Interactions Between a Belowground Herbivore and Primary and Secondary Root Metabolites in Wild Cabbage

Abstract Plants are attacked by both above- and below-ground herbivores. Toxic secondary compounds are part of the chemical defense arsenal of plants against a range of an-tagonists, and are subject to genetic variation. Plants also pro-duce primary metabolites (amino acids, nutrients, sugars) that function as essential compounds for growth and survival. Wild cabbage populations growing on the Dorset coast of the UK exhibit genetically different chemical defense profiles, even though they are located within a few kilometers of each other. As in other Brassicaceae, the defensive chemicals in wild cabbages constitute, among others, secondary metabo-lites called glucosinolates. Here, we used five Dorset popula-tions of wild cabbage to study the effect of belowground her-bivory by the cabbage root fly on primary and secondary chemistry, and whether differences in chemistry affected the performance of the belowground herbivore. There were sig-nificant differences in total root concentrations and chemical profiles of glucosinolates, amino acids, and sugars among the five wild cabbage populations. Glucosinolate concentrations not only differed among the populations, but also were affect-ed by root fly herbivory. Amino acid and sugar concentrations also differed among the populations, but were not affected by root fly herbivory. Overall, population-related differences in plant chemistry were more pronounced for the glucosinolates than for amino acids and sugars. The performance of the root herbivore did not differ among the populations tested. Survival of the root fly was low (<40%), suggesting that other belowground factors may override potential differences in ef-fects related to primary and secondary chemistry