Patterns of secondary metabolite allocation to fruits and seeds in Piper reticulatum

Abstract

Little is known about the evolution, diversity, and functional significance of secondary metabolites in reproductive plant parts, particularly fruits and seeds of plants in natural ecosystems. We compared the concentration and diversity of amides among six tissue types of Piper reticulatum: leaves, roots, flowers, unripe fruit pulp, ripe fruit pulp, and seeds. This represents the first detailed description of amides in P. reticulatum, and we identified 10 major and 3 minor compounds using GC/MS and NMR analysis. We also detected 30 additional unidentified minor amide components, many of which were restricted to one or a few plant parts. Seeds had the highest concentrations and the highest diversity of amides. Fruit pulp had intermediate concentrations and diversity that decreased with ripening. Leaves and roots had intermediate concentrations, but the lowest chemical diversity. In addition, to investigate the potential importance of amide concentration and diversity in plant defense, we measured leaf herbivory and seed damage in natural populations, and examined the relationships between amide occurrence and plant damage. We found no correlations between leaf damage and amide diversity or concentration, and no correlation between seed damage and amide concentration. The only relationship we detected was a negative correlation between seed damage and amide diversity. Together, our results provide evidence that there are strong selection pressures for fruit and seed defense independent of selection in vegetative tissues, and suggest a key role for chemical diversity in fruit-frugivore interactions.