Chemical defenses in tropical fruits: Quantifying variation in toxicity across fruit development and within fruit of vertebrate- and wind-dispersed canopy plants

Background/Question/Methods  The selective pressure of mutualists and antagonists has resulted in a myriad of plant adaptations, including nutritional fruit traits in response to seed dispersers and defenses in response to seed predators and pathogens. In fruit, chemical defenses are expected to be higher in immature compared to mature fruit, where they may mediate disperser interactions as well as protect seeds. The objective of this study is to quantify how chemical defenses vary between immature and mature fruit and within fruit (seed vs. pericarp) in eleven canopy species, and for five species, how chemical defenses in fruit compare to those in leaves. Chemical defenses were measured as reduced growth or survivorship of bioassay organisms across three concentrations of crude extract and controls. Crude extracts were prepared from immature and mature fruit from both wind- and vertebrate-dispersed species. Bioassays were performed with two foliar fungal pathogens (Phoma sp and Fusarium sp) and brine shrimp (Artemia franciscana), a commonly used organism for determining insecticidal properties of compounds. The responses of bioassay organisms (hyphal growth relative to controls and proportion of surviving brine shrimp) were fit to nonlinear curves to estimate the concentration at which there is a fifty percent reduction in growth or survivorship.

Results/Conclusions  Toxicity depended on fruit part, stage of development, and bioassay organism. In the brine shrimp bioassay, eight species exhibited higher toxicity in mature compared to immature fruits, whereas two species showed higher toxicity in immature fruit. In mature fruit, seeds of eight species were either similarly or more toxic than the pericarp. In the Phoma bioassay, the pericarp of mature Anacardium excelsum and the seed of mature Nectandra purpurascens significantly reduced hyphal growth, while the majority of crude extracts increased hyphal growth. In the Fusarium bioassay, nine species were more toxic in immature rather than mature fruit and the component of the fruit that was chemically defended varied among species. For five species, crude extracts from young leaves had similar or lower toxicities compared to fruit. The results of this study suggest that the majority of fruit in this system are chemically defended against natural enemies either in the seed or pericarp at some stage in development. The adaptive value of secondary metabolites in fruit is controversial, but recent studies suggest their potential adaptive role in defending seeds against microbes. Understanding the role of secondary metabolites in plant-animal and –microbe interactions will help resolve the evolution of fruit-frugivore interactions.