Locally and systemically induced glucosinolates follow optimal defence allocation theory upon root herbivory

1. Herbivore-induced defences in plants are considered a strategy to manage multiple interactions while saving resources. The optimal defence theory (ODT) is one of the most prominent theoretical frameworks to explain the defence allocation patterns within plants. It was recently shown that the ODT generally applies to constitutive glucosinolate (GSL) allocation in shoot and root organs. Previous studies showed that both root and shoot herbivore feeding may alter defence allocation over plant organs. For shoots, the effect depends on where the herbivores feed. It is as yet unknown whether similar principles apply to root-herbivore induced GSLs. 2. To analyse the effects of root localized herbivore feeding on GSL allocation, we conducted a pot experiment using Anomala cuprea grubs and four Brassicaceae; Brassica rapa, B. nigra, B. oleracea, and Sinapis alba. Individuals of these four plant species were grown in dedicated mesocosms. The grubs were confined either to the bottom soil, the m...