Plant defensive compounds have been established as important mediators of interactions between plants and their natural enemies. Many plant enemies, however, only attack specific tissues within any individual plant. Thus, the Optimal Defense Theory posits that allocation to defense of plant tissues occurs in relation to tissue value and probability of attack. Some previous studies have supported this, showing preferential allocation of defensive compounds to reproductive tissues. These studies, however, are generally snapshots in time. Shifts in allocation through time may be a successful strategy for plants defending against a variety of enemies. While it is known that these shifts can happen, there is little information about how the onset of phenological events such as flowering and fruiting correspond with changing allocation patterns among tissues. We investigated allocation of the defense compounds iridoid glycosides to leaves, flowers, fruits, and seeds in the honeysuckle Lonicera x bella (Caprifoliaceae) at important phenological time-points throughout a growing season. Seasonal allocation changes are especially unexplored for woody shrubs that flush only once in a season. In particular, we tested how allocation of iridoid glycosides to seeds and fruits changes as seeds develop and how this corresponds with changes in allocation to leaves.
Results/Conclusions
Leaves differed from reproductive tissues in both the concentrations of iridoid glycosides and the identities of individual iridoid compounds. Leaves, on average, contain lower concentrations of defensive compounds and fewer individual compounds. Preliminary results showed significant changes through time in some but not all individual iridoid glycosides. These results support the importance, when studying the allocation of chemical defense, of considering temporal variation as well as quantifying individual defensive compounds. Our preliminary data on patterns of allocation show some interesting differences from those found in iridoid glycoside-producing forbs, perhaps due to the different growth patterns. For example, these shrubs do not produce new leaves throughout the growing season and so must alter the defense levels of existing tissues. These data suggest that the suitability of different tissues for herbivores, dispersers, and pathogens changes throughout a growing season. This is likely to impact the outcomes of plant-enemy and plant-mutualist interactions, so temporal changes in allocation patterns may be an important component of plant chemical defense.