1. Optimal defence allocation theory (ODT) is one of the most prominent theoretical frameworks to explain the allocation of defence compounds within plants. It predicts that the most valuable and vulnerable plant organs have the highest levels of chemical defence. The ODT has been well worked out and experimentally tested for shoot defences, but not for root defences. To assess if ODT principles apply similarly to roots and shoots, we examined glucosinolates in aboveground and belowground organs of nine plant species belonging to two families. 2. In order to evaluate whether ODT equally applies to shoot and root organs, we designed a conceptual model in which aboveground and belowground organs were assigned to orders of importance to plant performance. We hypothesized that organs constituting the plant’s core structure are better protected than more distal organs. 3. The nine plant species were cultivated, and their roots and shoots were harvested and divided in three orders for glucosinolate analysis. Using a specialist (Delia radicum) and a generalist (Amphimallon solstitiale) root herbivore, we also experimentally tested the hypothesis that the generalist herbivore prefers to feed on fine roots with a low glucosinolate concentration, while the specialist prefers taproots with a high glucosinolate concentration. 4. We found that both in roots and shoots the higher-ordered core structural organs (taproots and stems) had the highest levels of glucosinolates. Belowground, taproots and lateral roots were better protected than the more distal, and less costly, fine roots in seven out of nine species tested. The specialist root herbivore preferred feeding on the highly defended taproots, which is in line with what has been found for aboveground specialist herbivores. Moreover, the glucosinolate concentration in roots overall was significantly higher than that in shoots. 5. Synthesis. These results support the hypothesis that ODT generally applies to glucosinolate allocation in aboveground and belowground organs and may mainly serve to maintain the integrity of the main plant structure. Moreover, it suggests that aboveground and belowground insect herbivores independently exert similar selection pressures on defence allocation patterns in roots and shoots.

Glucosinolate and insect feeding preference dataData.xlsx