Interactions among plant defense compounds: a method for analysis

Summary. Plants contain an enormous diversity and quantity of secondary metabolites, some of which are toxic and deterrent to herbivores and pathogens. This impressive diversity of plant compounds suggests a high probability of interactions among them. Synergistic interactions are those in which the combined activity of two or more chemicals is greater than that expected given their individual activities. On the other hand, antagonistic interactions are those in which the combined activity of two or more chemicals is less than that expected given their individual activities. Synergistic interactions could increase plant fitness whereas antagonistic interactions could decrease plant fitness. Interactions are thus potentially very important, not only in explaining the diversity of defense compounds within individual plants, but also in providing insight into plant defense strategies. Although synergistic interactions have received increased attention in the ecological literature in the last decade, the number of documented cases of synergy remains small and antagonistic interactions are rarely considered. The primary reason for this scarcity may be the difficulty of detecting, analyzing and displaying such interactions. Analysis by ANOVA, though sometimes used, often is not appropriate. We introduce a simple technique, isobolographic analysis, that is used in pharmacology for detecting and rigorously quantifying synergy and antagonism and provide an example using the brine shrimp toxicity assay. More statistically sophisticated approaches, such as isobolographic analysis, will allow ecologists to effectively document the role of chemical synergy and antagonism in interactions between species. Such chemical interactions may ultimately provide insight into longstanding, ecological questions. Received 26 October 1998; accepted 24 March 1999.