible to a variety of fungal diseases, including root, stem, and fruit rot caused by the pathogen Phytophthora palmivora. Since resveratrol at 1.0 mM inhibited mycelium growth of P. palmivora in vitro, we hypothesized that papaya resistance to this pathogen might be increased by transformation with the grapevine stilbene synthase construct pVst1, containing the Vst1 gene and its pathogen-inducible promoter. Multiple transformed lines were produced, clonally propagated, and evaluated with a leaf disk bioassay and whole plant response to inoculation with P. palmivora. RNA transcripts of stilbene synthase and resveratrol glycoside were induced in plant lines transformed with the grapevine pVst1 construct shortly after pathogen inoculation, and the transformed papaya lines exhibited increased resistance to P. palmivora. The immature transformed plants appear normal and will be advanced to field trials to evaluate their utility.

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Kava (Piper methysticum), a perennial shrub native to the South Pacific islands, has been used to relieve anxiety. Recently, several cases of severe hepatotoxicity have been reported from the consumption of dietary supplements containing kava. It is unclear whether the kava constituents, kavalactones, are responsible for the associated hepatotoxicity. To investigate the key components responsible for the liver toxicity, bioassay-guided fractionation was carried out in this study. Kava roots, leaves, and stem peelings were extracted with methanol, and the resulting residues were subjected to partition with a different polarity of solvents (hexane, ethyl acetate, n-butanol, and water) for evaluation of their cytotoxicity on HepG2 cells based on the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay and lactate dehydrogenase and aspartate aminotransferase enzyme leakage assays. Organic solvent fractions displayed a much stronger cytotoxicity than water fractions for all parts of kava. The hexane fraction of the root exhibited stronger cytotoxic effects than fractions of root extracted with other solvents or extracts from the other parts of kava. Further investigations using bioassay-directed isolation and analysis of the hexane fraction indicated that the compound responsible for the cytotoxicity was flavokavain B. The identity of the compound was confirmed by (1)H and (13) C NMR and MS techniques.