Print PageOptimal defense theory predicts that plant parts that have the highest fitness value, such as flowers and fruits, should be protected with high levels of chemical defense, but that, in animal-dispersed fruits, these defenses should diminish upon ripening. Changes in fruit chemistry with ripening that match these predictions are common; however, many ripe fruits still contain potentially toxic levels of secondary compounds. It has been debated whether these compounds should be explained adaptively, or as a result of physiological constraints on their exclusion from ripe fruit tissue. One adaptive hypothesis is that secondary compounds in ripe fruits function mainly as a defense against fruit antagonists, such as insect seed predators and fungal pathogens. In this case, plants may experience a trade-off between attraction of seed dispersers and fruit defense. We examined intraspecific variation in secondary chemistry of leaves, unripe, and ripe fruits in three populations of Lonicera x bella (Caprifoliaceae), focusing on a class of compounds with known importance in plant defense—the iridoid glycosides. We also monitored rates of fruit removal by vertebrates and fruit damage by pests at two-week intervals throughout a complete fruiting season, and examined relationships among these ecological interactions and the occurrence of fruit iridoid glycosides.
Results/Conclusions
Overall, predictions of optimal defense theory were supported; average levels of iridoid glycosides were highest in unripe fruits (14.9% dry weight), diminished in ripe fruits (9.4% dry weight), and lowest in leaves (5.3% dry weight). There was considerable variation among individuals and populations, particularly for unripe fruits, with levels of total iridoid glycosides ranging from 4-29% dry weight. Frequent mismatches in the composition of individual compounds and a lack of correlation in concentrations between leaves and fruits on the same plant suggest that there are no strong physiological constraints on the exclusion of particular foliar compounds from reproductive tissues. In addition, our field monitoring showed that only certain compounds were related to patterns of fruit removal and damage, and some compounds that influence removal rates do not seem to influence damage. These results are an important first step in understanding the relative influence of selective forces versus constraints on the evolution of fruit chemical traits and the implications for both mutualist seed dispersers and antagonist fruit pests.
