Rooted in place, plants often rely on secondary metabolites to mediate interactions with other organisms. Both attraction of mutualists and defense against antagonists are thought to be mediated by secondary metabolites, but the chemical ecology of wild, fleshly fruits is relatively unexplored. Piper is one of the largest genera of flowering plants, containing about 1,000 species. Previous investigations into the genus have described the presence of a broad range of secondary metabolites in leaves, including amides, alkaloids, lignans, terpenes, and steroids. However, few studies have described the occurrence of secondary metabolites found specifically in the infructescences of Piper. This study describes the secondary metabolites occurring in the infructescences of Piper sancti-felicis and their functional significance in ecological interactions. We focus on one group of compounds: alkenylphenols. Structurally-related compounds that have been described from several other species in the genus have known antibacterial and cytotoxic effects in vitro. This study has three specific objectives: 1) to elucidate the structures of the major alkenylphenol compounds present in the infructescences of P. sancti-felicis; 2) to describe the natural variation in alkenylphenol composition throughout infructescence development and across individual plants; and 3) to test the ecological significance of the alkenylphenols in fruit defense against fungi.
Results/Conclusions
We found ten alkenylphenols present in the infructescences of P. sancti-felicis. Results suggest that alkenlyphenol concentration in infructescences significantly differed among individual plants, developmental stages, and individual compounds. Alkenylphenol concentration was clearly higher in unripe infructescences compared to inflorescences. Findings from leaf analysis also suggest that alkenlyphenol concentrations in leaves differ among individual plants, developmental stages, and individual compounds. Expanding leaves had more alkenylphenols compared to young and mature leaves, where the compounds were relatively absent. Intraspecific variation of alkenylphenols across individual plants was clearly high. Results from the microdilution bioassays revealed that as alkenylphenol concentration increases, absorbance decreases. Our results suggest that the alkenylphenols in P. sancti-felicis are functioning as defensive secondary metabolites, possessing anti-fungal properties. As secondary metabolites may be expensive to produce, the plant invests more in the fruiting structure over time, with the highest concentrations found in unripe infructescences. Because defensive compounds tend to be less palatable, the alkenylphenol concentration drops slightly in ripe infructescences to avoid deterring seed dispersers.