The correlation between leaf-surface and leaf-tissue secondary metabolites: a case study with pyrrolizidine alkaloids in Jacobaea hybrid plants

Introduction: Usually whole plant or whole leaf extracts are analyzed to study the chemical ecology of insect-plant interactions. For herbivore species the contact with the leaf surface enables them to estimate the quality of the plant. The relationship between the leaf-surface and leaf-tissue secondary metabolites (SMs) could offer important new insights in insect-plant interactions mediated by SMs. Pyrrolizidine alkaloids (PAs), typical defense chemicals in Jacobaea species, are repellent for generalist herbivores but are attractive to specialists. Objectives: Explore whether the PAs on the leaf surface are a reliable representation of the PAs in the leaf tissue in PA-containing plants. Method: The concentration of individual PAs present on the leaf surface and in the corresponding leaf tissue from 37 genotypes (one plant from each genotype) of an F2 generation of a cross between Jacobaea vulgaris and Jacobaea aquatica was measured by high performance liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS). PAs were removed from the leaf surface by extraction with a slightly acidic aqueous solution. Results: The total amount of PAs present on the surface of the leaves was only 0.015% (range: 0.001-0.163%) of the total amount present in the leaf tissue. Most PAs present in the leaf tissue were also found on the surface, except for jaconine, dehydrojaconine, dehydrojacoline and usaramine N-oxide. Positive correlations between leaf-surface and leaf-tissue concentrations were found for most of the jacobine-like and otosenine-like PAs, but correlations for total PA, senecionine- and erucifoline-like PAs were not significant. Conclusion: These results indicate that PA variation on the leaf surface only partially reflects the PA variation in the leaf tissue. Because most herbivores are affected in a different manner by individual PAs, this result means that the leaf surface does not give a reliable estimate of plant quality to herbivores.