The optimal defense theory (ODT) of chemical defense provides a predictive framework for the distribution of anti-herbivore defenses in plants. One of its predictions is that chemical defenses will be allocated within a plant as a function of tissue value, where value is correlated with the cost of having that tissue removed. While many studies have examined intra-plant variation in defensive chemistry, these results have rarely been compiled quantitatively to assess the question of whether defense allocation is consistent with the prediction of ODT that more valuable tissues should be more defended than less valuable tissues.
We performed a formal meta-analysis of published and unpublished studies to examine the predictive utility of ODT. Specifically, we examined whether defensive chemicals occur at higher concentrations in flowers versus leaves and in younger leaves compared to older leaves, under the assumption that younger leaves are more valuable than older leaves. We also examined whether the expansion status of younger leaves, nodal position of the leaves, growing conditions, and chemical class of defensive compounds affected the mean effect sizes.
Results/Conclusions
We found that tissues with higher assumed value had significantly higher concentrations of defensive chemicals than tissues with lower value. In particular we found that younger leaves had higher concentrations of defensive chemicals than older leaves, consistent with the predictions of ODT. The magnitude of this difference was higher in the younger leaf/older leaf comparison than in the flower/leaf comparison, with no evidence that flowers were more defended than leaves. The overall results were not affected by chemical class, young leaf expansion, growing conditions, or leaf position on the plant.
Our results indicate that ODT is particularly useful in predicting the relative defensive chemical concentrations in younger leaves versus older leaves. It appears that plants are not diluting defensive chemicals in leaves as they age because there was no difference in the effect size between studies examining expanded versus unexpanded young leaves. Flowers might not have higher concentrations of defensive compounds because they often serve to attract insects, and thus plants might be under selection to balance the costs of herbivory versus the benefits of pollination service.
