Abstract 1. Plant growth and chemical defence compounds in four Lotus corniculatus genotypes exposed to factorial combinations of ambient and elevated carbon dioxide, and herbivory by caterpillars of Polyommatus icarus were measured to test the predictions of the carbon/nutrient balance hypothesis.
2. Shoot and root biomass, allocation to shoots versus roots, and carbon-based defence compounds were greater under elevated carbon dioxide. Pupal weight of P. icarus was greater and development time shorter under elevated carbon dioxide.
3. Herbivory decreased shoot growth relative to root growth and production of nitrogen-based defence (cyanide). Young leaves contained more defence compounds than old leaves, and this response depended on carbon dioxide and herbivory treatments (significant interactions).
4. Genotype-specific responses of plants to carbon dioxide and herbivory were found for the production of cyanide. Furthermore, maternal butterfly-specific responses of caterpillars to carbon dioxide were found for development time. This suggests the existence of genetic variation for important defence and life-history traits in plants and herbivores in response to rising carbon dioxide levels.