Optimal distribution of defences: Are plants smart investors?

1. Young leaves often contain higher concentrations of chemical defences than mature leaves. They may be better protected against herbivores because they are more valuable. 2. The youngest leaves of the rosette plants of Cynoglossum officinale, contain 50-190 times higher concentrations of pyrrolizidine alkaloid (PA) than old leaves. PAs act as defences against generalist herbivores. 3. We examined a model, which predicts optimal defence allocation in plants, based on the assumptions that photosynthetic activity declines with leaf age, herbivory decreases with increasing PA concentration and PA production is costly but can be reallocated without costs. For optimally distributing plants we predict how the concentration of defences decreases with leaf age. 4. The concentration of defences decreases with increasing resource availability and effectiveness of the defence. Reallocation of defences is most advantageous in slow-growing plants. The distribution of PAs over leaves of C. officinale observed in the field can be explained with the model, suggesting that plants indeed are 'smart' investors.